Introduction

In the vacuum heat treating world, where critical components are often near-net-shape with minimal to zero stock removal, the surface aesthetics of the final product are critical to the end user. Across industries such as aerospace, medical devices, and power generation vacuum processing has become increasingly valued—not only for its precision, but also for its ability to eliminate downstream operations, ultimately saving cost and time.

Given these benefits, customers are frequently willing to pay a premium for “bright, clean work.” To achieve these pristine results, vacuum heat treaters insist that incoming parts must be “clean and oil-free.” However, what qualifies as “clean” in a manufacturing environment rarely meets the exacting standards required for vacuum thermal processing. As a result, many commercial heat treaters adopt secondary cleaning measures—not only to ensure part cleanliness but to protect their vacuum furnaces from contamination by machining oils, lubricants, Dykem, oxidation, or polishing compounds.

Pre-Heat Treatment Cleaning: Traditional Challenges

Before any vacuum heat treatment, components must be thoroughly cleaned to remove organic and inorganic contaminants. Common practices include solvent immersion/ drying, and vapor degreasing, all designed to eliminate residues that can volatilize and redeposit within the vacuum furnace, potentially compromising part quality and damaging the vacuum furnace hot zone and cold wall.

However, these cleaning agents are often:

• Flammable
• Toxic
• Environmentally regulated
• Costly to dispose of when spent

Given that commercial heat treaters process parts from thousands of upstream operations, each introducing its own set of contaminants, cross-contamination becomes a significant risk. Stainless steel foil wrapping is often used as a defensive measure, isolating parts from the furnace environment. While wrapping is often effective, this method is:

• Labor-intensive
• Expensive
• Hazardous – Even with the proper PPE, the foil edges are razor-sharp. Foil wrapping continues to be our number one health and safety concern for our valuable employees.

The MIM Furnace: A Catalyst for Innovation

Five years ago, 91��ý of Western Pennsylvania was tasked with sintering pre-sintered metal injection molding (MIM) parts at 2200°F. The binders present in these parts volatilized during processing and heavily contaminated the vacuum furnace, resulting in extensive downtime and maintenance.

Instead of constructing a traditional cold trap to capture volatiles, CEO William Jones developed a more innovative solution: a “hot trap” designed to divert and capture contaminants before they could deposit inside the furnace. This proactive adaptation has proven to drastically improve part quality while eliminating the laborious and frequent cleaning of hot zones and cold walls.

After that MIM job ended, the underutilized furnace prompted experimentation. It was proven how well this adapted furnace performed on unwanted binders. We set out to test how this same system could be adapted to remove impurities from everyday production parts. After extensive trials using non-critical PH-grade stainless steel components, a fully integrated, vacuum-based cleaning and aging cycle was perfected. This development has since replaced traditional expensive pre-cleaning methods and dangerous foil wrapping, producing consistently clean and bright 17-4 PH aerospace components.

The Self-Cleaning Vacuum Furnace: How It Works

The key innovation lies in a dual roughing pump configuration:

Pumping System #1- Initial Pump-Down and Contaminant Removal:

• Components are loaded into the furnace unwrapped and uncleaned.
• Only Roughing Pump #1 is activated during the initial pump-down.
• A slow temperature ramp allows contaminants to vaporize and exit the hot zone through a heated port into Pump #1.
• Contaminants are safely trapped in the pump’s oil—the “hot trap”.

Pumping System #2 -Transition to Heat Treatment:

• After off gassing is complete, Pump #1 is isolated.
  Pump #2 system, which includes a roughing pump, booster, diffusion, and holding pump, takes over.
  The chamber is then brought to 1 x 10⁻⁵ Torr and the standard vacuum thermal cycle proceeds.

This two-stage pumping sequence cleans both the parts and the chamber prior to heat treatment—without ever opening the furnace door.

Results and Benefits

This newly developed vacuum furnace and process produces:

Cleaner Parts: Vacuum cleaning penetrates blind holes, threads, and keyways more effectively than traditional solvent or vapor methods.
Injury Reduction: The process eliminates the need for hazardous foil wrapping, significantly improving employee safety.
Environmental & Cost Advantages:

• Reduces or eliminates chemical solvent use.
• Cuts labor associated with pre-cleaning and wrapping.
• Reduces hazardous waste and disposal costs.

Furnace Maintenance Improvements:

• Hot zones and cold walls remain pristine—no weekly tear-downs.
• Pump #1 oil is changed biweekly, eliminating seizure concerns due to contaminated oil.

Conclusion: A Game-Changer for the Industry

Historically, part cleanliness in vacuum heat treating has been a persistent challenge—one often addressed through costly labor, chemicals, and stainless steel or titanium foil. 91��ý’ innovative dual-pump vacuum cleaning system, integrated seamlessly with a standard vacuum heat treatment cycle, redefines industry best practices.

This “self-cleaning furnace” concept not only delivers superior part finishes, but also enhances safety, reduces environmental impact, and cuts operating costs. In a world where precision, cleanliness, and sustainability matters more than ever, this advancement may very well represent the “Holy Grail” of clean vacuum processing.

Author: Bob Hill, President, 91��ý of Western PA and Michigan

As published in Heat Treat Today Magazine:

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NEO – Vacuum Oil Quenching Introduction

Despite decades of relentless innovation, the constraints of high-pressure gas quenching have become increasingly evident. Even with the utilization of specialized inert gas blends and heightened gas pressures, the gas cooling efficacy compared to liquid quenchant cooling particularly for heavier cross sections has its limitations. It is also undeniable that certain aerospace alloy steels governed by stringent liquid quench AMS specifications will never change.

Instead of consistently “no quoting” AMS governed oil quench alloys over the years at 91��ý of Western PA, Solar Manufacturing along with 91��ý engineers embarked on a journey of ingenuity during the tumultuous period of the pandemic. Their dedication culminated in the birth of the NEO vacuum oil quench furnace. With a unique 36″ x 36″ x 48″ hot zone that operates up to 2000°F maximum while accommodating a weight capacity of 2000 pounds, the NEO represents a paradigm shift in vacuum oil quenching technology.

Oil Quenching Challenges:

The creation of the NEO was not without its formidable obstacles. Foremost among these challenges was the development of a robust transfer mechanism capable of seamlessly relocating heavy workloads from the hot zone to the oil quench chamber under high vacuum conditions. Solar Manufacturing engineers triumphed over this hurdle with the implementation of a groundbreaking “lift and place” mechanism, which has since demonstrated flawless performance for nearly two years.

Additionally, concerns regarding oil back streaming in the new multi chambered vacuum system were meticulously addressed. With Solar’s strong acumen in vacuum technology, a solution was discovered. The hot zone remains pristine and oil-free to this day.

Metallurgical Benefits of Vacuum Oil Quenching:

The NEO heralds a new era of metallurgical excellence. By effectively eliminating any surface contamination, both intergranular oxidation (IGO) and decarburized or carburized surfaces on oil quenched components is eliminated. These critical metallurgical features are unattainable in traditional gas fired endothermic batch furnace equipment.

52100 Material

Furthermore, the NEO provides the metallurgical engineer the ability to finally thermocouple oil quenched parts in accordance with AMS 2750 Rev G standards. Being able to monitor part temperature with up to twelve (12) data points ensures thorough and precise thermocouple monitoring, bolstering control and repeatability.

At the completion of the automated austenitizing cycle, the newly designed transfer mechanism delivers precisely heated parts from the hot zone to the 3000-gallon oil quench chamber consistently within 20 seconds- all without the expulsion of flames and the discharge of smoke!
Since the NEO is a hermetically sealed furnace, the Solar engineers wanted to give the furnace operator “eyes” into the furnace. An internal camera was designed for the operator to watch the load transfer in real time from the control panel.

To eliminate the potential of part cracking, quench oil temperatures are maintained between 140°F to 180°F ±5°F which enhances consistent and repeatable metallurgical results. In addition, the quench oil recirculates within a closed loop oil to air cooling system, never allowing water contamination to infiltrate the oil.

Finally, the NEO consistently produces bright, clean work. This ultimately leads to less costly downstream processing for the customer.

Production Benefits of Vacuum Oil Quenching in the NEO:

Processing within an atmosphere totally devoid of oxygen, the NEO finally eliminates the need to match atmospheric carbon potentials to the carbon content of the alloys being processed. This not only eliminates costly oxygen probe purchases, but it also gives the operator the ability to mix and match various materials of various customers. Production efficiencies have been realized when multiple materials with varying carbon contents, which are similar in cross section and austenitizing temperature, are processed in one load.

This newfound flexibility is augmented by practices such as cold loading and unloading, which not only prevents detrimental oxidation but also extend the lifespan of the hot zone, thus minimizing downtime.

While building the NEO, Solar’s R&D department performed their own laboratory experiments on the vaporization of various quench oils at different pressures and temperatures. It was decided to purchase 3000 gallons of traditional Houghton G quench oil versus the costly “vacuum only” quench oils that are currently on the market today.

Health and Safety Benefits of the NEO Vacuum Oil Quenching:

At Solar, the safety and well-being of our workforce are paramount. By operating within a hermetically sealed furnace environment, the NEO effectively eliminates the risks associated with open flame exposure, explosivity, and skin burns.

Moreover, at the completion of every cycle, the NEO opens at both ends to the atmosphere. Full air exchange mitigates the potential hazards of confined spaces.

Most recently at our Hermitage PA facility, a power outage was experienced at the exact moment a 1500-pound load at 1550° F was being transferred to the oil quench. The hydraulically controlled transfer mechanism stopped on a dime, the internal door from the hot zone to the oil quench vestibule remained open, and the hot load vacuum cooled harmlessly under vacuum. The environs to the furnace remained unchanged- no smoke no flames. With pneumatically controlled traditional Batch IQ furnaces, the loss of air pressure during such an event often causes doors and elevators to drop unexpectedly with loads in precarious positions. The chance of an accident increases exponentially with unexpected power loss. Not the case with the NEO. Once the power was restored, the load was successfully reprocessed with a decarburized-free surface.
Unfortunately, the heat-treating industry has not been immune to disasters in the past. There have been multiple “total losses”, mostly due to oil quench fires and explosions. Recently, it is well known that if an insurance adjuster sights a flame or smoke within a plant, they are reluctant or may even refuse to write the policy. With the NEO this concern is eliminated.

Sustainability Benefits of Vacuum Oil Quenching:

In alignment with the global imperative to combat climate change, the NEO assumes a pivotal role in reducing the heat treatment industry’s carbon footprint. According to a 2019 article by Kanthal, an estimate of 80% of fuel used for heat treatment could ultimately be replaced by electricity, thus drastically reducing CO2 emissions. “When you burn something that contains carbon, you get carbon dioxide that you either must take care of or release into the atmosphere. With electric heating, you do not have any exhaust”. [1]
The second column of the chart below addresses the multiple environmental concerns associated with traditional batch IQ gas-fired oil quenching furnaces. The third column outlines the advantages of the NEO, which embraces electric heating as a sustainable alternative to fossil fuels. As sustainability pressures continue to mount, governments, customers and primes alike will continue to flow down requirements on how heat treaters plan to reduce their carbon footprints.

NEO Vacuum Oil Quenching Conclusion:

As the demands for metallurgical precision, safety, and environmental sustainability continue to mount, the NEO emerges as the undisputed vanguard of vacuum oil quenching technology. While gas-fired batch IQ furnaces remain entrenched, the NEO heralds a new dawn characterized by unparalleled efficiency, precision, and sustainability. Solar’s unwavering commitment to innovation ensures that the NEO will continue to lead the industry toward a future defined by cleanliness, safety, and environmental stewardship.

References:

[1] Kanthal (2019, 04) Heat Treatment CO2 Emissions cut by 50 percent by using electricity. https://www.kanthal.com/en/knowledge-hub/inspiring-stories/heat-treatment-co2-emissions-cut-by-50-percent-by-using-electricity/

[2] Aichelin Group (2024 01) CO2 Footprints and the Heat Treat Industry/The Monty

Author: Bob Hill, President, 91��ý of Western PA and Michigan

As published in Thermal Processing Magazine:

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Generally, electric power consumption is insidious because it is not seen and is not considered enough by operating personnel. The following is a summary of power consumed in a typical heat treat plant.

Electric motors

• Water recirculation pumps for cooling purposes
• Vacuum pumps
• Circulation pumps for oil used in quench tanks
• Fans
• Quench motors

Cost of Running Electric Motors @ 10 cents / KWh

*kVA rating is based on operating currents (amps). Note: efficiency of small motors is approximately 60%, increasing to 80% for larger motors, based on 746 watts/hp. Source: Glover Pocket Reference Book, October 1996 edition.

High Vacuum Diffusion pumps

• Operate with no noise, so operating costs are more or less “out of sight”.
• Ensure that all diffusion pumps are set up to run on the ConserVac^® Full power (all three phases on) during high vacuum cycle. Partial pressure cycles running on half power (one or two phases off). When possible, turn off/ shut down the diffusion pump.

Operating Costs for Diffusion Pumps, Varian

Gas blowers

• If the gas blowers are set up with variable speed drives, use them to reduce power consumption wherever possible, especially at the end of a cooling cycle when heat of compression is noticeable.
• Program all quench cycles to shut down the blower at the lower temperature as required on the work thermocouples (i.e., 125°F).
• Do not run gas blowers for excess time.

Building lighting

• Turn off all office and shop lights when not needed, except for night lights.
• Make sure all office lights are off during non-working hours and weekends.
• Building lights should be the newer, high efficiency type.

Office heat and air conditioning

• Office heating and air conditioning should be programmed for setback, same situation as office lighting.

Lesson to be learned: turn off any electric motor or light whenever it is not in operation.

Furnace heating rate

• No furnace should be heated any faster than 15°F to 30°F/minute or 900°F to 1800°F/hour, unless with specific instructions mandate otherwise.

Power demand

• Utility company provides an electric power meter for kWh and demand kWh.
• Meter contains a power demand register
  • Record of electric power usage over a specific time increment
  • Instantaneous demand peak recorded each month
  • Result is the total kW hours registered in one part of the bill and the second part of the bill is kW electrical power demand

• Ideal situation: instantaneous power demand would be flat with no demand peaks for the month, not the norm.
• Batch type electric furnaces can produce major demand peaks
• Major electric power savings are possible if equipment can be controlled so the peak demands can be staged, i.e., over several furnaces
• Schedule heavy production cycles to “off peak hours” usually during the evening, i.e., after 6 to 8 pm.
• Some utility companies will not penalize for the demand factor during “off peak hours”
• Electric furnaces that operate with “on-off” control using electric contactors are offenders
• The furnace will call for full power in the “on mode”
• When temperature reaches the set point, power will be completely turned off
• Electric power can easily be peaked if several batch furnaces operate in this mode together
• Solution: replace the electric on-off contactors with SCR (silicon controlled rectifier) power supplies
• SCR controllers will provide a proportional power control, minimizing peaked power demand

Power factor conclusions

• Can significantly affect the electric power bill if the electric utility charges a penalty for operating at power factor less than unity or bills in kVA rather than kW.

Power factor penalty, simplified.

Power factor can best be understood as consumed power that does no work. This is usually the result of electric motor loads or reactive magnetic loads like transformers. Phase angle fired SCRs also fall into this category.

It can be measured or calculated as:

Power factor = kW (real power)

������������������������������������������������������������������ kVA (apparent power)

• Electric furnaces that operate with resistance heating elements connected directly across the power line, or incandescent lighting in the plant operate at near unity power factor.
• Utility companies penalize users in different ways for power factor and varies by location.
• Motors will have an average power factor of 0.8%.
• Furnace power supplies will have variable power factor depending on loading, average about 0.65.

In summary

There are many ways to conserve dollars in any ongoing manufacturing or heat treating plant. Other than “turning off the lights”, many other opportunities are available to operating personnel as outlined above.

Reference:�� Conserving Electric Power, Part I and Part II, William R. Jones

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Technological change in many industries is often driven by advances in basic science, a move to digital transformation or simply the disruption of the status quo. In manufacturing, the focus on the ability to maintain a process with precision and repeatability based on supply-chain management is also crucial and has led to important advances in industries that make safety and quality imperative. ��

This process focus is nowhere more important than in the area of medical devices. The medical device industry produces many vital products, like joint implants, that must meet or exceed specifications for safety and quality. One of the ways many of these products are able to meet or exceed their specifications is through the application of sophisticated heat treating to induce microstructural changes that affect the materials and give them versatile and useful properties, like strength or durability.

Some of the medical products or components on which heat treating is used are:

• Hypodermic needles
• Stents
• Heart wires
• Titanium staples
• Metallic joint replacements
• Implantable battery cases
• Surgical hand and robotic surgical tools
• Dental burs

A Bit of History on Medical Device Heat-Treating with MedAccred Accreditation

Driven by the goals of patient safety along with clinical performance, medical device manufacturers have traditionally performed their process management and supply-chain oversight in-house. As Ed Englehard, vice president of corporate quality at 91��ý, explained, “Prior to 2010, many medical OEMs were vertically integrated with their manufacturing. Over the years as costs and administrative overhead grew, however, they found it more economical to farm some of these manufacturing processes out of their plants. For many of them, however, managing a wide-ranging and global supply chain was becoming a difficult task.”

91��ý, founded in 1983, is a Souderton, Pa.-based commercial provider of vacuum heat-treating, brazing, carburizing and nitriding services. With operations in Pennsylvania, California and South Carolina, it serves the aerospace, medical device, defense and power generation industries.

The initial idea for the MedAccred program was explored in 2010 when a major medical device OEM was having issues with their heat-treated orthopedic products. They conducted a benchmarking study of their heat-treating suppliers and saw that their better-performing suppliers were accredited by Nadcap^®. Nadcap is an industry-managed approach to conformity assessment for the aerospace industry that brings together technical experts from industry and government to establish requirements for accreditation, accredit suppliers and define operational program requirements.

What the benchmarking study found was a very distinct difference in quality performance between the Nadcap-accredited suppliers and those who were not accredited. This information pointed to the need for a better way to oversee critical process suppliers, like heat treaters and others, throughout the medical device supply chain.

The MedAccred program was initially developed to improve the quality of heat-treated products for the medical device industry. What began as a way to accredit heat-treating suppliers has today expanded into many other critical process areas to improve product quality and, more importantly, patient safety.

In 2012, a medical device industry roundtable was held in Chicago. 91��ý, having been a long-time accredited supplier in heat treating and nondestructive testing with the Nadcap accreditation program for aerospace, was involved in the roundtable talks. The discussion group included many of the major medical device manufacturers. The MedAccred program concept was created following this meeting.

“The more we dealt with our various medical device customers, most of which are some sort of manufacturer, the more we realized that there was no one who really understood heat treating and how critical it is to the success of a medical device,” Englehard said.

While sophistication and transparency were apparent with 91��ý’ aerospace clients, Englehard remarked that they did not see the same level with their medical accounts.

The MedAccred program was born out of a focused process of discovery and roundtable discussions and was modeled on the success of the Nadcap program. It was seen as a way for the medical device industry to get their arms around supply-chain management. In 2013, proof-of-concept audits based on the Nadcap program audit criteria were conducted. Briefings on the MedAccred concept were subsequently held with the FDA and included the importance of controlling the heat-treating process.

In 2014, the initial MedAccred technical task groups were created in various critical process areas, including heat treating; electronics – printed circuit board assemblies; electronics – cable and wire harnesses; sterilization; and welding.

The Souderton location of 91��ý received the first MedAccred accreditation globally in 2015 for heat treating. That year the FDA also subsequently recognized the aerospace heat-treating standard AMS2750, published by SAE International.

Heat-Treating Accreditation Today

Both Nadcap and the MedAccred program are administered by the Performance Review Institute, a not-for-profit trade association started in 1990 to administer critical process accreditation programs in industries where safety and quality are shared goals.

Like Nadcap in the aerospace sector, MedAccred also uses an industry-managed, rigorous technical audit approach to ensuring critical manufacturing process quality throughout the medical device supply chain. Technical experts from major medical device manufacturers work together to develop the audit criteria. MedAccred subscribing members include: Bausch Health, Baxter, Becton Dickinson, Boston Scientific, Edwards Lifesciences, Johnson & Johnson, Medtronic, Philips, Roche Diagnostics and Stryker.

“The MedAccred program gets right to the heart of what you say you are doing when you say you are a heat treater, or a welder or a printed circuit board manufacturer,” Englehard said. “We assume that the quality management system is there, but how you actually do the work in a technical manner is a different level of scrutiny than a quality management system. This seemed to be the area that was lacking in medical device supply-chain management up until MedAccred showed up.”

MedAccred uses SAE International standard AMS2750-Pyrometry as the cornerstone standard for heat-treating accreditation. This pyrometry standard included in the MedAccred audit is the first and only heat-treating standard granted complete recognition by the FDA. This standard covers pyrometric requirements for thermal-processing equipment used for heat treatment. The scope of the process and equipment audits include annealing, hardening and tempering, carburizing, nitriding, stress relieving, pyrometry, instrumentation and furnaces.

In addition to heat treating, MedAccred also accredits other medical device manufacturing processes, including:

• Cable and wire harness assembly
• Plastics extrusion
• Plastics injection molding
• Plastics mechanical assembly
• Printed circuit boards (bare boards)
• Printed board assembly
• Sterile device packaging
• Sterilization
• Welding

The Importance of “Flow Down” with MedAccred and Heat Treating

After seven years of experience with the MedAccred process, 91��ý is seeing progress with medical device OEMs in the area of supply-chain oversight to improve process quality. This progress is evident in the application of the concept of “flow down,” a contract provision where a prime contractor legally binds a subcontractor to the terms and conditions of the prime contract so that every supplier in the supply chain is accountable to the original contract standards.

“Medical customers are starting to enforce flow down so that the OEM designer’s original intent reaches down to the bottom of the supply chain in a level of technical sophistication and transparency that many of our direct customers were not accustomed to doing,” Englehard said. “In supply-chain management, contract flow down is hugely important, and it is an area that was weak in aerospace prior to Nadcap and definitely a weak area in medical supply-chain management prior to MedAccred.”

91��ý saw the development of the MedAccred process as a way to ensure that a medical device was going to be properly manufactured and, in this case, properly heat treated.

“At the end of the day, we all share exactly the same concern,” Englehard said. “This concern is that the place where a patient, a doctor and a medical device all come together should never be compromised by some form of manufacturing defect, and among those potential defects are those that might arise from critical suppliers like heat treaters. We felt that MedAccred was going to be a way to separate out those suppliers who were interested in patient safety from those who, frankly, were not.”

91��ý’ experience with the MedAccred process has yielded positive outcomes. According to Justin Hoffman, quality manager at 91��ý’ western Pennsylvania location, the MedAccred program has contributed to a reduction in the company’s in-house defect rate by 15-25% and also contributed to a reduction in defective product escaping to their customers.

Improving the Safety and Quality of Medical Devices Through Heat-Treating Process Accreditation – Conclusion

According to Fortune Business Insights¹, the global medical device market was valued at almost $489 billion in 2021, with growth of 5.5% expected annually through 2029. This growth is being fueled by an increasing number of patients undergoing diagnostic and medical procedures that emphasize early diagnosis and treatment. It is also being driven by medical industry investments in new technology and an increase in breakthrough device designations from the FDA.

Heat treatment as a critical process is a significant component in the manufacture of many medical devices, and its importance will grow as the market increases. Supplier accreditation in the heat-treating field, as in other areas covered by the MedAccred technical audit process, can support accountability up and down the medical device supply chain, providing a measure of added safety and security to patients worldwide.

For more information: Since being established as a not-for-profit trade association in 1990, PRI has become the global authority in facilitating industry-managed programs and administering critical process accreditation programs as well as developing web-based audit management software. Visit or call 724-772-1616.

References

Fortune Business Insights: “Medical Devices Market,” accessed at: . ��6/22/2022

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This paper explains reactions that can occur during a vacuum processing cycle and different methods of preventing these reactions. We will discuss how eutectic compositions can form while heat treating and how diffusion bonding can be a concern to heat treaters when two dissimilar materials are in close contact with each other in high vacuum at elevated temperatures.

To many people, the term “eutectic” is not well understood. The best way to think of a eutectic is a metallurgical meltdown. A eutectic reaction occurs when two components with different melting points and surfaces free of oxides come in contact with each other in the vacuum furnace. This can create an atomic diffusion. For some materials, when a specific atomic composition is reached, they will melt at a temperature much lower than the melting point of the individual metals. If that temperature is reached or exceeded during the heat treating cycle, melting will occur at the contact points. This is referred to as a eutectic melt.

The most common example of a eutectic reaction is with a tin/lead solder. Tin melts at about 450°F, while lead melts at about 620°F. When they are together as two components, the solder melts at 370°F. That is 80 degrees below where tin melts at and 250 degrees below where lead melts.

Another good example in vacuum heat treating is the titanium/nickel eutectic. In this scenario, titanium melts a little above 3000°F, whereas nickel melts at about 2650°F. If you heat treat them in a vacuum furnace, placing the titanium on a typical heat treat fixture such as a grid or stainless steel basket, the two materials would melt at around 1730°F. This could be a disaster if the technician does not understand the eutectic reaction.

We learned a good lesson on eutectic melting a couple years ago when a clean-up run was performed in one of our furnaces at 2400°F without prior removal of a work grid. The high nickel alloy cast fabricated grid was sitting on molybdenum support rods positioned on the furnace graphite support rails.

A form of a eutectic is diffusion bonding, a reaction that occurs well below the eutectic melting point and must be considered when two or more materials are in direct contact.

Diffusion bonding becomes more acute as the mass of the parts in contact increases. In vacuum brazing, a eutectic reaction can be a desirable formation when joining two pieces of metal using brazing filler metals. However, there are times when a eutectic reaction can occur with unintended and often damaging consequences.

Table 1 shows some alloy mating combinations and the eutectic melting points. ¹

Eutectic Barriers Used in Heat Treating

A eutectic barrier is the insertion of a material between the two mating metal surfaces that might be heading for a eutectic melt at elevated temperatures. This barrier material must be capable of withstanding the process’ maximum temperature to be effective.

The most effective barriers are:

• Refrasil cloth or Kaowool blanket
• Thin Ceramic plates
• Ceramic fixtures of different shapes
• Stop-off paints of different types

Refrasil is a clothlike material consisting of more than 96% silica (SiO2) that resists oxidation and most reactive melting. Most Refrasil textiles will not melt or vaporize until temperatures exceed 2650°F.

Kaowool is a good insulation and barrier when used in a blanket form.

Ceramic, high purity alumna can take the form of flat plates or individually designed fixtures to fit a particular application.

Stop-offs��are designed to protect metal surfaces from the flow of molten brazing filler metal, or to prevent metal surfaces from adhering or sticking to each other.

The following are illustrations of how barriers are used in actual applications.

1. Kaowool Barrier: Typical load of Titanium ingots on Kaowool with graphite fixturing. Processing temperature is 2350°F for 24 hours in high vacuum.

2. Refrasil Cloth Barrier: Separates the bars from the supporting graphite plate. This eliminates the possibility of a eutectic reaction when hardening the bars at 2125°F.

3. Ceramic Plate: Demonstrates the use of a thin ceramic plate to separate the fastening nuts from the graphite support plate. This process is for a sintering procedure to 2500°F.

4. Ceramic Plate: Typical load of nickel iron alloy, MuMetal, fixtured on Alumina sheet with graphite support. Processing temperature is 2150°F.

5. Ceramic Fixture: Using a special ceramic fixture to support the all-thread parts of high speed steel for a hardening cycle at 2100°F. The ribbed ceramic fixture separates the parts from the graphite support plate.

6. Stop-Off Barrier: Typical load of 420 stainless steel molds processed on graphite plate utilizing Wall Colmonoy Inc. white stop-off. Processing temperature is 1900°F.

7. Green Stop-Offs: Typical load of orthopedic implants fixtured with alloy 330 screens utilizing Wall Colmonoy stop-off fixtured in CFC grids. Processing temperature is 2175°F.

8. Other Barrier: Graphite plate supporting the load separated by a high temperature aluminum barrier developed by GMI.² This barrier would be similar to using the green stop-off material. This particular hardening process has an upper temperature of 2200°F.

These are just a few examples of how barriers are used to eliminate the possibility of eutectic, or sticking, reactions from forming.

Also, check out our detailed booklet on critical melting points.

Vacuum Heat Treating: Critical Melting Points Booklet

Written by:�� Real J. Fradette, Senior Technical Consultant, 91��ý, Inc., Roger A. Jones, FASM, CEO Emeritus, 91��ý, Inc. ����������������������

References:

1. Critical Melting Points and Reference Data for Vacuum Heat Treating Sept 2010, 91��ý, Inc.
2. Graphite Machine Inc., Topton, PA

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“Symptoms: The patient is completely overcome by the metal titanium. He or she tends to eat and sleep titanium, pushing all other metals out of his or her system. The patient will talk for hours about the virtues of titanium, extolling its remarkable qualities. Any blemish on titanium’s image, any negative characteristic will tend to be dismissed. Titanium’s feast-or-famine existence seems to only intrigue the patient.

Earliest known causes: In the 1950s, a number of patients were overcome with titanium, describing it as the “Wonder Metal.” The side effects of the “Wonder Metal” syndrome took many years to disappear.

Similar disease: See infatuation.

Length of disease: Lifetime.

Cure: None known.”

After working with titanium for more than two decades, I have fallen victim to the “titanium disease.” What makes this metal so unique? With a quick look at the history and distinctive properties, one can easily recognize the attraction.

History of Titanium

Titanium was discovered by an English pastor named William Gregor in the 1700’s. In the 1800’s small quantities of the metal were produced. Before World War II, titanium as a useful metal was only a tantalizing laboratory curiosity. At that time, titanium was only valuable as an additive to white paint in its oxide form. It took the long and expensive arms race between the United States and the Soviet Union in the 1940’s to create the need to solve many of the titanium complex problems.

Since the end of the Cold War, titanium has matured primarily as an aerospace material. However, this “Wonder Metal” has expanded to commercial markets such as artificial body implants, golf clubs, tennis rackets, bicycles, jewelry, heat exchangers, and battery technologies.

Titanium’s unusual metal attributes include a strength comparable to steel but 45% lighter. It is twice as strong as aluminum but only 60% heavier. It is both biologically and environmentally inert. It will not corrode. The metal is nonmagnetic and can hold strength at high temperature because it has a relatively high melting point. Finally, titanium has a very low modulus of elasticity and excellent thermal conductivity properties. For thermal processors, these “spring like “properties allow titanium to be readily formed or flattened with heat and pressure.

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Problems with Titanium

For all of its outstanding attributes, titanium is still the problem child of the metallurgical family. It is exceedingly difficult to obtain from its ore, which commonly occurs as black sand. If you scoop up a handful of ordinary beach sand and look closely, you will likely see that some of the grains are black; this is a titanium ore. In certain places in the world, especially Africa and Australia, there are vast black sand deposits. Although titanium is the ninth most abundant element on the earth, turning that handful of sand into a critical jet engine blade or body implant is a significant undertaking. The refining process is about 10,000 times less efficient than making iron, which explains why titanium is costly.

Titanium never occurs alone in nature, and it is a highly reactive metal. Known as a “transition metal,” it can form bonds using electrons from more than one of its shells or energy levels. Therefore, titanium is known as the “streetwalker metal.” Metallurgists are aware that titanium is renowned to “pick up” other elements quite readily during many downstream thermal and chemical processes. These reactions are often harmful to the advantageous properties of titanium and should be avoided at all times.

Vacuum Heat Treating Titanium is the Solution

Since titanium has a tremendous affinity to pick up other elements at elevated temperatures, primarily oxygen and hydrogen, the only way to heat treat titanium successfully is to utilize high vacuum atmospheres. High vacuum levels of x10^-5 Torr minimum and low leak rates of five microns per hour maximum are the parameters needed to retain this metal’s desired properties. An oxygen-rich atmosphere results in a�� hard “alpha case” surface condition. A hydrogen atmosphere results in a hydrided condition, which makes titanium very brittle to the core. Both conditions can be extremely detrimental to any critical titanium component.

With high pumping capability and tight pyrometric controls, vacuum heat treating processes successfully provide various treatments on the “wonder metal” while avoiding the “streetwalker” syndrome. Some vacuum heat treating processes include stress relieving, solution treating, aging, and degassing treatments. After proper processing, bright and clean parts with low hydrogen content and zero alpha case are the norm.

The recycling of titanium is of a different order of magnitude than it is for other metals due to its value. It took a shortage of titanium in the 1980s, and some innovative metallurgy, to transform valuable titanium scrap back into a qualified ingot. To do this, metallurgists used the reactiveness of the metal to their advantage.�� Since titanium is very ductile and extremely hard to grind into powder, metallurgists learned how to use hydrogen to their advantage. Adding hydrogen to turnings and scrap makes the titanium brittle and enables the material to be pulverized into fine powders. The final product must then be thoroughly degassed or dehydrided to enter back into the revert stream because every pound of titanium is precious.

The reactiveness of titanium also assists the metallurgist to apply various surface treatments. Nitrided and carbide surfaces, when used, add further protection to the titanium while making the exterior harder.

Titanium Alloys

Titanium alloys are divided into four distinct types: commercially pure, alpha, beta, and alpha-beta. Commercially pure grades have no alloy addition, and therefore they have very little strength. This grade of titanium is used when corrosion resistance is of greater importance. Alpha alloys are created with alpha stabilizers such as aluminum. They are easy to weld and provide a reliable strength at elevated temperatures. Beta alloys use stabilizers such as molybdenum or silicon which makes these alloys heat treatable to higher tensile strengths. Finally, the most used titanium alloy are the alpha-beta alloys. These heat treatable alloys are made with both alpha and beta stabilizers creating an excellent balance between strength, weight, and corrosion resistance.

Summary of Titanium

For all the advances, titanium and its many alloys, has not reached its apex in popularity in the world. Is there any other element that calls to mind the notion of strength quite like titanium? For what reason has this metal, named after the Titans of Greek mythology, never reached its full potential? If it were not for the expense, we would undoubtedly have titanium cars, houses, jets, bridges, and ships. Unfortunately, the cost of titanium keeps the “Titanium Disease” at bay.

Author: Robert Hill, FASM – President, 91��ý of Western PA and Michigan

The post Titanium: A Fascinating History and Future appeared first on 91��ý.

There is an age-old adage that exists in the heat treating world. That supposition states that “the smaller the vacuum furnace, the faster it will quench.” Our study compared the cooling rates of two distinctly sized High Pressure Gas Quenching (HPGQ) vacuum furnaces- a large 10-bar vacuum furnace equipped with a 600 HP blower motor versus a smaller 10-bar vacuum furnace equipped with a 300 HP motor. Both furnaces, one with 110 cubic feet, the other with a 40 cubic foot hot zone respectively, were exclusively engineered and manufactured by Solar Manufacturing located in Sellersville, PA.

History of High Pressure Gas Quenching in Heat Treatment

High Pressure Gas Quenching in the heat treatment of metals has made tremendous strides over recent years. Varying gas pressures within the chamber have been shown to be more governable than their oil and water quenching counterparts. The number one benefit of gas cooling versus liquid cooling remains the dimensional stability of the component being heat treated. In addition, using gas as a quench media mitigates the risk of crack initiation a component dramatically. This is primarily due to the temperature differentials during cooling. Gas quenching cools strictly by convection. However, the three distinct phases of liquid quenching (vapor, vapor transport and convection) impart undo stress into the part causing more distortion (Figure 1).

There are multiple variables involved with optimizing gas cooling. These include the furnace design, blower designs, heat exchanger efficiency, gas pressure, gas velocities, cooling water temperatures, the gas species used and the surface area of the work pieces. Whenever these variables remain constant, the relative gas cooling performance of a vacuum furnace typically increases as the volume of the furnace size decreases.

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The Heat Treating Furnace

Solar Manufacturing has built multiple high pressure gas quenching furnaces of varying sizes over the years ranging from 2 to 20 bar pressure. We have learned that vacuum furnaces, rated at 20 bar and above, became restrictive in both cost constraints and diminishing cooling improvements. Therefore, Solar Manufacturing engineers began to study gas velocities to improve cooling rates. They determined that by increasing the blower fan from 300 horsepower to 600 horsepower, along with other gas flow improvements, would substantially increase metallurgical cooling rates. The technology was reviewed and was determined to be sound. A 48” wide x 48” high x 96” deep HPGQ 10-bar furnace equipped with this newest technology was purchased by 91��ý of Western PA located in Hermitage, PA.

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The Test when Heat Treated

Once this new vacuum furnace was installed, a cooling test was immediately conducted. A heavy load would be quenched at 10-bar nitrogen in an existing HL 50 sized furnace (36”x 36” x 48”). The same cycle was repeated in the newly designed vacuum furnace almost three times its size! (See Pictures 1 and 2)

The load chosen for the experiment was 75 steel bars 3” OD x 17” OAL weighing 34 pounds each. The basket and grid system supporting the load weighed 510 pounds. The total weight of the entire load was 3,060 pounds. Both test runs were identically thermocoupled at the four corners and in the center of the load. All five thermocouples were deeply inserted (6” deep) into ¼” holes at the end of the bars (See Picture 3). Each load also contained two 1” OD x 6” OAL metallographic test specimens of H13 hot working tool steel. These specimens were placed near the center thermocouple to ensure the “worst case” in terms of quench rate severity. All tests were heated to 1850°F for one hour and 10-bar nitrogen quenched.

Vacuum Heat Treating Results

The comparative cooling curves between both HPGQ vacuum furnaces are shown in Chart 1. Table 1 reveals that in the critical span of 1850°F to 1250°F for H13 tool steel, the cooling rate in the larger furnace with more horsepower nearly matched the cooling rate of the furnace was three times smaller in size.

Micrographs of the H13 test specimens processed in each load were prepared (Pictures 4 & 5). The microstructure of each test specimen is characterized by a predominantly tempered martensitic microstructure with fine, undissolved carbides. The consistency of the microstructure across both trial loads further demonstrates that while the larger furnace utilized the higher horsepower, both resulted in a critical cooling rate sufficient to develop a fully martensitic microstructure.

Conclusions of Vacuum Gas Cooling

These tests prove that the greatest impact on the cooling performance in a vacuum furnace is to increase the gas velocity within that chamber. This was achieved primarily by increasing the horsepower of the blower fan. By doing this, the ultimate cost to the customer is significantly less than manufacturing a higher pressure coded vessel. This newly designed vacuum furnace has proven to be a game changer.

Part II of this article will discuss real life case studies and how both Solar and Solar’s customers have mutually benefited from this newest technology.

Written by:

The post Vacuum Gas Cooling – Is Pressure or Velocity Most Important? (Part 1) appeared first on 91��ý.

The vacuum furnace industry has searched for many years for the ideal material to be used in fixtures and grids for processing workloads at elevated temperatures. The support structures should be lightweight to achieve desired metallurgical results during the cooling phase of the process cycle. These lighter weight supporting members will also result in overall lower processing costs due to shorter heating and cooling portions of the overall furnace cycle.

The latest and most successful material used in graphite vacuum furnace fixtures, grids and leveling components is a Carbon/Carbon Composite (C/C) structure. Graphite is an allotrope and a stable form of carbon.

Carbon/Carbon Composite Materials for Fixturing

Carbon fiber reinforced carbon matrix composites (C/C Composites) have become one of the most advanced and promising engineering materials in use today. These C/C Composites consist of two primary components, carbon fibers and a carbon matrix (or binder). They are among the strongest and lightest high-temperature engineered materials in the world. Compared to other materials such as graphite, ceramics, metal, or plastic. It is lightweight, strong and can withstand temperatures over 2000°C without any loss in performance.

Typical Carbon/Carbon Composite Two-Tier Fixture

Properties of Carbon/Carbon Composites

Carbon/Carbon Composites are a two-phase composite material where both the matrix and reinforced fiber are carbon. Carbon/Carbon can be tailored to give a wide variety of products by controlling the choice of fiber-type, fiber presentation and the matrix Carbon/Carbon. It is primarily used for extreme high temperature and friction applications.

Carbon/Carbon combines the desirable properties of the two-constituent carbon materials.�� The carbon matrix (heat resistance, chemical resistance, low-thermal expansion coefficient, high-thermal conductivity, low-electric resistance, low-specific gravity) and the carbon fiber (high-strength, high-elastic modulus) are molded together to form a better combined material. The reinforcing fiber is typically either continuous (long-fiber) or discontinuous (short-fiber) carbon fiber type.

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Summarizing Properties of Carbon-Carbon Composites

• Excellent thermal shock resistance
• Low coefficient of thermal expansion
• High modulus of elasticity
• High-thermal conductivity
• Low density (about 114 lb/ft³)
• High strength
• Low coefficient of friction (in the fiber direction)
• Excellent heat resistance in non-oxidizing atmosphere. C/C Composites retain their mechanical properties up to 4982°F (2750°C)
• High abrasion resistance
• High electrical conductivity
• Non-brittle failure

The carbon fiber matrix can be used to create racks, plates, grids, and fixtures for vacuum heat treating applications.

CFC Design Fixturing for Medical Implants

Comparing the C/C Composite to other fixture and grid materials, we can create the following list:

1. Comparing to Basic Graphite

• High strength and rigidity
• High resistance to fracture

2. Comparing to Metals

• High heat resistance
• Low thermal expansion
• Lightweight (1/5 of metal)
• Excellent resistance to corrosion

3. Comparing to Ceramics

• High resistance to fracture
• High thermal shock resistance
• Precision machinable

Various Configurations of C/C Used as Fixtures and Grids for Heat Treating

Below are several examples showing different applications of how graphite materials are used in typical vacuum furnace applications.

• Titanium Ingots

10-2-3 Titanium ingots homogenized at 2350°F for 24 hours in high vacuum, 10^-5 Torr. Each ingot weighs about 10,000 pounds. The fixturing serves two purposes: it keep the ingots from rolling during the heat treatment process, and it also contours to the shape of the ingot so there are no flat spots after the homogenization.

• Steel Aerospace Components

4340M aerospace components hardened and tempered in partial pressure nitrogen. Graphite fixturing was used to minimize distortion and holes were machined into the graphite plates to help with the cooling phase of the cycle.

• Titanium Strips

Titanium strips annealed at 1450°F and aged in high vacuum, 10^-5 Torr. Strips were placed on a laser leveled graphite plate to maintain flatness during the run.

• 347 Screens

347 screens that were annealed at 1875°F in partial pressure nitrogen. The screens were too wide for our normal furnace grid, so we used graphite fixturing to get the screens into the center of the furnace to accommodate the width. The graphite also allows for the screens to settle flat during the heat treating.

• Ingot Fixtures

These are graphite support members that are used to process the ingots in Image 1.�� They maintain the shape of the ingots while providing support.

• Titanium Aerospace Component

Very intricate and precise graphite fixturing designed to minimize warpage during the solution age heat treatment of these 5-5-5-3 titanium aerospace components. The fixturing was manufactured by 5-axis machining equipment and it allows the part to move during the heat treatment and then settle back into the exact contour of the fixture.

The above images are just a small sample of the many supporting graphite designs that have become so critical in vacuum furnace processing. The graphite material can be readily machined for special shapes and applications.

We look forward to finding many more ways to successfully use these graphite components.

Written by:�� Real J. Fradette, Senior Technical Consultant, 91��ý, Inc.

The post The Use of Graphite for Vacuum Furnace Fixturing appeared first on 91��ý.

With approximately 100 production vacuum furnaces in operation each day, the 91��ý Group of Companies has amassed a vast body of knowledge and experience on the subject of vacuum furnace leaks, which we are pleased to make available to the rest of our industry. This article provides a detailed explanation of the various types of leaks that can occur, where they typically occur, and methods of locating and correcting these problems. Particular safety concerns relating to leak checking will also be discussed.

Production vacuum furnaces are designed to provide the ideal atmosphere for processing different materials, and to achieve excellent metallurgical results without surface contamination. However, when a vacuum leak occurs, the resulting integrity of the material being processed can be compromised.�� Therefore, it is most important that the furnace meet minimal leak rate standards for consistent treatment of any material.

All owners and operators of vacuum furnaces will eventually experience some type of leak affecting product quality or even damaging internal components of the vacuum furnace hot zone. ��Large leaks in a vacuum furnace are usually very obvious. They often result in the furnace not pumping down or the processed material showing clear signs of oxidation. Small leaks, however, often go undetected as the pumping system can offset the gas rate of the leak.�� As a result, the vacuum gauge might still show acceptable levels, misleading the system operators into continuing the process, which could lead to major scrap or damage.

What Is a Vacuum Leak in a Heat Treat Furnace?

Normal Leak

A normal leak can be identified as some type of opening, hole, or crack that allows air to be admitted or escape from a confined vessel.�� On a vacuum furnace, it is essential to prevent air from entering the chamber.�� With many components of the furnace requiring a seal, there are several areas at risk for leaks, such as threaded and brazed joints, improperly installed fittings, or O-rings. O-rings, particularly, should be examined regularly, as they can become cut or worn, flat or dirty, or lose elasticity, especially around doors and rotating or reciprocating assemblies. Because a vacuum furnace operates in deep or partial pressure vacuum during the heating phase of the cycle, and near atmospheric or positive pressure during the cooling phase of the cycle, proper sealing of all the above referenced components must be continually established.

Other Types Of Leaks

When a furnace fails to achieve an acceptable leak rate, based on a leak rate test over a one hour period, it does not necessarily mean there is a serious real leak occurring. The testing individual must determine if this is indeed a real leak or some other situation causing the failure.�� The problem could be caused by high gas loads, outgassing, back streaming of vacuum pump oil or a virtual leak occurring within the overall system.

Outgassing is defined as the release of vapor-pressure materials present in the vacuum system. Also, outgassing is often made obvious during the heating cycle by a large vacuum “spike” or rise in pressure during heating.

Sources for outgassing or virtual leaks could include:

1. Residual solvents or water remaining from cleaning, preventive maintenance, or water in the workload.
2. Leaks such as cooling water from vacuum chamber or heat exchanger.
3. Trapped volumes of gas due to poor vacuum construction practices.
4. Trapped spaces under non-vented hardware.
5. Gasses in spaces with poor conductance.
6. Porous material exposed to the atmosphere such as low density graphite.

Also, many sources of virtual leaks arise from an original manufacturing procedure or a repair on a chamber or system. ����As an example, small volumes can be trapped that have a connection to the vacuum side and yet, given the conductance restriction, cannot be pumped out easily or quickly. ��Some common examples of virtual leaks caused by poor design follow.

Our first example, shown as Figure 1, illustrates how a small volume of air or gas can be trapped at the bottom of the threaded hole.�� This small volume will eventually slowly leak out through the threaded area when the furnace reaches a certain vacuum level and will continue to slowly affect the vacuum performance of the furnace.

A second example of a possible virtual leak is shown in Figure 2.�� In this situation some trapped volume could occur if the O-ring groove is a dove tail design or if there are two O-rings and the space between them is not vented.

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A third example of a virtual leak is related to improper or incorrect weld design. In this situation, a space could be left between members being butt welded. Should a small crack eventually appear in the weld area, leakage could occur and cause this undesired and difficult to find leak.�� This is illustrated in Figure 3.

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It should be noted that most virtual leaks will not be detected in a normal leak checking cycle.�� The presence of a virtual leak will become apparent during the system pump down cycle when the ultimate pressures cannot be reached, i.e. hours, ��or it takes an excessive amount of time to reach blank off pressure.

In most cases, physical leaks and outgassing are present simultaneously where the leak is a flat pressure and the outgassing is constantly reducing.

Figure 4 highlights three examples of what can result during pump down. As is illustrated, a real leak reaches a limit while an outgassing occurance continues to achieve better vacuum over time.

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Determining a Vacuum Heat Treat Furnace Leak Rate

It is essential to know that a vacuum furnace is leak tight.�� Measuring how tightly the vacuum chamber is maintained is accomplished by performing a “Leak-Up-Rate” test.

There are various ways to prepare a furnace for a leak-up-rate test.�� Our recommendation on normal production furnaces is as follows:

Heat the empty chamber to approximately 100^oF over the most recent highest process temperature for one (1) hour to thoroughly clean the furnace of product contaminants. Then nitrogen or argon gas cool the furnace to ambient temperature.�� Without opening the chamber, continually re-pump down the furnace to establish an ultimate vacuum level.�� This should continue for an extended period, usually, but no more than, two hours.�� After that time, the vacuum pumps are valved off and the system allowed to sit while the rise in vacuum level is measured.�� This leak rate is typically measured in microns/hour with certain standards established as acceptable for a given system. Normally a leak rate of 10 microns per hour is acceptable for most vacuum furnaces but may be lower or higher depending on the application or volume of the vacuum chamber.

A typical rate of rise curve could look similar to the one in Figure 5.�� If we break down this curve, we can conclude that the first 15 minutes represent a total of any real leak and outgassing occurring in the furnace.�� As things begin to stabilize, the rate of rise begins to straighten out slightly to represent the real leak that is occurring.�� If we analyze what occurred over the previous 30 minutes, we see a rise of approximately 6 microns, or extending this, 12 microns per hour. However, customarily we start from time zero, thus 10-20 microns per hour.

This would represent a marginally tight furnace with consideration given to possibly finding a potential leak, especially if critical product is being processed.

One should always remember that the results of observing a pump down cycle and then performing a leak-uptest are most often affected by the overall cleanliness of the furnace, and might not lead to an immediate detection of problems. Also, the rate of rise curve and other vacuum decay tests will not locate the leaks but will only indicate the relative magnitude of all leaks.

When pump down fails or an excessive leak rate is present, this suggests the need for a long high temperature bake-out on the hot zone emptied of work, fixtures, or work grids, to eliminate any suspected contaminants.�� However, this should be avoided as hot zone damage can occur in the bake-out if the true problem is a real leakage of air.

If the furnace fails the leak rate test, something is wrong regardless of prior leak testing methods. The leak simply has not been isolated or found.

Examples of Where Vacuum Furnace Leaks Might Occur

Based on the above and our experience, listed below are many examples of where leaks could occur.

1. Furnace Door O-ring Seal
2. High Vacuum Main Valve Seal
3. Gas System Flanges
4. Heat Exchanger Feedthroughs
5. Nasty Water Leaks in the Heat Exchanger (i.e., water to vacuum)
6. Internal Furnace Chamber Welds
7. Pneumatic Cylinder Feedthroughs
8. Power Feedthroughs
9. Heating Element Power Feedthroughs
10. Compression Seals as in thermocouple assemblies, wire feed-through glands, or thermocouple sheaths.
11. Welds and Brazed Joints
12. Shaft Seals on Vacuum Valves and particularly vacuum blower motors.
13. Flexible Connectors in Piping
14. Threaded Joints (Vacuum Plugs, Vacuum Gauge)
15. Gasket Seals on Feedthroughs, Manifolds, Sight Ports, Cylinder Glands
16. Other O-ring Seals
17. Nasty Water Jacket to Vacuum Leaks

Safety Concerns in Leak Checking a Vacuum Heat Treating Furnace

Quench Gas Concerns

Often, leak checking requires an individual to enter the open vacuum furnace chamber during the checking process.�� This is especially true on larger furnaces such as car bottom furnaces. Maintenance and leak checking of any furnace chamber internals should only be attempted when the chamber has been completely purged of any residual gas remaining from a prior cycle. Residual quench gases still in the chamber even when the door is opened can cause asphyxiation and death.

The hazard is most critical if argon was used in the prior quenching cycle.�� Since argon is heavier than air, it can remain in low-lying areas such as a quench tank or service pit for many hours. It has no discernible odour and there is usually no advance warning before unconsciousness occurs.

Diffusion Pump Heaters

On furnaces equipped with oil diffusion pumps, maintenance and leak checking should only be attempted with care.�� As illustrated in Figure 6, the diffusion pump operates by boiling oil to form a vapor.�� Heated by electric heating elements at the bottom of the pump, the oil reaches a temperature of approximately 5000^oF and higher, thus providing a very hot external surface until the pump has cooled.�� Any leak checking around this area of the furnace must be done with care. Never air release a hot diffusion pump to air, i.e. to check oil level, as it may explode and cause massive furnace damage.

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Further Defining Leaks in Vacuum Heat Treat Furnaces

When a leak exists on a vacuum furnace system, the type of leak can be defined relative to the amount of flow occurring through the leak.

Turbulent Flow (Very rough vacuum)

Turbulent flow typically describes a gross leak situation. These leaks usually occur in unsoldered joints, open welds, open valves and plugs left off pipes.�� These types of leaks are usually the easiest to find.

Laminar Flow (Rough vacuum)

This term is commonly used to describe a small or fine leak.�� In this case, the leak is proportional to the differential pressure where doubling the pressure across the leak doubles the leak rate.

Molecular Flow (High vacuum)

This describes an extremely fine leak such as a virtual leak, or a membrane leak, as the result of a defective diaphragm in a valve or vacuum gauge.�� A leak like this occurs when the mean free path of the molecule is greater than the diameter or opening of the hole where the leak is escaping.�� In this situation, an increase in pressure has very little effect on the leak rate.

Also, gasses may penetrate through some solid materials such as elastomers (O-rings). These leaks are called permeation leaks. They are different from real leaks because the only way to prevent or stop them is to change to a less permeable material.�� These types of leaks usually occur in the 1×10^-6 Torr range or higher vacuum.

Methods of Leak Checking Vacuum Heat Treat Furnaces

There are three commonly used methods of detecting vacuum furnace leaks: (1) via sound emanating from the leak; (2) vacuum decay highlighted on the vacuum gauges when using a solvent to penetrate the leak; and (3) the use of a Helium Leak Detector (mass spectrometer).

Noisy Leaks

When a vacuum furnace demonstrates difficulty pumping down to expected rough vacuum levels, it typically indicates a fairly large leak.�� This could be as a result of some changes or modifications that have recently been made to the furnace. As the furnace is struggling to pump down, walk around the furnace and listen for any hissing or high-pitched whistling, an indication that air is penetrating a seal.�� As this may be a very small sound, it is important that surrounding noises are eliminated.

Since most furnaces cool under positive gas pressure, closing the furnace and backfilling to maximum backfill pressure will allow you to check for pressure decay and any sound coming from exiting gas.�� This will often show areas where proper seals have been broken or need tightening.

Some operators or leak checking personnel will often use a stethoscope when checking for leaks, as its ability to transmit low-volume sounds and eliminate external noise is exceptional. Often in gross leaks or some smaller leaks, a standard paint brush and soapy wash solution is helpful. Brush the solution around suspected leak areas. Big leaks will blow big bubbles at the leak while smaller leaks will form very fine foam.

Vacuum Gauge/Solvent Detection

For many years, operators and leak checking personnel have been using solvents to assist in finding vacuum leaks. Most solvents used today are either acetone (preferred) or alcohol.

The vacuum gauge of the vacuum furnace instrument panel is a very sensitive instrument. In the solvent method, the operator sprays or brushes the solvent around the suspected leaking area and watches for movement in the vacuum gauge.�� If a leak exists, the entering acetone will affect the gauge reading level since the gauge is calibrated for air, not acetone. The operator should allow sufficient time for the acetone to affect the gauge, normally 20 seconds is sufficient.

The solvent procedure is more sensitive when the furnace is at lower pressures (below 1 Torr) and is most commonly used to find gross leaks. This also allows the furnace to reach a vacuum range where the technician can then employ a helium leak detector to uncover smaller leaks.

If a gross leak is found, it can be temporarily sealed to continue leak checking using a sealant such as “duck seal,” Glyptal® red alkyd lacquer, or Tek-Seal clear vacuum sealant.�� However, the leak checker must realize that this is a temporary fix and must be permanently repaired prior to running any gas quenching cycle at positive pressures.

Helium Leak detecting

The preferred method of leak detection is with the use of a Helium Mass Spectrometer Leak Detector, a highly accurate instrument for locating leaks, especially in hard-to-reach areas. A mass spectrometer can detect extremely small amounts of helium. Helium is the tracer gas of choice because it is inert, nontoxic, relatively inexpensive (in small quantities), and not easily absorbed. Helium also easily flows through small leaks and has only a trace presence in air (usually 5 ppm).

The Spectrometer is connected into the vacuum system; Helium gas is sprayed on any area where a leak is suspected.�� The leak detector will sense the presence of Helium passing through the leak. Helium leak detectors usually work best at levels from the system’s ultimate vacuum to approximately 10 Torr. In some instances, it is necessary to “bag” or isolate a specific area on the furnace and inject helium into the contained space. The Spectrometer is sensitive enough to locate leaks even in moving or transition (vacuum to pressure) seals.

Figure 7 is the standard portable leak detector available through Varian Inc.

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To connect a leak detector:

Connect the device between the blower and mechanical vacuum pump. You’re taking advantage of the high helium pumping speed of the high vacuum pump and blower, so response time is very good. Since the helium pumping speed for the mechanical pump is less than the blower, less helium is lost and sensitivity is good.

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Typical Procedure For Leak Checking a Vacuum Furnace

A typical procedure for finding a leak in a vacuum furnace:

1. Before starting any testing, make sure the helium leak detector is calibrated (checked with a calibrated leak) and working properly. Make sure there are no leaks within the leak detector itself by spraying a small amount of flow into the cabinet. In addition, leak check the connection point from the leak detector to the furnace.
2. Connect the leak detector to the port on the roughing line with the ball valve closed.
3. Make sure water is turned on to the diffusion and roughing pumps.
4. Start the holding and roughing pumps.
5. Manually open the foreline to check for leaks prior to turning on the diffusion pump. Be sure to close the foreline valve following this test.
6. Slowly open the ball valve allowing the helium leak detector into the system.
7. Leak check all welds, threaded fittings, and flanges on the foreline and roughing system by spraying helium sparingly into these areas.
8. Repair any leaks resulting from this initial testing.
9. If any leaks were found and repaired, repeat steps E through H and leak check again.
10. If no more leaks are found, close the leak detector ball valve, and turn on the diffusion pump, allowing it to heat up for 45 minutes.
11. Start the process cycle and let the furnace pump down to optimum level.
12. If the furnace cannot cross over to high vacuum, open the ball valve slightly to the leak detector and leak check all flanges, threaded fittings, welds on the chamber, gas blower can, overhead piping, and vacuum switch copper lines. Fix any leaks found during this process.
13. Now go over the entire system by spraying helium at a high rate of flow.
14. If no more leaks are found, close the ball valve to the leak detector and remove it from the system.
15. The furnace should now be fully checked and should be ready to return to production.
    1. If no external helium leaks are found and the furnace still has a high leak rate as revealed by the leak rate test, there are several other possibilities: Gas leaking into the furnace via the gas backfill valve or partial pressure valves, i.e. nitrogen or argon. Take these valves apart and look for dirt or defective seals or diaphragms.
    2. A possible water-to-vacuum leak from the gas cooling heat exchanger. Take the heat exchanger out of the furnace and look for water deposits at brazed joints. Pressurize the heat exchanger with approximately 100 PSIG air pressure and soap bubble all brazed joints.�� If necessary, submerge the heat exchanger in a tank of water and look for air bubbles. A heat exchanger that has been submerged in water must undergo an oven bake-out prior to being re-installed into the furnace system.�� Do this at 300^oF for four (4) hours to boil out any residual water.
16. When all else fails, consider that water may be leaking from the chamber water jacket to the vacuum internal. It could also be from some other water jacket-to-vacuum enclosure such as the gas blower or heat exchanger enclosure. Remove all components of the hot zone and inspect for possible water deposits that could result from water leaks. If there are no apparent deposits observed, air pressurize the jackets to approximately 30 PSIG and soap bubble all welds and joints.�� Sometimes leaks can occur as cracks or even corrosion in the center of the chamber tank. All chamber leaks must be weld repaired and tested, with no resulting bubbles present.
17. Water leaking from a heat exchanger or vacuum chamber or other jacket into the vacuum will normally turn the roughing pump oil a milky color from a bad leak, or slightly discolored from a smaller leak. In a serious water leak, the vacuum chamber will not pump down below 1 Torr.�� If free water builds up in the bottom of the chamber or heat exchanger enclosure, over time the free water will freeze and form ice.�� Sometimes this will appear as frost on the air side of the chamber or heat exchanger tank.

Please remember that when using the helium leak detector, the time it takes for the leak detector to respond indicates the general area of a leak and further evaluation must be made to find the exact location.�� There is often the possibility of more than one leak within the same area.

If no leaks are found with the helium leak detector, this does not mean there are no leaks. Remember, the final test is the rate of rise test.

Conclusions on Vacuum Heat Treating Furnace Leaks and Detection Techniques

Leak detection is complicated and a detailed subject. All vacuum furnace users must eventually become experts in the “art” of leak checking or their operations will suffer difficult times.

All leak checking personnel should know and understand the following:

• The types of leaks that can occur, including the very difficult “virtual” leaks.
• How to properly perform a furnace leak rate check.
• Understand the safety aspects of leak checking.
• How to use the vacuum pumps and gauges while using a solvent solution to find simple leaks.
• Understand and have access to a helium leak detector for finding the more difficult vacuum leaks.
• How to properly repair or seal any vacuum leaks with consideration of both vacuum and positive pressure cooling cycles.
• Before starting a leak check, always review prior maintenance. Often a loose fitting like a replaced T/C improperly installed is the problem.�� Look for the obvious first before creating a more difficult situation.

Written By:

Real J. Fradette, Senior Technical Consultant, 91��ý Inc.

Contributor:

References:

1. Varian, Inc, VS Series Leak Detectors for Vacuum Furnaces
2. Daniel Herring, Finding and Fixing Vacuum Leaks, Industrial Heating, 2011
3. Maintenance Procedures for A Vacuum Furnace, Jeff Pritchard, Steel Times International 2009
4. Meyer Tool & Manufacturing, Vacuum Chamber Design: What is a Virtual Leak?
5. Daniel Herring, Atmosphere Heat Treatment, Principles/Applications/Equipment, Volume 1 & Volume 2
6. JM Lafferty, Foundations of Vacuum Science & Technology
7. Basic Vacuum Practices, 3/E by Varian, Inc, reprinted by 91��ý
8. Figures 1-3,
9. Figure 6,

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As additive manufacturing (AM), or 3D printing, continues to evolve, many challenges still plague this exciting new technology.�� By re-melting and re-solidifying metals in powder or wire form, each printing machine essentially becomes its own foundry.�� Therefore, precisely defining exact and repeatable mechanical characteristics of the uniquely printed components continues to be the major hurdle for the metallurgist. (Picture 1)

In addition, the vacuum heat treating of printed components is also evolving every day. Since additive manufactured parts display vastly different mechanical behaviors when compared to conventionally produced parts, it is logical that heat treatments associated with this process also vary.�� Since the majority of metallic printing processes involves high temperature melting along with rapid cooling rates, parts typically possess extremely high internal stresses. These inherent stresses can either be an advantage or a drawback to the design of the part. When internal stresses are detrimental to the end product, vacuum stress relieving seems to be the most prevalent and essential thermal process performed on 3D printed parts.�� Other less popular vacuum thermal processes include vacuum annealing and vacuum sintering.

Equipment Needed for Heat Treating Additive Manufactured Components

As additive manufacturing continues to become more popular, the manufacturer will often contract with the commercial heat treater to perform the critical thermal processing for their jobs.�� The AM manufacturers continue to seek out only those facilities possessing a wide variety of vacuum furnaces.�� These furnaces must be equipped with diffusion pumps to attain deep vacuum levels and must exhibit extremely tight temperature uniformities (±5°F). (Picture 2)

Since the ultimate goal of printing any 3D component is to produce a part that is “near net shaped,” the vacuum furnace atmosphere must be pristine.�� Surface contamination found on finished printed titanium or a nickel based alloy part could deem the part scrap.�� Therefore, often manufacturers will insist that only expensive all-metal hot zoned vacuum furnaces must be used to process their parts.�� Vacuum heat treaters must heed the warning! One must determine the risks associated with such requirements.�� The main purpose of this article is to inform the heat treating industry of some of those inherent dangers that we have encountered.

Dangers of Vacuum Heat Treating Additive Manufactured Parts

Direct Metal Laser Sintered Processing of Powders

Over the past two years it is obvious that the Direct Metal Laser Sintered (DMLS) method of printing AM parts has become the most prevalent technology used in the world today.�� During this “powder bed fusion” process a laser or electron beam is used to melt and fuse metallic powders together.�� A layer, typically 0.1mm thick of powder, is spread over the reusable build plate or platform.�� After a laser source fuses the first layer of powder, a new layer of powder is spread across the previous layer using a roller.�� Further layers or cross sections are fused and added until the entire model is created.�� Loose and unfused powder is inherent in this process and remains in position. It is imperative that all of this powder not only be removed from the obvious external surfaces, but also from all internal cavities, blind holes, and cooling passages. When these manufacturing precautions are not taken, the vacuum heat treater suffers. ��Picture 3 shows the resultant damage of vacuum heat treating heat treating a DMLS printed job that still contained loose powders within the builds.�� This was a very expensive mistake- a molybdenum hot zone that was severely damaged due to powders escaping from the internal cavities during heat treating. The corrective action is always to invert the plates, blow the entire build out with nitrogen, and to only process DMLS printed products in all metal hot zones that have no empty spaces within the solid build.

Binder Jet Processing of Powders

The Binder Jet Process (BJP) utilizes two materials- metallic powders and a binder.�� The binder acts as an adhesive between the powder layers.�� After printing, the components are generally “de-lubed” within special atmospheric furnaces.�� It is within these furnaces where “most” of the binders are burnt off or eliminated – but not entirely.�� Upon subsequent high temperature vacuum sintering treatments, the remaining binders immediately evolve and evaporate.�� These detrimental binders will seek to attach themselves to the coldest areas of the vacuum furnace which are typically the water cooled chamber or the pumping systems (See Picture 4).�� If one is planning to vacuum heat treat BJP printed parts, a specifically designed vacuum furnace equipped with a cold trap to collect the unwanted binders is a necessity.

Additional Additive Manufacturing Heat Treating Lessons Learned

The printing engineer must understand that the best results will be attained when the building platforms (or build plates) match the same material composition of the powder being adhered to it.�� In addition the maximum thickness of the printed part should closely resemble the thickness of the build plate.�� Matching build plate compositions and dimensions helps to provide a more stable crack-free printed part. (Picture 5)

Additionally, it is most important to have direct thermocouple placement within the printed part. Since AM allows for any design, we often request a 1/16” minimum printed hole to be incorporated within the thickest cross section of the printed part.�� If that is not allowed then a predrilled matching maximum thickness dummy block should be placed on top of the build plate (Picture 6). Note- the build plate itself typically will never match the temperature profile of the printed part. Thus the build plate should never be drilled and wired.

Since printing metallic parts produces unique geometries, one must be very aware of any unvented blind holes, cavities, pockets, or sealed cooling channels. Any differential pressure forces that are built up during vacuum processing may cause a printed part to crack or even explode.�� Therefore all printed internal geometries must have a clear path of evacuation.

Prior to vacuum processing any AM parts, the vacuum furnace should have been properly baked out at a minimum of 2400 °F and the known leak rates should be less than 5 Microns per hour. As previously noted the stresses of many printed builds are immense. Always employ multiple set point temperature holds and slow heating and cooling rates.

Often the customer will contract the heat treater to creep flatten the build plate in conjunction with the prescribed heat treatment. This is done very carefully with additional weights (Picture 7), however never underestimate the value that you are adding to the customer. This plate creep flattening service ultimately allows multiple reuses after the printed parts are excised from the plates.

Conclusion: Risky Business – The Vacuum Heat Treating of 3D Printed Components

While additive manufacturing continues to be a boon for specialized metal aerospace and medical device components, other industries being somewhat affected by this transforming technology still may seem a way off.�� Currently AM heat treating accounts for only 2% of this commercial heat treaters total heat treat sales.�� However this is 2% more than our total AM sales of the preceding two years!

The ultimate goal of any 3D process is reproducibility.�� If this can be controlled it will drive down the cost of production and thus boost reliability. Vacuum heat treating is proving to play a very large role in the success of the entire additive manufacturing process. As with any new technology, all downstream processors must learn to adapt.�� However it is also imperative that certain known information which is critical to the thermal processing of the printed components gets communicated back to the customer. As we have witnessed, if this information is not shared, the heat treating of AM parts could become risky business.

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