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, 51���� 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. 51����’ 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, 51���� of Western PA and Michigan

As published in Heat Treat Today Magazine:

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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, 51����, Inc., Roger A. Jones, FASM, CEO Emeritus, 51����, Inc. ����������������������

References:

1. Critical Melting Points and Reference Data for Vacuum Heat Treating Sept 2010, 51����, Inc.
2. Graphite Machine Inc., Topton, PA

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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 51���� 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:

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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, 51����, Inc.

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The “Hot Zone” or, furnace internal, on vacuum furnaces has evolved throughout the years, from an all-metal shielded design to insulated, employing various forms of thermal insulation. Both types of hot zones – the all-metal and the insulated – have their acceptable use based on final vacuum and thermal requirements. Though most modern furnaces in operation today use some type of graphite insulated hot zone, the all-metal hot zone is still necessary for processing certain materials which require a super-clean, non-contaminating environment.

In this article, we will highlight some of the essential design requirements needed to provide the proper all-metal furnace for these critical applications.

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Historical Trend – All-Metal vs. Insulated Vacuum Heat Treating Furnaces

Figure 1 (above) illustrates the variance in use of this type of vacuum furnace hot zone over the past 60 years. Initially, more than 80% of the furnaces incorporated an all-metal hot zone, however, toward the middle of the 1980’s that number was reduced significantly, with both types of hot zones (insulated vs. all-metal) earning approximately 50% of the processing industry.

By 2005, the insulated hot zone surpassed the all-metal hot zone in industry usage, peaking at approximately 87% insulated, compared to 13% all-metal. Since then, however, the all-metal furnace has made a comeback, thanks in part to new critically clean application requirements. It now represents just over 20% of new furnace installations.

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The Modern Insulated Heat Treat Furnace Hot Zone

Typical modern graphite insulated hot zone, as shown in Figure 2 (right), consists of:

1. A graphite foil hot face
2. Layers of PAN or rayon graphite felt insulation – 2” to 2.5” for 2400°F applications.

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Note: Our studies indicate that rayon felt is cleaner and less moisture-absorbing than PAN felt, providing better vacuum and producing cleaner work. The early use of carbon felt in graphite-type insulation is no longer acceptable due to its tendency to rapidly absorb air during furnace loading and unloading.

1. A stainless steel supporting ring structure
2. Curved graphite heating elements

Typical operating performance of graphite insulated hot zone, as seen in Figure 3, (right):

1. Slower vacuum pump down than all-metal hot zone, due to higher surface area of felt insulation and air entrapment
2. Normal ultimate vacuum – low 10^-5 Torr range
3. Any retained water from felt insulation could result in formation of CO and CO₂ which could affect some workload properties
4. Acceptable for most vacuum processing applications

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The Modern All-Metal Hot Zone for Vacuum Heat Treating

Typical all-metal hot zone for 2400°F operations, (Figure 4, right) includes:

1. Metallic shields – three molybdenum inner shields backed by two stainless steel outer shields.
2. Stainless steel support ring
3. Circular molybdenum sheet-type elements

Typical Operating Performance for all-metal hot zone, shown here in Figure 5 (right):

1. Faster and deeper vacuum performance due to much lower surface area of all-metal shields versus graphite insulation
2. Ultimate Vacuum – Low 10^-6 Torr range or better
3. Reduced water retention results in minimal formation and contamination of carbon gasses
4. Produces clean, non-reacted work

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Other Features Included in the All-Metal Furnace for High Purity Processing

The modern vacuum furnace incorporates a gas cooling system to rapidly cool the workload for metallurgical requirements, and to return the load to room temperature for unloading. This cooling system can be either internally attached at the rear of the furnace (Figure 6), or in a separate housing outside of the chamber (Figure 7).�� The following explains why the external system provides for cleaner performance.

An External Gas Cooling System with Isolation Valves

The external gas cooling system with isolation valves provides for isolating the cooling gas blower motor/heat exchanger assembly during the vacuum pumping and heating portions of the cycle.

This provides for better and deeper vacuum, resulting in an overall cleaner cycle needed for critical workloads.

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Location of Isolation Valves

The isolation valves are located on the cooling gas exit piping and the gas inlet piping.

During initial pump-down valves are open, then closed with blower housing kept under vacuum with a separate holding pump.

This design reduces overall pumping surface areas for faster and deeper vacuum performance. Backfill gas introduced simultaneously to blower housing and furnace chamber at cooling initiation to balance pressure prior to opening valves.

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Pump Down Comparison With and Without Cooling System Isolation Valves

Our next curve (Figure 8, below) shows the relative pump-down performance of a furnace with and without the gas cooling isolation valves.

As is illustrated, the pump-down speed of the furnace is at least 30% faster when the isolation valves are included on the system. Additionally, we are able to achieve a deeper vacuum level.

Comparing Residual Gas Trends of Two Different Hot Zones

Laboratory studies were conducted to establish the relative residual gasses of the all-metal versus the insulated hot zones at different vacuums and temperatures.�� Results, illustrated in Figures 9 and 10, were as follows:

All-Metal Hot Zone

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Insulated Hot Zone

Analyzing the above we can state:

• Water vapor is the dominant gas remaining at ambient temperature.
• Approximately 20% less water vapor in all-metal design.
• Above 1500°F, carbon monoxide begins to exceed water as dominate gas.
• Approximately 50% less carbon monoxide for all-metal versus graphite insulated.
• Both hot zones capable of producing contaminate-free work with proper techniques, including:
  1. Initial clean work
  2. Low furnace leak rate
  3. Initial pump down level
  4. Pre-cycle bake-outs
• All-Metal furnaces have inherently lower vacuum levels.

Purer Processing – Dedicated Climate Controlled Room for Titanium and Alloy Heat Treating

Another aspect of keeping the product as clean as possible is to isolate the front loading part of the furnace from the pumping system and cooling components.

Figure 11 (below) shows one of our dedicated humidity and temperature controlled rooms.

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Materials, Components, and Industries Requiring the All-Metal Furnace for Vacuum Heat Treating

Based on our many years of vacuum furnace processing, we can state the following:

Critical Materials Which Must Be Processed in the All-Metal Hot Zone:

1. Reactive metal parts with any finished machine surfaces, especially those to be welded after heat treatment. This includes all types of medical components.
2. Medical implant parts of any material with surfaces that may not be machined after heat treatment.

Other Materials Which Should Be Processed in the All-Metal Hot Zone:

1. Semi-finished reactive metal parts having cosmetic requirements.
2. Medical implement parts (surgical and diagnostic tools) requiring good cosmetic appearance. Materials include 15-5, 17-4, 17-7PH SS, Carpenter 304, 420, 440, 455, 465, etc.
3. Anything to be metallically boxed or wrapped in order to remain bright in a graphite hot zone.

Examples of Critical Parts Requiring an All-Metal Heat Treating Process

Heart Pacemaker Housings

Heart Stents

Titanium Components

Other Medical Components

Eliminates Wrapping Required in Insulated Furnaces

In order to maintain a pristine product in an insulated furnace, very often the entire tray of components must be wrapped in thin stainless steel sheet to protect the load from possible contamination. This not only adds additional processing cost but can be dangerous to the person wrapping, due to the very sharp corners of the sheet when folded. The all-metal hot zone furnace eliminates this type of wrapping.

Summation on the Need For the All-Metal Vacuum Furnace Hot Zone and Specific Application Advantages

The all-metal shielded furnace undeniably has an important place in the vacuum processing world. Providing the purest environment available, it produces a pristine end product to meet the most critical applications.

Written by:

Real J. Fradette – Senior Technical Consultant, 51���� Inc.

The post The Returning Need For the All-Metal Vacuum Furnace Hot Zone and Specific Application Advantages appeared first on 51����.

Dew Point Considerations in Vacuum Heat Treating

Since the majority of commercial and captive heat treat facilities do not typically operate under controlled environments, the temperature and humidity swings can often be drastic. The environmental changes are especially substantial when comparing the seasonal averages, monthly averages, weekly averages (See Figure 1), or even day light to nighttime daily averages. Temperature, along with adsorption and de-sorption within the gas sampling system also affects the reliability of dew-point readings.

The precision required of any dew-point analyzer for determining water content within an open atmospheric furnace is much more forgiving. However, when analyzing very dry specialty gases that are used for partial pressures and quenching in vacuum processing, a greater precision is needed. The sensitivity of dew point measurement within very dry ranges is often meaningless. For example, the difference between -110°F and -95°F dew-point is only 2 ppm of water (See Figure 2). Therefore, the accuracy of a dew point instrument within that normal operating range for specialty gases is questionable.

Oxygen Analysis Considerations in Vacuum Heat Treating

The present capability to simultaneously compare and contrast dew point readings adjacent to a Trace Oxygen Analyzer on a daily basis has definitely proven the superiority of one measurement versus the other (See Figure 3). Having a consistent signal that is totally independent of temperature and humidity provides a more reliable measurement of the dryness of the gas species that is being analyzed.

Calibration

Often a dew point sensor can fail without warning. Typically, an erroneous signal would eventually read extremely dry (e.g. -120° to -150°F). This failure could degrade over a long period of time or overnight. Semiannual calibration of the sensor must be outsourced and maintaining an inventory of a backup instrument can be costly.

Unlike the dew point instruments, the Oxygen Trace Analyzer instrument can be calibrated right on the shop floor by procuring a cylinder of nitrogen gas with exact known oxygen content (See Picture 1).

Calibration of the Oxygen Trace Analyzer instrument using a higher value of Oxygen (30.3 ± 2ppm) versus the typical production testing range of 0 to 5 ppm was tested (See Figure 4). This was done so that the operator could observe the instruments upscale reaction during the weekly calibration schedule. This in-situ oxygen calibration process, which cannot be done with a dew point instrument, has already identified several failing dew-point cells.

Oxygen and Water in Vacuum Systems

It is well known by any vacuum practitioner that the primary residual gas that is prevalent during a pump down of a vacuum furnace is dominated by water vapor.��The water molecules from the atmosphere, bombards and clings to the inner surfaces of a vacuum system anytime a vacuum furnace door is opened.��The longer the vacuum door is left open the more saturation will occur.��Besides being detrimental to materials being heat treated, water vapor desorption or pumping time can be a major time-wasting problem in vacuum processing.

Once the vacuum furnace is completely pumped down and water vapor is minimized, the relative leak tightness of the vacuum furnace needs to be addressed.��A vacuum furnace with a high leak rate will always possess higher residual oxygen content.�� Therefore, depending upon the size of the furnace, leak rates should never exceed 10 microns per hour at a pressure of 70 microns or less.�� A leak tight system, one where the volume of oxygen leaking into the furnace is sufficiently low, is paramount for consistent and accurate heat treatments of any materials.

Oxygen Versus Water – What is More Forgiving

Thankfully, oxygen and water vapor is always found in the air around us.��However, the amount of water vapor will vary with temperature and atmospheric pressure.��The earth’s atmosphere has the maximum capacity to hold a maximum volume of 2% water, regardless of temperature and pressure.��In contrast, the earth’s oxygen content is 10 times that of water content (see Table 1). Additionally, the vacuum processing of critical components can only tolerate up to 2.5ppm of oxygen, compared to 10ppm of water vapor.��Therefore, water vapor is much more forgiving in vacuum processing than oxygen.

Conclusion with Dew Point Versus Oxygen Content in Vacuum Heat Treating

It is indisputable that water vapor (dew point) and oxygen are both problematic actors in vacuum processing. Currently all technical specifications are only addressing one of these measurements in our specialty gas usage, dew point. Given the natural inconsistencies of dew point readings, the lack of in process calibration of dew point instrumentation, and the abundance of oxygen in our atmosphere – are we really analyzing the correct elements?

For specification requirements, this author continues to record daily dew point readings for every specialty gas. However, for true verification of critical clean and dry inert gases being used in vacuum processing, this author exclusively assesses the oxygen content results derived from the Oxygen Trace Analyzer instrument.

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Dew Point Versus Oxygen Content In Vacuum Processing Introduction

It is well known that accurate measurement of any heat treating atmosphere can have a significant effect on the quality and process yield of heat treated components.�� Traditionally, dew point analysis has always been the bellwether in determining our heat treating atmospheric conditions. This is because it was discovered very early on that moisture parameters can have a tremendous impact on carbon potential and thus on final properties.�� Since the main objective of any neutral furnace atmosphere is to prevent detrimental effects such as carburization, de-carburization, hydrogen embrittlement, oxidation, and soot formation, one must analyze much more than moisture content of a furnace atmosphere. Contending with such constituents as CO, CO₂, H₂, H₂O, N₂ and hydrocarbons (e.g. CH₄), a better more robust instrument was needed to analyze endothermic or exothermic atmospheres. Today, either Oxygen Probes or Three Gas Analyzers are the industry’s preferred analytical instruments to determine carbon potential within an atmospheric furnace.

Problems with Dew Point Content In Vacuum Processing

As the analysis and controls of atmospheric furnaces have evolved over the years, this author has questioned the antiquated method of measurement of purity of specialty gases within the vacuum processing arena. Why is dew point measurement still the bellwether of specialty gas providers, and more importantly within the vacuum heat treating industry? With the advent of such innovative manufacturing processes as Additive Manufactured components, the new metallurgy that comes with AM parts requires ultra-clean atmospheres. (See Picture 1) Is the dew point analyzer really the best instrument that we have in our toolbox?

Analyzing Dew Point in Vacuum Heat Treat Furnaces

Examine this picture of a rose (see Picture 2).�� The appearance of liquid water on this rose occurs only because of one simple fact:�� the temperature on the surfaces of the rose petals collecting the dew is below the dew point of the surrounding air.�� Likewise, the amount of water within specialty gas sampling lines in terms of the number of molecules or mass of water determines at what temperature the water vapor starts to go into liquid phase.

Therefore, an “exact” dew point measurement is most dependent upon the surrounding temperature. Note how the dew point values within the environs of a state of the art heat treatment plant process gas lines, i.e. N₂, AR, etc., vary from summer to winter (see Graph 1).�� The fact is today’s heat treatment plants are probably the worst environment to test for dew point. The majority of heat treating facilities in the world experience tremendous swings of ambient temperatures and relative humidity even within a 24 hour period of time. (See 51����’ Publication No. 3 in its Vacuum Furnace Reference Series entitled “Operating a Vacuum Furnace Under Humid Conditions,”).

Trace Oxygen Analyzers

A trace oxygen analyzer is a versatile microprocessor-based instrument used for detecting parts per million (ppm) levels of oxygen. Oxygen sensing instruments are typically sealed units and require reasonably regulated sample pressures (.2 to 2.4 slpm). The response time is dependent upon the flow rate, e.g. a low flow rate will result in a slower response to O₂. More importantly a trace oxygen analyzer results in a signal that is independent of temperature.

51���� engineers realized the advantages of trace oxygen instruments versus dew point instruments and decided to build a combination instrument that would employ both methods (See Picture 3). ��Solenoid controls automatically sample each of the four specialty gases (nitrogen, argon, helium and hydrogen) utilizing both instruments every six hours 24 hours a day. All dew point and trace oxygen results are recordable and traceable along with the plant’s ambient temperature and relative humidity. Alarm features are set for any values of dew point above -60° F and /or values of oxygen greater than 5 ppm in the process gas feed lines.

After one full year of side by side operation it was very clear to see the trace oxygen analyzer is the instrument of consistency (See Graph 2).

Specifications for Dew Point Measurements

So why has the vacuum heat treating community, when it comes to determining purity of their specialty inert gases, been slow to react when compared to the atmospheric heat treating community? It is this author’s opinion we are all being driven blindly by the specifications that govern us. Many specifications require only dew point measurements for gas purity.

More recently some specifications, such as Boeing Aircraft, acknowledge the Compressed Gas Association’s designations which include dew point AND ppm of oxygen for various gases (See Table 1). However, AMS 2769B paragraph 3.2.1.1 counteracts this allowance by addressing only dew point issues, more specifically the installation of sampling lines, the location of the dew point cell, and recording frequencies (See Table 2). ��This paragraph is often fertile ground for “findings” by any auditors who perform outside accreditation audits on heat treating facilities.

Dew Point Versus Oxygen Content In Vacuum Processing Conclusions

When vacuum heat treating metal alloys that oxidize readily in the presence of small concentrations of water vapor or oxygen, data suggests that dew point should not be the stand-alone gas purity analyzer.�� Dew point only measures the water vapor, not oxygen in the gas line.�� Adding an oxygen analyzer as an additional quality tool provides the heat treat shop greater assurance that the process gas entering the furnace is of the highest purity and meets the specifications of the customer.

Written by:

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• Thermocouple Construction
• Types of Thermocouples
• Vacuum Furnace Control Thermocouple Placement
• Temperature Uniformity in a Vacuum Furnace
• Use and Placement of Work Thermocouples
• How Work Material Properties Affect Heating Rates – Emissivity, Surface Finish, Mass, Surface Area
• Using Thermocouple Dummy Blocks and Their Placement
• Non-electrical Temperature Monitoring Devices

Thermocouple Construction

A thermocouple is normally defined by its’ junction type.�� These include as shown in Figure 1:

1. Exposed Junction (Bare Wire) has the fastest response time – ideal for measuring rapid temperature changes.
2. Ungrounded Junction (Sheathed) has a welded junction – ������insulated from the protective sheath and is electrically isolated. ��Longer in response time.
3. Grounded Junction (Sheathed) has a junction welded to tip of sheath. Wires are completely sealed from contaminants. Good response time.

Note: Sheathed thermocouples with a small diameter have a faster response time while larger diameter sheathed thermocouples have longer life and are better for measuring higher temperatures (heavier gauge internal TC wire).

The sheath material of a thermocouple is based on application and temperature range requirements.�� Typical thermocouple sheath materials include:

1. Inconel® 600 Sheath is ideal for severely corrosive environments and elevated temperatures. ��It resists progressive oxidation. ��Maximum operating temperature: 1148°C (2100°F).
2. 304 SS Sheath is for general-purpose use, is corrosion-resistant, and good for general heat treating applications. Maximum operating temperature: 1148°C (2100°F).
3. 316 SS Sheath has higher corrosion resistance than 304 SS. Withstands some strong acids. Maximum operating temperature:�� 1148°C (2100°F).

Types of Thermocouples for Heat Treating

There are many types of thermocouples and based on the type of wire alloys used, they are defined by an ANSI Type Symbol.�� Our next chart (Figure 2) highlights the most common types used in vacuum furnace applications.

Further defining the additional characteristics of the above thermocouple types, we have:

• Type K – is used protected or exposed in in oxidizing, inert or dry reducing atmospheres; exposure to vacuum limited to short time periods; reliable and accurate at high temperatures
• Type N – is used protected or exposed in oxidizing, inert or dry reducing atmospheres; very reliable and accurate at high temperatures
• Type S – Normally used as control and over-temperature thermocouples with alumina protection tubes; very reliable and accurate at high temperatures
• Type W3 – Normally used for control of very high temperature applications; offers the advantage of ductility over pure tungsten thermocouples; Accurate at extremely high temperatures

��

Vacuum Heat Treating Furnace Control Thermocouples and Their Placement

Most vacuum furnaces in operation today use a Type S TC for Control and Over-temperature protection.�� They are rigid and normally with an Alumina (Mullite) Sheath.�� They are quite fragile and must be handled with care. See Figure 3 below.

The Type S TC is typically used in operations up to 2800^oF.�� For higher applications, a Type B TC can be used for operations to 3200^oF or a Type W3 for higher temperature applications.

The placement of the control TCs within the vacuum chamber is most critical.�� Using proper vacuum seals on the TCs to penetrate into the chamber, they must extend into the hot zone, a minimum of 2.5 – 3.0 inches beyond the heating elements of the furnace to minimize the effect of the heating element on the TCs.

The adjacent photo (Figure 4) illustrates a typical furnace with three zones of heating with a control TC penetrating beyond the elements at each zone and an over-temperature TC in the center also extending beyond the elements.

Temperature Uniformity in a Vacuum Heat Treating Furnaces

In order to produce acceptable work in a typical vacuum furnace, it is essential that the furnace meet certain temperature uniformity standards within the hot zone area. The uniformity standard per ASM-2750E provides for class designations based on the acceptable uniformity for the class. The uniformity tolerances by furnace classes are:

• Class 1 – +/- 5^oF
• Class 2 – +/- 10^oF
• Class 3 – +/- 15^oF
• Class 4 – +/- 20^oF
• Class 5 – +/- 25^oF
• Class 6 – +/- 50^oF

Most vacuum furnaces are horizontal in configuration and a temperature uniformity survey (TUS) to meet the testing requirements required by ASM-2750E typically use a minimum of nine (9) TCs.

AMS 2750E allows for the use of racks specifically designed to accomplish the placement of the thermocouples for the TUS.�� A thermocouple heat sink attached to the rack is allowed for the thermocouple provided that the cross-section of the heat sink does not exceed 0.50” or is not thicker than the thinnest material being processed.�� The key to any rack structure is to be able to correctly place and rigidly support the T/Cs at the true dimensions of the furnace work zone.

Racks can be specifically designed as they apply to the expected use of the vacuum furnace.�� Also, baskets can be stacked to represent a full load configuration with the T/C heat sinks positioned on the on the inner eight corners of the baskets with a ninth T/C positioned in the center position of the load.

Solar has designed several T/C racks similar to the image shown in Drawing A (Figure 5). Typically this is a box configuration fabricated using stainless steel angle members with heat sink blocks located at the corners and center position.�� This structure will satisfy many commercial applications but has certain limitations when conforming to AMS 2750E and is not universal for multiple sized furnaces all requiring TUS.

Also, these types of racks require considerable storage space when not in use.

The newest, most acceptable and universal TUS support structure that has been designed, consists of individual stainless steel corner pipe members that sit vertical with a bottom support structure (Figure 6).�� The vertical height of each member can be adjusted with different center lengths to accommodate various size furnace hot zones.�� Each structure can be accurately positioned to the outer extremes of the hot zone to locate the eight corner T/Cs.�� An additional center structure is provided to properly locate the center T/C.�� All main pipe corner vertical structures will have top and bottom T/C heat sinks welded to the inner surface.�� Each heat sink may be 1/2” diameter by 3” long with a hole drilled on center to accept the T/C.�� This type of support structure configuration should be used to meet the most critical aerospace applications.

Our photo to the right (Figure 7) illustrates our rack structure positioned in the furnace ready to initiate a temperature survey.

The survey frequency on any furnace is dependent on the furnace class.�� The following frequency of TUS shall be required:

Class 1 & 2:�� An initial survey performed prior to production followed by three monthly periodic surveys for a total of four as the initial frequency.�� Subsequent surveys may then be reduced to quarterly provided prior surveys are all acceptable.

Classes 3 – 6: An initial survey performed prior to production followed by three quarterly periodic surveys for a total of four at the initial frequency.�� The frequency may then be reduced to semi-annually provided all prior surveys are acceptable.

Use and Placement of Work Thermocouples in Heat Treating

Work Thermocouples

Work thermocouples (Figure 8) measure heat transfer from heating elements to the “work”.�� They are typically placed in or on the part being processed.

The factors affecting heat transfer include:

1. Thermal conductivity of the part.
2. Emissivity/absorptivity of the part.
3. Mass of the work load
4. Surface area of parts

Typical work thermocouples are shown in the adjacent photo (Figure 9).�� From left to right, these would include:

• A bare wire with an alumina sleeve covered with a Refrasil sheath. Tip is ½” hard twist.
• A bare wire with an alumina sleeve covered with a Stainless Steel Mesh. Tip is ½” hard twist.
• This is a 1/16” Inconel sheathed TC.

Most work thermocouples (Figure 10) used in vacuum furnaces are:

• Type K – most common and least expensive yet very accurate. ��Currently losing favor to Type N.
• Type N – tends to now show improved accuracy and seems to provide extended life and multiple re-use.
• Both of the above are a concern on re-use when having operated over 2150^oF as they tend to drift.
• Aerospace industry requires new Type K thermocouples after each run.

Most vacuum furnaces incorporate an internal work thermocouple jack panel (Figure 11) positioned outside the furnace hot zone.�� This eliminates the need to continually provide a good vacuum seal into the vacuum chamber for the thermocouples.

The jack panel can be a source of error and must be cleaned regularly.�� In addition, any TC jacks that are not used during a particular cycle should be sealed with dummy plugs to eliminate contamination.

Placement of Work Thermocouples

Since the configuration of a workload typically varies with each cycle,�� it is essential that the thermocouples be placed in the optimum position to reflect the accurate temperature of the workload. Basic rules on placement of the TCs would be:

• 

  Place the load thermocouples in existing holes or crevices with the tip or hot junction in contact with the metal.

• Work thermocouples must be inserted deep into the center of the workload, especially on dense loads. (Figure 12)
• Consider an area on the workload that is most shielded from the radiation or the thickest cross-sectioned part in the load (slowest portion to reach equilibrium.
• Our example to the above illustrates TCs deeply imbedded during the annually process for brass tubing.

Our two photos to the right (Figures 13 & 14) demonstrate workloads that are difficult to thermocouple.�� These require that the thermocouples be fully imbedded into the center of the load.

Examples of load that are poor conductors of heat would be:

• Loosely coiled and stacked sheet
• Loosely rolled screen
• Loosely coiled wire
• Small fasteners/ ball bearings
• Powders

Common workload thermocouple problems could include the following:

• Crossed wires: If a thermoelectric circuit is backwards, the instrument will read backwards.
• Loose screw: A loose screw on the mounting plug will cause a poor connection and possibly shorting.
• Uncompensated junction: If materials are used that are not the same as the wires, an error will be introduced.
• Twisted wires: If wires touch at a point between the hot and cold junctions, a new hot junction will be produced causing a measuring error.
• Damaged insulation: If the insulation is damaged, and wires touch foreign materials, an error will be introduced. In addition, this easily allows twisted wires to touch.
• Poor hot junction: A loosely twisted or dirty junction will introduce an error.
• Dirty jack panel or extension wires: A dirty jack panel is a typical source of error and must be cleaned regularly.

How Work Material Properties Affect Heating Rates – Emissivity, Surface Finish, Mass, Surface Area

The ability of a part or material to accept heat at a certain rate is based on several factors.�� These include:

• Emissivity – The ability of a surface to emit radiation
• Absorptivity- ability of a surface to absorb radiation
• At thermal equilibrium, the emissivity of a body (or surface) equals its absorptivity
• A perfect black body (absorbs 100% radiation)

In order to demonstrate the above factors we created a test study.�� The test included preparing six (6) same sized dummy blocks of various surface conditions (Figure 15 below) and then subjecting them to the same heat treat cycle to record the heating profiles.�� The blocks were 2.5” cubes of carbon steel with the surface modified or plated as shown in our next photo.�� A thermocouple was inserted half way down the center of each block.

The blocks were first heated to 1000^oF, held for four (4) hours and then heated to 1700^oF and held for one (1) hour. The curves below (Figure 16) illustrate the serious time difference for the various blocks to reach temperature.

Using Thermocouple Dummy Blocks and Their Placement When Heat Treating

There are many workloads that are processed in a vacuum furnace that do not allow for work thermocouple to be placed in or in proper contact with the parts.�� This leads us to the introduction of dummy thermocouple blocks placed in the furnace to simulate the workload.

Based on our prior study illustrated above, when preparing or manufacturing dummy blocks, the following rules should be observed.

• Drill TC holes into center of the block.
• Match cross-section to largest workload part.
• Match the mass of the work
• Match thermal conductivity
• Match the surface condition
• Match emissivity
• Match the material of the parts

Our photo to the right (Figure 17) shows various dummy blocks used in varying load configurations.

Often it is not practical to produce a dummy thermocouple block to be an exact duplicate of the workload parts.�� An example is shown to the right (Figure 18) where the dummy block represents approximately one third of the actual parts height.�� However the critical cross-section dimensions have been simulated to represent true heating rate.

Some conclusions relating to the use of thermocouple dummy blocks would be:

• Radiation heating of work is dependent on mass and surface condition
• Heat rate : Bright and polished much slower than dull and dark
• Rough surfaces heat faster than smooth, reflective surfaces
• Dummy blocks not in direct contact with parts must have similar mass and surface area to mimic the heat rate of load
• Dummy blocks should be periodically re-conditioned to maintain proper surface smoothness and appearance
• One load of material, with a particular surface condition, compared with a load of the same material with another surface condition, could take as much as twice as long to reach the desired temperature

Non-electrical Temperature Monitoring Devices for Heat Treating

Recent and new developments have produced some interesting alternatives to work thermocouples to measure part temperature in a vacuum furnace.�� The latest is an easy and cost effective method to monitor process temperature – Orton Temp Tabs (Figure 20).

What are Orton Temp Tabs?�� They are ceramic disks that sinter at a controlled rate over a range of temperatures.�� The shrinkage is then correlated to the maximum temperature reached in the furnace.�� Therefore, they record peak temperature only.

Using and Measuring Temp Tabs

• 

  Temp Tabs are measured after they exit the thermal process (Figure 21).

• Best accuracy is achieved using a Temp Tab desktop gauge.
• Place the Temp Tab in the gauge and measure the diameter.
• Other measuring devices can be used as long as they measure to .01 mm.
• Enter the diameter measured into the Temp Trakker software or look up the temperature on a printed chart.

A typical Temp Tab chart (Figure 22) to correlate the exact temperature provided by the tab shrinkage dimensions is shown to the right.

Although Temp Tabs only record peak temperature, they can still be useful in a vacuum furnace to confirm thermocouple readings and feedback.

Understanding Vacuum Furnace Temperature Measurement Issues Conclusions

As has been highlighted above, the accurate measurement of temperature within a vacuum furnace can be a fairly complicated subject. Summarizing critical areas, we have:

• Both control and work thermocouples must be positioned properly to accurately reflect the true temperature conditions of the furnace and the workload.
• The type of thermocouple used is dependent on the temperature range and the application.
• Different types of sheathed thermocouples react at different heating rates based on their construction.
• Furnace temperature uniformity requirements are based on the Class type and application per ASM-2750E.
• Workload heating rate is determined by several factors including cross-section, mass, emissivity, surface condition and material being processed.
• When using dummy thermocouple block to represent the workload, it is essential that the dummy block be designed to satisfy all the factors in our prior paragraph.
• Temp Tabs can be useful in certain applications to measure and confirm peak furnace temperature.

Written By:

Real J. Fradette – Senior Technical Consultant

The post Understanding Vacuum Furnace Temperature Measurement Issues appeared first on 51����.

This paper will summarize the following:

• What is a Residual Gas Analyzer (RGA) and how does it work?
• What are the residual gases when analyzing a vacuum furnace in various conditions?
• What are the residual gases when comparing an all-metal to graphite insulated hot zone?
• What are the energy and maintenance comparisons of all-metal to graphite insulation?

What is a Residual Gas Analyzer (RGA)?

The RGA is a type of mass spectrometer that can detect atomic mass ranges from 1 to 300.�� However, it typically looks for atomic masses from 1 to 50.

The RGA can sample atmospheric pressures down to the 5×10^-12 Torr range but normally samples at the 1×10^-6 Torr vacuum level for our equipment.

The RGA provides a semi-quantitative measurement of the remaining gases in a vacuum system.�� It does not provide absolute values but compares relative amounts of residual gases that remain in the system.

RGAs can also be used as sensitive helium leak detectors. With vacuum systems pumped down to lower than 10^-5 Torr—checking of the integrity of the vacuum seals and the quality of the vacuum—air leaks, virtual leaks and other contaminants at low levels may be detected before a process is initiated.

Components of a Typical RGA Set-Up

A proper set-up for an RGA is to position the sensor to an extension from the vacuum chamber with a valve initially isolating the sensor from the chamber.�� Providing the vacuum to the sensor would be a turbo molecular pump backed by an oil sealed, rotary vein pump.

Initially the valved-off sensor connection is pumped down to 5×10^-5 Torr or better prior to opening the valve to the main chamber.

The RGA does not detect solids, only gases with less than 100 AMU.

Atmosphere Versus Vacuum Molecular Comparison

The above two figures illustrate the difference in a typical chamber at atmospheric pressure and a chamber under deep vacuum.�� The “perfect vacuum” does not exist. Trace amounts of gases are always present in a vacuum chamber or system.

A typical atmosphere would have approximately 1×10²⁰ molecules per cubic centimeter while a typical high vacuum would have approximately 1×10¹⁰ molecules per cubic centimeter.

The RGA analyzes the residual gas molecules remaining in the vacuum system which can include water vapor.

The following Period Table of Elements reflects the various atomic mass numbers for the various elements and gases to be discussed.

Comparing Types of Gases in Air and Vacuum

The following charts illustrate the difference in gases of atmospheric air and high vacuum:

Typical RGA Scans of a Basic Vacuum Heat Treat Furnace

We have analyzed a typical vacuum furnace that has been pumped down to the 10^-4 Torr range and have shown the residual gases on both Log and Linear curves on the two charts that follow:

• RGA Log Scale Chart of Residual Gases

Please note that on the above chart, water has an atomic mass of 18.�� Since water is H₂O, we have Hydrogen = 1 times 2 = 2 plus Oxygen�� = 16 for a total of 18.

��

• RGA linear Scale Chart of Residual Gases

As shown above, all the residual gases as related to the residual water vapor are better reflected in the log scale chart while the linear scale highlights water as the serious major player.

Illustration of Two Types of Vacuum Heat Treat Furnace leaks

Air Leak

A RGA can be used to help determine what type of leak is occurring in a vacuum furnace.�� Most leaks are some type of air leak due to bad seals or joints.�� An acceptable furnace leak is normally less than 20 microns per hour while a leak rate of greater than 20 microns per hour indicates a more serious problem.

The following chart illustrates the two different RGA plots that reflect the acceptable and unacceptable conditions.

As is shown, with a serious air leak, nitrogen and oxygen (major components of air) are the atomic masses that peak.

Water Leak

Our next chart illustrates the RGA plot of a furnace water leak.

As expected, the highest peak is for our water residual component.

Comparing an All-Metal Hot Zone Heat Treat Furnace Versus a Graphite Hot Zone Furnace

Many vacuum furnace processes require a very deep vacuum and minimal residual carbon gas in order not to contaminate the final product.�� In order to illustrate the relative residual gases remaining in two different types of insulated furnaces, we selected two of our identical sized lab furnaces for comparison. One had a graphite insulated hot zone with graphite elements awhile the other had an all-metal molybdenum/stainless steel shielded design with molybdenum elements.�� Photos of both are shown below.

The two furnaces had identical pumping systems with Varian 8” diffusion pumps and hot zones that measured 10” diameter by 18” high.

Prior to proceeding with a RGA analyzes, both furnaces were prepared as follows:

1. Bake out furnace at 2250°F for 2 hours and vacuum cool to <125°F.
2. Open furnace door for 5 minutes.
3. Pump down to 5 x 10^-5
4. Ramp at 20°F / minute to 2200°F.
5. Hold for 1 hour.

The Ambient Temperature RGA Comparison at 5×10^-5 Torr on a Log scale is shown below. The all-metal is identified as Moly while the insulated design is defined as Graphite.

The results above indicate that air components and carbon are definitely more prevalent in the graphite insulated furnace.�� This should be expected by the higher surface area of the graphite felt material than the all metal design and its retention of water vapor and air components when the furnace is open and also the graphite carbon molecules contributing to the carbon dioxide residual gas.

A liner RGA plot of the above ambient temperature tests is shown on our next chart.

Authors:

Real Fradette, Consultant

The post The Use of a Residual Gas Analyzer (RGA) to Determine Differences in Graphite and All-metal Hot Zone Vacuum Operation appeared first on 51����.

A recent process development test relating to carburizing illustrated the need to better understand the effect of surface emissivity and the proper use of dummy thermocouple test blocks when heat treating.�� The testing involved carburizing areas of a partially copper plated alloy steel part.�� The copper plating covered areas of the part that were not to be carburized.�� Since the configuration of the part made it impossible to place a thermocouple within the part, a dummy test block made of carbon steel with the approximate same cross-section was used for the process thermocouple without proper consideration of the surface condition of the test block.

Using the test block as the control, carburizing was initiated at the proper temperature based on the test block having reached that temperature.�� At the completion of the test, the part was examined for carburizing results and found in the non-copper plated areas, the depth of the carburized case to be shallow.�� This indicated that the cycle performed did not initially hold the part long enough at the correct temperature prior to carburizing.�� This resulted in the conclusion that when using dummy test blocks for controlling process times and temperatures, many factors must be considered including surface emissivity.

Download Understanding Emissivity and Thermocouple Test Blocks When Heat Treating

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