CARBURIZING TECHNICAL CONTENT

Elevate Your Knowledge: 5 Need-to-Know Case Hardening Processes

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OCYour parts need heat treated to herculean surface hardness but with a soft, ductile core. That is to say, you are looking at case hardening processes, most likely one of these: gas carburizing, low-pressure carburizing, carbonitriding, gas nitriding, and ferritic nitrocarburizing.

Mike Harrison at Gasbarre Thermal Processing Systems brings us a Technical Tuesday article about what case hardening is and how five of the most common processes vary by (1) comparing the specific guidelines for each temp and time, (2) identifying equipment used to perform each process, and (3) providing a chart (at the end!) to understand different process considerations.

Mike Harrison
Engineering Manager of Industrial Furnace Systems Division
Gasbarre Thermal Processing Systems

Case hardening falls into a class of heat treatment processes that typically involve the addition of carbon and/or nitrogen to the material through solid-gas reactions at the surface followed by diffusion. These processes are performed for any number of reasons that generally include increasing strength and wear resistance, but in all cases the end result is a harder, higher-strength surface with a softer, more ductile core.

Case hardening processes can be divided into two subsets: those that include quenching to harden, such as gas carburizing, low-pressure carburizing (LPC), and carbonitriding; and those that do not include quenching, such as gas nitriding and ferritic nitrocarburizing (FNC). This article will provide a brief look into each process, the types of equipment used, and considerations for implementation.

Diffusion + Quenching Processes

These processes involve heating the workload to austenitizing temperature, which is above the upper critical temperature for the material in question, then supplying and allowing the desired element(s) to diffuse into the part surface, followed by rapid cooling (quenching) to create a phase change to martensite that strengthens the material. Tempering is then performed to create a material that has the desired final strength and ductility properties. The result is a high concentration of added elements on the surface that continually decreases through diffusion until eventually matching the same concentration as the base material; this gradient similarly produces a hardness that is higher at the surface, gradually diminishing until reaching the core. Higher alloyed steels may also see a microstructural change in the core from quenching that produces a core with higher hardness than the previously untreated material, but lower than the surface hardness produced.

Atmosphere Gas Carburizing

Gas carburizing is a process where carbon is added to the material’s surface. The process is typically performed between 1550-1750°F, with carburizing times commonly between 2-8 hours; of course, these values can vary depending on the material, process, and equipment. The most common atmosphere used for atmosphere gas carburizing is endothermic gas with additions of either natural gas or propane to increase the carbon potential of the furnace atmosphere. Common case depths achieved are around 0.005-0.040”, with deeper cases possible through a combination of longer treatment times and/or higher temperatures.

Fig. 1 – Integral quench furnace: "The atmosphere gas carburizing process can be performed both in batch and continuous equipment."

The atmosphere gas carburizing process can be performed both in batch and continuous equipment. On the batch side, traditionally an integral quench (IQ) furnace is used (Fig. 1); it consists of a heating chamber where the workload is heated and exposed to the carburizing atmosphere, then the workload is transferred to an attached quench tank for cooling. The entire furnace system is sealed and under protective atmosphere to preserve the part surface and maintain safe control of any combustible gases. For batches of large product, a pit furnace can be used for carburizing with the workload being transferred via an overhead crane into and out of the furnace to a quench tank.

For continuous processing, a belt furnace can be used. The product is placed on a belt and then progresses through the furnace at the desired temperature and atmosphere composition; the carburizing time can be varied by adjusting the belt speed through the furnace. At the end of the furnace, the parts drop off the belt into the quench tank. Then, a conveyor pulls the parts out of the tank and drops them on another belt to be washed and tempered. For continuous processing of heavier loads pusher furnaces, rotary retort, rotary hearth, and roller hearth furnaces can be used.

Fig. 2 – Endothermic gas generator: "To achieve a carburizing atmosphere endothermic gas is typically used, which is produced by an endothermic gas generator that heats a combination of natural gas and air to create a mixture that is approximately 40% hydrogen, 40% nitrogen, and 20% carbon monoxide."

To achieve a carburizing atmosphere endothermic gas is typically used, which is produced by an endothermic gas generator (Fig. 2) that heats a combination of natural gas and air to create a mixture that is approximately 40% hydrogen, 40% nitrogen, and 20% carbon monoxide. This mixture is generally considered carbon-neutral, meaning it will neither add nor deplete carbon from the surface. To increase the carbon concentration the endothermic gas needs to be enriched with a gas (typically natural gas or propane) that will help produce additional carbon monoxide, which will “boost” the carbon potential and drive carbon diffusion into the material.

A less common carburizing atmosphere comes from a nitrogen-methanol system, where nitrogen gas and liquid methanol are combined and injected into the furnace. Upon exposure to the high furnace temperature the methanol will decompose to hydrogen and carbon monoxide. Natural gas or propane additions are still required in order to provide carbon for absorption into the surface of the steel.

Low-Pressure Carburizing

Low-pressure carburizing (LPC), or vacuum carburizing, is a variation of carburizing performed in a vacuum furnace. Instead of the atmospheres mentioned previously, a partial pressure of hydrocarbon gas (such as propane or acetylene) is used that directly dissociates at the part surface to provide carbon for diffusion. After LPC, the workload is transferred to a quench system that could use oil or high-pressure gas, typically nitrogen. LPC with gas quenching can be an attractive option for distortion prone complex geometries as the cooling rates are slower than oil quenching; however, given the slower cooling rate, it becomes very important to choose a higher alloyed steel that will achieve the desired hardness.

Fig. 3 – Vacuum furnace with oil quench

LPC typically provides faster carburizing times when compared to traditional gas carburizing. This can be attributed to a more efficient reaction of the hydrocarbon gas used and to the option of using higher carburizing temperatures, typically up to 1900°F. This is made possible by the type of internal furnace construction of vacuum furnace design, although care must be taken at higher temperatures to avoid undesirable grain growth in the material. LPC also has the benefit of eliminating the potential for intergranular oxidation, since it is running in a vacuum system.

LPC is typically performed in a single-chamber vacuum furnace, with oil quenching or high-pressure gas quenching done in a separate chamber (Fig. 3). Continuous vacuum furnaces can also be used in applications that require increased throughput (Fig. 4).

Fig. 4 – Continuous vacuum furnace

Carbonitriding

Despite its name, carbonitriding is more closely related to carburizing than it is to nitriding. Carbonitriding is a process where both carbon and nitrogen are added to the material surface. This process is typically performed in a range of 1450-1600°F and generally produces a shallower case depth than carburizing. Carbonitriding is used instead of carburizing for plain carbon steels that do not contain enough alloying content to respond well to quenching, as the added nitrogen can provide a higher hardenability in the case to allow for proper hardness development.

Atmosphere carbonitriding can be performed in the same equipment as is used for carburizing. The furnace atmosphere is still typically endothermic gas-based and includes the addition of ammonia to provide the nitrogen. Vacuum carbonitriding with both hydrocarbon and ammonia additions can also be performed in the same equipment as used for vacuum hardening and low pressure carburizing.

Diffusion Only Processes

These processes involve heating the workload to a temperature below the austenitizing temperature, allowing the desired element(s) to diffuse into the part surface, then slow cooling. The increase in hardness at the material surface comes only from the addition of the diffused element(s), and not from a phase change due to quenching. As these processes are performed below the lower critical temperature (i.e., below the austenitizing range), the desired core hardness and microstructure need to be developed through a separate heat treatment prior to case hardening. Generally, the process temperature selected should be at least 50°F below any prior treatment temperatures to avoid impact to the core properties.

Gas Nitriding

Gas nitriding is a process where nitrogen is added to the material surface. The process is typically performed between 925-1050°F; cycle times can be quite long as the diffusion of the nitrogen is slow at these temperatures, with nitriding times typically ranging from 16 - 96 hours or more depending on the material and case depth required. Nitriding can be performed in either a single or two-stage process and has the potential to produce two types of case, the first being a nitrogen-rich compound layer (or “white layer”) at the surface that is extremely hard and wear-resistant but also very brittle. This compound layer depth is dependent on processing time. In the more traditional two-stage process, the case depth produces a gradient of hardness from surface to core that commonly ranges from 0.010-0.025”, with minimal white layer, typically between 0-0.0005”. Nitriding is typically performed on higher alloyed steels or steels specifically designed for the nitriding process (e.g., Nitralloy®) as it relies on the formation of nitrides to create the increased hardness, which is achieved through the use of nitride-forming alloys such as aluminum, molybdenum and chromium. Pre and post oxidation treatments can be incorporated into the cycle to achieve certain benefits. Since the process does not require quenching to harden, it has the potential of producing a product that is more dimensionally stable and may not require any post-process finishing.

Fig. 5 – Horizontal retort nitriding furnace: "Traditionally, pit furnaces have been used for nitriding as they can accommodate larger load sizes and can be easier to seal as gravity helps keep the lid sealed; however, horizontal designs have gained in popularity in recent years."

This process is most commonly performed in batch equipment; while it is possible to use a continuous furnace, keeping the ends of furnace sealed to contain the atmosphere can be challenging. Traditionally, pit furnaces have been used for nitriding as they can accommodate larger load sizes and can be easier to seal as gravity helps keep the lid sealed; however, horizontal designs have gained in popularity in recent years (Fig. 5). In either case, the furnaces are usually a single-chamber design with the load sealed inside an Inconel or stainless steel retort.

To achieve a nitriding atmosphere, ammonia (not nitrogen) is used to supply the atomic nitrogen necessary for diffusion. At the process temperatures used, ammonia does not readily dissociate on its own; rather, it dissociates when exposed to a heated steel surface (iron acting as a catalyst) into atomic nitrogen and hydrogen. To control the amount of nitrogen available for nitriding, the dissociation rate of the ammonia can be measured with high dissociation rates (high hydrogen content) providing a lower nitriding potential and low dissociation rates (low hydrogen content) leading to more nitriding potential. The depth of the compound layer can be varied through control of the nitriding potential, with higher nitriding potentials producing a thicker compound layer.

For more precise atmosphere control, an ammonia dissociator can be used to provide gas to the furnace that has already been split to dilute the atmosphere with hydrogen to more quickly achieve a high dissociation rate in the furnace. The ammonia dissociator is a heated box with a small retort inside; the ammonia is passed through this retort that contains a catalyst to promote the dissociation of the ammonia, and the resulting gas mixture is cooled and then injected into the furnace.

Ferritic Nitrocarburizing

In the author’s opinion, just like with carbonitriding, ferritic nitrocarburizing (FNC) is named incorrectly as it is more closely related to nitriding than it is with carburizing. FNC is a process that is still mostly nitrogen-based but with a slight carbon addition as well. The added carbon helps promote compound layer formation, particularly in plain carbon and low alloy steels that do not contain significant nitride-forming alloys. This process is typically performed in a range of 1025-1125°F with cycle times much shorter than nitriding, typically 1-4 hours. The compound layer produced is usually much deeper than nitriding at 0.0005-0.0012”, with case depths reaching up to 0.025”, although in many applications a case depth may be difficult to measure. FNC is usually performed instead of nitriding in applications where the deeper compound layer is needed to increase wear resistance, but the added strength of a deep case depth is not as critical.

FNC can be performed in the same equipment used for nitriding, as long as a hydrocarbon gas is available to the furnace such as carbon dioxide or endothermic gas. FNC can also be performed in an IQ furnace using a mixture of ammonia and endothermic gas; for cooling, the parts can be oil quenched or slow cooled in a top cool chamber (if equipped).

Considerations

Case hardening processes are some of the most common heat treatments performed, but each process has its own unique needs. The table below provides a summary of the considerations that need to be made when selecting the optimum process. This list is by no means exhaustive; it is encouraged to work with a furnace manufacturer familiar with each process to help select the correct process and equipment needed.

Screenshot 2023-12-27 at 1.19.41 PM

About the Author: Mike Harrison is the engineering manager of the Industrial Furnace Systems division at Gasbarre. Mike has a materials science and engineering degree from the University of Michigan and received his M.B.A. from Walsh College. Prior to joining Gasbarre, Mike had roles in metallurgy, quality, and management at both captive and commercial heat treat facilities, gaining nearly 20 years of experience in the thermal processing industry. Gasbarre provides thermal processing equipment solutions for both atmosphere and vacuum furnace applications, as well as associated auxiliary equipment and aftermarket parts & service.

For more information: Contact Mike at mharrison@gasbarre.com

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Controls and Simulation: Heat Treat on Demand

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Best of the WebSource: Super Systems

Carburizing. It must happen sometimes, and if your heat treat division truly understands the impact of the atmosphere, more power to them. In this article by Jim Oakes of Super Systems, you will learn how seeing simulated data with real-time data can help you predict the amount of carbon available to the steel surface.

An excerpt:

“It is important to understand the model and specific variations caused by temperature, furnaces, agitation, fixturing, and part composition. Variations include alloying effects on the diffusion modeling based on certain alloy components, such as chromium and nickel.”

Read more at: “Understanding Atmosphere in Carburizing Applications Using Simulation and Real-Time Carbon Diffusion”

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Heat Treat Operators: How Well Do You Understand Dry Pumps?

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Source: VAC AERO International Inc.

With the popularity of dry pumps in furnace operations, vacuum furnace operators need to "have a handle" on how to operate them.

In this best of the web feature, the author explains the principles of operation, screw pump design, and various other screw pump characteristics. Learn about the 5 phases of dry pump operation and more in this succinct article.

An excerpt:

"Dry pumps represent a technology that is of interest to many heat treaters as they strive to increase performance and minimize cost and downtime. The advantages of these pumps are comparable to their oil sealed rotary vane cousins, and in certain applications, offer distinct advantages."

Read more at "Dry Pumps: Screw Type"

Heat Treat Operators: How Well Do You Understand Dry Pumps? Read More »

Washing and Heat Treat: What Did You Miss?

/ BRAZING TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT, PARTS CLEANING TECHNICAL-CONTENT

OCIn January 2021, Hubbard-Hall hosted a free webinar with Thomas Wingens of Wingens International and Michael Onken of SAFECHEM. These two experts described the influencing factors for technical cleanliness and some solutions for washing. This Technical Tuesday, we are sharing an Original Content overview of what happened at the virtual event.

To learn more, watch the recorded webinar here. Additionally, listen to Thomas Wingens on "Parts Washing" on Heat Treat Radio.

This year, we are seeing a lot of online-adapted education for the heat treat industry. One of these webinars was "Solving The 4 Most Common Metal Cleaning Challenges In Heat Treatment" hosted for free by Hubbard-Hall. Jeff Davis, SVP of business development and distribution at the chemical supplier, introduced experts Thomas Wingens, longtime metallurgist with a lifetime of exposure in the heat treating industry, and Michael Onken, market development manager at SAFECHEM. Here is a brief rundown of what they talked about.

The concluding slide from Hubbard-Hall's webinar, Tuesday February 2, 2021.
Source: Screen shot from Hubbard-Hall Webinar February 9, 2021

How Do You Clean | Why Do You Clean | Who Cleans

The audience indicated that if they cleaned, they overwhelming used water-based cleaners on their products.

The experts then gave four clear reasons why heat treaters should clean:

  • Optics -- get rid of stains
  • Achieve Uniformity -- resolve soft spots and stop-off paint issues
  • Brazing Voids -- prevent the appearance of bubbles on your part
  • Contamination of the Furnace -- all furnaces, even vacuum furnaces, are susceptible to contamination
  • Smoking Parts -- if not cleaned well, left-over oil on a part can smoke up

With all of these reasons and with the specificity of the part, all heat treaters should pay attention to how they clean their products, but especially commercial heat treaters. The reason? Commercial heat treaters are in the most challenging situation with cooling fluid contamination, corrosion protection, chips, dirt, and dust as they treat a variety of different parts at their facility. As a note, the experts noted that commercial heat treaters could remove these contaminants with sandblasting, pickling, and sputtering.

4 Challenges - 4 Solutions

One by one, Wingens shared a cleaning challenge that Onken immediately responded to.

1 - Residual Contamination Results in Insufficient Hardening (T.W.)

Residual contamination may be because the cleaning method you are using is insufficient or non-existent. Still, Wingens noted there is clear evidence that insufficient cleaning for nitriding and ferritic nitro caburizing (FNC) leads to white spots. This, among other things, is a cause for concern and may compromise the part quality.

1 - Consider Cleaning Factors, Regulations, and Requirements (M.O.)

If you are running into this cleaning challenge, you have to first consider specific factors, regulations, and requirements for implementing optimal cleaning, says Onken.

  1. Time. You want cleaning to be as short as possible because "time is money."
  2. Temperature.
  3. Mechanics of the cleaning machine.
  4. Chemistry of the Cleaning Agents. Alkaline, neutral, or and organic solvents? You must know what type of contaminations you have on the surface -- if it's polar or non-polar -- in order to use the correct solvent in cleaning the part.
    • Are the contaminants fat, resins, oil, petroleum or salts, emulsions, emulsions?

Additionally, there are several factors of the part itself, pricing, and Environmental Health & Safety standards that do come into play, as Onken lists in the slide below.

Michael Onken sharing factors influencing technical cleanliness.
Source: Screen shot from Hubbard-Hall Webinar February 9, 2021

2 - Surface Stains on Finished Product (T.W.)

This is a pretty straight forward challenge: you don't want the surface stain, so what do you do?

2 - Type of Contamination: Polarity (M.O.)

First, you want to clean "like-with-like." That is, if you have a water insoluble/non-polar contaminant like petroleum or wax, you want to clean with an insoluble/non-polar cleaner like halogenated solvents. Likewise, if the contaminants are water soluble/polar like salts or emulsion residues, then you clean with water-based cleaners. Check out the chart below that Onken shared at the webinar to see how specific cleaners are non-polar, polar, or even hybrid.

Polarity of cleaners and contaminants presented by Michael Onken at SAFECHEM.
Source: Screen shot from Hubbard-Hall Webinar February 9, 2021

Additionally, the way your load is situated can influence what cleaner you use. For a basket load, you'll want to use a cleaner with low surface tension like solvents since those can penetrate and move through the complex geometry of the load.

3 - Inconsistent Cleaning (T.W.)

The impact of a cleaner decreases in strength over time, particularly with solvents, leading to an oily surface. (See the example below.) What to do?

Oily parts before hat treating and after quenching.
Source: Screen shot from Hubbard-Hall Webinar February 9, 2021

3 - Process Stability (M.O.)

There are preventative measures, Onken highlights, that emphasizes process stability that can handle high through-put that will clean all of the parts you have uniformly:

  • Solvents: These are 100% composed of solvent with a stabilizer. Monitor build up of acids only, not the concentration of cleaner itself.
  • Water cleaner: These are 90-99% water mixed with other chemical(s). Therefore, they are much more complex. Check out alkalinity.

Bottom line: keep an eye on how your cleaners are doing so that you always know their quality before you use them.

4 - The Cost. (T.W.)

Wingens pointed out that it is costly to invest in a cleaner, and so how is a heat treater to mitigate this practical challenge?

4 - Efficient Product Use (M.O.)

First, look at efficiency of aqueous cleaning. Solvent cleaning is now in closed machines, not open machines. It is simply not that efficient to use an open machine because a lot of the cleaner disperses into the atmosphere when it is in use. That is why it is more common to see closed cleaning process. Vacuum Tight Machines close the processes even more.

Do what can to conserve material and keep the process efficient and effective.

Final Comments

The experts left the live webinar with a few final comments, noting that there is a move away from water-based cleaning because of the constraints of being able to do batch part cleaning (see solution #2). Additionally, they reiterated that investment costs are higher for closed system with a vacuum; but due to their efficiency, that investment can be paid-off fairly quickly.

If you are interested in catching the next webinar, "Do You Know Your Real Cost of Cleaning?" is happening next week, February 23, 2021 at 2:00pm ET. Again, the recorded webinar can be accessed here.

 

All images were captured during the live webinar on February 2, 2021.

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Case Study: The Low-Pressure Carburizing Process Improvement for a Ring Gear

/ CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT

Justin Sims
Lead Engineer
DANTE Solutions

OC“The original LPC schedule, consisting of six boost-diffuse steps, was producing large amounts of carbides during the process. With large amounts of primary carbides in the case of the heat treated gear, rolling contact fatigue performance was decreased.”

Heat Treat Today‘s Technical Tuesday feature, “Low Pressure Carburizing Process Improvement for a Ring Gear: Controlling Carbide Formation during LPC,” explores a case study, written by Justin Sims, lead engineer at DANTE Solutions, about how software modeling aids heat treaters in improving their low pressure carburizing process. Enjoy today’s Original Content.

Introduction

Low pressure carburizing (LPC) processes are becoming more widespread throughout industry due to the reduced cycle times and the control over the carbon profile through the case. Unlike gas carburizing, which utilizes a constant carbon potential to maintain the available carbon on the part surface at a specific value, LPC utilizes a series of boost and diffuse steps. A boost step involves the temporary addition of a carbon carrying gas to the furnace chamber, usually acetylene, to increase the surface carbon to the saturation limit of austenite. If not properly controlled, the carburized case may have an excessive amount of carbon, which damages the final microstructure. After a requisite amount of boost time, generally half minute to several minutes, the carbon carrying gas is evacuated from the chamber. The concentrated carbon in the shallow surface layer from the boost step is then allowed to diffuse into the part, reducing the surface carbon. These two steps are then repeated until the required case depth and carbon profile are achieved.

For steel alloys that do not contain a significant amount of strong carbide forming elements, the LPC process is relatively easy to control. However, with the advent of high strength steels for the aerospace industry, most of which contain substantial amounts of strong carbide forming elements, such as chromium, molybdenum, and vanadium, the LPC process can be challenging. The primary carbides formed during the LPC process, if not properly dissolved, can damage fatigue performance.

While Fick’s Second Law describes the diffusion of carbon through a low alloy steel with reasonable accuracy, the same is not true of medium and high alloy steels. This is due to the presence of carbides forming and dissolving during the LPC process. During a boost step, the carbides formed increase the total amount of carbon into the surface. During the diffuse step, as the carbon that is in solid solution diffuses into the part, reducing the carbon in austenite, the carbides can dissolve to provide more carbon to the solid-state solution. If the carbides are not allowed to fully dissolve or shrink to a significantly small size before the next boost step begins, they will continue to grow. In order to properly predict the carbon profile of medium and high alloy steels, the carbide formation and dissolution must be considered. The heat treatment simulation software DANTE has implemented this feature.

The following is a case study for redesigning a LPC schedule of a ring gear using DANTE. The original LPC schedule, consisting of six boost-diffuse steps, was producing large amounts of carbides during the process. With large amounts of primary carbides in the case of the heat treated gear, rolling contact fatigue performance was decreased.

Image 1

Geometry and Model

Part: Ring Gear

  • Material: Ferrium C64
  • Outer Diameter: 5.5 inches
  • Inner Diameter: 4.5 inches
  • Height: 0.060 inches
  • Number of Teeth: 40

Model: Single Tooth

  • Cyclic Symmetry: Carbon boundary conditions
    act uniformly on all teeth
  • Number of Elements: 233,850 linear hexagonal
  • Number of Nodes: 245,055
  • Higher mesh density near surface to capture
    steep carbon gradients

 

LPC Experiments vs. Prediction

Image 2

  • Experimental data versus DANTE prediction for 3 LPC runs
  • LPC experiments conducted using a cylinder with a 4-inch OD and a 4-inch height made of Ferrium C64
  • 3 different boost-diffuse schedules executed
    • 6 boost-diffuse steps
    • All 3 schedules used the same first 11 steps
    • Final diffuse time increased for each run, with Run 1 having the shortest and Run 3 having the longest
  • LECO used to measure the carbon profile of the test coupons
  • DANTE model parameters for carbon diffusivity, carbide formation, and carbide dissolution fit from experimental data
    • Simulation matches experimental data reasonably well

 

Baseline (Original Carburizing Process)  Model Results

  • The case depth originally was designed for 0.75 mm (0.030 inch) on the flank of the tooth, with a carbon value of 0.3% resulting in a hardness value of 50 HRC for Ferrium C64 when tempered at 495°C (925°F).
  • The contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the process for the baseline model:
    • Areas above 0.011 carbon contain primary carbides
    • Tip contains a high amount of primary carbides
  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the baseline model over the total time of the process.
    • Carbides present at a depth of 0.25 mm (0.010 inch)
    • Case depth ~0.35 mm deeper than required

Image 3

Image 4

 

 

 

 

 

 

 

  • Contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the 3rd boost and diffuse steps
  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the baseline model over the total time of the process
    • Carbides formed during the first boost step continue to grow as the process progresses, indicated by the increasing carbon in carbide
    • Final diffuse not long enough to fully dissolve carbides

Image 5

Image 6

Image 7

Redesigned Carburizing Process Model Results

Image 8

  • To ensure the primary carbides dissolve completely before hardening, a new schedule was developed with the aim of reducing the carbon in primary carbide form:
    1. 3 boost-diffuse steps were removed, and the diffuse times increased substantially.
    2. An increase in diffuse time increased the schedule by approximately one-half hour, which is acceptable given the positive results.
  • The contour plot shows all carbon, the carbon in the austenite matrix, and the carbon in primary carbide form at the end of the process for the redesigned process model.
  • Line plot shows the carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbide) from the surface of the flank towards the core for the redesigned model at the end of the process
  • Small carbides (negligible) at a depth of 0.1 mm (0.004 inch)

Image 9

  • Easily removed with finish grinding operation
  • Contour plot shows all carbon, the carbon in the austenite matrix and the carbon in primary carbide form, at the end of the 2nd boost and diffuse steps for the redesigned process
    • Primary carbides are nearly fully dissolved, even in the tip (carbon is higher, but it is not in carbide form), at the end of the diffuse step

Image 10

Image 11

 

 

 

 

 

 

 

  • Line plot shows the predicted carbon in the austenite matrix (Carbon) and the carbon in primary carbide form (Carbon in Carbides) at the surface of the flank for the redesigned model over the total time of the process
    • Carbides are nearly fully dissolved after each diffuse step

 

Image 12

Summary

  1. The heat treatment simulation software DANTE model parameters for carbon diffusivity, carbide formation, and carbide dissociation fit from experimental data.
    • Any steel alloy and LPC equipment can be fit to the DANTE carburizing model.
  2. The software successfully predicted the results of a low-pressure carburizing process that was resulting in poor part performance during rolling contact fatigue:
    • Model showed that large primary carbides exist at a depth of 0.25 mm (0.010 inch).
    • Model showed that the carbides do not have time to dissolve during the boost steps.
  3. The software was used to successfully redesign the boost-diffuse schedule to improve rolling contact fatigue performance:
    • Model showed that small primary carbides (negligible) exist at a depth of 0.1 mm (0.004 inch).
    • Model showed that the carbides nearly fully dissolve during the diffuse steps.
    • Small carbides were removed during the finish grinding operation.
    • Rolling contact fatigue performance improved due to the absence of primary carbides near the surface.
  4. Additionally, the software is not limited to Ferrium C64 with respect to primary carbide formation during LPC:
    • Continually updating the material database with carbide behavior for different alloys
    • Continually validating the model with experiments

 

About the Author: Justin Sims is a lead engineer at DANTE Solutions. For more information, contact Justin at DANTE Solutions

All images were provided by DANTE Solutions.

 

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Changing the In-Line Heat Treating Paradigm

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Evolution of ideas and transitions to more innovative and efficient methods of heat treating are common themes in this ever-changing world. In this article, Dennis Beauchesne, General Manager at ECM USA, Inc., explores the integration of heat treatment for in-line machining cells and the benefits and efficiencies experienced.

This article article first appeared in the latest edition (June 2020) of Heat Treat Today’s Automotive Heat Treat  magazine.

Introduction

Dennis Beauchesne, General Manager, ECM-USA, Inc.

Heat treating in the automotive industry has evolved tremendously over the last 20 years.  From the dinosaur pusher furnaces of yesterday to the low-pressure carburizing and high-pressure gas quenching of today, we are now embarking on new concepts with not only in-line processing, but also automated single piece and bulk loading in small batches. Modern heat-treating equipment is now being sized to fit into single-piece flow lines with small batches and in-line with pre- and post-machining centers. This article will examine the integration of heat treatment for in-line machining cells, and the influences for the customer to provide an overall quality system. These details will be compared to batch or continuous batch heat treatment as commonly known in the automotive industry.

Over the last 20 years, low pressure vacuum carburizing (LPC) has been proven as the choice for carburizing high production parts in a variety of markets all over the world. It is the process of choice for many high-fatigue and low-distortion parts; thus, it can be used in conjunction with vacuum oil quenching (VOQ) and, in most cases, high pressure gas quenching (HPGQ). Advantages of this process include the fact that equipment is more easily maintainable, flexible, and independent from operators’ intervention than traditional atmosphere carburizing. In addition, reduction of effluents from the process are significantly decreased. A benefit of this process is that the furnace equipment is used more along the lines of a machining cell, which has most commonly been reserved for induction heat treating in the past. With the added benefits of LPC, the importance of strength and fatigue life have increased beyond previous process capabilities; in turn, LPC in-line processing has been considered more frequently.

The rapid shut-down/cool-down (5-6 hours) of a vacuum system is a significant advantage compared to the convoluted days of cooling an atmosphere system. Simply shutting down one day per week due to scheduled maintenance, without the need for supervision, or use of additional utilities required for idling during downtime, is also highly desirable. Other facets of the equipment and process have allowed vacuum furnace equipment to be more conducive to high production manufacturing. This includes recipe ease, process repeatability, and load-to-load processing flexibility in a continuous flow environment. These additional benefits allow the use of part-specific “recipes” while allowing for high production through the system and insuring individual metallurgical requirements.

High pressure gas quenching (HPGQ), using a dedicated quenching cell, is often linked with LPC for several reasons. Some of the most important reasons are:

  • to provide a cleaner environment in the plant
  • to remove oil-quenching tanks and the need for oil on a production floor
  • to eliminate the need for pits in the floor or managing oil containment
  • to obtain a safer, more ergonomic environment via the elimination of open flame and hot surfaces
  • to achieve more precise distortion control of dimensionally critical parts
  • to reduce or eliminate post-heat treat machining needs
  • to eliminate post-heat treat blasting (for cleanup)
  • to eliminate post-heat treat washing (for quench oil removal)
  • to eliminate post-heat treat washer effluent (sludge) removal

Plan view of in-line heat treatment system

 ECM Technologies has been manufacturing LPC and HPGQ systems for over 25 years and designed an innovative in-line system, called NANO.  The name NANO is appropriate in terms of the equipment design as the load size is smaller than traditional loads and the physical equipment is more compact than furnaces more commonly used today.  The system has also been designed with maintenance and expandability in mind. The premise of the design is to be able to process loads as quickly as

In-line heat treatment system being fully-tested prior to shipment to customer

every 7.5 minutes per load. The system is modular in production capacity with growth from three to six heating cells, which can be supplied. The system is ready for a low number of loads, with flexible needs for varying part requirements, or can be maximized to provide throughput for modern high-demand production needs. The NANO consists of four basic modules, the heating module (typically two), the transfer module, and the high-pressure gas quench module. The gas quench module uses a 20 bar gas quench system. With a smaller chamber size and the ability to quench at 20 bar, materials that were not able to be quenched in gas are now applicable for gas quenching. This gas-quenching method will open up this technology to more applications that were limited by core and surface hardness requirements in the past.

 

Figure 1. - In-line heat treat installation showing automated robotic loading/unloading

 

Using the NANO as the heat treat equipment base, automation completes the overall single- piece flow system to provide true in-line heat treating. The parts can be presented to the system in bulk or in single-piece trays. These parts can then be loaded and virtually tracked through the system using vison systems.

In-line heat treat installation showing robotic arm facilitating automated loading/unloading

The NANO accommodates a workload size of 24” wide x 20” deep x 10” high (600mm wide x 500mm deep x 250mm high); see Figure 1. Workload parts can be processed on industry-typical alloy fixturing or more preferably on CFC fixtures. The goal of designing the system, beyond better maintenance accessibility and gradual production increase, is low distortion and in-line production flow.

By processing loads with less work pieces, part uniformity and distortion are identical from part to part as the “3D” or 3-sided heating elements heat the parts uniformly and homogenously. These elements are designed not to sag onto the parts and provide adequate clearance for the automated loader to perform accurate transfers within the system.

Automation and Integration

The system has been designed for manual or automated loading. Manual loading can be as simple as manually loading basic fixtures or baskets, and manually loading through the system and subsequent processes. Automated loading can be from a simple robot platform that loads parts onto a small fixture from a single part flow to the handling of many pieces in a bulk load. The robot can handle all functions of the installation from loading single parts on to the fixture to placing the loads in the furnace, and then, cryogenic treatment, tempering and eventually back to a single-piece flow. The system is also capable of checking surface hardness and registering the data to be kept with the load report. This total integration can allow for a smaller footprint and less manpower in the heat treat area.

In addition, automation can handle typical small bulk-loaded parts. Some of these parts are traditionally processed using mesh belt furnaces but can now be processed by this vacuum furnace in-line system. Bulk loads are loaded into basket-like CFC trays and can be weighed and processed as needed to ensure the quality of each load.

 

Low pressure carburizing installation showing easy access for maintenance

In-line heat treating is not just for carburized products, it can be for hardening, brazing, annealing, and integrated with post treatments—such as cryogenic and complex tempering operations. This allows the NANO to fit into various markets for many different types of heat treatments, not only steel parts, but for special heat-treating processes as well.

Equipment layouts are typically developed to accommodate specific applications. They can be as simple as a manual load station to a robot loading single-piece flow parts onto smaller fixtures or loading bulk parts into baskets for processing.

Maintenance Features

In-line processing, as well as bulk processing, together with automation to load and unload single-piece production are not the only key items in this design. Maintenance and operation were high on the list of criteria as well. Maintenance features such as ease of access are important on production equipment, but especially within heat treating. With the smaller load sizes and equipment, cool-down is significantly faster resulting from the smaller, water-cooled heating zones. Once cooled and released to atmospheric pressure, the system can then be opened via the full opening maintenance access door for easy, internal service effort. However, this is rare because all mechanisms that require control and quick review are located on the exterior of the system and outside the vacuum chamber. This allows for ease of access to all major components and reduces the need to stop or interrupt production.  Additionally, for hot zone service, each heating module can be rolled away from the central transfer module to allow easy access to all hot zones in that module. This allows for easy-open access without the hindrance of confined spaces.

Distortion Evaluation

Figure 2. Differential gear used for distortion evaluation

Using a differential gear (Figure 2), we monitored two characteristics, which are usually requested for this type of gear, and tabulated the results. This evaluation was done using a current day load that will be illustrated as a large batch (FLEX) and those processed in the NANO will be a small batch (NANO). The two characteristics that have the most influence: (1) Cylindricity (circularity) of the outside diameter and the runout between the same diameter and (2) “Backface” flatness of the gear teeth.

Backface flatness (Figure 3) shows greater variance through the large batch with further distance from the nitrogen used in quenching in the range of 11.9 µm.  For the small batch results, the backface flatness distortion range was limited to only 1.2 µm. This uniform result is directly connected to the uniform quenching in the smaller load. This allows for parts to be processed closer to actual machined dimensions (near net shape), as well as being handled on an in-line production basis.

The runout analysis (Figure 4) shows a uniformity for the small batch with a spread of only 3.2 µm across the load. In the large batch, you will see a wider range of distortion uniformity using 10 bar nitrogen quenching with a spread of 24 µm. This is most likely due to the lower layers being further from the vertical down flow of gas through the load. These results are quite extraordinary for a full-size load.

Figure 4. Backface flatness comparing large batch processing (FLEX LOAD) and one-piece processing (NANO)

Conclusion

In-line processing can now be a common thought in the layout of future facilities. In practice, it is a growing aspect of the heat-treating world. With the new NANO vacuum furnace system and automation options, better part-to-part quality can be achieved along with better control of metallurgical parameters and results. Overall, streamlining heat treating into production cells throughout the facility allows for better part flow, and optimally sizing products for the production of particular throughput requirements.

References

[1] Beauchesne, D., “FNA2016 - LPC with OIL & GAS Quenching” (2016)

[2] Esteve, V. & Lelong, V., “LPC - What Does it Mean to Metallurgy” (ASM-Mexico 2016)

[3] Welch, A. & Lelong, V., “How it’s done and Why: Transitioning Parts from Atmosphere Carburizing to Low Pressure Vacuum Carburizing” (HT 2015)

 

About the author: Dennis Beauchesne is the general manager of ECM USA and brings experience of over 200 vacuum carburizing cells installed on high pressure gas quenching and oil quenching installations. He has worked in the thermal transfer equipment supply industry for almost 30 years, 18 of which have been with ECM USA.

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Bill Disler on Carburizing Trends in the Automotive Heat Treating World

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, AUTOMOTIVE HEAT TREAT, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, OP-ED

Bill Disler President & CEO AFC-Holcroft

Automotive part designs and heat treating processes have undergone many changes over the years, especially the powertrain. By looking back at the progress of these changes, we can learn more about emerging trends in automotive heat treating today.

In this Heat Treat Today Technical Tuesday feature, Bill Disler, president and CEO of AFC-Holcroft, brings his familiarity with big atmosphere carburizing systems and LPC automotive cell carburizing systems and looks at how the evolution of equipment and process requests says a lot about the trends we see today in automotive heat treating.

This article originally appeared in Heat Treat Today’s June 2019 Automotive print edition.

Although many components undergo heat treatment processes, the powertrain—specifically, gears— typically requires more carburizing time than other automotive parts. Not surprisingly, the powertrain has also seen many changes in heat treatment trends.

Not only have powertrain designs gone through tremendous transformations but so has the equipment being used to process those evolved components. Having spent years on the supplier side of atmosphere furnaces, vacuum carburizing, and gas quench as well as induction systems, I find it interesting to look back at some of the drivers that have helped morph this industry’s heat treat needs.

Traditional Continuous Atmosphere Furnace

Large atmosphere pusher furnaces produced nearly all of the powertrain gears 20+ years ago. Today, cellular low-pressure carburizing (LPC) and gas quench systems carry the load, although the results have not been cost saving. Moving from high volume gas heated carburizing equipment to small batch carburizing in electrically heated furnaces did not reduce utility costs per part; instead, other areas adjusted to compensate. Eliminating the expense of hard grinding transmission gears was an acceptable rationale for this increase in both capital expense and operating costs. Eventually, streamlining the overall gear manufacturing process, combined with locating heat treat within machining lines, produced positive measurable results. Plant traffic decreased, minimizing safety risks. Cooler and cleaner furnace systems were designed. And installations were made easier. Many agreed the changes were justified.

Integrated Vacuum Heat Treat Cells

As we look back, many of these drivers for change proved valid. Others, not so much. In most cases, consumer preference for quiet powertrains necessitates hard grinding of gears. Green is in and talk of the absolute need for zero intergranular oxidation (IGO) in carburized gears has slowed. LPC/Gas post quenched parts are perceived as cleaner and leaner; however, it is often difficult to differentiate green parts from processed parts, so it has become a best practice to add part marking after carburizing and hardening to avoid even the remote risk of sending soft parts down the line to the next stage of manufacturing. Shot peening is still common for strength reasons. The ability to nest large cellular LPC systems within machining has been a success, but rarely are the installations as quick and easy as promised.

Hybrid Furnace Concepts

Conventional atmosphere furnace technology has advanced as well, although at a slower pace, in step with a renewed interest in energy efficiency, particularly in the U.S. where gas is cheap and electric is not. Combustion systems operate cleaner and at much higher efficiency than in the past. Having said that, it is curious how little interest end users have in trading cost-saving gas-heated systems for the easier to install, neater looking electric heating options. In addition, it is no longer common to use water for cooling conventional atmosphere furnace systems as end users do not want to deal with the cost and complications that accompany this option. The market is polarized over this. LPC systems rely on large water volumes for cooling, and they are small batch, electrically heated systems. At the same time, gas quench systems consume huge quantities of water and require giant 300 HP plus motors that are tough to manage in plant power systems.

Flexible and Re-deployable Heat Treat Systems

It is my observation that the automotive market is anticipating the next iteration of heat treat equipment. One type of process or equipment style will not fit all needs, yet all hope for the perfect single part flow solution—an elusive dream due to physics. The cost/time equation still does not balance, and carburizing offers the benefits many manufacturers are looking for, despite the desire to design the process out of practice. Many automotive transmission parts that were originally processed in LPC and gas quenched now use gas nitriding instead, even though gas nitriding is another long process, and nitriding introduces ammonia back into the process—something most automotive plants are not enthusiastic to have in their plants. Two steps forward and one step back.

Repackaging Continuous Furnace Systems

With the widening range of processes and solutions under exploration, as well as ever changing powertrain systems designed to accommodate supplemental electric motors, lighter weights, smaller cars, and larger SUVs, all we can be certain of is ongoing change. I believe that we have witnessed major adjustments in automotive heat treat processing as the pendulum has swung from big, multi-row atmosphere pushers with salt or oil quench to electric-heated cellular LPC and gas quench units. One surprising result has been the resurgence of salt quenching, which controls distortion of high-pressure gas at a much lower cost with less complexity. Salt, like gas, is a single-phase quench media: It does not boil in these processes like oil does, and it can be used at temperatures that support martensitic quench with far less thermal shock and much higher heat transfer than the options. Older processes carry the baggage of tarnished past reputations, but I no longer count them out. Today’s automation, process control technology, and innovation can provide the foundation for brand new concepts, repackaging of older ideas, and hybrids of multiple technologies. Together, these create building blocks that heat treat equipment suppliers will use to meet changing trends in automotive carburizing and heat treatment. It will be interesting to be involved in the journey as these changes take place.

About the Author: Bill Disler is president and CEO of AFC-Holcroft, part of the Aichelin Group located in Vienna, Austria. He is a member of the Board of Trustees -Metal Treating Institute (MTI), and a member of the Board of Advisors at Lawrence Technical University, College of Engineering in Southfield, Michigan. This article originally appeared in Heat Treat Today’s June 2019 Automotive print edition.

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Temperature Monitoring and Surveying Solutions for Carburizing Auto Components: AMS2750E and CQI-9 Temperature Uniformity Surveys

/ AUTOMOTIVE HEAT TREAT, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS

Dr. Steve Offley (“Dr. O”), Product Marketing Manager PhoenixTM

This is the final installment in a 4-part series by Dr. Steve Offley (“Dr. O”) on the technical challenges of monitoring low-pressure carburizing (LPC) furnaces. The previous articles explained the LPC process and explored general monitoring needs and challenges (part 1), the use of data loggers in thru-process temperature monitoring (part 2), and the thermal design challenge (part 3). In this segment, Dr. O discusses AMS2750E and CQI-9 Temperature Uniformity Surveys. You can find Part 1 here, Part 2 here, and Part 3 here. 

A significant challenge for many heat treaters is the need to provide products certified to either AMS27150 (aerospace) or CQI-9 (automotive). To achieve this accreditation, furnace Temperature Uniformity Surveys (TUS) must be performed at regular intervals to prove that the furnace set-point temperatures are both accurate and stable over the working volume of the furnace. Historically, the furnace survey has been performed with great difficulty trailing thermocouples into the heat zone. Although it’s possible in a batch process when considering a semi-batch or continuous process, this is a significant technical challenge with considerable compromises as summarized below.

Trailing Thermocouple TUS Process Steps

Figure 1. Typical TUS thermocouple. Positions — 9 point survey. Furnace void corners and center

  • TUS is often carried out using long or ‘trailing’ thermocouples that exit through the furnace door.
  • Furnace often needs to be cooled, then de-gassed so TUS frame can be set up in the furnace.
  • Thermocouples are then led out through the furnace door and connected to a data logger or chart recorder.
  • The furnace is then heated to surveying temperatures.
  • The survey is then carried out, after which furnace is cooled, and thermocouples are removed.

 

Disadvantages of Traditional TUS Process

  • Lots of furnace downtime may be involved (can be up to 24 hours).
  • Thermocouples have to exit the furnace door.
    • This may involve “wedging” the door up, or “grooving” out the hearth to get thermocouples out.
    • Or thermocouples may get caught in the furnace door.
  • A significant amount of the technician’s time is taken up preparing report.

Applying the “Thru-Process” approach to TUS, the measurement system is transferred into the furnace with the survey frame allowing the setup process to be done quickly, safely, and repeatable. (See Figure 2)

Figure 2. PhoenixTM System loaded into a furnace as part of a TUS frame. Thermocouples pre-fitted to the 8 vertices of the cube frame and center. Furnace ambient temperature recorded either with a virgin exposed junction thermocouple (typical MI) or with heat sink damper fitted.

Operating the system with RF telemetry, TUS data is transferred directly from the furnace back to the monitoring PC where at each survey level temperature stabilization and temperature overshoot can be monitored live with TC and logger correction factors applied. The Thermal View Software is developed to ensure that the final TUS report complies fully to the AMS2750E/CQI-9 standards.

Figure 3. PhoenixTM Thermal View Survey Software showing a TUS profile at three set survey temperatures. The probe map shows exactly where each probe is located and easy trace identification. Detailed TUS report generated with efficiency.

Features incorporated into the Thermal View Software to provide full TUS capability include the following:

TUS Level Library – Set-up TUS level templates for quick efficient survey level specification (Survey Temp °F, Tolerance °F, Stabilization, and Survey Times)

TUS Frames Library – Show clearly exact TUS frame construction and probe location using Frame Library Templates – Frame Center and 8 Vertices.

Logger Correction File – Create a logger correction file to compensate TUS readings automatically from the logger’s internal calibration file.

Thermocouple Correction File – Create the thermocouple correction file and use to compensate TUS readings directly.

TUS Result Table & Graph View – For each TUS temperature level, see from the graph or TUS table instantaneously full survey results.

Furnace Class Reporting – Report the specified furnace class at each temperature level.

 

 

Overview

The PhoenixTM Temperature Profiling System provides a versatile solution for both performing product temperature profiling and furnace TUS in industrial heat treatment. It is designed specifically for the technical challenges of low-pressure carburizing (LPC) whether implementing either high-pressure gas quench or oil quench methodology, providing the means to Understand, Control, Optimize and Certify the LPC Furnace and guarantee product quality and process operation efficiency and certification.

 

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Temperature Monitoring and Surveying Solutions for Carburizing Auto Components: Thermal Barrier Design

/ AUTOMOTIVE HEAT TREAT, AUTOMOTIVE HEAT TREAT NEWS, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, INSULATION TECHNICAL CONTENT

This is the third in a 4-part series by Dr. Steve Offley (“Dr. O”) on the technical challenges of monitoring low-pressure carburizing (LPC) furnaces. The previous articles explained the LPC process and explored general monitoring needs and challenges (part 1) and the use of data loggers in thru-process temperature monitoring (part 2). In this segment, Dr. O discusses the thermal barrier with a detailed overview of the thermal barrier design for both LPC with gas or oil quench. You can find Part 1 here and Part 2 here. 

Low-Pressure Carburizing (LPC) with High-Pressure Gas Quench – the Design Challenge

A range of thermal barriers is available to cover the different carburizing process specifications. As shown in Figure 1 the performance needs to be matched to temperature, pressure and obviously space limitations in the LPC chamber.

 

Fig 1: Thermal Barrier Designed Specifically for LPC with Gas Quench.

(i) TS02-130 low height barrier designed for space limiting LPC furnaces with low-performance gas quenches (<1 bar). Only 130 mm/5.1-inch high so ideal for small parts. Available with Quench Deflector kit. (0.9 hours at 1740°F/950°C).

(ii) Open barrier showing PTM1220 logger installed within phase change heatsink.

(iii) TS02-350 High-Performance LPC barrier fitted with quench deflector capable of withstanding 20 bar N2 quench. (350 mm/13.8-inch WOQD 4.5 hours at 1740°F /950°C).

(iv) Quench Deflect Kit showing that lid supported on its own support legs so pressure not applied to barrier lid.

The barrier design is made to allow robust operation run after run, where conditions are demanding in terms of material warpage.

Some of the key design features are listed below.

I. Barrier – Reinforced 310 SS strengthened and reinforced at critical points to minimize distortion (>1000°C / 1832°F HT or ultra HT microporous insulation to reduce shrinkage issues)

II. Close-pitched Cu-plated rivets (less carbon pick up) reducing barrier wall warpage

III. High-temperature heavy duty robust and distortion resistant catches. No thread seizure issue.

IV. Barrier lid expansion plate reduces distortion from rapid temperature changes.

V. Phase change heat sink providing additional thermal protection in barrier cavity.

VI.  Dual probe exits for 20 probes with replaceable wear strips. (low-cost maintenance)

LPC or Continuous Carburizing with Oil Quench – the Design Challenge

Although commonly used in carburizing, oil quenches have historically been impossible to monitor. In most situations, monitoring equipment has been necessarily removed from the process between carburizing and quenching steps to prevent equipment damage and potential process safety issues. As the quench is a critical part of the complete carburizing process, many companies have longed for a means by which they can monitor and control their quench hardening process. Such information is critical to avoid part distortion and allow full optimization of hardening operation.

When designing a quench system (thermal barrier) the following important considerations need to be taken into account.

  • Data logger must be safe working temperature and dry (oil-free) throughout the process.
  • The internal pressure of the sealed system needs to be minimized.
  • The complexity of the operation and any distortion needs to be minimized.
  • Cost per trial has to be realistic to make it a viable proposition.

To address the challenges of the oil quench, PhoenixTM developed a radical new barrier design concept summarized in Figure 2 below. This design has successfully been applied to many different oil quench processes providing protection through the complete carburizing furnace, oil quench and part wash cycles.

Fig 2: Oil Quench Barrier Design Concept Schematic

(i) Sacrificial replaceable insulation block replaced each run.

(ii) Robust outer structural frame keeping insulation and inner barrier secure.

(iii) Internal completely sealed thermal barrier.

(iv) Thermocouples exit through water/oil tight compression fittings.

In the next and final installment in this series, Dr. O will address AMS2750E and CQI-9 Temperature Uniformity Surveys, which often prove to be challenging for many heat treaters. "To achieve this accreditation, Furnace Temperature Uniformity Surveys (TUS) must be performed at regular intervals to prove that the furnace set-point temperatures are both accurate and stable over the working volume of the furnace. Historically the furnace survey has been performed with great difficulty trailing thermocouples into the heat zone. Although possible in a batch process when considering a semi-batch or continuous process this is a significant technical challenge with considerable compromises." Stay tuned for the next article in the series of Temperature Monitoring and Surveying Solutions for Carburizing Auto Components.

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Comparative Study of Carburizing vs. Induction Hardening of Gears

/ AEROSPACE HEAT TREAT, AEROSPACE HEAT TREAT NEWS, CARBURIZING TECHNICAL CONTENT, FEATURED NEWS, INDUCTION HEATING TECHNICAL CONTENT

Modern rotary-wing aircraft propulsion systems rely on different types of gears to transmit power from the turbine engines to the rotors. The basic requirements of these gears are that they are high strength, sustain long life, meet weight considerations, and have a high working temperature and low noise and cost, among others.

Most importantly, these gears require a hard, wear-resistant surface with a ductile core.

Gas carburizing is the current heat treat method used to produce these aircraft quality gears, but this method of heat treatment is costly due to the large number of process steps, huge footprints, energy consumption, and environmental issues. Moreover, the final grinding of gear teeth to correct distortion produced during quenching reduces effective surface compressive stresses.

An investigation into low-cost alternatives for surface hardening aerospace spur gears was conducted where specimens of the selected gears were induction hardened using a patented process. Dimensional and microstructural analyses were conducted, and residual stress studies were performed. This article is a summary of the steps and observations of the case study that resulted from this investigation, which can be summarized this way:

The proposed induction process is a low-cost alternative to conventional gas carburization. In some applications, a 25% savings is estimated.

The first step to gear manufacturing demands a total understanding of aerospace gear requirements. As the gear transmits torque, the teeth are subjected to a combination of cyclic bending, contact stresses, and different degrees of sliding or contact behavior. It is, therefore, critical for a gear to have the proper case and core structure to withstand these loading conditions.

With every revolution, a cyclic bending load is applied, resulting in tensile stress at the root region of the gear. The core of the gear has to be soft to absorb impact load and prevent brittle failure. Due to high-speed contact between adjacent gear teeth, peak shear stresses generated at the surface act in the normal direction to the surface. Pitting, spalling, or case crushing types of failures can occur due to low residual stress or inadequate case depth.

For aircraft quality gears, typical surface hardness is around 58Rc to 60Rc. The case depth is in reference to 50Rc and is controlled by diametral pitch.

Carburization

Carburization hardening is the most widely used technique for surface hardening of aerospace quality gears. A brief introduction to carburization is necessary to understand the potential benefits of this process and how other surface transformation can improve on some of the drawbacks of this commonly used process.

After raw material is received, it is forged to achieve proper grain structure and core hardness. The alloy most commonly used is ASM 6260 (AISI 9310). This low carbon alloy steel exhibits high core toughness and ductility.

Parts are loaded in a furnace and heated to 1650ºF – 1750ºF in a carbon rich atmosphere, where approximately 1% carbon potential is maintained. The depth and level of carbon absorption depend on carbon potential, temperature, time inside the furnace, and the alloy content of the material. After the desired carbon gradient is achieved, the gears are cooled slowly. Then the parts are heated to austenitizing temperature and quenched.

The process depends on the size, geometry, dimension tolerances, and other gear requirements.

The heat treat cycles shown above are two commonly used carburization processes. The difference in post carburization steps depends on the alloy used and final product requirement.

The characteristic of carburization is the inherent distortion associated due to the difference in cooling rates between the thin web and thicker rim. Distortion can occur as a size growth, a change in involute profile, or the loss of crown in spur gears.

Case Hardening by Selective Heat Treatment

The number of process steps required to case carburize a gear can be significantly reduced only if the gear tooth surface areas are heat treated.

Processes for locally heating only the tooth surface include induction, flame, laser, and electron beam.

In order to use induction, steel with a minimum of 0.5% carbon must be used. Several different alloy steels were experimented with, such as AMS 6431, AlSl 6150, and AlSl 4350/4360/4370. These steels were selected due to their combination of toughness, temper resistance, hardenability, and strength. The hardened case is obtained by heating a specific volume of the tooth surface above the transformation temperature for that material. Rapid contour heating produced a case of martensitic structure around the profile-hardened area, resulting in high compressive residual stress at the surface at the root fillet. This compressive stress increases the tooth bending fatigue life, where tensile stress exists due to tooth bending.

Transformation hardening allows a significant reduction in process steps and associated fabrication costs, due to two different factors:

  1. Since sufficient carbon is already present in the base material, copper masking, plating, stripping and carburization steps are eliminated.
  2. In selective hardening, the area of the heated zone is limited to only the hardened sections, and distortion is minimal and predictable.

Surface hardening applications are generally controlled by three process parameters, namely frequency, power level, and time. In this respect, several different hardening processes have been used for gear hardening. The proposed method discussed in this presentation is known as Dual Pulse Induction Hardening (DPIH).

DPIH Process

The DPIH is a patented process (U.S. patent #4,639,279). The process uses single frequency for both the preheat and final heat cycles. Two different power levels are used. This allows the entire process to be performed in one setup, using a single solid-state power supply.

The DPIH process consists of the steps described below:

 

 

The heat treatment process steps for both the carburized and DPIH processes for the aircraft gear are compared below:

 

 

An 85% reduction in heat treat process steps occurs when the gear hardening method is changed from conventional gas carburization to DPIH.

 

Conclusion:

Comparison of the above data and the conventional carburization process to DPIH process.

Carburizing grade material has to be changed from low carbon to medium carbon steel for induction hardening. In both the processes, surface hardness achieved is comparable, but the characteristic of induction hardening is that the gear section maintains a constant hardness value from the surface up to the transition zone, where it rapidly drops to core hardness levels, unlike a more gradual decrease in hardness in case of carburized gears. Low distortion of induction hardening gear is also a major cost reducing factor.

 

Acknowledgment:

This work was performed at AGT, Division of General Motors.

Madhu Chatterjee is founder and president of AAT Metallurgical Services LLC in Michigan with extensive experience in advanced engineering, research and development, and process and product improvement. He is also one of the original dozen consultants that inaugurated Heat Treat Today’s Heat Treat Consultants resource page. You can learn more about Madhu Chatterjee here.

 

 

 

 

Look for more on aerospace heat treating in the upcoming special aerospace manufacturing edition of Heat Treat Today.

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