e begin with an interesting tale of what hap-pens if we naively follow along. Just ask the four young Oysters who become enthralled with the seemingly idle chatter of the Walrus and the

Carpenter, ending up as the main course at dinner.

The time has come, the Walrus said,To talk of many things:Of shoes and ships and sealing wax Of cabbages and kings And why the sea is boiling hot And whether pigs have wings.

The Walrus and the Carpenter, Louis Carroll, Through the Looking-Glass and What Alice Found There, 1872.

Atmosphere gas carburizing is a process so familiar to most heat treaters it is too often taken for granted. We trust our oxygen-probe readings to keep us safe, and we expect the outcome of the process to never change. But occasionally we get in trouble, and when we do, valuable lessons emerge. Lets learn more. We will start by looking at various external and internal factors that can affect the carburizing process, uncover issues related to process and/or equipment variability, discover where the pitfalls might lie and talk about what we can do to avoid them.

Part LoadingMany times, variation in case depth and other carburizing prob-lems can be traced back to how parts are loaded in baskets and x-tures. Loading arrangements generally fall into one of two broad categories: weight-limited or volume-limited. In either case, when loading parts in furnace baskets or onto racks, our rst instinct is to maximize loading ef ciency. However, as heat treaters must also be concerned with proper part spacing (i.e. positioning parts within the load for optimal heat transfer), atmosphere circulation, temperature uniformity and heat extraction during quenching (to minimize distortion). And while trial and error is often the most prudent path, we must also take into consideration: Furnace-induced factors (often a function of the style of fur-

nace in use). Being aware of the process limitations induced by

a given design is an invaluable aid when things go wrong. Part geometry and orientation factors. We need to ask ourselves

questions such as, How much space should be left between parts? and Is random loading (Fig. 1) or nesting possible or even prudent?

For example, bearing races of various diameters a typical volume-limited load con guration are often nested inside one another, producing an optically dense workload that is dif cult to uniformly heat in many cases. In this instance, the cycle must be adjusted to allow enough time for the interior parts to reach temperature. Here, the furnace fan (type, speed, rotational di-rection, location) plays a signi cant role in the heating process. Fasteners are another example of where random loading in either continuous or batch-type (Table 1) units is most often used to handle the sheer volume of parts to be run. In this case, atmo-sphere penetration throughout the load, cleanliness of the parts

entering the furnace and allowing adequate time at temperature are considerations that must be factored into the process. If parts are not bulk loaded, a good rule of thumb is that the gap around a part should be no less than 25% and no greater than 75% of the parts envelope diameter (Table 2).

Atmosphere Gas Carburizing Case Studies, Lessons Learned (Part 1)

Daniel H. Herring | 630-834-3017 | heattreatdoctor@industrialheating.com

The Heat Treat Doctor

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24 September 2012 - IndustrialHeating.com

Table 1. Part surface area to load-size relationship for typical integral-quench furnaces

Load Size (width length height) mm (inches)

Maximum Part Surface Area m2 (ft2)

600 x 900 x 600 (24 36 24) 16.723.2 (180250)

760 x 1200 x 760 (30 48 30) 27.937.2 (300400)

900 x 1200 x 900 (36 48 36) 37.246.4 (400500)

900 x 1800 x 900 (36 72 36) 58.067.4 (625725)

Table 2. Part spacing requirements for typical batch loading

Part Diameter Horizontal Spacing (inside)Vertical Spacing

(inside)

mm inches mm inches mm inches

) 25 ) 1 619 0.250.75 1319 0.50.752550 12 1338 0.51.5 1925 0.751

5075 23 1957 0.752.25 2538 11.5

75100 34 5775 2.253 3850 1.52

* 100 * 4 * 75 * 3 * 50 * 2

This month we begin a podcast conversation called the IH Monthly Prescription with The Heat Treat Doctor. Every month,Dan Herring sits down with IHs editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at reed@industrialheating.com. Find the podcast on our website or use the Mobile Tag on page 26. IH Monthly Prescription is sponsored by SECO/WARWICK Corp.

Part CleaningAlthough atmosphere gas carburizing demands only a moderate level of cleanliness (compared to many other processes or indus-tries), contamination, such as cutting oils and residues left on parts, can cause signi cant problems both in our equipment (Fig. 2)and on the parts themselves. Carburizing and carbonitriding tend to be far more forgiving with respect to the amount of contamina-tion (e.g., oils, water, cleaning residues, etc.) that can be tolerated without interfering with case development and the quality of the resultant microstructure. Still, it is important to remember that cleaning must be done to at least a level appropriate for the in-tended application.

Carburizing Process Problems and Their SolutionsInadequate Case DepthNot achieving the desired case depth (Fig. 3) can be due to a num-ber of factors, some of which are carburizing at too low a carbon potential (i.e. too lean a furnace atmosphere), partial or complete decarburization of the part surface from air in ltration due to a leaky furnace, processing at the wrong temperature perhaps due to malfunctioning or improperly located thermocouples, retained austenite in the case region or a slack quench. Steps that can be taken to correct these maladies include in-creasing the carburizing potential of the furnace atmosphere (par-ticularly if boost/diffuse carburizing is being performed), changing

the carburizing process (e.g., carburizing and slow cooling fol-lowed by a subcritical anneal prior to reheat and quench), subzero treatments and selecting the proper tempering temperature.

Shallow Case or No Case DepthProducing shallow case depth or areas where there is no case devel-opment points to incomplete surface preparation prior to carburiz-ing, the presence of surface contaminants or possibly the misap-plication of selective carburization methods (i.e. stop-off paints or poorly adhering copper plate). Another area of concern is how the parts are being received from upstream operations. Dirty dunnage and suspect transport methods may add a level of contamination (e.g., rust) that is unacceptable to the carburizing process. Solutions to these problems include controlling the cleaning process, cleaning the parts washer as well as replacing its solution on a frequent basis, and handling parts with clean gloves.

Coming UpIn part 2, we will discuss problems associated with retained aus-tenite, decarburizing/de-alloying, intergranular oxidation, case leakage, case cracking/separation, case crushing, untempered/tempered martensite effects and other issues. IH

References1. Herring, D. H., How to Load Parts in Furnace Baskets, Heat Treating

Progress, November/December 2003.2. Herring, D. H., Its Time to Clean Up Our Act!, Industrial Heating,

January 2008.3. Weires, Dale J., Gear Metallurgy, Effective Heat Treating and Harden-

ing of Gears Seminar, SME Short Course, 2007.4. Mr. Darwin Behlke, Twin Disc, Inc., private correspondence.

26 September 2012 - IndustrialHeating.com

Fig. 1. Example of random loading of fasteners on a mesh belt prior to carbonitriding

Fig. 2. Internal furnace contamination sodium deposits in the form of a glassy coating

Fig. 3. Low case hardness[3]

HRC 60

HRC 50 Required

Possible decarburization or retained austenite

Case too lean or tempered too highH

ardn

ess

Case depth

Fig. 4. Damaged gear teeth due to lack of adequate carburization

Areas without case

Go directly to this months podcast by using this Mobile Tag.

e continue the discussion started last month on atmosphere carburizing, namely the problems we encounter in the heat-treat shop and the solutions that must be implemented to achieve

a successful outcome. Lets learn more.

Carbides and Carbide NecklacesThe formation of grain-boundary (i.e. massive) carbides and carbide necklaces (Fig. 1) has been the subject of a great deal of study but one that is directly related to process variables that are out of control. These include too high a carbon potential of the atmosphere during the boost portion of the cycle, insuf cient diffusion time, too short a soak time at temperature and

hardening from too low a temperature, to name a few. Fortunately, the formation of carbides can be minimized by steps such as controlling the carbon potential, adding more diffusion time to the recipe and changing the hardening temperature (or time). This is one of the reasons metallurgists are so concerned about verifying the oxygen (carbon) probe readings by use of a three-gas analyzer to determine the actual CO value, performing shim-stock testing to determine actual surface carbon and taking dew-point measurements to compare with historical information.

Retained AusteniteAustenite that does not transform to martensite upon quenching is called retained austenite (RA). RA occurs when steel is not quenched to its Mf (martensite nish) temperature (i.e. low enough to form 100% martensite). Since the Mf drops below

room temperature in alloys containing more than 0.30% carbon, signi cant amounts of untransformed (retained) austenite may be present, intermingled with martensite at room temperature (Fig. 2). Causes for high percentages of RA include a carbon potential that is too high and direct quenching from carburizing temperature. Leaning out the carbon potential, slow cooling followed by a sub-critical anneal (optional), and reheating and quench from a lower hardening temperature are solutions as well as introducing a subzero treatment, typically in the range of -62 to -100C (-80 to -150F). RA is problematic because it is metastable. Stress, elevated temperature or time will cause RA to transform into untempered martensite. In addition, a volume change (increase) accompanies this transformation and induces a great deal of internal stress in a component, increasing the likelihood of cracking.

Decarburization and DealloyingIf a steel part is exposed to elevated temperatures in the presence of air (Fig. 3), carbon will be depleted from the surface of the part (i.e. decarburization) and/or alloying elements such as manganese and chromium will be oxidized at the surface (i.e. dealloying). These effects generally occur when air leaks are present in the equipment, an improper carbon potential (too low) is used during the hardening process for the alloy in question, when preheating in air prior to loading into a protective atmosphere furnace is done above 370C (700F), or when parts are hardened without adequate atmosphere protection. Proper furnace maintenance, including checking radiant tubes for pinhole leaks and periodic pressure testing, combined with proper atmosphere control typically eliminate equipment variables related to this problem. Copper plating or selective

Atmosphere Gas Carburizing Case Studies, Lessons Learned (Part 2)

Daniel H. Herring | 630-834-3017 | heattreatdoctor@industrialheating.com

The Heat Treat Doctor

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16 October 2012 - IndustrialHeating.com

This month we begin a podcast conversation called the IH Monthly Prescription with The Heat Treat Doctor. Every month,Dan Herring sits down with IHs editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at reed@industrialheating.com. Find the podcast on our website. IH Monthly Prescription is sponsored by Praxair.

Fig. 1. Bearing race corner exhibiting retained austenite (white areas)

Fig. 2. Bearing race corner exhibiting retained austenite (white areas)

Fig. 3. Total decarburization on a steel part surface

stop-off paints (if used) must be adherent and properly applied.

Intergranular OxidationIntergranular oxidation (IGO) and inter-granular attack (IGA) are commonly associated with oxygen present during the carburizing portion of the cycle. In atmosphere carburizing, some IGO/IGA is unavoidable, typically 0.013 mm (0.0005 inches) or less, but can negatively affect mechanical properties such as bending fatigue life. Corrective action involves improved atmosphere control, being sure that the furnace is leak-free and/or switching to an alternative carburizing method such as low-pressure "vacuum" carburizing. Post-heat-treatment solutions often involve grinding of the surface to remove this effect.

Low Case HardnessLow hardness in the carburized case (Fig. 4) is often caused by carburizing with a carbon potential that is too lean, higher than normal amounts of RA, partial decarburization, a slack quench or over tempering. The surface-hardness drop can typically be corrected by using one of the following methods: increasing carburizing boost time (e.g., higher carbon potential in the atmosphere); carburizing, slow cooling, sub-critical annealing (optional), reheating and quenching from a lower hardening temperature; introducing a subzero treatment; and/or selecting the correct tempering temperature.

Selected Carburization and Case LeakageDuring carburizing, a variety of stop-off paints and/or copper-plating methods (i.e. masking techniques) may be used to selectively carburize certain component areas. If these techniques prove faulty, the carburizing atmosphere can leak under the protective layer. Causes include surface contamination or improper surface prep-aration (i.e. oils, greases, dirt remaining on the surface) leading to blisters or irregularities; in-adequate drying time; attempting to paint in too high a relative humidity atmosphere; improper copper-plating methods (e.g., adherence issues such as aky surfaces, too thin a layer of cop-per); and overly aggressive blasting after plating. Selecting the proper stop-off technique and material for the job, preparing surfaces properly,

18 October 2012 - IndustrialHeating.com

Fig. 4. Low case hardness Fig. 5. Case/core separation in a gear tooth

HRC60

HRC50

Required

Case too lean or tempered

too high

Possible decarburization or retained austenite

Hard

ness

Case Depth

allowing adequate drying time, performing a low-temperature bake at 150C (300F), using controlled cleaning (after and prior to carburizing) and baking of parts after copper plating will ensure a proper outcome. When post-nital-etch checking of gears, for example, suspect areas appear as irregular, dark-gray indications in an area that should be light gray in appearance. Case Cracking/Case Separation/Case CrushingOccasionally, cracks (Fig. 5) are found to occur within the case (typically originating in the sub-surface). This phenomenon is known as case/core separation (or case cracking/case separation) and often leads to case crushing (Fig. 6) the inability of the case to support the applied load. In gears, this is not to be confused with pitting, a form of surface fatigue failure of a gear tooth. Microcracking near massive carbides is also reported to cause case cracking. Case/core separation is often due to improper part geometry (e.g., thin and thick sections on the same component) and/or carburizing to a case depth that is too deep. Eliminating high carbon concentrations at edges and in corners, allowing adequate stock allowance (for possible post-heat-treat material removal) and selecting the proper carburized case depth are all ways to help eliminate this phenomenon.

Tempering Effects The question is often asked of a carburized part, should the tempering temperature be selected to achieve the targeted hardness in the case, the core or both? As it turns out, the case is much more sensitive to the tempering temperature selected than the core. Tempering temperature, time at temperature and, in some instances, cooling rate after tempering are important factors to consider. The goal is to produce a tempered-martensite structure in the carburized-case region while maintaining proper surface hardness.

Other IssuesFor the most part, the problems with atmosphere carburizing are well known as are their solutions. It is the enemy we know, which is somehow a comforting thought. Control of process- and equipment-induced variables combined with a robust quality-assurance program will avoid the problems discussed here as well as others that might arise. So, there you have it. Enough information about carburizing problems/solutions to avoid the pitfalls of taking the process for granted and assuming nothing can go wrong. Remember, the old oyster in the oyster bed remained where he was and didnt wander off with the Walrus and the Carpenter. Experience kept him off the dinner table. IH

References and Fig. 6 available online