Atmosphere Gas Carburizing - Case Studies, Lessons Learned (Part 2)
Carbides and Carbide Necklaces
The 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, insufficient 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.
Austenite that does not transform to martensite upon quenching is called retained austenite (RA). RA occurs when steel is not quenched to its Mf(martensite finish) temperature (i.e. low enough to form 100% martensite). Since the Mfdrops below room temperature in alloys containing more than 0.30% carbon, significant 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 -100˚C (-80 to -150˚F).
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 Dealloying
If 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 370˚C (700˚F), 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 stop-off paints (if used) must be adherent and properly applied.
Intergranular 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 Hardness
Low 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 Leakage
During 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 preparation (i.e. oils, greases, dirt remaining on the surface) leading to blisters or irregularities; inadequate drying time; attempting to paint in too high a relative humidity atmosphere; improper copper-plating methods (e.g., adherence issues such as flaky surfaces, too thin a layer of copper); and overly aggressive blasting after plating.
Selecting the proper stop-off technique and material for the job, preparing surfaces properly, allowing adequate drying time, performing a low-temperature bake at 150˚C (300˚F), 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 Crushing
Occasionally, 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.
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.
For 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 didn’t wander off with the Walrus and the Carpenter. Experience kept him off the dinner table. IH
Fig. 6 available online
1. Herring, D. H., Fundamentals of Carburizing and Carbonitriding, Practical Learning Series, ASM International, 2001.
2. Weires, Dale J., Gear Metallurgy, Effective Heat Treating and Hardening of Gears Seminar, SME Short Course, 2007.
3. Mr. Darwin Behlke, Twin Disc Inc., private correspondence.