Carburizing is a case hardening process that introduces carbon into the surface of a component, forming an engineered hardened outer layer—or “case”—around a tougher, more ductile center or “core”. This is ideal for steel parts that require fatigue resistance and strength by utilizing a hard, wear-resistant surface in combination with a strong yet ductile core to prevent spalling and cracking.
Achieving the right balance between case depth and core strength depends on two key actions: selecting the appropriate steel grade and maintaining precise control throughout the carburizing, hardening, and tempering process sequence.
Like other hardening methods, carburizing involves three basic steps: heating, quenching, and tempering. What sets it apart is a carbon‑rich atmosphere that allows the diffusion of carbon into the steel’s austenitic matrix during the heating step.
Most often, the entire surface of a part is carburized. If desired, masking by copper plating or one of various commercial carbon stop-off “paints”, can prevent absorption of the carbon in selective regions of a part.
The part is held at temperature long enough for the carbon to boost and then diffuse into the material to the desired depth. Temperature plays a strong role in the process—the higher the temperature, the shorter the required time for a given case depth.
For more information, refer to data in the Metallus’ Practical Data for Metallurgists.
Here’s what happens to a part during the carburizing process:
The entire part’s microstructure transforms from ferrite/pearlite to austenite. At the surface of the part, carbon in the atmosphere diffuses into the austenite. This creates a high carbon surface and a progressively decreasing carbon gradient from that near the surface to where it meets the core carbon content.
The carbon-rich case region of the part transforms to a microstructure generally consisting of high carbon martensite with some level of retained austenite while the core transforms to lower carbon martensite and ferrite. Due to the differences in the Martensite Start (Ms) temperatures of the case and core, the case region transforms later than the core and creates the formation of beneficial residual compressive stresses in the case.
This step affects both the case and core regions making the microstructure more uniform and improves the ductility/toughness characteristics of both the case and core. Fine temper carbides form in the higher carbon regions of the case, with the carbon gradient from case to core progressively decreasing.
How do you know what to aim for in terms of carburization? It starts with knowing the loads a component will face in service. These will be the surface contact loads/stresses and the resultant subsurface shear stress profile and the strength profile of the case. From there, you can determine the required case depth and surface hardness. The process is then adjusted to achieve specific properties.
| Characteristic | Hardness | Wear Resistance | Fatigue Resistance | Corrosion Resistance |
| Effect | increased | increased | material dependent | material dependent |
Three primary factors control the outcome of carburizing: temperature, time, and chemistry.
The longer a part is held at temperature, the deeper the carbon penetrates—so case depth is governed by both time and temperature. Chemistry plays a role too: the carbon potential (or the amount of free carbon in the atmosphere) determines how much carbon diffuses into the surface and the steel composition also influences how readily that carbon is absorbed.
Carburizing should primarily be considered for parts subject to line contact situations such as roller bearings, spherical bearings, and gear teeth. If desired, masking allows the process to be applied only where wear resistance is needed, for example, to harden the running surface of a bearing race. (Fortunately, many/most applications do not require the time, cost, and control of such selective treatment. But it is certainly a viable option.)
Even when the process seems dialed in, carburizing can produce unexpected results. Identifying the root cause comes down to knowing how variables interact. There are many factors to consider, but in general, an improper microstructure leads to improper properties—and that’s going to affect the performance of the component. When troubleshooting, consider:
Both insufficient and excess temperature can create improper microstructures and could keep the part from achieving its intended properties.
If the part is held at temperature too long or not long enough, the microstructure needed for the desired properties may not properly develop.
Controlling carbon potential is essential to achieving the desired case depth; too little carbon and the case will not form properly. Gas selection is critical.
If a chosen grade is near its practical hardenability limit, switching to a different steel may resolve issues with softness. In many cases, trying to force a low hardenability, cheaper grade to perform can cost more than using a higher‑hardenability grade steel that carburizes and hardens more readily.
Atmosphere control issues can disrupt proper case formation, while contaminated quenchant may affect case hardness or prevent the core from achieving sufficient strength.
Distortion is an inherent risk whenever heat is involved. It cannot be eliminated but may be reduced with appropriate process design, fixturing, or by using press quenching.
Carburizing typically reduces stainless performance, but some high‑alloy steels—despite slowing carbon diffusion—may still carburize to meet both hardness and corrosion-resistance requirements. (Sullivan stocks several grades of this nature.) In other situations, other case hardening methods such as nitriding or induction may be satisfactory alternative options.
Carburizing is a straightforward process, but juggling all the variables—and figuring out what went wrong if issues arise—is not easy. At Sullivan, we support customers with a range of low-carbon, high-alloy steels for carburizing, including 8620, 9310, Jethete M152, M50NiL, CX13VDW, and NC310YW.
Our product experts can help diagnose why a part may be underperforming. With every customer, we offer technical guidance on selecting the appropriate material, heat treatment, process adjustments, and more. We stand behind the steel we sell.
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