When a part must resist wear at the surface yet stay tough at its core, case hardening wins—and carburizing services are the most common way to get there. Carburizing diffuses carbon into a low-carbon steel surface, then quenches it into a hard martensitic case over a ductile interior. The result: gears, pins, camshafts, and bearing seats that shrug off abrasion while absorbing shock. Done right, it is one of the most cost-effective upgrades you can specify, turning a few cents of steel into a component that outlasts the machine around it. It is also remarkably scalable, from a handful of repair parts to millions of transmission components.
What Carburizing Is
Carburizing heats the steel (typically 900–950 °C) in a carbon-rich atmosphere so carbon migrates into the outer layer. After a controlled soak, the part is quenched and tempered, leaving a high-carbon case (often 0.8–1.0% C) that hardens to 58–62 HRC while the low-carbon core stays soft and tough. Our heat treatment line integrates carburizing with straightening, tempering, and clean-up so parts arrive ready for finishing. The microstructural magic is in the gradient: a hard skin for wear, a resilient center for impact, and no brittle through-hardened fracture path.
Gas vs Vacuum Carburizing
Gas carburizing is the workhorse for high volume: endothermic atmospheres and precise flow control give uniform cases at low cost. Vacuum (low-pressure) carburizing uses acetylene pulses in a vacuum furnace—cleaner, with less oxidation and tighter case control, ideal for critical aerospace and medical components. Both beat pack carburizing for repeatability. If you are comparing hardening routes, our heat treatment services overview covers the full menu, including nitriding and induction hardening, so you can see where each process fits on the cost-versus-control curve.
Steel Grades That Respond
Low-carbon and alloy steels take carburizing best: 1018, 1020, 8620, 4118, and 4320 are staples. Alloy grades like 8620 add hardenability for thicker sections and are the default for automotive gears. Avoid high-carbon steels—they already have too much carbon to benefit and risk cracking during quench. After hardening, surface grinding restores datums and removes decarburized skin on critical faces, ensuring the case depth you measured is the case depth you keep.
Distortion and Grinding Allowance
Quenching warps thin or asymmetric parts. Shops control it with fixture straightening, restrained quenching, and generous stock allowance so grinding removes the case irregularity without cutting into the soft core. CNC turning and milling before treatment must leave this allowance, which is why DFM input at quote time matters more than almost any other variable. A part designed for carburizing leaves extra material on bores and flat faces; a part designed for through-hardening does not, and the difference shows up as scrap.
Masking and Post-Quench
Bores, threads, and press fits that must stay soft are copper-plated or coated with stop-off before carburizing. After quench and temper, parts may get black oxide for mild corrosion resistance or zinc plating for aggressive environments. Salt-spray and hardness mapping confirm the case depth and quality. Selective masking is an art: done well, only the wear surfaces harden and the assembly features stay machinable and dimensionally stable.
When to Carburize vs Nitride
Carburizing gives the deepest, toughest case (0.5–2.0 mm) for heavy wear and fatigue. Nitriding, covered in our nitriding services post, runs cooler with less distortion and a thinner case—better when dimensions are tight and volumes are lower. For shafts that also need surface hardening without a full soak, our induction hardening services localize the heat to specific zones, hardening only the bearing journal while the rest of the shaft stays tough.
Common Carburizing Defects to Avoid
Most carburizing problems are designed in, not processed in. The first is insufficient case allowance: if you machine the part to final size before treatment, grinding to remove distortion eats into the soft core and you lose the hard case where it matters. Always leave stock on case-critical surfaces. The second is poor masking—unmasked threads and bores harden and become impossible to mate or tap, so specify masked zones explicitly. The third is carburizing the wrong grade; high-carbon steels crack on quench and gain nothing, while a low-carbon grade with the wrong hardenability may not reach target hardness at depth. Porosity in PM (powder metal) parts can carburize unevenly, so PM isn’t a default candidate. Quench cracks show up on sharp corners and heavy sections transitions—round those edges and avoid abrupt section changes. Finally, skip the post-quench temper and you get a brittle part that fails in service. A supplier who reviews the drawing for case depth, masked zones, and quench-risk geometry before loading the furnace will save you far more than one who simply quotes the print.
FAQ
How deep is a carburized case?
Typical case depth is 0.5–2.0 mm, set by soak time and temperature, and verified by microhardness.
Does carburizing distort parts?
Some, especially thin sections; fixture straightening and grinding allowance manage it predictably.
Which is better: gas or vacuum carburizing?
Gas for cost and volume; vacuum for cleanliness, control, and critical parts.
Can stainless steel be carburized?
Standard austenitic stainless resists it; special ferritic grades can be, but nitriding is usually preferred.
What comes after carburizing?
Temper, straighten, grind datums, then optional black oxide or plating for corrosion resistance.
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