Case Hardening of Steel: Carburizing vs Nitriding vs Induction Hardening

Hardenability in steel comes from carbon. When a medium or high-carbon steel is heated and quenched, carbon trapped in the crystal structure forces the formation of martensite — the hard, brittle phase that gives hardened steel its wear resistance. Low-carbon steels like 1018 (0.18% C) and 8620 (0.20% C) simply do not carry enough carbon for that transformation to bite: quench them and the surface stays soft. That is a problem, because these same steels are cheap, weldable, easy to machine, and tough — everything you want in a gear blank or shaft except surface hardness.

Case hardening solves the contradiction from two directions. Thermochemical processes — carburizing, carbonitriding, and nitriding — change the surface chemistry, diffusing carbon or nitrogen into the outer layer so it can harden while the low-carbon core stays tough. Surface-heating processes — induction and flame hardening — take a medium-carbon steel like 1045 or 4140 and harden only a shallow zone by heating and quenching the surface faster than the interior can respond. Either way, you end up with the combination no single heat treatment of steel can deliver on its own: a case at 55–64 HRC that shrugs off wear, over a core that bends before it breaks. That hardness-versus-toughness split is exactly what a stress-strain curve shows — the hardened case gives up elongation, while the core keeps it.

There is a second, less obvious benefit. Because the carburized case occupies more volume than the core beneath it, quenching leaves the surface in compressive residual stress — and fatigue cracks struggle to initiate and grow through a compressed layer. This is a large part of why carburized gears outlast through-hardened ones under repeated contact loading, and why the automotive industry carburizes its transmission gears as a matter of course. Classic case-hardened parts include gears, camshafts, shafts, splines, pins, bearing races, and wear pads — anything that sees rolling or sliding contact but must also survive shock. If a part needs that compressive surface layer but cannot take a quench, shot peening puts it there mechanically, with no furnace time at all.

“Case harden” on a print is not a specification — it is an argument waiting to happen. A workable callout has three elements: surface hardness, case depth with its measurement basis, and any features that must stay soft. A typical production note reads: “Carburize, harden, and temper. Surface 58–62 HRC. Effective case depth 0.8–1.2 mm to 550 HV. Threads and bore to remain un-carburized.” The 58–62 HRC range is the standard behind AGMA and ISO gear ratings: hard enough to resist pitting, not so brittle that grinding cracks or chipping follow.

The measurement basis matters because a carburized case has no sharp boundary — carbon content and hardness decline gradually into the core. Standards cut through the ambiguity: ISO 2639 defines effective case depth as the distance from the surface to the 550 HV hardness level; SAE J423 defines it for carburized and carbonitrided cases as the depth to 50 HRC (approximately 513 HV). Effective case depth is always less than total case depth, so if the drawing just says “case depth 1 mm” with no qualifier, the shop and the inspector can measure the same part and honestly disagree. State the basis, or better, name the standard.

One more subtlety catches buyers: if the part is ground after hardening, grinding removes case. A part carburized to 1.2 mm effective depth and then ground 0.3 mm per side arrives at roughly 0.9 mm of finished case. When the final case matters, specify “effective case depth after grinding” so the heat treater adds enough stock to land on the requirement. Verification is done by microhardness traverse on a sectioned coupon or a witness sample run with the load — worth calling out on critical parts, and worth pairing with the dimensional inspection points covered in our CNC machining tolerances guide.

Match the steel to the process, because each family exists to serve one. For carburizing, the classics are 8620 and 1018. 8620 is a nickel-chromium-molybdenum carburizing steel — the alloy content gives the case and core hardenability that plain carbon steel lacks, which is why it dominates gears, camshafts, and drive pins. It carburizes at 880–960°C, is typically reheated to 860–900°C and oil-quenched, then tempered at 150–200°C. 1018 and 1020 are the budget option: cheap, available everywhere, perfectly adequate for pins, bushings, and light-duty wear parts where the core strength of 8620 is not needed.

For nitriding, the answer is 4140. Nitriding needs alloying elements that form stable nitrides — chromium, molybdenum, and aluminum — and 4140’s chromium-molybdenum chemistry delivers a surface around 58–65 HRC (up to roughly 1,000–1,200 HV on dedicated nitriding grades like Nitralloy 135, which carries about 1% aluminum). The steel is quenched and tempered first to establish core strength, finish-machined, stress-relieved, and then nitrided. 4340 nitrides well too when the core needs more strength. One caution: any machining or grinding after nitriding must stay within the shallow case — cut through it and the hard surface is gone for good.

For induction and flame hardening, look to medium-carbon steels like 1045 and 1141, or pre-hardened 4140: enough carbon to reach 55–62 HRC at the surface, no furnace atmosphere required. These grades appear constantly in shafts, spindles, and wear rails. For a wider view of how these and other grades compare on machinability and cost, our steel materials page covers the grades we machine daily, and our guide to free-machining steel explains the resulfurized grades when cycle time matters more than hardenability.