Case Hardening
What is the core metallurgical idea behind case hardening?
To increase surface hardness while retaining a tough core, so the part resists wear at the surface but still tolerates impact and cyclic loading in the bulk.
When would you choose case hardening over through-hardening?
When the primary failure mode is surface wear, sliding contact, or contact fatigue and the part still needs a ductile core; through-hardening is more appropriate when uniform hardness through the section is required.
Why is carburizing common for CNC-made gears?
It produces a hard tooth flank and root surface while maintaining a load-bearing core, which is why it is widely used for gears and drivetrain parts.
Case hardening is a surface-hardening heat treatment that creates a hard, wear-resistant outer layer while leaving a tougher, ductile core. This allows parts like gears, camshafts, and drive components to resist wear and contact fatigue without becoming brittle through their full section. Common processes include carburizing, nitriding, and induction hardening.
In CNC manufacturing, case hardening is typically applied after near-final machining. The common workflow involves machining the part close to final geometry, sending it for carburizing or another process, then inspecting distortion, hardness, and case depth before any final corrective grinding or lapping on critical fits. The hard layer is created by diffusion and subsequent quenching/tempering. This process is chosen when surface wear is the dominant failure mode but the part still needs load-bearing toughness in the interior. For gears, camshafts, splines, and bearing contact zones, the practical purpose is to put the hardest material exactly where contact happens while preserving core toughness to avoid brittle fracture under impact or cyclic loading. Typical targets are about 58–62 HRC at the case with a softer core, and effective case depth is often in the 0.1–3.0 mm range depending on the process.
Machining to final size before heat treat: Case hardening can distort parts during quench and temper. If bores, journals, or datums are machined finished beforehand, the part may return out of round or off-center, requiring rework or scrap.
Specifying the wrong steel grade: Carburizing suits low-carbon steels, while nitriding depends on alloy chemistry. Using the wrong base material can produce shallow case depth or poor hardness, causing teeth or splines to bruise, gall, or pit early despite appearing hardened.
Assuming a hard case fixes poor geometry: Case hardening improves wear resistance but does not correct misalignment, poor flank contact, or undersized sections. Those issues still drive bending stress and fatigue, leading to spalling or microcracking even when surface hardness tests pass.