Carburizing
Carburizing is a thermochemical case-hardening process that diffuses carbon into the surface of low-carbon or alloy steel at 850–950°C, followed by quenching to form a hard martensitic case (HRC 58–62) while preserving a tough, ductile core (HRC 25–45). It is commonly applied to CNC-machined parts like gears, camshafts, and bearings requiring wear resistance and shock resistance.
In a CNC shop, carburizing is planned after rough machining and before final finishing because the thermal cycle causes distortion, growth, and shape changes that must be corrected by finish grinding or machining. The process is chosen for parts that endure rolling contact, sliding wear, or repeated impact—such as gears, splines, and bearing races—where the hardened case handles surface loads and the softer core absorbs shocks. Case depth, typically 0.2–1.5 mm, is controlled by time, temperature, and atmosphere; precision-critical parts require extra stock for post-heat-treat grinding. Machinists fixture parts to minimize movement during heating and quenching, then grind critical surfaces like bearing journals, tooth flanks, and locating faces to restore tolerances. This approach is especially vital for thin or intricate geometries prone to warp.
Why carburize instead of through-hardening the whole part?
Carburizing provides a hard outer wear layer while preserving a tough core, which is better for gears and impact-loaded parts than making the entire section hard and brittle.
What microstructural change creates the hard case?
Carbon enrichment at the surface followed by quenching promotes martensite formation in the case, producing high hardness while the lower-carbon core remains comparatively ductile.
What process controls matter most on the shop floor?
Temperature, soak time, atmosphere control, quench severity, fixturing, and final grind allowance are the main controls because they determine case depth, hardness, and dimensional stability.