Carburizing

Carburizing hardens low-carbon steel by diffusing carbon into the surface at high temperature, creating a wear-resistant case over a tough core.

Overview

Carburizing (case hardening/carburization) is a heat treatment that adds carbon to the surface of low-carbon steel at ~900–950°C in a carbon-rich atmosphere, then quenches to form a hard martensitic case over a tougher, lower-carbon core. Typical case depths range from ~0.25–2.0 mm depending on time and specification, with surface hardness commonly in the 58–64 HRC range for suitable steels.

Choose carburizing for parts needing high wear and contact-fatigue resistance without making the whole section brittle—gears, shafts, cams, and bearing surfaces. It fits low to medium production well, and high volume when paired with controlled furnaces and repeatable specs.

Tradeoffs: the high-temperature cycle can cause distortion and size change, so finish grinding/honing after heat treat is common. Deep cases increase cycle time and cost. Sharp edges can over-harden; thin sections risk warping or through-hardening.

Common Materials

  • AISI 1018
  • AISI 1020
  • AISI 8620
  • AISI 9310
  • 20MnCr5
  • 16MnCr5

Tolerances

±0.003" to ±0.010" after carburize/quench (plan finish grind for tighter)

Applications

  • Transmission gears
  • Pinions and gear shafts
  • Cam lobes and followers
  • Bearing races and journals
  • Automotive differential components
  • Wear sleeves and bushings

When to Choose Carburizing

Carburizing makes sense when you need a hard, wear-resistant surface with a tough core on low-carbon steels, especially for rolling/sliding contact. Plan for post-heat-treat finishing if the part has tight fits, critical runout, or gear geometry sensitivity. It’s a strong choice when the drawing can call out case depth and hardness requirements clearly for repeatable production.

vs Annealing

Choose carburizing when you need a hardened surface for wear and contact fatigue; annealing is for softening, stress relief, and improving machinability. Carburizing is typically done after most machining, with grinding/honing reserved for final sizing.

vs Normalizing

Choose carburizing when surface hardness and a controlled case depth drive performance; normalizing mainly refines grain and improves uniformity of properties through the section. If the part’s failure mode is wear at the surface, normalizing alone won’t deliver the needed hardness.

vs Quenching and Tempering

Choose carburizing when you want a hard case with a tougher core on low-carbon steel and don’t want the entire cross-section at high hardness. Quench and temper hardens through-section (material-dependent), which can reduce toughness in thick parts or create excessive hardness where you only need it at the surface.

vs Nitriding

Choose carburizing when you need deeper case depths and high load-carrying capability on economical low-carbon steels. Nitriding runs at lower temperature with less distortion and often no quench, but typically produces shallower cases and requires nitride-forming alloy content for best results.

Design Considerations

  • Specify effective case depth, surface hardness range, and core hardness/strength target; include the measurement method/standard if you have one
  • Avoid sharp edges and pointed gear tips; add edge breaks/radii to reduce over-carburization and quench cracking risk
  • Expect distortion—leave grind stock on critical diameters, journals, and gear flanks; call out which surfaces will be finish-ground
  • Control thin walls and large section changes; abrupt transitions and asymmetry drive warpage during quench
  • Define masking requirements for no-case surfaces (threads, bores, seal lands) and allow access for stop-off or copper plating
  • Select a carburizing steel grade intended for the process (e.g., 8620/9310/20MnCr5) and avoid using plain low-carbon steel when core strength is critical