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Atmospheric carburizing

Carburizing is a case hardening process that introduces carbon into a solid ferrous alloy. This is accomplished by heating the metal in a carbonaceous atmosphere above transformation temperature for a pre-determined time. Subsequent to carburizing, parts are quenched to harden the surface carburizing layer. The core remains unaffected.

It is a widely used surface hardening process for low carbon steel. The industrial importance of carburizing is expressed in its market share, as one third of all hardening heat treatment is covered by carburizing and hardening.

Carburizing and quench produce hard surfaces which are resistant to wear. Moreover, failure from impact loading is avoided due to a softer core. Unlike case hardening processes, this process is usually used for deep case depths.

Typical applications include transmission gears and shafts for automotive, wind turbine and pump components and all applications where components are required to operate for long periods and under high impact loads. A wide variety of steels can be carburized. The unique combination of a hard wear resistant surface and a tough core can be controlled by the choice of alloy elements and process parameters.

Examples:

DIN SAE BS
1.7131 - 16MnCr5 5115/5117 590H17
1.7243 - 18CrMo4 4118H 708M20
1.6523 - 20NiCrMo2-2 8620H 805H20
1.5752 - 15NiCr13/14NiCr14 3310 655H13

Carburizing is a thermochemical diffusion process which adds carbon to the surface of a low carbon steel (typically 0.25% carbon) with other alloying elements. The carbon diffusion depth and the related effective case hardening depth (ECD) can vary from shallow, often less than 2mm, to deeper depths of 4 to 6mm. The total process is applied in three phases:

  • A thermochemical process in a chamber furnace with integrated quench to enrich the surface in a carbonaceous environment in a temperature range of 880 to 980°C.
  • Hardening is achieved after quenching either in oil, polymer/water solution or salt.
  • A tempering operation follows. Tempering temperature and time at heat depend on product requirement and application. The tempering process minimises peak stresses and reduces crack initiation.