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Surface treatment and tribology

A gear made of low-carbon steel (about 0.2%C) is carburized and quenched. Which statement about this treatment is correct?

  1. ACarbon is diffused into the surface in the austenite region (about 900–950 °C) before quenching, giving hard martensite at the surface and a tough core
  2. BCarburizing in the ferrite region (about 550 °C), which dissolves much carbon, hardens the surface without quenching and with little distortion
  3. CThe carbon-rich surface transforms before the core, leaving tensile residual stress there after quenching and lowering fatigue strength
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Answer: A. Carbon is diffused into the surface in the austenite region (about 900–950 °C) before quenching, giving hard martensite at the surface and a tough core

Carburizing is done in the austenite region (about 900–950 °C), where carbon solubility is large (about 1.2–1.4% in the 900 °C range) and diffusion is fast. When the surface carbon content is raised to around 0.8% and the part is quenched, the surface becomes hard martensite and the core a tough low-carbon structure, combining wear resistance with impact resistance. The carbon-rich surface has a lower martensite start temperature and transforms later than the core, so its volume expansion is constrained by the core and compressive residual stress remains in the surface, which also raises fatigue strength. The solubility limit of carbon in ferrite is at most about 0.02%, and hardening without quenching in the low-temperature ferrite region is a feature of nitriding. Carburizing and quenching requires measures against quench distortion.

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