Microstructure of pure iron treated by nitriding and quenching process

Austenite formation on the surface of pure iron during nitriding in a mixture of NH ₃ and H ₂ gases and surface hardening due to formation of iron-nitrogen martensite by subsequent quenching were investigated. In nitriding at 973 K at a partial pressure of NH ³(∫NH ₃) of 5% and 10%, austenite layer is formed at the surface while double-layered structures consisting ε-Fe _2∼3N and austenite are developed at of 20%. In addition, at high ∫NH ₃, voids are formed in ε-Fe _2∼3N nitride as well as austenite with high nitrogen content. At ∫NH ₃ of 5% and 10%, the growth rates of austenite accords with parabolic growth law and become faster with increasing ∫NH ₃. On the other hand, after prolonged treatment at ∫NH ₃ of 20%, thickness of the nitrided zone is less than that for 10% due to substantial formation of voids and resultant emission of nitrogen from the surface. The hardness of the nitrided zone is nearly the constant at 800HV in the specimen nitrided at ∫ _NH3 of 5% whereas softening near the surface occurs in the specimens nitrided at ∫NH ₃, of 10% and 20% due to formation of the retained austenite and the voids.