A comparative study of salt-bath nitrocarburizing and gas nitriding followed by post-oxidation used as surface treatments of H13 hot forging dies

• A wide study comparing salt bath nitrocarburizing versus gas nitriding + post-oxidation was performed.
• Wear micromechanisms developed on both surface treatments under conditions simulating shop forging were determined.
• No relevant stress relaxation or softening were observed until 500 ºC but gradual relaxation and partial softening at 600 ºC.
• A cryogenic treatment has notably increased the wear resistance of plain H13 steel.

The use of surface treatments is essential to increase the in-service lifetime of hot forging dies due to their improvement in terms of wear and fatigue resistances. Two thermochemical surface treatments (salt-bath nitrocarburizing and nitriding followed by post-oxidation) applied to a H13 steel have been studied in this paper. The effect of a deep cryogenic treatment on wear resistance has also been assessed.It was shown that the two applied surface treatments markedly improved the wear resistance of the steel as a result of surface hardening and the compressive residual stress field thus induced, along with the formation of surface layers of nitrides and oxides with excellent tribological features. These surface treatments also led to good performance when tested at high temperature simulated working conditions: no relevant stress relaxation and softening until 500 °C, but gradual stress relaxation and partial softening after prolonged exposure at 600 °C was observed. Furthermore, the application of a deep cryogenic treatment showed a significant reduction in the wear rate compared to that of conventional quenched and tempered H13 steel.