Damage Mechanisms in Salt Bath Nitro-carburised and Plasma Nitrided Hot Forging Dies of H11 Tool Steel.

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  • Devi M. Uma
    Research and Development Division, Tata Iron & Steel Co. Ltd.

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Studies have been made to compare the effect of salt bath nitro-carburising and plasma nitriding treatments, on the working life of hot forging dies made of H11 grade of tool steel. Working life of the surface modified dies, that are used for the production of bearing races by high speed closed die forging, was evaluated in the real plant operation. Salt bath nitro-carburised tools are found to give an average life of 12 000 races with a very wide variation between 1 000 to 35 000 races. In sharp contrast, plasma nitrided tools showed a consistent two to four fold higher life than the salt bath nitro-carburised tools. The surface of worn out dies was examined by scanning electron microscopy (SEM) to understand the nature of damage processes involved in the dies subjected to two different surface treatments. Fatigue cracks, resulting from cyclic thermal and mechanical loading during operation, are the dominant modes of damage in the case of salt bath nitro-carburised as well as plasma nitrided dies. The severity of die damage by mechanical fatigue was more in the salt bath nitro-carburised dies. Abrasive wear due to micro cutting was observed to result in significant damage of the salt bath nitro-carburised die. Die damage due to micro cutting was not observed in the plasma nitrided dies. However, signatures of chipping and plastic deformation were noticed at few locations. The results obtained on service life and wear mechanisms after the two different surface treatments are examined in the light of the characteristics of the modified surfaces.

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