Experimental Characterisation of Tool Hardness Evolution Under Consideration of Process Relevant Cyclic Thermal and Mechanical Loading During Industrial Forging

The near-surface layer of forging tools is repeatedly exposed to high thermal and mechanical loading during industrial use. For the assessment of wear resistance of tool steels, in previous work thermal cyclic loading tests were carried out to investigate changes in hardness. However, actual results of time-temperature-austenitisation (TTA) tests with mechanical stress superposition demonstrated a distinct reduction of the austenitisation start temperature indicating a change in the occurence of tempering and martensitc re-hardening effects during forging. Therefore, the superposition of a mechanical compression stress to the thermal cyclic loading experiments is of high interest. Tests are carried out in this study to analyse hardness evolution of the tool steel H11 (1.2343) under consideration of forging process conditions. The results show that the application of compression stresses on the specimen during the temperature cycles is able to restrict tempering effects while increasing the amount of martensitic re-hardening.