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Dies and molds are subjected to severe mechanical and thermal loads during injection or forging operations, As fatigue is one of the most important cause of tool steels damage, we studied isothermal (between 200°C and 600°C) fatigue behaviour and damage of two grades (SMV3 and ADC3) of a tempered martensitic 5 % Cr steel. More than 100 fatigue tests were performed in order to analyse mechanical loading, temperature and initial hardness influences. We observed a softening during all lifetime which is typical of tempered martensitic steels Softening amplitudes are increasing with increasing total strain amplitude, and decreasing with increasing temperature (till 550°C) and hardness (between 42 HRC and 50 HRC). Influences of temperature and hardness are limited when observed through a Manson-Coffin diagram, Conversely, increasing test temperature or decreasing hardness leads to a strong reduction of lifetime (factor 1000 for a 8 HRC decrease!) when results are plotted in a Basquin diagram. Interrupted tests showed that crack initiation controls lifetime for a number of cycles to failure around 10 5 cycles. Three sites of crack initiation were identified and discussed. - non metallic inclusions (200°C-400°C); - lath boundary (200°C-400°C), - prior austenitic grain boundary (500°C-600°C).

Comportement et durée de vie en fatigue isotherme de l’acier X 38 CrMoV 5