Fatigue thermique des aciers à outils de forge à chaud

37èmes journées du cercle d'études des métaux, 27-28 mai 1998, Albi, FRANCEInternational audienceThermal fatigue of X 38 CrMoV 5 (formerly Z38CDV5), a 5% Cr steel, is investiga ted, in as quenched and tempered (47 HRC) as well as annealed conditions. A thermal fatigue rig usinghighfrequency induction heating is developed. Tubularspecimens are used. By modi fying the specimen geometry, various thermal gradients and therefore different thermo-mecha nical loading i.e. mechanical strain versus temperature loops are generated. Finite element calculations of the thermo-mechanical strains and stresses reveal that the stress state in the centre of the external surface of the thermal fatigue specimen is quasi bi-axial and does not change for the different geometries used. Outside of this region, the stress ratio (i.e. hoop stress (σθθ) over axial stress (σzz)) arises progressively to about 2 to 2.5 (uni-axial condition). Depending upon the maximum temperature of the thermal cycle and the amplitude of the mecha nical strain generated by the thermal gradient, bi-axial oxide-scale spalling or heat checking were observed. Heat checking (bi-axial cracking) was predominantly observed in the centre of the specimens while towards to the ends of the specimens, the uni-axial cracking proceeds. Microhardness measurements at room temperature reveal a thermal fatigue softening in as quenchedand temperedsteel (47 HRC). A higher maximum temperature of the thermal cycle and a higher mechanical strain increases thermal fatigue softening. The role of the number of the thermal cycles was overshadowed by the more important effect of the amplitude of the mechanical strain and the temperature. A quasi linear softening is observed over few milli metres beneath the external surface. The hardness achieve then the initial hardness ofthe steel. The softening rate i.e. hardness over the thermo-mechanically affected zone width is controlled by the thermal gradient and thus the thermo-mechanical loading. No softening was observed in annealed steel