Fatigue thermique des aciers à outils pour travail à chaud

Abstract

Thermal fatigue of X 38 CrMoV 5 (formerly Z38CDV5), a 5% Cr steel, is investigated, in as quenched and tempered (47 HRC) as well as annealed conditions. A thermal fatigue rig using high frequency induction heating is developed. Tubular specimens are used. By modifying the specimen geometry, various thermal gradients and therefore different thermo-mechanical 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 (σ θθ ) over axial stress (σ 22 )) 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 mechanical 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 quenched and tempered steel (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 millimetres beneath the external surface. The hardness achieve then the initial hardness of the 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.