Heat-checking of hot work tool steels

摘要

Thermal fatigue (TF) is one of the life-limiting factors of the surface of the hot work tool steels. Bi-axial thermal strains and stresses are the main driving forces for the bi-axial cracking of tools well known as the heat checking. A TF rig using tubular specimens, induction heating and pressure air-cooling is developed. Two tempered martensitic steels 55NiCrMoV7 and X38CrMoV5 are investigated (47 HRC hardness) under a TF between 500-650°C. The effect of the specimen thickness on the softening (X38CrMoV5) is revealed by the post mortem room temperature microhardness measurements, X-ray residual stress and width broadening evaluations. The initial compressive residual stresses (due to the machining) become tensile very early under TF cycling. The X-ray width decreases with the number of thermal cycle. Cyclic inelastic straining and cyclic temperature tempering explain this softening. Cracking seems start when the oxide scale achieves a 《critical thickness》. Depending on the thermomechanical stress-state, the damage feature changes from a parallel multi-cracking near the extremities of the specimens (uni-axial loading) to a "cell-type" (or "square-type") cracking at the centre of the specimens (bi-axial loading). Uni-axial cracks can be very much extended. Under TF cycling, secondary cracks initiate progressively perpendicular to major axis of the uni-axial cracks. By increasing the longitudinal over the hoop stress ratio from the extremities (R=σzz/σθθ)=O) to the centre (R≈1), the density of the secondary cracks enhances. The 55NiCrMoV7 steel presents a lower heat checking resistance. In X38CrMoV5 the localised heat checking and the oxide-scale spalling are observed while in 55NiCrMoV7 the heat checking covers well the whole surface of the TF specimen. Oxidation-TF interactions play an important role in crack initiation and propagation.