Effect of local stress on the heat-checking morphology in high temperature tool steels under thermal fatigue: Transition from multi-axiality to uniaxiality

• Thermal fatigue of a X38CrMoV5 steel is investigated under various test conditions.
• A microscopic crack network forms on the surface of the tubular specimen.
• The cracking morphology changes from a cell- to a parallel-type along the specimen.
• A local stress amplitude ratio Δσzz/Δσθθ near 1 leads to multi-axial heat-checking.
• Uniaxial cracking results when Δσzz/Δσθθ is close to or below 0.6.

Thermal fatigue experiments are performed on a high temperature tool steel X38CrMoV5 (AISI H11), under various maximum temperatures and heating rates. A microscopic interconnected crack pattern (named “heat-checking”) forms on the oxidised surface of the laboratory tubular specimen. A gradual transition is observed, from a “cell-type” cracking at the centre of the specimen to a “parallel cracking” at its extremities. This variation of the morphology is well demonstrated by geometrical and topological characteristics of the crack network (micro-crack orientations, cell shape and node density), which change along the longitudinal axis of the specimen. The thermo-elasto-plastic loading of the specimen is estimated by Finite Element Calculations using ABAQUS™. Whatever the thermal fatigue conditions, a linear correlation can be established between the longitudinal and hoop stress amplitude ratio Δσzz/Δσθθ and the hoop and longitudinal inter-crack spacing ratio dθθ/dzz. It is shown that a stress amplitude ratio close to 1 results in a multi-axial heat-checking, while a uni-axial cracking is generated when Δσzz/Δσθθ is close to or below 0.6. This means that the morphology of the heat-checking pattern (cell or parallel type) can be used as an indicator for the local stress ratio of the thermal fatigue specimen or industrial real tools.