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 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 (TM). Whatever the thermal fatigue conditions, a linear correlation can be established between the longitudinal and hoop stress amplitude ratio Delta sigma(zz)/Delta sigma(00) and the hoop and longitudinal inter-crack spacing ratio d(00)/d(zz) 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 Delta sigma(zz)/Delta sigma(00) 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. (C) 2013 Elsevier Ltd. All rights reserved.