Strain-based approach to crack growth and thermal fatigue life of hot work tool steels

Thermal fatigue cracking is one of the most important life-limiting tool failure mechanisms in die casting of aluminium and brass. It results from the rapid alternating heating and cooling of the die surface during the casting process, and it is observed as a network of fine cracks on the tool surfaces exposed to thermal cycling. The crack pattern deteriorates the surface finish of the tool and, therefore, that of the cast products. Hot work tool steels are frequently used as die materials to minimise this tool damage. In this study, thermal fatigue cracking of two hot work tool steel grades, hardened and tempered to various conditions, were evaluated using an experimental thermal fatigue test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a noncontact laser speckle technique. The thermal fatigue damage was characterised with respect to crack length and density of cracks. Based on the experimental findings estimations of crack growth and thermal fatigue life, and their temperature sensitivity were made. In addition, the crack growth and thermal fatigue life were represented using strain-based models. It was found that the resistance against thermal cracking improves with initial tool steel hardness, in spite of the fact that thermal fatigue causes considerable softening, and that the initial ranking in hardness among the different steels is unaffected by thermal cycling. The cyclic thermal crack growth rate increases roughly with one order of magnitude when increasing the maximum cycle temperature from 700 to 850degreesC. In addition, an increase in the maximum cycle temperature from 600 to 700degreesC and from 700 to 850degreesC reduces the thermal fatigue life with roughly one order of magnitude, respectively.