H13 tool steel was tested on a Gleeble 300 to simulate the rapid thermal and mechanical conditions of the friction-stir-welding plunge stage. Thermal cycling between 100 °C and 620 °C at 500 °C s⁻¹ for up to 80 cycles caused negligible changes in tensile strength (1567–1667 MPa) and ductility (15–16%), confirming excellent thermal stability. Tensile tests at 620 °C under strain rates of 2.54–1016 s⁻¹ revealed marked strain-rate sensitivity: maximum stress rose from about 1040 MPa at 2.54 s⁻¹ to 1105 MPa at 254 s⁻¹ with larger, deeper dimples and a wider plastic zone, indicating strain-rate hardening. At 1016 s⁻¹, the stress dropped slightly to 1082 MPa, and both plastic-zone size and dimple dimensions decreased, showing that adiabatic thermal softening localized deformation. These mechanical and microstructural trends demonstrate that, while H13 resists thermal cycling, extreme strain rates during plunging can initiate and propagate microcracks, leading to premature failure of the friction stir welding tool unless plunge parameters are controlled.
Djimaoui et al. (Sun,) studied this question.