Research of fracture mechanism of steel grade P91 on impact bending after preheat treatment before welding

The influence of modes of preliminary heat treatment (HT) of P91 steel on crack formation during impact bending test has been investigated. Curves of dependence of force variation in time and specimen deflection variation depending on the force have been experimentally obtained, by which the following were determined: crack nucleation energy, crack development energy, maximum value of specimen deflection before fracture, maximum force and fracture force, strain rate, time before fracture, amount of fibrous component, impact toughness, stress intensity coefficient. The fractures were investigated by optical microscopy methods. It is shown that standard pre‑welding heat treatment (HT) of P91 steel – double normalization significantly reduces impact toughness, transforming the fracture into a quasi‑brittle state. The use of different variants of thermocyclic treatment (TCT) increases the total fracture energy and minimizes the amount of brittle component, while after TCT the fracture has mainly intracrystalline character.