Stress rupture behaviour of simulated weld joint microstructures of P91 steel

X10CrMoVNb 9 10 (P91) steel is being used in sub critical and supercritical boilers in welded condition for the past many years. Welding results in different microstructural features in the heat-affected zone (HAZ) and the performance of the weld joint during service depends on the above microstructural conditions. Failure often takes place in the intercritical region of the HAZ. The aim of the study has been to simulate various microstructural conditions encountered during welding through heat treatment at different temperatures and correlate them with stress rupture properties. To achieve the weld heat affected zone (HAZ) microstructures viz. over tempered, intercritical, fine and coarse grained the P91 steel samples were heat treated in the temperature range of 1073 to 1573 K. Creep rupture properties were lowest for P91 steel heat treated at 1173 K which represents intercritical zone and are very high for the material heat treated at 1473 or 1573 K. Creep rupture properties, room and high temperature tensile properties increased with increasing grain size when austenitised above 1173K, whereas the impact properties exhibited an opposite trend and decreased with increasing grain size (i.e. austenitising temperature). TEM studies have revealed that in all conditions M23C6 carbide is present but in the material heat treated at higher austenitising temperatures, the dislocation density is higher and martensitic lath size is finer which has resulted in higher mechanical properties and creep strength Considering the impact strength of similar to 120 J and creep rupture properties of P91 steel austenitised at 1373 K, it can be inferred that the austenitising temperature of this steel can be beneficially increased if higher creep rupture properties are required at the cost of slight reduction in impact strength.