SOME STUDIES ON THE EFFECT OF HEAT TREATMENT AND DIFFUSIBLE HYDROGEN ON P91 WELDS

Abstract

The weldability of P91 steel depends upon suitable heat treatment and welding procedures. With respect to varying heat treatment temperature regimes the nature of precipitation strengthening and mechanical properties of P91 steel vary. The present research work deals with the effect of heat treatment procedures on the mechanical properties and microstructural behaviour of P91 steel base metal and welded joints. Effects of weld groove geometries on the weldability of the aforementioned steel were also studied. An optimum combination of heat treatment procedure for normalizing and tempering of P91 steel was determined during the current investigation. Since the P91 steel is used for high temperature applications, it’s high-temperature Charpy impact strength with respect to different tempering conditions was also determined. The optimum combination of weld properties in terms of Charpy impact strength, short-term creep strength, hardness and homogeneity of microstructure were determined for varying heat treatment conditions. The effect of varying post welding heat treatment (PWHTs) on type IV cracking behaviour were also studied. It is known that, delayed cracking in high strength steels such as P91 occurs as a consequence of combined effect of high residual stress, high diffusible hydrogen content and propensity of microstructure to crack. In practical welding situation, in spite of all the precautions, hydrogen pick-up may occur which can adversely affect the weldability of P91 steel. In the present research work, the effects of diffusible hydrogen content on the flexural strength, lower critical stress and tensile strength of P91 steel welds were also determined with respect to different electrode conditions. The consequence of the welding such as shrinkage of the P91 welds, shrinkage stress and residual stresses were also studied in details.