Microstructure and tensile properties of modified 9Cr-1Mo steel (grade 91)

The influence of different soaking temperatures in the range 973-1623 K (below Ac-1 to above Ac-4) before oil quenching and tempering, on the microstructure, hardness, grain size, and tensile properties of modified 9Cr- 1Mo steel has been studied. This was done in an effort to assess the tensile behaviour of the different microstructures likely to be encountered in the heat affected zone of a fusion welded joint of the steel. The steel developed predominantly martensitic structure after quenching. Soaking of steel in the intercritical temperature range (between Ac-1 and Ac-3) reduced the prior austenitic grain size and hardness, Soaking temperatures above Ac-3 increased the grain size and hardness of the steel until the formation of delta ferrite at temperatures above Ac-4, The delta ferrite formation at soaking temperatures above Ac-4 reduced the grain size and hardness of the steel. The tensile strength of the steel exhibited a minimum for soaking in the intercritical temperature range where the ductility was highest, Strength increased and ductility decreased with further increases in soaking temperatures above Ac-3 The formation of delta ferrite at soaking temperatures above Ac-4 improved the ductility, The tensile properties have been correlated with the microstructures. (C) 2001 IoM Communications Ltd.