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

Abstract

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 δ ferrite at temperatures above Ac ₄. The δ 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 δ ferrite at soaking temperatures above Ac ₄ improved the ductility. The tensile properties have been correlated with the microstructures.