Influence of Carbides Precipitated by Low-temperature Tempering on the Room-temperature Mechanical Properties of Grade 91 Steel

Abstract

The precipitation behavior of carbides in modified 9Cr-1Mo steel (Grade 91) subjected to low-temperature tempering and the influence of those carbides on the mechanical properties at room temperature were investigated. An as-quenched sample (AQ) contained a small amount of metal carbide (MC) in its martensite microstructure. On low-temperature tempering at 300–500°C, intended to suppress the recovery and growth of the dislocation substructure, three types of carbide were formed; these were identified as Fe₄C, hP8-type Fe₃C, and oP16-type Fe₃C by using replica samples for transmission electron microscopy and extracted-residue analyses. Samples subjected to double low-temperature tempering at 500°C for 5 min and then at 300°C for 1 h (DLTT) contained large amounts of carbide compared with the AQ sample, but had similar a lath width. The hardness of the DLTT sample was higher than that of the AQ sample, whereas its tensile strength was slightly lower than that of the AQ sample, regardless of the strain rate. The reason that precipitation strengthening did not increase tensile strength is considered to be the early formation of microvoids due to delamination at the carbide/matrix interfaces during tensile testing.