Delayed Fracture Characteristics of Tempered Bainitic (BIII-type) and Tempered Martensitic Steels

Abstract

The influence of cementite morphology on the delayed fracture characteristics of medium-carbon low-alloy high strength steels was investigated in 0.1N-HCl aqueous solution at room temperature.
Three different types of test were made depending on the characteristic values of delayed fracture in question. The variation of carbide morphology was attained by two kinds of heat treatment. One is isothermal transformation followed by tempering and the other quench-tempering which formed tempered bainite (BIII-type) and tempered martensite with same tensile strength respectively.
There was no remarkable difference in threshold stress intensity factor K_ISCC and the ratio of incubation time τi to total time to fracture t_F between the two structures. In regard to crack propagation rate dC/dt, however, tempered bainite (BIII-type) showed much smaller value than tempered martensite, which resulted in longer time to fracture in tempered bainite.
With K_I around K_ISCC typical prior-austenite grain boundary fracture was found in both structures but with increased K_I, the fraction of quasi-cleavage fracture surface increased in tempered bainite (BIII-type), while in tempered martensite the intergranular fracture still remained dominant with increasing K_I. Reduced susceptibility to intergranular cracking with intermediate K_I value in tempered bainite (BIII-type) is likely to be due to the suppression of cementite precipitation along prior-austenite grain boundaries as shown by transmission electron microscopy, resulting in the reduced crack propagation rate.