An experiment-based model of combined hardening and non-hardening embrittlement in an interstitial free steel

The fracture appearance transition temperatures (FATTs) are evaluated for the samples of a Nb-stabilized and P-strengthened interstitial free (IF) steel. Based on the measurements, the combined effect of hardening and phosphorus grain boundary segregation on the embrittlement of the steel is investigated. Both hardening and P boundary segregation can raise the FATT of the steel, causing hardening embrittlement and non-hardening embrittlement, respectively. Meanwhile, the grain size influences both kinds of embrittlement, i.e., there is a synergistic effect between grain size and hardening or P boundary segregation on the FATT of the steel. With the aid of the Taylor expansion along with the experimental data, a combined hardening and non-hardening embrittlement equation is established, being expressed as FATT = 2.1Cp + 3.48σs − 22.36d−1⁄2 + 0.64(Cp − 14)(d−1⁄2 − 3.06) + 0.896(σs − 14)(d−1⁄2 − 3.06) − 13.7, where FATT is the fracture appearance transition temperature in °C, Cp is the phosphorus boundary concentration in at%, σs is the yield strength in 10 MPa, and d is the grain size in mm. A comparison of the calculated and measured FATTs is made, demonstrating that the calculated FATTs are well consistent with the measured ones.