Effect of nano-carbide formation on hydrogen-delayed fracture for quenching and tempering steels during high-frequency induction heat treatment

The amount, size, and spheroidization of nano-carbides were evaluated to determine their effect on the hydrogen-delayed fracture (HDF) properties of high-strength steel wires produced by quenching and tempering (QT) during high-frequency induction heat treatment. The steel wires had a microstructure of tempered martensite showing a tensile strength of 1.2 GPa. The size of the carbide decreased and the spheroidization increased with the tempering temperature. The 0.3C–Si steels that have needle-like carbides were more susceptible to HDF than the other steels (0.2C–Cr and 0.2C–Cr–Mo) that have carbides of high spheroidization rate. Calculation of the activation energy using thermal desorption spectrometry (TDS) defined the hydrogen trapping site. The trapping sites of diffusible hydrogen were the grain boundary and lath boundary for all three wires. Carbide/matrix interfaces trapped nondiffusible hydrogen for all specimens. The 0.2C–Cr steel exhibited the largest resistance to HDF because of the nano-sized and spheroidized carbide.