Effect of tempering temperature and inclusions on hydrogen-assisted fracture behaviors of a low alloy steel

The tensile properties and fracture behaviors of the pre-charged hydrogen low alloy steel subjected to various tempering temperatures were investigated by slow strain rate tensile tests. Hydrogen was introduced into the samples by electrochemical method in 0.5 mol L−1 NaOH solution with 1 g L−1 CH4N2S for 24 h at room temperature (298 K). The results show that with an increase in tempering temperature, the resistance to hydrogen embrittlement increases. For the sample tempered at 200 °C, hydrogen-assisted microcracks initiate at the lath boundaries and the interfaces between inclusions and matrix. Moreover, the void around the inclusion initiates at the matrix-inclusion interface and grows in the form of the interface decohesion for hydrogen free sample, while the void also starts at the interfaces and propagates into the steel matrix along vertical tensile stress direction in the presence of hydrogen. Fractography reveals that for hydrogen-charged samples, the crack originates from mixed O-Si-Al-Ca inclusions in the samples tempered at 200 °C and 400 °C, while it is not located at inclusions in the sample tempered at 650 °C.