Effect of tempering temperature on microstructure evolution and Cl − corrosion resistance of 4Cr13 steel

The effect of tempering temperature on microstructure and Cl − corrosion resistance of 4Cr13 steel was investigated using XRD, SEM, electrochemical and immersion testing. The results showed that with increasing tempering temperature, the matrix evolved from tempered martensite to tempered troostite, and finally to tempered sorbite. The amount of retained austenite (RA) gradually decreased and became negligible after tempering at 500 °C. A small amount of primary carbides identified as M 23 C 6 carbides survived after quenching. Granular Cr-rich M 23 C 6 carbides were precipitated during tempering at 400 °C-600 °C. Electrochemical and immersion tests revealed that with increasing tempering temperature, both the corrosion potential and pitting potential initially decreased and then increased, whereas the corrosion current density exhibited the opposite trend. The average corrosion rate and the corrosion weight loss firstly increased and then decreased. The massive precipitation of Cr-rich M 23 C 6 carbides upon tempering at 500 °C-550 °C promoted the formation of Cr-depleted zones and markedly deteriorated the corrosion resistance. Tempering at 600 °C accelerated coarsening of the Cr-rich M 23 C 6 carbides and lowered their distribution density, which alleviated the Cr-depletion effect and partially restored the surface passivation capability. Tempering at 200 °C prevented the precipitation of Cr-rich carbides and yielded the highest corrosion resistance.