Effect of Al on martensite tempering: comparison with Si

Both Si and Al are known to have negligible solubility in cementite and therefore retard cementite precipitation. The effect of Si on carbides formation during martensite tempering has been extensively studied, whereas that of Al has attracted little attention. The aim of the present study is to shed light on the effect of Al on martensite tempering. Various advanced characterization techniques like FEG-SEM, TEM, in situ synchrotron XRD and APT have been employed to investigate the microstructural evolution during martensite tempering of 0.25C–2.1Mn steels with or without Al and Si additions. It is revealed that Al addition promotes ε-carbide formation, whereas Si addition has no significant effect. Al addition has weaker influence than Si addition to retard θ-carbides formation. At lower tempering temperature or short tempering time, Si addition retards more efficiently θ-carbide growth, whereas at higher temperature tempering or longer tempering time, Al addition becomes more efficient to retard θ-carbide growth. As a consequence, Si addition resists better martensite softening during tempering at lower temperature or for shorter time, whereas Al addition resists more strongly martensite softening during tempering at higher temperature or for longer time. Based on the present experimental results, mechanisms are proposed to explain the effect of Al on θ-carbide formation and growth as compared with Si.