Effect of Tempering Temperature on Microstructure and Mechanical Properties of D406A Steel

Abstract

D406A steel serves as a critical structural material for load-bearing components in aerospace solid rocket motors. To achieve an excellent strength-plasticity balance of D406A steel in this work, the quenched specimens austenitized at 890 °C were subjected to tempering treatments at 320 °C, 350 °C, 380 °C and 410 °C, respectively. SEM and EBSD characterization were adopted to systematically investigate the effects of tempering temperature on the microstructure, grain boundary characteristics, local strain, Schmid factor and mechanical properties. The results reveal that the lath martensite gradually undergoes recovery and disintegration with increasing tempering temperature, while the fraction of low-angle grain boundaries rises first and then falls, reaching the maximum value of 48.2% for the specimen tempered at 350 °C. At this tempering temperature, the KAM distribution is uniform, the Schmid factors shift toward the medium-to-high range, the proportion of grains with soft orientation increases, and the deformation coordination capacity is optimal. The specimen tempered at 350 °C exhibits an ultimate tensile strength of 1633.9 MPa, a yield strength of 1262.5 MPa and a Vickers hardness of 485.4 HV, achieving the optimal synergy between strength and plasticity.