The effect of austempering temperature on crystallographic features and mechanical properties is investigated in low-carbon bainitic steel, focusing on the relationship between microstructure and mechanical properties. After isothermal holding at, above, and below martensite start (M_S) temperatures and tempering, a mixed microstructure of martensite/bainite and martensite/austenite (M/A) constituents is obtained. The fraction of M/A constituents increases as the austempering temperature increases, while the density of block boundaries decreases. The instantaneous work hardening rate exhibits continuous decay without a notable transition because of the retained austenite in the M/A constituents. The toughness decreases with increasing austempering temperature, which is related not only to the fraction of M/A constituents but also to the density of block boundaries. Isothermal treatment below the M_S temperature enables the formation of structures with fewer M/A constituents and high-density block boundaries, through which excellent toughness can be achieved.

The microstructure and impact toughness in an ultra-high-strength casing were investigated, attempting to reveal the reason for toughness fluctuations along the casing in terms of crystallography. The morphological structures at the head of the casing are lath bainite, while those at the end are granular bainite and lath bainite. The head exhibits a higher density of high-angle grain boundaries dominated by block boundaries and a higher impact toughness than the end, showing an inhomogeneous microstructure and causing toughness fluctuations in the casing. The higher density of block boundaries in the head resulted from weaker variant selection because of its higher cooling rate than the end. Therefore, the underlying reason for the toughness fluctuations lies in the varying densities of the block boundaries along the casing triggered by uneven cooling.

The effects of different pre-strain levels on the dislocation density, twinning behavior, resultant tensile properties, and cryogenic impact toughness of a high-manganese austenitic steel for low-temperature service were investigated. The results indicate that the dislocation density and volume fraction of twins are sharply increased when the pre-strain exceeds 15%, leading to an increase in yield strength and a decrease in impact toughness. At a 5% pre-strain level, few mechanical twins are observed while the dislocation density increases, resulting in enhanced yield strength whilst maintaining the toughness. The dislocation and grain refinement strengthening effects dominate the yield strength at various pre-strain levels. The initial mechanical twins and increased dislocations induced by pre-straining adversely affect the impact toughness. These findings validate the potential of controlling the mechanical twins and dislocations via pre-strain treatment as an effective approach to tailoring the mechanical properties of high-manganese austenitic steel.