Tempering of Direct Quenched Low-Alloy Ultra-High-Strength Steel, Part II - Mechanical Properties

摘要

Direct quenched untempered ultra-high-strength structural steels can possess good toughness and weldability when based on low carbon contents. In this study, the influence of tempering at 500°C has been investigated to evaluate the possibilities of widening the range of strengths that can be produced from one 0.1% C alloy composition. The study covered the four microstructural states presented in part I: direct quenched (DQ), reheated and quenched (RQ) and their tempered variants (DQ-T and RQ-T). In addition to tensile testing, the Charpy-V transition temperature T28J and the fracture toughness reference temperature T_0 were determined for 6 mm thick specimens. The hardness of the DQ and RQ states was identical at ca. 400 HV. However, on tempering, the DQ state retained its hardness better than the RQ state with hardness values 346 HV (DQ-T) and 327 HV (RQ-T). The yield strengths of the DQ materials were ca. 100 MPa higher than those of the RQ materials both as-quenched and after tempering. Despite the higher strength of the DQ and DQ-T states, both had lower T_(28J) temperatures than the RQ and RQ-T states mainly due to their finer effective grain sizes. The widely used correlation between the T_(28J) and the T_0 temperatures was not obeyed and the reasons for this are discussed.