Microstructure and impact toughness evolution in high-strength anchor chain steel: A Jominy test-based investigation

• Impact toughness correlates to ductility for the entirely ductile fracture. • Impact toughness correlates to effective grain size refinement for mixed fracture. • The relationship between impact toughness and Jominy distances is elucidated. • A method for predicting impact toughness of anchor chain is proposed. Ensuring the impact toughness of anchor chains remains challenging due to inhomogeneities in microstructure and mechanical properties induced by variations in cooling rates during quenching. In this research, the evolution of microstructure and impact toughness at different distances from the quenched end ( d qe ) of the Jominy test for a high-strength anchor chain steel were analysed though instrumented Charpy impact tests, microstructure characterization, phase transformation kinetics, and crystallographic analysis. Our results reveal that with the increase of d qe , the transformed microstructure gradually transits from full martensite (1.5 mm, d qe hereafter the same) to martensite dominant (9 mm) and bainite dominant (13–40 mm), accompanied by a monotonic decrease in Vickers hardness. The impact absorbed energy at 0℃ increases from 172.83 J (1.5 mm) to 215.24 J (9 mm) despite the reduced density of high angle grain boundary, due to the higher ductility at d qe of 9 mm enhancing both the crack initiation and propagation energy. Then the impact absorbed energy continuously decreases from 215.24 J (9 mm) to 124.97 J (40 mm), attributed to the higher ductile–brittle transition temperature for coarser microstructure at larger d qe , which increases the proportion of brittle fracture surface. The coarser microstructure at larger d qe can be explained by the severe variant selection during phase transformation.