The effect of variation of microstructure on fracture mechanics parameters of HSLA-100 steel

The variations in the FCGR curve, the J–R curve, the blunting-line slope and the stretch zone width (SZW) for systematic variation of microstructure in Cu-strengthened HSLA-100 steel has been studied. The microstructural variation in the steel has been introduced through ageing at various temperatures after an initial quenching treatment. This has resulted in progressive tempering of the as-quenched martensitic matrix, accompanied by nanoscale precipitation of coherent Cu particles that gradually coarsen and loose coherency with overageing. It was observed that although there was a systematic trend, the FCGR curves were least sensitive to microstructural changes. The variation of fracture toughness, characterized by JQ in most cases and by KQ for the microstructures displaying highest strengths, correlated well with the inverse relationship between fracture toughness and strength. A systematic trend was also observed for the pre-exponent and exponent of the power-law tearing curve (for cases in which brittle fracture was precluded), the blunting-line slope and the SZW. The effect of coherency of precipitates in restricting plastic flow, as implied from the variation of mechanical properties with ageing temperature is thought to be responsible for the effects observed.