Microstructure and mechanical properties of 1000 MPa grade steel plate for hydropower station in different quenching processes

Strength and toughness of the ferrum‐1.2manganese‐1.4nickel steel plates in the DQT (direct quenching→tempering) and DQQT (direct quenching→reheated quenching→tempering) processes were investigated. This ferrum‐1.2manganese‐1.4nickel which possessed a relatively low alloy content and improved purity was used as the 1000 MPa grade high‐strength steel plates for hydropower station. The flat prior austenite grains with the dislocation density of 4.86×1014 m−2 and nano‐scaled precipitates were acquired in the DQT process. The grains and precipitates grew up and the dislocation density decreased to 4.26×1014 m−2 as a result of reheated quenching in the DQQT process. After tempering, the yield strength of the specimen treated by the DQT process was ∼996 MPa, which was ∼34 MPa higher than that of the DQQT sample. The difference of yield strength between these two quenching processes was primarily ascribed to grain boundary strengthening and partly caused by dislocation strengthening or precipitation strengthening. Due to the circular propagation path of cracks and matrix softening, the impact energy at −60 °C in the DQQT process was superior to that of the DQT process.