Sensitivity of the Impact Toughness and Microstructure of 15CrNi3MoV Steel Under Different Quenching Rates

The sensitivities of the mechanical properties and microstructure of 15CrNi3MoV alloy steel under different quenching rates were investigated in the present study.After subjection to quenching with four different cooling rates(water cooling,forced air cooling,static air cooling and furnace cooling)followed by tempering,the microstructure was characterized by scanning electron microscopy(SEM),electron back-scattered diffraction(EBSD),and transmission electron microscopy(TEM);and the low-temperature(−20°C)impact toughness was evaluated.The results showed that the tempered microstructure and mechanical properties had high sensitivity to the quenching rate.With a decrease in the quenching rate,the low-temperature impact energy of tempered specimens decreased with increasing fluctuation.Correspondingly,the fracture morphology changed from completely ductile to brittle.In addition,as the quenching cooling rate decreased,the as-quenched matrix changed from a lathy to a polygonal structure with the presence of carbides and martensite-austenite(M-A)constituents,and the effective grain size increased.Tempered martensite with dispersed fine carbides was found in the tempered water cooling specimen,and tempered bainite with a polygonal structure containing large carbides and rare incomplete undecomposed M-A constituents was found in the tempered forced air cooling,static air cooling and furnace cooling specimens.The small effective grain size and fine carbides contributed to the good temperature impact toughness of the tempered water cooling specimens.