Characterization of Precipitated Phases and Carbides’ Coarsening in DH36 Shipbuilding Steel during Tempering Process

Abstract

The effect of tempering temperature on precipitates in DH36 steel was studied by using OM, SEM and TEM analysis. The results showed that DH36 steel after tempering was composed of ferrite, pearlite and precipitated phases. The grain sizes of ferrite have not increased obviously with increasing of tempering temperature. There were two types of carbides: (Nb,Ti)C and cementite, the sizes of which were more than 80 nm and less than 80 nm, respectively. The precipitation temperature of them were 1370 K and 977 K after calculation. As the tempering temperature increased, the average sizes of precipitated phases increased, while volume fractions and precipitation strengthening decreased. Cementite coarsened much faster than (Ti,Nb)C, which made strength and toughness of steel decrease sharply in 973 K. Meanwhile, moderate amount of precipitates made the toughness reach summit at 873 K. To sum up, the steel after 10800 s at 773 K tempering has the best mechanical properties and finest dispersed precipitates.