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The effects of rare earth (RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite (GB) composed primarily of bainitic ferrite and martensite and/or austenite (M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700 °C for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature (DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.

The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 °C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180° to 650°C before the standard tempering at 700 °C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 °C tempering. The experimental results show that 455 °C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 °C tempering. Microstructural observations reveal that during the 455 °C pre-tempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 °C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.