Influence of Ti addition on the microstructure and mechanical properties of a 5% Cr–Mo–V steel

• The Ti addition largely increases hardness to toughness balance.
• Ti effectively refines microstructure.
• Ti retards the precipitation kinetics of secondary M23C6 carbides.

A systematic study to determine the effect of Ti addition on the microstructure and mechanical properties of a 5% Cr–Mo–V tool steel has been made in terms of optical and electron microscopy, as well as X-ray diffraction, thermal analysis, and small angle neutron scattering. With the characterization of the as-quenched structure, the microstructural variation as a function of aging time was investigated. The Ti addition effectively refines the prior austenite grain structure as expected. More importantly, it can change the precipitation behavior of secondary carbides formed upon aging. As this change is attributed to the resisted Cr diffusion into cementite, the resultant precipitation of M23C6 carbides was restricted. Hence this leads to a significant improvement of the hardness to toughness balance, which is coupled with grain refinement. This variation in precipitation behavior was considered to have a thermodynamic aspect, especially the driving force for nucleation of equilibrium carbides involved in the present alloy system.