Effect of tempering time on microstructure and mechanical properties of high Ti microalloyed quenched martensitic steel

With the development of Ti microalloying technology, the application and theory research of Ti in microalloyed steels are becoming more deeply and widely. However, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched martensitic steel has been rarely touched upon, meanwhile, it has long been inconclusive whether precipitated phases coarsening or the recovery and softening of martensitic matrix is the dominant role resulting in the decrease of hardness along with long time tempering of microalloyed steel. In this work, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched steel was systemactically investigated by TEM, XRD and Vickers-hardness test, and the interaction between precipitation hardening and microstructural softening of the high Ti microalloyed steel was also studied. The results indicate that the hardness increases for Ti microalloyed steel with tempering time 10~ 300 s, which is attributed to the fact that the precipitation hardening by nano-sized TiC particles is greater than the recovery and softening of matrix. With the tempering time from 300 s to 10 h, nano-sized TiC particles precipitate more and more and the mass fraction of TiC with the size less than 5 nm increases, owning to the precipitation hardening produced by tiny TiC which offsets the hardness decrease due to the gradual softening with recovery of matrix, and therefore, the hardness can keep a long platform; in addition, with the tempering time 10~20 h, the hardness decreases significantly and the deacreasing rate of hardening for steel with Ti microalloying is higher than that for steel without Ti microalloying. The average particle size of TiC increases from 2.76 nm at 10 h to 3.15 nm at 20 h. Calculation results show that the decrease of hardness caused by coarsening of TiC is 11.94 HV, while caused by recovery of matrix is 24.56 HV. It is shown that the recovery of matrix is the dominating factor for reduction in hardness, but coarsening of tiny TiC speeds the decrease of hardness and is also an important factor resulting in the decrease of hardness. © Copyright.