Effect of processing route on microstructure and tensile properties of hot isostatic pressed 30CrMnSiNi2A steel: Comparison of prealloy and master alloy routes

Abstract

The prior particle boundaries (PPBs) are non-negligible powder metallurgy defect, which seriously weaken the mechanical properties of hot isostatic pressed (HIPed) steels. In present study, the master alloy route is proposed to improve the PPBs in HIPed 30CrMnSiNi2A steel, and the Fe-4.40Cr-4.80Mn-4.60Si-6.80Ni-1.00Mo-0.40V-1.12C master alloy system with low solidus temperature and wide solid-liquid temperature interval is designed. The microstructure, tensile properties and fracture behavior of HIPed compacts prepared by prealloy and master alloy routes are systematically compared. The results show that the structure in prealloyed sample is granular bainite (GB); and substantial Si oxides distributed along particle boundaries constitute serious PPBs. The yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) of the sample are 723 ± 10 MPa, 1238 ± 16 MPa and 12.3 ± 0.4%, respectively. In master alloy route, the formation of local liquid disrupts the distribution of Si oxides along particle boundaries and promotes the coarsening of oxides, which results in dispersed distribution of oxides and significant reduction in oxide density. In addition, since the structural uniformity depends on element diffusion, a relatively uniform structure is obtained through the optimization of powder size. The structure of optimized sample mainly consists of GB and martensite; and good tensile properties with YS of 798 ± 7 MPa, UTS of 1310 ± 8 MPa and TE of 15.3 ± 0.6% are obtained. The higher tensile properties are mainly due to the improvement of PPBs, which avoids the initiation of microcracks along PPBs and further avoids premature failure of material.