Microstructure characterization and tensile properties of hot isostatic pressed China ultrahigh strength steel

摘要

Powder metallurgy hot isostatic pressing (HIP) is a promising technology for producing complex ultrahigh strength steel (UHSS) components. In the present study, a kind of China UHSS, 30CrMnSiNi2A steel, was prepared by HIP with different processing temperatures. The microstructure and tensile properties of HIPed compacts were analyzed in depth via scanning electron microscopy, X–ray diffraction, electron back–scattered diffraction, transmission electron microscopy, tensile testing and nanoindentation experiment. Results show that all HIPed compacts are granular bainitic structure composed of bainitic ferrite matrix and M–A islands. With the HIP temperature increasing, the volume fraction of M–A islands decreases, and the average grain size increases. Besides, a harmful structure (that is prior particle boundaries, PPBs), which is composed of a great number of Mn–rich and Si–rich oxide inclusions, is formed in the HIPed compacts. Notably, increasing HIP temperature is beneficial to eliminate PPBs. The volume fraction of M–A islands, grain size and the state of PPBs together determine the tensile properties of HIPed compacts, in which the state of PPBs has a most significant impact. With the HIP temperature rising, the tensile fracture mode changes from inter–particle debonding to transgranular quasi–cleavage, which benefits from the elimination of PPBs.