Effect Of Powder Characteristics On Microstructure And Mechanical Properties Of Hot Isostatically Pressed 30CrMnSiNi2A Steel

30CrMnSiNi2A steel is a kind of low alloy ultra-high strength steel developed by China,which is widely used in aerospace,military defense and other fields.Hot isostatic pressing technology is an effective method for preparing highly dense,high-performance powder metallurgical steel components.As the raw material of powder metallurgy parts,the characteristics of particle shape,particle size distribution and surface state will affect the performance of products,while the effect of raw powder material characteristics on the organization and mechanical properties of powder metallurgy low alloy ultra-high strength steel is not clear.Therefore,the study of the effect of powder characteristics on the organization and properties of powder metallurgical 30CrMnSiNi2A steel is of great significance for the preparation of high performance powder metallurgical steels.In this paper,30CrMnSiNi2A steel was prepared by hot isostatic pressing technique using three kinds of atomized powders: water atomization(WA),gas atomization(GA)and plasma rotating electrode atomization(PREP),and the relationship between powder characteristic and metallographic organization,inclusions defects,tensile and impact properties of 30CrMnSiNi2A steel was mainly investigated.Furthermore,the effects of powder oxygen content and powder ball milling pretreatment on the microstructure and properties of powder metallurgy30CrMnSiNi2A steel were investigated.The main findings are as follows:(1)The morphology,particle size distribution,and fluidity of the three atomized powders,WA,GA,and PREP,differ significantly.WA powder has an extremely irregular shape and the worst fluidity;GA powder is nearly spherical with hollow and satellite powders;PREP powder is a spherical powder with no internal defects,excellent fluidity,and the lowest oxygen content.The sintered samples of the three atomized powders are all granular bainite,but show different characteristics of primitive particle boundaries(PPBs).The size of PPBs inclusions decreases in an order from the samples sintered with WA,GA and PREP powder,and the inclusions composition transform from Al-rich and Si oxides to Al-rich oxides.(2)The powder oxygen content significantly affects the PPBs morphology and mechanical properties of 30CrMnSiNi2A steel.The oxygen content of PREP powder increases and the Fe oxide layer thickens after oxidation treatment.As the powder oxygen content increases,the size of PPBs inclusions increases.When the oxygen content increased to 650 ppm,inclusions with sizes >0.5 μm appeared,leading to a significant decrease in the elongation and impact toughness of the steel.(3)PREP powder ball milling pretreatment can significantly improve the impact toughness of 30CrMnSiNi2A steel.With the increase of ball milling time,the impact toughness showed a trend of increasing and then decreasing.The sintered sample with the powder ball-milled for25 h achieves the optimal strength and toughness match in which the tensile strength and impact toughness are 1337 MPa and 31.6 J,respectively.The impact toughness was improved by 49.8% compared to the sample consolidated using powder without ball milling,attributed to the breakage of the oxide layer on the powder surface and the formation of harmonic structures in the sintered compact.