Study On Microstructure Evolution And Mechanical Properties Of H13 Steel Fabricated By Selective Electron Beam Melting

Traditional processing technology for manufacturing molds has problems such as long cycle time,high cost and tedious process,and difficulty preparing molds with complex cavity structures.As one of the most promising additive manufacturing technologies,Selective Electron Beam Melting（SEBM）breaks the limitations of traditional processing technology and can produce cost-effective mold products to meet the requirements of high performance and complexity in small quantities in a short time.In this paper,it was systematically investigated for SEBM fabricated H13steel that the microstructure evolution,carbide precipitation,and mechanical properties of deposited specimens at different heights.It inferred from the observation of microstructure combined with the calculation results that carbide precipitation and solid-state phase transformation during solidification.The main conclusions of the study of the 40 mm high samples at the 2 mm,20 mm,and 40 mm high positions are as follows.（1）The main microstructure of H13 steel formed by SEBM consists mainly of the bainite/martensite complex phase and a small amount of residual austenite and a small amount of carbide distributed out of the grain boundaries.with the decrease of deposition height（40 mm→2mm）,the content of martensite and residual austenite in the specimen decreases from 43.1%and1.8%to 36.4%and 0.4%,respectively;the content of bainite increases from 56.6%to 62.9%.The typical cell-like structure formed at the top of the specimen,and the middle and bottom of the specimen gradually disappear as the deposition height decreases.（2）The primary carbides in as-deposited SEBM-H13 steel are mainly rich-V MC,rich-Mo M₂C and rich-Cr M₇C₃ carbides.The nano-secondary carbides precipitated at the grain and cell boundaries are rich-V MC carbides,rich-Mo M₂C carbides and rich-Cr M₂₃C₆ carbides.The average particle size and content of secondary carbides at the top,middle and bottom were 40.6 nm,1.51%,45.8 nm,3.02%and 51.3 nm,6.13%,respectively.（3）The top surface of the as-deposited SEBM-H13 steel specimen has the highest microhardness of 565.6±5.8 HV_0.5,and the gradient along the deposition direction is stable at506.7±3.3 HV_0.5.The top area of the specimen has the highest tensile strength of 1338.1±30.8 MPa and elongation of 2.02%;its value decreases with the deposition height to 1172.4±44.3 MPa,and the elongation increases to 2.6%.The fracture mode also changes from brittle fracture to a mixed fracture mode with brittle fracture mainly accompanied by ductile fracture.Grain boundary strengthening,dislocation strengthening,solid solution strengthening and precipitation strengthening are the main strengthening mechanisms of SEBM-H13 steel.