Research of Niobium-Bearing Cold Work Die Steel with High Vanadim Content by Powder Metallurgy
|School||Iron and Steel Research Institute|
|Keywords||powder metallurgy cold work die steel microstructure mechanicalproperties wear resistance|
As manufacturing industry is shifting to China, the national economy shows a growing demand for high-quality tool and die materials. At present, most of the die steels in China are produced by casting and forging process, during these processes, the alloy elements segregation and coarse structure can be possibly induced, which are harmful to the properties of the die steels. In this dissertation, two kinds of cold work die steel with high vanadium content and different compositions corresponding to2.45%C-5.25%Cr-1.3%Mo-9.75%V-0.9%Si (AHPV10) and2.45%C-5.25%Cr-1.3%Mo-8.64%V-1.7%Nb-0.9%Si (AHPV9Nb2) were produced by powder metallurgy, the thesis focuses on the alloy composition design, microstructure transformation during the processes, heat treatment, mechanical properties and wear resistance.Under thermodynamic equilibrium condition, phases in AHPV10and AHPV9Nb2have been identified and phases transformation have also been studied. Thermodynamic equilibrium phase diagram of AHPV10has been calculated by using thermal-Calc software and TCFE database. The type, volume and compositions of the phases from tested are in good accordance with calculation results. The results reveal that Niobium has a strong influence on phases transformation of alloys with high vanadium content, and when the content of niobium is in the range of0to1.7%, the structure tends to be more homogenous, in this thesis, AHPV9Nb2with niobium content of1.7%has been studied.Powdered AHPV10and AHPV9Nb2are composed of martensite, residual austenitic, MC carbide. Niobium of powdered AHPV9Nb2mainly exists in vanadium rich MC carbides and forms vanadium-niobium compound MC carbides. Niobium lowers the nucleation barrier of MC carbides and promotes smaller carbides forming. The experimental results indicate that the D50of carbides are0.47μm and0.37μm corresponding to powdered AHPV10and AHPV9Nb2respectively.The powder has been compacted to ingot with100%relative density after hot isostatic pressing (HIP). The phases of AHPV10and AHPV9Nb2are ferrite, MC carbide. During HIP process, carbides precipitate and grow up, the mean radius of initial carbides precipitated during atomization stage in AHPV10increases to0.35μm, the mean radius of carbides precipitated during HIP process is0.037μm, and the volume percent is1.97%of all carbides; the mean radius of initial carbides precipitated during atomization stage in AHPV9Nb2increases to0.28μm, the mean radius of carbides precipitated during HIP process is0.058μm, and the volume percent is0.31%of all carbides.The structure of the forged ingots after annealing process has been studied. AHPV10and AHPV9Nb2are composed of ferrite, MC carbide. The principle structure of MC carbides is VC, which partially transforms to V6C5due to the ordering of carbon-vacancy. The forged ingots have been processed under different heat treatment, MC carbides precipitate during tempering process. Carbides in AHPV10have been electro-extracted and studied. According to the results, after quenching at1120℃and tempering at540℃, the D50is1.03μm for the primary MC carbides, and21.77nm for the secondary MC carbides, both carbides have the same structure, but the secondary MC carbides contain higher Cr, Fe alloy elements compared to the primary MC carbides. The mass percent of the primary carbides and secondary carbides is14.0%and5.9%respectively.The mechanical properties of AHPV10and AHPV9Nb2have been studied. After quenching at1120℃and then tempering at540℃, the highest hardness of HRC63.9and HRC64.3have been obtained for AHPV10and AHPV9Nb2respectively. When the tempering temperature increases, the fracture surfaces of the bending samples change from ductile rupture to brittle rupture. AHPV10and AHPV9Nb2have the same yield strength. AHPV9Nb2gives higher bending strength due to its higher bearing to deformation before rupture occurres. The impact toughness of AHPV10and AHPV9Nb2increases as tempering temperature increases, the fracture is brittle rupture.The influence of the hardness and carbide size on wear resistance has been studied. When the hardness of AHPV10and AHPV9Nb2is relatively high, the main wear mechanical is adhesive wear, AHPV9Nb2gives higher adhesion wear resistance; When the hardness of AHPV10and AHPV9Nb2is relatively low, the main wear mechanical is abrasive wear, AHPV10gives higher abrasive wear resistance.