Study On Microstructure Evolution And Properties Of PM High Vanadium High Speed Steel

In this paper, nitrogen atomization + super-solid liquid phase sintering(SLPS) process is adopted to prepare high vanadium high spee d steels(HVHSS) with 9.5% vanadium. In order to make comparative analysis, high chromium cast irons(HCCIs) are prepared with nitrogen gas atomization +SLPS process. Comparative studies are conducted on the aspects of microstructure, hardness, impact toug hness, flexural strength, bending strength and wear resistance to analyze the effects of manufacturing procedure on microstructure and properties of powder metallurgy(PM) HVHSS.Firstly, the size, morphology, microstructure, and phase composition of HVHSS powders are studied and results show that, the HVHSS powder particles, with a size of about-200 mesh, are spherical and carbide particles are spherically dispersed in the microstructure, and XRD diffraction analysis shows that the phase composition of HVHSS powder is ?+VC.Secondly, suppression performance of HVHSS powders is studied. The results show that the forming property of HVHSS powder is poor and it could be effectively improved by adding a forming agent. Styrene-butadiene rubber(SBR) and zinc stearate, as forming agents, are added into the powders respectively, and different effects of SBR and zinc stearate on the forming performance are studied. The results show that SBR outperforms zinc stearate in this process. The specimen density rises with the increase of compacting pressure; however, when the pressure exceeds 300 MPa, there is no obvious rise in specimen density, and when the pressure reaches 400 MPa, specimen defects such as stratification and crack appear. The suitable pressing process could be done with: 1.1wt%SBR, pressure of 300 MPa, where the corresponding density specimen density could reach 5.65g/cm3.Thirdly, SLPS vacuum sintering technology was adopted to obtain high density products. DSC differential thermal analysis shows that the HVHSS sintering temperature ranges from1230? to 1300?. Results show that the microstructures of sintered HVHSS are mainly martensite, residual austenite and spherical VC. The carbides are uniformly distributed in the grain boundary and inside the grain. At the same holding time, with the increase of sintering temperature, grain and carbide gradually grow up, the amount of carbides in unit area is gradually reduced and carbides distribution uniformity decreases gradually. And at the same sintering temperature, with the increase of holding time, the m icrostructure evolution illustrates the same rule as above. The best sintering process is 1265 ?×120min, and the product density is 7.59g/cm3, hardness, impact toughness, and bending strength are respectively HRC68.2, 6.6J/cm2, and 1706 MPa.And then, the quenching temperature and tempering temperature on microstructure and mechanical properties of H VHSS were studied and the phase composition under different heat treatment conditions was analyzed. The results show that the phase composition of the quenching HVHSS is quenched martensite, residual austenite and VC, and the phase composition of tempered HVHSS is tempered martensite, residual austenite and VC. The brittleness appear at about 550?,so this temperature range should be avoid during tempering. After tempering, the best performance values are: hardness HRC67.5, impact toughness 7.0J/cm2, and bending strength 1967 MPa.After that, the differences in microstructure and mechanical properties of PM HVHSS and HCCIs are studied and analyzed. The results show that the microstructure matrix of PM HVHSS and HCCIs are martensite and austenite, and the main differences are grain sizes and carbides. The grain size of PM HVHSS is bigger than that of PM HCCIs, while the carbide is mainly spherical VC in HVHSS and mainly rod(Fe,Cr)7C3 in HCCIs, which results in a better performance of HVHSS concerning hardness, impact toughness and bending strength.And finally, the relationship between sintering temperature and impact abrasive wear resistance, as well as the wear resistance of HVHSS and HCCIs was studied. The results show that as the sintering temperature increases, the wear resistance of HVHSS samples illustrates a trend of first increasing and then decreasing; and with the increase of the impact energy, the wear resistance of both HVHSS and HCCIs decreases. Through comparative analysis of results from HVHSS and HCCIs impact abrasive wear test, it is found that the im pact abrasive wear resistance of HVHSS is better than that of HCCIs and under the same conditions, the impact abrasive wear resistance of HVHSS is about 3 times better than that of HCCIs. Through the observation and analysis of the worn surface of the samp les, it is found that HVHSS impact abrasive wear mechanism is mainly fatigue wear, and the wear mechanism of HCCIs is the combination of cutting wear and fatigue wear.