Study on Process and Microstructure Properties for Canned Hot Extrusion of M32High Speed Steel Powder
|School||Harbin Institute of Technology|
|Keywords||PM HSS hot extrusion microstructure properties heat treatment numerical simulation|
High speed steel is widely used in cutting tool, cold and hot work mould, otherhigh wear-resisting, high temperature resistant cutting materials and structure parts field,which has high hardness, wear resistance, toughness and high cutting toughness. Highspeed steel can be manufactured according to the melting method, but the materialscontain carbide segregations and the defect of abnormal grain growth. Compare to thistraditional method of manufacturing, powder metallurgy high speed steel can eliminateor decrease these defects and enhance combination property of materials. In order tosolve the defects of powder metallurgy high speed steel manufacture such as complexprocess, difficult plastic process, thick carbides and inferior performances, this articlemanufactured bars for M32powder metallurgy high speed steel by employing thenumerical simulation, powders canned hot-extrusion and follow-up heat treatmenttechnology routes.Water-atomized M32steel powder, sample of extrusion and different treatmentprocess were systematically characterized by the X-ray diffraction （XRD）, scanningelectron microscope （SEM）, and particle-size analyze （PAS）, optical metallographicmicroscope （OM）, analytical balance （AB） and universal electronic test machine （UTM）.First, M32HSS powder canned hot extrusion was analyzed by using theDEFORM-2D finite element numerical simulation technology. This two dimensionalfinite element analysis can not only qualitative analyze HSS powder flow law and thedistribution of the field of hot extrusion process, the covering powder hot extrusionforming law was revealed, but simulate the effects of different sheath size on extrusioneffect. The simulation results show that powders canned hot-extrusion effect is in themost ideal condition when sheath bottom and wall thickness are5mm.Water-atomized M32high speed steel powder used in this study was irregular inshape. The average particle size of the powder is98μm. The irregular powder particleseasily achieve a more solid mechanical and metallurgical combination in the conditionof three hydrostatic pressures and therefore make it easier for a better densification. Asystematic study of the microstructure and properties of the M32high speed steelpowder in different extrusion temperatures was conducted. It is shown that theorganization of extruded samples is based on martensitic and a small amount of residualaustenite; and two types carbide dispersed on it, one type is MC carbides with V for rich,another type is M6C carbides with W, Fe for rich. After extruded at optimum extrusiontemperature of1240℃, the organization is small, uniform and with less empty, thedensity is high （98.04%）, which shows the best mechanical properties. Its hardness is 45.6HRC and the bending strength is2170.77MPa. Characteristics of vertical andhorizontal cross-section organization are totally different. There are cavities in itsvertical section and carbides which distributed into a streamline, while the horizontalcross-section is dispersed irregularly. Hardness in vertical section is slightly higher thanthe horizontal section and the small anisotropy is also found.In order to explore the influence of heat treatment system on the organization andperformance of high speed steel, the heat treatment was applied to the high speed steelwith better mechanical properties after the powder metallurgy extrusion. The resultsshow that under the conditions of the1180℃quenching and560℃×3tempering, theM32high speed steel powder can obtain optimal mechanical properties, which has thehardness of66.7HRC, bending strength of3721.83MPa, uniform distribution ofcarbides and fine grain size.