Study Of Processing, Microstructure And Properties Of T15Powder High Speed Steel

In this thesis, gas atomized T15high speed steel powders were used, and theirmicrostructure and phase evolutions during rapid solidification were systematicallyinvestigated. The bulk samples were consolidated via powder metallurgy route, andsubsequent heat treatment was adapted to improve their properties. Thereby, T15powder metallurgy high speed steel with good strength, hardness and wear resistancewas produced.The solidification characteristics of gas atomized T15high speed steel powderswere studied. The powders exhibited spherical shape, while their sizes presentedGaussian distribution, and their cooling rates lay between104to106K/s. The resultsfrom SEM, XRD and TEM analysis showed that, with the decreasing of the powder size,equally means the increasing of cooling rates, the as-solidified microstructures of T15high speed steel changed from equiaxed grains to dendritic grains and finally to cellulargrains. The crystallization phase changed from martensite to austenite and finally toferrite. Carbides of cubic MC type riched in element V and closed-packed hexagonalM2C type riched in element W existed at grain boundaries in continuous network.For comparison,3different processing routes were used, T15high speed steelsamples by HIP, cold-pressing sintering and alloy melting were produced. Among them,the HIP sample showed the finest grains. Itsmicrostructure is homogenous andsomepartial dislocations were found. Carbides in the forms of MC, M6C and M23C werefound in HIP sample. The hardness of the sample was37.6HRC, which was lower thanthe melt sample. However, the HIP sample had a bending strength of2400MPa, atensile strength of1300MPa and an elongation of7%at ambient temperature. Theseproperties were all superior to those of melt samples. The HIP sample thus had a goodcombination of strength and plasticity. The HIP samples showed bigger deflectionduring bending. Its fracture toughness reached43MPa·m1/2, which was much higherthan that of the melt sample. The HIP sample fractured in ductile mode and showeddimple fracture surface. The melt sample on the other hand exhibited cleavage facetsafter fracture. In spite of its finalized grains, the cold-pressing sintered T15steel had arelatively lower density and stress concentration would be induced after loaded. Therefore, it has similar toughness and strength with the melt sample.The base of HIP T15high speed steel is quenched martensite, retained austeniteand unsolved carbides. Overheated microstructures appeared when quenched at1260℃.Secondary carbides precipitated from grains after tempering. The quenchingtemperature and tempering temperature were directly related to the morphology of themartensite, the grain size and the amount of carbide. The microstructure of T15highspeed steel consisted of finalized lath martensite with round carbides of MC and M6Ctypes after quenching at1210℃along with tempering at550℃for three times. Theoptimal mechanical properties were obtained because of the solution hardening ofmartensite and the dispersion strengthening by carbides. The bending strength reached4200MPa. The hardness was920HV and the fracture hardness was20MPa·m1/2. Thesecondary carbides had better thermal resistance and maintained a relatively highhardness at high temperatures produced by friction. The friction coefficient and wearloss were both less than those of M2and W2high speed steel. Its wear mechanismswere mainly adhesive wear and oxidation wear.