Study On Preparation Of A New High-alloyed Tool Steel By MA-SPS

High-alloying tool steels are widely used in the industrial production of tools and dies due to their high strength,good ductility and excellent wear resistance. However,the conventional processes are very complex with long production runs because of the high alloying contents. A new kind of high-alloying tool steel was prepared by mechanical alloying and spark plasma sintering,obtaining high hardness and relative density,fine grain size and uniform microstructures without carbide segregation.In this work,HGSF01 high-alloying tool steel powders were prepared by mechanical alloying,and the characteristics of ball-milling powders were analyzed by means of SEM,XRD and DSC. The experiments for preparation of HGSF01 high-alloying tool steel powders with different ball milling time and weight ratio of balls to powders were performed. The ball milling time was 40h,60h and 80h and the weight ratio of balls to powders was 7.5:1,10:1 and 15:1. The results show that the powders have been alloyed by ball-milling,after ball-milling a new phase (Fe,Cr)3C carbide appears in the small powder particles. The sintering temperature is selected as about 1100℃because the solidus temperature of the ball-milled high-alloying tool steel powders is 1085℃. A reasonable ball milling time and weight ratio of balls to powders for the preparation of HGSF01 high-alloying tool steel were 10:1 and 60h.The spark plasma sintering technology for the prepared HGSF01 of high-alloying tool steel powder was also investigated. The orthogonal experiments with 5 factors and 3 levels were studied for preparation of HGSF01 high-alloying tool steel powders by spark plasma sintering. The results show that the sintering temperature and holding time have the greatest effect on relative density of the samples in the sintering process. The best reasonable sintering parameters was that: milling time for 40h,weight ratio of balls to powders for 7.5:1,sintering temperature for 1100℃,heating rate for 100℃/min and holding time for 10min.Annealing pretreatment was taken and nitrogen quenching at 1100℃as well as double tempering at 560℃were taken as the final heat treatment for the sintered samples. The microstructure of the sintered samples before and after the heat treatment was observed by optical microscope and scanning electron microscopy. The relative density,Rockwell hardness,bending strength and friction and wear properties of the sample before and after the heat treatment was analyzed by Archimedes' principle,Rockwell hardness tester,and CMT5105 computer controlled electronic universal testing machine and the M-2000 friction and wear tester,respectively. The results show that the properties of the sintered sample obtained by SPS are as follows. Fine grains with a mean grain size of 5μm and very small carbide segregation uniform disperse in the matrix; a small number of holes exist in the sample with the diameter not more than 1μm. Before heat treatment the hardness is HRC 62,while after the final heat treatment the hardness change to HRC 61. The maximum bending strength of samples both before and after treatment is about 1500 MPa. Moreover,the wear resistance of the sintered samples is better than that of the ESR samples,and a better wear resistance of the samples after heat treatment is obtained by compared the wear scar width of sample after friction and wear. The bending strength and wear resistance of HGSF01 high-alloy tool steel is improved evidently by WC particles reinforced.