Study On Spray-formed Nb-containing M3 High Speed Steel: Microstructures And Properties Optimization, Application

With high hardness, high wear resistance and good toughness, AISI M3 high speed steel (HSS) is extensively used as cutting tools and cold molds. HSSs made by conventional ingot casting contain coarse network of eutectic carbides at grain boundaries. Even after certain forging degree, the carbide particles are arranged in bands parallel to forging direction, which deteriorating the isotropic properties and toughness of HSSs. For the high cost of powder metallurgy (PM) HSSs as well as complex processing steps, the application of PM HSSs is still limited. New technology or processes are desired to produce high performance HSSs with low costs. The rapid solidification technique combined with adding strong carbide forming elements is an important way to improve the microstructures and properties of HSSs. Our group has studied the combining effect of spray forming and Nb alloying to improve the performance of M3:2 HSS, and revealed the related mechanism Nb alloying and microstructural refinement. However, the processes such as spray forming, hot deformation and heat treatment were not studied systematically as well as the effect of Nb on these processes. Based on the investigation of microstructure evolution and property changes induced by above processes, an optimizing principle for microstructures and processing techniques of spray-formed HSSs were provided.The optimized spray forming parameters for M3 HSS was as follows:super heat:160-180℃, atomization pressure:0.45-0.5MPa, gas-to-melt flow rate ratio (GMR):0.7-0.75. The hot processing constitutive equations, kinetics of dynamic recrystallization and processing maps were established. And the hot-working parameters were obtained:deformation temperature:1050-1150℃, strain rate: 0.001-0.1s-1, forging ratio:6.25. The heat treatment for the spray-formed HSS was: quenching at 1200℃ and tempering at 560℃ for three times.Two distinguished microstructure features in the deposits were found: equiaxed grains (mean diameters:45μm) and porosity, with the plate-like M2C eutectic carbides mainly containing Mo and blocky MC carbides mainly containing V at grain boundaries. The porosity was eliminated and the plate-like M2C carbides were broken while the grains were refined to 10μm by dynamic recrystallization during hot processing. The secondary hardening of the spray-formed HSSs can be attributed to the precipitation of the secondary carbides, needle-shaped M2C and rod-liked MC, both which obey the Baker-Nutting and Pitsch-Schrader orientation relationships, respectively.Niobium addition can significantly refine the lamellar sizes and decrease the thermostability of M2C eutectic carbides, so that they can be easily decomposed into finer and uniform M6C carbides, which will be dissolved much more easily during subsequent quenching, and ultimately increase the secondary hardening ability. Nb addition also increased the amount of MC carbides, and the content of W/Mo in MC phase, and it can replace partial V into MC. These factors promoted the increased hardness of MC and the improvement of the wear resistance of spray-formed Nb-containing M3 HSS that would mainly improve the tool performance.