The Evolution Of Carbides In Spray-formed Nb-containing M3High Speed Steels And Corresponding Mechanisms

The tool steels greatly required for manufacturing industries are mostly produced by conventional cast technique, and large strain deformation as well as complex heat treatments are subsequently applied to refine the size and distribution of the carbides, so as to improve the comprehensive properties of steels. However, high-grade high alloyed high speed steels (HSSs) are difficult to be produced by conventional cast technology, but instead of complex and high-cost powder metallurgical (PM) technology. The spray forming technology with its unique technical advantages, however, can substitute PM technology to produce high alloyed HSSs. Our team has been focusing on the preparation of HSSs by spray forming for some years, and we have successfully prepared Nb-containing M3HSSs by spray forming, which exhibits better hardness and bending strength than those of ASP23HSS. However, the related mechanism about the effect of Nb to the microstructures and properties is not clear and uncovering these mechanisms should be necessary and important to develop the Nb-containing spray-formed M3HSSs. Based on the investigation and understanding of microstructure evolution and precipitation behavior of carbides during deposition, hot-working, quenching and tempering processes of the spray-formed Nb-containing HSSs, the mechanism of Nb to the microstructure evolution and formation, transformation of carbides in these steels were uncovered presently. This will provide a theoretical basis for the performance stabilization and application of spray-formed high alloyed HSSs with Nb addition. Main conclusions are as follows:Introducing Nb can greatly affect the component, morphology and distribution of carbides in the as-deposited M3HSSs. With0.5-1wt%Nb addition, the irregular VC (5-10μm) in M3HSS can be substituted by high number density and even-distributed spherical or nearly spherical V-Nb complex MC carbides (4-6μm), and the amount and thickness of M2C, as well as the grain sizes also can be decreased. Because the crystallization temperature of MC carbides can be increased with Nb addition, leading to a large number of MC carbide precipitated at high temperature during solidification, which can limit the growth of γ dendrites and promote the formation of spherical MC carbides and refined grains and M2C eutectic carbides.With increasing Nb content to2wt%, NbC and M6C carbides appeared besides of further grains refinement. This is due to that the crystallization of MC phases will be initiated above the peritectic temperatures for2wt%Nb addition, contributing to the peritectic transformation and refinement of austenite grains. Meanwhile, the very fine M2C carbides appeared in the droplets can be transformed into the W-riched M6C carbides and NbC in the mushy zones, leading to increased NbC content.The lamellar sizes and stability of M2C eutectic carbides can be decreased significantly with Nb addition, so that they can be easily decomposed into a mixture of fine M^C and MC carbides during hot deformation, and the hot workability of M3HSS will be effectively improved.Partial V in MC carbides and less V/Mo in M6C carbides can be substituted by Nb, and this accounted for the further increased thermal stability of MC and M6C carbides together with high W levels in M6C induced by spray deposition, and both two carbides can pin the grain boundaries and inhibit grain growth more effectively.Compared with M3HSS, the coarsening and dissolution rate of M2C precipitates in2wt%Nb alloyed steel was relatively slow after long-time aging at600℃. It showed that the M2C precipitates in this steel are considerably more concentrated than those in M3steel during tempering at560-600℃. Prolonging tempering times to96h at600℃resulted in decreased amount of M2C precipitates in M3HSS, accompanying with coarsened needle sizes to30-40nm, but the size of M2C precipitates in2wt%Nb alloyed HSS remained10-20nm. After tempering at600℃for168h, the matrix of both steels were decomposed, and the M2C precipitates in M3HSS are completely dissolved. In contrast, plenty of fine M2C carbides with size of20-50μm were still present in2wt%Nb-containing HSS, so that its red hardness was enhanced. The reason is that the content of Mo in the primary carbides was decreased obviously with2wt%Nb addition, leading to the Mo concentration that can be dissolved into the matrix were increased, which can improve the antiplatelet of M2C precipitates and benefited the maintenance of fine and high-density M2C precipitates.