Investigation into microstructure of spray formed V4 cold working mould steel and its roiling and annealing

The material selected for this work was the spray formed Vanadis4 high alloy cold working mould steel （abbreviated to V4 steel）. Its microstructure, hot rolling process, and annealing treatment have been investigated. Observed from the optical and electron microscopes, the as-sprayed V4 steel had the finer microstructure of uniform and equiaxial grains ,while after hot rolling for densification and spheroidized annealing, the V4 steel obtained an excellent spheroidized structure that is favorable to subsequent quenching and tempering treatment. The spheroidized structure and level of annealed hardness of the V4 steel are almost the same as expensive imported powder metallurgy the V4 steel. It is difficult to produce V4 steel with the conventional ingot metallurgical technique, so the multi-step and high-cost powder metallurgy method is generally used at present. Compared to the powder metallurgy technique, using the spray forming technique to produce the V4 steel has obvious advantages and potential market competitiveness in reducing production costs, simplifying working process, and shortening the production cycle.

Microstructure of the spray formed Vanadis4 cold work die steel and its evolution during hot rolling and heat treatment

A high alloyed Vanadis4 （V4） steel was produced by spray forming process in Baosteel, and the microstructure of the as-sprayed steel and its evolution during hot rolling and annealing processes were characterized. It was found that the as-sprayed V4 steel has a homogeneous and fine scale rrficrostructure with uniform dispersion of carbides. The hot rolling temperature is the key factor in controlling the evolution of type, morphology and distribution of carbides, as well as the matrix microstructure of the as-rolled steels. The optimized processing parameters for the as-sprayed V4 steel are rolling at 1050℃ and then annealing at 900℃. The microstructural evolution mechanisms during hot rolling and annealing are determined according to the microstructural analyasis of the material at different stages.

Microstructure characteristics of spray-formed high vanadium and cobalt high speed steel

The microstructure of high vanadium and cobalt high speed steel （high-V/Co HSS ） and the morphology of its carbides were analyzed by optical microscope （OM）,scanning electron microscope （SEM） ,electron probe microanalysis （EPMA）, X-ray power diffraction （XRD）, atomic force microscopy （AFM） and single phase erosion （SPE）. The results suggest that the as-sprayed high-V/Co HSS has fine equiaxed grains （about 20 μm in size）, which were homogeneously distributed. The carbides have two classical morphologies- one is fine particles （ about 2 μm in size） distributed along the grain boundaries and the other is needle-like one, distributed on the grain boundaries. There are MC carbides, M2C carbides,M6C carbides and Cr2WC2 carbides in the as-sprayed high-V/Co HSS samples,however,in the as-cast high-V/ Co HSS ,there are MC carbides and CrzWC2 carbides only. The SPE results show that there are two types of MC carbides in the as-sprayed HSS ：the sphere one and the particle-like one. The former is about 2 ixm in size and the latter is less than 1 μm, dispersed inside the grains,quite different from the MC carbides in the as-cast HSS. According to the AFM results, the skeleton-like M6C carbides of the as-sprayed high-V/Co HSS are embedded in the matrix along the grain boundaries. It is found that there are sharp membrane pieces on the carbides. Some small bamboo-shoot-like MC carbides grow from the matrix and are dispersed inside the grain. Those larger MC carbides are spherical particles embedded at the grain boundary junctions.