The Research On Microstructure And Properties Of New Type Of High-boron High Speed Steel

High boron high speed steel which has peculiarities of high hardness, good hardenability, outstanding wear resistance and thermal fatigue resistance is a new kind of wear-resistant material with wear-resistant phases of boro-carbide.The addition of boron can partly replace traditional precious metals in high speed steel, so the research and development of high boron high speed steel has become the hot topics in the study of low-cost and high-performance HSS。 However, Boron brittle caused by boron, especially thick boron carbide along the grain boundary formation, makes the high boron HSS to a major topic of research and application at home and abroad.The influence of carbon content and heat treatment, especially the small amounts of cerium for high boron high speed steel’s properties and microstructure is studied by means of metallographic testing (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), microstructure analysis, hardness testing, abrasion testing and differential thermal analysis (DSC).The main conclusions are as follows:(1) High boron HSS’s hardness increased with the increasing carbon content. The value of low-carbon high boron HSS’s hardness after heat treatment is40HRC, while the high-carbon high boron HSS’s hardness value reached62HRC. The increase of carbon content was favorable for the formation of boro-carbide, and the volume fraction of boro-carbide in high-carbon high speed steel is much higher than low-carbon high boron high speed steel. In the low-carbon high boron high speed steel, the grain size is bulky and the boro-carbide’s distribution is net-like. However,the grain size is small, the boro-carbide which in small numbers persent by feathery flake and herringbone mostly present by graininess and its distribution is uniformity.(2) A small amount of cerium addition can significantly improve the comprehensive performance of high-carbon high boron high speed steel, especially the impact toughness and wear resistance performance. The impact toughness values (ak) increases from6.7J/cm2to14.3J/cm2.The friction coefficient from0.6057down to0.5243. Microstructure research results show that a small amount of cerium addition can refine primary austenite grain size in high boron the high speed steel, improve the distribution of boro-carbide and obviously refine boro-carbide. After joined rare-earth cerium, granular boro-carbide distributed small dispersion, flake boro-carbide is small and relatively even distributed, and mesh boro-carbide disappeared basically.(3) Distribution regularity of high-boron high speed steel alloy elements as follows:V, Ti and other elements are mainly distributed in the granular boro-carbide;Mo, W, Cr are mainly distributed in layers of flake, plume and herringbone boro-carbide;Cr are in the matrix, Si are mainly distributed in the matrix, B and alloy element formate to boro-carbide, mainly in the matrix, C have distribution in the boro-carbide and matrix.(4) A new type of high-boron high speed steel which has Fair comprehensive performance’s(After annealing hardness is48HRC, ultimate hardness is62HRC, impact toughness is29.2J/cm2, friction coefficient is0.4907) boron and carbon content is:Content of carbon is1.1%, content of boron is2.2%, Content of Ce is0.5%. The further increases of boron and carbon severely reduces the impact toughness and wear resistance of high-boron high speed steel. Boro-carbide in the as-cast organization of new type of high boron HSS is less, and its distribution is uniform. After heat treatment, there is a lot of boro-carbide, the microstructure consists of martensite, a small amount of residual austenite and boro-carbide; Its grain size is small; Boro-carbide is mainly granular and plume, its distribution is very uniform dispersion, the mesh boro-carbide basically disappeared.(5) The tempering system in that new type of high-boron high speed steel is relatively appropriate is500℃tempering twice. After500℃tempering twice, the precipitation of boro-carbide is more, granular boro-carbide which mostly separate out of matrix indispersion distribution is given priority, and granular boro-carbide is Isolated and dispersed distribution. The impact toughness value and hardness value of materials reach to maximum after500℃tempering twice. The air cooling is the best way to quench high-boron high speed steel.