Research Of Microstructure And Properties Of Spray Formed New Type Hot Work Tool Steel

H13hot work tool steel is still widely used in different countries in the world, but to some extent its range of application and tool life are limited for its poor isotropy and insufficient high-temperature strength above600℃. It is generally considered that the poor isotropy has a lot to do with the inevitable coarse primary carbides and the segregation of alloying elements because of the relatively slow cooling rate during the traditional casting process. And the insufficient high-temperature strength above600℃is mainly related to the alloy composition and also to the types, amounts and distribution of the carbides. For a high Cr content in H13steel, M7C3type carbide becomes the key precipitated carbide during tempering and this carbide is easy to coarsen above600℃, inducing sharp decrease of the high-temperature strength of H13steel. Presently, the fine and uniform microstructures induced by spray forming technique are applied to improve the isotropy while the composition optimization aims to enhance the high-temperature strength. Meanwhile the influence of alloying elements and the spray forming technology itself on microstructure and properties was studied systematically, and following conclusions were drawn:A series new hot work tool steels with excellent high temperature strength and toughness were designed based on the composition of H13steel, and prepared via spray forming. Compared to H13steel, some properties of these spray formed new hot work tool steels were enhanced remarkably without a compromise of toughness, including tempering resistance, high-temperature strength, thermal stability, high-temperature wear resistance and thermal fatigue resistance, especially the high-temperature strength and thermal stability. Specifically, high-temperature strength under700℃was averagely increased by200MPa and the hardness after holding at660℃for12h was still kept above33HRC instead of26HRC for the commercial H13steel. The most representative composition of the spray formed new hot work tool steel was (wt%): Fe-0.37C-0.8Si-0.3Mn-3.0Cr-2.4Mo-1.5V-0.04Nb, with the best high-temperature strength and high-temperature wear resistance. In particular, the high-temperature tensile strength under700℃reached up to530MPa,240MPa higher than that of H13steel, and the high-temperature wear volume under400℃was reduced to 1/7of H13steel.The optimal spray forming parameters for these new hot work tool steels and H13steel was as follows: pouring temperature among1600-1625℃, atomizing pressure among0.45-0.50MPa. Different from the coarse dendrites in the as-cast H13, fine and homogeneous equiaxed grains without coarse primary carbides were gained in the as-deposited steels, and the grain sizes were decreased to20-30μm, significantly improving the segregation of carbides and alloying elements. The corresponding heat treatment for these spray formed new steels was chosen:quenching temperature among1040-1070℃, tempering temperature among620-640℃, so as to ensure the excellent comprehensive performance.The studies showed that the recovery degree of the martensite and the strengthening effect of carbides were mainly responsible for the improvement of high-temperature strength of these spray formed hot work tool steels. When the H13steel was tempered at650℃, the martensite was almost completely recovered and a large amount of Cr7C3and M6C type carbides with a size of about120nm were precipitated along the original martensite lath boundaries and grain boundaries, greatly reducing the strength. But for the spray formed new type hot work tools, after650℃tempering the lath martensite with high dislocation density was still presence, and intensive plate-like VC type carbides with a length of about20nm and thickness about2.5nm were found to be uniform-distributed within the martensite lath which could inhibit the martensite recovery so as to increase the strength.For the alloying elements, it showed that tempering resistance and high-temperature strength at700℃can be remarkably enhanced by only decreasing the Cr content of H13steel to3.0wt%which could obviously inhibit the precipitation of Cr7C3and greatly promote the precipitation of VC during tempering. This case also can be made by increasing the Mo content that can inhibit the precipitation of Cr7C3and promote the precipitation of VC during tempering so as to further increase the high-temperature strength at700℃. Although the effect of W was similar to Mo, the ability to increase high-temperature at700℃of Mo is better than that of W, by which the thermal stability can be effectively enhanced. Since the diffusion rate of W is much less than that of Mo, the coarsening speed of carbides during high-temperature treatment was slower if W appeared in carbides, inducing an improvement of thermal stability. Accordingly, alloy7with0.6wt%W addition had the best thermal stability, and maintained a hardness of37HRC after tempered at660℃for12h. The increase of V content not only further inhibited the precipitation of Cr7C3and M6C type carbides during tempering but also greatly promoted the precipitation of VC as well as its stability, thus further enhancing high-temperature strength at700℃and inhibiting grain growth during quenching. When increasing the content of V and Mo together, high-temperature strength at700℃also can be enhanced due to the inhibition of Cr7C3precipitation and the increase of VC precipitation as well as its stability. Besides, the amount of undissolved carbides after quenching was larger, so that the high-temperature wear resistance was enhanced but toughness was reduced. The grain growth can be inhibited during quenching with0.04wt%Nb addition, but the coarse primary VC and NbC carbides will be precipitated along grain boundaries after increasing Nb content to0.12wt%, and the segregation of alloying elements, alloy strength and toughness will be deteriorated.In conclusion, new type hot work tool steels with excellent high-temperature strength and toughness were prepared successfully via spray forming technique, not only obviously improving the segregation of carbides and alloying elements, but also significantly enhancing the high-temperature strength above600℃, thereby a new way for developing high-performance hot work tool steel could be offered. Moreover, the effects of alloying elements including Cr、Mo、 W、 V、 Nb can provide a certain theoretical reference to alloy design for high-performance hot work tool steels.