| The surface of hot work mold steel will be oxidized in the high temperature and big impacting conditions. It will take thermal fatigue crack under the repeated heating and cooling conditions. All of these can cut down the life of mold. Then it is essential to take the surface treatment of hot mold steel. As one of the surface modification technology, TD salt-bath has been concerned widely since it is invented in Japanese Toyota research center. In abroad, the process has become a skilled surface treatment. Using TD salt-bath process can become chromium carbide layer in the surface of mold. The strength between layer and base is stronger than PVD and CVD process. And then the strong resistance of oxidation above 500℃, high hardness comform to the working conditions of hot mold and little thermal expansion value is helpful for reducing thermal fatigue crack. But numerous kinds of alloy is contained in hot mold steel. And then there is not any data which is specialized in hot mold steel. So the reaserch of TD salt-bath chromizing of hot mold steel has deep implication.In this paper, TD salt-bath chromizing of hot mold material was a preliminary study. First, through the full array test, steel, processing temperature and processing time were analyzed, and we can get the law of factors and determine optimum process. Furthermore, the organization morphology, phase structure of the TD chromizing coating have been observed and analyized by X-ray diffraction (XRD), SEM and energy spectrum analysis and other modern analysis methods. And then, we carried out a system of testing and analysis of coating, such as hardness, high temperature oxidation resistance and corrosion resistance. The valuable conclusions are followed here based on the experimental study and comprehensive analysis.1.In the full array test, taking the coating thickness as indicator, we analyze the steel, processing temperature and processing time on coating thickness. Among them, the alloying elements have greater impact on austenitizing temperature, then affect the coating thickness. Carbon content and processing time have not obvious effect on coating thickness. Processing temperature should be consistent with the quenching temperature of steel. At last, the final treatment process were 1000℃×6h or 1050℃×4h(3Cr2W8V),910℃×4h(5CrNiMo),1050℃×6h(W6Mo5Cr4V2).2.Coating phase mainly include Cr7C3 and (Cr, Fe)7C3.The coating had uniform thickness, good density and continuity and formed metallurgical bond with base. Cr7C3 is a carbide which has complex structures. The reason is rC/rM>0.59.As the size of the C atoms is larger than voids of close-packed lattice, so the lattice of metal atoms must be changed appropriatly basing on the hexagonal.3. After the salt-bath chromizing, the hardness of the surface coating can increse 1200-1600 (HV0.05) and is not reduced with the post heat treatment. Through gradient microhardness test of the cross-section, we discovered that there is gentle hardness fall in layer, but it's steep drop between layer and base. According to the hardness distribution, we can infer that there is not appear carbon depleted zone.4. Chromizing coating has good high temperature oxidation resistance. Under the 700℃, chromizing samples have obvious advantages comparing with ordinary samples in oxidation resistance.The form of chromizing samples is secondary oxidation in 100 hours.5. Performance of the corrosion resistance of chromizing samples is very prominent in nitric acid.The corrosion resistance is 20.2 time than ordinary samples in 12 hours, and there is still 8.3 times in 96 hours. Corrosion resistance is equivalent with 1Cr18Ni9Ti.
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