The Research Of Low Temperature Solid Power Chromizing Mechanism

In this paper, low temperature solid powder chromizing(LTSPC) technology was exploratory researched, Cr-diffusion layers were get on annealed and quenched 20&45 steel. Then the layers'morphology, component, phases, formation mechanism were discussed.Through the research, it's proved that the chromizing layers could form on annealed and quenched 20&45 steel at 580-650℃for 3-5h, with the help of chromizing agent, the layer is about 2-8μm. The layers'quality was related by carbon content, pre treatment, heating preservation time and so on. Through the experiments, the layer's thickness, compactness, homogeneity will be better when time lasts longer; under the same treatment condition, layers formed on quenched samples are thicker than layers on annealed samples of the same carbon content; it also should be rolled periodically to keep the layers homogeneous, whatever, the frequency couldn't be too fast, or there will be loose and holes.The recrystallization on poor-carbon layer between quenched samples and Cr-diffusion layer caused by stress in Cr-diffusion layer, which removes the stress' influence largely, and strengthen the sample and layers.Through theoretical calculation, it's proved that in LTSPC, vacancy mechanism plays important role to atoms'diffusion, layers'formation and growth;what's more, it also does great influence to the thickness of poor-carbon layers, which makes the calculations match the factual thickness in some extent.Through the XRD and field emission analyze, when the experimental perimeters don't change, the layers are different under different pre treatment conditions;the chief phases of layers on annealed samples are a-(Cr, Fe), Fe-Cr, Fe3C,it's maybe because carbon in annealed samples diffuses slowly at 580～650℃in 3-5h, for Fe3C has to break down firstly, so the layer's chief phase is (Fe, Cr) solid solutions;the chief phases of layers on quenched samples areα-(Cr, Fe), Cr7C3, Fe3C, and Cr7C3 could improve the layers'hardness, intensity, so wears will be improved too, it's maybe because carbon diffuses faster in quenched samples. Things above seems that quench as a kind of pre treatment could make up low carbon content's consequence and do good to the chromizing layers.In the process of LTSPC, carbon diffuses form the sample to the layers, it's uphill diffusion, however, carbon's diffusion in the layers vary in different samples: in the layers on annealed samples, carbon content from the middle to the sides is degressive, that makes the component curve to be symmetric parabolic. And Fe3C's appearance in layers reduces carbon's chemical potential, which provides the driving power for uphill diffusion;quench produces lots of defections, which fasten carbon diffusion in quenched samples, what's more, this provides enough carbon to form chromium carbide. But atoms diffuse much more difficult in layers when the layers get thicker, finally, it's distributes from the layers'surface to samples.Through the research, LTSPC is proved to be viable. The experiment could succeed on middle and low carbon steel, which seems it's also viable to other kinds of steel. In this paper, the heating temperature is low and time is short, which greatly reduces energy and natural resource consumption, what's more, it can replace stainless steel in some fields;as the works' last treatment process, it could improve performance, reduce content of alloy elements. So it could make up the defections on works' surface, which reduces scrap rate and cost of production.In a word, through the research, LTSPC and it's great application value is proved to be true, meanwhile, it might be the reference for other alloy layers' technology.