Research On Surface Nano-crystalline And Related Metallography Problems In Pure Iron And Its Alloys

With the development of research on the technique and properties of nanostructured(NC)materials, it is reasonable to achieve surface modification by the generation of a nanostructured surface layer so that the overall properties and behavior of the material are significantly improved. It is validated that surface nanocrystallization(SNC) is one of the most important techniques, which can apply the NC materials into practice engineering. A certain depth of nanostructured surface layer has been successfully obtained in many material systems. Nevertheless, up to now, clear scenery of the process, microstructures and properties of the nanostructure surface layer is still lacking. There are many basical problems need to solve. Therefore, it is important to study the underlying mechanism for grain refinement by plastic straining.In this work, Strain, strain rate and stress of surface layer of Q235 low carbon steel sample during SMAT were investigated by finite element method, the effect of strain rate on deformation and grain refinement mechanism of Q235 low carbon steel during SMAT was proposed. The phenomenon of counterintuitive behavior which refers that permanent deflection is in the opposite direction of the load is found in the process of SMAT. In this paper, detailed numerical analysis on dynamic counterintuitive response of plates under current conditions i.e. subjected to continued short pulse loading have been studied. This dynamic behavior shows sensitivity to those factors such as plate depth, impact frequency and process time. The effects of these factors have been discussed.Porosity-free and contamination-free nanostructure surface layers with thickness about 40μm were fabricated on pure iron and Q235 low carbon steel plate respectively by using SMAT. Microstructural characterization of the nanostructure surface layer was investigated by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and optical microscope(OM).Electrochemistry experiment, tensile test, hardness measurements, friction and wear test have been carried out to investigate the properties of the fabricated surface nano-crystalline layer. Ti ion was implanted into the SMA treated samples and untreated coarse-grained samples by using Metal Vapor Vacuum Arc (MEVVA) source implanters, concentration distribution of Ti along the depth of sample was measured by means of Auger electron spectroscopy (AES). The effect of SNC on ion implantation in pure iron and Q235 low carbon steel was investigated.And a layer of Fe/Ni alloy surface layer was fabricated by appending Ni powder into the container during the process of pure iron specimen SMAT. To accelerate the inter-diffusion of Fe and Ni and the alloying process, the specimens have been annealed at different temperatures. The friction and wear behavior of alloy surface layers by appending different powders were studied, and the friction and wear mechanism were discussed.The main results are given as follows:1 Strain, strain rate and stress gradually decrease along the depth of sample during SMAT. During SMAT, the strain rate of the 20μm top surface layer can reach 681s-1. There involve two stages during SMAT: elastic-plastic deformation stage and dynamical recovery stage. Strain and strain rate play an important role in the grain refinement process and final stabilized grain size after SMAT.2 Detailed numerical analysis have validated that the anomalous response of the plate after SMAT is counterintuitive behavior, which could be considered as the result of coupling of material nonlinearity and geometry nonlinearity. The counterintuitive behavior shows intense to many factors, especially to the manner of the loads. Under the conditions offered by SMAT, the increase of process time should enhance the original kinetic energy of the plate. So that only with smaller load, the plate can obtain a certain deformation and adequate energy. Accordingly, the loading range can be enlarged and the counterintuitive behavior can be achieved more easily under current conditions.3 Equiaxed nanocrystallines with random crystallographic orientations were obtained in the 40μm surface layer of pure iron and Q235 low carbon steel by means of SMAT, the average grain size of the nanocrystallines ia approximates 30nm for Q235 and 15nm for pure iron. Based on the gradient variation of microstructure, the microstructure of the plastic deformed surface layer can be subdivided into four sections along depth from the top treated surface: nanostructure region,submicro-grained region, transition region and the base.4 After SMAT, the micro-hardness of the surface layer in the copper samples is evidently enhanced. The hardness decreases gradually with the increase in depth, and the micro-hardness of the surface layer is twice more than that of the original sample. The increased micro-hardness on the surface layer of nanocrystalline may be attributed to the refinement of grains and work-hardening.5 After SMAT, the corrosion property is downgrade for pure iron and that is upgrade for Q235 low carbon steel specimen. The main reason for the worsen corrosion resistance for the SMATed pure iron is that pure iron is with lower hardness and weaker competence of withstanding deformation, which cause finer grain size and much more micro-crack in the surface layer. Pure iron surface shows coarser than that of Q235 after SMAT. In the opposites, low carbon steel Q235 specimen is with higher hardness which induces the higher counter-distortion force during SMAT. The degree of deformation after SMAT is lower than pure iron. The grain size of nano-crystalline is larger than that of pure iron. The surface has been rolled and with little micro-cracks. Thus SMATed low carbon steel specimen exhibit better corrosion resistance.6 Compared with the original material, the SMATed pure iron specimen's elastic modulus decreases, yield strength and tensile strength increases while elongation decreases. After a short time of low temperature anneal treatment, the specimen's elastic modulus increased once more. The reason may be the decrease of the residual stress and density of defects grain boundary disorientation during anneal treatment. The main reason for the improvement of yield strength and tensile strength is that the surface grains have been refine to nano-grade during SMAT. Accompany SMAT, surface layer has been rolled, there exist remainder compress stress in the surface layer, which can effectively counteract the form of crack. At the same time, the surface nano-grains'layer can effectively block off the development of the slippages in the base metal and prevent the rigidification of the base metal.7 After Ti has been implanted into pure iron and Q235 low carbon steel, compared to untreated coarse-grained samples, implantation concentration of Ti in the samples after SMAT increases dramatically. The main reason maybe that the solid solubility of implanted element has been enhanced, which is caused by the interaction of the implanted elements and the defects produced during the SMAT process such as vacancies, grain boundaries and dislocations. And at the same time the ion diffusion coefficient has been enlarged after SMAT,which may contribute to the increase of implantation concentration. The implantation depth changes little. 8 The friction and wear properties of the pure iron and Q235 were improved and the friction coefficients clearly decreased by means of SMAT. The nanocrystalline layer may reduce the effect of fatigue wear.9 Nickel powders were added to the sample container during SMAT for a pure iron plate. After 100min, Ni powders were welded into the surface of iron plates and a homogeneous layer of Fe/Ni alloy with 100μm depth is formed on the surface of Fe base. Studies on the interface microstructure indicated that there was significant atomic diffusion and formation of an alloying layer during SMAT process. And the degree of alloying is enhanced after proper heat treatment process. A new method of surface alloying of metals has been developed.10 Surface nano-alloy reduces the degree of plastic removal and surface fatigue fracture under the moderate load respectively. The friction and wear properties of the pure iron have been improved by means of surface nano-alloy. With the same loading, the ZrO2 alloy layer shows the best wear resistance, the WC, C and Ni alloy layers'wear resistance show descend in turn, and higher than the SMATed pure iron. The improvement in friction and wear properties may be attributed to the harder surface alloy layer and NC surface layer which reduces the press depth, the moving resistance, the degree of plowing and micro-cutting during the friction and wear test.