There are two possibilities for the formation of surface defects of cold-drawn steel pipes: one is that the material itself is not plasticity enough in the deformation process, leading to the formation of cracks and folds; the other is that surface defects are caused by oxidation of the material surface, and surface defects magnify into cracks and folds in the deformation process. Experiments 1: Thermal simulation tensile test results and analysis. In order to study the high temperature plasticity of materials, a series of thermal simulation tensile experiments were carried out.
It can be found that the high plasticity zone of 9Ni steel at 900-1200 C can reach 90% of its tensile deformation. It is not difficult to find that both piercing and cross rolling steps are in the high plastic zone by comparing the deformation amount and deformation temperature at each stage of pipe rolling, and the deformation amount is much less than the deformation capacity of the material. Although the final stage temperature of the sizing step is below 900 C, the previous analysis has shown that the defects on the outer surface of the tube are formed before sizing. Therefore, it can be considered that the small external bending and cracks in this rolling process are not caused by the poor plasticity of the material itself.
Experiment 2: Experimental results and analysis of high temperature oxidation
The morphology of the samples was oxidized at 100 C for different time.
It can be seen that although the surface of the oxide sample is smooth, fine grain boundary oxidation occurs between the oxide layer and the metal interface after one hour, as shown in Figure 4 (b). With the prolongation of oxidation time, the oxidation depth of grain boundary is further deepened, as shown in Figure 4 (c). (d). At this time, the oxidation rate of grain boundary is greater than the propulsive rate in the oxide phase. When the oxidation depth of grain boundary reaches a certain level, the oxide thickness increases further with the prolongation of oxidation time, but the oxidation depth of grain boundary does not increase further, as shown in Figure 4 (e). It can be seen that the speed of grain boundary oxidation and internal propulsion of oxide phase metals reached equilibrium.
The results show that the brittle surface and micro-cracks on the outer surface of cold-drawn steel tube caused by grain boundary oxidation always exist under high temperature and heat preservation conditions. Such a surface will inevitably cause surface defects in the deformation process of seamless stainless steel pipe processing.
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