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冷拔钢管表面缺陷形成的实验结果及分析

2018-11-28 08:42:20 常州市星火铝艺制品有限公司 阅读

冷拔钢管表面缺陷的形成有两种可能性:一种是材料本身在变形过程中塑性不够,导致裂纹与外折形成;另一种是材料表面氧化引起表面缺陷,表面缺陷在变形过程中放大成为裂纹与外折。实验一:热模拟拉伸实验结果及分析.为了研究材料高温塑性,进行了一系列热模拟拉伸实验。可以发现900-1 200℃为9Ni钢的高塑性区,其拉伸变形量可达90%以上。对比轧管各个阶段的变形量与变形温度,不难发现穿孔与斜轧两个步骤都在高塑性区,且变形量远小于材料的变形能力。定径步骤最后阶段温度虽然低于900℃,但是前面的分析已经表明,管体外表而的缺陷形成在定径之前。因此可以认为,本次轧制中出现的小外折与裂纹不是由于材料本身塑性不佳引起的。

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.

实验二:高温氧化实验结果及分析

Experiment 2: Experimental results and analysis of high temperature oxidation

在 100℃经不同时间氧化样品的形貌。

The morphology of the samples was oxidized at 100 C for different time.

可见,虽然为氧化样品表面光滑,但是1h后氧化层与金属界面之间就出现了细小的晶界氧化,见图4(b)。随着氧化时间延长,晶界氧化深度进一步加深,见图4(c).(d)。此时晶界氧化速度大于氧化层相金属内推进速度。当晶界氧化深度达到一定程度以后,随着氧化时间延长,氧化层厚度进一步增加,但是晶界氧化深度不再进一步加大,见图4(e)。可见此时晶界氧化及氧化层相金属内部推进的速度达到了平衡。

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.

这一结果显示,在高温保温条件下,冷拔钢管外表面由晶界氧化导致的脆性表面及微裂纹一直存在。这样的表面在无缝不锈钢管钣金加工的变形过程中势必会引起表面缺陷。

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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