埋弧焊

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焊缝裂纹的识别

裂缝可能是所有焊接缺陷中最糟糕的,可以在焊接关节中找到,因为它们可以开始小但随着时间的推移而生长。裂缝可以在几乎所有类型的焊接,焊料或钎焊接接头中找到,并BOB体育平台下载网址且实际上每个位置包括粘合区域,HAZ(热影响区域)和母金属。有多种工具可用于检测包括非破坏性技术的裂缝,例如超声波,X射线和CT扫描;但这些技术并不总是适用于特定的关节设计,或者对于频繁使用非常昂贵。用于检测裂缝的最常用的技术是焊缝的金相评估。但这种技术有自己的问题,这使得难以检测到这一时事通讯的重点的原因难以检测。首先是样品制备过程的效果,其可以将材料涂抹到裂缝中。涂抹是一种常见问题,特别是在铜和铝等软合金中;软合金抛光需要实践,并不是一种琐碎的操作。软合金抛光确实需要用环氧树脂安装样品,使得它们可以在抛光期间保持在固定平面上; polishing has to be performed in gradual decreasing increments of sandpapers grades and then with colloidal ceramic powders such as alumina and diamond. Careful polishing is essential to ensure there is no smearing on the surface where material can get pushed around and ends up smothering the crack. The second reason it is difficult to detect cracks as their appearance is not always like a straight line as is show in schematics but can meander around grain boundaries making it difficult to differentiate between the two. In the absence of any stress trying to open up the crack, the crack width can be very fine and sometimes indistinguishable from grain boundary. The third reason cracks are difficult to identify is that the metallographic section is a 2-dimensional cut, while the actual crack may have 3- dimensional profile which is only partially captured the plane of sectioning. If you suspect that the crack is bigger than it appears, one option is to keep polishing the section down and track the crack along the z-direction. Cracks can sometimes have path or plane which is aligned to a specific orientation as relates to the joint or applied stress and perhaps you may have to section multiple samples along different planes to get a good understanding on the crack morphology. The final and often the main reason for difficulty in identifying cracks is due to the lack of sufficient contrast in metallographic sections that are often etched before review. Since cracks will often resemble features in a metallograph such as a long grain boundary or a boundary between two phases, some of the finer cracks may be missed altogether. Hence, I recommend that metallographic sections be photographed twice; first after fine polishing, and then, after etching. A photograph of an unetched but finely polished section will provide sufficient contrast for positive identification of a crack. Comparison of the un-etched and etched photographs will help identify location and potential reasons for crack formation

上面的图1显示了一个弧焊前(左)和后(右)蚀刻。裂纹在未蚀刻的部分很明显,但在蚀刻的部分不太明显。在这种情况下,裂纹是一条显示缺乏熔合的线。

以上图2为类似情况下的激光焊接断面,在左侧未腐蚀断面裂纹更明显,而在右侧腐蚀断面裂纹不明显。

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