焊接失效分析-确定焊接缺陷的根本原因

即使焊接缺陷的原因看起来很明显, careful forensic investigation reveals opportunities to save time and money. The case studies below illustrate how weld 失效分析 results can pinpoint vulnerabilities and areas for improvement in the welding process. 

焊接失效的电子显微镜分析

优质的冶金焊缝，可靠, 健壮的, 完全适合服役对人类安全绝对是至关重要的. 在航空航天领域尤其如此, 运输, and energy sectors where metallurgical weld failures often result in catastrophic events that are extremely costly; and, 更糟糕的是, 以夺人性命告终. The weld-metal itself; however, should not always be the very first thing to blame. Correctly and accurately identifying the root cause of weld failures requires an objective and thorough forensic investigation that almost always hinges on the application of electron microscopy (EM).

Below are a couple of detailed examples of forensic analyses where EM and optical workflows have been used to confirm that defective and/or contaminated weld material was the root cause of failure. In each case, the most likely pathway to the observed defects in the weld were also identifiable.

焊缝孔隙率对焊接失效的影响

CASE 1: Visually obvious and large voids within the weld metal (a) resulted in multiple origin fatigue fracture [progression lines and the coalescence of multiple fatigue cracks are indicated with yellow arrows in (a)]. This large series of void structures in the weld metal was initially thought to be formed because of contamination; and, lower spatial resolution imaging via backscattered electrons (b) did at first suggest this could be true {because of the apparent Z-contrast at the bottom of the voids}.

Moderate resolution imaging (c) at ~7500X coupled with energy dispersive spectroscopy (EDS); however, showed that contamination was not the case as many areas (blue triangles) showed mainly Fe/Mn being present that was known to be characteristic of the weld metal. This approach further revealed that the surface morphology at the base of large primary, 二次, 第三纪空洞是晶体学的, 在上雕琢平面的, 高度方向, and non-contaminated (yellow ellipses); thus confirming weld metal solidification occurred within void spaces that were filled with entrapped 气体.

检查 within these micro-voids (d) at even higher spatial resolution (~30,000X)进一步揭示了富铁纳米晶(i.e. not contaminated) weld metal had directionally solidified as “fingers” within the micro-voids; additionally confirming that high-velocity 气体 impingement most likely occurred during welding.

用良好的焊接规范防止焊接失效

CASE 2: The girth weld on a large diameter pipeline was fractured and the origin of the break was identified by tracing convergent steps and arrest lines to the location shown in (a). Stereo optical microscopy then revealed that the weld metal exhibited burn-through (black ellipse), and this confirmed that the field operator laid the weld bead too slowly thus allowing time for burn-through.

The optical inspection also showed that contamination was present at the origin of fracture. Looking at the origin of the break via SEM/EDS then confirmed that gross contamination was present, 这是由于Ti, Si, Ca, O, 和ai(蓝色三角形). Final confirmation that the observed contamination on the break surface was embedded within the weld metal was then obtained via cross-sectioning through the origin (c). 然后通过SEM/EDS进一步分析该位置(d).

Taking the extra step to analyze the cross-section (white diamond) via EM additionally revealed that iron oxide platelets were embedded within Ti/Si/Ca rich contamination. Taking the time for this additional level of analysis on the cross section not only confirmed contamination was in the weld, 但也那一片锈迹斑斑, 脏焊条很可能是在现场被抓的, 匆忙地清洗, 并用于制造环焊缝的其余部分.

This analysis case highlights the importance of good welding practice and how simple oversight of a single step can lead to a catastrophic release. 

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