In practice, we can face a problem — to understand the causes of the appearance of systematic defects in welded joints.
It is assumed that the welding technology (WPS) and welders have already passed certification, but when production starts defects appear suddenly and systematic.
What can be understood as «systematic»? Let’s say that this is when the proportion of joints with repairs welded over a short period of time (for example, a week, a shift depending on the number of joints) exceeds 20%.
Note: In industry, it is recommended to use control charts for statistical analysis. however, it is often enough to simply analyze the proportion of repair joints.
To facilitate the task of finding the root cause of the defects, I prepared a special mind map. Here, at the first level, groups of possible causes are presented, with the most likely to be selected from the font size, from which to start, and the orange color represents the actions that should be taken to confirm or disprove the hypothesis about the cause.
Note: You could submit it in the form of Ishikawa diagram, but I’m more used to using a mind map.
Such a diagram does not take into account, in fact, the types of defects, but only offers an exhaustive list of parameters and conditions that should be checked in order to clarify the root cause in the general case.
Also, the diagram does not describe the steps that should be taken to eliminate the found cause. For each case, this will be its own set of measures, for example, if errors are committed through the fault of the welder, then he will removed from the job and will be sent for re-certification.
I pay special attention to the fact that the cause of the emergence of systematic «defects» can be incorrectly performed non-destructive testing (NDT). So the problem can be in non-compliance with the requirements of the procedure, an error in the calibration, the subjective factor of operator reviewing the data.
In the case of automated ultrasound control, a good verification method may be:
* Sending the results of the inspection to a level III for review
* Repeated inspection of the seam with a defect
* Repeated inspection by rotating the scanning device 180 degrees
I hope this scheme can help a respected audience in practice, and I will be grateful for the comments and additions to this scheme.