Weld porosity is expected and can be controlled. Normal locations where porosity can be found are at the starts and stops of a weld pass. Therefore, when trying to develop full strength in a confined space repair, it is important to keep the starts and stops away from the final area that will carry the load.
The picture at the left shows a weld repair on a rotary kiln stiffening ring. The crack went all the way through the stiffening ring and travelled into the thicker shell section directly underneath the ring. It spread outwards from there. One might argue that there is a lot of extra wasted weld metal here, and there is a bit extra ... ok ... A LOT EXTRA (maybe more like overkill). However, the priority was on keeping the starts and stops far away from the final ground down shape.
The final ground shape is shown in the picture to the left (red circle). This is a different crack repair that did not go into the shell as in the upper left picture.
Because it is important to develop the strength in every portion of the compact stiffening ring, porosity cannot be tolerated. If all of the welding starts and stops were located along the actual perimeter of the ring it would be considerably weaker. Porosity acts as tiny stress risers and devalues the overall strength that would normally be achieved if the cross section was of high quality metal.
The other part to this repair is highlighted in the article "Machine Design - Case No. 87: The Wrong Way of Reinforcing the Shell on a Rotary Kiln."
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