TPT September 2014

Article

Caparo

Mismatch defects in ERW steel tubes By Abhishek Bhartim, engineer, Caparo Engineering India Limited and Sitaram Bainsla, intern, Caparo Engineering India Limited

1 Introduction ERW tubes are manufactured by joining the two edges of skelp. That the two edges remain parallel to each other while welding is crucial. The uneven joining of the edges amounts to “mismatch defect”. Most tube mills employ scarfing tools to remove the extra material squeezed out from the outer portion as well as from the inner portion. If mismatch defect is present in the tubes, this will subsequently lead to significant thinning of the tubes in the weld area, particularly, where the wall thickness of the tube is smaller, say 0.9mm. The thinning is not just because of the material removal by scarfing tools, but it may be present because of other processing steps like corrugation or bending. This may pose serious implications if a mismatch defect is also present. When this defect is present in the tube, it will show a tendency towards cracking or splitting open when subjected to external pressure like inward flanging or spinning (shock absorbers), because of the mismatch defect. A lot of analysis has been done to address this issue of weld failure in electric resistance welded (ERW) tubes, and an assortment of issues and root causes has been listed. We, in our capacity, have tried to study the same problem. In the present study, our focus is on “mismatch defect”, which we believe is the prime cause behind such failures. Herein, we are discussing the defect, where it is emanating from, and how is it going to affect the weld quality or weld integrity in the ERW tubes. These days the supply chain is increasingly demanding zero defect supplies. Everyone among tier 1 and tier 2 suppliers in the automotive sector is endeavouring towards producing defect free material. This, in the case of welded mild steel tubes, has proved to be a chimera. Despite every effort directed towards the best manufacturing systems and with best technology available, it is often seen that the tubes somehow fail the customer’s needs, particularly in cases where there is severe cold work being done on the tubes, like inward flanging for reserve tubes (shock absorbers), or drifting (in case of fuel filler tubes).

Mismatch effect

This defect is quite elusive – ie it is very difficult to detect in daily operation as, for example, at x = 0mm you may find a sound weld, but at x = 5mm mismatch defect might be there to seriously and adversely affect the integrity of weld. This attribute of the tube makes it susceptible to failures at the customer’s end. 2 Possible reasons of mismatch Here we attempt to list the various reasons that might be attributed to the mismatch defect: • Slit handling/improper slitting • Work done by the break down passes • Improper design for extremely light or heavy gauges • Mill alignment • Position of guide rolls • Storage in the accumulator • Forces applied by the squeeze rolls 2.1 Slit handling/improper slitting – Improper slit handling or cutting leads to edge damage in the skelp. The damaged edge while welding cannot remain parallel to the other edge in the tube mill at welding stage. This subsequently leads to mismatch defect in the tubes.

Edge damage due to handling

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September 2014