Distortion and its Control in Welding of Stainless Steel Structures - Case Studies

Abstract

Distortion is a perennial problem faced by engineers engaged in welding fabrication. The shape change deformations and change in the dimensions that occur after welding is termed as distortion, leading to various undesirable consequences. So there exists a necessity to control distortion within limits. When distortion exceeds acceptable limits, correction of distortion after the complete fabrication results in major reworking operation that consumes both fabrication time and cost. Distortion control in complex structures has always been a challenge to fabrication engineers, dealing with stainless steels. To arrive at an appropriate control method, an in-depth analysis of the shape change deformation behavior of the component and adaptation of appropriate methods to control distortion are essential. In this study, two circular components made of stainless steel have been taken up for control of distortion. The first structure is the labyrinth used in hydro turbine assembly that consists of a ring type of flange made of martensitic stainless steel (ASTM grade A240 S41500) welded with a shell type web using grooved fillet welds. The size of the welded assembly is huge and the quantum of weld is very high leading to heavy distortion that often required to be corrected by way of weld build-up and subsequent machining. The second one is the vortex finder assembly which is a part of cyclone separator that is used to separate the fine particles of coal from coarse ones for use in the combustion chamber of typical Circulating Fluidised Bed Combustion (CFBC) boilers. This is again a cylindrical structure with three shells joined with three flanges and supported by stiffeners. The structure is made of special grade Austenitic stainless steel to withstand high temperature. In both the structures, most of joints are of Tee-joints with fillet welds. During fabrication of these assemblies, the angular tilt, local waviness and the diameter variation were the major issues. Correction after fabrication is not viable considering the configuration and shape of the component. This calls for control of distortion during fabrication itself. Towards this, a suitable welding procedure using an appropriate weld sequence was evolved after studying the weld details of the components. This weld sequence was implemented in-situ in actual components. During fabrication on-line monitoring was carried out and course corrections were provided. After implementation of the weld sequence, it was found that the distortion of the component could be reduced and contained well within the permissible limits. This made the extra rework redundant thus leading to a significant reduction in the cycle time of manufacture of the component. The details of the methodology adopted are described in this paper. Keywords: Labyrinth assembly, vortex finder assembly, stainless steel welding, distortion, T joint, grooved fillet weld, angular tilt, weld sequence.