technical case study
Fixtures should therefore support a sound welding strategy rather than compensate for over-welding, poor fit-up or an unbalanced sequence. On thin sheet, water-cooled or heat-sink fixtures can combine restraint with rapid heat removal. These systems require careful application to avoid excessively rapid cooling or adverse effects on weld quality and metallurgy. Control heat without compromising the procedure Distortion control is closely linked to heat input, but “less heat” does not simply mean welding as quickly as possible. Current, voltage, travel speed, electrode size, deposition rate and welding position must remain within the qualified procedure. Positioners can increase flat-position welding, allowing efficient deposition and consistent travel. The objective is to deposit the required weld metal efficiently without overheating a large surrounding area. Peening may be permitted in selected applications to counter weld contraction, but it requires engineering approval and must comply with the applicable procedure. Root and final passes are generally excluded because of cracking and inspection concerns. Thermal stress relief may reduce residual stresses in suitable weldments, but stainless steel grade, component design and service requirements must be considered before any heat treatment is specified. The workshop control plan Before striking the arc, the fabrication team should confirm that it has: • Avoided over-welding and minimised groove volume • Achieved consistent fit-up and root gaps • Used intermittent welds where permitted • Balanced welds around the neutral axis • Planned an alternating, symmetrical welding sequence • Limited the number of passes • Selected a qualified, controlled heat-input procedure • Used positioners where these improve welding efficiency • Provided appropriate clamps, fixtures or strongbacks • Preset components for predictable shrinkage • Planned subassembly and final-assembly sequencing • Allowed the fabrication to cool sufficiently before releasing restraint Distortion cannot be eliminated entirely, but it can be controlled. In the panel example, reducing weld volume, alternating the stiffener welds, presetting the plate and maintaining restraint during cooling prevents individual shrinkage forces from accumulating in one direction. The result is a straighter assembly, more accurate connections and substantially less corrective work. The central lesson is simple: distortion control belongs in the welding plan, not in the repair bay.
of movement is known, the components can be preset, pre-bent or pre-sprung in the opposite direction. Welding shrinkage is then used to pull the assembly towards its required final position. The correct preset may be determined from previous production data, a representative trial weld or procedure development. For repeat fabrication, recording the actual movement of the first unit enables the workshop to refine the preset and sequence for subsequent assemblies. Identical weldments may also be clamped back to back so their shrinkage forces oppose one another. They should remain restrained until they have cooled sufficiently to minimise movement when the clamps are released. Restraint is not a complete solution Clamps, fixtures, jigs and strongbacks help maintain fit-up and alignment during welding. Strongbacks are particularly useful on butt-welded plate, where clips and wedges can hold plate edges in position throughout the welding cycle. However, restraint does not remove shrinkage. It can convert visible movement into residual stress. Some movement may occur after the restraint is released, particularly if the component remains hot.
19
Issue 3 – 2026
Made with FlippingBook - professional solution for displaying marketing and sales documents online