Residual Stresses and Distortion After Welding

A precise understanding of these thermal mechanisms makes it possible to develop effective preventive measures from the initial design and assembly stages.

How Can Welding Distortion Be Prevented?

From an economic perspective, preventing welding distortion during the design and assembly stages is significantly more cost-effective than carrying out expensive repairs afterward.

Optimizing Design, Fit-Up, and Weld Metal Volume

To limit thermal movement, the primary rule is to minimize the amount of deposited weld metal and strategically position the joints. Eliminating unnecessary welded connections is the first line of defense. Using bent plates or rolled sections can eliminate numerous weld beads and remove the very source of distortion. When welding is essential, groove geometry plays a decisive role. On thick components, a symmetrical double-V (X) groove reduces the volume of deposited weld metal by half compared with a single-V groove. This joint preparation balances shrinkage forces on either side of the neutral axis. In addition, overwelding must be avoided. Excessive fillet weld throat thickness does not increase the required mechanical strength but unnecessarily increases distortion. To anticipate the behavior of complex structures before fabrication, engineers use numerical welding simulation. Finally, the accuracy of the initial fit-up determines the final result. Using professional welding tables ensures a perfectly flat reference surface. For pipe welding, using pipe alignment clamps ensures a consistent root gap and prevents variations in heat input.

Controlling Tack Welding, Restraint, Heat Input, and Welding Sequence

Applying practical workshop techniques helps distribute heat input and limit movement during welding. Alternating welding sequences should be prioritized to balance forces on both sides of the neutral axis. The back-step welding technique involves depositing short weld beads in the direction opposite to the overall progression of the joint. This method divides the heating process into smaller sections and reduces overall longitudinal shrinkage. Angular presetting also provides an effective solution. It involves positioning components at an initial angle opposite to the expected distortion before welding, allowing shrinkage to bring the structure into the desired geometry. Mechanical restraint is another commonly used method. Using clamps, fixtures, or stiffeners prevents geometric movement of the assembly. However, restraint limits external distortion at the expense of increased internal residual stresses. Reducing heat input offers an additional approach. The use of pulsed MIG/MAG welding provides precise control over metal transfer and reduces shrinkage. To securely hold components in place before assembly, the technical team must understand how to use welding clamps. Installing the complete assembly on a welding table with an S355 steel tabletop provides the rigidity needed to hold components securely. Nevertheless, reducing heat input must never compromise weld penetration, fusion, or compliance with the qualified Welding Procedure Specification (WPS).

Even when all preventive measures are rigorously applied, a final dimensional inspection remains essential to confirm that the component meets the required specifications.

How to Inspect and Correct a Component After Welding?

Geometric quality control ensures manufacturing compliance, while correcting any deviations requires the application of strictly qualified procedures.

Measuring Geometry and Determining Acceptability

Component inspection must establish a fundamental distinction between measuring external dimensional deviations and evaluating internal residual stresses. Dimensional inspection is performed using geometric measuring instruments, straightedges, or angle gauges. The measured values are compared with the tolerances specified in the fabrication drawings or applicable construction code. ISO 13920 establishes general tolerances for welded structures. However, this standard applies only when explicitly referenced in the engineering drawings or contract. Information to be verified or validated before publication.

Choosing Between Mechanical Straightening, Thermal Straightening, and Heat Treatment

When geometric tolerances are exceeded, the workshop must choose between mechanical straightening, thermal straightening, and stress-relief heat treatment.

Mechanical straightening involves applying an external force using a press or hydraulic jacks. The principle consists of deforming the component in the opposite direction beyond its yield strength. This plastic deformation compensates for elastic recovery (spring-back). This method carries risks of work hardening, surface indentation, and crack initiation if excessive force is applied.

Thermal straightening (also known as heat shrinking) relies on creating a localized heated zone. The hot metal attempts to expand but is restrained by the surrounding cold metal. As a result, it undergoes plastic deformation under compression. During cooling, the heated zone contracts and generates a tensile force that pulls the component back toward its intended geometry. Heating can be applied using spot, line, or wedge-shaped patterns. Wedge heating is suitable for structural sections and thick assemblies. The length of the wedge should equal two-thirds of the plate width. The base of the triangle should measure one-sixth of this length. The operator must heat from the apex toward the base while ensuring uniform heat distribution through the material thickness. For carbon-manganese steel, the heating temperature must be strictly limited to between 600°C and 650°C (dark red heat). Exceeding 650°C may alter the metallurgical structure and affect the material's mechanical properties.

Post-weld heat treatment (PWHT), also known as stress-relief heat treatment, involves heating the entire structure in a temperature-controlled furnace. This process reduces the level of internal residual stresses. However, while heat treatment reduces internal stresses, it does not automatically straighten a distorted component.