RESIDUAL STRESS AND DISTORTION IN WELDING

TYPES OF DISTORTION:

Longitudinal
Transversal
Angular
Bowing& Dishing
Buckling

FACTORS EFFECTING DISTORTION:

Parent Metal Property: Co-efficient of thermal expansion & specific heat per unit volume has influence on distortion. As distortion is determined by expansion & contraction of material, so Co-efficient of thermal expansion plays important role.

RESTRAINT: If a component is welded without any external restraint, it distorts to relieve the welding stresses. So, using restraints distortion can be stopped but risk of HAZ cracking is possible.

JOINT-DESIGN: Both fillet & butt joints are prone to distortion. It can be minimized by adopting a balanced thermal stresses in plate thickness. Ex: using a double-v butt joint instead of single-v butt joint & welds are deposited at same time.

PART FIT-UP: Fit-up should be uniform to produce predicted shrinkage. The joint gap may increase if fit-up is poor and degree of distortion is increased with this. So the joints should be adequately tacked to prevent relative movement between the parts during welding.

WELDING PROCEDURE: Degree of distortion is mainly through effect of the heat input. As general the weld volume, welding sequence & technique should aim to balance the thermally induced stresses around the axis of the component.

DISTORTION PREVENTION:

PREEVENTION BY PRE-SETTING, PRE-BENDING & USE OF RESTRAINTS:

PRE-SETTING: The parts are preset and left free to move during welding. The amount of distortion is assumed before welding and parts are set according to the amount of assumed distortion. Unfortunately, as it is difficult to predict the amount of pre-setting required to accommodate shrinkage a number of trials is needed.

PRE-BENDING: Pre-bending or Pre-springing the parts before the welding is a technique used to pre-stress the assembly to counteract shrinkage during welding. Releasing the wedges after welding will allow parts to move back into alignment. Unfortunately, as it is difficult to predict the amount of pre-bending required to accommodate shrinkage a number of trials is needed.

USE OF RESTRAINT: Because of the difficulty in applying pre-setting & pre-bending restraints is the widely used and practiced technique. The basic principle is that all the components are held under restraints in correct position until completion of weld and use of balanced welding sequence the distortion is minimized.

DISTORTION PREVENTION BY DESIGN:

Using extrusion & bending methods instead of welding to minimize distortion.
Placing & balancing of weld is important to minimize distortion.
Reducing the amount of weld metal to minimize distortion
Reducing the number of weld runs by using continuous welding process like TIG to minimize distortion
Using balanced welding techniques.

DISTORTION PREVENTION BY FABRICATION PROCESS:

Tack welds are ideal for setting & maintaining the joint gap and used to resist transverse shrinkage.
By clamping two components back-to-back & welding of both components can be balanced around the neutral axis of the combined assembly to minimize distortion.
Longitudinal stiffeners in the form of flats or angles, welded along the each side of the seam are effective to minimize distortion.

DISTORTION PREVENTION BY WELDING PROCESS:

Use of back-step welding by depositing short adjacent weld length in the opposite direction to the general progression can be helpful in minimizing distortion.
Use of Skip welding by laying short weld lengths in a pre-determined, evenly spaced & sequence along the seam can be helpful in minimizing distortion.

DISTORTION CORRECTIVE TECHNIQUE:

Mechanical technique: Mechanical techniques principally are hammering & pressing the component till the component is assumed to be in its correct shape.

Thermal technique:

- Spot heating is used to remove Buckling Distortion.
- Line heating is used to correct Angular Distortion.
- Wedge shaped heating is used to straightening curved plates. For thicker section it may be necessary to use two torches one on each side of the plate.