Welding Defects & Distortion

Porosity - Holes/bubbles in the weld. This is created when gas becomes trapped in the weld pool as it solidifies. Causes include:

Lack of weld shielding gas - (Mig/Mag & Tig Welding) INCREASE GAS FLOW.

Too much weld shielding gas - (Mig/Mag & Tig Welding) If the flow rate, or pressure, of gas is too high turbulence is created over the weld. This turbulence allows air to penetrate the shield, this then means you do not have an adequate gas shield around the weld pool. REDUCE GAS FLOW.

Damp Welding Electrodes - (MMA). USE DRY WELDING ELECTRODES.

Oil/grease on the workpiece - As contaminates burn off they create excess gas, which gets trapped in the weld. CLEAN WORKPIECE before welding.

Surface coating on the workpiece - Paint or plating burning off will create excess gas which gets trapped in the weld. REMOVE SURFACE COATING before welding.

Tall Narrow Welds - Caused by a lack of heat input. INCREASE POWER

Flat Wide Welds - Caused by excessive heat input. REDUCE POWER

Undercut - Caused by excessive heat input. REDUCE WELDING POWER

Slag Inclusion - (Arc Welding only) Primary cause is slag from a previous weld not being fully removed before second weld pass is overlaid. Undercut will make slag removal more difficult and increase the risk of slag inclusion. REMOVE ALL SLAG PRIOR TO OVERWELDING.

Weld Distortion is caused by two factors, the general uneven heating of the workpiece that welding causes, and the weld metal itself shrinking. This later cause makes sense, when a weld is created you are producing a moulten pool of metal, like most materials, this hot metal will be in an expanded state. Thus, as it solidifies and cools, it contracts, or shrinks. As the weld metal cools and shrinks it pulls on the metal around it, creating stresses and thus distortion.

Welding Distortion can be corrected by stretching the weld metal to relieve the stress caused by shrinking. For butt and outside corner welds, this can be done by peening. Do this by holding a metal block behind the weld and peening evenly along the weld with a hammer. Be careful to avoid over peening as this will overstretch the weld and re-distort the workpiece, this cannot be corrected easily!!! So peen the whole weld a little, assess the effect, then if necessary, peen a little more.