Some quite harmful weld defects are due purely to incorrect technique and early recognition of their cause and effect can assist in establishing good practices.

Undercut

Cracks aside, undercut is usually considered as the worst defect. Undercut is the term given to a sharp narrow groove along the toe of the weld due to the scouring action of the arc removing the metal and not replacing it with weld metal. It reduces cross sectional area (and strength) but more importantly it provides a notch into the heat affected area of the joint which will act as a stress raiser and possible point of crack initiation. This defect is particularly detrimental in shafts and beams in rotating or flexing service, giving rise to fatigue failure. The causes are usually associated with incorrect electrode angles, incorrect weaving technique, excessive current and too fast travel speed.

Lack of Fusion

In this defect, weld metal lies adjacent to unfused base material or previous runs without admixture i.e. the two sections are not welded together. This is usually associated with the opposite situation which causes undercut in that too much molten metal is flowing within the joint area without sufficient direct arc action on the base metal beneath. Usual causes are too slow a rate of travel, incorrect electrode manipulation or current too low.

Slag Inclusions

Slag may be associated with undercut, incomplete penetration and lack of fusion in addition to its presence within a bead. Insufficient cleaning out of slag along an undercut toe of a multipass weld and incorrect electrode manipulation can leave pockets of slag and unfused sections along the weld joint. Excessive weaving and the use of too large an electrode in a narrow groove or too low amperage can also cause slag pockets. Slag inclusions not only reduce cross sectional area strength of the joint but may serve as an initiation point for serious cracking, particularly in the harder steels.

Incorrect Profile

This defect is one not only relating to appearance but also to overall strength of the joint. Excessive concavity results in insufficient throat thickness in relation to the nominated weld size. Excessive convexity results in poor weld contour which in multilayer welds can give rise to slag inclusions while in the finished weld it provides a poor stress pattern and a local notch effect at the toe of the weld. Selection of correct size and type of electrode with correct current and electrode manipulation will not give these defects.

Incomplete Penetration

The general purpose manual arc welding electrodes in common use are not noted for their penetration. Joints must therefore be prepared to permit full and proper access to the electrode and weld metal so as to achieve the full throat thickness of the weld. A butt weld or fillet weld where the weld metal does not penetrate to the root resulting in insufficient throat thickness suffers from incomplete penetration and reduced joint strength. Insufficient root gap, too great a land, too large an electrode, current too low or incorrect angle of electrode can all contribute to this complaint.

Cracks

Cracks can occur in both the base metal and the weld metal as a result of welding. Aspects of base metal and weld metal composition are very important as is also the need for low hydrogen electrodes to be dry. However incorrect technique can also be a cause either directly or indirectly. For instance too high a current producing excessive concavity will reduce throat thickness as will slag entrapment on the root of the weld due to too large an electrode, too slow a rate of travel or current too low. Insufficient throat thickness can lead to weld cracking in a shrinking weld and a restrained joint. Cracked tack welds ‑ too small for the job ‑can lead to cracked final welds if not removed.

Porosity

Porous welds may arise as a result of coating breakdown due to excessive current, excessive moisture pickup by the electrode (particularly low hydrogen types), and impurities absorbed from the base metal. Using wet electrodes is bad practice. A bake in the kitchen oven for an hour at 110°C (230°F) for general purpose types and 250°C (480°F) for low hydrogen types will improve the situation.

When metal is heated it expands in all directions and as it cools down it contracts. As it becomes hotter most common steels become softer and more easily worked, a factor we use when hot forging components to a required shape, bending etc.

These two factors, working together can result in warping or distortion away from the original or expected shape where the areas being heated are restrained from movement in one or more directions either by their own shape with uneven localised heating or by some externally applied force.

A simple workshop example is to take a section of metal, say, 75mm of 25mm x 3mm and clamp it lightly in the vice. Now apply the torch to the centre of the bar and heat until the centre section is a bright red to orange colour and allow to cool. The steel tries to expand but, restrained lengthwise by the jaws of the vice, it gives in the soft hot area and, on cooling, the natural contraction will result in the final length being shorter and the bar will fall from the jaws. Repeat the same experiment with several successive runs of arc welding across the centre of the bar.

Imagine the case where the ends of the steel bar were welded to each jaw of the vice first. The bar would then not be free to contract and on cooling it would be carrying an internal tensile pull, acting in the form of a stretched spring between the two jaws. Such locked up or residual stresses may in some structures gain such magnitude that they can seriously impair the load carrying capacity of a member. On the other hand, we use the same effect to advantage when we camber the longitudinal members of a tray body against the bending effect of the load and thus increase its load capacity.

It is important in welding and cutting operations to be aware of these factors and plan the placement of welds, preheats, the use of holding jigs etc., so that both distortion and locked up stresses are kept to a minimum. It will be realised that each successive bead of weld metal not only has a heating effect on the metal beneath but that in cooling from the molten state high contraction forces are present within the bead. The following are useful hints to minimise unwanted distortion or stresses:

1.   Wherever possible, particularly in low ductility materials like cast iron, have the components free to move and set up out of position so that the contraction pulls them into position. (see also page 10).

2.   Peening or hammering of the weld metal is a compressive action that will help balance out the tensile pull of a contracting weld. Again this is useful in cast iron or heavy butt welds that must be welded from one side only.

3.    Do not use any greater heat (amperage or electrode size) or volume of welding than is necessary. Over welding is expensive and adds to distortional problems. Often intermittent welds rather than a continuously welded seam are all that is required.

4.   Balance welding on both sides of the joint in a sequence that will have weld bead being deposited offsetting the distortional effect of the previous bead.

5.   In many instances it is desirable to clamp materials to substantial strong backs so as the overall dimensions are held during welding and cooling.

6.   Avoid excessive local heat buildup. Short runs and the use of back step techniques are two methods of reducing cumulative effects.

7.    Use the right joint preparation and avoid excessive gaps involving large widths of molten pools under cooling contraction.

8.     For lighter sheet tack more frequently to hold the plates in alignment.