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Welding Repairs That Last

Making your arc welds permanent

By Graham Andrews

Published in Town and Country Farmer

Welding equipment is now an important part of many home workshops. It can be used to build a box trailer, make items for the garden such as trellises, archways and wrought ironwork, and mend broken metal items around the home. A weld should be at least as strong as the item being welded, but when a weld fails, the homeowner is left wondering 'why?' At best, weak welds are unsightly, but at worst they can be dangerous, even lethal.

A very common mistake is to weld metal that is too thin to take manual arc welding. Thin metal is easily burned through; the blade of a shovel, for example, is probably too thin to weld with a standard electric welder. Brazing, using oxyacetylene equipment, is a better method.

However, there are many other causes of bad welds and understanding these causes should allow you to minimise or eliminate defective welds. Most failures can be attributed to:

Preparation

For strength, the weld metal must penetrate the full thickness of the parent metal. Incomplete weld metal penetration may not be apparent until after the repair fails.

The main causes of poor penetration are:

Although the neatest and most logical way to repair the item may appear to be to push the broken parts tightly against one another, this usually results in weld metal only being deposited on the surface. If the excess is ground down afterwards, only the thinnest section of weld metal is left to hold the pieces together.

As a general rule, if the parts being repaired are less than three millimetres thick, leaving a space between the two parts is essential. The gap should be about the thickness of the metal.

If the metal is more than three millimetres thick, bevel both edges to an angle of 45º.

Intrusions

The weld is weakened in proportion to the amount of impurities present. Impurities can take the form of paint, metal primer, zinc or zinc oxide from galvanising or galvanic coatings applied to prevent corrosion, rust, oils, dirt or slag from a previous run of weld metal. It takes only a few minutes to clean the surfaces adequately with a solvent or remove rust with a wire brush, steel wool or emery paper. If the rust is severe, grinding the surface with an angle grinder will remove the problem quickly. Grease can be removed with suitable non-greasy solvents such as turpentine and the surface wiped clean and dry.

Dampness and moisture

Electrodes should be stored in a dry place and should be handled carefully to avoid damage to their flux coating. Arc welding electrodes consist of a metal core surrounded by a coating of flux. If the coating is damaged, striking the arc can be difficult or impossible, If the electrodes are damp, dry them out for about an hour in an oven set on 110 - 120°C. Using damp electrodes can result in a fiery arc, with the weld metal laid down spasmodically and scattered, rather than deposited in a neat row.

On items, such as garden furniture, that have been outside for a long time, water can be trapped in the tubing or pipes, or on the back of the seam being re-welded. Steam, generated by the heat, becomes trapped as the weld metal is deposited and small bubbles of water vapour appear in the molten metal, weakening the joint and possibly allowing corrosion.

Electrodes

Using the correct electrode is essential for all welding tasks.

There are now so many types available that there is virtually an electrode for every possible application.

General purpose electrodes, those rated E6012 or E6013, are useful for welding mild steels in most situations. However, some electrodes are designed for overhead use, others for downward welding. Even with general purpose electrodes, using an electrode, that is intended for downward welding, in an overhead position can result in poor deposition of weld metal and a weld that is liable to break.

Electric current

Poor contact between the earthing clamp and the work results in insufficient current available to melt and carry the weld metal.

The arc will be intermittent, with the weld metal laid down in small blobs instead of a neat straight row. The earthing clamp can look tight, but if there is any grit under it, poor electrical contact will be prevented. Clean the metal to remove all the grime, grit and other insulating matter that could prevent proper electrical contact.

The smaller welders found in home workshops usually have finger-grip heads, but with use, these can work loose. Always check that they are secured tight before welding.

Amperage

Set the welder on an amperage that is appropriate to the electrode and the joint to be welded.

Electrode packets usually have the recommended amperage printed on them. These will give a high and a low reading, the low end of the scale being for thin material, the higher setting for thicker metal. Too little current will be evident by frequent 'sticking' of the electrode to the metal.

Too long an arc, i.e. the electrode is held too far from the surface, will produce a lot of heat, which can burn through the metal, particularly on thinner sections.

The long arc is characterised by excess splatter and a louder crackling than is usually produced by an arc of the correct length. An arc that is too short will become obvious when the tip of the electrode becomes buried in the molten weld metal and slag.

Rate of travel

The speed at which the weld metal is deposited determines its quality. If the rate of travel is too fast, the weld metal will be thin and spindly, and perhaps intermittent.

If the rate of travel is too slow, the pool of weld metal will deposit an excess of weld metal.

Undercutting

The presence of undercutting weakens the weld. The bead of weld metal should slightly bulge above the level of the base metal.

When undercutting has occurred, the edge of the bead of weld metal will be slightly below the level of the base metal and will appear to have cut into it.

The causes of undercutting are:

Distortion

Because of the high temperatures involved in welding, the base steel will expand and can easily twist out of shape. Such distortion not only leads to unsightly welds, but the weld metal can crack as everything cools or the base metal can remain permanently distorted after cooling.

If the crack or join to be welded is long, deposit the weld metal in short bursts (about 25-50 mm at a time worked in different directions and, where possible, from different sides) rather than a continuous run. This will help to minimise overheating and consequent buckling.

Lack of fusion

Lack of fusion occurs when the weld metal fails to fuse, or adhere, to the base metal and, in accordance with Murphy's Law, usually happens at the worst possible time. It is caused by insufficient attention being taken to direct the weld metal or welding electrode, towards the base metal. This can be overcome by using the correct rate of travel, the correct amperage, correct electrode, and the operator keeping his or her mind on the job!

Cracking

Cracks in welds have several possible causes, such as:

Even tack welds can crack if they are too small. Tack welds about 10 mm long are ideal.

Another common cause of cracked welds is cooling the metal too quickly after the welding has been completed. It is always tempting to use cold water to cool the steel down, but the weld should be allowed to cool slowly to avoid distortion and cracking.

Repairing cast iron

Cast iron is a splendid material with unique properties. Its flow characteristics when molten enable it to be cast into many useful items, such as car engine blocks and water pumps, and outdoor furniture such as tables and benches. However, cast iron can break if handled too roughly and though it can be welded, the methods are different to welding mild steel.

To repair cast iron, use an electrode that is manufactured specifically for welding cast iron. The usual preparations that apply to most other welding repairs are also required here: cleaning, removal of paint and, particularly for automotive parts, removal of all traces of grease, oil and water. To remove moisture, use mild but even heat.

Cracks in cast iron creep with stress, such as heat distortion, and it is not impossible for the crack to grow faster at one end than you can weld at the other. To overcome this, drill a small hole at each end of the crack before welding commences.

Since the carbon content of cast iron is so high, improper welding results in carbides of iron that are extremely hard. If the repair has to be machined or ground afterwards, these carbides will be a problem. The correct method for welding cast iron is to melt the weld metal, but NOT the cast iron. Use a short arc, but the electrode shouldn't touch the base metal.

Deposit the weld metal in stages and allow the cast iron to cool between each short run to keep the temperature and the stresses to a minimum. After welding, allow the cast iron to cool VERY slowly. Don't cool it with water or even leave it in a strong cold wind.

Finishing off

The completed weld should be at least as strong as the original metal and should be finished off to a professional appearance to minimise any possibility of corrosion or cracking. Grind off the excess weld metal, apply a metal primer to prevent rust forming and paint the surface.

If you have taken care to avoid the problems listed above, your welds should last indefinitely.