After a brief look at the welding on the hitch of my father's truck recently, I was struck by his apparent lack of concern over the quality of his welding. In fact it was plain he was not only happy with it, but proud of it. My evaluation, though gentle, had to be tough: It was full of defects. It was bad, very bad. If it had not also been well bolted on, I would have had to stop him from pulling anything another mile. Even though my father has been dabbling with welding for some time, his novice approach made me shrink to think of the thousands of others like him that are out there. Weld defects can cost much money, injuries and lives every year.
Experienced and novice welders alike are guilty of making welds full of defects. Defects are not always visible as “Ugly” welds. There are several classes of weld defects that the weldor should be aware of while welding. It should be pointed out that most welds contain some slight defect or another, but the slight defect or a minor amount of them do not constitute a defective weld. TIG, MIG, or Stick welding, whether welding Steel, Aluminum, or virtually any metal, will produce serious defects if not performed properly. The following are several main types of weld defects encountered regularly in nearly every weld.
1. Metal grain growth. When metal is unevenly heated or there is a large difference in the weld puddle temperature and the surrounding metal that is not visible to the naked eye. This area is known as the heat affected zone. Large “grains” of metal form, making a weak spot in the metal. Growth of the metal grain can be controlled by proper preheating and slow cooling. Particularly if it is high carbon steel improper heating and fast cooling can leave the weld hard and brittle from the grain size increase in the HAZ resulting in weld failure.
2. Blowholes and Inclusions. Sometimes due to improper electrode or torch manipulation, large gas blowholes or slag inclusions will be trapped in the metal when the weld cools too fast after the metal has been deposited. Both blowholes and inclusions introduce areas of potential failure where cracks can form under stress. Careful attention to travel speed, and rod or torch height/angle can reduce the chance of inclusions and blowholes. Proper shielding gas coverage can help in the case of MIG or TIG welding. Post flow cooling will help TIG welders from having blowholes at the end of the weld as well.
3. Porosity. A large number of small gas pockets creates a condition of porosity. This is often a result of poor shielding from the atmosphere, either through poor slag coverage of the weld while Stick welding or improper shielding gas coverage, either through turbulence, improper gas selection, or gas contamination.