Usage:
Repairing Cracks and Defective Welds
Cracks often develop in components that are used for unusually severe applications or are approaching the limit of their service life. Defective welds, on the other hand, are usually caused by careless welders who fail to use good welding techniques. The most common defects in welds are: underbead cracks, slag inclusions, porosity (gas holes and pockets), cold lap, undercut and slugged welds. The procedures for repairing cracks and replacing defective welds are similar.
Figure 47 - Underbead cracks are caused by hydrogen gas being absorbed by the seal.
Underbead cracks occur between the weld deposit and the parent metal or in the heat affected zone under the weld deposit in carbon steels (Figure 47). The cracks are usually caused by hydrogen gas being absorbed by the parent metal and being released as the metal cools. The gas builds up pressure in the minute voids of the parent metal and forces the metal to crack. Underbead cracking is most common with steels having a carbon content in excess of 0.30%, but it often occurs when low carbon steels are welded in ambient temperatures below 32° F without preheating to remove moisture.
Underbead cracks are not visible from the surface of the joint and can be detected only by X-ray or magnetic particle inspection. Using hydrogen electrodes that have been stored properly to keep them dry will eliminate most underbead cracking.
Slag inclusion refers to entrapment of slag within the weld metal (Figure 48). The inclusions arise mainly from failure to clean the weld deposit before the next pass is made. Too slow a travel speed also causes slag inclusions, because the volume of slag becomes too large to be floated out of the puddle of molten metal.
Figure 48 - Slag inclusions arise from failure to clean the weld deposit.
Porosity usually is caused by the oxidation of foreign matter such as grease or paint that has been left on the joint (Figure 49). The foreign matter is vaporized by the molten metal and forms gas pockets in the weld deposit. Porosity can also be caused by excessive moisture in any type of electrode coating or by excessive magnetic arc blow. Careful cleaning of joints prior to welding, using dry electrodes and controlling arc blow will prevent porosity.
Figure 49 - Porosity is caused by oxidation of foreign matter on the joint.
Cold lap is caused by improper fusion of the weld metal to the parent metal, arising from insufficient heating of the joint members (Figure 50). Instead of bonding to the parent metal, the weld deposit lies on top of it, giving rise to a definite crack or void. Cold lap can be prevented by proper manipulation of the electrode to bring all parts of the joint up to a molten state. A slightly longer arc or slightly higher welding current will also help to combat the problem of cold lap by increasing the amount of heat liberated at the joint.
Figure 50 - Cold lap is improper fusion of parent metal and weld.
An undercut is a groove melted in the parent metal adjacent to the toe of a weld that is not filled with weld metal (Figure 51). Undercutting occurs most often in other than flat positions and is caused by excessive heat in the joint, coupled with failure to wash the weld metal up onto the vertical plate. It can be prevented by reducing the welding current or shortening the arc and by manipulating the electrode properly. Undercuts can be remedies by making another pass on the joint to fill the groove in the parent metal with weld metal.
Figure 51- Undercuts will arise from excessive heat in the joint.
A slugged weld is a weld made in a joint with a wide gap that has small pieces of steel placed in the gap to fill up space (Figure 52). The external appearance of a slugged weld may be perfect, but the joint is weakened considerably because of improper fusion of the slug to the parent metal and poor penetration of the weld metal. Besides being a shoddy practice, weld slugging contributes nothing at all to the welding speed or electrode economy. If a back-up plate is used with a wide gap joint (Figure 53), a larger electrode can be used and the joint can be welded properly in less time than it can be if slugs are added. Slugging a weld is an intolerably poor welding technique and should never be employed in any type of service welding.
Figure 52- A slugged weld has small pieces of steel in the gap to fill up space.
Figure 53- A back-up plate will allow a wide gap joint to be welded properly.
Cracks and defective welds (except undercuts) must be removed completely, and the resulting gap must be filled with a sound weld deposit in order to restore a component to its original strength. Any portion of a crack that is left will initiate further cracking in the new weld. The best way to remove a crack or a defective weld is to grind it out with a portable grinder or gouge it out with an Arcair torch. The grinding or gouging must extend at least an inch past the visible ends of a crack in order to remove all traces of the crack and any fatigued metal.