Residual stress
The magnitude of stress that can be formed from welding can be roughly calculated using: ET
Where E is Young's modulus, is the coefficient of thermal expansion, and T is the temperature change. For steel this calculates out to be approximately 3.5 GPa (510,000 psi).
Arc strike cracking
Arc strike cracking occurs when the arc is struck but the spot is not welded. This occurs because the spot is heated above the materials upper critical temperature and then essentially quenched. This forms martensite, which is brittle, and micro-cracks. Usually the arc is struck in the weld groove so this type of crack does not occur, but if the arc is struck outside of the weld groove then it must be welded over to prevent the cracking. If this is not an option then the arc spot can be postheated, i.e., the area is heated with an oxy-acetylene torch, and then allowed to cool slowly.
Cold cracking
Residual stresses can reduce the strength of the base material, and can lead to catastrophic failure through cold cracking, as in the case of several of the Liberty ships. Cold cracking is limited to steels, and is associated with the formation of martensite as the weld cools. The cracking occurs in the heat-affected zone of the base material. To reduce the amount of distortion and residual stresses, the amount of heat input should be limited, and the welding sequence used should not be from one end directly to the other, but rather in segments.
Cold cracking only occurs when all the following preconditions are met:[citation needed]
susceptible microstructure (e.g. martensite)
hydrogen present in the microstructure (hydrogen embrittlement)
service temperature environment (normal atmospheric pressure): -100 to +100 F
high restraint
Eliminating any one of these will eliminate this condition.
Hot cracking
Hot cracking, also known as solidification cracking, can occur with all metals, and happens in the fusion zone of a weld. To diminish the probability of this type of cracking, excess material restraint should be avoided, and a proper filler material should be utilized. Other causes include too high welding current, poor joint design that does not diffuse heat, impurities (such as sulfur and phosphorus), preheating, speed is too fast, and long arcs.
Underbead crack
An underbead crack, also known as a heat-affected zone (HAZ) crack, is a crack that forms a short distance away from the fusion line; it occurs in low alloy and high alloy steel. The exact causes of this type of crack are not completely understood, but it is known that dissolved hydrogen must be present. The other factor that affects this type of crack is internal stresses resulting from: unequal contraction between the base metal and the weld metal, restraint of the base metal, stresses from the formation of martensite, and stresses from the precipitation of hydrogen out of the metal.
Longitudinal crack
Longitudinal cracks run along the length of a weld bead. There are three types: check cracks, root cracks, and full centerline cracks. Check cracks are visible from the surface and extend partially into weld. They are usually caused by high shrinkage stresses, especially on final passes, or by a hot cracking mechanism. Root cracks start at the root and extent part way into the weld. They are the most common type of longitudinal crack because of the small size of the first weld bead. If this type of crack is not addresses then it will usually propagate into subsequent weld passes, which is how full centerline cracks (a crack from the root to the surface) usually form.
Reheat cracking
Reheat cracking is a type of cracking that occurs in HSLA steels, particularly chromium, molybdenum and vanadium steels, during postheating. It is caused by the poor creep ductility of the heat affected zone. Any existing defects or notches aggravate crack formation. Things that help prevent reheat cracking include heat treating first with a low temperature soak and then with a rapid heating to high temperatures, grinding or peening the weld toes, and using a two layer welding technique to refine the HAZ grain structure.