Existing flaws or notches exacerbate fracture development. Heat treatment with a low temperature soak followed by quick heating to high temperatures, grinding or peening the weld toes, and utilizing a two-layer welding process to improve the HAZ grain structure are all things that can assist avoid reheat cracking. Preheating of the metal before welding starts will also help reduce stress during solidification.
The best way to prevent crack formation during welding is to select proper welding procedures. Avoid heat-affected zones (HAZs) by welding over large areas instead of in small spots. Also, be sure to clean and preheat your workpiece before welding begins to remove surface impurities that may become incorporated into the HAZ. Finally, use caution not to burn through or melt the base material when using oxyacetylene or plasma cutting/soldering techniques, as this will cause reheat cracking.
If you do encounter reheat cracking during welding, the solution is quite simple: stop welding immediately and allow the crack to cool before proceeding. There is no need to repair reheat cracks because they will always reappear if welding is continued.
Welding is a very effective method for joining metals together because it produces a joint that is strong enough to carry most forms of mechanical loading. However, like any other technique, welding can be used inappropriately and this may lead to failure.
How can you put a halt to hot cracks?
Hot cracking occurs as a result of weld metal embrittlement or the partly melted zone (PMZ) cooling between the liquidus and solidus temperatures or just below the solidus (Fig. 4). Hot cracking can also occur during welding if the filler material is softer than the base metal (for example, stainless steel filler material used to fill holes or cracks in carbon steel parts). The presence of hot cracks indicates that there is some degree of weld metal embrittlement. Embrittlement occurs when the amount of solute in the weld puddle exceeds a certain threshold level. As more solute is added to the molten pool, the viscosity increases, which leads to a slow-down in heat transfer from the melting point to the surrounding atmosphere and substrate. This causes the weld puddle to heat up slowly, which results in excessive heating of the metal near the surface.
The main factors that determine whether or not hot cracking will occur are the type and amount of impurities present in the weld metal. For example, if the weld metal contains large amounts of solutes such as zinc or magnesium, then it is likely that hot cracking will occur. On the other hand, if the weld metal has little to no impurity content, then this would reduce the chance of hot cracking occurring.
To avoid cold cracking, consider pre-heating the base material to slow down the cooling process. You may also utilize low-hydrogen welding consumables to reduce the amount of hydrogen that is dispersed into the weld. Finally, apply post-weld heat treatment to restore some of the strength lost due to cold working.
Aluminum has the potential to develop serious cracks if it's subjected to cold stress. To prevent this from happening, protect aluminum from exposure to low temperatures by either heating it up before you start welding or pre-heating the base material. This will help delay or eliminate any cold cracking that may occur.
Cracking is also reduced when using high quality welding consumables. The type of metal in the filler rod affects how much cold work is done to the joint. For example, stainless steel rods produce a smoother finish but they can also cause cracking if used in excess. Cast iron rods provide an extremely rough surface and are great for heavy fabrication work but they cannot be used in areas where grinding fluid might leak out because they will absorb the fluid and become soft.
Aluminum alloys are strengthened by adding small amounts of certain elements such as magnesium, silicon, zinc, and copper. These additions increase the stiffness of the alloy and promote solidification at lower temperatures. However, they also reduce the ability of the material to absorb thermal energy during welding.
The creation of shrinkage cracks during the solidification of weld metal is known as hot cracking. Almost all metals exhibit this behavior. Hot shortness, hot fissuring, solidification cracking, and liquidation cracking are all terms for hot cracking. The amount of heat added to the metal during the welding process causes the metal to expand differently than it does at room temperature. As the metal begins to cool down, it tries to return to its original state, but because there is still a large amount of heat left in the metal, any difference in density between the two parts of the material cause stress. This stress can become great enough to break down part of the metal molecule, starting with the outer layer.
Hot cracks usually do not affect the strength or performance of the weld joint. They are relatively easy to spot when viewed under light microscopy. Laser beam scanning electron microscopy (LSEM) allows us to see deeper into the weld metal, revealing defects such as voids and inclusions that might otherwise go undetected. Cracks also can be seen using optical microscopy after the weld metal has been polished and etched.
Hot cracks can be found in all types of welding processes, but they are most common in laser beam welding, gas tungsten arc welding (GTAW), and plasma cutting.
Cold cracks typically occur along weld and fusion lines after welding and during the "maturing" process. Their development process is connected with high tensile residual stresses and structural alterations that occur in the material over time. These stresses can cause a small section of the metal to crack when it reaches its yield point. The crack will then grow until it comes into contact with another defect or impurity, such as a grain boundary or inclusions, causing further growth.
Impurities that cause cold cracks include but are not limited to: oxide inclusions, nitrogen inclusions, and carbonaceous deposits. Oxide inclusions occur due to contamination from air inside the welding shop while nitrogen and carbonaceous deposits result from using shielding gases such as Ar or He during welding.
The type of metal being welded also affects the formation of cold cracks. For example, if high-carbon steel is being welded, then cold cracking may occur because heat treatment (hardening) of the steel increases its susceptibility to stress corrosion. On the other hand, if low-carbon steel is being welded, then cold cracking does not usually occur because there is not enough substance to form oxides. Instead, fusion defects are likely to form as these two types of metals have very different melting points (2300°F for high-carbon steel vs 1450°F for low-carbon steel).
Hot cracking, also known as solidification cracking, occurs in aluminum welds when significant levels of heat stress and solidification shrinkage are present when the weld is solidifying to varying degrees. As the weld cools, the difference in temperature between the molten metal and the surrounding environment causes the alloy to contract more than the environment it is cooling into. This mismatch in contraction rates results in stress within the alloy body which eventually leads to failure. Stress can be reduced by adding certain elements such as copper to the aluminum welding pool.
Welding creates a large amount of heat that must be removed from the workpiece. The most efficient method of removing heat is through radiation into space. However, because aluminum has low thermal conductivity, much of this heat remains within the material itself. The higher the concentration of aluminum in the alloy, the greater its resistance to deformation at high temperatures. Therefore, to improve its resistance to hot cracking, less reactive metals such as zinc or copper are added.
Aluminum alloys contain various amounts of zinc, magnesium, silicon, and copper. Although these additional elements provide stronger welds, they also increase the difficulty of melting the alloy and the risk of contamination from the filler metals used during fusion welding.