- Materials & Industrial Technology
Different Types of Welding Defects
Welding Defects Happen
As much as we want our finished welds to be perfect, sometimes that is not the case. Welding defects occur even on welds that are created by the most experienced and skillful welders around. A welding defect is any flaw that compromises the integrity of the finished weld. There are many different types of weld defects that can occur, some you may be familiar with and some you may not. Below we have listed many different types of defects and what causes them.
Arc Strike Cracking
Arc strike cracking occurs when the arc is struck but there is no weld. This happens primarily because of the weld is heated above the materials upper critical temperature and is then basically quenched. When this happens brittle martensite and micro-cracks develop. To avoid this defect from occurring, the arc is struck in the weld groove, when it is struck outside of the weld groove then it must be welded over to avoid cracking from taking place.
Cold cracking occurs when residual stresses reduce the strength of the base material which can lead to catastrophic failure. Cold cracking is limited to steels and is commonly associated with the development of martensite as the weld area cools. The cracking appears in the heat-affected area of the base metal. In order to reduce the amount of residual stress and distortion the amount of heat should be limited and the welding sequence should be performed in segments instead of running the weld from end to end continuously.
Crater crack develops when a crater is not properly filled before the welding arc is broken. When this happens it causes the outer edges of the crater to cool faster than the interior of the crater which causes sufficient stresses to form a crack.
Hat cracks typically begin to form at the fusion line and extend up through the length of the weld. This type of crack is typically caused by using too much voltage and not enough speed during the welding process.
Hot cracking, also known as solidification cracking, can happen in all metals and tends to occur at the site of the fusion weld. To help avoid this from happening excess material restraint should be avoided and proper filler should be used. This defect can also happen when too high of a welding current is used, arcs that are too long, when the welding speed is too fast and when impurities are present in the welding material.
Underbead cracking, also referred to as heat-affected zone cracking, is a crack that develops a short distance from the fusion line. It can occur in either a low alloy or high alloy steel. The cause of this type of crack has not been recognized, but it is known that the presence of dissolved hydrogen must be present in order for the crack to develop. Another reason that this type of crack may occur is internal stress caused by unequal contraction between the base metal and the weld metal.
A longitudinal crack usually runs along the length of a weld bead. There are three types of longitudinal cracks; check cracks, root cracks and full centerline cracks. Check cracks can be seen on the surface and may extend into the weld. Check cracks are typically caused by high shrinkage stresses, especially on final passes. Root cracks tend to develop at the start of the weld and travel partially into the weld. This is the most common type of longitudinal crack primarily because of the small size of the first weld bead. If a root crack is not taken care of right away it will usually spread into subsequent weld passes, which is the cause of a full centerline crack.
Reheat cracking occurs in HSLA steels, particularly in chromium, molybdenum and vanadium steels during the post heating process. They are caused by poor creep ductility of the heat affected zone. To prevent this type of crack from occurring, you should heat treat the steel first with a low temperature soak and then again with a rapid heating to high temperature. You may also want to grind or peen the weld toes using a two layer welding technique to refine the HAZ grain structure of the steel.
Don't Be Discouraged By Welding Defects
These are a few of the most common types of welding defects that can occur. If you take your time and properly prepare your materials, the chances of a crack occurring are lessened. But remember, no weld can be perfect all of the time and imperfections and defects can occur. This doesn't make you less of a welder, but just a human being who can't be perfect all of the time. Welding defects can lead to dangerous situations, but if you do your best to try to catch them early you can help make the outcome less dangerous.
No welding technique is perfect, there will always be imperfections, and the important thing is to not let them hinder you from continuing to weld. The amount of defects that occur will typically decrease as you become more experienced, but never become insensitive to the possibility that they may occur. Always check your finished welds and make sure that they are of the highest quality and stand up to the requirements of their primary use. Attention to detail is crucial when welding.
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