Macro Defects in Steel - Welding defects

Welding defects

During the process of welding, various types of defects can occur. Some defects are due to the quality and hardness of the weld metal, while others are due to lack of skill of the welder. The most common defects occurring during welding are given below.

Porosity – Molten weld metal has a considerable capacity for dissolving gases which come into contact with it such as hydrogen, oxygen and nitrogen. As the molten weld metal cools its ability to retain the gases reduces. With the change from the liquid to the solid state, there is reduced solubility with falling temperature. This causes evolution of gas at a time when the metal is becoming mushy and therefore incapable of permitting the gas to escape freely. Entrapment of the gas causes gas pockets and porosity in the final weld. The porosity can be of three types namely (i) fine porosity, (ii) blow holes, and (iii) piping. Fine porosity consists of small bubbles of gas usually of diameter less than 1.5 mm. Blow holes are usually gas pores larger in dimension while ‘piping is an elongated or tubular cavity. Piping is usually almost perpendicular to the weld surface. It can result from the use of wet powdered flux or from inadequate control of the welding current. Another typical form of pipes has the appearance of a branch of a tree. Porosity may be scattered uniformly throughout the weld, isolated in small groups, or concentrated at the root of the weld. Various causes of porosity include excessive moisture content of the electrode covering, incorrect electrode current, defective gas shielding, contamination of joint surface or filler wire, and rapid cooling of the weld metal or the composition of the electrode core wire or parent steel.

Non-metallic inclusions – These are the result of weld metal contamination by substances on the surface of the joint or by the atmosphere. However, the normal source is the slag formed by the electrode covering or flux used in the welding process. Some slag is trapped in the deposited metal during its solidification, particularly if the metal fails to remain liquid for a sufficient period to permit the slag to rise to its surface. In multi-pass welding, insufficient cleaning between weld passes can leave a portion of the slag coating in place to be covered by subsequent passes. A particular characteristic of slag inclusions is the ‘slag line’, intermittent or continuous. Such slag lines are frequently accompanied by a pronounced lack of fusion to the base steel. In general, inclusions can be due to any one of several reasons which include (i) failure to clean the surface of the joint, (ii) failure to remove slag from a previous deposit, (iii) incorrect edge preparation, (iv) incorrect handling of the electrode, (iv) insufficient arc shielding, and (v) improper rate of cooling.

Tungsten inclusions – These are particles of metallic tungsten embedded in the weld metal which originate from the tungsten electrode used in tungsten arc welding. Causes are excessive welding current allowing the melting and deposition of tungsten in the weld and incorrect polarity of electrode using a DC source. Tungsten inclusions can also be caused from dipping the electrode into the molten weld metal or by touching the filler rod to the electrode during welding. Tungsten inclusions normally occur at the start of welds when the electrode is usually cold. Small globular and widely scattered tungsten inclusions are sometimes permissible, but sharp edged inclusions are dangerous.

Lack of fusion – This is due to the lack of union in a weld between the weld metal and parent steel or between parent steel and parent steel or between weld metal and weld metal. Hence, the lack of fusion can be of three types namely (i) lack of side fusion, (ii) lack of root fusion and (iii) lack of inter-run fusion. The defect results mainly from (i) the presence of slag, oxides, scale, or other non-metallic substances, (ii) too low a welding current, or (iii) incorrect edge preparation. Incomplete fusion can also arise from too high a welding current when the high rate of melting encourages the welder to use excessive welding speed. The defect greatly reduces the strength of a joint subjected to static loading and under cyclic or shock loading which is quite serious.

Incomplete root penetration – In butt welding, a root opening is usually left at the bottom of the groove (in single side welding) or at the centre of the weld (in two-side welding). If the opening between the two plates is narrow, it is difficult to achieve complete penetration and fusion at the root of the weld. Hence there can be a lack of fusion in the root of the weld or a gap left by the failure of the weld metal to fill the root of a butt weld. It is caused by the electrode held at an incorrect angle, an electrode too large in diameter, a rate of travel too fast, an insufficient welding current, or an improper joint preparation (e.g. joint misalignment).

Cracks – These are defined as a discontinuity produced either by tearing of the steel while in a plastic condition (hot crack) or by fracture when cold (cold crack). Cracks can occur in either the weld metal or parent steel. In weld metal they are classified as longitudinal, transverse, crater, and hairline cracks. In the parent steel, it is cracking in the parent plate with the origin in the heat-affected zone of the weld. The strength of a welded joint under any conditions of loading is seriously reduced by the presence of a crack. Weld metal cracks are caused by high localized stresses in the joint arising from the shrinkage of weld metal, by resistance of movement of the parts, or by vibration of the structure during welding. Hence, it is important that each weld run is strong enough to withstand the shrinkage and allows as much freedom of movement as possible. Longitudinal weld cracks usually occur in the root run and, if left as such, will eventually propagate through subsequent runs. Incorrect finishing of a weld run can form a crater and possibly lead to a crater crack. Parent steel cracking is associated with the welding of medium carbon and alloy steels. A considerable amount of work has been done into the techniques for welding these steels and it is most important that instructions regarding type and condition of electrode, the degree and extent of preheat and the restrictions on size of single pass welds are strictly followed to avoid this form of cracking.

Undercut – During the final or cover pass the exposed upper edges of the bevelled weld preparation tend to melt and to run down into the deposited metal in the weld groove. The result is a groove which may be either intermittent or continuous, with more or less sharp edges along the weld reinforcement.

Concavity at the root of the weld – A concave surface at the root of the weld can occur specially in pipe welding (without a cover pass on the root side). In overhead welding this condition is a consequence of gravity which causes the molten metal to sag away from the inaccessible upper surface of the weld. It can also occur in down-hand welding with a backing strip at the root of the weld groove if slag is trapped between the molten metal and the backing strip.

Excessive penetration – In welds molten metal sometimes runs through the root of the weld groove producing an excessive reinforcement at the back side of the weld. In general this is not continuous but has an irregular shape with characteristic hanging drops of excess metal.

Overlap – it is an imperfection at the toe or root of a weld caused by an overflow of weld metal onto the surface of the parent metal, without fusing with the latter. It is caused when the welding rod has been used at an incorrect angle, the electrode has travelled too slowly, or the current was has been low.

Reference: Website http://ispatguru.com/metallurgical-processes-and-defects-in-steel-products, 2017.