Cracking is one of the most frequently observed defects in welding. It is due to excessive mechanical stress in the weld bead. The most common types of cracking are hot and cold cracking.

Hot cracking

As indicated by its name, this type of cracking appears when the metal is still hot and in the process of solidifying. The design of the weld assembly is one the factors that favor the appearance of this type of defect. This can be explained by the fact that the more the bevel is narrow, the greater will be the mechanical stress resulting from the metal solidifying. If this stress is too high, cracking is liable to occur during the change in phase. Therefore, it is important for bevel angles to be determined correctly. When the bevel is made by hand, it cannot be mastered totally accurately. For this reason, it is better to use suitable apparatus for machining edges, so that an accurate, constant bevel angle can be obtained.

Cold cracking

Cold cracks appear after welding (immediately, several hours or even several days later).

This type of defect is caused by a simultaneous combination of three factors: a temper structure (hard and fragile), residual mechanical stress (related to clamping for example) and the presence of diffusible hydrogen in the weld bead. This last factor may be related to poor preparation of the edges. It must be remembered that, when welding is done on a rusty or poorly degreased part, the hydrogen present in the rust or the hydrocarbons will decompose in the weld. Stresses will then appear on the atomic scale when the metal is cooling down. If ever this concentration of stresses becomes too high, the metal will crack.

When accompanied by other precautions such as drying the electrodes in ovens or preheating parts to be welded, dry machining the joint edges will enable will enable welding to be done on hydrogen-free material and, in this way, reduce the probability of cold cracking.

Wird ein Werkstück vor dem Schweißen bearbeitet, muss unbedingt sichergestellt werden, dass die verwendeten Spannbacken und Schneidwerkzeuge für die Bearbeitung vonhochlegiertem Chrom-Nickel-Stahl geeignet sind (zum Beispiel Spannbacken aus diesem Werkstoff) und vorher nicht für die Bearbeitung von unlegiertem Stahl verwendet wurden (oder in diesem Fall anschließend gründlich gereinigt wurden).

Iron contamination

Iron contaminations concerns stainless steels. Whenever this type of steel finds itself in contact with iron particles and an electricity conducting medium (damp air for example), a galvanic corrosion mechanism comes into being. As a result, the passive layer of stainless steel will deteriorate progressively and risks of pitting are then liable to occur.

Generally, these iron particles come from using inappropriate equipment. This may concern:

• Shaping equipment: Presses, bending machines, etc.
• Cleaning equipment: Metal brushes, cleaning rags that have been used on carbon steel, etc.
• Machining equipment: Cutting and beveling tools, clamping jaws, etc.

This type of contamination may also come from grinding operations carried out on carbon steel in the vicinity of the stainless steel parts.

Incomplete or excess penetration may also be caused by a bevel with a land thickness that is not compatible with welding parameters. These parameters are determined beforehand depending on the material to be welded, the geometry of the welded joint and the welding process used. However, correctly mastering the welding parameters does not mean that a penetration defect can be totally avoided. Variable land thickness due to poor preparation may also deteriorate the quality of the welded joint. For example: parameters that are correct for a land 1.5mm thick may lead to excess penetration if the land is only 0.5mm thick or incomplete penetration on a land 2.5 mm thick. Mastering this thickness, by carrying out an internal counter-boring operation or profile tracking on pipes that are out-of-round will increase the final quality of the weld.

Lack of fusion, also called cold-lapping, is characterized by a non-molten contact zone between the filler metal and the base metal.

Once again, poor preparation of the contact surfaces is one of the causes of this defect. With a bevel that is too thin compared with the diameter of the electrode, the arc may be attracted by one of the walls. As a result, fusion occurs on one of the edges and the bevel is filled with molten metal. However, as the arc has not directly reached the root (or the previous pass) and the other side of the bevel, the zones in question are not melted but just covered by an extra layer of filler metal. The weld may appear to be good, but, in fact, the metallurgical continuity required when assembling by a welding process is not achieved. As these defects are generally located inside the welded joint, they can rarely be seen by the naked eye and require the use of special control techniques such as ultrasound or radiography.

Proper determination of the bevel angle and accurate machining at a constant angle reduce cold-lapping risks.

Assemblies welded in this way do not require bevels but extremely accurate facing on part ends. For example, preparation for laser welding will be acceptable if the alignment fault is less than 1/10th of a millimeter. Moreover, parts must be totally free of any contamination for using these technologies successfully, especially electron beam welding. As electron beam welding operations are carried out under vacuum conditions, no water or hydrocarbon residues must be allowed to enter the chamber as their presence is liable to make creating a vacuum a more delicate task.

Using these ultra hi-tech methods requires the presence of suitable machining equipment capable of perfectly preparing the ends to be welded.