DEFECTS RELATED TO POOR PREPARATION OF THE EDGES BEFORE WELDING

DEFECTS RELATED TO POOR PREPARATION OF THE EDGES BEFORE WELDING

For almost 100 years, industries in all sectors have been using welding processes for their metal assemblies ...

For almost 100 years, industries in all sectors have been using welding processes for their metal assemblies. Techniques and equipment are being perfected year by year for ever more efficiency. Despite the use of state-of-the-art welding machines, the pre-welding stage, ie the preparation of the ends to be welded, is often neglected or performed with unsuitable techniques. However, among the many factors that can affect the quality of a weld, poor preparation is a recurring factor. This article details these flaws and how a properly made preparation reduces the risk of seeing them appear.

CRACKS

Cracking is one of the most common defects in welds. It is due to excessive mechanical stresses in the welded bead. The most commonly observed cracks are hot cracking and cold cracking.

Hot cracking

As its name suggests, this type of crack appears when the metal is still hot, at the time of solidification. Among the factors favoring the appearance of such defects, there is the design of the welded joint. Indeed, the smaller a chamfer, the greater the mechanical stresses due to the solidification of the metal. If they are too high a crack may form during this change of state. It is therefore important to correctly determine the angle of the chamfer. When done manually, the chamfer can not be precisely mastered. We will therefore favor a machining of the edges using a suitable machine that will achieve a precise and consistent angle.

Cold cracking

A cold crack appears after the welding operation (immediately or several hours or several days later). Such a defect is caused by the simultaneous combination of three factors: a quenching structure (hard and brittle), residual mechanical stresses (related to clamping for example) and the presence of diffusible hydrogen in the weld bead. This last factor can be linked to poor preparation of the edges. Indeed, during a welding operation on a rusted or poorly degreased part, the hydrogen present in the rust or hydrocarbons will decompose in the weld. Constraints will then appear at the atomic scale during the cooling of the metal. When this concentration of stress is too great, the metal will crack.

Accompanied by other precautions such as electrode drying or preheating parts, the machining of the edges (without lubrication) will allow welding on a material free of hydrogen and thus reduce the probability of cold cracking.

BLOWHOLES

In addition to promoting cold cracking, a poorly prepared surface can also cause blistering in the weld. Indeed, the presence of water, rust or fat on the part will cause the formation of gas bubbles that may be trapped inside the cord. As for cold cracking, proper machining (without lubrication) of the welding end of the part will significantly reduce the risk of occurrence of this type of defects.

FERROUS POLLUTION

Ferrous pollution concerns stainless steels. When such steel is in contact with ferrous particles and an electrically conductive medium (eg moist air), a mechanism of galvanic corrosion is created. The passive layer of stainless steel will then gradually deteriorate and pitting corrosion may occur.

These ferrous particles generally come from the use of inappropriate equipment. This concerns:

• forming equipment: presses, benders, etc.
• cleaning equipment: wire brushes, rags that have been used on carbon steel, etc.
• machining equipment: cutting tools, chamfering tools, clamping jaws, etc.

This pollution can also come from grinding operations on carbon steel made near the stainless steel parts.

In the context of a machined part before welding, it will be essential to ensure that the clamping jaws and the cutting tools used are compatible with the machining of stainless steel (for example stainless steel or alloy jaws). light) and have not been used before for carbon steel work (or have been decontaminated since).

PENETRATION FAULT

Lack and excess of penetration

The lack of penetration is mainly characterized by an unfused zone at the root of the weld. The excess corresponds to a surplus of molten metal at the base of the welded joint. These two faults are caused by bad welding parameters (intensity, tension and speed of advance) but also by a play between the parts or a thickness of heel poorly mastered.

Too narrow a game will result in a lack of penetration whereas a game too important risks to create an excess of penetration. A very precise positioning of the parts will not be sufficient in all cases to eliminate this type of anomalies. Indeed, if the parts to be welded are not perfectly parallel because of a bad preparation, an irregular game between the parts could then generate an excess or a localized lack. Precise training of the end makes it possible to overcome one of the causes of appearance of this defect.

Good preparation, to ensure a constant play or regular heel, is essential when using automated processes such as orbital TIG or robotic MIG. Indeed, the welding being carried out without human intervention, the defects of alignments are not made up by the machine as could an experienced welder.

Collage or lack of fusion

The lack of melting, also known as bonding, is characterized by a zone of unmelted contact between the deposited metal and the base metal.

Here too, the preparation of the edges is one of the causes of appearance of this defect. In a chamfer too narrow with respect to the diameter of the electrode, the arc can be attracted by one of the walls. There is thus fusion of one of the edges and the chamfer is filled with molten metal. However, since the arc did not directly reach the root (or the previous pass) and the opposite side of the chamfer, these areas were not melted and were simply covered with filler metal. The appearance of the weld may then seem suitable but in reality, the metallurgical continuity sought during assembly by welding is not respected. These defects are generally located inside the welded joint, they are rarely visible to the naked eye and require the use of control techniques such as ultrasound or radiography.

Proper chamfer angle determination and precise machining at a constant angle will reduce the risk of sticking.

ALTERATION OF MATERIAL PROPERTIES

To make the cut of a metal part, two techniques are mainly used. The cutting by heat input (torch, plasma, laser, etc.) and cutting by mechanical machining (grinding wheels, saws, orbital cutting machines, etc.).

The consequences of imprecise preparation related to the use of machining solutions such as grinding wheels or saws have been detailed previously.

In the case of cutting by heat input, the quality of the cut can be satisfactory when performed by a skilled operator or using an automated system. However, these techniques produce in most cases a thermally affected zone (ZAT) near the cut. In this zone, the physical characteristics of the material are greatly modified. If a weld is made directly on this altered material, the quality of the weld and its mechanical strength could then be compromised. It will therefore be essential to take over this machining area in order to eliminate this ZAT and to ensure the final quality of the weld.

The use of orbital machines makes it possible to overcome the problems of precision related to the use of grinding wheels or saws and the constraints of ZAT related to cutting techniques by heat input. This equipment allows cutting and chamfering in one operation without ZAT and with optimal and repetitive quality.

SPECIAL CASE OF HIGH ENERGY DENSITY PROCESSES

The term "high energy density processes" refers to laser welding and electron beam welding (FE). These methods use a beam (laser or electron bombardment) focused so that the energy provided to the workpiece is such that it melts the metal. Thanks to these techniques, it is possible to weld several hundred millimeters thick in a single pass and realizing a cord rarely exceeding 5mm in width.

Assemblies welded in this way do not require chamfering but extremely precise dressing of the ends of the pieces. For example, a preparation for laser welding will be acceptable if the misalignment is less than 1 / 10th of a millimeter. In addition, these technologies, and particularly FE welding, require parts that are free of pollution. Since the electron beam operation is carried out under vacuum, no water or hydrocarbon residue must be introduced into the chamber, otherwise the vacuum may become more difficult to obtain.

The use of these highly advanced methods requires the use of appropriate machining equipment capable of achieving perfect preparation of the end to be welded.

Welding is always a delicate operation. Many parameters must be known and mastered to guarantee an optimal result. In the opposite case, they are as many causes of appearance of defect during or after the realization of the weld seam which alter the quality of the assembly.

Despite the improvement of techniques and knowledge in the field, manufacturers are regularly confronted with problems of non-compliant welded joints. The consequences of a poorly executed weld can be disastrous for the good progress of a project, as well in terms of delays as in terms of budget. Indeed, eliminate a weld bead and make a new one can take several hours and even several days in the case of large assemblies.

Proper preparation of the welded edges will never eliminate 100% of the causes of appearance of defects but will in 100% of cases increase the final quality of the welded assembly.

Thomas Regourd, Sales Engineer North America for: Protem SAS, Etoile sur Rhone / France, contact@protem.fr