Introduction – cracks
As you have read in the introductory article on welding imperfections, the classification of the various imperfections is laid down in the standard NEN-EN-ISO 6520-1:2007. In this article we go deeper into the group of cracks.
Cracks in a weld can vary in size. Some cracks can be seen with the naked eye. Other cracks are so small that they can only be identified under a microscope. A crack, regardless of its size, has a major impact on the integrity of the weld. That is why they are unacceptable in almost all cases.
Within this group we can distinguish two types of cracks:
- Hot cracks
- Cold cracks
As the name suggests, hot cracks occur at higher temperatures due to the shrinkage stresses that occur when the weld metal and the adjoining base material cool down. During the solidification of the weld metal, solid crystals are formed surrounded by a still liquid film of weld metal. The shrinkage stresses can no longer be absorbed, resulting in a crack in the part of the weld that solidifies last. Usually in the middle of the weld. This crack formation is stimulated by, for example, the presence of phosphorus and sulphur. Elements that are common in dirt and grease. In order to prevent cracks, it is therefore important that the weld seam is properly cleaned before welding.
Additional measures against hot cracking
When welding non-ferrous alloys, there is a greater chance of hot cracks forming. This is also the case with austenitic stainless steel. If attention is paid to a number of additional measures, cracking can often be prevented. Consider, for example:
- a special welding sequence to reduce the shrinkage stresses
- an appropriate composition of the welding consumables
- additional cleaning prior to welding
Cold cracks occur after the weld has solidified and are the result of a combination of the following factors:
- (welding) stresses
- a brittle hard structure
- the presence of hydrogen
- a temperature below 150 °C
Tensions will always be present in a finished weld and its surroundings due to shrinkage stresses. Measures to reduce these stresses are an adapted welding sequence, weld bead thickness and preheating. Of course, the type of material to be welded and the requirements for the welded joint must be taken into account.
The formation of a brittle hard structure in the heat-affected zone (HAZ) next to the weld mainly occurs when welding steel, especially where the strength of the steel increases. This is due to a higher percentage of carbon in the alloy or the elements that promote the formation of a brittle structure after cooling. This brittle structure can be prevented by preheating prior to welding whenever there is a chance.
Addtitional measures against cold cracking
The element hydrogen has a major influence on the occurrence of cold cracks. It is possible to prevent this element from being present too much in the weld. This can be done by using welding consumables in which little hydrogen is present. Hydrogen is present as moisture in the atmosphere, which is why this welding filler material must be stored under dry conditions.
At a temperature below 150 °C, hydrogen can escape very poorly from the metal (diffusion). Hydrogen can even cause cracking quite some time after welding. In critical cases, a weld or a construction with welded joints may be subjected to low hydrogen annealing at a temperature of approximately 250 °C for a number of hours to allow the hydrogen to escape.
The occurrence of cold cracks is always due to a combination of the above factors, of which hydrogen has a very large influence. If one of the factors is less present, such as the chance of the formation of a brittle structure, one can allow a larger supply of hydrogen.