Cold cracking is a more common type of cracking in welding production, which occurs when the weld is cooled to a lower temperature, for low-alloy high-strength steels, around the martensitic transformation temperature. The three elements of cold crack formation are the hardening tendency of the steel, the hydrogen content of the weld and its distribution, and the stress state of the welded joint.
Hardening tendency of steel depends mainly on its chemical composition and cooling conditions. The greater the hardening tendency of the steel, the more likely to produce cold cracking when welding. Because the greater the hardening tendency means that the weld will produce more martensite organization when heated, and martensite deformation capacity is low prone to brittle fracture. The hardening tendency of welded joints, in addition to the chemical composition, cooling conditions, but also with the welding process, the structure of the plate thickness.
Among them, the influence of chemical composition on the hardening tendency of steel can be roughly estimated using the carbon equivalent method , as follows.
CE (IIW) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15
For example, for steel plates less than 20 mm thick, the hardening tendency is not significant when CE < 0.4%.
Metal with a large hardening tendency will form a large number of lattice defects under the condition of thermal imbalance, and under the condition of stress and thermal imbalance, it will form crack sources and even expand to form macro cracks.
If hydrogen is present in the weld and heat-affected zone, it will reduce its toughness and produce hydrogen embrittlement. The high-carbon martensitic hardened tissue is very sensitive to hydrogen embrittlement and cold cracking sensitivity. The maximum hardness of the heat-affected zone is commonly used in welding to assess the hardening tendency of certain high-strength steels.
Hydrogen is one of the important factors causing the formation of cold cracking in high-strength steel welding, and make it has a delayed character, usually hydrogen-induced delayed cracking called "hydrogen cracking" or "hydrogen-induced cracking". The reason for the "delay" is that it takes a certain amount of time for hydrogen to diffuse in the steel, gather at microscopic defects, generate stresses, and crack.
The higher the hydrogen content in the welded joint of high-strength steel, the greater the susceptibility to cracking, and when the hydrogen content is greater than a certain critical value, cracking will begin to appear, the size of the critical value varies from case to case.
When the concentration of hydrogen in the welded heat-affected zone is high enough, there will be further embrittlement of the martensitic tissue (if any), and thus the formation of cracks.
High-strength steel welding cold cracking not only depends on the steel hardening tendency, the harmful effects of hydrogen, but also depends on the stress state of the welded joint, and sometimes the stress state even plays a decisive role. The thermal stress (uneven heating and cooling), phase change stress (volume change of the organization during phase change) and the form of the structure, welding sequence, etc. of the welded joint can form the constraining force.
The above-mentioned three elements of the formation of cold cracking, each has its own intrinsic law, but also affect each other. In general, the heat-affected zone and the hardening tendency of the weld metal are intrinsic factors for cracking, while hydrogen can play its harmful role in inducing cracking only when there is hardened tissue formation in the steel.