Delayed crack problem?
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There is a question asking about materials that have a tendency to develop delayed cracks in their welds: Q345R, Q370R, 13MnMoNiR, Cr-Mo steel, etc. I’ve heard for the first time that Q345R welds have such a tendency to develop delayed cracks – could this be a mistake?
The Q345R welds were confirmed to have no tendency for delayed cracking.
This should be a single-choice question, and the answer is 13MnMoNiR. 13MnMoNiR is an alloy similar to Japan’s BHW35; it is a steel suitable for use in thick-walled structures with good weldability.
Does X7Ni9 steel have a tendency to delayed cracking?
In the presence of hydrogen, it has a tendency to develop hydrogen-induced delayed cracking.
20mm can practically be ruled out; Q345R with a thickness of 70 or 80mm is not suitable either, not to mention the tendency for delayed cracking. Delayed cracking refers to cold cracking. Cold cracking generally occurs as a result of diffused hydrogen coming together to form molecular hydrogen; this leads to a rapid increase in volume, and the combined stresses cause hydrogen-induced cracking. Since it takes time for atomic hydrogen to diffuse and aggregate into molecular hydrogen, the cracking occurs at a delayed time after welding, which is why it is called delayed cracking. Generally, delayed cracking is related to three main factors: hardening tendency, diffused hydrogen, and degree of constraint. The hardening tendency is not only associated with the carbon equivalent but also directly related to the plate thickness; an increase in plate thickness not only enhances the hardening tendency (as plate thickness affects the cooling rate, which in turn influences the formation of the M structure) but also results in poorer deformation capacity, higher stress levels, and an increased degree of constraint. Furthermore, for the same plate thickness, with different structures, corner joints have a higher degree of restraint than butt joints. Therefore, the tendency for delayed cracking depends not only on the material composition but also on thickness and joint type. For Q345R, preheating is not required when only considering the C-equivalent value; however, when the thickness exceeds a certain level, preheating becomes necessary. For even greater thicknesses, all three measures – preheating, post-heating, and heat treatment – must be taken into account, as this is directly related to an increased tendency for delayed cracking