Regarding materials with a tendency to develop delayed cracks

This post was last edited by Loulan Meng on 2010-6-4 21:48. Seen elsewhere: 1 Delayed cracks 1.1 Definition of delayed cracks Cracks that occur after a certain period of time following welding are known as delayed cracks. Delayed cracking is a common defect of cold cracking; it does not occur immediately after welding, but appears hours, days, or even longer after welding. 1.2 Materials prone to delayed cracking: 15MnNbR, 18MnMoNbR (difficult to obtain), 13MnMoNbR (a copy of Japan’s BHW35; it is a single-layer thick-walled steel with good weldability but at a high price), 07MnCrMoR, 07MnNiMoDR, and Japan’s CF-62 series steels. 2 Thermal Cracks 2.1 Definition of Thermal Cracks Cracks that occur during welding at temperatures above 300°C are known as thermal cracks. Thermal cracks generally include solidification cracks that occur at a temperature slightly below the solidification temperature, with a few occurring within the solidification temperature range. 2.2 Causes of thermal cracks Thermal cracks arise due to the effect of welding tensile stress on the low-melting eutectics at the grain boundaries. Welding stress is the external factor that causes cracks, while the eutectic with a low melting point is the internal condition that leads to cracks. The relatively high levels of S and Fe in the weld can form low-melting-point eutectics; therefore, the high level of S is the main factor. In pressure vessel welding, reducing the line energy or using multi-pass welding are effective methods to prevent hot cracks. 3 Reheat Cracks 3.1 Definition of Reheat Cracks Cracks that occur when a welded joint is heated again within a certain temperature range after welding is completed (as part of stress-relief heat treatment or other heating processes) are known as reheat cracks. The reheat cracks that occur during stress-relief heat treatment are also known as stress-relief treatment cracks, or SR cracks. 3.2 Causes of reheat cracking There are two reasons for the occurrence of reheat cracking: one is related to the carbide-forming elements contained in the steel (such as Cr, Mo, Ti, and B). Elements such as those in pearlitic heat-resistant steels can significantly increase SR crack sensitivity ； Secondly, it is related to the heating rate and heating time; different steel grades have distinct temperature ranges in which they are prone to reheat cracking. Therefore, when developing the post-weld heat treatment process, the residence time of the welded parts within the sensitive temperature range should be minimized as much as possible. The former are internal factors, while the latter are external causes. Where conditions permit, increase the heating rate as much as possible to quickly pass through the reheat crack sensitivity zone, thereby preventing the formation of reheat cracks. However, when the heating rate is too fast, a large temperature difference between the surface and the interior of the container leads to significant thermal stress, which can cause deformation and cracking of the welded parts. Therefore, GB150-1998 sets limits and specifications on the heating rate and the temperature difference of the welded parts in clause 10.4.5.1. Similarly, the cooling rate should also be controlled. It is the responsibility of manufacturing units to develop advanced, reasonable, simple, and feasible heat treatment procedures tailored to different welded components, which also reflects their experience and technical capabilities. 3.3 Special cases requiring a lower heating rate Weldments that meet one of the following conditions should be heated at a lower rate; otherwise, it may also lead to cracking of the weldment: 1) Weldments with poor thermal conductivity ； 2) Welded parts with complex shapes and large differences in thickness ratios ； 3) Welded parts with very large thicknesses. Clause 10.4.5.1 of GB150-1998 stipulates that the minimum heating rate is 50°C/h, and the temperature of the welded parts when they enter the furnace shall not exceed 400°C. If the temperature entering the furnace is too high, it increases the heating rate, resulting in a large temperature difference between the inside and outside of the welded part. Under the stress caused by such a large temperature difference, the welded part is prone to deformation and cracking. 3.4 Materials prone to reheat cracking: 15MnR, 15MnNbR, 18MnMoNbR, 13MnMoNbR, 07MnCrMoR, 07MnNiMoDR, and Japan’s CF-62 series steels. That is, some precipitation-hardening high-alloy steels; for such steels, the heat treatment temperature must be controlled: if it is too low, stress cannot be released ； If it gets too high, it will crack. It is specifically controlled by the manufacturer through a heat treatment process, with a recommended temperature of 580°C ± 20°C. 4 Materials with high sensitivity to cold cracking: It is generally believed that materials with an Rm value of 450 MPa or higher are prone to developing cold cracks. Such as heat-resistant steels, martensitic stainless steels, welded Ni-containing low-alloy steels, welded joints of dissimilar steels, special structural steels, and surfacing layers.

Thank you, I get it! It is usually 16MnR!

There seems to be a mistake with the 2l floor! Regarding the clarification of knowledge related to NDT for 16MnR: this material is not very sensitive to cold cracks (i.e., delayed cracks).

The two main factors that affect the occurrence of delayed cracking (as far as I know) are: 1. Thickness – the greater the thickness, the higher the probability. 2. C-equivalent – a higher value means a higher probability. Method of elimination: preheating before welding. Materials prone to reheat cracking are mainly those with high levels of Cr, Mo, and V elements (stainless steels are excluded), such as CrMo steel.

Reply to 5# jinboldin: Is there anything else that’s wrong? Could you also upload your information to share it? Personally, I also think that 16MnR (Q345R) does not exhibit a tendency to develop delayed cracks. Moreover, our company has never treated it as a material prone to delayed cracking during the manufacturing process. I obtained the above information from elsewhere, and I cannot guarantee its accuracy; I hope everyone will have their own opinions. Please point out any mistakes, or simply edit them out.

Based on our factory’s many years of experience, Q345R materials with a thickness of over 40 mm still exhibit a certain tendency to develop delayed cracks; thinner materials generally don’t have this issue. Everyone can share their observations based on what happens over time! !

Reply to 6# baictq: Materials that tend to develop reheat cracks are mainly those containing high levels of Cr, Mo, and V elements (stainless steels are excluded), such as CrMo steel. ????? I don’t understand – aren’t materials containing Cr, Mo, and V elements still considered stainless steel? ? ? Is the author referring to low-alloy steel? ? ?

16MnR should not have a tendency for delayed cracking; we generally don’t take such factors into consideration here.

The two main factors that affect the occurrence of delayed cracking (as far as I know) are: 1. Thickness – the greater the thickness, the higher the probability. 2. C-equivalent – a higher value means a higher probability. Method of elimination: preheating before welding. Materials prone to reheat cracking are mainly those with high levels of Cr, Mo, and V elements (stainless steels are excluded), such as CrMo steel