thePetroStreet

What are the conditions under which delayed cracking can occur? How can it be prevented?

There are four conditions under which delayed cracking can occur. Even one of these is absent, delayed cracking would not occur.
1. Presence of hydrogen
2. Susceptible microstructure
3. Presence of stress
4. Low temperature
In order to prevent, each of the above factors has to be addressed.
1. Use low-hydrogen electrodes, properly back them in an oven which should not be overloaded. Cleanliness of the weld joint, consumables from dirt grease or other lubricant is must. Prehaeting the weld joint would also help in diffusing the hydrogen from the weld joint.
2. Cooling of weld joint shall be taken care off as increased cooling rate would be detrimental as it increases the hardness levels of the weld joint due to formation of martensite. Increasing the preheat and interpass with controlled cooling after welding would help.
3. Restraint on weld joints shall be avoided to the maximum possible but it is always not practical. Normally, closing weld joints face higher restraints causing unwanted stress on the weld.
4. Normally, delayed cracking occur at a temperature below 100 deg C. Preheat, interpass and post heating controls can help in diffusing the hydrogen from the joint and eliminating the risk of delayed cracking.