July 16, 2014
Editor’s Note: This is the second column in a two-part series.
Q: I work in a midsized job shop, and we weld with the FCAW, GMAW, and SMAW processes. Occasionally we encounter weld cracking issues. What causes weld cracking, and what can I do to prevent it?
A: Stress cracking is similar to hot cracking because it usually produces a longitudinal crack, but it can also be transverse, especially in circumferential welds where the stress is even higher. Stress cracks result when the residual stress of welding exceeds what the weld or base metal can tolerate. In many cases, it also can be attributed to an undersized weld bead.
You can address stress cracking by increasing the weld size, improving joint fit-up to eliminate gaps, and applying or increasing preheat temperature. Preheating slows down the cooling rate, allowing the stresses in the welded area to relieve themselves over a longer period of time. All these suggestions address the reduction of residual stresses during welding.
Hydrogen cracking is sometimes referred to as delayed cracking because it can occur up to 72 hours after the welding is completed. These cracks can occur in the weld metal or heat-affected zone (HAZ), which is the area adjacent to the weld that didn’t melt. Hydrogen cracking runs transverse to the weld 99 percent of the time. It occurs in the presence of four criteria:
To prevent hydrogen cracking, apply preheating or increase the preheating and interpass temperature. This will slow down the cooling rate and allow the excess hydrogen to diffuse before being trapped in the weld metal. The amount of preheat needed will depend on the material type, thickness, and the amount of stress in the weldment. Recommended preheat tables for the various types of steel are available from you filler metal or steel supplier. Depending on its size, you could also hold the weldment above 300 degrees F for a few hours after the welding is completed.
Finally, reduce the amount or source of available hydrogen. This could be as simple as keeping the base material clean and dry and using a filler metal that is low in hydrogen.
Practical Welding Today® was created to fill a void in the industry for hands-on information, real-world applications, and down-to-earth advice for welders. No other welding magazine fills the need for this kind of practical information. Subscriptions are free to qualified welding professionals in North America.