50 Underbead Cracks

Underbead cracks are cracks in the heat-affected zone (HAZ) that generally do not extend to the surface of the base metal. Underbead cracks may be longitudinal or transverse, depending on the direction of the principal stresses in the weldment. Underbead cracks are cold cracks and are usually short and discontinuous.

Under bead cracks could be hydrogen cracks that occur in steels susceptible to hydrogen embrittlement during welding. Dissolved hydrogen, which is released from the electrode or from the base metal, combines with martensite formed in the HAZ during rapid cooling, creating a narrow region that is extremely brittle and sensitive to cracking from residual stresses.

Under bead cracks are detected by ultrasonic examination (UT) or radiographic examination (RT) because the crack is usually below the surface and immediately adjacent to the weld. Because of their tightness and short length, under bead cracks may be difficult to detect.

Underbead Crack Prevention

A drying procedure must be used to remove moisture that can absorb into the coatings on some types of electrodes when exposed to humid atmospheres. The procedure involves storing electrodes in a low-temperature oven, preheating the surface before welding to remove moisture, and post-heating immediately to encourage hydrogen to escape.