Stress-Relief Cracking of a Welded Alloy Steel Tube

A thick-walled tube that was weld fabricated for use as a pressure vessel exhibited cracks. Similar cracking was apparent at the weld toes after postweld stress relief or quench-and-temper heat treatment. The cracks were not detectable by nondestructive examination after welding, immediately prior to heat treatment. Multiple-pass arc welds secured the carbon-steel flanges to the Ni-Cr-Mo-V alloy steel tubes. Investigation (visual inspection, metallographic analysis, and evaluation of the fabrication history and the analysis data) supported the conclusion that the tube failed as a result of stress-relief cracking. Very high residual stresses often result from welding thick sections of hardenable steels, even when preheating is employed. Quenched-and-tempered steels containing vanadium, as well as HSLA steels with a vanadium addition, have been shown to be susceptible to this embrittlement. Manufacturers of susceptible steels recommend use of these materials in the as-welded condition.