99 Diseases of Pressure Equipment: Welding Flaws and Defects

After a pressure equipment or piping failure, it’s not uncommon to find out during the failure analysis part of the investigation that the failure initiated at a welding flaw of some sort. Many of the “99 Diseases”, including cracking and pitting mechanisms initiate from welding flaws, including cracks and other sharp discontinuities (e.g., hydrogen cold cracks, hot cracks, incomplete fusion, incomplete penetration, undercutting, arc strikes, crater cracks, etc.)

These types of flaws/defects act as “mechanical notches” which provide a ready source of initiation from which many of the 99 Diseases can then propagate to failure. These mechanical notches can act as significant stress raisers, which are needed by many of the cracking mechanisms in the 99 Diseases to begin their “dirty work” in the industry’s pressure equipment and piping. In addition to these mechanical notches, there are other types of welding flaws and discontinuities that can provide “metallurgical notches” that may become initiation sites for several types of the 99 Diseases. These discontinuities include things such as: hardness variations, residual stress variations, microstructural variations, and macrostructural variations. Such metallurgical variations also can play a significant role in creating initiation sites for crack propagation in-service.