As welding professionals, we strive for strong, sound welded connections that stand the test of time. However, the reality is that welds, like any manufactured component, can be susceptible to imperfections. These imperfections, often referred to as defects, can happen during the welding process itself or manifest later during the service life of the welded structure. Understanding these potential pitfalls is crucial for ensuring the integrity and safety of weldments.

In this article we’ll touch upon two broad categories of welding defects: those related to the fabrication process and those that emerge during service.

Fabrication-Related Defects

Fabrication-related defects are those that occur directly as a consequence of the welding operation. These issues can stem from the inherent metallurgical behavior of the materials being joined, as well as from the specific welding procedures and techniques employed.

Think of it this way: the intense heat and rapid cooling cycles involved in welding create a dynamic environment where various types of cracking can occur. Some of these “hot cracks” form while the weld metal is still partially or fully liquid. Others, known as “warm cracks,” develop at elevated solid-state temperatures. Then there are “cold cracks,” often associated to the presence of hydrogen, which can appear at or near room temperature, sometimes even hours after the welding is complete.

Beyond these metallurgical cracking issues, fabrication can also introduce non-metallurgical defects. These often result from a lack of precise control over the welding process. Examples include a failure to properly fuse the weld metal with the base material (lack of fusion), the creation of a notch along the weld bead (undercut and overlap), or insufficient penetration into the joint (incomplete penetration). Even the presence of slag trapped within the weld (slag inclusions) falls into this category. Furthermore, improper joint design or poor fit-up of the parts before welding can lead to geometric defects such as “hat cracks.”

While these fabrication flaws can significantly impact the structural integrity of a weld, many can be minimized or eliminated through careful attention to welding parameters, joint preparation, and material cleanliness.

Service-Related Defects

Once a welded structure enters service, it is subjected to various environmental and mechanical stresses. These conditions can give rise to a different set of defects, known as service-related defects.

Welds, by their very nature, often possess microstructures and residual stresses that differ from the surrounding base material. This makes them more vulnerable to failure under certain service conditions.

Common service-related defects often manifest as cracks that develop due to factors like the presence of hydrogen absorbed during welding, exposure to corrosive environments, or the gradual relaxation of internal stresses over time. Cyclic loading can lead to fatigue cracks, while prolonged exposure to high temperatures and stress can cause stress rupture or creep. The combined effects of corrosion and cyclic loading can also result in corrosion fatigue. Finally, exceeding the design limits of the weld can lead to mechanical overload failures.

It’s important to recognize that pre-existing fabrication defects can often act as initiation points or accelerators for service-related failures, particularly in cases of fatigue. For example, a welded component may be designed in such a way that it provides infinite life – meaning that it will never fail due to cyclic loading. However, if the fabrication process introduces a notch, such as undercut during welding, it now creates a stress riser that can provide the perfect conditions for crack initiation as it is a stress riser. Additionally, variations in the weld and heat affected zone microstructure, such as localized hard zones, can influence its resistance to mechanical overload or brittle fracture.

How do we prevent these failures

While understanding the different types of welding defects is crucial, the ultimate goal of a welding engineer is prevention. By grasping the mechanisms that lead to these failures, we can develop effective strategies to minimize their occurrence. This might involve adjusting or implementing welding procedures, implementing preheating or post-weld heat treatment, or even selecting different materials altogether.

While understanding the landscape of potential welding defects is foundational, our primary aim as welding professionals is to build robust, defect-free structures from the outset. Therefore, our next article will move beyond this introduction to spotlight specific examples. We will examine a common fabrication-related defect, detailing its origins during the welding process. We will also explore a typical service-related defect, illustrating how it can emerge during the operational life of a weldment. For each of these examples, we will outline preventative measures that a welding engineer, or whoever holds responsibility for welding operations, can implement to avoid these issues altogether.

As you’ll see, the preventive measures that will be presented can be used to prevent almost every single welding-related issue that can occur in a fabrication shop.

References:

​Welding Metallurgy and Weldability, John C. Lippold ​

​Weld Troubleshooting for Non-Welding Engineers