Knife Line Attack on Stabilized Austenitic Stainless Steel (ASS): A Welding Engineer's Perspective

Introduction

As a welding engineer, it is crucial to understand the various challenges associated with different types of steels and their welding processes. One such challenge is the occurrence of knife line attack on stabilized Austenitic Stainless Steel (ASS). In this blog, we will delve into the causes, consequences, and possible remedies for this issue.

Understanding Knife Line Attack

Knife line attack refers to a very narrow band or line that forms in an area adjacent to the fusion boundary during the welding process. This phenomenon specifically occurs in stabilized ASS. The affected area experiences a thermal cycle close to the fusion boundary, where temperatures rise significantly above the solidus temperature.

Causes of Knife Line Attack

Knife line attack in stabilized ASS is primarily attributed to the behavior of titanium (Ti) and niobium (Nb) carbides. Normally, during welding, these carbides should reprecipitate by leaving carbon in the solution. However, under certain circumstances, such as reheating due to stress relieving or during service, the carbon is not effectively removed, leading to sensitization. This sensitization is what ultimately results in knife line attack. It is important to note that knife line attack can also occur during multi-pass welding.

Consequences of Knife Line Attack

Due to its lower thermal conductivity and higher coefficient of thermal expansion compared to mild steel, ASS is prone to severe residual stress during welding. When exposed to a corrosive environment, this combination of residual stress and sensitization can lead to trans-granular cracking, known as stress corrosion cracking (SCC). SCC can significantly compromise the structural integrity of the welded component.

Remedies for Knife Line Attack

To mitigate the occurrence of knife line attack on stabilized ASS, several remedies can be implemented:

Addition of Rare Earth Metals: Adding rare earth metals, such as cerium and lanthanum, can accelerate the precipitation of carbides. This helps to prevent the sensitization of the material and reduces the risk of knife line attack.

 Using Low Carbon Grades: Utilizing ASS grades with lower carbon content can minimize the likelihood of carbide precipitation and subsequent sensitization. This approach is particularly effective in preventing knife line attack during welding.

 Post Weld Solutionizing and Quenching: Applying a heat treatment process called solutionizing followed by quenching can help dissolve any existing carbides and restore the material's stability. This treatment is typically performed after welding to alleviate sensitization and minimize the risk of knife line attack.

Conclusion

As a welding engineer, it is vital to be aware of the challenges associated with different types of steels. Knife line attack on stabilized Austenitic Stainless Steel (ASS) is a critical issue that can lead to stress corrosion cracking and compromise the structural integrity of welded components. By understanding the causes, consequences, and remedies for knife line attack, engineers can implement appropriate measures to prevent its occurrence. Whether it's through the addition of rare earth metals, using low carbon grades, or employing post-weld heat treatments, careful consideration and proper implementation can ensure the longevity and reliability of welded ASS structures.