Stress Relaxation Cracking in Industrial Systems: Key Insights for Reliability

Stress relaxation cracking (SRC) is a critical issue for industries that operate with high-temperature environments, such as petrochemicals, power generation, and refining. SRC can lead to unexpected failures, causing downtime and costly repairs.

What is Stress Relaxation Cracking?

Creep-resistant materials generally have high creep ductility and can accommodate inelastic strains at elevated temperatures by relaxation. However, many of these materials undergo a process known as age hardening when exposed to temperatures between 430-850 °C (806-1560 °F). Age hardening occurs when fine particles precipitate within the material’s grain structure, increasing the material’s hardness while reducing its ductility. Once this embrittlement occurs, the material becomes more vulnerable to SRC. Cracking typically occurs in the area of highest stress, though this stress is often largely residual.

This type of cracking is known by several terms, including reheat cracking, stress relief cracking, and creep embrittlement cracking, as well as stress-assisted grain boundary oxidation cracking. Cracking can also occur during post-weld heat treatment, where the process is typically referred to as reheat cracking. In service, however, it is more commonly identified as stress relaxation cracking.

While 1 ¼ Cr materials can suffer reheat or creep embrittlement cracking as discussed in API 938-A, the following two case studies focus on SRC, which is commonly seen in austenitic stainless steels and nickel-based alloys, especially when these materials are welded. The residual stresses from welding, combined with the high operational temperatures, make the heat-affected zones particularly susceptible to cracking.

Case Study #1: High-Temperature Reactor Vessel Cracking

A reactor vessel operating at approximately 524 °C (975 °F) was found to have stress relaxation cracking near key welds. The vessel, made from 347H stainless steel, developed cracks in the heat-affected zone (HAZ). Further analysis revealed that the weld filler material was ERNiCrMo-3. This weld filler had a higher creep strength than the parent metal, leading to local strain concentration in the HAZ and the development of SRC.

Key Takeaways:

• Crack Location: The crack appeared near the weld joint connecting the vessel’s head to its shell, following the HAZ,  a common site for SRC as grain structure alterations and age hardening occur during welding.
• Microstructural Evidence: Metallurgical analysis showed intergranular cracking, precipitate formation inside the grains, and chromium depletion at the grain boundaries.
• Material Mismatch: The filler metal used in the welding process had a higher creep strength than the vessel, causing a stress differential between the parent and filler material.

Proposed Solution: To prevent future occurrences, repairs were proposed using 16-8-2, a filler metal more compatible with the parent material. Improved welding techniques, such as controlled deposition welding, were also recommended to be employed to reduce residual stress.

Case Study #2: SRC in Convection Section Coils

In another case, cracking was discovered in convection section coils. The cracks appeared at connection points between the coils and the manifold, specifically at the weldolet to manifold welds. The coils were 304H stainless steel and the weld material was 308L stainless steel. Finite Element Analysis confirmed high stress concentrations in the areas adjacent to the weldolet-to-manifold weld toes.

Key Takeaways:

• Design Issues: The design of the coil itself, as well as the weldolets, created stress concentration points at the weld joints, increasing the risk of SRC.
• Welding Residual Stresses: The welding method used generated high residual stresses, which, when combined with high operating temperatures, exacerbated the cracking.

Proposed Solution: It was recommended to replace the manifold incorporating in some design modifications to reduce stress concentrations, and switch to a lower-stress welding process to prevent further occurrences of SRC. Post-weld heat treatment was also recommended as part of the manufacturing process.

As can be seen from the outlined case studies, a number of factors contribute to SRC. These include:

• Material Susceptibility: All creep-resistant materials with <25% Cr, including stabilized grades like 347/347H or 321/321H, are more prone to SRC due to the presence of carbide formers, such as niobium or titanium.
• Welding Practices: Residual stresses from welding, especially in the heat-affected zone, significantly contribute to SRC, particularly when materials with mismatched creep strengths are used.
• Operational Stress: High operational temperatures combined with rigid designs, thick sections, and high stress concentration points, such as in weldolets or connection joints, increase the likelihood of SRC.

Practical Strategies to Prevent SRC

Given the complexities of SRC, effective management involves a holistic approach that addresses materials, design, and welding techniques. Here are key strategies to mitigate the risk of SRC:

• Material Selection: Choose materials that are less prone to stress relaxation cracking in high-temperature environments. Non-stabilized grades, such as 304H or 316H, may offer better resistance than stabilized grades like 347H.
• Optimized Design: Avoid creating stress concentration points in critical areas. Designs that allow even stress distribution, such as tapered connections, reduce the risk of cracking. A stress analysis can be useful in identifying high stress connections.
• Refined Welding Practices: Use controlled deposition welding techniques to minimize residual stress in critical areas. Limit the size of weld passes and the interpass temperature to reduce heat input.
• Routine Monitoring: Implement regular non-destructive testing methods, such as ultrasonic testing or dye penetrant testing, to detect early signs of SRC and address them before significant damage occurs.

Conclusion: Safeguarding Your Equipment from SRC

Stress relaxation cracking poses a significant risk to industrial systems operating with high temperatures, but with the right preventative strategies, it can be effectively managed. By focusing on material selection, design optimization, and careful control of welding practices, you can greatly reduce the likelihood of SRC and ensure the long-term reliability of your equipment.

At Becht, our experts have decades of experience in analyzing and predicting SRC problems, as well as preventing SRC from occurring through improvements in design and fabrication practices. Contact us today to learn how we can help safeguard your equipment.