Zhuojin Pipe Fitting Co., LtdAnalysis of Surface Cracks in Stainless Steel Overlay Layer of Hydrogenation Reactor and Repair Process Discussion
Literature Overview
The paper by Pan Zhihao, Li Xufeng, and Fu Ruwen, published in "Guangzhou Chemical Industry" (2020, Volume 48, Issue 16, pages 131-133), presents a case study of surface crack analysis and repair process development for the austenitic stainless steel overlay layer on a hydrogenation reactor in an ethylene cracking unit. Conducted at the Guangdong Special Equipment Inspection and Research Institute, this work addresses a critical safety issue in the petrochemical industry: the integrity of stainless steel overlay layers on pressure vessels that are exposed to harsh operating conditions including high temperatures, high pressures, and corrosive media.
Incident Description and Inspection Findings
The hydrogenation reactor in question is a second-stage unit in an ethylene cracking plant, where it is subjected to hydrogen-containing atmospheres at elevated temperatures and pressures. During a scheduled inspection, the following defects were identified:
- Circumferential surface cracks were found on the austenitic stainless steel overlay layer at the ring weld joints between the upper and lower heads and the cylindrical shell.
- Additional surface cracks were observed on the inside of the manhole, also on the stainless steel overlay layer.
- The cracks were described as diffuse and distributed circumferentially around the weld joints.
The inspection employed a combination of non-destructive testing methods including penetrant testing, phased array ultrasonic testing, and ferrite testing, along with physical and chemical analysis of the affected areas.
Crack Cause Analysis
Based on the inspection results, the authors conducted a systematic analysis of the crack causes using the following approach:
| Analysis Method | Finding | Implication |
|---|---|---|
| Penetrant testing | Circumferential surface cracks distributed along the overlay layer | Cracks are surface-breaking and oriented in the circumferential direction |
| Phased array ultrasonic testing | No significant subsurface defects detected | Cracks are primarily surface defects rather than volumetric defects |
| Ferrite testing | Ferrite content in the overlay layer was within acceptable limits | Cracking is not primarily due to excessive ferrite content |
| Physical and chemical analysis | Overlay layer composition was consistent with the specified grade | Cracking is not due to material composition deviation |
| Microscopic examination | Cracks propagated through the weld metal and heat-affected zone | Cracks are related to welding residual stresses and thermal cycling |
The analysis concluded that the primary cause of the surface cracks was the combined effect of welding residual stresses, thermal cycling during operation, and the hydrogen environment. The residual stresses from the overlay welding process created a tensile stress state in the overlay layer, which was exacerbated by the thermal cycling and hydrogen embrittlement during operation. The circumferential orientation of the cracks is consistent with the tensile residual stress pattern that develops during longitudinal welding of the overlay layer.
Repair Process Development
The authors developed a targeted repair process that included the following steps:
- Crack removal: The cracked overlay layer was ground away to a depth that ensured complete removal of the crack and affected material, as verified by penetrant testing.
- Surface preparation: The exposed base metal surface was cleaned and prepared for re-overlay welding.
- Re-overlay welding: A qualified welding procedure was used to deposit a new overlay layer, with careful control of the welding parameters to minimize residual stresses.
- Post-weld heat treatment: A stress relief heat treatment was performed to reduce the residual stresses in the repair area.
- Final inspection: The repaired area was inspected using penetrant testing and phased array ultrasonic testing to verify the integrity of the repair.
The results of the final inspection confirmed that the repair was effective, with no new cracks detected in the repaired area.
Engineering Practice Implications
This case study provides valuable lessons for the inspection and repair of stainless steel overlay layers on pressure vessels in the petrochemical industry. The systematic approach to crack analysis, combining multiple non-destructive testing methods with physical and chemical analysis, is a model for thorough defect investigation. The development of a targeted repair process that addresses the root cause of the cracking, rather than merely treating the symptoms, is a critical aspect of effective repair.
The paper also highlights the importance of hydrogen embrittlement in the cracking of stainless steel overlay layers in hydrogen-containing environments. Engineers must be aware of this mechanism and design overlay welding procedures and post-weld treatments that minimize the susceptibility to hydrogen-induced cracking.
Study Insights
This paper is a practical and instructive case study that demonstrates the application of systematic defect analysis and repair methodology in a real-world industrial setting. The combination of multiple inspection techniques, the thorough root cause analysis, and the development of a targeted repair process provide a comprehensive approach to addressing overlay layer cracking. For engineers involved in the inspection and maintenance of pressure vessels with stainless steel overlay layers, this paper offers a valuable framework for identifying, analyzing, and repairing surface cracks, ultimately contributing to the safety and reliability of critical process equipment.