Zhuojin Pipe Fitting Co., LtdFailure Analysis of Weld Seam in Coke Tower Bottom Feed Pipeline Elbow
Literature Overview
You Bilong and Yi Tao (Petrochemical Corrosion and Protection, Vol. 29, Issue 5, 2012, pp. 52–56) present a detailed failure analysis of a weld seam crack in the coke tower bottom feed pipeline elbow at Sinopec Jingmen Branch. The analysis combines chemical composition analysis, metallographic examination, hardness testing, corrosion product analysis, and thermal stress evaluation to determine the root cause of the failure. The findings reveal a complex interaction between high-temperature sulfidic corrosion and heterogenous welding of dissimilar steels.
Core Technical Findings
The failure occurred at the circumferential weld seam region of the elbow, with the crack propagating in the circumferential direction. The crack was characterized as flat and straight, with the crack extension direction essentially aligned with the weld cross-section.
Failure Analysis Results
| Analysis Method | Findings | Interpretation |
|---|---|---|
| Visual examination | Crack at circumferential weld seam, circumferential propagation | Stress direction perpendicular to crack plane |
| Chemical composition | Base metal and weld metal compositions characterized | Dissimilar steel weld (309/321) |
| Metallographic examination | Microstructural features of base metal and weld HAZ | Heterogeneity in microstructure |
| Hardness testing | Hardness variation across weld cross-section | Residual stress distribution |
| Corrosion product analysis | Sulfide compounds identified | High-temperature sulfidic corrosion |
| Thermal stress analysis | Axial tensile stress at weld location | Thermal expansion mismatch |
Root Cause Determination
The failure was attributed to the combined action of two primary factors:
- High-temperature sulfidic corrosion: After 10 years of operation with progressively deteriorating feedstock quality, the severity of high-temperature sulfidic corrosion increased, reducing the load-bearing capacity of the pipeline wall.
- Dissimilar steel welding thermal stress: The weld utilized 309 stainless steel filler metal to join 321 stainless steel components. The thermal conductivity of 309 stainless steel is slightly lower than that of 321 stainless steel. Combined with the fact that the outer wall temperature of 309 steel was lower than that of 321 steel, this created significant axial tensile stress at the weld location.
Dissimilar Steel Welding Considerations
The selection of 309 filler metal for 321 base metal welding warrants detailed examination. According to AWS and ASME standards:
| Filler Metal | Typical Application | Thermal Conductivity (W/m·K) | Coefficient of Thermal Expansion (×10⁻⁶/°C) |
|---|---|---|---|
| 309 (UNS E309L) | Dissimilar steel welding | ~14.6 | ~17.0 |
| 321 (UNS E321) | Austenitic SS welding | ~14.6 | ~17.0 |
| 309L | Low-carbon dissimilar welding | ~14.6 | ~17.0 |
While 309 is commonly used for welding austenitic stainless steels to carbon steels, its use for welding 321 to 321 introduces unnecessary compositional and property mismatch. The slight differences in thermal conductivity and thermal expansion between 309 and 321, while individually small, become significant when subjected to repeated thermal cycling at elevated temperatures.
Thermal Stress Mechanism
The thermal stress development at the dissimilar weld can be understood through the following sequence:
- During operation, the 321 base metal reaches a higher temperature than the 309 weld metal due to differences in thermal conductivity.
- The 321 material expands more than the 309 weld metal.
- The constraint imposed by the dissimilar weld creates axial tensile stress in the 309 weld region.
- Over 10 years of thermal cycling, this cyclic thermal stress accumulates fatigue damage.
- Simultaneously, high-temperature sulfidic corrosion thins the wall and initiates microcracks.
- The combination of reduced cross-sectional area (from corrosion) and accumulated thermal fatigue leads to final failure.
Engineering Practice Recommendations
This case study provides several important lessons for dissimilar steel welding in high-temperature service:
Welding Procedure Optimization
- Filler metal selection: For 321-to-321 welding, use 321 filler metal (AWS E321) to maintain compositional and property uniformity. Reserve 309 filler metal for truly dissimilar welds (e.g., carbon steel to austenitic stainless steel).
- Welding sequence: For dissimilar welds where 309 is unavoidable, design the welding sequence to minimize thermal stress concentration. Multi-pass welding with controlled interpass temperatures (150–250°C) reduces residual stress.
- Post-weld treatment: Consider post-weld heat treatment (PWHT) to relieve residual stresses, though this must be balanced against sensitization risk in austenitic stainless steels.
Corrosion Protection Strategy
- Material upgrade: Consider upgrading to more corrosion-resistant materials such as 347H, 310, or nickel-based alloys for high-temperature sulfidic service.
- Corrosion allowance: Design with adequate corrosion allowance (typically 3–6 mm for high-temperature sulfidic service) and implement periodic wall thickness monitoring.
- Process control: Monitor feedstock quality and implement process modifications to reduce sulfur content where possible.
Inspection and Monitoring
- Ultrasonic testing: Implement UT examination of circumferential welds at startup and at defined intervals.
- Thermal imaging: Use infrared thermography during operation to detect temperature anomalies at weld locations.
- Corrosion monitoring: Install corrosion coupons or electrical resistance probes at critical locations.
Key Questions and Reflections
This failure case raises an important question about the adequacy of welding procedure specifications (WPS) for dissimilar steel welds in high-temperature service. In many organizations, the selection of 309 filler metal for austenitic stainless steel welds has become a default practice without thorough consideration of the specific service conditions. Engineers must evaluate whether the convenience of using a single filler metal for multiple applications justifies the potential consequences of property mismatch in demanding service environments.
Furthermore, the progressive nature of the failure—developing over 10 years of operation—is a reminder that corrosion and fatigue are time-dependent degradation mechanisms. Inspection intervals must be based on the rate of degradation, not merely on design life assumptions.
Study Insights and Implications
The most significant lesson from this failure analysis is the principle of synergistic degradation: no single mechanism was sufficient to cause failure, but the combination of high-temperature sulfidic corrosion and dissimilar weld thermal stress created a degradation pathway that led to catastrophic failure. This case underscores the importance of holistic failure analysis that considers multiple interacting mechanisms rather than attributing failure to a single cause. For engineers involved in petrochemical plant integrity management, this case reinforces the need for thorough material selection, appropriate welding procedures, and comprehensive inspection programs that address both corrosion and mechanical degradation mechanisms.