Zhuojin Pipe Fitting Co., LtdCrack Failure Analysis of Cracking Furnace Dilution Steam Pipe Tee
Literature Overview
This 2024 study by Chen Jing and colleagues from the Institute of Metal Research, Chinese Academy of Sciences, published in Pressure Vessel (Vol. 41, No. 7), presents a comprehensive failure analysis of a 304H heat-resistant stainless steel tee that cracked after only one year of service in an ethylene cracking unit dilution steam pipeline. The study integrates material composition analysis, mechanical property testing, metallographic examination, residual stress measurement, and fracture surface characterization to identify the root cause of failure.
Failure Characteristics
The investigation reveals that cracks initiated from both the inner and outer walls of the tee, with most crack locations exhibiting intergranular corrosion features. The crack propagation mode is predominantly intergranular, indicating that the failure mechanism is stress corrosion cracking (SCC) rather than fatigue or overload failure.
| Analysis Method | Key Finding | Significance |
|---|---|---|
| Chemical composition | Within 304H specification | Material selection not the primary issue |
| Mechanical properties | Within specification | No mechanical property degradation |
| Metallographic examination | Sensitization present | Chromium carbide precipitation at grain boundaries |
| Residual stress (axial, main pipe) | Maximum 147 MPa | Reached 50% of yield strength |
| Residual stress (hoop, near intersection) | Maximum 114 MPa | Reached 50% of yield strength |
| Fracture surface | Intergranular corrosion products | O, S, Cl acidic corrosive media detected |
| Crack propagation | Intergranular | Confirms SCC mechanism |
Root Cause Analysis
The root cause is identified as stress corrosion cracking resulting from the combined action of sensitization (chromium carbide precipitation at grain boundaries during welding or heat treatment), residual tensile stresses, and corrosive media (O, S, Cl-containing acidic species). The sensitization process depletes chromium at grain boundaries, making them susceptible to intergranular attack. The residual stresses, which reached approximately 50% of the material's yield strength, provided the driving force for crack initiation and propagation.
| Contributing Factor | Status | Role in Failure |
|---|---|---|
| Material sensitization | Present | Created susceptible microstructure |
| Residual tensile stress | 147 MPa axial, 114 MPa hoop | Provided crack driving force |
| Corrosive media (O, S, Cl) | Present in corrosion products | Enabled intergranular attack |
| Service environment | Dilution steam (high temperature) | Accelerated sensitization and corrosion |
Engineering Recommendations
The study recommends two primary countermeasures: material substitution (selecting a more SCC-resistant alloy such as 316H or duplex stainless steel) and stress relief treatment (solution heat treatment to eliminate sensitization and reduce residual stresses). For existing in-service tees, engineers should implement a comprehensive inspection program including eddy current testing for intergranular corrosion, residual stress measurement, and periodic corrosion monitoring.
Study Insights and Practical Implications
This case study is particularly instructive because it demonstrates that even a material within specification can fail prematurely if sensitization and residual stresses are not properly managed. The 304H alloy, while suitable for high-temperature service, is susceptible to sensitization in the 500-800°C range, which is relevant to dilution steam piping temperatures. Engineers should ensure that all 304H components undergo solution heat treatment after fabrication and that residual stress levels are verified to be below critical thresholds. The detection of both O and S and Cl in corrosion products suggests that the dilution steam may contain trace contaminants that accelerate intergranular corrosion, warranting steam quality monitoring as part of the preventive maintenance program.
This failure analysis underscores the critical importance of considering the combined effects of microstructure, residual stress, and environment in pressure component design and maintenance. For engineers responsible for pressure vessel integrity, this case should serve as a reminder that material selection alone is insufficient; the entire fabrication and heat treatment history must be evaluated to ensure long-term reliability in aggressive service environments.