Zhuojin Pipe Fitting Co., LtdStress and Crack Analysis of SHS-Centrifugal Stainless Steel Lined Composite Pipe
Literature Overview
This paper by Xi Wenjun, Yin Sheng, and Lai Heyi from University of Science and Technology Beijing, published in Iron and Steel (Vol. 36, No. 4, 2001, pp. 57–61), investigates the thermal stress development and cracking mechanisms in stainless steel inner-lined composite pipes manufactured using the Self-Propagating High-temperature Synthesis (SHS) combined with centrifugal casting method. The study addresses a critical quality issue in the production of corrosion-resistant composite pipes, where cracking in the stainless steel liner layer compromises the integrity and corrosion resistance of the final product.
Manufacturing Process Background
The SHS-centrifugal method is a technique for producing composite steel pipes with a corrosion-resistant stainless steel inner liner bonded metallurgically to a carbon steel outer shell. The process involves:
- Preheating the carbon steel pipe to a specific temperature.
- Placing a mixture of reactants (typically containing iron powder, chromium oxide, nickel oxide, and aluminum powder) inside the pipe.
- Initiating the self-propagating reaction, which generates temperatures exceeding 2000°C.
- Centrifuging the molten reaction products against the inner wall of the carbon steel pipe.
- Allowing the composite pipe to cool, forming a metallurgical bond between the stainless steel liner and the carbon steel shell.
The resulting composite pipe combines the mechanical strength of carbon steel with the corrosion resistance of stainless steel, making it suitable for aggressive chemical environments.
Thermal Stress Analysis
The primary focus of the study is the thermal stress development during the cooling phase after the SHS reaction and centrifugation. During cooling, differential thermal contraction between the stainless steel liner and the carbon steel shell generates residual stresses. The key factors influencing thermal stress include:
| Factor | Effect on Thermal Stress | Typical Range |
|---|---|---|
| Temperature difference during cooling | Higher ΔT increases stress | 1500–2000°C initial |
| Thermal expansion coefficient mismatch | Greater mismatch increases stress | Stainless steel: ~17×10⁻⁶/°C; Carbon steel: ~12×10⁻⁶/°C |
| Liner thickness | Thicker liner increases constraint | 3–10 mm typical |
| Cooling rate | Faster cooling increases stress gradient | Varies with process |
| Phase transformations | Can either increase or relieve stress | Depends on composition |
The thermal stress in the stainless steel liner is predominantly tensile in the circumferential direction and compressive in the radial direction, with the maximum tensile stress occurring at the outer surface of the liner (the interface with the carbon steel shell). When this tensile stress exceeds the tensile strength or fracture toughness of the stainless steel at the given temperature, cracking initiates.
Crack Mechanism Analysis
The study identified two contributing factors to cracking in the stainless steel liner:
- Inclusion-induced embrittlement: Non-metallic inclusions (oxides, sulfides) in the stainless steel liner reduce the effective cross-sectional area and act as stress concentrators. These inclusions originate from the SHS reaction products and may not be fully removed during the reaction process. The presence of inclusions reduces the ductility of the liner, making it more susceptible to cracking under thermal stress.
- Thermal stress: The residual thermal stress from differential cooling provides the driving force for crack initiation and propagation. The stress state is complex, involving both elastic and plastic deformation depending on the temperature at which the stress develops.
The interaction between these two factors creates a synergistic effect: inclusions reduce the fracture resistance, while thermal stress provides the driving force. Neither factor alone may be sufficient to cause cracking, but their combination leads to a significant reduction in the crack-free threshold.
Countermeasures and Process Optimization
The authors proposed several measures to eliminate or minimize cracking in the stainless steel liner:
- Accelerating phase separation: Promoting the separation of harmful phases during the SHS reaction reduces the concentration of embrittling elements in the final microstructure.
- Reducing inclusions: Improving the purity of the SHS reactants and optimizing the reaction conditions to minimize oxide and sulfide formation in the stainless steel liner.
- Improving ductility: Adjusting the alloy composition of the stainless steel liner to enhance its ductility at elevated temperatures, thereby increasing its tolerance to thermal stress.
- Increasing austenite content: Promoting a higher fraction of austenite in the stainless steel microstructure improves ductility and reduces the tendency for brittle fracture. This can be achieved through alloy design (e.g., increasing manganese or carbon content within specified limits) or through controlled cooling rates.
Technical Insights and Practical Considerations
From a metallurgical perspective, this study highlights the challenges of producing sound composite pipes using reactive synthesis methods. The SHS process generates extremely high temperatures that can cause significant intermixing and reaction between the liner and the shell, but the subsequent cooling introduces complex residual stress states that must be managed carefully.
The finding that inclusions play a critical role in cracking has implications for quality control. Standard non-destructive testing methods may not detect all inclusion populations, particularly fine or dispersed inclusions. Metallographic examination of sample coupons is therefore essential for verifying the soundness of the stainless steel liner.
The recommendation to increase austenite content is particularly relevant from a corrosion resistance standpoint. Austenitic stainless steels (such as 304 and 316 grades) offer superior corrosion resistance compared to ferritic or martensitic grades, and the presence of austenite in the liner also improves its ductility and crack resistance. However, excessive austenite can reduce the hardness and wear resistance of the liner, so an optimal balance must be struck.
Summary
This study provides valuable insights into the cracking mechanisms in SHS-centrifugal stainless steel lined composite pipes, identifying the combined effect of inclusion-induced embrittlement and thermal stress as the root cause. The proposed countermeasures—accelerating phase separation, reducing inclusions, improving ductility, and increasing austenite content—offer practical pathways for process optimization. Engineers involved in the design and production of corrosion-resistant composite pipes should pay careful attention to the metallurgical quality of the liner, the thermal stress management during cooling, and the alloy design that balances corrosion resistance with crack resistance.