Thermal fatigue failure occurs when heating tubes repeatedly undergo cyclic temperature rise and fall. Alternating thermal expansion and contraction generate periodic tensile and compressive thermal stress inside the pipe body. After numerous cycles, microcracks initiate on the surface and gradually extend inward to form penetrating cracks, resulting in medium leakage. This damage mainly concentrates on parts with large temperature gradients and structural discontinuities. It belongs to fatigue damage induced by cyclic thermal load, which is common in heating equipment with frequent startup and shutdown.
1. Thermal Fatigue Crack Initiation and Propagation Mechanism
When the pipe wall is heated rapidly, the surface layer expands first and is restrained by the low-temperature inner metal, producing surface tensile stress. During cooling, the outer layer shrinks faster and suffers compressive stress. Repeated switching between tension and compression forms cyclic thermal alternating stress. Surface tiny defects such as scratches and weld defects become crack sources. Under continuous cyclic load, fatigue cracks expand along the direction perpendicular to principal stress. As cracks penetrate the pipe wall thickness, medium leakage happens. Different from static stress corrosion cracking, thermal fatigue is driven purely by mechanical alternating stress from temperature fluctuation without mandatory corrosive medium participation.
2. Typical Positions Vulnerable to Thermal Fatigue Damage
Weld joints and heat-affected zones with uneven metallographic structure and stress concentration;
Inlet sections where cold medium directly mixes with hot pipe body to form sharp temperature difference;
Flange root, variable diameter and elbow structural mutation positions;
Liquid level alternating areas where gas phase and liquid phase temperature differ greatly;
Rigid fixed pipe sections unable to freely expand and deform with temperature change.
3. Key Inducing Factors Accelerating Thermal Fatigue
Frequent emergency startup and shutdown, cold medium directly injected into high-temperature pipeline;
Unreasonable pipeline fixing method with rigid clamping limiting thermal displacement;
Excessive local temperature gradient caused by uneven heating of heating components;
Surface machining burrs, scratches and welding defects acting as crack initiation points;
Material grain coarsening after long-term high-temperature service reducing fatigue resistance.
4. Full-Link Preventive Control Technical Measures
① Implement gradient temperature rise and cooling operation specifications
Slowly heat up before startup and cool down step by step when stopping to reduce instantaneous temperature difference.
② Optimize pipeline support structure
Adopt sliding supports and expansion joints to release thermal displacement and locked thermal stress.
③ Improve surface finish and eliminate surface defect sources
Grind and polish weld seams and pipe surface to remove scratches and sharp stress concentration points.
④ Optimize heating layout to avoid local overheating
Distribute heat sources evenly to prevent partial pipe wall from bearing excessive temperature gradient.
⑤ Select fine-grained stable stainless steel for frequent alternating temperature working conditions
Improve material intrinsic thermal fatigue resistance via incoming material grade optimization.
5. Prevention Effect Comparison Table
表格
| Operation & Installation Mode | Thermal Fatigue Risk | Application Suggestion |
|---|---|---|
| Frequent abrupt temperature change + rigid fixed installation | Rapid crack generation after short cycle operation | Add expansion compensation and standardize temperature control process |
| Gradual temperature regulation + flexible support + surface finishing | Greatly slow down fatigue crack propagation | Standard design and operation specification |
| Expansion joint configuration + homogenized heating + regular flaw detection | Minimum thermal fatigue failure risk | Intermittent frequent start-stop equipment preferred scheme |
Conclusion
Thermal fatigue cracks of 316 stainless steel heating tubes stem from cyclic alternating thermal stress generated by repeated temperature alternation, leading to fatigue crack nucleation and expansion. Core prevention approaches are alleviating temperature mutation amplitude, reserving thermal expansion compensation space, eliminating surface stress concentration defects and improving material fatigue performance. Strict process operation and structural optimization can effectively prevent penetrating leakage accidents caused by long-term thermal cyclic fatigue.
