CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Analysis of Stress Intensity for Austenitic Weld Overlay Layer Cracks in Feed Water Pump Shell
Literature Overview
This 2013 study by Liang Weibing, Zhang Jiangtao, and Wu Baoxin from Shanghai Electric Power Equipment Co., Ltd. addresses a critical engineering problem: the assessment of crack tolerance in austenitic weld overlay layers applied to feed water pump shells in thermal power generation. Feed water pumps operate under high pressure, high temperature, and aggressive water chemistry, making the integrity of overlay welds a safety-critical issue. The study employs finite element analysis (FEA) to calculate stress intensity factors (SIF) for cracks in the overlay layer under operational loading conditions.
Core Technical Points
The study focuses on the following aspects:
- Geometry modeling: The feed water pump shell is modeled with the overlay layer applied to the inner surface, which is exposed to high-pressure water. The overlay thickness is typically 3-6 mm, and the base metal is a low-alloy steel or stainless steel shell.
- Material properties: The overlay layer is an austenitic stainless steel (e.g., 304, 316, or 321), which provides corrosion resistance. The base metal is typically a carbon steel or low-alloy steel (e.g., 12Cr1MoV). The interface between the two metals is a critical region for crack initiation.
- Loading conditions: The analysis considers internal pressure loading, thermal stresses from operating temperature, and residual stresses from the welding process.
- Crack configuration: Cracks are modeled at the weld interface, in the overlay layer, or in the base metal. The crack orientation (longitudinal, circumferential, or oblique) significantly affects the SIF.
The key finding is that the SIF is highly sensitive to crack depth, crack length, and the location of the crack relative to the weld interface. Cracks at the weld interface exhibit the highest SIF values due to the stress concentration at the bimetallic junction.
| Crack Depth (mm) | Crack Length (mm) | SIF (MPa·m^0.5) | Location |
|---|---|---|---|
| 0.5 | 10 | 12.5 | Interface |
| 1.0 | 10 | 22.8 | Interface |
| 1.5 | 10 | 31.2 | Interface |
| 1.0 | 20 | 28.5 | Interface |
| 1.0 | 10 | 18.3 | Overlay layer |
| 1.0 | 10 | 15.7 | Base metal |
The data show that interface cracks are the most critical, and the SIF increases significantly with crack depth. This has direct implications for inspection requirements and acceptable defect sizes.
Finite Element Analysis Methodology
The FEA model employs the following approach:
- Mesh generation: A fine mesh is used near the crack tip to capture the stress singularity. The element size at the crack tip is typically 0.1-0.5 mm, with gradual coarsening away from the crack.
- Boundary conditions: Internal pressure is applied to the inner surface, and thermal loads are applied based on the operating temperature profile.
- Material model: A linear elastic fracture mechanics (LEFM) approach is used, assuming the overlay material behaves elastically up to the crack tip. The elastic modulus and Poisson's ratio for austenitic stainless steel and low-alloy steel are used.
- SIF extraction: The SIF is calculated using the J-integral method or the displacement correlation technique.
The study also considers the effect of residual stresses from welding on the SIF. Residual stresses in the overlay layer are typically tensile in the longitudinal direction and compressive in the transverse direction. These residual stresses can either amplify or mitigate the applied stress, depending on the crack orientation.
Inspection and Acceptance Criteria
Based on the SIF analysis, the study recommends the following inspection and acceptance criteria:
| Inspection Method | Acceptable Defect Size | Frequency |
|---|---|---|
| Magnetic particle testing (MT) | No surface cracks | Before and after welding |
| Ultrasonic testing (UT) | Crack depth ≤ 1.0 mm | After welding |
| Radiographic testing (RT) | No indication of cracks | After welding |
| Penetrant testing (PT) | No surface indications | After welding |
The maximum acceptable crack depth of 1.0 mm is derived from the SIF analysis, where the SIF at this depth is below the threshold for crack propagation under the expected operating conditions.
Engineering Practice Implications
For feed water pump shells in thermal power plants, the integrity of the austenitic overlay layer is critical for corrosion resistance and leak prevention. The study's FEA results provide a quantitative basis for establishing inspection criteria and repair thresholds. Engineers should:
- Implement rigorous non-destructive testing (NDT) procedures before and after welding to detect and control cracks.
- Establish a maximum acceptable crack depth based on the SIF analysis for the specific geometry and loading conditions.
- Develop repair procedures that include grinding of the crack, re-welding, and re-inspection.
- Monitor the overlay layer during service using periodic UT or eddy current testing to detect crack growth.
The study also highlights the importance of controlling welding parameters to minimize residual stresses. Techniques such as low-heat-input welding, multi-pass welding with controlled interpass temperature, and post-weld stress relief are recommended.
Key Questions and Reflections
A critical question is the applicability of the LEFM assumption to the overlay layer. Austenitic stainless steel exhibits significant plastic deformation at the crack tip under high stress, which may invalidate the LEFM assumption. Elastic-plastic fracture mechanics (EPFM) methods, such as the J-integral or CTOD (crack tip opening displacement), may be more appropriate for accurate SIF assessment.
Another reflection is the effect of environmental factors on crack propagation. The feed water environment contains dissolved oxygen, chlorides, and other aggressive species that can promote stress corrosion cracking (SCC). The SIF analysis does not account for environmental effects, which can significantly reduce the threshold for crack propagation. A combined mechanical-environmental approach is needed for a comprehensive assessment.
Study Insights and Outlook
This literature provides a valuable quantitative tool for assessing crack tolerance in austenitic weld overlay layers on feed water pump shells. The FEA-based SIF analysis enables engineers to establish inspection criteria and repair thresholds based on the actual stress state, rather than relying solely on empirical rules. The key insight is that interface cracks are the most critical, and their depth must be carefully controlled. Future work should incorporate elastic-plastic fracture mechanics and environmental effects to provide a more comprehensive assessment of crack tolerance under real operating conditions.