Zhuojin Pipe Fitting Co., LtdNumerical Analysis of Stress Intensity for Austenitic Surfacing Layer Cracks in Feed Water Pump Body
Literature Overview
This paper by Liang Weibing, Zhang Jiangtao, and Wu Baoxin from Shanghai Electric Power Repair and Manufacturing General Plant Co., Ltd., published in Thermal Power Generation (2013, Vol. 42, No. 2, pp. 54–57), presents a finite element analysis (FEA) of stress intensity factors for cracks in austenitic stainless steel surfacing layers on feed water pump bodies. The study combines thermal-stress coupled analysis, structural optimization, and Larson-Miller parameter-based aging assessment to evaluate and improve the structural integrity of surfaced pump components.
Core Technical Content
Problem Statement and Background
Feed water pumps in power plants operate under high-pressure, high-temperature conditions and are susceptible to erosion-corrosion on internal surfaces. Austenitic stainless steel surfacing is commonly applied to protect pump bodies from erosion-corrosion. However, cracks have been observed in the surfacing layers of some pump bodies, posing a risk of catastrophic failure. The authors conducted a comprehensive FEA study to understand the crack initiation and propagation mechanisms and to optimize the pump body geometry to improve structural integrity.
Finite Element Model and Analysis
The FEA model incorporated the following elements:
- Geometry: Three-dimensional model of the pump body with surfacing layer, including bolt holes and threaded connections
- Materials: Elastic-plastic material properties for the pump body (carbon steel) and surfacing layer (austenitic stainless steel)
- Boundary conditions: Internal pressure, thermal loading, and mechanical loads representative of operating conditions
- Coupled analysis: Thermal-stress coupled analysis to account for thermal gradients and residual stresses
Key Analysis Results
The FEA results revealed several critical findings:
- Stress distribution: The maximum stress in the surfacing layer occurs at the plane where the wrap angle θ = 0°, corresponding to the location of maximum bending moment.
- Stress concentration: The stress at point P1 (near the bolt hole root) is significantly higher than at points P2 and P3, indicating that bolt hole geometry is a critical factor in crack initiation.
- Crack propagation tendency: The crack has a tendency to propagate outward from the threaded hole root, driven by the high stress intensity factor at this location.
- Structural optimization: The pump body geometry was optimized by adjusting the bolt hole distribution radius (R1) and the maximum outer radius of the surfacing layer (R2). The optimal configuration was identified as 2×R1 = 685 mm and 2×R2 = 605 mm, which provides the highest safety factor.
Larson-Miller Parameter Analysis
The Larson-Miller parameter (LMP) is a widely used method for predicting the long-term creep behavior of materials at elevated temperatures. The LMP is defined as:
LMP = T × (C + log₁₀(t))
where T is the absolute temperature (K), t is the rupture time (hours), and C is a material constant (typically 20 for austenitic stainless steels).
The authors used LMP analysis to evaluate the aging behavior of the surfacing layer under operating conditions. The key findings include:
- The LMP values at the surfacing layer locations correspond to material lives of several thousand hours, indicating that creep is not the primary failure mechanism.
- The thermal fatigue and stress corrosion cracking (SCC) are more likely failure modes for the surfacing layer under the given operating conditions.
- The structural optimization significantly improves the LMP values, extending the predicted service life of the pump body.
Structural Optimization Results
| Parameter | Original Design | Optimized Design | Improvement |
|---|---|---|---|
| Bolt hole distribution radius (2×R1) | 660 mm | 685 mm | 3.8% increase |
| Surfacing layer max outer radius (2×R2) | 580 mm | 605 mm | 4.3% increase |
| Maximum stress at P1 (MPa) | 450 | 380 | 15.6% reduction |
| Safety factor | 1.8 | 2.2 | 22.2% improvement |
| Predicted service life | Baseline | 30–50% increase | Significant |
The optimization results demonstrate that modest geometric changes can significantly improve the structural integrity of the pump body and surfacing layer.
Engineering Practice and Application
Surfacing Procedure for Pump Bodies
The following surfacing procedure is recommended for feed water pump bodies based on the study findings:
| Parameter | Recommended Value |
|---|---|
| Surfacing material | Austenitic stainless steel (e.g., 308L, 316L) |
| Welding process | GTAW or FCAW-G |
| Preheat | 100–150 °C |
| Interpass temperature | ≤ 200 °C |
| Number of passes | 2–3 |
| Post-weld treatment | Stress relief at 600–650 °C |
| NDT | PT + UT for surfacing layer |
Quality Control and Inspection
The following quality control measures are essential for ensuring the integrity of the surfaced pump body:
- Pre-weld inspection: Verify base metal condition, cleanliness, and geometry.
- Welding procedure qualification: Qualify the surfacing procedure per ASME Section IX or equivalent.
- In-process monitoring: Monitor welding parameters, interpass temperatures, and bead quality.
- Post-weld inspection: Conduct PT and UT for crack detection; perform hardness testing to verify surfacing layer properties.
- In-service monitoring: Implement periodic inspection programs to detect crack initiation and propagation.
Key Questions and Reflections
- Crack initiation mechanism: Is the primary crack initiation mechanism thermal fatigue, stress corrosion cracking, or mechanical overload? Each mechanism requires different mitigation strategies.
- Surfacing layer thickness: What is the optimal surfacing layer thickness for balancing erosion-corrosion protection and structural integrity? Excessive thickness may increase residual stress and crack susceptibility.
- Residual stress management: How can residual stresses in the surfacing layer be minimized? Techniques such as low-heat-input welding, stress-relief welding, and vibration stress relief should be considered.
- In-service monitoring: What are the most effective non-destructive testing methods for detecting cracks in the surfacing layer during service? Phased array ultrasonic testing (PAUT) and thermography may offer advantages over conventional UT.
Study Insights and Implications
This paper provides a comprehensive approach to evaluating and improving the structural integrity of austenitic stainless steel surfacing layers on feed water pump bodies. The integration of FEA, structural optimization, and LMP-based aging assessment offers a powerful methodology for predicting and preventing surfacing layer failures. The key insight is that modest geometric optimizations can significantly improve the safety factor and service life of surfaced components.
For contemporary power plant engineering, the principles established in this study—thermal-stress coupled analysis, structural optimization, and long-term aging assessment—are directly applicable to the design and maintenance of surfaced components in high-pressure, high-temperature service. The methodology can be extended to other components (e.g., reactor pressure vessels, heat exchangers, and piping) where surfacing is used for erosion-corrosion protection. Engineers involved in power plant maintenance and component design should adopt this integrated approach to ensure the long-term reliability of surfaced components.