CLADDING TECHNOLOGY SHANXI CO., LTDCrack Analysis and Improvement of Stellite Alloy Weld Overlay Layer on Domestic Steam Turbine Pump Balance Disc - Technical Study Reflection
Literature Overview
This technical study presents a detailed root cause analysis of cracking in a Stellite alloy weld overlay layer deposited on the balance disc of a domestic steam turbine pump, along with the corrective measures implemented to eliminate the cracking problem. This topic represents a classic case study in weld overlay quality assurance and defect prevention, directly within my area of expertise in cladding and bimetal product manufacturing.
The balance disc is a critical component in steam turbine pumps that controls axial thrust by maintaining pressure balance across its faces. The Stellite alloy overlay layer provides wear resistance and corrosion resistance to the balance disc surface, which is subjected to high-temperature, high-pressure steam service. Cracking in the overlay layer can lead to catastrophic pump failure, making the prevention of such defects a matter of safety as well as reliability.
Root Cause Analysis Methodology
The study employs a systematic root cause analysis approach, incorporating the 5W2H methodology and Failure Mode and Effects Analysis (FMEA) principles to identify the contributing factors to the cracking problem. The analysis examines both material-related factors (base metal composition, overlay material composition, welding consumable quality) and process-related factors (welding parameters, preheating, interpass temperature, post-weld treatment).
The analysis also incorporates metallographic examination, fractography analysis, and chemical composition analysis to establish the metallurgical basis for the cracking. The findings reveal that the cracking is primarily caused by a combination of high residual stresses, hydrogen embrittlement, and the formation of brittle intermetallic phases at the overlay-base metal interface.
Defect Characterization
| Defect Type | Location | Morphology | Cause |
|---|---|---|---|
| Surface cracks | Overlay surface | Radial, 0.1-0.5 mm wide | High residual stress + HIC |
| Intergranular cracks | Overlay grain boundaries | Intergranular, branching | Sensitization + stress |
| Interface cracks | Overlay-base metal interface | Planar, along interface | Dilution + thermal mismatch |
| Undercut cracks | Overlay edge | Edge cracking | Stress concentration |
| Hot cracks | Overlay weld beads | Transgranular, dendritic | Low-melting-phase segregation |
Contributing Factors Analysis
| Factor Category | Specific Factor | Contribution to Cracking | Severity |
|---|---|---|---|
| Material | Base metal carbon content | Promotes carbide formation at interface | High |
| Material | Overlay material Cr content | Promotes sensitization and HIC | High |
| Material | Base metal hydrogen content | Promotes hydrogen embrittlement | Medium |
| Process | Insufficient preheating | Increases cooling rate and residual stress | High |
| Process | Excessive interpass temperature | Promotes grain growth and sensitization | Medium |
| Process | Inadequate post-weld stress relief | Leaves high residual stresses | High |
| Process | Welding sequence | Creates unfavorable stress distribution | Medium |
| Design | Overlay thickness | Affects residual stress magnitude | Medium |
| Design | Base metal geometry | Creates stress concentration | Low |
Corrective Measures Implemented
Based on the root cause analysis, the following corrective measures were implemented:
- Material optimization: Selected a lower-carbon base metal grade and specified a modified Stellite alloy with reduced chromium content to minimize sensitization and intermetallic formation.
- Process parameter optimization: Established a controlled preheating temperature of 200-250 °C, limited interpass temperature to below 250 °C, and implemented a multi-pass welding sequence to distribute residual stresses more uniformly.
- Post-weld treatment: Implemented a two-stage post-weld heat treatment consisting of solution treatment at 1050-1100 °C followed by aging at 850-900 °C to dissolve harmful intermetallic phases and promote fine carbide precipitation.
- Hydrogen control: Implemented strict hydrogen control measures including consumable drying, base metal degreasing, and post-weld bake-out at 200-250 °C to eliminate absorbed hydrogen.
- Quality control enhancement: Established enhanced NDT requirements including UT or TOFD inspection of the overlay layer and bond interface, metallographic examination of representative samples, and hardness mapping across the overlay thickness.
Process Improvement Summary
| Parameter | Before Improvement | After Improvement |
|---|---|---|
| Preheating temperature | None or 100 °C | 200-250 °C |
| Interpass temperature | Uncontrolled | Below 250 °C |
| Post-weld treatment | None | Solution + aging |
| Hydrogen control | Basic | Comprehensive |
| NDT coverage | Visual only | UT/TOFD + MT + PT |
| Cracking rate | 15-25% | Below 1% |
| Overlay hardness | Variable | Uniform 380-420 HV |
| Bond strength | Marginal | Excellent (> 500 MPa) |
Connection to Cladding Engineering Practice
This case study directly addresses several fundamental challenges in cladding and weld overlay operations that I encounter regularly in bimetal pressure vessel fabrication:
- Residual stress management: The cracking problem was primarily driven by high residual stresses, which are a universal challenge in cladding operations. The corrective measures emphasize the importance of preheating, controlled interpass temperatures, and post-weld stress relief.
- Interface metallurgy: The formation of brittle intermetallic phases at the overlay-base metal interface is a common problem in dissimilar metal cladding. The corrective measures emphasize the importance of material selection and post-weld heat treatment to control interface microstructure.
- Hydrogen control: Hydrogen-induced cracking is a significant concern in weld overlay operations, particularly with nickel-based alloys. The corrective measures emphasize comprehensive hydrogen control from consumable preparation through post-weld bake-out.
- Systematic quality control: The case study demonstrates the importance of integrating multiple NDT methods and metallographic examination into the quality control program for weld overlay operations.
Key Insights and Reflections
The most valuable insight from this case study is the recognition that cracking in weld overlay layers is rarely caused by a single factor but rather by the interaction of multiple contributing factors. The root cause analysis reveals that the cracking problem was caused by the combined effects of insufficient preheating, inadequate post-weld treatment, hydrogen contamination, and unfavorable material selection. Addressing any single factor in isolation would not have been sufficient to eliminate the cracking problem.
This principle is universally applicable across all cladding operations. The prevention of defects in weld overlay layers requires a systematic approach that considers the entire process chain from material selection through welding execution to post-weld treatment and final inspection. The case study reinforces the importance of FMEA and systematic root cause analysis in identifying and addressing the multiple factors that contribute to weld overlay defects.
For the cladding engineer, the key takeaway is that the prevention of cracking in weld overlay layers requires a comprehensive approach that addresses material selection, process parameters, post-weld treatment, and quality control simultaneously. The case study provides a practical roadmap for implementing such a comprehensive approach, and the corrective measures described offer a proven solution to a common and critical problem in hardfacing operations.
Summary
The case study of cracking analysis and improvement in Stellite alloy weld overlay layers on steam turbine pump balance discs provides critical engineering insights for cladding practice. The systematic root cause analysis reveals that cracking is caused by the interaction of multiple factors including residual stress, hydrogen embrittlement, and interface metallurgy. The corrective measures implemented demonstrate that a comprehensive approach addressing material selection, process parameters, post-weld treatment, and quality control can effectively eliminate cracking problems. For the cladding engineer, the key takeaway is that the prevention of defects in weld overlay layers requires a systematic, multi-faceted approach that considers the entire process chain and addresses all contributing factors simultaneously. The case study serves as a valuable reference for engineers dealing with similar cracking problems in hardfacing operations across various industries.