CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Cladding Method Combinations on HAZ Microstructure of Low Alloy Steel Corrosion-Resistant Cladding
Literature Overview and Background
This study investigates how different combinations of cladding welding methods affect the heat-affected zone (HAZ) microstructure of low alloy steel base metals used in nuclear power plant applications. The corrosion-resistant overlay layers are essential for protecting low alloy steel components from aggressive chemical environments, but the welding process used for cladding can significantly influence the HAZ properties and, consequently, the long-term integrity of the component.
The authors systematically compare multiple cladding methods—including submerged arc welding (SAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and plasma transferred arc welding (PTA)—and examine how sequential combinations of these methods influence the HAZ microstructure, hardness distribution, and corrosion resistance.
Core Technical Findings
HAZ Microstructure Comparison Across Cladding Methods
The study presents detailed metallographic analysis of the HAZ produced by different cladding methods:
| Cladding Method | Heat Input (kJ/mm) | HAZ Width (mm) | Primary Phase | Carbide Distribution | Hardness (HV) |
|---|---|---|---|---|---|
| SAW | 15-25 | 4.0-6.0 | Coarse bainite | Coarse M₇C₃ | 320-380 |
| GMAW | 8-15 | 2.5-4.0 | Fine bainite | Fine M₇C₃ | 350-420 |
| GTAW | 3-8 | 1.0-2.5 | Martensite + tempered | Fine MC | 400-480 |
| PTA | 5-10 | 1.5-3.0 | Fine martensite | Very fine MC | 380-450 |
The results clearly demonstrate that lower heat input methods (GTAW, PTA) produce narrower HAZ with finer microstructure and higher hardness. However, higher hardness in the HAZ can increase susceptibility to hydrogen-induced cracking and reduce ductility.
Sequential Method Combinations
The study examines several sequential combinations of cladding methods:
Combination A: GTAW (first pass) → SAW (subsequent passes)
- First pass GTAW creates a fine-grained transition layer
- Subsequent SAW passes build thickness with lower cost
- HAZ shows gradient microstructure: fine martensite near surface transitioning to coarse bainite at depth
- Overall HAZ hardness: 350-450 HV (gradient)
Combination B: PTA (first pass) → GMAW (subsequent passes)
- First pass PTA creates a very fine-grained, high-hardness transition layer
- Subsequent GMAW passes provide good deposition rate
- HAZ shows more uniform microstructure than Combination A
- Overall HAZ hardness: 380-420 HV (relatively uniform)
Combination C: GTAW (first pass) → PTA (subsequent passes)
- First pass GTAW creates a fine transition layer
- Subsequent PTA passes maintain fine microstructure throughout
- HAZ shows fine microstructure with minimal coarse grain regions
- Overall HAZ hardness: 400-460 HV (uniform, high)
Corrosion Resistance of HAZ
The study evaluates the corrosion resistance of the HAZ produced by different method combinations using potentiodynamic polarization testing in simulated nuclear service conditions:
| Method Combination | Corrosion Potential (mV vs. SCE) | Corrosion Current (μA/cm²) | Pitting Potential (mV) |
|---|---|---|---|
| SAW only | -200 | 15 | -100 |
| GMAW only | -150 | 8 | -50 |
| GTAW only | -100 | 4 | 0 |
| Combination A (GTAW+SAW) | -120 | 6 | -30 |
| Combination B (PTA+GMAW) | -80 | 3 | 20 |
| Combination C (GTAW+PTA) | -50 | 2 | 50 |
The results show that Combination C (GTAW + PTA) provides the best corrosion resistance in the HAZ, with a pitting potential 50 mV above the solution potential, indicating excellent resistance to localized corrosion.
Process-Structure-Property Correlations
Heat Input and HAZ Properties
The study establishes clear correlations between heat input and HAZ properties:
- Heat input < 5 kJ/mm: Martensitic microstructure, high hardness (400-480 HV), low toughness, good corrosion resistance
- Heat input 5-15 kJ/mm: Bainitic microstructure, moderate hardness (350-420 HV), good toughness, moderate corrosion resistance
- Heat input > 15 kJ/mm: Coarse bainitic/ferritic microstructure, low hardness (320-380 HV), high toughness, poor corrosion resistance
Recommended Method Combinations for Nuclear Applications
Based on the study findings, the following method combinations are recommended for nuclear power plant applications:
| Application | Recommended Combination | Rationale |
|---|---|---|
| High corrosion resistance required | GTAW (first) + PTA (subsequent) | Best HAZ corrosion resistance, fine microstructure |
| High toughness required | GMAW (first) + SAW (subsequent) | Moderate HAZ hardness, good toughness |
| Balanced properties required | PTA (first) + GMAW (subsequent) | Good balance of hardness, toughness, and corrosion resistance |
| Cost-sensitive applications | GTAW (first) + SAW (subsequent) | Good properties at lower cost |
Study Insights and Engineering Implications
The most valuable insight from this research is the demonstration that the cladding method combination can be strategically selected to optimize HAZ properties for specific service requirements. This provides cladding engineers with a rational basis for process selection rather than relying on empirical approaches.
The study also highlights the importance of the first pass in determining HAZ microstructure. The first pass creates the initial thermal cycle that establishes the grain structure and phase composition in the HAZ. Subsequent passes have limited influence on the already-established HAZ microstructure, but they can modify the overlay layer properties.
From a standards perspective, the study provides technical data that can support welding procedure qualification under ASME IX or NB/T 47014. The HAZ hardness and corrosion resistance data can be used to demonstrate compliance with acceptance criteria for nuclear-grade cladding welds.
In summary, this research provides a comprehensive understanding of how cladding method combinations influence HAZ microstructure and properties, offering practical guidance for process selection in nuclear power plant cladding applications.