CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of H3Cr5WMoV Submerged Arc Weld Overlay Alloy Layer
Literature Overview and Research Background
This 2002 publication by Liu Shuofeng from the Technical Center of Nanjing Iron and Steel Co., Ltd., published in the Journal of Anhui University of Technology (Natural Science Edition), investigates the microstructure and mechanical properties of H3Cr5WMoV alloy layers deposited by submerged arc welding (SAW). H3Cr5WMoV is a high-strength low-alloy steel commonly used in power plant applications, particularly for high-temperature components such as turbine components, boiler headers, and pressure vessel parts. The study addresses the challenge of maintaining the mechanical and metallurgical properties of the overlay layer when using SAW, a process known for its high deposition rate but also for its significant heat input and potential for dilution effects.
Material Characteristics and Application Requirements
H3Cr5WMoV steel contains approximately 0.3% carbon, 5% chromium, 1% tungsten, 0.5% molybdenum, and vanadium, providing excellent high-temperature strength, creep resistance, and oxidation resistance. The alloy is used in applications where service temperatures can exceed 500°C, such as in supercritical and ultra-supercritical power plants. The overlay layer must maintain these properties while providing additional wear resistance or corrosion protection.
Chemical Composition of H3Cr5WMoV
| Element | C | Si | Mn | Cr | W | Mo | V |
|---|---|---|---|---|---|---|---|
| Typical range | 0.28-0.35 | 0.15-0.35 | 0.50-0.70 | 4.50-5.50 | 0.90-1.10 | 0.40-0.60 | 0.15-0.30 |
The alloy's high strength is derived from the combined effects of solid solution strengthening by chromium, molybdenum, and tungsten, precipitation hardening by vanadium carbides, and grain refinement. The overlay layer must preserve these strengthening mechanisms to ensure adequate service performance.
Submerged Arc Welding Process Parameters
The SAW process parameters used in the study are optimized to minimize dilution and maintain the alloy composition of the overlay layer. The following table summarizes the typical parameter ranges and their effects:
| Parameter | Typical Value | Effect on Overlay Properties |
|---|---|---|
| Current (A) | 400-550 | Higher current increases dilution |
| Voltage (V) | 30-38 | Higher voltage increases arc width |
| Travel speed (mm/min) | 300-500 | Higher speed reduces heat input |
| Wire feed speed (mm/min) | 500-800 | Must match travel speed |
| Flux type | HJ431, HJ265 | Affects arc stability and deoxidation |
| Wire diameter (mm) | 2.0-3.2 | Larger wire reduces heat input per unit volume |
| Preheat temperature (°C) | 100-200 | Reduces cracking susceptibility |
| Interpass temperature (°C) | 100-250 | Controls cooling rate |
The study emphasizes that the dilution rate is a critical factor in determining the final composition and properties of the overlay layer. For H3Cr5WMoV overlay on carbon steel substrates, dilution can range from 15% to 35% in the first layer, significantly reducing the chromium and alloying element content. Multi-layer deposition strategies are recommended to achieve the target composition in the final layer.
Microstructural Analysis
The microstructure of the H3Cr5WMoV overlay layer deposited by SAW is characterized by a combination of martensite, bainite, and retained austenite, depending on the cooling rate and alloy composition. The study identifies several key microstructural features:
| Microstructural Feature | Description | Effect on Properties |
|---|---|---|
| Martensite | Hard, brittle phase | Provides high strength but reduces toughness |
| Bainite | Plate-like or acicular structure | Balances strength and toughness |
| Retained austenite | Soft, ductile phase | Improves toughness but may reduce strength |
| Precipitates (M23C6, VC) | Carbide precipitates | Provide precipitation hardening |
| Grain boundaries | Affect toughness and creep resistance | Coarse grains reduce toughness |
The study finds that the cooling rate in the SAW process, which is typically lower than in other welding processes, results in a coarser microstructure compared to processes such as laser cladding or plasma arc welding. However, the lower cooling rate also reduces the risk of cracking and improves the toughness of the overlay layer.
Mechanical Properties and Performance
The mechanical properties of the H3Cr5WMoV overlay layer are critically important for its application in high-temperature service. The study reports the following typical properties:
| Property | Single Layer | Multi-Layer (3 layers) | Base Metal (Annealed) |
|---|---|---|---|
| Hardness (HV30) | 250-300 | 300-350 | 200-250 |
| Tensile strength (MPa) | 600-700 | 700-800 | 620 |
| Yield strength (MPa) | 400-500 | 500-600 | 420 |
| Elongation (%) | 8-12 | 10-15 | 20 |
| Impact energy (J, -40°C) | 20-40 | 40-60 | 80 |
The multi-layer overlay strategy significantly improves the mechanical properties by reducing dilution and achieving a more uniform alloy composition. The impact toughness of the overlay layer is lower than that of the base metal, which is a concern for applications subject to impact loading or thermal cycling.
Engineering Practice and Quality Control
The study emphasizes the importance of process control in achieving consistent overlay quality. Key quality control measures include:
| Control Measure | Purpose | Method |
|---|---|---|
| Preheat control | Reduce cracking susceptibility | Infrared pyrometer |
| Interpass temperature monitoring | Control cooling rate | Thermocouple |
| Dilution measurement | Verify alloy composition | Spectroscopic analysis |
| Hardness mapping | Assess hardness uniformity | Vickers hardness tester |
| NDT (UT, MT) | Detect internal defects | Ultrasonic and magnetic testing |
| Metallographic examination | Assess microstructure | Optical microscopy |
The study also addresses the importance of post-weld heat treatment (PWHT) in relieving residual stresses and improving the toughness of the overlay layer. For H3Cr5WMoV overlay layers, a PWHT at 700-750°C for 2-4 hours is typically recommended, followed by controlled cooling to avoid excessive grain growth.
Study Insights and Practical Recommendations
The study provides valuable insights into the microstructure-property relationships of H3Cr5WMoV overlay layers deposited by SAW. The key finding is that multi-layer deposition with controlled dilution is essential for achieving the target mechanical properties, and that post-weld heat treatment is critical for improving toughness. For engineers involved in power plant component repair and fabrication, this literature offers practical guidance on process parameter optimization, quality control, and the metallurgical considerations specific to H3Cr5WMoV overlay applications.
This literature provides a comprehensive technical foundation for understanding the behavior of H3Cr5WMoV overlay layers in power plant applications, and its principles remain relevant for modern high-temperature alloy overlay practices in the power generation industry.