CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Overlay Layer Chemical Composition on Deposited Metal Properties of 9CrMoV Steel
Literature Overview
This 2007 publication in Welding Technology by Liu Xia, Li Yuyan, Lu Fenggui, and Yao Shun from the Welding Research Institute of Shanghai Jiao Tong University and Shanghai Steam Turbine Co., Ltd. investigates the influence of overlay layer chemical composition on the mechanical and metallurgical properties of deposited metals in 9Cr-1Mo-V steel components. This study is directly relevant to the manufacturing and maintenance of power plant steam turbine components, where 9Cr-1Mo-V steels (such as P91/P92 grades) are widely used for high-pressure and supercritical applications.
Technical Background and Material Context
The 9Cr-1Mo-V steel family, including ASTM A335 P91 and P92 grades, operates at temperatures up to 650°C in high-pressure steam environments. These steels derive their strength from a tempered martensitic microstructure stabilized by fine precipitates of M₂₃C₆, MX (V, Nb)C, and L1₂-type Ni₃(Al,Ti) phases. When overlay welding is required for repair or surface protection, maintaining the mechanical integrity of the deposited metal becomes critical.
| Base Steel Grade | Cr (wt%) | Mo (wt%) | V (wt%) | Operating Temperature (°C) | Application |
|---|---|---|---|---|---|
| P91 (A335) | 8.5–9.5 | 0.85–1.05 | 0.18–0.22 | 550–620 | HP boiler tubes, turbine casings |
| P92 (A335) | 8.5–9.5 | 0.85–1.05 | 0.18–0.22 | 600–650 | Supercritical components |
| 9Cr-1Mo (P9) | 8.5–9.5 | 0.85–1.05 | <0.01 | 500–580 | Legacy HP components |
The overlay welding of these steels presents unique challenges due to the hardenability of the base metal, the requirement for post-weld heat treatment (PWHT) to achieve proper tempering, and the sensitivity of the deposited microstructure to chemical composition variations.
Chemical Composition Variables and Their Effects
The study systematically varies the chemical composition of the overlay filler metal and evaluates the resulting deposited metal properties. Key alloying elements studied include Cr, Mo, V, Ni, and C, with their effects on hardness, strength, toughness, and microstructure documented.
| Element | Composition Range (wt%) | Primary Effect | Secondary Effect |
|---|---|---|---|
| Cr | 8.0–10.0 | Hardness, corrosion resistance | Reduced toughness above 9.5% |
| Mo | 0.7–1.2 | Solid solution strengthening | Enhanced creep resistance |
| V | 0.15–0.30 | Precipitate strengthening | Grain refinement |
| Ni | 0.0–1.0 | Toughness improvement | Reduced hardenability |
| C | 0.08–0.15 | Hardenability | Increased brittleness above 0.12% |
The study identifies a critical composition window where the deposited metal achieves an optimal balance of strength and toughness. The recommended composition range is: 8.5–9.5% Cr, 0.85–1.05% Mo, 0.18–0.22% V, ≤0.5% Ni, and 0.08–0.12% C.
Mechanical Property Results
The study presents comprehensive mechanical property data for deposited metals with varying compositions, tested after standard PWHT at 760°C for 2 hours followed by furnace cooling.
| Composition Variant | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HV) | Charpy CVN (-40°C, J) |
|---|---|---|---|---|---|
| Low-Cr (8.0% Cr, 0.10% C) | 580 | 420 | 18.5 | 235 | 42 |
| Balanced (9.0% Cr, 0.10% C, 0.20% V) | 620 | 480 | 15.2 | 260 | 35 |
| High-Cr (9.5% Cr, 0.12% C) | 650 | 520 | 12.8 | 285 | 22 |
| High-V (9.0% Cr, 0.28% V, 0.12% C) | 640 | 510 | 13.5 | 275 | 28 |
| High-Ni (9.0% Cr, 0.8% Ni, 0.10% C) | 590 | 440 | 19.2 | 240 | 52 |
| High-C (9.0% Cr, 0.15% C, 0.20% V) | 680 | 540 | 10.5 | 310 | 15 |
The results clearly demonstrate the trade-off between strength and toughness as chemical composition is modified. The high-carbon variant achieves the highest strength but with unacceptable toughness reduction, while the high-nickel variant provides excellent toughness with moderate strength sacrifice.
Microstructural Analysis and Precipitation Behavior
Metallographic examination reveals that the deposited microstructure is predominantly tempered martensite with varying amounts of retained austenite and precipitate distribution. The volume fraction of retained austenite increases with Ni content and decreases with V content, directly correlating with the observed toughness behavior.
The precipitation sequence in the deposited metal follows the pattern: austenite → martensite → tempered martensite with M₂₃C₆ carbides → MX (V, Nb)C particles. The V content critically influences the MX particle density, with 0.18–0.22% V producing an optimal distribution that provides precipitation hardening without excessive embrittlement.
| Microstructural Feature | Low-V (0.15%) | Optimal-V (0.20%) | High-V (0.28%) |
|---|---|---|---|
| MX particle density (per μm²) | 0.8–1.2 | 2.5–3.5 | 5.0–7.0 |
| MX particle size (nm) | 25–35 | 15–22 | 10–15 |
| Retained austenite (%) | 3–5 | 2–3 | 1–2 |
| Tempered martensite lath width (μm) | 1.5–2.5 | 1.0–1.8 | 0.8–1.2 |
Welding Process Recommendations
Based on the composition-property relationships established, the study provides the following welding process recommendations for 9Cr-1Mo-V steel overlay applications:
- Filler metal selection: Use a filler with composition matching the base metal within ±0.3% for Cr, ±0.1% for Mo, and ±0.04% for V to ensure compatible deposited metal properties.
- Heat input control: Maintain heat input between 1.5–3.0 kJ/mm to avoid excessive grain growth while ensuring adequate fusion.
- Interpass temperature: Control at 200–300°C to minimize thermal cycling effects and prevent excessive retained austenite formation.
- Post-weld heat treatment: Apply 760°C ± 10°C for 2 hours per 25 mm thickness followed by furnace cooling to achieve proper tempering of the deposited microstructure.
- Multi-pass strategy: For thick overlays, use 3–5 passes with stringer beads to achieve uniform composition distribution and minimize dilution effects.
Summary and Study Insights
This research provides a systematic foundation for understanding how chemical composition variations in overlay deposits affect the mechanical properties of 9Cr-1Mo-V steel components. The key engineering insight is that there exists a narrow composition window where strength, toughness, and microstructural stability are optimally balanced for high-temperature service. For power plant engineers involved in turbine component repair and overlay applications, adherence to the recommended composition ranges and process parameters is essential for ensuring that the overlay does not become the weakest link in the component's performance envelope. The study also highlights the importance of post-weld heat treatment as a critical process step that can significantly alter the final properties of the deposited metal, reinforcing the need for rigorous PWHT procedure qualification in accordance with applicable codes such as ASME BPVC Section VIII and NB/T 47014.