CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Cladding Layer Chemical Composition on Deposited Metal Properties of 9CrMoV Steel
Literature Overview
This study, published in 2007 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 how the chemical composition of the cladding layer influences the mechanical and metallurgical properties of deposited metal on 9CrMoV steel substrates. The research is particularly relevant to the power generation industry, where 9CrMoV (equivalent to 9Cr-1Mo-V or P91-type) steels are widely used for high-temperature pressure parts such as boiler headers, superheater tubes, and turbine components. The study addresses a critical engineering challenge: achieving a metallurgically sound cladding layer that maintains the creep strength and thermal stability of the base 9CrMoV material while providing corrosion or erosion resistance where required.
Core Technical Findings
The authors systematically varied the chemical composition of the cladding consumable — specifically adjusting chromium, molybdenum, vanadium, and carbon content — and evaluated the resulting deposited metal through tensile testing, hardness measurement, microstructural examination, and high-temperature strength characterization. The key finding is that the balance of alloying elements in the cladding layer has a decisive effect on the deposited metal's resistance to temper embrittlement and intergranular fracture at elevated temperatures.
The study demonstrates that an optimal cladding composition closely matched to the base 9CrMoV steel (approximately 9% Cr, 1% Mo, 0.15–0.20% V, and controlled carbon content of 0.08–0.12%) produces deposited metal with mechanical properties that approach those of the base material after appropriate heat treatment. Deviations from this composition window lead to either excessive softening or brittle intermetallic phase formation.
Key Technical Parameters
| Parameter | Optimal Range | Effect of Deviation |
|---|---|---|
| Cr content | 8.5–9.5 wt% | <8.5%: reduced creep strength; >9.5%: increased brittleness |
| Mo content | 0.9–1.2 wt% | <0.9%: reduced temper embrittlement resistance; >1.2%: excessive hardening |
| V content | 0.12–0.22 wt% | <0.12%: reduced high-temperature strength; >0.22%: carbide segregation |
| C content | 0.08–0.12 wt% | <0.08%: insufficient precipitation strengthening; >0.12%: increased brittleness |
| Heat treatment | 760°C × 2h + air cool | Below 700°C: incomplete tempering; above 800°C: grain coarsening |
Microstructural Analysis
The deposited metal microstructure in the optimized composition exhibits a tempered martensitic matrix with fine, uniformly distributed MX-type carbides (M(C,N) where M = V, Nb, Mo). These nano-scale carbides are critical for maintaining creep strength at temperatures up to 600°C. When the vanadium content falls below 0.12%, the MX carbide density decreases significantly, leading to a marked reduction in 650°C tensile strength — a critical concern for boiler tube applications operating at 580–620°C.
Conversely, excessive carbon content promotes the formation of larger M23C6 and M6C carbides at grain boundaries, which act as preferential sites for crack initiation under creep conditions. The study confirms through fractographic analysis that intergranular fracture becomes dominant when the carbon-to-vanadium ratio exceeds 0.8.
Engineering Practice Implications
From a fabrication standpoint, this research reinforces several critical practices for cladding 9CrMoV components:
- The welding consumable must be carefully selected to match or slightly exceed the base metal's alloy content, particularly for vanadium and molybdenum.
- Preheating to 250–300°C is essential to control cooling rates and prevent cold cracking, given the high hardenability of the 9CrMoV system.
- Post-weld heat treatment (PWHT) at 760°C is not optional but mandatory to achieve the target tempered martensitic microstructure.
- The cladding layer thickness should be kept to a minimum (typically 1–3 mm) to reduce thermal distortion and minimize dilution effects, which can shift the effective composition away from the designed window.
Key Questions and Reflections
A question that emerges from this work is whether the composition-matching approach can be extended to newer 9Cr-0.5Mo-V-Nb steels (P92/P92 equivalent), which have higher vanadium and niobium content. The fundamental metallurgical principles remain valid, but the tighter composition tolerances and the increased sensitivity to intergranular carbide precipitation in P92 suggest that the cladding consumable design must be even more precisely controlled.
Another practical consideration is the dilution effect. In actual fabrication, the cladding layer inevitably experiences 10–30% dilution from the base metal, depending on the welding process and parameters. The study's recommendations should therefore be interpreted as target compositions for the consumable that, after accounting for typical dilution, yield the desired deposited metal composition. This requires process-specific dilution modeling rather than a one-size-fits-all approach.
Summary
This 2007 study provides a rigorous, composition-based framework for designing cladding consumables for 9CrMoV steel components. Its core message — that precise control of Cr, Mo, V, and C in the deposited metal is essential for maintaining high-temperature mechanical properties — remains highly relevant to today's practices in power plant maintenance and repair welding. The research bridges the gap between metallurgical theory and field application, offering actionable composition windows that engineers can directly apply when selecting or developing welding consumables for 9CrMoV cladding operations.