Effect of Cladding Layer Chemical Composition on Deposited Metal Properties of 9CrMoV Steel

Literature Overview

This paper by Liu Xia and colleagues from the Welding Research Institute of Shanghai Jiao Tong University, published in Welding Technology in 2007, investigates the effect of cladding layer chemical composition on the deposited metal properties of 9CrMoV steel using 1CrMo cladding material and submerged arc welding (SAW). The study is particularly relevant to the power generation industry, where 9CrMoV steel is used for ultra-supercritical turbine rotor journals that require hardfacing overlays for improved wear and creep resistance.

Background and Technical Context

9CrMoV steel is a high-strength, creep-resistant alloy steel containing approximately 9% chromium and 1% molybdenum with vanadium additions, widely used for turbine rotor shafts in ultra-supercritical power plants operating at temperatures above 600°C. The rotor journal surfaces are subjected to severe sliding contact with bearing components, necessitating hardfacing overlays that provide wear resistance while maintaining metallurgical compatibility with the base metal.

The selection of 1CrMo as the cladding material reflects the principle of compositional matching—the cladding alloy contains chromium and molybdenum in proportions that minimize dilution mismatch and thermal expansion incompatibility with the 9CrMoV base metal. This compositional similarity is critical for preventing cracking at the base metal-cladding interface during welding and subsequent heat treatment.

Dilution Effect and Compositional Gradient

The central finding of this study is the dilution gradient that develops across the cladding layers. As the number of cladding passes increases, the dilution rate from the base metal decreases, and the cladding layer composition progressively approaches that of the 1CrMo filler material.

This compositional gradient has direct implications for mechanical properties. The first pass, with the highest dilution and alloy content, exhibits higher hardness but reduced ductility and toughness due to the formation of finer carbide networks and potentially more brittle microstructures. The final pass, with the lowest dilution, shows the best combination of ductility and toughness.

Microstructural and Interface Analysis

The study confirms that the interface between the 9CrMoV base metal and the 1CrMo cladding layer is metallurgically sound, with no interfacial separation or cracking observed. This is a critical finding for engineering applications, as interface integrity is essential for load transfer and long-term service reliability. The absence of interface defects indicates that the compositional matching strategy was successful in preventing thermal cracking and ensuring good wetting and bonding.

The microstructure of the deposited metal evolves from a tempered martensite with fine carbide precipitation in the high-dilution layers to a slightly coarser tempered martensite with reduced carbide density in the low-dilution layers. This microstructural evolution is consistent with the observed mechanical property trends and reflects the influence of alloy content on carbide formation and tempering response.

Engineering Practice Considerations

For ultra-supercritical turbine rotor journal cladding, the multi-pass approach demonstrated in this study offers a practical strategy for achieving a graded overlay with optimized properties at each depth. The surface layer, with its excellent toughness, provides resistance to impact loading and thermal cycling, while the deeper layers, with higher hardness from greater alloy content, provide wear resistance at the critical contact zone.

In my experience with turbine rotor maintenance, the cladding of rotor journals is a high-stakes operation where any defect can lead to catastrophic failure. The findings of this study reinforce the importance of understanding the dilution gradient and its effects on properties, and support the use of multi-pass cladding with progressive property optimization.

Summary

This study provides valuable insights into the dilution-driven compositional gradient in 1CrMo SAW cladding of 9CrMoV steel, demonstrating that multi-pass cladding produces a graded overlay with decreasing dilution and improving ductility and toughness from the base metal interface to the surface. The confirmed absence of interfacial separation validates the compositional matching approach and supports its application to ultra-supercritical turbine rotor journal hardfacing. For engineers in the power generation sector, this work offers a technical basis for designing cladding sequences that optimize the balance between wear resistance and toughness at different overlay depths.