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 and Shanghai Steam Turbine Co., Ltd., published in Welding Technology (2007, Vol. 36, Issue 5), investigates the influence of cladding layer chemical composition on the mechanical properties and microstructure of deposited metal in 9CrMoV steel applications. The research employed 1CrMo cladding material with submerged arc welding (SAW) and optimized the cladding process parameters through systematic experimentation.

Application Context

The research addresses a critical engineering need in ultra-supercritical steam turbine construction. The 9CrMoV steel is widely used for turbine rotor shaft necks, which are subjected to severe thermal cycling and mechanical loading during turbine operation. Cladding of these components is necessary to restore dimensions after machining and to provide a refined microstructure at the critical bearing surfaces. The 1CrMo cladding material is selected for its compositional similarity to the 9CrMoV base material, minimizing dilution effects and ensuring compatible mechanical properties.

Key Technical Findings

Dilution Rate Variation

The most significant finding of this research is the systematic variation in dilution rate with cladding layer position. As the distance from the base material increases, the dilution rate decreases, resulting in progressively lower alloy element content in the deposited metal. This has direct consequences for mechanical properties:

Cladding Layer Position	Dilution Rate	Alloy Content	Mechanical Behavior
Near base material	High	High	Lower ductility and toughness
Intermediate layers	Moderate	Moderate	Balanced properties
Far from base material	Low	Low	Improved ductility and toughness

Mechanical Property Evolution

The researchers observed that as the cladding layers progress away from the base material, the deposited metal exhibits improved plasticity and toughness. This is directly attributable to the reduced alloy element content resulting from lower dilution. The higher alloy content in the first few layers promotes the formation of harder but more brittle microstructures, while the subsequent layers develop more ductile microstructures as the composition approaches that of the pure 1CrMo filler metal.

Interface Integrity

A critical finding for engineering applications is that the base material and cladding layer exhibit good bonding performance with no interface separation observed. This confirms that the 1CrMo/9CrMoV combination is metallurgically compatible and that the SAW cladding process can produce sound, defect-free joints between the base material and cladding layers.

Process Optimization

The researchers optimized the SAW cladding process parameters through systematic experimentation. Key process considerations included:

Welding current and voltage: Must be controlled to achieve appropriate penetration depth without excessive dilution in the first layer.
Travel speed: Affects heat input and cooling rate, which influence microstructure and mechanical properties.
Layer thickness control: Critical for managing the dilution gradient and ensuring adequate cladding build-up.
Preheating and interpass temperature: Essential for preventing cracking in the high-alloy 9CrMoV base material and ensuring proper microstructure development.

Engineering Practice Integration

This research has direct applicability to turbine manufacturing and repair operations:

Dimensional restoration: The cladding process can restore worn shaft necks to specified dimensions while maintaining or improving mechanical properties.
Surface refinement: Multiple cladding layers provide a refined microstructure at the bearing surface, which can improve fatigue resistance and service life.
Process qualification: The research provides data for process qualification and procedure specification development for 9CrMoV turbine shaft cladding operations.
Quality control: Understanding the dilution gradient enables engineers to predict property variation through the cladding layers and establish appropriate inspection and testing protocols.

Critical Analysis and Reflections

The research provides valuable engineering data but raises several important considerations. First, the dilution rate variation through the cladding layers means that property testing must be performed at multiple depths to establish the complete property profile. Second, the improved ductility in outer layers comes at the potential cost of reduced hardness, which may be unacceptable for certain bearing applications. Third, the long-term stability of the cladding microstructure under thermal cycling conditions typical of turbine operation is not addressed in this research. Fourth, the effect of cladding on residual stress distribution in the shaft neck is not investigated, which is critical for fatigue performance assessment.

Study Insights and Implications

This research demonstrates the importance of understanding dilution effects in cladding applications and provides a practical framework for managing property variation through the cladding layers. The finding that dilution decreases with distance from the base material is fundamental to cladding process design and should be incorporated into all cladding procedure development. For turbine manufacturing engineers, this research provides confidence that 1CrMo SAW cladding can produce sound, well-bonded cladding layers on 9CrMoV shaft necks with predictable property variation. The systematic approach to process optimization and property characterization represents best practice in cladding research and should be emulated in similar applications. The research also highlights the value of understanding the composition-property relationship in cladding applications, as this knowledge enables rational selection of cladding materials and process parameters for specific engineering requirements.