Effect of Welding Process on Compatibility Between 1Cr16Ni4Mo2Cu2W1VN Steel and Co6B Alloy Cladding Layer

Literature Overview

This 2024 publication by Wu Weijian and Liu Chengmiao from Shanghai First Machine Tool Factory Co., Ltd., in collaboration with Li Quan from the Central Iron and Steel Research Institute Co., Ltd. Special Steel Research Institute, investigates the effect of welding process parameters on the metallurgical compatibility between 1Cr16Ni4Mo2Cu2W1VN steel and Co6B alloy cladding layers. This study addresses a highly specialized and technically demanding application: the cladding of a high-performance cobalt-chromium-tungsten hardfacing alloy onto a high-strength, high-toughness martensitic steel substrate.

Materials and Metallurgical Challenges

Base Material: 1Cr16Ni4Mo2Cu2W1VN Steel

The 1Cr16Ni4Mo2Cu2W1VN steel is a high-strength martensitic steel designed for demanding applications requiring a combination of high yield strength, good toughness, and wear resistance. The chemical composition and its implications for cladding compatibility are as follows:

Cladding Material: Co6B Alloy

Co6B is a cobalt-based hardfacing alloy containing approximately 5-7% chromium and 5-7% tungsten, with the balance being cobalt and iron. This alloy is known for its exceptional wear resistance, high-temperature strength, and excellent corrosion resistance in oxidizing environments. The key properties of Co6B include:

• Hardness: HRC 40-50 in the as-welded condition, HRC 50-60 after heat treatment
• Dilution tolerance: Can accommodate up to 20-30% base metal dilution without significant property degradation
• Thermal conductivity: Lower than steel, which affects the thermal cycle and solidification behavior
• Thermal expansion coefficient: Different from steel, creating residual stresses at the interface

The metallurgical challenge lies in the significant difference in composition between the base steel and the Co6B alloy. The high carbon, chromium, and tungsten content of the base steel can lead to the formation of hard, brittle carbides at the interface, while the cobalt-rich composition of Co6B creates a steep compositional gradient that affects solidification behavior and phase transformation.

Welding Process Effects on Compatibility

Process Selection

The study evaluates multiple welding processes for the cladding of Co6B onto 1Cr16Ni4Mo2Cu2W1VN steel. The key processes examined include:

1. Submerged Arc Welding (SAW): High deposition rate but high heat input, leading to significant dilution and potential cracking.
2. Gas Metal Arc Welding (GMAW/MIG): Moderate heat input and good process control, suitable for multi-pass cladding.
3. Plasma Transferred Arc (PTA) Powder Cladding: Low dilution and precise control, ideal for thin, uniform layers.
4. Gas Tungsten Arc Welding (GTAW/TIG): Low heat input but low deposition rate, suitable for thin layers and repair applications.

Heat Input and Dilution

The heat input is the primary process variable that governs the dilution rate and, consequently, the metallurgical compatibility. Higher heat input leads to greater melting of the base material, increasing the dilution rate and altering the composition of the cladding layer. The study demonstrates that the dilution rate must be carefully controlled to ensure that the Co6B layer retains its desired properties while maintaining a sound bond with the base steel.

Interface Microstructure and Phase Analysis

The microstructure at the cladding-base interface is the critical determinant of joint integrity. The study identifies several key microstructural features:

1. Dendritic solidification structure: The Co6B layer solidifies in a dendritic pattern, with the primary phase being FCC cobalt solid solution and the secondary phases being Cr7C3 and WC carbides.
2. Transition zone: A narrow region at the interface where the composition transitions from base steel to Co6B alloy. This zone may contain mixed phases including martensite, austenite, and carbides.
3. Carbide network: The high carbon and chromium content of the base steel promotes the formation of a continuous network of Cr-rich carbides at the interface, which can be detrimental to toughness.
4. Intermetallic compounds: Under certain conditions, brittle intermetallic phases may form at the interface, particularly if the thermal cycle is too slow or the dilution is too high.

Welding Sequence and Thermal Management

The welding sequence and thermal management strategy are critical for achieving a sound cladding layer. The study recommends the following approach:

1. Surface preparation: Machine the base steel surface to a depth of 2-3 mm to remove any scale or contaminated material.
2. Preheating: Preheat the base steel to 200-300°C to reduce the thermal gradient and prevent cracking in the heat-affected zone.
3. Undercutting: Create a 1-2 mm undercut groove at the edge of the cladding area to ensure proper overlap and avoid edge undercutting.
4. Multi-pass cladding: Apply 2-3 passes of Co6B alloy, with each pass overlapping the previous by 30-50%.
5. Interpass temperature control: Maintain the interpass temperature below 250°C to prevent excessive grain growth and carbide coarsening.
6. Post-weld heat treatment: Optional solution treatment at 1100-1150°C followed by rapid quenching to dissolve carbides and homogenize the microstructure.

Mechanical Properties and Performance

The mechanical properties of the cladding layer are directly influenced by the welding process parameters. The study reports the following typical values for the optimized process:

The non-optimized cladding shows significantly higher hardness but lower toughness, indicating the formation of excessive brittle carbides due to high dilution. The optimized process achieves a good balance between hardness and toughness, with a hardness of 450-550 HV and an impact energy of 30-60 J.

Engineering Applications

The 1Cr16Ni4Mo2Cu2W1VN/Co6B cladding system is particularly suitable for applications involving severe wear, erosion, and high-temperature service. Typical applications include:

• Excavator bucket teeth: Subjected to abrasive wear from soil and rock
• Cement mill liners: Exposed to grinding media and abrasive materials
• Pump impellers and wear plates: Subjected to erosion-corrosion in slurry service
• Mold cavities: Subjected to thermal fatigue and abrasive wear during metal forming

The Shanghai First Machine Tool Factory's involvement in this research underscores the practical importance of this cladding system in the machine tool industry, where wear-resistant surfaces are essential for extending component life and reducing maintenance costs.

Study Insights and Reflections

This 2024 study represents the latest state of the art in the cladding of cobalt-based alloys onto high-strength steels. The systematic investigation of process parameters and their effects on metallurgical compatibility provides valuable guidance for engineers designing cladding solutions for demanding applications. The key takeaway is that the welding process is not merely a means of depositing material, but a critical variable that determines the metallurgical quality and service performance of the cladding layer. The collaboration between an industrial manufacturer and a research institute exemplifies the industry-academia partnership model that is essential for advancing welding technology. Future research should focus on developing predictive models for interface microstructure and properties based on process parameters, as well as on exploring novel cladding materials and multi-layer cladding schemes that can further enhance the performance of this system.