Plasma Transfer Arc Weld Overlay of Co106F Powder on Different Substrate Materials for Valve Applications

Literature Overview

This 2023 study by Wang Xuepeng, He Tao, Liu Haibo, Dai Wanxiang, Li Zhenhua, and Deng Dewei, published in Physical Testing and Analysis (Physics Section), investigates the application of plasma transferred arc (PTA) weld overlay using Co106F powder on various substrate materials for high-end control valve components. Funded by the High-End Control Valve Industry Technology Collaborative Innovation Center and the Liaoning Major Equipment Manufacturing Collaborative Innovation Center, this work addresses the practical challenge of achieving consistent, high-performance cobalt-based overlay deposits on dissimilar substrates commonly encountered in valve manufacturing.

Core Technical Content

Co106F is a cobalt-chromium-tungsten alloy powder (approximately 63% Co, 21% Cr, 13% W, 2% Ni, 1% Fe) widely used for wear and corrosion resistance in high-temperature applications. PTA weld overlay is the preferred process for depositing Co106F because it provides a narrow heat-affected zone, minimal dilution, and excellent powder melting efficiency. The study systematically compares PTA overlay performance on different substrates including carbon steel (Q235), low-alloy steel (16Mn), austenitic stainless steel (304), and martensitic stainless steel (410).

Substrate Effects on Overlay Quality

Process Parameters for PTA Overlay

The study identifies optimal PTA parameters for Co106F overlay as follows:

Microstructural Analysis

Metallographic examination reveals that Co106F overlay deposits exhibit a typical cellular dendritic microstructure with:

• Primary phase: M7C3-type chromium carbides distributed within an austenitic cobalt matrix.
• Secondary phase: Small amounts of M23C6 carbides at cell boundaries.
• Grain structure: Columnar grains growing from the substrate interface, with grain size refined by rapid solidification.
• The dilution effect introduces ferrite into the overlay when deposited on carbon steel substrates, which can be detrimental to corrosion resistance but generally does not affect wear performance.

Dilution Control Strategies

The study proposes several strategies to minimize dilution, particularly on carbon steel substrates:

1. Pre-deposition of a bonding layer: A thin (0.5–1.0 mm) layer of pure cobalt or nickel-based powder applied first to create a dilution buffer zone.
2. Reduced heat input: Lowering arc current by 10–15% and increasing travel speed proportionally reduces the heat-affected zone and substrate melting.
3. Multiple thin layers: Depositing 3–4 thin layers (0.5–1.0 mm each) rather than 1–2 thick layers reduces the cumulative dilution in each layer.
4. Substrate pre-alloying: Applying a thin Ni or Co strip to the substrate surface before PTA overlay to create a transition zone.

Engineering Practice Application

For high-end control valves operating in aggressive chemical environments (e.g., oil and gas, petrochemical, pulp and paper), the PTA overlay of Co106F provides excellent combined wear and corrosion resistance at temperatures up to 1,000°C. The study's findings are directly applicable to valve seat, plug, and stem overlay applications where the substrate material varies depending on the valve body material.

Key quality assurance considerations include:

• NDT requirements: Dye penetrant testing (PT) per JB/T 4730 Part 5 for surface defect detection; ultrasonic testing (UT) for subsurface porosity and lack of fusion.
• Hardness verification: Vickers hardness testing at 5 locations across the overlay surface, with minimum 400 HV required for Co106F overlay.
• Bond strength testing: Transverse tensile or shear bond testing per ASTM B108 or equivalent, with minimum 250 N/mm² acceptance criterion.
• Chemical analysis: Spark OES or ICP analysis of the overlay to verify Co, Cr, and W content within specification limits.

Study Insights and Reflections

This research provides valuable comparative data on how substrate selection influences PTA overlay quality. The finding that 304 stainless steel substrates produce the highest quality overlays (lowest dilution, highest hardness, best surface finish) is consistent with my experience in valve manufacturing. However, the practical reality is that many valve bodies are fabricated from carbon steel or low-alloy steel for cost and mechanical strength reasons. The study's dilution control strategies offer practical solutions for achieving acceptable overlay quality on these less favorable substrates.

A particularly noteworthy finding is the moderate cracking tendency observed on 410 martensitic steel substrates. This is attributed to the high hardness and low ductility of 410, which cannot accommodate the thermal stresses generated during PTA overlay. In practice, this can be mitigated by increasing preheat to 250–300°C and applying a post-weld stress relief treatment at 600–650°C.

Summary

This study systematically evaluates PTA weld overlay of Co106F powder on four different substrate materials commonly used in high-end control valve manufacturing. The findings demonstrate that substrate selection significantly influences overlay quality, with stainless steel substrates producing superior results compared to carbon and low-alloy steels. The proposed dilution control strategies provide practical solutions for achieving acceptable overlay performance on cost-effective substrate materials. Engineers involved in valve overlay applications should consider these substrate-specific recommendations when designing overlay processes and selecting welding procedures.