Microstructure and Water Erosion Performance of Stellite Plasma Surfaced Coatings on 1Cr12Ni2W1Mo1V Stainless Steel

Literature Overview

This study by Zhang Youyi, Sun Xuejie, and Ran Chuanhai from Sichuan Vocational and Technical College of Engineering, published in Materials Protection (2019, Vol. 52, No. 1, pp. 88-91), investigates the application of plasma arc surfacing to deposit Stellite alloy coatings on 1Cr12Ni2W1Mo1V martensitic stainless steel substrates. The primary objective is to enhance the water erosion resistance of this corrosion-resistant alloy used in demanding hydraulic and marine environments where particulate-laden water flow causes accelerated material degradation. The work addresses a practical engineering challenge faced in pump impellers, valve components, and hydroelectric turbine runners where base material erosion rates are unacceptable.

Core Technical Findings

The study employs plasma arc surfacing (PAS), a process characterized by high energy density and controlled dilution rates, to achieve metallurgical bonding between the Stellite overlay and the substrate. The key findings reveal that the coating achieves excellent metallurgical bonding with the base metal, with no visible interfacial defects such as cracks, pores, or lack of fusion observed under optical microscopy. The microstructure of the Stellite overlay consists of a dendritic Co-Cr solid solution matrix with uniformly distributed carbide particles in the interdendritic regions. The primary carbide phases identified are M7C3-type chromium carbides and a smaller fraction of WC particles, which together provide the hard phase reinforcement responsible for erosion resistance.

Microhardness Distribution Analysis

The average microhardness of the Stellite overlay at 382.38 HV4.9 is approximately 1.96 times that of the base material at 195.29 HV4.9, demonstrating substantial hardness enhancement. The maximum measured hardness of 421.00 HV4.9 indicates localized regions of high carbide concentration. Notably, the second surfacing layer exhibits significantly higher hardness than the first layer, which can be attributed to the reduced thermal input per pass in subsequent layers and the resulting finer grain structure due to faster cooling rates.

Erosion Performance Mechanism

The water erosion resistance improvement is attributed to the synergistic effect of the hard carbide phases (M7C3 and WC) providing resistance to particle impact and the Co-Cr solid solution matrix offering good ductility to accommodate deformation without brittle fracture. The erosion rate of the coated surface is substantially lower than that of the uncoated base material, confirming that plasma surfacing with Stellite alloy is an effective strategy for extending service life in water erosion environments.

Process and Standards Analysis

Plasma arc surfacing offers several advantages over conventional arc surfacing processes including SMAW and SAW for this application. The process parameters typically employed include arc current in the range of 180-250 A, arc voltage of 25-30 V, travel speed of 200-350 mm/min, and shielding gas (argon) flow rate of 15-25 L/min. The dilution rate between coating and substrate is generally maintained below 15-20%, which is critical for preserving the alloy composition and properties of the Stellite coating.

From a standards perspective, the plasma surfacing process must comply with relevant qualification procedures under ASME Section IX (QW-461 for plasma arc surfacing) or equivalent national standards such as NB/T 47014 for nuclear applications. The coating thickness for water erosion applications is typically 3-6 mm, with multi-pass surfacing recommended to achieve uniform properties throughout the coating thickness. The bond strength test per ASTM B733 or equivalent should demonstrate no delamination at the coating-substrate interface under shear loading.

Engineering Practice Integration

In practical applications, the selection of Stellite alloy grade is critical. Stellite 6 (Co-Cr-W with 5-7% C) is commonly used for general water erosion service, while Stellite 21 (Co-Cr with lower carbon) may be preferred where lower hardness but better ductility is required. The surface preparation of the 1Cr12Ni2W1Mo1V substrate before surfacing must include thorough cleaning to remove scale, oil, and contamination, followed by appropriate preheating to 200-300°C to prevent cold cracking in the martensitic base metal.

The multi-layer surfacing strategy is particularly important. The first layer, which experiences the highest dilution rate, should be deposited with slightly higher heat input to ensure adequate wetting and bonding. Subsequent layers can be deposited with reduced heat input to minimize grain growth and maintain fine microstructure. The hardness gradient observed between layers suggests that process parameter optimization for each pass is beneficial rather than using identical parameters throughout.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation. First, the long-term stability of the coating under cyclic erosion-corrosion conditions has not been fully addressed. While static immersion tests demonstrate good corrosion resistance, the combined effect of mechanical erosion and chemical attack may accelerate degradation mechanisms not captured in single-mode testing. Second, the residual stress state within the multi-layer coating is a concern, as the mismatch in thermal expansion coefficients between the Co-based coating and the martensitic steel substrate may induce tensile stresses that promote cracking under thermal cycling.

From a metallurgical perspective, the presence of M7C3 carbides, while providing hardness, may be susceptible to preferential dissolution in aggressive aqueous environments, potentially creating microgalvanic cells between the carbide and the matrix. This raises concerns about the coating's performance in chloride-containing waters, where localized corrosion at carbide-matrix interfaces could initiate premature failure.

Study Insights and Implications

This research provides valuable quantitative data for engineers designing overlay solutions for water erosion service. The 1.96× hardness improvement and demonstrated erosion rate reduction provide concrete justification for selecting plasma surfacing with Stellite alloy over alternative approaches such as thermal spray coatings or hardfacing with conventional arc processes. The plasma process's ability to achieve low dilution rates and fine microstructure makes it particularly suitable for precision surfacing applications where dimensional control is important.

For engineering practice, the key takeaway is that plasma arc surfacing of Stellite on 1Cr12Ni2W1Mo1V stainless steel represents a reliable and effective technology for water erosion protection. The metallurgical bonding quality, controlled microstructure, and significant hardness enhancement collectively deliver a coating system that substantially extends component service life. Engineers should consider this technology when specifying repair or upgrade solutions for hydraulic equipment, particularly where the base material's inherent erosion resistance is insufficient for the operating conditions.