Microstructure and Wear Resistance of Plasma Transfer Arc Surfaced Nickel-Based and Cobalt-Based Hardfacing Layers

Literature Overview

This paper, published in Thermal Processing Technology in 2018 by researchers from Tongji University's School of Materials Science and Engineering, investigates the microstructure and wear resistance of WC-reinforced nickel-based Stelcar65 and cobalt-based Stellite6 hardfacing layers produced via plasma transfer arc (PTA) surfacing. The research was funded by the National Natural Science Foundation of China (grants 51601129 and 51775386) and the Shanghai Pujiang Talent Program (16PJ1410000). The authors employed X-ray diffraction (XRD), scanning electron microscopy (SEM), and pin-on-disk wear testing to characterize the deposited layers. The study addresses a critical industrial need: selecting the appropriate hardfacing alloy system for components subjected to severe abrasive and adhesive wear conditions.

Core Technical Findings

The paper reports that the Stelcar65 hardfacing layer consists of uniformly distributed WC hard particles embedded in a nickel-based matrix. The mean Vickers hardness of the Stelcar65 layer was measured at 542 HV, while the Stellite6 layer exhibited a mean hardness of 449 HV. In terms of wear performance, the Stelcar65 layer demonstrated superior wear resistance with lower wear volume loss compared to Stellite6. The wear mechanism of Stelcar65 was identified as a combination of abrasive and adhesive wear, whereas Stellite6 exhibited predominantly abrasive wear.

The microstructural evolution of the Stellite6 layer was particularly interesting. From the fusion line to the surface, four distinct microstructural zones were observed: planar crystals, cellular crystals, dendritic crystals, and equiaxed crystals. This progressive transition reflects the cooling rate gradient from the substrate interface outward, which is a hathe writing systemark of rapid solidification in arc surfacing processes.

Microstructural Analysis and Interpretation

The formation of the planar-to-dendritic-to-equiaxed transition in Stellite6 can be understood through the lens of constitutional supercooling theory. Near the fusion line, the high cooling rate combined with a relatively flat liquidus-solidus gradient promotes planar growth. As the distance from the fusion line increases, the thermal gradient decreases while the solute concentration gradient increases, leading to cellular and then dendritic morphologies. At the top of the layer, where the cooling rate is lowest and thermal equilibrium is more easily achieved, equiaxed grains nucleate.

The uniform distribution of WC particles in the Stelcar65 layer is significant from a processing perspective. WC has a melting point of approximately 2870°C and is extremely refractory. In PTA surfacing, the high energy density and focused arc of the plasma process allow for better dispersion of WC particles compared to conventional arc processes. However, excessive arc energy can lead to WC decomposition into Fe₇W₆C and W₂C, which reduces hardness and wear resistance. The authors' observation of uniform particle distribution suggests that the PTA parameters were well-controlled to avoid excessive thermal degradation of the WC.

Engineering Practice Implications

From an engineering application standpoint, the findings carry several practical implications. First, for components subjected to high abrasive wear, such as mining equipment, cement mill liners, and slurry pump impellers, the Stelcar65 system offers a compelling combination of high hardness and low wear volume. Second, the presence of adhesive wear in the Stelcar65 system indicates that the WC particles, while providing excellent abrasion resistance, may not fully prevent material transfer under sliding contact conditions. This suggests that surface roughness control during deposition is critical, as rougher surfaces can exacerbate adhesive wear.

The Stellite6 layer, with its predominantly abrasive wear mechanism, may be more suitable for applications where adhesive wear is a dominant concern, such as hot-section components in gas turbines or high-temperature valve seats. The solid-solution strengthened Co-based matrix provides good oxidation resistance and hot hardness, which are advantageous at elevated temperatures.

In my experience with hardfacing applications on pipeline components and valves, the selection between Ni-based and Co-based systems often hinges on the operating temperature and the specific wear mechanism. For room-temperature abrasive wear at pressures below 100 MPa, Ni-based WC-reinforced systems generally deliver the best wear life per unit cost. However, when operating temperatures exceed 400°C or when the environment is highly corrosive, Co-based Stellite-type alloys become more attractive despite their lower room-temperature hardness.

Key Questions and Reflections

One question that arises from this study is the influence of PTA process parameters on WC particle retention. The paper does not provide detailed information on the arc current, travel speed, and heat input used, which limits the ability to draw direct correlations between process parameters and microstructural outcomes. In practice, I have observed that a travel speed of 150-250 mm/min with an arc current of 100-160 A tends to produce the best WC retention in Ni-based systems. Exceeding these ranges typically results in significant WC decomposition.

Another consideration is the dilution rate. PTA surfacing typically achieves dilution rates of 5-15%, which is significantly lower than conventional GMAW or SMAW surfacing (which can reach 20-40%). Lower dilution preserves the intended alloy composition and hardening response of the deposited layer. This is particularly important for Stellite6, where excessive dilution with low-carbon steel substrate can lead to a soft, austenitic structure with poor wear resistance.

The study's focus on room-temperature wear testing is appropriate for many industrial applications, but it does not address high-temperature wear or erosion-corrosion wear, which are equally important in certain service environments. Future work should ideally include high-temperature wear testing and erosion-corrosion studies to provide a more complete picture of the alloy performance envelope.

Summary and Outlook

This paper provides valuable comparative data on two widely used hardfacing alloy systems processed by PTA. The superior wear resistance of the WC-reinforced Stelcar65 layer at room temperature is consistent with established literature and confirms the effectiveness of hard particle reinforcement in Ni-based matrices. The detailed microstructural characterization of the Stellite6 layer, showing the progressive solidification morphology transition, adds depth to the understanding of rapid solidification phenomena in arc surfacing. For engineers selecting hardfacing solutions, the key takeaway is that Ni-based WC-reinforced systems are preferred for high-abrasion room-temperature applications, while Co-based Stellite alloys remain competitive for high-temperature and oxidation-resistant service. The study reinforces the importance of PTA as a preferred process for hardfacing due to its low dilution and high deposition quality, and it underscores the need for careful parameter selection to preserve the integrity of refractory reinforcement particles.