Effect of Heat Treatment on Microstructure and Properties of Plasma-Surfaced Cobalt-Based Alloy on PH17-4 Martensitic Precipitation-Hardening Stainless Steel

Literature Overview

This study, published in the journal "Welding" (2012, Issue 10, pp. 65-69), investigates the influence of post-weld heat treatment on the microstructure and mechanical properties of a Stellite 12 cobalt-based alloy overlay deposited onto PH17-4 martensitic precipitation-hardening stainless steel via plasma arc surfacing. The research was conducted by Deng Dewei, Chen Rui, and Wang Dongying from Dalian University of Technology and Shenyang Blower Works Group Co., Ltd. The work is particularly relevant to engineers working on repair and overlay applications in the power generation, petrochemical, and compressor industries where PH17-4 components require enhanced surface wear and corrosion resistance.

Core Technical Findings

The investigation employed optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and Vickers microhardness testing to characterize the as-welded and heat-treated conditions. The plasma surfacing process produced a well-bonded interface between the Stellite 12 overlay and the PH17-4 substrate, with a uniform overlay microstructure and a heat-affected zone (HAZ) approximately 2 mm wide near the fusion line.

The key findings can be summarized as follows:

• As-welded condition: A distinct HAZ of approximately 2 mm width formed in the PH17-4 substrate adjacent to the fusion boundary, indicating significant thermal influence from the plasma arc process.
• Solution treatment at 1050°C: The HAZ in the substrate was completely eliminated, the substrate microstructure became homogenized, the overlay microstructure was refined, and microhardness increased.
• Aging treatments at 480°C, 540°C, and 620°C: Hard phases precipitated in the substrate, resulting in increased microhardness. The microhardness decreased with increasing aging temperature, consistent with coarsening of precipitate phases at higher temperatures.

Process and Standards Analysis

The choice of 1050°C for solution treatment is critical. PH17-4 typically undergoes solution treatment in the range of 1010-1065°C, and 1050°C represents the upper end of this range, which is sufficient to dissolve carbides and precipitates in both the substrate and the overlay. This temperature also ensures complete homogenization of the HAZ, effectively erasing the thermal damage caused by the surfacing process.

The aging temperature selection reflects standard PH17-4 precipitation-hardening practice. The standard aging temperatures for PH17-4 are 480°C (H900 condition), 540°C (H1025 condition), and 620°C (H1150 condition), corresponding to progressively lower strength and higher ductility. The observed decrease in microhardness with increasing aging temperature is consistent with the well-established relationship between aging temperature and precipitate size in Ni-precipitation-hardening stainless steels.

Integration with Engineering Practice

From a practical standpoint, this study addresses a common engineering challenge: how to restore or enhance the surface properties of PH17-4 components without degrading the bulk material's mechanical performance. In the compressor and pump industry, PH17-4 impellers, shafts, and casing components often require cobalt-based overlay for improved wear and corrosion resistance. However, the heat input from surfacing processes can create a HAZ that compromises the precipitation-hardened microstructure of the substrate.

The study demonstrates that a properly designed post-weld heat treatment sequence can:

1. Eliminate the thermally damaged HAZ through solution treatment.
2. Refine the overlay microstructure for improved hardness and wear resistance.
3. Restore and potentially enhance the substrate's strength through controlled aging.

This approach has direct implications for the repair of PH17-4 components in service, where complete component replacement may be economically or logistically impractical.

Key Questions and Reflections

Several questions arise from this study that merit further consideration:

1. The study does not report detailed dilution rates between the overlay and substrate. In practice, dilution affects the final composition and properties of the overlay, and for Stellite 12 on PH17-4, the dilution could introduce significant Ni and Cr into the overlay, potentially affecting its wear resistance.
2. The effect of multiple surfacing passes on the HAZ width and the subsequent heat treatment response is not addressed. Multi-pass surfacing is common in practice, and each subsequent pass re-heats the previous pass, potentially modifying the thermal history.
3. The study focuses on microhardness but does not address corrosion resistance, which is equally important for cobalt-based overlays in aggressive environments. The effect of heat treatment on the corrosion performance of the overlay-substrate system would be valuable information.
4. Residual stress after heat treatment is not discussed. Post-weld residual stresses can significantly affect the fatigue performance and dimensional stability of the repaired component.

Study Insights and Implications

The most significant insight from this study is the demonstration that a well-designed heat treatment cycle can not only repair the thermal damage to the PH17-4 substrate but also simultaneously improve the overlay microstructure. This dual benefit makes the approach particularly attractive for industrial applications where both surface and bulk properties are critical.

For engineers involved in component repair and refurbishment, this study reinforces the importance of integrating surfacing process parameters with post-weld heat treatment design. The plasma arc surfacing process, with its relatively low heat input compared to conventional arc processes, provides a favorable starting condition for subsequent heat treatment. The elimination of the HAZ through solution treatment effectively resets the thermal history of the substrate, allowing the precipitation-hardening mechanism to be re-established through controlled aging.

The practical recommendation from this study is that plasma surfacing of cobalt-based alloys on PH17-4 should always be followed by a solution treatment at approximately 1050°C, followed by aging at the temperature appropriate for the required mechanical properties. This approach ensures that the full benefit of both the overlay and the substrate is realized, maximizing the service life of the repaired component.