Plasma Hardfacing Process Optimization of Fe90 Alloy on Low Carbon Steel

Literature Overview

This paper by Lu Hailong (Jilin Institute of Industry and Technology) and Kang Jiandong (Liaohe Oilfield Exploration Bureau Mechanical General Factory, Panjin, Liaoning) was published in "Hot Working Technology" (Vol. 41, No. 15, 2012, pp. 174–175). The study focuses on plasma arc hardfacing of Fe90 iron-based alloy onto low carbon steel substrates, investigating the effect of welding current on the microstructure, hardness, and wear resistance of the hardfacing overlay.

Core Technical Findings

The authors applied plasma arc hardfacing to deposit Fe90 alloy onto low carbon steel and characterized the resulting overlay using Rockwell hardness testing, wet sand-rubber wheel abrasive wear testing, and optical microscopy (OM) and scanning electron microscopy (SEM) for microstructural analysis.

Key results include:

The study also found that the surface performance of the hardfacing layer is superior to that of the side surface, which is an important practical observation for applications where the surface is the primary wear zone.

Interpretation of Technical Points

Plasma Arc Hardfacing Process Characteristics

Plasma arc hardfacing is a high-energy-density process that produces a narrow, deep weld pool with low dilution of the base metal. Compared to conventional GTAW or SAW hardfacing, plasma arc welding offers better control of the molten pool geometry, which is critical for achieving high alloy content in the overlay and minimizing dilution-related softening.

The Fe90 alloy is a high-carbon, high-chromium iron-based hardfacing material, typically containing approximately 9% carbon equivalent and significant chromium content. The high carbon and chromium content produces a microstructure rich in carbides (primarily M7C3 and M23C6 type chromium carbides), which provides excellent abrasion resistance but limited ductility.

Effect of Welding Current on Microstructure and Performance

The welding current directly controls the heat input and the size of the molten pool. At lower currents, the heat input is insufficient to fully melt the filler material, resulting in incomplete dilution and a microstructure that may contain unmelted particles or incomplete alloying. At higher currents, the heat input increases, leading to greater dilution of the base metal into the overlay, which reduces the effective alloy content and carbide volume fraction in the hardfacing layer.

At the optimal current of 180 A, the balance between sufficient melting and controlled dilution produces a microstructure with high carbide density and uniform distribution, yielding the maximum surface hardness of 67.7 HRC and the best wear resistance. The surface hardness exceeding the side hardness is attributed to the higher cooling rate at the surface, which promotes finer grain structure and potentially more martensitic transformation in the iron-based alloy system.

Surface versus Side Performance Difference

The observation that surface performance exceeds side performance is consistent with the directional solidification behavior in plasma arc hardfacing. The surface of the weld experiences the fastest cooling rate due to direct exposure to the atmosphere, promoting finer microstructures. The side surface, being closer to the base metal, experiences slower cooling and potentially greater dilution, resulting in coarser microstructures and lower hardness.

In practical applications, this surface-side performance gradient must be considered when designing the hardfacing geometry. For applications where the side surface is also subject to wear, additional passes or a different welding sequence may be required to achieve uniform performance.

Process Parameters and Optimization

The following table summarizes the typical process parameter ranges for plasma arc hardfacing of iron-based alloys on carbon steel:

The study's focus on current as the primary variable is appropriate for initial process development, but a comprehensive optimization would also consider travel speed, arc voltage, and wire feed rate to establish a full process window.

Engineering Practice Integration

In oilfield equipment applications such as pump shafts, valve stems, and wear rings, Fe90 hardfacing is widely used to extend component life in abrasive environments. The hardness of 67.7 HRC achieved in this study is well-suited for applications involving sliding wear against hard particles such as sand and mineral scale.

However, engineers must be aware of the limitations of Fe90 hardfacing in impact loading environments. The high carbide content and martensitic microstructure make the overlay susceptible to chipping and cracking under shock loading. For applications involving both abrasion and impact, a two-layer approach may be preferable: a Fe90 surface layer for wear resistance over a more ductile transition layer to absorb impact energy.

Key Questions and Reflections

The study does not report the dilution rate or the carbon and chromium content of the actual overlay, which are critical for understanding the microstructure-property relationship. A dilution rate of 15–25% is typical for plasma arc hardfacing, and this would significantly affect the carbide volume fraction and hardness.

Additionally, the wear test methodology (wet sand-rubber wheel) simulates a specific type of abrasive wear. The wear resistance of Fe90 may vary significantly under different wear mechanisms such as adhesive wear, erosive wear, or tribochemical wear. Engineers should validate the applicability of the reported wear performance to their specific service conditions.

The lack of microhardness traverse data across the overlay thickness is a notable omission. A hardness profile from the surface to the fusion line would reveal the gradient in microstructure and performance, which is essential for predicting real-world wear behavior.

Study Insights and Implications

This research confirms that plasma arc hardfacing of Fe90 alloy can achieve surface hardness exceeding 67 HRC on low carbon steel substrates, with the optimal welding current at 180 A. The surface-side performance gradient highlights the importance of welding sequence design for multi-pass hardfacing applications.

For engineering practice, the key takeaway is that plasma arc hardfacing offers a viable route for producing high-hardness iron-based overlays with controlled dilution, provided that process parameters are carefully optimized. The study provides a starting point for process development but requires further investigation into dilution control, multi-pass welding strategies, and performance under various wear mechanisms for comprehensive application guidance.

Conclusion

The plasma arc hardfacing of Fe90 alloy onto low carbon steel at 180 A welding current achieves an excellent surface hardness of 67.7 HRC with superior wear resistance. The surface-side performance difference underscores the need for careful welding sequence design in practical applications. Engineers should use this study as a baseline for process parameter optimization while recognizing the need for additional characterization of dilution, hardness profiles, and wear behavior under service-representative conditions.