Zhuojin Pipe Fitting Co., LtdEffect of Chromium Content on Microstructure and Properties of Plasma Surfaced Fe-Cr-Ti-C Alloy
Literature Overview
This paper by Yang Yang, Zong Lin, Zhou Jian, Wang Xuezhuo, Xu Junyao, and Wang Ming, published in the Journal of Shenyang University of Chemical Technology in 2023 (Vol. 37, No. 5, pp. 457-462), investigates the influence of chromium mass fraction on the microstructure and mechanical properties of plasma-transferred arc (PTA) surfaced Fe-Cr-Ti-C alloy coatings on carbon steel substrates. The research was supported by the National Natural Science Foundation of China (Grant No. 51901141) and the Liaoning Provincial Department of Education Youth Project (Grant No. LQ2017014).
Experimental Design and Methodology
The authors designed and prepared multiple Fe-Cr-Ti-C alloy powders with varying chromium mass fractions, which were then deposited onto carbon steel substrates using the PTA surfacing technique. The characterization methods employed included:
- X-ray diffraction (XRD): To identify the phase composition of the surfacing layers.
- Scanning electron microscopy (SEM): To examine the microstructural morphology and phase distribution.
- Microhardness testing: To evaluate the hardness distribution across the surfacing layer.
This multi-technique approach provides a comprehensive understanding of the relationship between composition, microstructure, and mechanical properties.
Microstructural Evolution with Increasing Chromium Content
The surfacing layers exhibited a complex microstructure composed of austenite (γ), martensite (M), M7C3 carbide, and TiC particles. The evolution of these phases with increasing chromium content followed a non-linear trend:
| Cr Mass Fraction | Dominant Phase | M7C3 Morphology | TiC Morphology |
|---|---|---|---|
| Low | Austenite + Martensite | Discontinuous network | Flower-like |
| Medium | Increased Martensite | Increasing quantity | Flower-like or blocky |
| 15.0% | Optimal balance | Hexagonal primary | Blocky |
| High | Decreased Martensite | Abundant hexagonal | Blocky |
A key observation is that the martensite content first increases and then decreases with rising chromium content. This non-monotonic behavior can be attributed to the competing effects of chromium on the austenite stability and the cooling rate during PTA surfacing. Chromium is a strong austenite stabilizer, which promotes the retention of austenite at room temperature. However, the presence of titanium and carbon also influences the phase transformations, and the interplay between these elements creates a complex phase evolution.
Hardness Distribution and Performance Optimization
The microhardness distribution across the surfacing layer exhibited a gradient profile, with the highest hardness values at the top surface of the coating. The surface hardness increased progressively with chromium content, reaching an optimal value at 15.0% Cr mass fraction, where the average microhardness at the top of the surfacing layer reached HV 0.2 1186.
The hardness gradient is a characteristic feature of PTA surfacing, resulting from the rapid cooling rates at the surface compared to the interior of the coating. The surface, which cools fastest, develops a finer microstructure with higher martensite content and more dispersed carbides, resulting in higher hardness.
Phase Analysis and Metallurgical Interpretation
The formation of M7C3 carbides is strongly influenced by chromium content. At low chromium levels, M7C3 appears as a discontinuous network along grain boundaries. As chromium content increases, the M7C3 quantity grows and its morphology transforms from a network to primary hexagonal plates. This transformation is metallurgically significant because primary hexagonal M7C3 plates are generally harder and more wear-resistant than network-type carbides, but they can also act as crack initiation sites if they are too large or too interconnected.
The TiC particles, which are extremely hard (approximately 28 GPa on the Vickers scale), appear in flower-like or blocky morphologies. These particles provide excellent resistance to abrasive wear and contribute significantly to the overall hardness of the surfacing layer. However, the distribution and morphology of TiC must be carefully controlled to avoid brittleness and cracking.
Process-Structure-Property Relationships
The PTA surfacing process is characterized by extremely high cooling rates (typically 100-1000 K/s), which promote the formation of metastable phases such as martensite and fine carbides. The process parameters—including arc current, travel speed, powder feed rate, and standoff distance—directly influence the thermal cycle and consequently the microstructure and properties of the surfacing layer.
The chromium content acts as a key composition variable that modulates the phase equilibrium and transformation kinetics during rapid solidification. At the optimal chromium level of 15.0%, the balance between hardening carbides (M7C3 and TiC) and the ductile matrix (austenite and martensite) is achieved, resulting in the highest surface hardness without compromising the structural integrity of the coating.
Engineering Practice Implications
For engineering applications requiring high-wear-resistant surfacing layers on carbon steel components, the Fe-Cr-Ti-C alloy system with 15.0% Cr represents a promising composition. Applications include:
- Pump shafts and impellers in chemical processing
- Valve components in high-wear environments
- Mining equipment wear parts
- Turbine components subject to erosion
The PTA surfacing technique offers several advantages over conventional arc surfacing, including lower dilution with the base metal, thinner surfacing layers with better property gradients, and the ability to deposit layers with composition and microstructure that cannot be achieved through conventional casting or rolling.
Reflections and Technical Insights
The non-monotonic relationship between chromium content and martensite formation is a fascinating metallurgical phenomenon. At low chromium levels, the austenite stability is relatively low, and rapid cooling promotes martensitic transformation. As chromium increases, the austenite stability rises, which initially promotes more martensite by allowing the austenite to survive longer during cooling before transforming. However, beyond a critical chromium level, the austenite becomes so stable that it persists to room temperature, reducing the martensite fraction.
The hardness value of HV 0.2 1186 achieved at 15.0% Cr is remarkably high for an iron-based surfacing alloy. This level of hardness is comparable to some cobalt-based and nickel-based surfacing alloys, which are typically more expensive. The Fe-Cr-Ti-C system therefore offers a cost-effective alternative for high-wear applications, provided that the toughness and crack resistance are adequate for the intended service conditions.
One concern that warrants further investigation is the residual stress state of the PTA surfacing layers. The rapid thermal cycling inherent in PTA surfacing can generate significant tensile residual stresses, which may compromise the fatigue resistance and service life of the component. Post-deposition stress relief heat treatment may be necessary, but this must be carefully controlled to avoid softening the hard carbide-bearing microstructure.
Summary
This research systematically demonstrates that the chromium content in Fe-Cr-Ti-C plasma surfacing alloys has a profound and non-linear influence on the microstructure and hardness of the resulting coatings. The optimal chromium content of 15.0% produces a surface hardness of HV 0.2 1186, achieved through a favorable combination of martensite, M7C3 carbides, and TiC particles. The findings provide valuable guidance for the design of high-wear-resistant iron-based surfacing alloys and highlight the importance of composition optimization in achieving desired property combinations.