Microstructure and Properties of Iron-Based Alloy Powder Block Coatings Deposited by Carbon Arc Surfacing and Laser Cladding

Literature Overview

This study, published in Hot Working Technology (Vol. 46, Issue 18, 2017, pp. 149–152) by Hu Jiao and colleagues from Liaoning University of Technology, investigates the microstructure, phase composition, microhardness, and wear resistance of Fe-05 alloy powder block coatings deposited on Q235 steel substrates using two distinct thermal processes: carbon arc surfacing (CAS) and laser cladding (LC). The research was supported by the National Natural Science Foundation of China (Grant No. 51201084). The work is particularly relevant to engineers working on surface engineering solutions for pipe components, fittings, and structural elements where enhanced wear resistance is required without full replacement of the base material.

Core Technical Findings

Microstructural Comparison

Both processes produced coatings with a characteristic two-phase morphology: a dark matrix phase with dispersed white secondary phases. The dark phase was identified as an α-(Fe,Cr) solid solution, while the white phases included a complex array of carbides and borides.

The presence of multiple carbide and boride phases indicates that the Fe-05 alloy powder block contains significant amounts of carbon, chromium, boron, nickel, and silicon. The refinement of the microstructure in laser cladding coatings, attributed to the rapid solidification rates inherent to the process, is a critical differentiator.

Hardness and Wear Performance

The laser cladding coating achieved an average microhardness of approximately 989 HV, significantly exceeding that of the carbon arc surfacing deposit. This is consistent with the well-established relationship between solidification rate, grain refinement, and hardness in rapidly solidified alloys. The finer grain structure and more uniformly distributed hard phases in the laser cladding coating contribute to both higher hardness and improved abrasive wear resistance.

Crack Analysis

Both processes produced transgranular cracks in the coatings. This is an important observation from a quality control perspective. Transgranular cracking in hardfacing deposits is typically associated with:

• Excessive cooling rates causing high residual tensile stresses
• Phase transformation stresses during solidification (austenite-to-martensite transformation)
• Mismatch in thermal expansion coefficients between the coating and the Q235 steel substrate

The occurrence of cracks in both processes suggests that the inherent brittleness of the carbide-rich microstructure is a fundamental challenge, regardless of the deposition method employed.

Engineering Practice Implications

Application to Pipe Fitting Surface Engineering

For pipe fittings—particularly elbows, tees, and reducers used in abrasive slurry service—the choice between carbon arc surfacing and laser cladding has significant economic and performance implications.

Carbon arc surfacing offers:

• Lower equipment investment (no laser system required)
• Greater flexibility in field application
• Adequate performance for moderate wear environments
• Higher dilution with base metal, which may be acceptable for Q235 substrates

Laser cladding offers:

• Superior microhardness (~989 HV) and wear resistance
• Lower dilution rates (typically 2–5% versus 15–30% for CAS)
• Better metallurgical bonding with the substrate
• Suitable for critical components where coating life extension is paramount

Process Selection Criteria for Engineering Applications

Key Questions and Reflections

The study raises several important questions for practical engineering application:

1. Crack mitigation strategies: Since both processes produce transgranular cracks, what preheat temperatures, interpass temperature controls, or post-weld heat treatment (PWHT) protocols could effectively reduce cracking? For carbon arc surfacing, a preheat of 200–300°C and interpass temperature control below 400°C might be considered. For laser cladding, adjusting the laser power and scanning speed to optimize the thermal cycle is essential.
2. Dilution control: The high dilution in carbon arc surfacing may compromise the wear resistance of the final coating. Engineers must consider whether multiple layers of deposition are necessary to achieve the desired surface composition.
3. Substrate compatibility: Q235 steel is a mild carbon steel with relatively low carbon and alloy content. The large difference in thermal properties between the Q235 substrate and the hardfacing deposit is a primary contributor to residual stresses and cracking. For pipe fittings made from higher-grade steels (e.g., API 5L X70, X80), the dilution effects would be different and require separate investigation.
4. Service temperature considerations: The study does not address the high-temperature stability of the coatings. For pipe fittings in oil and gas applications where temperatures may exceed 200°C, the stability of the carbide phases and the potential for temper embrittlement need to be evaluated.

Study Insights and Implications

This research provides valuable comparative data that engineers can use when selecting surface engineering processes for pipe components. The key takeaway is that laser cladding offers substantially superior hardness and wear resistance compared to carbon arc surfacing for the same Fe-05 alloy powder block, but at significantly higher equipment cost. For field repairs of large-diameter pipe fittings or pipeline components, carbon arc surfacing remains a practical and economical choice, provided that crack mitigation measures are implemented. The identification of specific hard phases—particularly the combination of (Fe,Cr)₇C₃, Cr₂₃C₆, and borides—provides insight into the wear mechanism and helps in predicting service life under abrasive conditions. Engineers should note that the presence of borides (Fe₂B, Fe₃B, CrFeB) contributes significantly to hardness but also increases brittleness, which must be balanced against the need for toughness in impact-loaded applications such as pipe fitting installation and handling.