Development of Flux-Cored Wire for Hardfacing Applications

Literature Overview

Wang Hong, Wang Shuanglin, Yao Fengdi (Rizhao Vocational and Technical College, Shandong Province), and Zhou Qinglin (Suzhou Tiantai Welding Materials Co., Ltd.) published a study in Welding (2007, No. 1, pp. 55–57) on the development of a flux-cored wire for hardfacing applications, specifically targeting the repair of rolling mill rolls and similar components. The study uses H08A thin steel strip as the outer sheath and systematically varies the alloying elements in the flux powder, including Cr, Mo, V, W, and B, to optimize the hardness and wear resistance of the overlay deposit. The authors employ a "multi-element, small-amount" alloying strategy to achieve a balanced combination of hardness, toughness, and wear resistance.

Core Technical Analysis

Alloy Design Philosophy

The "multi-element, small-amount" (多元少量) alloying strategy is a well-established approach in hardfacing alloy design. The principle is that adding multiple carbide-forming elements in small amounts produces a greater number of fine, dispersed carbide precipitates than adding a single element in a large amount. This results in:

• Higher hardness: Due to the increased volume fraction of hard carbide particles.
• Improved wear resistance: Due to the fine dispersion of carbides, which provides uniform wear protection.
• Better toughness: Due to the avoidance of large, brittle carbide networks that can initiate cracks.
• Refined grain structure: Due to the nucleation effect of multiple carbide phases on austenite grain growth.

The following table summarizes the alloying elements used in the study and their effects:

Microstructure and Hardness

The overlay deposits were examined using optical microscopy and SEM/EDS, and their hardness was measured using Vickers and Rockwell hardness testers. The results showed that:

• The microstructure consists of a martensitic matrix with dispersed carbide particles.
• The carbide particles are primarily M7C3 and M2C type, with minor M6C and M23C6 phases.
• The hardness of the overlay deposit ranges from 550 to 650 HV, depending on the alloy composition.
• The optimal composition (highest hardness and wear resistance) contains approximately 8% Cr, 4% Mo, 2% V, 4% W, and 0.3% B.

Wear Resistance Testing

The wear resistance of the overlay deposits was evaluated using a pin-on-disc test rig under dry sliding conditions. The results showed that:

The addition of V, W, and B progressively improved the wear resistance, with the multi-element alloy showing a 3-fold improvement over the baseline 4Cr-2Mo alloy.

Engineering Practice Integration

The developed flux-cored wire is particularly suitable for the repair of rolling mill rolls, which are subjected to severe abrasive and adhesive wear from contact with hot steel slabs. The following process parameters and practices are recommended:

1. Preheat: 200–300 °C to prevent cracking in the HAZ.
2. Welding process: GMAW (MAG) with a travel speed of 200–400 mm/min and a heat input of 1.5–3.0 kJ/mm.
3. Number of passes: 2–3 passes, with a final pass of the overlay alloy to ensure the required hardness and composition.
4. Interpass temperature: ≤ 250 °C to control the microstructure and avoid excessive grain growth.
5. Post-weld stress relief: Optional, at 500–600 °C for 1–2 hours, to reduce residual stresses.
6. Post-weld inspection: MT for surface cracks, hardness mapping for uniformity, and a wear test on a coupon if required.

The use of flux-cored wire for hardfacing offers several advantages over solid wire or SMAW:

• Higher deposition rates due to the higher deposition efficiency of the flux-cored process.
• Better alloy transfer due to the flux, which allows for a wider range of alloy compositions.
• Improved arc stability and weld quality.
• Reduced spatter and better operator comfort.

Study Insights and Reflections

This paper demonstrates the effectiveness of the "multi-element, small-amount" alloying strategy in hardfacing alloy design. The systematic approach to varying alloy composition and evaluating the resulting properties provides a clear methodology that can be applied to other hardfacing applications.

One key insight is that the addition of boron, even in small amounts (0.1–0.5%), has a disproportionate effect on hardness and wear resistance due to the formation of extremely hard boride phases (B4C, Fe2B) and the grain-refining effect of boron. This suggests that boron could be a valuable alloying element in other hardfacing alloys, provided that its detrimental effects on weldability (e.g., increased cracking susceptibility) are managed through appropriate process control.

Another important finding is that the multi-element alloy produces a more uniform microstructure with finer and more dispersed carbides compared to single-element alloys. This uniformity is critical for wear resistance, as it ensures that the overlay surface wears evenly and does not develop localized wear tracks that can lead to premature failure.

The study also highlights the importance of matching the overlay alloy to the specific wear mechanism. For abrasive wear (as in rolling mill rolls), a high-carbon martensitic alloy with hard carbides is appropriate. For erosive wear (as in slurry pumps), a tougher austenitic alloy with carbides may be more suitable. The "multi-element, small-amount" strategy can be adapted to both applications by adjusting the base composition and the type and amount of carbide-forming elements.

In summary, the development of a multi-element flux-cored wire for hardfacing applications represents a practical and effective approach to improving the wear resistance and service life of industrial components. The "multi-element, small-amount" alloying strategy, combined with systematic property evaluation, provides a robust methodology for hardfacing alloy development that can be applied to a wide range of industrial applications, from rolling mill roll repair to mining equipment and power generation components.