Effect of TiC Addition on High-Chromium Surfacing Layer Microstructure and Wear Resistance

Literature Overview

Published in Hot Working Technology in 2012, this research by Lu Debin and colleagues from Xiangtan University investigates the influence of TiC addition on the microstructure, dilution behavior, and abrasive wear resistance of high-chromium surfacing layers deposited via self-shielded flux-cored wire open arc surfacing on Q235A steel substrates. The work was supported by the Hunan Provincial Natural Science Foundation and Xiangtan Municipal Government Joint Natural Science Fund (Grant No. 11JJ9015).

Technical Methodology

The experimental approach employs flux-cored wire self-shielded open arc surfacing, a process that offers significant advantages for field repair and large-scale surfacing operations compared to shielded metal arc or submerged arc methods. The Q235A carbon steel substrate was selected as a representative low-carbon structural steel commonly used in industrial equipment construction.

The TiC was introduced as a pre-formed carbide addition within the flux-cored wire composition. Unlike the in-situ formation approach described in the companion study on TiC-VC electrodes, this method relies on mechanical incorporation of TiC particles into the flux core, which are then transferred to the weld pool during arc melting.

Microstructural Evolution with TiC Content

The most significant finding concerns the effect of TiC addition on dilution-related microstructural features. In conventional high-chromium surfacing without TiC addition, the substantial dilution from the Q235A substrate introduces excess carbon and reduces the chromium concentration in the weld metal, leading to the formation of:

• Chromium-enriched α-Fe solid solution: These regions form where local chromium concentration remains above the solubility limit but insufficient for carbide formation.
• Network-type carbides: Chromium carbides (Cr7C3, Cr23C6) form along grain boundaries in a continuous network, creating crack initiation sites.
• Dendritic carbide distributions: During rapid solidification, carbides form preferentially along dendrite arms, creating anisotropic microstructures.

As TiC content increases in the flux-cored wire, these dilution-induced microstructural features progressively diminish and eventually disappear. The mechanism involves several factors:

1. Dilution reduction: The TiC particles increase the overall alloy content of the deposited metal, reducing the relative impact of substrate dilution on the final composition.
2. Nucleation site provision: TiC particles serve as heterogeneous nucleation sites for the chromium carbides, promoting a more uniform and finer distribution rather than network or dendritic morphologies.
3. Anisotropy reduction: The more uniform carbide distribution eliminates the directional solidification patterns that create anisotropic mechanical properties.

Wear Mechanism Analysis

The wet sand abrasive wear testing revealed that the dominant wear mechanism is micro-flaking (micro-exfoliation). This is particularly relevant because:

• Micro-flaking occurs when thin layers of material are detached from the surface by the combined action of abrasive particle impact and cyclic stress.
• The presence of uniformly distributed TiC particles within a chromium carbide matrix provides excellent resistance to this mechanism by interrupting the propagation of micro-cracks that precede flaking.
• The reduction in network-type carbides eliminates the preferential crack initiation and propagation paths that would otherwise accelerate micro-flaking.

Engineering Significance for Pipeline Applications

For pipeline and equipment repair operations, the self-shielded flux-cored wire open arc process offers practical advantages:

• Field applicability: No external gas shielding required, making it suitable for outdoor and remote pipeline repair operations.
• High deposition rate: The flux-cored wire format enables higher metal deposition rates compared to solid wire processes.
• Substrate adaptability: Effective on various carbon steel substrates commonly encountered in pipeline construction.

The elimination of network carbides through TiC addition is particularly important for pipeline applications where the surfacing layer must withstand cyclic loading and thermal cycling. Network carbides create brittle intergranular fracture paths that can initiate under fatigue loading conditions typical in pipeline service.

Key Technical Parameters and Recommendations

• TiC particle size: Optimal particle size for flux-cored wire application is typically 5-50 μm, balancing dispersibility against agglomeration tendency.
• Flux core design: TiC particles should be uniformly distributed within the flux powder blend to ensure consistent transfer to the weld pool.
• Welding parameters: Open arc surfacing typically employs currents of 180-260 A, arc lengths of 8-12 mm, and travel speeds of 150-300 mm/min for high-chromium surfacing.
• Layer thickness: Multiple thin passes (2-3 mm each) are preferred over single thick deposits to minimize residual stress and ensure uniform microstructure throughout the buildup.

Study Insights

The most valuable contribution of this research is the demonstration that TiC addition serves not merely as a hard phase reinforcement but as a microstructural modifier that fundamentally alters the dilution behavior of high-chromium surfacing systems. This insight opens new possibilities for surfacing design on low-alloy steel substrates where dilution is typically a major concern. The elimination of anisotropic features through TiC modification also has implications for the directional properties of the surfacing layer, which is particularly relevant for components subjected to multiaxial stress states.