Zhuojin Pipe Fitting Co., LtdEffect of TiC on High-Chromium Surfacing Layer Microstructure and Wear Resistance
Literature Overview
This research by Lu Debin, Gong Jianxun, Xiao Yifeng, and Ding Fen from Xiangtan University was published in "Hot Working Technology" in 2012 (Volume 41, Issue 9, pages 165-168). Funded by the Hunan Provincial Natural Science Foundation and Xiangtan Municipal Science and Technology Joint Fund, the study investigates the influence of titanium carbide addition on the microstructure and wear resistance of high-chromium surfacing deposits produced using flux-cored wire self-protected open-arc welding on Q235A substrate. This work addresses a critical issue in high-chromium surfacing technology: the detrimental effects of substrate dilution on deposit properties.
Technical Methodology and Experimental Design
The researchers employed flux-cored wire self-protected open-arc welding (also known as submerged flux cored arc welding or self-shielded flux cored arc welding) to deposit high-chromium alloy layers on Q235A carbon steel substrates. This welding method is widely used in field conditions and heavy industrial applications because it does not require external shielding gas and can be used in outdoor environments.
The experimental variable was the TiC content in the flux-cored wire, with multiple levels tested to establish a systematic relationship between TiC addition and resulting deposit properties. Characterization methods included optical microscopy, scanning electron microscopy, X-ray diffraction, and microhardness testing. Wear testing was conducted using wet sand abrasion testing, with worn surface morphology analysis performed by SEM.
Core Findings: Dilution Effect Mitigation
The most significant finding of this study is that TiC addition effectively reduces the adverse effects of substrate dilution on high-chromium surfacing deposits. In conventional high-chromium surfacing without TiC addition, the dilution of chromium by the carbon steel substrate leads to the formation of undesirable phases including chromium-rich alpha-iron solid solution and network or dendritic carbides. These phases are detrimental because they reduce the overall wear resistance and introduce anisotropy into the deposit.
| TiC Content Effect | Microstructural Change | Wear Performance Change |
|---|---|---|
| Low TiC | Partial reduction of dilution-induced phases | Moderate improvement in wear resistance |
| Medium TiC | Significant reduction of alpha-Fe solid solution and dendritic carbides | Substantial improvement in wear resistance |
| High TiC | Near-elimination of dilution-induced undesirable phases | Optimal wear resistance with reduced anisotropy |
The mechanism by which TiC achieves this effect is related to the thermodynamic stability of TiC. Titanium carbide is extremely stable and resistant to dissolution during the welding process. As a result, TiC particles survive the welding thermal cycle and remain as discrete hard phases in the final deposit. Their presence effectively "anchors" the chromium content in the deposit by reducing the effective dilution rate, since the TiC phase does not participate in the dilution process.
Microstructural Analysis and Wear Mechanism
The microstructural evolution with TiC addition follows a clear pattern. Without TiC, the high-chromium deposit shows significant dilution effects: chromium-rich alpha-iron solid solution forms as a result of chromium being diluted below the level needed to form primary chromium carbides, and network or dendritic carbides form along grain boundaries and interdendritic regions. These features create anisotropic properties and weak interfaces susceptible to crack initiation.
With increasing TiC addition, these dilution-induced phases progressively diminish and eventually disappear. The resulting microstructure becomes more homogeneous, with TiC particles distributed within a high-chromium matrix. This homogenization significantly reduces the anisotropy of the deposit, which is important for applications where the deposit must resist wear from multiple directions.
The wear mechanism identified through surface morphology analysis is micro-flaking (micro-spalling). In this wear mechanism, small fragments of the deposit material are detached by the abrasive particles, creating a surface characterized by shallow, relatively uniform pits. This is generally considered a more benign wear mechanism compared to ploughing or cutting, as it indicates that the deposit material is cohesive and the wear process is distributed rather than concentrated.
Engineering Implications for Pipeline and Component Applications
For engineers working with pipeline components, pump impellers, and other equipment subject to abrasive wear, this study provides valuable guidance. The high-chromium surfacing system with TiC addition offers several advantages: first, the reduced dilution sensitivity means that the deposit properties are more predictable and consistent across different substrate geometries and thicknesses. Second, the reduced anisotropy means that the deposit can resist wear from varying directions, which is important for components experiencing multidirectional wear. Third, the elimination of network carbides reduces the risk of intergranular cracking during thermal cycling.
The flux-cored wire self-protected open-arc welding method used in this study is particularly relevant for field repair applications where external shielding gas is not available. This makes the technology accessible for pipeline maintenance, mining equipment repair, and other field conditions.
Critical Reflections and Practical Considerations
From a practical standpoint, the study demonstrates that TiC addition is an effective strategy for overcoming one of the most persistent problems in high-chromium surfacing technology: substrate dilution. The approach is elegant because it does not require changes to the welding process itself, only to the consumable composition. This means that existing welding equipment and procedures can be used, minimizing the barriers to adoption.
However, several practical considerations remain. The optimal TiC content for a given application requires careful determination, as excessive TiC can affect weld bead shape and may introduce its own set of processing challenges. The cost of TiC addition relative to the performance improvement should be evaluated on a total cost of ownership basis, considering the extended service life of the protected component. Additionally, the study focuses on wet sand abrasion testing, which may not fully represent all service conditions, particularly those involving combined wear-corrosion or high-temperature wear.
This research contributes meaningfully to the understanding of how to design high-chromium surfacing systems that maintain their intended properties despite substrate dilution. The practical impact is significant for engineers seeking reliable abrasion-resistant coatings on carbon steel components where field welding conditions prevail.