Effect of Titanium on Microstructure and Wear Resistance of High-Chromium Alloy Surfacing Layers

Literature Overview

This investigation by Liu Yue and colleagues from Henan University of Science and Technology, published in Chinese Journal of Materials Research (2013, Vol. 27, No. 6, pp. 641-646), examines the influence of titanium addition on the microstructure and tribological performance of high-chromium alloy surfacing layers deposited on Q235 carbon steel substrates. Funded by the Henan Provincial Science and Technology Key Project (112102213117), the research addresses the critical engineering challenge of developing cost-effective wear-resistant surfacing solutions for industrial applications involving severe abrasive wear conditions.

Experimental Methodology

The study employs a combined wire-and-powder surfacing technique, where a welding wire provides the base deposition while alloy powder blocks are introduced into the weld pool to achieve the desired high-chromium composition with controlled titanium content. This hybrid approach offers advantages over pure powder-based surfacing methods, including better deposition efficiency and more uniform alloy distribution.

The characterization methodology is comprehensive, employing:

• Optical microscopy (OM) for general microstructural examination
• X-ray diffraction (XRD) for phase identification
• Spectrometric analysis for chemical composition verification
• Auger electron spectroscopy (AES) for surface composition analysis
• Scanning electron microscopy (SEM) for detailed microstructural characterization
• Abrasive wear testing machine for tribological evaluation
• Rockwell hardness testing for mechanical property assessment

Key Technical Findings

The optimal titanium content of approximately 3% achieves a hardness of 60 HRC with a wear weight loss of 0.2066 g under the specified abrasive wear test conditions. The microstructure at this composition consists of an α-Fe and γ-Fe matrix with finely distributed Ti(N,C)y carbonitride particles.

Microstructural Evolution with Titanium Addition

The fundamental microstructural features of the high-chromium surfacing layer include:

1. α-Fe phase: The ferritic matrix provides ductility and toughness to the composite microstructure.
2. γ-Fe phase: Austenitic regions contribute to thermal stability and may form under specific cooling conditions.
3. Carbides: Chromium-rich carbides (Cr7C3, Cr23C6) provide the primary wear resistance mechanism through their high hardness.
4. Ti(N,C)y carbonitrides: Titanium carbonitride particles, formed at the optimal 3% Ti content, serve as additional hard phases that enhance wear resistance through a synergistic mechanism.

Wear Mechanism Analysis

The wear behavior of the high-chromium surfacing layer can be understood through the following mechanisms:

• Micro-cutting: Abrasive particles mechanically cut into the surface, removing material through ploughing and cutting actions.
• Adhesive wear: Localized welding and tearing between the surfacing surface and the counterface occurs under high contact pressure.
• Fatigue wear: Cyclic contact stresses initiate subsurface cracks that propagate to the surface, causing material removal.

The hard carbide and carbonitride particles function as a "wear skeleton" within the softer matrix. During abrasive wear, the hard particles resist micro-cutting by the abrasive particles, while the matrix absorbs energy and prevents catastrophic fracture. This composite-like behavior, where hard particles are embedded in a tougher matrix, is the fundamental principle behind the enhanced wear resistance of the surfacing layer.

Titanium's Role in Microstructure and Performance

Titanium addition influences the surfacing layer through multiple mechanisms:

1. Nucleation site provision: Titanium nitride (TiN) and titanium carbide (TiC) particles formed during solidification serve as heterogeneous nucleation sites for chromium carbides, promoting a finer and more uniformly distributed carbide morphology.
2. Carbide stabilizer: Titanium has a strong thermodynamic affinity for carbon and nitrogen, forming stable TiC and TiN phases that prevent the coarsening of chromium carbides during solidification and subsequent cooling.
3. Grain refinement: The presence of TiN particles inhibits grain growth during solidification, resulting in a finer matrix grain structure that contributes to overall hardness and wear resistance.
4. Carbonitride formation: The Ti(N,C)y phase represents a unique titanium carbonitride that combines the hardness of TiC with the stability of TiN, providing an optimal balance of wear resistance and thermal stability.

Engineering Practice Implications

The findings have direct applications in several industrial sectors:

• Mining equipment: Excavator buckets, conveyor rollers, and crusher components benefit from the enhanced wear resistance of Ti-containing high-chromium surfacing layers.
• Cement industry: Mill liners, hopper linings, and grinding elements experience severe abrasive wear from cement clinker and raw materials.
• Agricultural machinery: Plow shares, disc blades, and tillage equipment operate under conditions of intense soil abrasion.
• Power generation: Coal handling equipment, including chutes, hoppers, and conveyor systems, faces continuous abrasive wear from coal and ash.

The 60 HRC hardness level achieved at 3% Ti content places the surfacing layer in the category of "ultra-hard" surface treatments, suitable for applications requiring resistance to severe abrasive wear while maintaining acceptable impact resistance.

Critical Reflections and Study Insights

The combined wire-and-powder surfacing technique used in this study offers practical advantages for industrial implementation. Pure powder surfacing methods, such as plasma transfer arc surfacing (PTA), require specialized equipment and consumables, while the hybrid approach can be executed with more conventional submerged arc or gas metal arc surfacing equipment, reducing capital investment requirements.

However, several practical considerations merit attention:

• Powder feed uniformity: The introduction of alloy powder blocks into the weld pool must be carefully controlled to ensure consistent Ti content across the entire surfacing layer. Variations in powder feed rate can lead to compositional non-uniformity and inconsistent wear performance.
• Multi-pass consistency: Industrial applications typically require multiple surfacing passes to achieve adequate layer thickness. Maintaining the optimal 3% Ti content across all passes requires precise control of both wire and powder feed rates.
• Heat input management: High heat input can cause coarsening of the Ti(N,C)y particles and chromium carbides, degrading the wear resistance. The study's results should be interpreted within the context of the specific heat input conditions used.

The 0.2066 g wear weight loss value, while indicating good wear resistance, should be compared with industry benchmarks for specific applications to assess practical performance. The wear test conditions (abrasive particle type, load, sliding distance, lubrication) significantly influence the absolute wear rate and must be documented for meaningful comparison with field performance data.

Reference Value and Outlook

This study demonstrates that titanium addition to high-chromium surfacing alloys is an effective strategy for enhancing wear resistance through the formation of Ti(N,C)y hard particles that function as a wear skeleton within the matrix. The optimal 3% Ti content represents a practical target for alloy design, balancing wear resistance, hardness, and processability. For engineers developing wear-resistant surfacing solutions for industrial applications, the findings provide a clear metallurgical rationale and quantitative performance data to support alloy selection and process optimization decisions.