Microstructure and Wear Resistance of Multi-Layer Argon Arc Surfacings with TiC Particle Reinforcement

Literature Overview

This study by Song Si-li, Zou Zeng-da, Wang Xin-hong, and Li Qing-ming from the School of Materials Science and Engineering at Shandong University investigates the microstructure and tribological performance of multi-layer, multi-pass tungsten inert gas (TIG) surfacing coatings reinforced with TiC particles. Published in the Transactions of the Welding Journal (2007, Vol. 28, No. 4, pp. 33-37), the research addresses the surface engineering of ordinary carbon steel through the in-situ formation of titanium carbide particles during the arc surfacing process. The work was supported by the Ministry of Education Doctoral Fund (Project 20020422032), reflecting its significance in advancing composite surface technologies.

Technical Methodology and Process Parameters

Surfacing Process Description

The technique involves pre-coating a mixture of titanium iron (TiFe) and graphite (C) powder onto the surface of carbon steel substrates, followed by multi-layer, multi-pass TIG melting. This approach is distinct from conventional surfacing methods in several important ways:

• The reinforcement phase (TiC) is not added as pre-formed particles but is formed in situ during the arc melting process through the reaction between titanium from TiFe and carbon from graphite.
• The multi-layer approach builds up the coating thickness progressively, allowing for gradient microstructure development.
• TIG welding provides a stable, controlled heat source suitable for surface alloying without excessive dilution from the base metal.

Process Parameters and Their Effects

The in-situ formation of TiC is a metallurgical advantage over ex-situ particle addition because it eliminates the need for expensive pre-formed TiC powder and ensures good interfacial bonding between the reinforcement and the matrix. However, it also introduces complexity in controlling the TiC particle size, distribution, and volume fraction.

Microstructure Analysis

Phase Composition

The surfacing layer microstructure consists of four primary phases:

1. Ferrite: The soft, ductile matrix phase inherited from the carbon steel base metal through dilution. Ferrite provides toughness and prevents catastrophic brittle failure under impact loading.
2. Low-carbon martensite: Formed during rapid cooling of the surfacing layer. The martensitic transformation occurs because the cooling rate from the arc temperature (approximately 2000-3000°C) to ambient temperature exceeds the critical cooling rate for the local composition. Low-carbon martensite (0.1-0.4% C equivalent) offers good hardness without the brittleness associated with high-carbon martensite.
3. In-situ TiC particles: The primary reinforcement phase. TiC has an extremely high hardness (approximately 2800 HV) and excellent thermal stability. The particle size and distribution are critical for wear resistance; optimal performance is achieved with particles in the 1-10 μm range, uniformly distributed throughout the matrix.
4. Other carbides: Including Fe₃C, Fe₇C₃, and possibly Ti₇C₃, which form from the interaction between iron, carbon, and titanium in the melt pool. These secondary carbides contribute additional hardness and wear resistance.

Hardness Distribution

The surface hardness reaches HRC 57 or above (equivalent to approximately 600 HV), with a gradient distribution characteristic. This gradient is a natural consequence of the multi-layer surfacing process:

• The outermost layers experience the highest cooling rates and lowest dilution, resulting in the highest hardness.
• Deeper layers experience greater dilution from the base metal and lower cooling rates, resulting in progressively lower hardness.
• The transition zone between the coating and the base metal provides a smooth hardness gradient that prevents stress concentration and spalling.

The gradient hardness profile is actually advantageous for wear applications because it combines the high surface hardness needed for wear resistance with the toughness of the base metal underneath, preventing delamination under impact or cyclic loading.

Tribological Performance

Friction Coefficient Behavior

The sliding wear test results reveal an important characteristic of TiC-reinforced coatings: the friction coefficient is unstable during the wear process, exhibiting large fluctuations. This behavior can be attributed to the following mechanisms:

The instability of the friction coefficient is a direct consequence of the heterogeneous microstructure. When a TiC particle is exposed at the wear surface, it creates a hard asperity that increases local friction. When the particle is worn down or fractured, the softer matrix is exposed, reducing friction. This alternating exposure creates the observed fluctuations. While this may be undesirable in applications requiring stable friction (such as precision machinery), it is generally acceptable in heavy-duty wear applications where the primary concern is material removal rate rather than friction stability.

Wear Volume and Wear Mechanisms

The most significant finding is that the wear volume of the surfacing layer is 15-20 times smaller than that of the base metal. This dramatic improvement is attributed to the combined effects of:

1. Pinpoint effect of TiC particles: The hard TiC particles resist wear by the counterface, preventing material removal from the surrounding matrix.
2. Pinning effect: TiC particles anchored in the matrix resist crack propagation and material detachment.
3. Matrix hardening: The presence of TiC and associated carbides increases the overall matrix hardness, reducing adhesive and abrasive wear.
4. Gradient structure: The hardness gradient prevents catastrophic spalling by accommodating thermal and mechanical stresses gradually.

Wear Mechanism Analysis

The wear mechanisms operating in the TiC-reinforced coating can be categorized as follows:

Engineering Applications and Relevance to Pipe Manufacturing

Surface Engineering for Pipe and Fitting Components

The multi-layer TIG surfacing technique described in this study has direct applications in the oil and gas, mining, and chemical processing industries where pipe components are subjected to severe wear conditions:

• Drill collars and downhole tools: These components experience extreme abrasive wear from formation contact and are prime candidates for TiC-reinforced surfacing.
• Valve seats and trim components: Wear resistance is critical for maintaining seal integrity over extended service periods.
• Pump impellers and casing components: Slurry service requires surfaces that resist both abrasive and erosive wear.
• Mixer shafts and agitator components: Continuous sliding contact with abrasive slurries demands high wear resistance.

Comparison with Conventional Surface Treatments

The TiC TIG surfacing approach offers a favorable balance of performance, cost, and applicability to large pipe components, particularly where the component geometry allows access for TIG torch positioning.

Critical Assessment and Process Challenges

Dilution Control

One of the primary challenges in TIG surfacing is controlling dilution from the base metal. Excessive dilution reduces the TiC volume fraction and degrades the coating properties. The multi-layer approach partially addresses this by depositing the first layer with high dilution and subsequent layers with lower dilution. However, achieving a consistent TiC volume fraction throughout the coating requires careful parameter control.

TiC Particle Morphology

The in-situ formation of TiC during TIG surfacing typically produces particles with irregular shapes and varying sizes. While this is generally acceptable for wear applications, the particle morphology affects the uniformity of wear performance. Post-weld heat treatment or controlled cooling can influence the TiC particle size and distribution.

Residual Stress

Multi-layer surfacing introduces residual stresses from thermal cycling. These stresses can be tensile in the coating surface, potentially leading to cracking, especially in thick coatings or under impact loading. Stress relief treatments or controlled interpass temperatures can mitigate this issue.

Study Insights and Implications

This study demonstrates that a relatively simple and cost-effective TIG surfacing technique, using inexpensive TiFe and graphite powders as feedstock, can achieve a dramatic 15-20 fold improvement in wear resistance over the base metal. The in-situ TiC formation approach is elegant in its simplicity—requiring no pre-formed reinforcement particles—and produces a well-bonded, gradient-structured coating with excellent tribological properties.

For pipe manufacturing engineers, the key takeaway is that surface engineering through arc surfacing can significantly extend the service life of wear-critical pipe components without the need for expensive alloy upgrades or complex manufacturing processes. The technique is particularly attractive for repair applications where replacing an entire component is impractical or uneconomical. The gradient hardness profile is a natural advantage that provides both surface protection and structural integrity, making it suitable for components subjected to combined wear and impact loading.

The instability of the friction coefficient, while potentially undesirable in precision applications, is generally acceptable in heavy-duty wear environments where the primary performance metric is material removal rate rather than friction stability. Engineers should, however, consider this characteristic when selecting surface treatments for applications with strict friction requirements.