Abrasive Wear Behavior of Fe-Cr-C Wear-Resistant Surfacing Alloy

Literature Overview

This study by Wang Zhihui and colleagues from Beijing University of Technology, published in Welding Journal (2010, Vol. 31, Issue 11, pp. 73-76), investigates the abrasive wear behavior of an Fe-Cr-C wear-resistant surfacing alloy. The research was funded by the Beijing Municipal Education Commission Science and Technology Program (Grant No. 00900054R5004). The study is directly relevant to engineers designing overlay welding solutions for abrasive wear applications.

Core Technical Content

Material and Methodology

The researchers prepared an Fe-Cr-C wear-resistant alloy on Q235 carbon steel substrate using submerged arc surfacing (SAW). Wear testing was conducted using an MLS-225 wet rubber wheel abrasive wear tester with wet quartz sand as the abrasive material. The wear mechanism was analyzed through scanning electron microscopy (SEM) observation of the worn surface and energy dispersive spectroscopy (EDS) analysis of the wear debris and wear features.

Wear Mechanism Findings

The study identified two primary wear mechanisms operating in the Fe-Cr-C surfacing alloy under the test conditions:

1. Spalling removal mechanism (dominant): Micro-cracks initiate and propagate in the surface layer, leading to the spalling off of material. This is the primary wear mechanism.
2. Micro-cutting removal mechanism (secondary): Ploughing and ploughing-creasing caused by abrasive particles results in material removal through micro-cutting. This mechanism is present but less significant than spalling.

Role of Carbides in Wear

A critical finding of this study is the direct relationship between carbide particles and the spalling wear mechanism. EDS analysis of the spall pits revealed that the Cr element content in the spall pits corresponds to the Cr content range of (Cr, Fe)7C3 carbides. This indicates that the spall pits are formed by carbide fracture.

This is a significant finding because it suggests that the very carbide particles that provide wear resistance through hardness are also the source of wear damage through fracture. This creates a fundamental tension in the design of high-carbon, high-chromium surfacing alloys: increasing carbide content improves hardness but may also increase susceptibility to spalling wear.

Key Technical Parameters

Engineering Practice Integration

The finding that carbide fracture is the primary cause of spalling wear has important implications for surfacing alloy design and process optimization:

1. Carbide size and distribution: Smaller, more uniformly distributed carbides are less likely to fracture under impact loading than large, isolated carbides. This suggests that process parameters promoting fine carbide precipitation should be used.
2. Matrix ductility: A ductile matrix can accommodate the stress concentrations around carbide particles, reducing the likelihood of crack initiation. The balance between matrix hardness and ductility is critical.
3. Layer thickness: Thicker surfacing layers may provide more material to absorb impact energy before reaching the brittle substrate interface. However, thicker layers are more prone to cracking during cooling.
4. Preheating and interpass temperature: Controlling the thermal cycle during surfacing can influence the carbide morphology and matrix microstructure, which in turn affects wear resistance.

The use of submerged arc surfacing for this application is practical, as SAW offers high deposition rates and good productivity. However, the high heat input of SAW can lead to coarse microstructures. For applications where fine carbide distribution is critical, processes with lower heat input such as plasma arc surfacing or GTAW surfacing may be more appropriate.

The wet quartz sand wear test simulates a specific wear condition—wet, moderate-abrasivity wear. This is relevant to applications such as slurry pumps, mining equipment, and cement plant components. However, the wear mechanism may differ under dry, high-abrasivity conditions, where micro-cutting might become more dominant.

Wear Mechanism Analysis

The spalling wear mechanism can be understood through the following sequence of events:

1. Impact loading: Abrasive particles impact the surfacing layer surface, creating localized stress concentrations.
2. Crack initiation: Stress concentrations at carbide-matrix interfaces or at carbide particle boundaries initiate micro-cracks.
3. Crack propagation: Micro-cracks propagate through the matrix and along carbide-matrix interfaces.
4. Carbide fracture: Under continued loading, carbide particles fracture, creating voids.
5. Spalling: The fractured carbide particles and surrounding matrix material spall off the surface.

The EDS evidence showing Cr content in spall pits corresponding to (Cr, Fe)7C3 confirms that the spalling originates from carbide fracture. This is distinct from matrix-dominated wear, where the spall pits would show the matrix composition.

Study Insights and Reflections

This study provides a valuable contribution to our understanding of the wear mechanisms in Fe-Cr-C surfacing alloys. The identification of carbide fracture as the primary cause of spalling wear is a critical insight that has direct implications for alloy design.

One important consideration is that the wear behavior of surfacing alloys is highly dependent on the specific wear conditions. The wet quartz sand test conditions used in this study are relatively mild compared to, for example, dry silica sand or hard rock abrasives. Under more severe conditions, the wear mechanism might shift from spalling to micro-cutting or even to adhesive wear.

The study also highlights the complexity of optimizing surfacing alloys for wear resistance. Simply increasing the hardness of the overlay does not necessarily improve wear resistance if it leads to increased brittleness and carbide fracture. The optimal design requires a careful balance between hardness (provided by carbides) and toughness (provided by the matrix).

Another consideration is the role of the substrate. Q235 steel is a relatively soft carbon steel, and the interface between the hard surfacing layer and the soft substrate can be a weak point. Under high impact loading, the substrate may deform, leading to delamination of the surfacing layer. This is a practical concern that should be addressed in engineering design.

Summary

This study by Wang Zhihui and colleagues provides important insights into the abrasive wear behavior of Fe-Cr-C surfacing alloys, identifying carbide fracture as the primary cause of spalling wear under wet quartz sand abrasion. The EDS evidence linking spall pits to (Cr, Fe)7C3 carbide composition is compelling and has direct implications for alloy design. Engineers should consider that optimizing wear resistance requires balancing carbide content for hardness against matrix ductility for toughness, and that the wear mechanism is highly dependent on the specific service conditions.