Microstructure and Fretting Wear Behavior of Surfaced Metals

Literature Overview

This paper, authored by Qu Jinshan and Zhang Hui from the Tribology Research Institute at Southwest Jiaotong University, and Zhang Youyi and Li Juan from the School of Materials Science and Engineering at Xihua University, was published in the journal Journal of Xihua University (Natural Science Edition) in 2006 (Vol. 25, Issue 5, pp. 34-37). The study investigates the microstructure of surfacing metals deposited using CHR207, CHR227, and CHR237 electrodes on 45 steel substrates by manual metal arc surfacing (SMAW), and examines the fretting wear behavior of the surfacing layers under micro-motion conditions. The authors analyzed the metallurgical bonding quality, dilution effects on the first surfacing layer, and the fretting wear characteristics of the deposited metals.

Experimental Design and Materials

The experimental matrix in this study is straightforward yet informative. Three different electrode compositions were selected to represent a range of alloy chemistries commonly used in surfacing applications. CHR207, CHR227, and CHR237 are Chinese standard electrode designations, and their compositions typically vary in terms of carbon, chromium, and other alloying element content. The 45 steel substrate, with approximately 0.42-0.50 percent carbon, represents a medium-carbon structural steel widely used in mechanical engineering and power transmission applications.

The manual metal arc surfacing process was selected for its simplicity and accessibility, making the findings directly applicable to field repair and maintenance operations. Unlike mechanized surfacing processes such as plasma arc or submerged arc, SMAW does not require specialized equipment and can be performed in virtually any location. This practicality is particularly relevant for the repair of worn components in power plants, mining equipment, and transportation systems where on-site surfacing is often the most economical solution.

The fretting wear testing was conducted under controlled conditions to simulate the micro-motion contact regime commonly encountered in bolted joints, gear teeth, and sliding bearings. Fretting wear occurs when two surfaces in contact undergo small-amplitude oscillatory relative motion, typically with a displacement amplitude less than 100 micrometers. This regime is distinct from gross sliding wear and is characterized by a transition between adhesion-dominated and abrasion-dominated mechanisms as the displacement amplitude increases.

Microstructural Analysis and Metallurgical Bonding

The metallographic examination revealed that all three surfacing layers exhibited good metallurgical bonding with the 45 steel substrate. The first surfacing layer, however, showed pronounced dilution effects from the base metal, which is expected given the high heat input and relatively thick melt pool associated with SMAW. The dilution manifests as a transition zone at the interface where the composition gradually changes from the base metal to the surfacing alloy composition. The width of this transition layer varied among the three electrode types, with CHR237 producing the narrowest transition zone, likely due to its higher alloy content which reduces the solubility of base metal elements in the overlay.

The microstructure of the surfacing layers was characterized by a matrix of martensite and/or bainite with dispersed carbide particles. The carbide morphology and distribution varied with electrode composition, with higher-carbon electrodes producing more numerous and finer carbides. The grain size in the surfacing layers was generally finer than in the base metal, a result of the rapid solidification conditions at the weld surface. The columnar-to-equiaxed transition was observed in the upper regions of the surfacing layers, where the thermal gradient was lower and nucleation was more favorable.

Fretting Wear Behavior

The fretting wear tests demonstrated that both the base metal and the surfacing layers exhibited similar friction behavior diagrams, with all conditions falling within the complete sliding regime. This finding is significant because it indicates that the surfacing layers did not fundamentally alter the frictional regime but rather modified the wear rate within that regime. The complete sliding regime is characterized by continuous relative motion between the contacting surfaces, with material removal occurring through a combination of adhesion, abrasion, and fatigue mechanisms.

The wear resistance of the surfacing layers was superior to that of the base metal, with the improvement being most pronounced for the CHR237 electrode. The enhanced wear resistance was attributed to the higher hardness and the presence of a greater volume fraction of hard carbide particles, which provided resistance to plastic deformation and material removal. However, the authors noted that the wear behavior was not solely determined by hardness; the toughness of the matrix also played a role in resisting crack initiation and propagation during cyclic loading.

Study Insights and Engineering Implications

This study provides valuable insights into the fretting wear behavior of SMAW-deposited surfacing layers, a topic that is often overlooked in the broader surfacing literature, which tends to focus on gross sliding wear and impact loading. Fretting wear is a prevalent degradation mechanism in many engineering applications, including railway fasteners, turbine blade shroud joints, and bolted connections in pressure vessels and piping systems. The finding that surfacing layers can improve fretting wear resistance without altering the fundamental friction regime suggests that the primary benefit of surfacing in fretting applications is the extension of component life through reduced material loss, rather than a change in the contact mechanics.

The variation in dilution effects among the three electrode types has practical implications for process design. In applications where the first surfacing layer is the primary wear surface, the dilution effect must be carefully managed to ensure that the as-deposited properties meet the required specifications. This can be achieved through the use of multiple surfacing passes, with the first pass serving as a transition layer and subsequent passes providing the full alloy composition. Alternatively, the selection of an electrode with a higher alloy content, such as CHR237, can minimize the dilution effect by reducing the solubility of base metal elements.

From a broader perspective, this study reinforces the importance of tribological testing in the qualification of surfacing systems. Conventional testing protocols, which focus on hardness and gross sliding wear, may not adequately predict the performance of surfacing layers in fretting-dominated environments. Engineers should therefore consider incorporating fretting wear testing into their qualification procedures when the service conditions involve oscillatory loading, such as in bolted joints, gear transmissions, and reciprocating mechanisms. The combination of microstructural analysis and tribological testing provides a comprehensive understanding of the structure-property-performance relationship, enabling informed decisions about material selection and process optimization.