Microstructure and Fretting Wear Study of Surfacing Metals

Literature Overview

This study, conducted by Qu Jinshan and colleagues from the Tribology Research Institute at Southwest Jiaotong University and the School of Materials Science at Xihua University, was published in the Journal of Xihua University (Natural Science Edition, 2006, Vol. 25, No. 5, pp. 34–37). The research investigates the microstructure of surfacing deposits made with three different electrode types (CHR207, CHR227, and CHR237) on 45 steel substrates, and examines their fretting wear behavior under reciprocating sliding conditions.

Experimental Configuration

Materials and Process Parameters

The selection of three different electrode types allows systematic comparison of how consumable chemistry influences both microstructure and tribological performance. The CHR series electrodes represent different alloy compositions designed for various surfacing applications, with variations in carbon content, alloying elements, and base matrix structure.

Microstructural Analysis

Metallurgical Bonding Quality

The study confirms that surfacing metals achieve good metallurgical bonding with the 45 steel substrate across all three electrode types. This is a fundamental requirement for overlay performance, as inadequate bonding leads to delamination under mechanical loading.

Dilution Effects

A key finding is that the first surfacing layer exhibits significant dilution by the base metal. This is expected and well-documented in overlay welding literature—substrate melting inevitably occurs during the first pass, introducing base metal composition into the overlay. The dilution level affects:

• Overlay hardness and composition
• Corrosion resistance properties
• Mechanical property uniformity
• Phase constitution of the overlay

Interface Transition Layer

The width of the metal transition layer at the interface varies with electrode type. This transition zone represents the region of compositional gradient between substrate and overlay, and its characteristics influence:

• Crack initiation resistance at the interface
• Thermal cycling fatigue life
• Stress distribution under load

The variation in transition layer width among the three electrode types suggests that different alloy compositions and melting characteristics produce different dilution profiles, which has direct implications for process design in multi-pass surfacing operations.

Fretting Wear Behavior

Friction Characteristic Diagrams

Both the substrate metal and surfacing layers exhibit similar friction characteristic diagrams, with fretting occurring in the complete slip regime. This finding is significant because it indicates that the surfacing deposits do not fundamentally alter the frictional regime—they operate in the same tribological state as the base material but with potentially different wear rates.

Fretting Regime Classification

The operation in the complete slip regime means that fretting wear is dominated by abrasive mechanisms and material removal through delamination. This regime is typical of high-amplitude, high-cycle applications such as bolted joints, bearing surfaces, and press-fit connections.

Wear Performance Comparison

While the paper confirms that all materials operate in the same friction regime, the implications for wear rate differences are important. In the complete slip regime, wear rate is primarily governed by:

1. Surface hardness of the overlay
2. Hardness ratio between mating surfaces
3. Surface roughness and texture
4. Presence of protective oxide layers
5. Subsurface microstructure (grain size, phase distribution)

The three electrode types, with their different alloy compositions, would be expected to produce overlays with different hardness levels and phase constitutions, leading to different wear rates even within the same friction regime.

Engineering Practice Implications

Selection Criteria for Surfacing Electrodes

Based on the findings of this study, electrode selection for fretting-critical applications should consider:

• Dilution control: Electrodes that produce more uniform multi-pass deposits with controlled dilution
• Transition layer characteristics: Electrodes that minimize the transition layer width for improved interface integrity
• Hardness uniformity: Consistent hardness across the overlay thickness for predictable wear behavior
• Microstructural stability: Resistance to phase transformations during service thermal cycling

Application Context

Fretting wear is a significant concern in many industrial applications including:

• Railway wheel-rail contacts (relevant to Southwest Jiaotong University's expertise)
• Turbine blade root attachments
• Bolted and riveted structural joints
• Press-fit bearing assemblies
• Electrical connector contacts

The use of 45 steel as a substrate is representative of common structural and mechanical components that require fretting-resistant surfacing treatment.

Study Insights and Reflections

The confirmation that surfacing metals and base metals exhibit similar friction characteristic diagrams, with operation in the complete slip regime, has important implications for surface engineering design. It suggests that the primary benefit of surfacing in fretting applications comes from wear rate reduction rather than friction regime modification. This aligns with the general principle that surface coatings and overlays in fretting applications must be designed to resist material removal rather than to change the fundamental friction behavior.

The variation in transition layer width among different electrode types is a finding that deserves further investigation. The transition layer represents a zone of microstructural and compositional heterogeneity that could serve as a preferential site for fretting fatigue crack initiation. Understanding how electrode chemistry influences transition layer characteristics could lead to improved design of multi-pass surfacing procedures for fretting-critical components.