Metallographic Structure and Fretting Wear of Overlay Metals

Literature Overview

This paper by Qu Jinshan, Zhang Hui, Zhang Youyi, and Li Juan, published in the Journal of Xihua University (Natural Science Edition) (2006, Vol. 25, No. 5, pp. 34-37), investigates the microstructure and fretting wear behavior of overlay metals deposited using CHR207, CHR227, and CHR237 welding electrodes on 45 steel substrates. The study examines the metallurgical bonding between overlay and substrate, the dilution effect on the first overlay layer, and the fretting friction characteristics of the deposited metals.

Core Technical Content

The authors conducted manual arc surfacing using three different welding electrodes (CHR207, CHR227, and CHR227) on 45 steel substrates, followed by fretting friction tests on the deposited overlay layers. The study focused on three key aspects: the microstructure of the deposited metals, the metallurgical bonding at the overlay-substrate interface, and the fretting wear behavior under reciprocating sliding conditions.

The metallographic analysis revealed that the overlay metals exhibited good metallurgical bonding with the base metal. The first layer of deposited metal showed significant dilution from the base metal, which is expected due to the relatively high heat input and the melting of base metal into the first weld pass. The width of the transition layer at the interface varied depending on the electrode type used, indicating that electrode composition and melting characteristics influence the dilution behavior.

Interpretation of Technical Points

The finding that all three overlay metals exhibited fretting behavior in the complete slip regime is significant. In fretting wear, the transition from partial slip to complete slip is associated with a change in the wear mechanism from adhesive wear to abrasive wear, with the latter typically resulting in higher material loss rates. The complete slip regime indicates that the relative displacement between the contacting surfaces exceeds the elastic limit, leading to plastic deformation and material transfer at the contact interface.

The dilution effect on the first overlay layer is a well-known phenomenon in surfacing processes, but this study quantifies its impact on fretting wear behavior. The first layer, being most affected by base metal dilution, has a composition that is intermediate between the electrode alloy and the base metal. This intermediate composition may affect the hardness, toughness, and wear resistance of the first layer, potentially making it a weak link in the overlay system under fretting conditions.

The variation in transition layer width with electrode type is also noteworthy. Different electrode compositions have different melting points, fluidity, and dilution characteristics, all of which influence the width and composition of the transition layer. A wider transition layer may indicate greater mixing between the overlay and base metal, which could either improve or degrade the interface strength depending on the specific metallurgical interaction.

Engineering Practice Integration

For piping and fitting applications where overlay surfaces are subject to reciprocating motion, such as valve stems, control rod drive mechanisms, and pump seals, the fretting wear behavior of the overlay is critical. The findings of this study suggest that:

1. The first overlay layer should be carefully controlled to minimize dilution and ensure adequate hardness and wear resistance.
2. Multi-pass surfacing with a transition layer may be beneficial, as the first layer can be designed to provide good bonding while subsequent layers provide the desired wear resistance.
3. The electrode selection should consider not only the desired overlay properties but also the dilution behavior and transition layer characteristics.

In the context of pipe fitting manufacturing, where overlay hardfacing is used to protect critical components such as valve seats, pump impellers, and bearing surfaces, the fretting wear behavior is particularly relevant. Components subject to reciprocating motion under load are susceptible to fretting fatigue, which can lead to crack initiation and component failure. The selection of overlay materials and process parameters should therefore consider the fretting wear resistance of the deposited metal.

Key Reflections and Study Insights

This study provides valuable insights into the fretting wear behavior of overlay metals, which is often overlooked in conventional wear testing that focuses on unidirectional sliding or impact wear. Fretting wear is a distinct wear mechanism that occurs under oscillatory loading conditions, and its characterization requires specialized testing methods and analysis approaches.

The observation that all tested overlay metals exhibited complete slip behavior suggests that the fretting conditions used in the study were sufficiently severe to exceed the elastic limit of all materials. In practical applications, the fretting conditions may be less severe, and the overlay metals may exhibit partial slip behavior with lower material loss rates. However, the complete slip regime provides a conservative assessment of the overlay's fretting resistance, which is appropriate for safety-critical applications.

The study also highlights the importance of considering the first overlay layer separately from subsequent layers. The first layer is subject to the greatest dilution and may have different mechanical properties and wear resistance than the upper layers. In multi-pass surfacing, the first layer can be optimized for bonding quality while subsequent layers are optimized for wear resistance, providing a balanced overlay system that addresses both interface integrity and surface performance.