Friction and Wear Properties of Cladding Surface on Q235 Structural Steel

Literature Overview

This paper by Li Cong, Chen Xueqin, Peng Daoheng, and Wang Jinfeng, published in Materials Protection in 2019 (Vol. 52, No. 10, pp. 70-74), investigates the friction and wear performance of a cladding layer deposited on Q235 structural steel using CO2 gas-shielded flux-cored wire arc welding. The research was supported by the Central Guidance Fund for Local Science and Technology Development and the Hubei Provincial Department of Education, and was motivated by the need to improve the surface durability of low-carbon structural steel components in demanding service environments.

Core Technical Findings

The authors examined the microstructure and tribological properties of three distinct zones: the base metal, the cladding layer, and the interface region. The key findings are summarized below:

Zone	Friction Coefficient	Wear Morphology	Dominant Microstructure
Base Metal	Highest	Large wear area, severe plastic deformation, "peeling pits"	Ferrite-pearlite
Cladding Layer	Intermediate	Moderate wear, some ploughing	Hard phases present
Interface Region	Lowest	Reduced ploughing, improved wear resistance	Fine acicular martensite, refined grains, hard phases

Microstructural Analysis of the Interface Region

The interface region between the base metal and the cladding layer exhibits the most interesting microstructural features. The authors observed fine acicular martensite with significantly refined grains in this zone. This refinement is attributed to the rapid cooling rates experienced during the welding process, which promote high nucleation rates and limit grain growth.

The interface region also shows evidence of intense elemental diffusion and metallurgical reactions, resulting in the formation of a high volume fraction of hard phases. These hard phases, likely including carbides and possibly intermetallic compounds, contribute significantly to the wear resistance of this zone. The combination of fine grain structure and hard phase dispersion creates a synergistic strengthening effect that enhances both hardness and wear resistance.

The microstructural gradient from the base metal through the interface to the cladding layer is critical for understanding the overall performance. The transition from soft ferrite-pearlite in the base metal to hard martensitic structures in the cladding layer creates a composite-like structure that can accommodate different types of mechanical loading.

Wear Mechanism Analysis

The wear morphology analysis reveals distinct wear mechanisms operating in each zone:

Base Metal Zone: The wear is characterized by severe plastic deformation, large wear areas, and the formation of "peeling pits." This indicates that the dominant wear mechanism is adhesive wear, where material transfer occurs between the contacting surfaces due to the relatively low hardness of the ferrite-pearlite structure.
Cladding Layer Zone: The wear morphology shows moderate ploughing and some plastic deformation. The presence of hard phases provides some resistance to wear, but the overall wear resistance is still limited by the matrix hardness.
Interface Region Zone: The wear morphology shows significantly reduced ploughing and plastic deformation compared to the cladding layer. The combination of fine grains and hard phases creates a more effective barrier against wear initiation and propagation. The hard phases act as load-bearing elements that resist penetration by the counterface material.

Engineering Practice Implications

The findings of this study have direct implications for the surface engineering of Q235 structural components:

Process Selection: CO2 gas-shielded flux-cored wire arc welding is a cost-effective and practical method for depositing wear-resistant cladding layers on low-carbon steel components.
Interface Optimization: The interface region, with its superior wear resistance, suggests that controlling the heat input and cooling rate during welding is critical for achieving optimal performance. Lower heat input and faster cooling rates promote finer grains and more hard phases in the interface.
Multi-Layer Strategy: Depositing multiple layers with decreasing heat input from the base metal outward may further enhance the microstructural gradient and improve overall wear resistance.

However, several challenges must be addressed in practical applications:

Residual Stress Management: The high hardness of the cladding layer, combined with the thermal stresses from welding, may lead to cracking or spalling during service. Post-weld stress relief or controlled cooling may be necessary.
Bond Strength: The metallurgical bond between the cladding layer and the base metal must be sufficient to withstand the mechanical loading in service. Interface quality testing is recommended.
Geometric Constraints: The welding process parameters may need to be adjusted for different component geometries and orientations.

Key Questions and Reflections

The observation that the interface region exhibits the lowest friction coefficient and the best wear resistance raises an important question: could this interface region be designed as the primary wear surface in a multi-layer cladding strategy? By controlling the welding parameters to create a thick interface-like zone with fine grains and hard phases, it may be possible to achieve superior wear resistance without relying solely on the cladding layer composition.

Another consideration is the effect of the CO2 shielding gas on the microstructure and properties of the cladding layer. CO2 is known to promote oxidation and may introduce oxygen into the weld metal, which could affect the carbide formation and overall microstructure. Alternative shielding gases, such as argon or mixed gases, might produce different microstructural outcomes.

Summary

This study provides valuable insights into the microstructural and tribological behavior of cladding layers deposited on Q235 structural steel. The identification of the interface region as the zone with the best wear resistance, due to the combination of fine acicular martensite and hard phases, offers a new perspective for optimizing cladding layer design. The findings highlight the importance of controlling the welding process to manage the microstructural gradient and achieve the desired balance between hardness, toughness, and wear resistance. This work contributes to the growing body of knowledge on surface engineering of low-carbon structural steels and provides practical guidance for engineers seeking to extend the service life of components in wear-critical applications.