Effect of Axial Pressure on Microstructure and Wear Properties of Friction Welding Overlay Layer on X65 Alloy Steel

Literature Overview

This study by Zhang Xu, Yang Xinqi, Cui Lei, and Du Bo from the College of Materials Science and Engineering, Tianjin University (2016), supported by the National Natural Science Foundation of China (Grant No. 51475327), investigates the influence of axial pressure on the microstructure and wear properties of a friction welding overlay layer on X65 pipeline steel. Published in the journal China Surface Engineering, this paper explores a relatively novel approach to surface hardening and wear protection: friction welding overlay.

X65 is a high-strength low-alloy (HSLA) steel widely used in oil and gas pipelines. It is susceptible to wear in certain service conditions, such as in pipeline installation, repair, and maintenance operations. Friction welding overlay offers a unique method to deposit a wear-resistant layer by bringing a consumable rod into frictional contact with the workpiece surface.

Core Technical Content

Friction welding overlay is a solid-state welding process that combines the principles of friction stir welding (FSW) and friction welding. A rotating consumable rod (typically made of a wear-resistant material such as high-carbon steel, tool steel, or a ceramic-reinforced composite) is pressed against the workpiece surface with a controlled axial force. The frictional heat generated at the interface softens the material of both the rod and the workpiece surface, and the axial pressure causes material transfer from the rod to the workpiece, forming an overlay layer.

The authors investigated the effect of axial pressure on the following aspects:

The key findings of the study are summarized below:

1. Overlay thickness: Increasing the axial pressure from 5 kN to 20 kN increased the overlay thickness from approximately 0.5 mm to 2.0 mm. The relationship between axial pressure and overlay thickness was approximately linear within the studied range.
2. Microstructure: At low axial pressures (5–10 kN), the overlay layer consisted of a mixture of rod material and base metal, with a relatively coarse microstructure. At higher axial pressures (15–20 kN), the overlay layer became more homogeneous, with a finer grain structure and a more uniform distribution of carbides. The interface between the overlay and the base metal showed evidence of plastic deformation and dynamic recrystallization, confirming the solid-state nature of the process.
3. Hardness: The hardness of the overlay layer increased with increasing axial pressure. At 5 kN, the overlay hardness was approximately 350 HV, while at 20 kN, it reached approximately 550 HV. This increase is attributed to the refinement of the microstructure and the increased volume fraction of hard carbide phases.
4. Wear resistance: The wear resistance of the overlay layer, evaluated by pin-on-disk tribological testing, improved significantly with increasing axial pressure. The wear rate decreased from approximately 8×10⁻⁶ mm³/N·m at 5 kN to approximately 2×10⁻⁶ mm³/N·m at 20 kN, representing a fourfold improvement.

Process Analysis and Microstructural Evolution

The microstructural evolution of the friction welding overlay layer is governed by the interplay of thermal and mechanical effects. The frictional heat generated at the interface causes plastic deformation and dynamic recrystallization of the rod material, while the axial pressure drives material transfer from the rod to the workpiece. The resulting overlay layer is characterized by a dynamically recrystallized microstructure with a fine grain size and a high density of dislocations.

At low axial pressures, the material transfer is limited, and the overlay layer consists of a thin layer of rod material with significant mixing of base metal. The microstructure is relatively coarse, and the hardness is lower. At high axial pressures, the material transfer is more complete, and the overlay layer is predominantly rod material with a fine, dynamically recrystallized microstructure. The high density of dislocations and the presence of fine carbide particles contribute to the high hardness and wear resistance.

A critical aspect of the microstructural evolution is the interface between the overlay and the base metal. The interface is characterized by a region of severe plastic deformation, where the grain size is refined to the submicron scale. This region provides excellent bonding between the overlay and the base metal, as confirmed by the absence of interfacial cracking during tensile testing.

Engineering Practice and Limitations

While friction welding overlay offers several advantages over traditional welding-based overlay methods, it also has limitations that must be considered in engineering practice:

1. Limited overlay thickness: The maximum overlay thickness achievable by friction welding overlay is typically limited to 2–3 mm, which may be insufficient for applications requiring thicker overlay layers.
2. Material compatibility: The consumable rod material must be compatible with the base material in terms of melting point and thermal conductivity. For X65 steel, high-carbon tool steels are suitable consumable materials.
3. Surface preparation: The workpiece surface must be clean and free from oxide scales, oils, and other contaminants. Any surface contamination can lead to poor bonding and reduced overlay quality.
4. Equipment requirements: Friction welding overlay requires specialized equipment capable of applying controlled axial pressure and rotation speed. This equipment is more complex and expensive than conventional welding equipment.

Despite these limitations, friction welding overlay is particularly suitable for applications where a thin, hard, wear-resistant layer is required on a specific surface area, such as pipeline joints, valve seats, and bearing surfaces.

Study Insights and Implications

This study demonstrates that axial pressure is a critical process parameter in friction welding overlay, with a direct and significant influence on the microstructure, hardness, and wear resistance of the overlay layer. The key insight is that the process is fundamentally a solid-state process, and the microstructural evolution is governed by plastic deformation and dynamic recrystallization rather than by melting and solidification.

The findings have important implications for the design and optimization of friction welding overlay processes. Engineers should carefully select the axial pressure based on the required overlay thickness and wear resistance, while also considering the equipment capabilities and the material compatibility.

This research contributes to the growing body of knowledge on friction-based surface engineering techniques and opens up new possibilities for the protection of HSLA steels in demanding service environments.