Effect of Axial Pressure on Friction Surfacing Microstructure and Wear Performance of X65 Alloy Steel

Literature Overview

This study by Zhang Xu and colleagues from Tianjin University's Key Laboratory of Modern Connection Technology, published in China Surface Engineering (2016, Vol. 29, No. 6, pp. 113-122), investigates the influence of axial pressure on the microstructure and wear resistance of friction surfacing (FS) layers deposited on X65 pipeline steel. Funded by the National Natural Science Foundation of China (Grant No. 51475327), the research systematically explores the relationship between axial force parameters and the resulting metallurgical and tribological characteristics of the friction surfacing layer.

Fundamental Principles of Friction Surfacing

Friction surfacing is a solid-state additive manufacturing process that deposits a consumable rod or wire onto a substrate by rotating the consumable at high speed and pressing it axially against the substrate surface. Unlike conventional arc welding surfacing processes, FS operates without melting of the substrate, relying instead on plastic deformation and adhesion under high temperature and pressure. The deposited material is transferred from the consumable to the substrate through a complex mechanism involving frictional heating, plastic flow, and oxide film fragmentation.

For X65 pipeline steel, which is widely used in long-distance oil and gas transmission pipelines, the surface often requires enhanced wear resistance for applications involving erosion from slurry flow, sand-laden gas, or mechanical contact with pipe supports. Friction surfacing offers a compelling solution because it avoids the dilution issues inherent in arc welding processes and produces a metallurgically bonded layer with excellent mechanical properties.

Key Process Parameters and Experimental Findings

The study identifies the following optimal process window:

The study reveals several important findings regarding the effect of axial pressure:

Microstructural Evolution

The friction surfacing joint consists of three distinct zones: the surfacing layer, the heat-affected zone (HAZ), and the base metal. A critical observation is that the surfacing layer microstructure differs significantly from that of a conventional friction weld joint. While friction welds typically exhibit fine-grained, elongated ferrite grains due to severe plastic deformation and rapid cooling, the FS layer displays a mixed microstructure of coarse lenticular and granular bainite. This is attributed to the relatively higher temperatures and slower cooling rates experienced during the surfacing process compared to welding.

The HAZ is divided into two sub-regions:

• Overheated zone: Characterized by bainitic microstructure, resulting from temperatures exceeding Ac3 but below melting.
• Recrystallization zone: Exhibiting fine ferrite grains due to complete recrystallization and grain growth.

Axial Pressure Effects on Geometry and Microstructure

As axial pressure increases from 59.7 MPa to 104.4 MPa:

• The surfacing layer width increases due to greater lateral spreading of the deformed material.
• The surfacing layer thickness decreases because higher pressure drives material laterally rather than building up vertically.
• Excessively high axial pressure does not increase the effective volume of the deposited layer; beyond a threshold, additional pressure only causes material spreading without meaningful deposition.
• The HAZ size is significantly affected by axial pressure, with higher pressures producing larger HAZ dimensions due to increased thermal input from greater frictional contact.
• The HAZ microstructure morphology remains relatively unchanged regardless of axial pressure, indicating that the phase transformation behavior in the HAZ is governed more by cooling rate than by the degree of plastic deformation.

Wear Performance

The most practically significant finding is that all friction surfacing layers exhibit superior wear resistance compared to the base X65 steel. The maximum reduction in wear volume is 33.3% compared to the uncoated substrate. The average hardness of the surfacing layer is consistently higher than the base metal across all tested axial pressures. This improvement is attributed to the work hardening effects of severe plastic deformation during the FS process, which refines the microstructure and increases dislocation density in the deposited layer.

Engineering Practice Implications

For pipeline engineers considering friction surfacing as a surface hardening solution for X65 pipeline components, several practical considerations emerge:

• Equipment requirements: The FS process requires a dedicated friction surfacing machine capable of precise axial force control (±5% accuracy) and stable rotational speed maintenance. The investment in specialized equipment must be justified by the application volume.
• Material selection: Using the same material (X65) as both substrate and consumable simplifies the supply chain and ensures metallurgical compatibility, but limits the ability to tailor surface properties through compositional engineering.
• Axial pressure optimization: The optimal axial pressure window of 59.7-104.4 MPa provides sufficient metallurgical bonding and wear resistance improvement. Operating outside this range risks either insufficient bonding (low pressure) or excessive material spreading without thickness gain (high pressure).
• Process limitations: FS is primarily suited for flat or slightly curved surfaces. Application to cylindrical pipe surfaces requires specialized tooling and may introduce geometric limitations.

Key Questions and Reflections

This study raises an important question for pipeline surface engineering: Is the 33.3% wear volume reduction achievable through FS sufficient for the demanding erosion conditions encountered in oil and gas pipeline service? For comparison, conventional hardfacing overlay welds using high-alloy consumables (e.g., Stellite-type alloys) can achieve wear resistance improvements of 5-10 times the base material. The trade-off is that FS preserves the substrate metallurgy and avoids dilution, while hardfacing introduces a dissimilar overlay with potentially superior wear properties but at the cost of metallurgical compatibility and residual stress concerns.

Another reflection point is the coarse bainitic microstructure observed in the FS layer. While this microstructure provides adequate wear resistance for moderate erosion conditions, it may not be optimal for high-temperature service where tempering and microstructural instability become concerns. Future research could explore the use of alloyed consumable rods (e.g., Cr-Mo-V or Ni-based alloys) in combination with FS to achieve both metallurgical bonding and enhanced wear resistance.

Study Insights

The systematic investigation of axial pressure as the primary variable, with rotational speed and travel speed fixed at pre-optimized values, demonstrates a well-designed experimental methodology. The finding that excessively high axial pressure does not proportionally increase deposited material volume is particularly valuable for process optimization — it establishes an upper bound for force application that prevents unnecessary energy input and equipment wear. For engineers working in pipeline maintenance and repair, this study provides a clear process window for implementing FS as a surface enhancement technology on X65 pipeline components.