Oxygen-Acetylene Flame Surfacing of Brass on Piston Components

Literature Overview

This technical paper by Ji Ping from Shaanxi Aircraft Hydraulic Components Factory, published in Welding Technology in 1989 (Vol. 18, No. 2, p. 48), describes the application of oxygen-acetylene flame surfacing to deposit a brass overlay on a piston component made of 45 steel. The piston was a spare part for an imported 1.7-meter rolling mill produced by Wuhan Steel. The requirement was to deposit a 1.0–2.0 mm thick, 40 mm wide brass overlay on the 250 mm diameter outer cylindrical surface, with a dilution rate of approximately 2%.

Technical Background and Requirements

The piston component in question served in the hydraulic system of a large rolling mill. The brass overlay was required to provide:

• Wear resistance: Brass offers excellent wear resistance against steel counterfaces, particularly in sliding contact applications.
• Anti-galling properties: The soft brass overlay prevents galling and seizure during sliding contact with steel components.
• Low friction coefficient: Brass has a lower friction coefficient than steel, reducing wear and energy consumption.
• Corrosion resistance: Brass provides adequate corrosion resistance in the hydraulic fluid environment.

The original imported component was analyzed to determine the overlay specifications:

The factory lacked submerged arc surfacing equipment, necessitating the development of an alternative process using available oxygen-acetylene flame surfacing capabilities.

Oxygen-Acetylene Flame Surfacing Process

Oxygen-acetylene flame surfacing is one of the oldest and most versatile surfacing methods, though it has largely been superseded by arc welding processes in many applications. However, it remains valuable in situations where specialized equipment is unavailable or where its unique characteristics are advantageous.

Process Characteristics

Process Steps

1. Surface preparation: The base metal surface must be thoroughly cleaned to remove oil, rust, and contamination. A shallow groove may be machined to provide mechanical keying for the overlay.
2. Preheating: The base metal should be preheated to a moderate temperature (typically 200–300 °C for steel) to reduce thermal gradients and minimize cracking risk.
3. Flame adjustment: A neutral or slightly carburizing flame is typically used. The flame should be adjusted to provide a concentrated, high-energy tip for efficient melting of the surfacing material.
4. Surfacing technique: The brass surfacing material (typically in the form of rod or strip) is fed into the molten weld pool created by the flame. The operator must maintain a consistent travel speed and filler wire feed rate to achieve a uniform bead profile.
5. Multi-pass buildup: Multiple passes are typically required to achieve the target overlay thickness. Each pass should be applied with proper interpass temperature control.
6. Post-weld treatment: The overlay may require machining to achieve the final dimensions and surface finish.

Technical Challenges and Solutions

The application of oxygen-acetylene flame surfacing to deposit brass on steel presents several technical challenges:

Dilution Control

Achieving the target dilution rate of 2% is critical for ensuring the overlay properties are not compromised by excessive base metal mixing. Strategies for minimizing dilution include:

• Using a shallow, wide weld pool geometry.
• Maintaining a low heat input per unit length.
• Applying multiple thin passes rather than fewer thick passes.
• Using a preheated base metal to reduce thermal gradients.

Cracking Prevention

The dissimilar metal junction between brass and steel is susceptible to cracking due to:

• Thermal expansion mismatch between the two metals.
• Formation of brittle intermetallic compounds at the interface.
• Residual stresses from differential cooling rates.

Countermeasures include:

• Preheating the base metal to reduce thermal stresses.
• Using a low-stress welding sequence.
• Applying thin overlay layers to limit residual stress buildup.
• Post-weld stress relief if necessary.

Surface Quality

The requirement for a bright, dense surface finish is challenging with flame surfacing, which typically produces a rougher surface than arc welding processes. Achieving the required surface quality may require:

• Post-weld machining or grinding.
• Careful control of the welding parameters to minimize spatter and porosity.
• Use of a clean, well-prepared surfacing material.

Comparison with Alternative Processes

Engineering Practice Insights

This case study illustrates the importance of process flexibility in industrial maintenance and repair operations. When the ideal process is unavailable, engineers must develop alternative solutions using available resources while maintaining acceptable quality standards.

Key lessons from this application include:

1. Thorough analysis of the original component: Understanding the original overlay specifications, including thickness, composition, dilution rate, and surface finish, is essential for developing an equivalent alternative process.
2. Process adaptation: The oxygen-acetylene flame surfacing process, while less commonly used today, offers flexibility and accessibility that can be valuable in maintenance environments.
3. Quality verification: The resulting overlay must be verified through dimensional measurement, metallographic examination, and performance testing to ensure it meets the required specifications.
4. Documentation: Detailed documentation of the process parameters, techniques, and results is essential for future reference and for establishing a repeatable process.

Key Questions and Reflections

Several questions arise from this case study:

1. How does the long-term performance of the flame-surfaced brass overlay compare with the original submerged arc-surfaced overlay in terms of wear life and reliability?
2. What are the intermetallic compound formation characteristics at the brass-steel interface, and how do they affect the overlay's adhesion and durability?
3. Could a hybrid approach (such as flame preheating followed by arc surfacing) provide a better balance between equipment availability and overlay quality?
4. What are the current best practices for brass surfacing on steel components, and how have they evolved since 1989?

Study Insights and Implications

This paper, though relatively brief, provides valuable insights into the practical challenges of industrial maintenance welding. The case study demonstrates that process constraints (such as equipment availability) must be considered alongside metallurgical requirements when developing surfacing solutions.

The most significant insight is that the oxygen-acetylene flame surfacing process, despite its limitations, can produce acceptable results for dissimilar metal overlay applications when properly executed. This finding is particularly relevant for maintenance operations in remote locations or facilities with limited welding equipment.

The paper also highlights the importance of reverse engineering—the analysis of existing components to determine their specifications and manufacturing processes. This skill is essential for maintenance engineers who must produce replacement or repair parts for equipment where original documentation may be unavailable.