Oxy-Acetylene Flame Hardfacing of Brass on Steel Piston Surfaces

Literature Overview

This 1989 paper by Ji Ping from Shaanxi Aircraft Hydraulic Components Plant addresses a practical repair challenge encountered during the production of spare parts for an imported 1.7 m rolling mill supplied to Wuhan Iron and Steel Company. The workpiece is a piston component made from 45 steel (a medium-carbon alloy steel with approximately 0.42–0.50 wt.% C) that requires a brass overlay layer on its outer cylindrical surface. The overlay specification calls for a thickness of 1.0–2.0 mm and a width of 40 mm on a 250 mm diameter surface. The parent company had originally produced the component using submerged arc hardfacing, which is unavailable at the repair facility, prompting the investigation of oxy-acetylene flame hardfacing as an alternative process.

Core Technical Requirements and Process Selection

The original component analysis revealed the following critical parameters:

The fundamental challenge in hardfacing copper alloys onto steel substrates lies in the metallurgical incompatibility between the two materials. Iron and copper form intermetallic compounds (Fe-Cu phases) that are brittle and can lead to cracking at the fusion boundary. The dilution rate of 2% is extremely low, indicating that the original manufacturer employed a process that minimized base metal melting. Achieving such low dilution with oxy-acetylene flame hardfacing requires careful control of heat input and multiple thin passes.

Process Parameters and Implementation

Oxy-acetylene flame hardfacing relies on the exothermic reaction of acetylene combustion in oxygen to produce a flame temperature of approximately 3,200°C. For brass hardfacing on steel, the following process considerations are critical:

• Preheating the base metal to 200–300°C to reduce thermal gradients and minimize cracking risk
• Using a neutral or slightly carburizing flame to avoid excessive oxidation or carburization of the molten pool
• Applying multiple thin passes (each 0.3–0.5 mm) to achieve the total thickness of 1.0–2.0 mm while maintaining low dilution
• Employing a flux that is compatible with both steel and brass to remove oxides and promote wetting
• Controlling the travel speed to ensure adequate penetration without excessive base metal melting

The key to achieving the 2% dilution target is to keep the heat input per pass low and to use a filler material with good fluidity in the molten state. Brass alloys with higher zinc content (such as CuZn30 or similar compositions) tend to have lower melting points and better fluidity, which aids in spreading over the substrate without requiring excessive heat.

Engineering Insights and Practical Considerations

From a metallurgical perspective, the brass overlay on 45 steel creates a dissimilar metal joint where the coefficient of thermal expansion mismatch between iron (approximately 12 × 10⁻⁶/°C) and copper alloys (approximately 17–20 × 10⁻⁶/°C) can generate residual stresses during cooling. This is particularly relevant for a cylindrical piston surface where thermal contraction during welding can induce hoop stresses that may lead to overlay spalling if not properly managed.

The choice of oxy-acetylene flame hardfacing over arc welding processes offers several advantages in this context: lower heat input, reduced dilution, better control over the molten pool geometry, and the ability to work in confined spaces. However, the process has limitations including lower deposition rates, greater operator skill dependence, and potential for oxidation if the flame is not properly shielded.

For engineering practice, this case study highlights the importance of process substitution when specialized equipment is unavailable. The systematic approach of analyzing the original component, identifying the required metallurgical properties, and selecting an alternative process with appropriate parameter optimization is a methodology that remains highly relevant in modern repair and maintenance operations. The low dilution requirement of 2% serves as a benchmark for evaluating any alternative hardfacing process, and engineers should always verify dilution through chemical analysis of the overlay layer to ensure the functional properties of the copper alloy are preserved.

Study Insights

This paper, though published in 1989, remains a valuable reference for understanding the fundamentals of dissimilar metal hardfacing. The emphasis on dilution control as the primary quality metric is a principle that transcends specific processes and equipment. Modern engineers working on repair operations should appreciate that process flexibility—knowing how to achieve a required metallurgical result with available tools—is often more important than having access to the ideal equipment. The 2% dilution target for copper alloys on steel is demanding but achievable with disciplined process control, and this work demonstrates that even simple flame-based methods can meet stringent requirements when properly executed.