Plasma Overlay Welding of Q235 Electrolytic Shell-Breaking Hammer Head Microstructure and Properties

Literature Overview

This study note examines a 2012 publication by Cao Hongmei and colleagues from Wuhan University, China Energy Investment Ningxia Qingtongxia Energy and Aluminum Group, and the 6456th Factory of the People's Liberation Army, published in the Chinese Journal of Surface Engineering. The research focuses on plasma transferred arc (PTA) overlay welding of Q235 carbon steel electrolytic shell-breaking hammer heads used in the aluminum smelting industry. This application addresses a critical component in the pot lining maintenance process of electrolytic aluminum cells, where hammer heads are subjected to extreme impact, abrasion, and thermal cycling.

Application Background and Technical Requirements

In the electrolytic aluminum production process, pot linings must be periodically broken and removed for maintenance. The shell-breaking hammer heads used for this operation experience:

• Impact forces of 5-15 kN during each strike
• Surface temperatures reaching 400-600°C from hot alumina and carbon lining contact
• Abrasive wear from carbon and alumina particles
• Thermal cycling between ambient and elevated temperatures
• Corrosive exposure to fluoride-containing compounds

These combined loading conditions result in rapid failure of conventional Q235 hammer heads, typically lasting only 20-40 strikes before replacement is required. The plasma overlay approach aims to extend service life by 3-5 times through application of a wear-resistant and heat-resistant surface layer.

Plasma Overlay Process Parameters

PTA overlay welding was selected for this application due to its advantages of low dilution, precise heat input control, and ability to produce dense, crack-free overlay layers. The following process parameters were optimized through systematic experimental study:

Microstructural and Mechanical Property Analysis

The plasma overlay layer produced with the optimized parameters exhibited the following characteristics:

Microstructure: The overlay consisted of a fine martensitic matrix with dispersed carbide particles (Cr7C3, Cr23C6, and Mo2C). The grain size in the overlay was significantly finer (5-10 μm) compared to the base metal (50-80 μm), attributable to the rapid cooling rate inherent to PTA processing.

Hardness profile: A hardness gradient was observed across the overlay cross-section, with surface hardness of HV 850-900 decreasing to HV 650-700 at the fusion boundary. The base metal hardness remained at HV 120-150, confirming minimal heat-affected zone influence.

Dilution rate: The measured dilution rate was 8-12%, significantly lower than conventional arc welding methods (typically 20-35%). This low dilution was critical for maintaining the intended microstructure and properties of the overlay alloy.

Bond strength: The overlay-to-base metal bond strength exceeded 450 MPa, well above the minimum requirement of 300 MPa specified for this application.

Wear and Impact Testing Results

The overlay-treated hammer heads demonstrated substantial improvements in service performance:

Defect Analysis and Quality Control

During the production trials, the following defects were identified and addressed:

• Cracking in overlay layer: Occurred at powder feed rates above 35 g/min due to excessive cooling rate; resolved by reducing feed rate and increasing preheat temperature.
• Porosity: Observed in early trials due to inadequate powder drying; eliminated by implementing strict powder storage and preheating procedures (150°C for 2 hours).
• Uneven overlay thickness: Caused by inconsistent torch-to-workpiece distance; controlled by implementing a mechanical torch positioning system with ±0.5 mm accuracy.
• Delamination: Rare but occurred when interpass temperature exceeded 250°C; prevented by monitoring and controlling interpass temperature below 200°C.

Study Insights and Engineering Recommendations

This research demonstrates the significant potential of PTA overlay welding for extending the service life of critical industrial components. The 4-5 fold improvement in hammer head life translates directly to reduced maintenance downtime and lower total operating costs in aluminum smelting operations.

The key insight is that PTA overlay provides a uniquely favorable combination of low dilution, high hardness, and good bond strength that is difficult to achieve with conventional welding methods for this specific application. The process is particularly well-suited for components where the base material properties must be preserved while the surface is significantly enhanced.

For engineering implementation, the following recommendations are derived from this study:

1. Powder quality control is paramount; moisture content must be maintained below 0.1% for consistent results.
2. Process monitoring should include real-time tracking of powder feed rate and arc voltage to detect deviations before defects develop.
3. Periodic hardness and thickness verification during production should be performed on witness coupons.
4. The overlay thickness should be designed to accommodate 10-15% grinding allowance for post-weld finishing.

This work provides a solid foundation for the systematic application of PTA overlay technology in the aluminum industry and can be extended to other harsh-service components such as pot rakes, anode lifting equipment, and furnace linings.