Plasma Hardfacing of Diesel Engine Valves Failure Analysis and Improvement Measures

Literature Overview

This paper, published in Welding (2005, No. 10, pp. 62–64) by Zhang Fubang, Hou Shanglin, Hu Chunlian, and Tong Xiangyang, analyzes the factors affecting the yield rate of plasma hardfacing on diesel engine valves for internal combustion locomotives and proposes improvement measures. The research is funded by the Gansu Provincial Natural Science Foundation (ZS032-B25-006). Diesel engine valve hardfacing is a critical manufacturing process, as valve performance directly affects engine reliability, fuel efficiency, and emission compliance.

Core Technical Analysis

Plasma Hardfacing Process for Valve Application

Plasma hardfacing (plasma arc surfacing) is a well-established technology for valve hardfacing. The process uses a high-velocity plasma jet to melt a consumable electrode or wire onto the valve surface, producing a hard, wear-resistant overlay. The plasma arc provides several advantages for this application:

• Concentrated heat input: The plasma arc is highly focused, allowing precise control of the melt pool
• Low dilution: The intense heat input with minimal substrate melting results in low dilution rates
• Atmospheric protection: The plasma arc provides a stable, controlled atmosphere that minimizes oxidation
• High deposition rate: Compared to TIG hardfacing, plasma hardfacing offers higher productivity

Yield Rate Factors

The yield rate (成品率) is a critical quality metric in valve hardfacing production. A low yield rate directly impacts manufacturing cost and production throughput. The paper identifies several factors that cause yield rate fluctuations:

Failure Modes

The primary failure modes in plasma hardfaced valves include:

1. Overlay spalling: The hardfacing layer separates from the substrate due to poor bonding or excessive residual stress.
2. Cracking: Cracks in the overlay or at the fusion line, often caused by excessive cooling rates or high carbon content.
3. Porosity: Gas pores in the overlay caused by inadequate shielding or contamination.
4. Insufficient hardness: Dilution with substrate material reduces overlay hardness below specification.
5. Uneven overlay thickness: Inconsistent deposition leading to non-uniform wear performance.

Improvement Measures and Implementation

Based on the failure analysis, the paper proposes a series of improvement measures:

Material Control

• Electrode qualification: Rigorous incoming inspection of hardfacing electrodes, including chemical composition analysis, hardness verification, and microstructure examination.
• Batch consistency: Ensuring consistent electrode composition from batch to batch to maintain uniform overlay properties.
• Storage and handling: Proper storage conditions to prevent electrode degradation and contamination.

Process Optimization

• Parameter standardization: Establishing and documenting optimal process parameters including arc current, arc voltage, travel speed, electrode feed rate, and shielding gas flow rate.
• Preheating protocol: Implementing controlled preheating of valve blanks to reduce cooling rates and minimize cracking risk.
• Post-weld heat treatment: Applying appropriate stress-relief heat treatment to reduce residual stresses in the overlay and substrate.

Equipment and Environment Control

• Nozzle maintenance: Regular inspection and replacement of plasma nozzles to maintain arc stability.
• Shielding gas quality: Ensuring high-purity shielding gas with appropriate flow rates.
• Environmental monitoring: Controlling workshop humidity and temperature to minimize atmospheric effects.

Quality Control Implementation

• In-process monitoring: Implementing real-time monitoring of arc parameters and deposition characteristics.
• Post-weld inspection: Hardness testing, visual inspection, and dimensional verification of every hardfaced valve.
• Statistical process control: Using SPC techniques to track yield rate trends and identify process drift.

Engineering Practice Implications

The valve hardfacing application presents unique challenges compared to other hardfacing applications:

• Small component size: Valves are relatively small components, requiring precise process control and minimal heat input.
• Complex geometry: Valve stems, heads, and seat areas have different geometry, requiring adaptable process parameters.
• High production volume: Locomotive diesel engines require large quantities of valves, making yield rate a critical economic factor.
• High reliability requirement: Valve failure can lead to engine damage and safety issues, requiring stringent quality control.

The plasma hardfacing process is particularly well-suited for this application due to its ability to produce thin, uniform overlays with controlled dilution. However, the small size of valves and the need for high production rates require careful optimization of process parameters and equipment configuration.

Key Questions and Reflections

The paper provides practical improvement measures but does not extensively discuss the metallurgical aspects of the overlay-substrate interface. For valve applications, the bond strength between the overlay and the valve material is critical, as the overlay must withstand repeated thermal cycling and mechanical loading during engine operation. The thermal expansion mismatch between the overlay and the valve substrate can generate significant interface stresses during each engine cycle.

Additionally, the paper does not address the long-term wear performance of the hardfaced valves under actual engine operating conditions. Laboratory hardness and wear test results may not fully predict in-service performance, which is influenced by factors such as lubrication, temperature cycling, combustion gas exposure, and mechanical loading.

The improvement measures proposed in the paper are largely process-oriented and quality control-focused. While these are essential, a more comprehensive approach might include metallurgical optimization of the overlay composition, consideration of residual stress management through process design, and evaluation of alternative hardfacing methods such as laser cladding or HVOF for comparison.

Study Insights and Conclusions

This paper provides a practical and systematic approach to improving the yield rate of plasma hardfaced diesel engine valves. The identification of key failure factors and the proposed improvement measures reflect a thorough understanding of the manufacturing challenges. The measures span material control, process optimization, equipment maintenance, and quality control—covering all aspects of the manufacturing process. For production engineers working on valve hardfacing operations, this paper offers a useful framework for yield rate improvement through systematic failure analysis and targeted process improvements. The emphasis on statistical process control and in-process monitoring reflects modern manufacturing best practices and provides a foundation for continuous improvement initiatives.