Influence of Plasma Arc Surfacing Process Parameters on Stellite Alloy Dilution Rate

Literature Overview

This paper by Zhu Kai, Que Meifu, Zhu Zhiyuan, Dai Anlun, Chen Hui, and Niu Wenming from Jiangsu University and KSB Valve Industry Changzhou Co., Ltd. was published in Welding Technology (2014, Vol. 43, No. 9, pp. 37-40). Supported by the Jiangsu Provincial Key Laboratory of Advanced Welding Technology, the study systematically investigates the effect of plasma arc surfacing (PAS) process parameters on the dilution rate of Stellite alloy cladding layers using orthogonal experimental design. The work is particularly relevant to valve manufacturing, where Stellite alloy hardfacing of valve trim components requires precise control of dilution to ensure the functional properties of the deposited layer.

Core Technical Findings

Orthogonal Experimental Design

The authors employed orthogonal experimental methods to evaluate the influence of key plasma arc surfacing parameters on dilution rate. The parameters investigated include powder feed rate, transferred arc current, and surfacing speed. The results reveal a clear hierarchy of parameter influence:

Optimal Process Parameters

The optimized process parameters and resulting performance are as follows:

Microstructural Analysis

The cladding layer microstructure consists of cellular and dendritic α-Co-based solid solution with interdendritic carbides. This microstructure is characteristic of Stellite-type alloys and provides the combination of high-temperature strength, wear resistance, and corrosion resistance that makes these alloys suitable for demanding service conditions.

Process Parameter Interaction Analysis

Powder Feed Rate Dominance

The dominance of powder feed rate in controlling dilution is physically intuitive: higher powder feed rates increase the volume of deposited material relative to the melted substrate, thereby reducing the proportion of base metal in the weld pool. However, excessively high feed rates can lead to incomplete powder melting, porosity formation, and poor metallurgical bonding. The optimal value of 31 g/min represents a balance between dilution control and deposition quality.

Current and Speed Interplay

The transferred arc current determines the thermal energy input, which governs both the melting rate of the substrate and the powder. Higher currents increase substrate melting, raising dilution, but also improve powder melting efficiency. Surfacing speed affects the residence time of the arc at any given location, with faster speeds reducing heat input per unit length and consequently reducing dilution. The interaction between current and speed is captured in the concept of linear energy input (E = I × V / v), which is a fundamental parameter in surfacing process design.

Engineering Practice Integration

Application to Valve Manufacturing

The study's industrial partner, KSB Valve Industry Changzhou, highlights the direct relevance to valve trim hardfacing. In high-pressure and high-temperature steam service, valve seats and guides are typically hardfaced with Stellite alloys to resist erosion-corrosion and galling. The dilution rate directly affects the chemical composition and phase balance of the cladding layer, which in turn determines the service life of the valve components.

Quality Control Considerations

The 2.2% dilution rate achieved in this study is exceptionally low and represents excellent process control. For comparison, typical dilution rates in plasma arc surfacing of Stellite alloys range from 5-15% in conventional practice, with values above 20% often considered unacceptable for critical applications. The low dilution achieved here is attributed to the optimized combination of high powder feed rate and moderate thermal input.

From a quality control perspective, the following monitoring points should be established in production:

1. In-process monitoring: Track powder feed rate and arc current continuously; deviations beyond ±5% should trigger parameter adjustment.
2. Post-weld verification: Perform metallographic examination of the cladding layer to confirm dilution rate by measuring the thickness of the base metal penetration zone.
3. Hardness verification: Surface hardness of HV400-HV450 is typical for Stellite 6 with low dilution; values below HV350 may indicate excessive dilution.

Key Questions and Reflections

The study focuses on dilution rate as the primary quality metric, but dilution is only one aspect of cladding quality. The interaction between dilution rate and microstructure evolution — particularly the type, morphology, and distribution of interdendritic carbides — is not fully explored. For instance, even at low dilution rates, excessive thermal cycling from multiple passes can lead to carbide coarsening and reduced hardness in the final cladding layer.

Additionally, the study does not address the effect of substrate preparation on dilution. Surface roughness, oxide scale, and preheating conditions can all influence the effective dilution rate. In production environments, substrate cleanliness and preparation consistency are often the most significant sources of dilution rate variation, and should be controlled independently of process parameters.

Study Insights and Implications

This paper provides a rigorous, data-driven framework for optimizing plasma arc surfacing parameters to achieve low dilution rates in Stellite alloy hardfacing. The orthogonal experimental approach is particularly valuable for production environments where systematic parameter optimization is needed without extensive trial-and-error. The finding that powder feed rate is the dominant parameter offers a practical lever for engineers to control dilution without compromising deposition efficiency. For valve manufacturers and other producers of hardfaced components, the target of 2-3% dilution with HV440 hardness represents an achievable quality benchmark that can be incorporated into specification requirements and process control plans.