Plasma Arc Overlay Welding of WC Enhanced High Chromium Cast Iron Microstructure and Performance

Literature Overview

This paper by Zhang Guodong, Li Li, and Cao Hongmei (2015, Chinese Surface Engineering, Vol. 28, No. 6) investigates the microstructural evolution and tribological performance of plasma arc powder overlay (PAO) deposits on Q235 carbon steel substrate. Two deposit compositions were compared: conventional high-chromium cast iron (HCCI) and WC-enhanced high-chromium cast iron (WC-HCCI). The study was funded by the National Natural Science Foundation of China (51374154) and the Hubei Provincial Natural Science Foundation (2014CFB707), indicating rigorous experimental design and peer-reviewed methodology.

Core Technical Content

The fundamental objective is to understand how exogenously added tungsten carbide particles influence the solidification microstructure, hardness distribution, wear resistance, and corrosion behavior of plasma arc overlay deposits. The plasma arc powder welding process is selected because of its high energy density, narrow dilution zone, and precise thermal input control, making it ideal for producing functionally graded surface layers with minimal substrate interaction.

Microstructural Analysis

The HCCI deposit microstructure consists of primary (Fe,Cr)₇C₃ carbides embedded in a eutectic matrix of austenite and carbide. The eutectic structure forms during the final stages of solidification when the remaining liquid reaches the eutectic composition. In contrast, the WC-HCCI deposit exhibits a three-component microstructure: primary carbides, intact and partially dissolved WC particles, and a reduced volume fraction of eutectic structure.

The key metallurgical insight is that WC particles act as heterogeneous nucleation sites during solidification, promoting the formation of primary carbides and suppressing the eutectic reaction. This effectively redirects the solidification pathway, increasing the volume fraction of hard primary phases at the expense of the softer eutectic matrix.

Hardness Distribution

The WC-HCCI deposit demonstrates significantly higher microhardness throughout the cross-section compared to HCCI. The hardness profile from the surface to the fusion line shows a characteristic gradient: high and relatively uniform hardness in the bulk deposit, followed by a sharp drop at the fusion line where substrate dilution occurs. This hardness cliff at the fusion line is a critical concern in engineering applications because the fusion zone represents the weakest link in the overlay system.

The addition of WC does not adversely affect the metallurgical bonding at the fusion line, as confirmed by line-scan analysis across the interface. This is an important finding because it demonstrates that the enhanced surface properties are achieved without compromising the bond integrity, which is essential for structural reliability under cyclic or impact loading.

Wear and Corrosion Performance

The wear resistance and corrosion performance of WC-HCCI surpass those of HCCI in both electrolytic corrosion and thermal corrosion environments. The superior wear resistance is attributed to the higher volume fraction of hard carbide phases, particularly the retained WC particles which provide a high-hardness reinforcement matrix. For corrosion resistance, the dense carbide network reduces the available surface area for corrosive attack and the WC particles contribute to a more passive surface chemistry.

Engineering Practice Integration

From a practical standpoint, this research has direct relevance to several industrial applications:

1. Mining equipment: Wear plates and chutes in mineral processing benefit from WC-enhanced overlay deposits where both abrasion and corrosion are present.
2. Petrochemical piping: Pipeline components exposed to sand-laden flow and corrosive media can utilize WC-HCCI overlay for extended service life.
3. Thermal power plants: Boiler tubes and heat exchanger surfaces subjected to ash erosion and hot corrosion can be protected with similar overlay systems.

The plasma arc powder welding process parameters for such applications typically involve current ranges of 150–300 A, arc voltage of 20–35 V, travel speed of 100–300 mm/min, and powder feed rate of 100–250 g/min. The substrate preheating temperature should be controlled between 150–250 °C for low-carbon steel to minimize cracking susceptibility.

Key Questions and Reflections

A critical question arises regarding the long-term stability of WC particles under severe thermal cycling. While the paper demonstrates excellent static properties, in-service conditions involving repeated heating and cooling could promote WC dissolution and carbide coarsening. Engineers should consider post-weld heat treatment strategies to stabilize the microstructure.

Another practical concern is the dilution effect at the fusion line. The sharp hardness drop indicates significant substrate metal incorporation. In multi-pass overlay welding, increasing the number of passes progressively reduces dilution, but this increases process cost. A multi-pass strategy with at least two to three passes is recommended for critical applications where the fusion zone strength is a design concern.

The research methodology follows a systematic approach: composition design, process parameter optimization, microstructural characterization, and performance evaluation. This PDCA (Plan-Do-Check-Act) cycle is exemplary for overlay welding research and should be adopted as a template for similar investigations in industrial settings.

Study Insights and Implications

The most significant finding is that WC particle addition transforms the solidification microstructure from eutectic-dominated to primary-carbide-dominated, which is the fundamental mechanism behind the performance improvement. This principle can be extended to other reinforcement systems such as TiC, SiC, or B₄C particles in overlay welding applications. The confirmation that bond quality is not compromised is particularly valuable for engineers who have historically been cautious about incorporating exogenous particles into overlay systems due to concerns about interface integrity.