Plasma Surfacing Welding Performance of Middle Trough in Coal Mining Equipment

Literature Overview

This paper, authored by Wang Cunfei and colleagues from Shenhua Shendong Coal Group and Xi'an Jiaotong University, published in Coal Mine Machinery (2019, Vol. 40, No. 9, pp. 46-49), investigates the welding performance of plasma surfacing applied to the middle trough of longwall shearer equipment. The middle trough is a critical structural component in coal mining longwall systems that endures severe abrasion from coal and rock debris, making its wear resistance a primary engineering concern. The study systematically examines macrostructure, microstructure, hardness distribution, and wear resistance of the plasma-clad layer, ultimately demonstrating that the cladding meets the required welding standards for middle trough repair.

Core Technical Content and Process Parameters

The authors employed plasma transfer arc (PTA) surfacing technology to build up a wear-resistant layer on the middle trough substrate. The key finding is that a cladding layer thickness of up to 8 mm can be achieved, which is substantial for repair applications in mining equipment. The alloy powder used contains carbon and chromium as the principal alloying elements, which contribute to wear resistance through two mechanisms: solid solution strengthening and dispersion strengthening of carbides.

The wear resistance improvement of 1.77 times over the base material, while modest, is considered sufficient for the operational conditions of mining equipment where the primary requirement is extended service life between maintenance intervals.

Metallurgical Analysis and Interpretation

The study examines both macro and micro structural features of the clad layer and the base metal. From a metallurgical perspective, the carbon and chromium elements in the alloy powder dissolve into the austenitic or martensitic matrix during the remelting process, creating a solid solution that elevates the base hardness. Simultaneously, chromium carbides (predominantly Cr7C3 and Cr23C6) precipitate during solidification and cooling, providing dispersion strengthening that resists abrasive particle indentation and plowing.

The plasma arc provides a concentrated heat source with high energy density, which results in rapid melting and solidification. This rapid cooling rate promotes fine grain formation and minimizes carbide coarsening, both of which are favorable for wear resistance. However, the high cooling rate also introduces residual stresses that must be managed through appropriate interpass temperature control and post-weld heat treatment.

Engineering Practice Implications

From a practical standpoint, this study addresses a common problem in coal mining operations: the repair of heavily worn middle trough components without complete replacement. The economic advantage of surfacing repair over replacement is significant, particularly for large structural components where logistics and downtime are critical cost factors.

The process window for plasma surfacing of the middle trough should account for:

• Preheating: Typically 150-250°C to reduce thermal gradients and residual stresses
• Interpass temperature: Maintained below 250°C to prevent excessive grain growth
• Powder feed rate: Must be calibrated to achieve uniform dilution control
• Travel speed: Critical for balancing deposition rate against heat input

A potential concern is the dilution effect. In single-pass surfacing, dilution can reach 20-30%, which may compromise the hardness of the cladding layer. Multi-pass deposition with controlled overlap helps maintain the desired alloy composition throughout the build-up thickness.

Key Questions and Reflections

The study raises an important question about the long-term performance of the cladding under cyclic loading conditions typical of mining equipment. While the wear resistance is demonstrated under laboratory conditions, the actual service environment involves impact loading, vibration, and occasional thermal cycling. The bond strength between the clad layer and the base material under these conditions warrants further investigation, particularly regarding the risk of interfacial cracking.

Additionally, the paper does not address the transition zone between the clad layer and the base material in detail. In engineering practice, the fusion zone is often the weakest link in surfacing applications, as it contains a gradient of composition and microstructure. Understanding the mechanical properties of this transition region is essential for predicting the service life of the repair.

Study Insights and Implications

The work demonstrates that plasma surfacing is a viable and effective repair technology for mining equipment components subjected to abrasive wear. The 1.77-fold improvement in wear resistance, combined with the ability to build up substantial thickness (8 mm), makes this approach particularly attractive for field repair operations. The reliance on chromium carbide dispersion strengthening as the primary wear resistance mechanism is well-established in the literature and provides a predictable, repeatable performance characteristic.

For engineers managing maintenance programs in mining operations, this study provides a technical basis for specifying plasma surfacing as a repair method for middle troughs and similar components. The key to successful implementation lies in controlling the process parameters to minimize dilution and ensure uniform microstructure throughout the cladding thickness. Post-weld stress relief treatment should be considered for components subjected to cyclic loading to prevent fatigue cracking at the cladding-substrate interface.