Effect of CaCO3 Addition on Microstructure and Wear Resistance of High-Chromium Alloy Surfaced with Composite Powder and Solid Wire

Literature Overview

This 2023 study by Gong Jianxun, Liu Chao, Huang Hongjiang, Ai Xiaowen, and Liu Shutong, published in Surface Technology (Volume 52, Issue 2, pages 215-224), investigates the influence of calcium carbonate (CaCO3) addition to composite powder on the microstructure and wear resistance of high-chromium alloy overlay coatings deposited via self-shielded open-arc welding. The research was supported by the Hunan Provincial Natural Science Foundation (Project No. 2021JJ30669). The study employed X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), high-speed arc photography, and wear testing to comprehensively characterize the effects of CaCO3 addition on the overlay welding process and resulting coating properties.

Core Technical Findings

Effect of CaCO3 on Arc Behavior and Deposition

The addition of CaCO3 to the composite powder fundamentally alters the arc behavior during open-arc surfacing welding. The decomposition of CaCO3 at welding temperatures produces CO2 gas, which expands the arc plasma and changes the arc morphology from a conical shape to a flat bell-shape. This expanded arc coverage area, approximately double that of the baseline, results in more uniform melting of the composite powder and significantly reduces the dilution ratio from 0.281 to 0.140.

Microstructural Evolution and Hardness Improvement

The transformation from a hypoeutectic to a hypereutectic microstructure is the key metallurgical change that drives the improvement in wear resistance. In the hypereutectic structure, the primary M7C3 carbide phase forms during solidification, resulting in a higher volume fraction of hard carbide particles distributed throughout the coating matrix. This increased carbide content directly enhances the hardness and abrasive resistance of the overlay coating.

Wear Mechanism Analysis

The wear mechanism analysis revealed two primary modes of material removal: micro-cutting and micro-spalling. The micro-cutting mechanism involves the ploughing of abrasive particles through the coating surface, while micro-spalling involves the detachment of small fragments from the coating surface due to fatigue or stress concentrations at the carbide-matrix interface. The improvement in wear resistance is attributed to the increased volume fraction of M7C3 carbides, which provide superior resistance to both cutting and spalling mechanisms.

Engineering Practice Implications

Application to Abrasive Wear Protection

High-chromium alloy overlay coatings are widely used in the steel pipe and fitting industry for protecting components against abrasive wear. The findings of this study have direct relevance to several applications:

• Mining and mineral processing: Slurry pipes, valve internals, and pump impellers that are subjected to severe abrasive wear from solid particles
• Cement and construction: Wear parts in crushers, mills, and conveyors that are exposed to abrasive materials
• Power generation: Coal handling systems and flue gas ducts that experience particulate erosion
• Oil and gas production: Sand-laden flow lines and completion equipment that require enhanced abrasive resistance

Process Optimization and Cost-Benefit Analysis

The most significant practical finding of this study is the minimal cost of CaCO3 addition relative to the substantial improvement in wear resistance. CaCO3 is an inexpensive, readily available material that can be easily incorporated into composite powder formulations. This makes the technology highly attractive for industrial applications where cost-effectiveness is a critical consideration.

Standards and Specification Considerations

Key Questions and Reflections

An important question that arises from this study is the effect of CaCO3 addition on the corrosion resistance of the high-chromium alloy coating. While the wear resistance is significantly improved, the presence of CaCO3-derived phases in the microstructure may affect the corrosion behavior, particularly in acidic or chloride-containing environments. This aspect requires further investigation before the technology can be recommended for applications where corrosion resistance is also a critical requirement.

Another consideration is the consistency of the CaCO3 addition effect across different welding conditions and equipment configurations. The arc behavior modification is sensitive to the specific welding parameters and equipment used, and the optimal CaCO3 addition level may vary depending on the welding setup. This variability must be addressed through systematic process characterization and optimization for each specific application.

Study Insights and Reference Value

This study demonstrates an elegant and cost-effective approach to improving the wear resistance of high-chromium alloy overlay coatings through the simple addition of CaCO3 to the composite powder. The mechanism of action is clear: CaCO3 decomposition expands the arc, increases powder utilization, reduces dilution, and promotes the formation of a hypereutectic microstructure with a higher volume fraction of primary M7C3 carbides. The resulting improvement in hardness and wear resistance is substantial, with a 2.3x increase in wear resistance achieved at minimal additional cost. For engineers involved in the surface engineering of steel pipes and fittings, this study provides a practical and economically viable solution for enhancing the abrasive wear resistance of overlay coatings. The methodology of combining high-speed arc photography with microstructural analysis and wear testing offers a comprehensive approach to understanding the process-structure-property relationships in overlay welding operations. This work exemplifies the principle that simple process modifications can yield significant performance improvements when the underlying metallurgical mechanisms are understood and controlled.