Effect of Molybdenum Content on Microstructure and Wear Resistance of High-Chromium Cast Iron Overlay Layer

Literature Overview and Research Motivation

This study systematically investigates the influence of molybdenum (Mo) content on the microstructure and wear resistance of high-chromium cast iron weld overlay layers. High-chromium cast irons, containing 10-30 wt% Cr, are widely used for overlay applications in mining, cement, and power generation industries due to their excellent abrasive wear resistance. The addition of molybdenum is a common practice to enhance hardness, improve corrosion resistance, and refine the microstructure, but the optimal Mo content for specific applications remains poorly defined in the literature.

The study examines Mo contents ranging from 0% to 6.0 wt% in a base composition of approximately 26 wt% Cr, 2.5 wt% C, 1.0 wt% Mo (baseline), with systematic variation of the molybdenum level. The overlay layers were deposited using submerged arc welding onto low-carbon steel substrates, and the resulting microstructures and wear properties were characterized using metallography, X-ray diffraction, and standardized wear testing.

Microstructural Evolution with Molybdenum Content

The microstructure of high-chromium cast iron overlays is primarily composed of a martensitic matrix with dispersed carbides. The type, size, and distribution of these carbides are critically influenced by the molybdenum content, which acts as a strong carbide-forming element and affects the solidification sequence.

The transition from M7C3 to M23C6 to M6C carbide types with increasing Mo content is a critical microstructural change. M6C carbides (Mo-rich) are harder and more wear-resistant than M23C6 carbides, which in turn are harder than M7C3 carbides. However, excessive Mo content promotes the formation of coarse, angular carbides that can act as crack initiation sites and reduce the toughness of the overlay.

Wear Resistance Characterization and Analysis

The wear resistance was evaluated using both dry sliding wear tests against alumina counterfaces and abrasive wear tests using standardized sand abrasion methods. The results clearly demonstrate that wear resistance increases with Mo content up to an optimum level, beyond which the benefits diminish or reverse.

The dry sliding wear rate decreased from approximately 8×10⁻⁴ mm³/N·m at 0% Mo to approximately 3×10⁻⁴ mm³/N·m at 3.0% Mo, with a marginal further improvement to 2.8×10⁻⁴ mm³/N·m at 4.0% Mo. Beyond 4.0% Mo, the wear rate increased slightly due to the reduced toughness and increased susceptibility to crack propagation. The abrasive wear results showed a similar trend, with the optimal Mo content falling in the range of 2.5-4.0 wt%.

The wear mechanism transitioned from adhesion-dominated wear at low Mo contents to abrasion-dominated wear at higher Mo contents. At low Mo levels, the softer M7C3 carbides were more susceptible to adhesive transfer, while at higher Mo levels, the harder M6C and M23C6 carbides provided superior resistance to abrasive material removal.

Engineering Implications and Optimal Design Guidelines

The study provides clear guidance for selecting Mo content based on the specific service conditions. For applications dominated by abrasive wear, such as ball mill liners, cone crusher mantels, and conveyor chutes, a Mo content of 2.5-4.0 wt% is recommended to maximize wear resistance. For applications involving significant impact loading, such as hammer mill hammers and primary crusher jaws, a lower Mo content of 1.0-2.0 wt% may be preferable to maintain adequate toughness and resistance to impact spalling.

The study also highlights the importance of controlling the cooling rate during welding, as faster cooling rates promote finer carbide distributions and higher hardness values. In multi-pass overlay applications, the interpass temperature should be carefully controlled to avoid excessive grain growth and carbide coarsening in previously deposited passes.

Study Insights and Practical Recommendations

This study provides valuable quantitative data on the Mo effect in high-chromium cast iron overlays, filling a gap in the technical literature where Mo optimization has been largely empirical. The key insight is that the optimal Mo content is not a single value but depends on the specific wear mechanism and loading conditions. Engineers should consider the entire service environment, including temperature, corrosion exposure, and impact severity, when selecting the Mo content for a particular application.

The practical recommendation is to use a Mo content of 2.0-3.5 wt% as a general-purpose choice for high-chromium cast iron overlays, with adjustments based on the specific application requirements. For high-temperature applications above 500°C, higher Mo contents of 3.0-4.0 wt% are beneficial due to the enhanced creep resistance and thermal stability of Mo-containing carbides. The study reinforces the principle that overlay design must balance hardness, toughness, and corrosion resistance, and that molybdenum is a versatile alloying element that can be tuned to achieve the desired property combination.