Effect of Molybdenum on Microstructure and Wear Resistance of High-Hardness Open-Arc Weld Overlay Alloys

Literature Overview

This study investigates the influence of molybdenum (Mo) addition on the microstructure, hardness, and wear resistance of high-hardness open-arc weld overlay alloys, which are widely used in severe abrasion environments such as mining, cement, and power generation equipment. The research systematically varies Mo content in the filler alloy system and evaluates the resulting metallurgical and tribological performance. The work is particularly relevant to engineers designing overlay consumables for components subjected to high-temperature abrasive wear, where conventional carbide-based alloys may suffer from thermal softening or insufficient hardness retention.

Core Technical Findings

The study demonstrates that Mo plays a multifaceted role in the overlay alloy system. Molybdenum acts as a strong carbide-forming element, promoting the precipitation of M7C3 and MC-type carbides in the martensitic or austenitic matrix. These carbides significantly enhance the hardness of the overlay deposit, often pushing it above 60 HRC in optimized compositions. Additionally, Mo contributes to solid solution strengthening of the matrix phase and improves high-temperature hardness retention through its effect on the TTT and CCT diagrams, delaying the onset of softening at elevated service temperatures.

The microstructural evolution with increasing Mo content follows a predictable trend. At lower Mo levels, the deposit consists primarily of tempered martensite with dispersed carbide particles. As Mo content increases, the volume fraction of hard carbides grows, and the matrix may transition toward a more austenitic character due to Mo's austenite-stabilizing effect when combined with sufficient carbon and nickel. This microstructural transformation directly correlates with improved wear resistance, as measured by pin-on-disk and dry sliding tests.

Key Process Parameters and Microstructural Analysis

Microstructural Evolution with Mo Addition

The metallographic analysis reveals distinct zones within the overlay deposit. The weld pool region exhibits a columnar dendritic structure with carbide segregation at the interdendritic regions. The heat-affected zone (HAZ) shows grain coarsening and potential martensite formation in the base metal, depending on the preheat and interpass temperature control. The dilution zone between the overlay and the substrate is critical, as excessive dilution from the carbon steel base metal reduces the effective Mo and carbon content, leading to a softer transition region that may become the wear initiation site.

Wear Mechanism Analysis

The wear resistance improvement attributed to Mo addition operates through three primary mechanisms. First, the hard carbide phases provide direct abrasive resistance through their high hardness (Vickers hardness of M7C3 carbides typically exceeds 1500 HV). Second, the refined microstructure resulting from Mo-induced nucleation sites increases the number of asperity interactions per unit area, distributing the wear load more evenly. Third, Mo improves the thermal stability of the microstructure, preventing softening during the localized heating that occurs at the wear interface during sliding contact.

Engineering Practice Implications

In practical applications, the selection of Mo content must balance wear resistance against weldability and cracking susceptibility. Excessive Mo content (above 12 wt%) can lead to increased hardness in the HAZ, raising the risk of hydrogen-induced cracking, particularly in thick-section components with high residual stresses. Engineers should adopt the following countermeasures:

• Implement controlled preheat (minimum 150°C for thick sections) to reduce cooling rates and hydrogen diffusion rates.
• Use low-hydrogen fluxes or gas-shielded consumables to minimize hydrogen pickup.
• Apply post-weld stress relief treatment (600–650°C for 2 hours per 25 mm thickness) to reduce residual stresses without significantly reducing overlay hardness.
• Control dilution by using overlay layers with reduced first-pass width and ensuring proper weld bead overlap (minimum 50% overlap between adjacent passes).

The study also highlights that Mo-containing overlay alloys are particularly effective in high-temperature abrasive wear scenarios, such as coal mill liners and cement kiln components, where service temperatures exceed 200°C. At these temperatures, conventional high-carbon martensitic overlays soften significantly, while Mo-strengthened alloys retain hardness values above 45 HRC even after prolonged exposure.

Key Questions and Reflections

One critical question that emerges from this study is the optimal balance between Mo content and toughness. While higher Mo levels improve wear resistance, they simultaneously reduce the fracture toughness of the overlay deposit. In applications where the component is subjected to impact loading in addition to abrasion, such as crusher hammers or conveyor components, a compromise composition with 8–10 wt% Mo may provide the best overall performance. Future work should investigate the effect of Mo in combination with other alloying elements such as tungsten and vanadium to create synergistic effects that maximize both hardness and toughness.

Summary and Study Insights

The systematic investigation of Mo effects on open-arc weld overlay alloys provides valuable guidance for consumable design and process optimization. The key takeaway is that Mo is not merely a hardness-enhancing element but a microstructure-tailoring alloying element that influences phase composition, carbide morphology, thermal stability, and ultimately wear performance through multiple interacting mechanisms. Engineers engaged in overlay repair and manufacturing should carefully consider Mo content selection based on the specific service conditions, including temperature, wear mode, and mechanical loading, rather than simply maximizing hardness without regard to the broader metallurgical consequences.