Manganese-Molybdenum Series Surfacing Trials on Rock Loader Bucket Teeth

Historical Context and Engineering Significance

The research by Li Yajiang from Shandong University of Technology and Zhang Yongxi from Shandong Coal Design Institute (1989) represents an early but significant contribution to the development of manganese-molybdenum (Mn-Mo) alloy surfacing systems for mining machinery. Rock loader bucket teeth are among the most severely worn components in underground coal mining operations, subjected to combined abrasive, impact, and fatigue loading from hard rock and coal masses. The development of effective hard-facing alloys for these components directly impacts mining productivity and equipment availability.

Alloy Design Philosophy

The Mn-Mo alloy system investigated in this study is based on the well-established high-manganese austenitic steel family (Hadfield steel and derivatives), with the addition of molybdenum to enhance specific properties. The design rationale includes:

• High manganese (12-18%): Promotes retained austenite content, enabling strain-induced martensitic transformation during impact loading, which increases surface hardness from approximately 200 HV (as-cast) to 500-700 HV (after deformation)
• Molybdenum addition (1.5-4.0%): Increases tempering resistance, promotes carbide precipitation (Mo2C, MoC), and improves high-temperature wear resistance
• Carbon content (1.0-2.0%): Ensures sufficient carbide formation and supports the austenitic microstructure
• Chromium (2-5%): Contributes to corrosion resistance and additional carbide hardening

Surfacing Process and Microstructural Results

The surfacing trials employed submerged arc welding (SAW) and flux-cored arc welding (FCAW) processes, which were the predominant industrial surfacing methods at the time. The surfacing deposits were applied to bucket teeth made of Q345 or 16Mn base steel, with the surfacing layer serving as the primary wear-resistant surface.

The microstructural analysis reveals a predominantly austenitic matrix with dispersed carbides (Mn3C, Mo2C, and complex carbides) and some retained ferrite. The strain-induced martensite transformation during service creates a work-hardening effect that progressively increases surface hardness at the most severely loaded regions.

Wear Performance and Failure Analysis

Field trials conducted in underground coal mines demonstrated that the Mn-Mo surfacing alloys significantly outperform conventional carbon steel bucket teeth. The wear life improvement of 2.5 to 4.5 times is attributed to the combined effects of:

1. Work-hardening capability: The retained austenite transforms to martensite under impact loading, creating a self-hardening surface that adapts to varying wear conditions
2. Carbide reinforcement: Molybdenum carbides provide hard particles that resist micro-ploughing by abrasive particles
3. Tough matrix support: The austenitic matrix provides excellent impact resistance, preventing brittle fracture that would terminate the work-hardening cycle

The primary failure modes observed include spalling of the surfacing layer from the base material due to insufficient bond strength, and progressive wear-through of the surfacing layer exposing the base material. Spalling was found to be the dominant failure mechanism in approximately 40 percent of failed components, indicating that interface quality is a critical control parameter.

Process Optimization and Defect Control

The surfacing trials identified several process parameters that significantly influence deposit quality and service performance:

• Travel speed: Optimal range of 150 to 250 mm/min for SAW; higher speeds reduce dilution but may cause incomplete fusion
• Wire feed speed: 4 to 7 m/min for FCAW; must be coordinated with travel speed to maintain consistent bead geometry
• Preheating: 100 to 200 degrees Celsius recommended for thick base materials to prevent cold cracking
• Inter-pass temperature: Limited to below 250 degrees Celsius to avoid excessive grain growth in previously deposited layers
• Flux selection: Low-hydrogen fluxes (moisture content below 1.0 percent) essential for preventing porosity

Critical Assessment and Modern Perspective

While the Mn-Mo surfacing system investigated in this 1989 study represents sound engineering practice for its era, modern surfacing technology has evolved considerably. Contemporary alternatives include cobalt-tungsten alloys, high-chromium white cast irons, and advanced nickel-based hardfacing compositions that offer superior wear life in many applications. However, the Mn-Mo system retains advantages in terms of cost-effectiveness, impact toughness, and work-hardening capability that make it suitable for specific applications where extreme hardness is not required but impact resistance is paramount.

The fundamental principle established by this research—that strain-induced martensitic transformation can be exploited for in-situ surface hardening—remains valid and continues to inform modern alloy design. Engineers working with manganese-based surfacing alloys today should note that the dilution ratio (typically 10 to 25 percent for Mn-Mo alloys on steel substrates) must be carefully controlled, as excessive dilution reduces austenite retention and diminishes the work-hardening effect. The study serves as a valuable historical reference and a reminder that material selection must always be matched to the specific loading and wear conditions of the application.