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

Literature Overview

The paper published in the journal "Iron and Steel" (2024, Vol. 59, No. 4, pp. 148-158) by Zhang Kai, Guo Zhi, Sun Dongyun, Li Yanguo, and Zhang Ming from Yanshan University and other institutions investigates the role of molybdenum addition in high chromium cast iron overlay deposits. The research is funded by the National Natural Science Foundation of China (projects 52122410, 51831008) and Hebei Provincial Science and Technology Research Programs. The authors designed a series of ultra-high carbon, high chromium overlay wear-resistant plates with Mo content ranging from 0.04% to 0.30% by mass fraction, and systematically studied the influence of Mo on microstructure, phase composition, hardness, and wear behavior using XRD, SEM, EDS, hardness testing, and dry sand abrasion tests.

Core Technical Findings

The overlay layer microstructure consists primarily of austenite (gamma phase) and M7C3 carbides (where M = Fe, Cr). The authors observed that as Mo content increases, the primary carbide content decreases while austenite content increases. The individual primary carbide size follows a non-monotonic trend, first decreasing and then increasing with Mo addition. The hardness of primary carbides increases with Mo content, but the macroscopic hardness of the overlay layer first increases and then decreases. After wear testing, the number of fracture cracks in primary carbides decreases, and spalling gradually reduces. However, in the 0.30% Mo specimen, eutectic carbide spalling appeared, indicating an adverse effect at higher Mo levels.

Interpretation of Technical Points

The fundamental mechanism identified is that Mo atoms incorporated into the primary carbide lattice raise the intrinsic hardness of individual carbide particles. However, Mo simultaneously reduces the volume fraction of primary carbides. This dual effect creates a competition: at low Mo levels, the hardening effect on carbides dominates and improves overall wear resistance; at high Mo levels, the reduction in carbide volume fraction becomes dominant, leading to decreased hardness and increased wear loss. The authors note that the refinement followed by coarsening of primary carbides suggests a complex nucleation and growth behavior that warrants further investigation.

From a metallurgical perspective, Mo is known to be a strong carbide former that stabilizes M7C3-type carbides. The EDS analysis confirmed Mo enrichment in the carbide phase, which is consistent with the thermodynamic tendency of Mo to preferentially partition into carbides in Fe-Cr-C systems. The reduction in primary carbide content with increasing Mo may be attributed to Mo's effect on the solidification path, potentially altering the eutectic reaction temperature and carbon activity.

Engineering Practice Integration

In industrial applications such as port handling equipment (the second author's affiliation with Qinhuangdao Port Company is notable), high chromium cast iron overlay layers are widely used as internal linings for large industrial equipment subject to abrasive wear. The finding that 0.13% Mo provides optimal wear resistance reduction of 13.6% has direct practical significance for welding consumable design. Engineers selecting overlay welding wires or electrodes for wear-resistant applications should consider Mo addition within the 0.04%-0.13% range to achieve the best balance between carbide hardness and carbide volume fraction.

However, the study's limitation is acknowledged: only three Mo levels were tested, and the transition from refinement to coarsening of primary carbides is not fully characterized. A more detailed Mo content gradient study would be beneficial for establishing a precise process window. Additionally, the study focuses on dry sand abrasion, while many industrial applications involve three-body or slurry abrasion where the optimal Mo content might differ.

Key Questions and Reflections

The non-monotonic behavior of hardness and wear resistance with Mo content raises an important question about the threshold mechanism. At what Mo concentration does the volume fraction reduction begin to dominate over the hardening effect? The 0.13% Mo level appears to be near this threshold, but more data points between 0.13% and 0.30% would help define the boundary. Furthermore, the observation of eutectic carbide spalling at 0.30% Mo suggests that excessive Mo may alter the eutectic morphology in a way that creates weaker interfaces, which is a critical concern for fatigue and impact loading conditions in service.

Study Insights and Implications

This research provides valuable guidance for the design of Mo-alloyed high chromium cast iron overlay welding consumables. The optimal Mo addition of approximately 0.13% represents a practical engineering target that balances carbide hardening against carbide volume fraction. For production environments, this suggests that welding wire formulations should be carefully controlled to maintain Mo within a narrow window, as both under-alloying and over-alloying degrade wear performance. The study also highlights the importance of understanding the competing mechanisms in alloy design, where a single alloying element can have multiple and sometimes opposing effects on microstructure and properties.