Zhuojin Pipe Fitting Co., LtdEffect of Molybdenum on Microstructure and Tribological Performance of Cr27 High Chromium Cast Iron CMT Surfacing Layer
Literature Overview
This paper by Zhang Wenyang, Hu Lei, and Yuan Lin, published in Surface Technology (2026, Vol. 55, No. 11, pp. 120-128), investigates the influence of molybdenum addition on the microstructure and friction-wear behavior of Cr27 high chromium cast iron surfacing deposits produced via Cold Metal Transfer (CMT) arc surfacing. The study is supported by the Guangdong Provincial Administration for Market Regulation Science and Technology Project (2025CT12). The work is particularly relevant to engineers working on surface engineering for wear-resistant components in mining, cement grinding, and material handling applications where high chromium cast irons are widely deployed.
Core Technical Findings
The researchers prepared two sets of surfacing specimens: one with Mo addition and one without, both based on Cr27 high chromium cast iron flux-cored wire using CMT arc surfacing. Characterization was carried out through penetrant testing (PT), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and pin-on-disk wear experiments.
Microstructural Evolution
Both surfacing layers exhibited the classic high chromium cast iron microstructure consisting of (Fe,Cr)₇C₃ eutectic carbides embedded in an austenitic matrix. However, the addition of Mo produced two significant microstructural changes:
| Parameter | Without Mo | With Mo | Change |
|---|---|---|---|
| Austenite grain size | Baseline | Reduced by ~38.6% | Significant refinement |
| Average hardness (HV) | 690 HV | 740 HV | +7% |
| M₇C₃ carbide volume fraction | Baseline | Increased | Notable improvement |
| Average friction coefficient | 0.645 | 0.395 | ~39% reduction |
| Wear loss | Baseline | Reduced by ~50% | Substantial improvement |
Mechanism of Grain Refinement
The authors propose a two-stage mechanism for Mo-induced grain refinement:
- Heterogeneous nucleation during solidification: Mo₂C, with a high melting point, precipitates early during solidification and serves as heterogeneous nucleation sites for austenite, thereby increasing the nucleation rate and reducing grain size.
- Grain boundary pinning during late-stage solidification: Residual Mo₂C particles at austenite grain boundaries act as Zener pinning obstacles, restricting grain coarsening during the final stages of solidification.
Carbon Partitioning Effect
Mo addition reduces the solubility of carbon in austenite, which drives more carbon into the eutectic reaction. This results in a higher volume fraction of M₇C₃ carbides. The combined effect of increased carbide content and refined grain size accounts for the hardness improvement from 690 HV to 740 HV.
Process Considerations and Defect Analysis
A critical engineering observation is that Mo addition led to significantly increased spatter during surfacing and induced transverse cracking. This is a practical concern for production environments where spatter management and crack-free deposition are essential for surface quality and subsequent machining.
| Defect/Issue | Cause | Mitigation Strategy |
|---|---|---|
| Increased spatter | Mo addition alters arc stability and droplet transfer characteristics | Optimize CMT parameters (wire feed speed, arc voltage); consider shielding gas composition adjustment |
| Transverse cracking | Thermal stress concentration exacerbated by Mo-induced phase transformation | Reduce heat input; implement interpass temperature control; consider post-weld heat treatment |
Engineering Practice Implications
From a practical standpoint, the 50% reduction in wear loss is a compelling economic argument for Mo-containing surfacing alloys in high-wear applications. However, the spatter and cracking issues must be carefully managed in production. Engineers should consider:
- Implementing strict pre-heat and interpass temperature protocols to control thermal gradients.
- Using low-hydrogen shielding gas mixtures to minimize hydrogen-induced cracking susceptibility.
- Conducting PT or MT inspection after surfacing to detect transverse cracks before component deployment.
- Evaluating the cost-benefit ratio of Mo addition against the increased process control requirements.
Study Insights and Reflections
The paper provides valuable mechanistic understanding of how Mo functions in high chromium cast iron surfacing systems. The dual role of Mo₂C as both a nucleation site and a grain boundary pinning agent is an elegant demonstration of how alloying elements can be leveraged for microstructural control. The 38.6% grain refinement achieved is substantial and directly correlates with improved tribological performance through both Hall-Petch strengthening and increased carbide volume fraction.
One area that warrants further investigation is the long-term wear behavior under realistic service conditions. Pin-on-disk testing provides controlled laboratory data, but field performance in cement grinding mills or mining crushers involves more complex loading conditions, including impact, abrasion, and possibly corrosion. The transverse cracking issue also needs more systematic study, particularly regarding its relationship with heat input, substrate preheat temperature, and wire composition.
The work contributes meaningfully to the design of advanced surfacing alloys for the cement and mining industries, where high chromium cast irons are workhorse materials for wear protection. Engineers involved in surface engineering specifications should pay attention to the Mo content optimization window, as excessive Mo could exacerbate cracking without proportional wear resistance gains.