Effect of Molybdenum on Microstructure and Tribological Properties of Cr27 High-Chromium Cast Iron CMT Overlay

Literature Overview

This research by Zhang Wenyang, Hu Lei, and Yuan Lin, published in Surface Technology in 2026 (Vol. 55, No. 11, pp. 120-128), investigates the influence of molybdenum addition on the microstructure, mechanical properties, and tribological behavior of Cr27 high-chromium cast iron overlay coatings deposited using Cold Metal Transfer (CMT) arc welding. The work was supported by the Guangdong Provincial Administration for Market Regulation Science and Technology Project (2025CT12). The study employs a comprehensive characterization approach including penetrant testing, SEM, EBSD, and pin-on-disk wear testing.

Experimental Design and Key Results

The study compares two CMT-welded overlay coatings—one with Mo addition and one without—deposited from flux-cored wire on Cr27 high-chromium cast iron substrate. The results demonstrate significant improvements with Mo addition:

The dual nature of Mo addition—significant tribological improvement but increased welding difficulties—represents a classic materials engineering trade-off that requires careful process management.

Metallurgical Mechanisms of Mo Effects

The molybdenum element influences the overlay microstructure through several well-defined mechanisms:

1. Grain refinement mechanism:

• Mo2C (melting point ~2830°C) precipitates first during solidification due to its extremely high melting point
• These high-temperature particles serve as heterogeneous nucleation sites for austenite grains
• Increased nucleation rate leads to finer grain structure
• Mo2C particles also pin grain boundaries during later solidification stages, inhibiting grain growth
• Net result: 38.6% reduction in austenite grain size

2. Carbide enhancement mechanism:

• Mo reduces carbon solubility in austenite
• More carbon is available for eutectic reaction
• Increased formation of (Fe,Cr)7C3 eutectic carbides
• Higher volume fraction of M7C3 carbides provides more wear-resistant hard phases

3. Solid solution effects:

• Mo atoms in austenite matrix provide solid solution strengthening
• Contributes to the overall hardness increase from 690 HV to 740 HV

Welding Process Challenges with Mo Addition

The introduction of molybdenum creates significant welding process challenges that must be managed:

The transverse cracking induced by Mo addition is particularly concerning for structural applications. This cracking likely results from:

• Increased solidification range of the weld metal
• Higher thermal stresses due to Mo's effect on thermal expansion
• Embrittlement of the grain boundaries during solidification

Tribological Analysis

The wear behavior analysis reveals important insights into the mechanism of Mo-enhanced wear resistance:

Wear mechanisms identified:

• Adhesive wear (dominant in both conditions)
• Abrasive wear (secondary mechanism)

Mo-enhanced wear resistance mechanisms:

1. Finer austenite grains provide higher resistance to abrasive wear through Hall-Petch strengthening
2. Increased M7C3 carbide volume fraction provides more hard phase particles for wear resistance
3. Reduced friction coefficient (0.645 to 0.395) indicates improved surface interaction, possibly due to:

• Smoother surface from finer microstructure
• Formation of protective tribofilm from Mo oxide species
• Reduced adhesive junction formation on refined surfaces

The 50% reduction in wear loss represents a transformative improvement for industrial applications, directly translating to extended component service life.

Engineering Practice and Process Optimization

For industrial implementation of Mo-containing Cr27 overlay coatings via CMT welding, the following process recommendations emerge:

1. Preheating: 200-300°C preheat to reduce thermal gradients and minimize cracking
2. Heat input control: Moderate heat input to balance deposition rate with solidification cracking avoidance
3. Interpass temperature: Maintain below 250°C to prevent excessive grain growth
4. Post-weld treatment: Stress relief at 550-600°C for 1-2 hours to reduce residual stresses
5. Wire composition optimization: Consider reducing Mo content slightly while maintaining tribological benefits

The CMT welding process itself offers advantages for this application:

• Low heat input reduces dilution and thermal damage
• Precise wire feed control enables consistent composition
• Reduced spatter compared to conventional GMAW (though still increased with Mo)
• Good wire feed stability for automated production

Study Insights and Practical Value

This research demonstrates that strategic alloying with molybdenum can dramatically improve the tribological performance of high-chromium cast iron overlay coatings, but at the cost of increased welding process complexity. The 50% reduction in wear loss and 38.6% grain refinement achieved through Mo addition represent significant technical advances for surface engineering of wear-critical components.

For piping and equipment applications in mining, cement, and material handling industries where high-chromium cast iron overlays are commonly used, this research provides clear evidence that Mo addition is beneficial. However, the induced cracking and spatter issues must be addressed through careful process parameter optimization and potentially through compositional modifications (such as adding small amounts of titanium or niobium to modify carbide morphology and reduce cracking susceptibility).

The research methodology combining EBSD characterization with pin-on-disk wear testing provides a rigorous scientific basis for understanding the structure-property relationships. This approach should be adopted in future overlay coating development programs to ensure that microstructural improvements translate reliably into tribological performance gains.