Effect of Molybdenum on Microstructure and Tribological Performance of Cr27 High Chromium Cast Iron Overlay Deposits

Literature Overview

This study published in Surface Technology (2026, Vol. 55, No. 11, pp. 120–128) by Zhang Wenyang, Hu Lei, and Yuan Lin investigates the influence of molybdenum addition on the microstructure and wear resistance of Cr27 high-chromium cast iron overlay deposits produced via CMT (Cold Metal Transfer) arc surfacing. The research was funded by the Guangdong Provincial Administration for Market Regulation Science and Technology Project (2025CT12). The work is particularly relevant to engineers dealing with wear-critical components in mining, cement, and material handling industries where high-chromium cast iron overlays serve as protective surfaces.

Core Technical Findings

Microstructural Evolution

Both Mo-containing and Mo-free overlay deposits consist primarily of (Fe,Cr)₇C₃ eutectic carbides and an austenitic matrix. However, Mo addition produced two significant microstructural changes:

Parameter	Without Mo	With Mo	Change
Austenite grain size	Baseline	~38.6% finer	Significant refinement
M₇C₃ carbide volume fraction	Baseline	Increased	Enhanced
Average hardness (HV)	690	740	+7%
Average friction coefficient	0.645	0.395	-38.8%
Wear mass loss	Baseline	~50% reduction	Substantial improvement

Mechanism of Grain Refinement

The study proposes a dual mechanism for austenite grain refinement through Mo addition:

Heterogeneous nucleation: During solidification, high-melting-point Mo₂C precipitates first, serving as nucleation sites that increase the nucleation rate of austenite grains.
Grain boundary pinning: In the late stage of solidification, Mo₂C particles pin grain boundaries and impede austenite grain growth.

Additionally, Mo reduces the solubility of carbon in austenite, forcing more carbon to participate in the eutectic reaction, thereby increasing the volume fraction of M₇C₃ carbides.

Process Challenges and Defect Analysis

A critical practical finding is that Mo addition significantly increases spatter during CMT surfacing and induces transverse cracking. This presents a real engineering challenge:

Spatter increase: The Mo₂C precipitation behavior alters the arc stability and droplet transfer characteristics, requiring process parameter adjustments.
Transverse cracking: Likely related to the increased carbon activity at grain boundaries and the brittleness introduced by Mo₂C network formation during solidification. Penetrant testing confirmed the presence of these defects.

Countermeasures for Engineering Practice

Defect	Root Cause	Recommended Countermeasure
Excessive spatter	Altered arc dynamics with Mo₂C	Reduce wire feed speed by 10–15%; increase arc voltage slightly
Transverse cracking	High carbon activity + Mo₂C network	Increase preheat temperature to 200–250°C; apply post-weld stress relief at 600°C for 2h
Surface irregularity	Non-uniform Mo distribution in flux	Use flux with controlled Mo content distribution; ensure consistent wire coating

Wear Mechanism Analysis

Both deposits exhibit a combination of adhesive wear and abrasive wear as dominant mechanisms. The Mo-containing deposit achieves superior wear resistance through:

Finer austenite grains providing more uniform load distribution
Higher M₇C₃ carbide content offering enhanced abrasive resistance
Lower friction coefficient reducing adhesive wear initiation
Increased hardness providing resistance to plastic deformation

The combination of grain refinement and carbide enhancement creates a synergistic effect that exceeds what either mechanism alone would achieve.

Study Insights and Engineering Implications

This research demonstrates that even small additions of Mo to high-chromium cast iron overlay systems can produce disproportionate improvements in tribological performance. The 50% reduction in wear mass loss is practically significant for components such as crusher liners, ball mill shells, and conveyor rollers where overlay life directly impacts maintenance scheduling and operational costs.

However, the transverse cracking issue raises important questions for production implementation. Engineers must balance the wear resistance gains against the increased risk of cracking, particularly in thick overlay applications where residual stress accumulation is more severe. A practical approach would involve limiting overlay thickness per pass and implementing interpass temperature control to manage thermal cycling effects.

The CMT process itself is well-suited for this application due to its low heat input and reduced dilution characteristics, which preserve the high-chromium chemistry of the overlay. The study provides valuable data for parameter optimization when Mo is introduced into the filler metal composition.

Reference Value and Outlook

The findings have direct applicability to the design of wear-resistant overlay systems for cement mills, mining equipment, and material handling components. Future work should investigate the effect of Mo content optimization (0.5%, 1.0%, 1.5%) to establish a quantitative relationship between Mo addition and both wear performance and crack susceptibility. Combining Mo with other grain-refining elements such as Ti or V may further improve the balance between hardness, toughness, and crack resistance in high-chromium cast iron overlay systems.