Effect of Molybdenum on Microstructure and Properties of Plasma Cladded Cobalt-Based Alloys

Literature Overview

This 2006 study by Hou Qingyu and Huang Zhenyi from Anhui University of Technology investigates the influence of molybdenum addition on the microstructure and properties of plasma cladded cobalt-based alloys. Supported by the Anhui Provincial Higher Education Young Teacher Research Fund (2006jql082), the research was published in the journal of Rare Metals. The study addresses a critical alloy design question for cobalt-based cladding materials used in high-temperature and wear-resistant applications.

Cobalt-based alloys are among the most effective materials for high-temperature wear resistance and corrosion resistance in severe service environments. The addition of molybdenum is a well-established strategy for enhancing the high-temperature strength and oxidation resistance of cobalt alloys, but its specific effects on plasma cladded deposits require systematic investigation.

Core Technical Content

Cobalt-based cladding alloys typically contain 50-70% Co, with the balance consisting of Cr, W, Mo, Ni, and C. These alloys are valued for their ability to retain hardness and strength at elevated temperatures, making them suitable for applications such as turbine components, hot dies, and severe wear environments. The plasma transferred arc (PTA) cladding process is particularly well-suited for depositing cobalt-based alloys due to its high energy density and low dilution characteristics.

The molybdenum addition affects the cladding microstructure through multiple mechanisms: solid solution strengthening of the cobalt matrix, formation of Mo-rich carbides and oxides, modification of chromium carbide morphology, and influence on the solidification behavior of the deposit.

Microstructural Evolution with Molybdenum Addition

At low molybdenum concentrations, the primary effect is solid solution strengthening of the face-centered cubic cobalt matrix. Molybdenum atoms dissolve in the cobalt lattice, increasing lattice distortion and impeding dislocation motion. This results in a gradual increase in hardness and strength without significant changes to the overall microstructural morphology.

As molybdenum content increases beyond approximately 3-4%, secondary phases begin to form. These include Mo-rich carbides (Mo2C, MoC) and possibly molybdenum oxide phases (MoO3, CoMoO4) depending on the oxygen content of the molten pool. The formation of these secondary phases contributes additional hardening through particle reinforcement mechanisms.

At high molybdenum concentrations (above 9-10%), the microstructure may become embrittled by the formation of coarse, brittle molybdenum-rich phases. These phases can act as crack initiation sites and reduce the toughness of the deposit. Additionally, excessive molybdenum may promote cracking during solidification due to the high carbon equivalent of the alloy.

Performance Characteristics and Service Behavior

The wear resistance of molybdenum-modified cobalt-based cladding deposits improves with molybdenum addition up to an optimal level, beyond which the benefits diminish or reverse. The enhanced wear resistance is attributed to the combined effects of increased matrix hardness, refined microstructure, and the presence of hard secondary phases that provide additional wear resistance through ploughing and cutting mechanisms.

High-temperature performance is significantly enhanced by molybdenum addition. The molybdenum atoms increase the activation energy for dislocation motion at elevated temperatures, resulting in superior hot hardness retention. This is particularly important for applications such as hot working dies, turbine components, and extrusion tooling where the material must maintain its properties at temperatures approaching 600-800°C.

Corrosion resistance is also affected by molybdenum addition. In oxidizing environments, molybdenum promotes the formation of protective oxide scales (MoO3, CoMoO4) that provide additional oxidation resistance. In reducing environments, the effect of molybdenum on corrosion resistance is less pronounced and may even be detrimental if excessive amounts promote pitting.

Process Considerations and Engineering Practice

The plasma cladding process parameters must be carefully controlled when depositing molybdenum-containing cobalt alloys. The increased carbon equivalent and potential for brittle phase formation require careful management of cooling rates and solidification conditions. Key process considerations include:

• Arc current and voltage: must be sufficient to ensure complete powder melting without excessive dilution
• Powder feed rate: must be synchronized with travel speed to maintain consistent composition
• Travel speed: should be optimized to achieve fine microstructure without excessive cooling rates
• Substrate preheating: may be required to prevent cracking in thick sections
• Interpass temperature: must be controlled to manage thermal cycling effects

The dilution ratio is particularly important for molybdenum-containing alloys because the molybdenum content of the deposit is highly sensitive to the base metal dilution. Even small variations in dilution can significantly affect the molybdenum concentration and, consequently, the microstructure and properties of the deposit.

Key Questions and Reflections

An important question arising from this research is the optimal molybdenum content for specific service conditions. The study suggests that the optimal molybdenum level depends on the balance between hardness, toughness, and high-temperature performance required for the application. For wear-resistant applications at room temperature, lower molybdenum contents may be sufficient, while high-temperature applications may require higher molybdenum levels.

Another critical consideration is the long-term stability of molybdenum-rich phases under thermal cycling. Molybdenum carbides and oxides may undergo phase transformations or coarsening during prolonged exposure to elevated temperatures, potentially degrading the properties of the deposit over time. Understanding these time-dependent effects is essential for predicting service life.

Additionally, the interaction between molybdenum and other alloying elements (Cr, W, Ni) in the deposit microstructure warrants further investigation. The synergistic or antagonistic effects of multiple alloying elements on microstructure and properties are complex and not fully predictable from single-element studies.

Study Insights and Implications

This research provides valuable insights into the role of molybdenum in enhancing the performance of plasma cladded cobalt-based alloys. The systematic investigation of molybdenum content effects on microstructure and properties offers guidance for alloy design in specific applications.

The findings demonstrate that molybdenum is an effective alloying addition for improving the high-temperature wear resistance and strength of cobalt-based cladding deposits. However, the benefits are not unlimited, and optimal molybdenum content must be determined based on the specific service requirements and processing constraints.

In conclusion, this study highlights the importance of molybdenum as a microstructure-refining and high-temperature-strengthening element in cobalt-based plasma cladding alloys, while emphasizing the need for careful optimization of molybdenum content to balance hardness, toughness, and long-term stability in demanding service environments.