Research on Microstructure and Properties of CO2 Cladding Iron-Based Alloy

Literature Overview

This study investigates the microstructure, mechanical properties, and performance characteristics of iron-based alloy cladding layers deposited using carbon dioxide (CO2) as the shielding gas. CO2 shielding gas is widely used in industrial welding and cladding applications due to its low cost, availability, and adequate shielding properties. However, the use of CO2 in cladding applications presents unique challenges related to oxidation, spatter, and microstructural control that differ from argon or mixed-gas shielding.

The study focuses on iron-based cladding alloys, which are commonly used for wear resistance applications in mining, construction, and heavy machinery. Iron-based alloys offer a good balance of cost, availability, and performance, making them attractive for large-scale industrial cladding operations. The research examines the influence of CO2 shielding on the microstructure evolution, mechanical properties, and wear performance of the cladding layer, providing practical guidance for process optimization.

Core Technical Points

Chemistry of CO2 Shielding in Cladding

Carbon dioxide is a reactive shielding gas that dissociates at high temperatures to form carbon monoxide (CO) and atomic oxygen (O). The dissociation reaction is:

CO2 → CO + O (at temperatures above 2000°C)

The atomic oxygen released during dissociation can react with alloying elements in the molten pool, leading to:

1. Oxidation of alloying elements: Elements such as manganese (Mn), silicon (Si), chromium (Cr), and aluminum (Al) are preferentially oxidized, depleting the cladding layer of these beneficial elements.
2. Formation of oxide inclusions: Metal oxides (MnO, SiO2, Cr2O3) form in the weld pool, potentially affecting the microstructure and properties.
3. Carbon pickup: The CO released during dissociation can increase the carbon content of the cladding layer, promoting carbide formation.
4. Spatter formation: The reactive nature of CO2 leads to higher spatter rates compared to argon or argon-based mixtures.

Alloy Composition Design for CO2 Cladding

The alloy composition must be designed to compensate for the oxidizing effects of CO2 shielding. The following composition ranges are recommended:

Microstructural Characteristics Under CO2 Shielding

The microstructure of iron-based cladding layers deposited under CO2 shielding is characterized by:

Primary phases:

• Martensite (M): The dominant phase in high-carbon compositions, providing high hardness. The martensite morphology is influenced by the cooling rate and carbon content.
• Austenite (A): Retained austenite phases contribute to toughness. The amount of retained austenite is influenced by the Ni content and cooling rate.
• Carbides: M7C3, M23C6, and complex carbides (M6C, M23C6 with Cr, Mo, V) provide wear resistance. The carbide morphology is affected by the CO2-induced carbon pickup.

Secondary phases:

• Oxide inclusions: MnO, SiO2, and Cr2O3 inclusions are more prevalent under CO2 shielding compared to argon shielding. These inclusions can act as crack initiation sites if not properly controlled.
• Bainite: In lower-carbon compositions or slower cooling conditions, upper bainite may form, providing moderate hardness with improved ductility.

Comparison with Argon Shielding

Mechanical Property Evaluation

Hardness Distribution

The hardness of the CO2-cladded iron-based alloy varies with depth and position:

The hardness gradient is influenced by the cooling rate, which is higher at the surface and lower near the interface. The CO2 shielding promotes slightly higher hardness due to carbon pickup, but this comes at the cost of reduced toughness.

Wear Resistance Performance

The wear resistance of CO2-cladded layers is comparable to argon-cladded layers, with the higher hardness partially compensating for the reduced toughness. The overall wear performance is acceptable for most industrial applications.

Impact and Fracture Behavior