CLADDING TECHNOLOGY SHANXI CO., LTDWear Resistance of Composite Overlay Coatings Deposited by CO2 Welding with High-Carbon Chromium Iron Alloy Powder
Literature Overview
This 2011 study published in the Transactions of the China Welding Institute by Yuan Kaifeng, Yin Ke, Wang Jun, and Li Muqin from Jiamusi University and Guilin Aerospace Industry College investigates the wear resistance of composite overlay coatings produced by CO2 gas metal arc welding (GMAW) using a combination of high-carbon chromium iron alloy powder and other reinforcing materials. The research was supported by the Ministry of Education Metal Wear-Resistant Materials and Surface Technology Research Center and the Heilongjiang Provincial Department of Education Innovation Base Fund.
Core Technical Content
The study addresses the practical challenge of developing cost-effective, high-performance wear-resistant overlay coatings using commercially available welding consumables and standard CO2 welding equipment. Unlike expensive specialized processes such as plasma arc cladding or laser cladding, CO2 GMAW is widely available in industrial workshops and offers high deposition rates, making it attractive for large-scale industrial applications such as mining equipment, construction machinery, and cement mill components.
The composite overlay material combines high-carbon chromium iron alloy powder, which serves as the primary alloying and hard phase source, with additional reinforcing elements. The high carbon content promotes the formation of cementite (Fe3C) and chromium carbides (Cr7C3, Cr3C) during solidification, while the chromium content enhances both hardness and oxidation resistance. The term "spraying" in the title likely refers to a powder feeding or spraying technique used to deliver the composite powder into the weld pool, possibly through a dual-wire or wire-plus-powder feeding system.
Microstructural Analysis
The overlay coating microstructure is expected to exhibit the following features:
- Matrix: A martensitic or bainitic iron-based matrix, hardened by carbon and chromium in solid solution and by transformation hardening during cooling.
- Hard phases: Chromium carbides (Cr7C3, Cr3C) and cementite (Fe3C) precipitates distributed throughout the matrix, providing the primary wear resistance mechanism.
- Microsegregation: The rapid solidification in GMAW welding creates significant microsegregation, with chromium and carbon enriched in the interdendritic regions, promoting the formation of carbide-rich zones.
- Cracking tendency: The high carbon and chromium content creates a cracking-sensitive coating, particularly susceptible to hot cracking due to low-melting-point eutectic phases at grain boundaries.
Process Parameters and Their Effects
| Parameter | Typical Value | Influence |
|---|---|---|
| Shielding gas | CO2 (100%) or CO2 + Ar mix | CO2 provides higher hardness but more spatter and oxidation |
| Wire current | 200–350 A | Higher current increases deposition rate but may increase dilution |
| Travel speed | 150–400 mm/min | Higher speed reduces heat input and dilution |
| Powder feed rate | 50–200 g/min | Controls composite phase content in the coating |
| Arc voltage | 22–32 V | Affects arc stability and powder melting efficiency |
| Number of passes | 1–5 layers | Multiple layers increase dilution and residual stress |
Wear Resistance Mechanisms
The wear resistance of the composite overlay is governed by multiple mechanisms operating simultaneously:
- Abrasion resistance: Provided by the hard carbide phases (Cr7C3, Cr3C, Fe3C) with hardness values ranging from 1,500 to 2,800 HV, which resist material removal during sliding contact.
- Oxidation resistance: Chromium enrichment at the surface forms a protective Cr2O3 scale during high-temperature wear, reducing oxidative material loss.
- Matrix support: The hardened martensitic matrix provides the necessary support for the hard phases, preventing their pullout during abrasive contact.
- Toughness contribution: The retained austenite or tempered martensite in the matrix provides some toughness to resist crack propagation.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Low-melting-point eutectics at grain boundaries | Reduce carbon content, add sulfur/copper modifiers, control dilution |
| Cold cracking | Hydrogen pickup from CO2 shielding gas | Use low-hydrogen flux, preheat substrate, post-weld bake |
| Excessive dilution | High heat input, large substrate thickness | Reduce current, increase travel speed, use backing bar |
| Poor adhesion | Oxidized substrate surface, contamination | Thorough surface preparation, grit blasting to Sa 2.5 |
| Uneven coating thickness | Inconsistent powder feeding, operator skill | Use automated powder feeding system, standardized procedures |
Engineering Practice Implications
The use of CO2 GMAW for depositing composite wear-resistant coatings is particularly attractive for repair applications where the component has already been in service and specialized equipment is not available. The high deposition rate (typically 5 to 15 kg/h) makes it suitable for rebuilding worn surfaces on large components such as excavator buckets, mining shovels, and cement mill liners. However, the relatively coarse microstructure and higher dilution compared to PTA or laser cladding limit the achievable hardness and wear resistance.
A critical engineering consideration is the residual stress state of the multi-pass overlay. Each subsequent pass reheats the previous layer, creating a complex thermal history that can lead to cracking, particularly in high-carbon, high-chromium coatings. The use of multiple thin layers with controlled interpass temperatures is essential to manage this risk. Post-weld tempering treatment at 550 to 650 °C for 1 to 2 hours is commonly applied to reduce residual stresses and improve the coating's resistance to cracking.
Study Insights
This research demonstrates that even with relatively simple welding equipment, it is possible to produce composite overlay coatings with improved wear resistance by incorporating high-carbon chromium alloy powder. The key challenge lies in managing the trade-off between hardness (which requires high carbon and chromium) and weldability (which requires controlled dilution and minimized cracking). Engineers should recognize that the performance of such coatings is highly dependent on process parameters and that systematic optimization is required for each specific application.