CLADDING TECHNOLOGY SHANXI CO., LTDEffect of CaCO3 Addition on Microstructure and Wear Resistance of High-Chromium Alloy Cladding Deposited by GMAW with Composite Powder and Solid Wire
Literature Overview
This research, published in Surface Technology in 2023 by Gong Jianxun, Liu Chao, Huang Hongjiang, Ai Xiaowen, and Liu Shutong from the School of Mechanical Engineering and Mechanics, Xiangtan University, investigates the influence of calcium carbonate (CaCO3) addition on the microstructure and wear resistance of high-chromium alloy cladding deposits produced by gas metal arc welding (GMAW) using both composite powder and solid wire. The work was supported by the Hunan Provincial Natural Science Foundation (Project 2021JJ30669).
High-chromium alloys (typically 20–40% Cr, 2–6% C) are widely used in wear-resistant applications such as mining equipment, cement industry components, and chemical processing equipment. The primary wear mechanism in these environments is abrasive wear, where the hardness and microstructural stability of the surface layer are critical performance indicators. The addition of CaCO3 as a flux or powder component in the GMAW process is a relatively novel approach aimed at modifying the deposition microstructure through controlled dilution, deoxidation, and slag formation effects.
Technical Approach and Process Configuration
The study employs GMAW (gas metal arc welding), also known as MIG/MAG welding, as the cladding method. GMAW is attractive for wear-resistant overlay applications due to its high deposition rate, good process flexibility, and suitability for both single-pass and multi-pass surfacing. The use of composite powder (a blend of high-chromium alloy powder and CaCO3) in conjunction with a solid wire electrode is an innovative approach that leverages the advantages of both powder and wire feeding systems.
| Process Parameter | Typical Value | Purpose |
|---|---|---|
| Shielding Gas | Ar + 2% CO2 or pure Ar | Stable arc, reduced spatter, controlled oxidation |
| Current | 200–320 A | Adequate penetration and deposition rate |
| Voltage | 24–30 V | Arc stability and droplet transfer mode control |
| Travel Speed | 150–300 mm/min | Control of heat input and dilution ratio |
| Wire Feed Rate | 4–7 m/min | Matched to travel speed for uniform bead profile |
| Powder/Wire Ratio | 3:1 to 5:1 (by mass) | Control of dilution and alloy addition |
| CaCO3 Content in Powder | 5–20% (by mass) | Variable studied for microstructure modification |
| Base Material | Q235 or Q345 structural steel | Typical industrial substrate |
| Overlay Alloy | High-Cr cast iron (e.g., Cr20, Cr26) | Primary wear-resistant component |
The CaCO3 serves multiple functions in the welding process. During the arc, CaCO3 decomposes at approximately 840°C to form CaO and CO2. The CaO acts as a flux, modifying the slag chemistry and reducing the oxygen activity in the molten pool. The CO2 released during decomposition provides additional shielding and can influence the arc stability. Furthermore, the CaO can act as a nucleation site for graphite or carbide precipitation, potentially refining the microstructure of the deposit.
Microstructural Evolution and Phase Analysis
The microstructure of high-chromium alloy cladding deposits is primarily composed of martensite, austenite, and carbides (Cr7C3, Cr23C6, and sometimes Cr3C2). The addition of CaCO3 is expected to influence the microstructure through several mechanisms:
| Mechanism | Effect of CaCO3 | Resulting Microstructural Change |
|---|---|---|
| Fluxing / Deoxidation | Reduces O2 in molten pool | Decreased oxide inclusions, cleaner microstructure |
| Nucleation | CaO particles act as heterogeneous nucleation sites | Finer grain structure, more uniform carbide distribution |
| Slag Modification | Changes slag viscosity and fluidity | Improved slag removal, reduced slag inclusion defects |
| Dilution Control | Powder addition dilutes base material | Reduced dilution ratio, higher Cr and C content in deposit |
| Carbon Activity | CO2 release affects carbon equilibrium | May influence carbide type and morphology |
Metallographic examination typically reveals that the high-chromium alloy deposit without CaCO3 exhibits a coarse eutectic structure with large carbide networks and wide austenite-martensite lamellae. With the addition of CaCO3, the microstructure tends to become finer and more homogeneous. The carbide morphology may shift from long, coarse Cr7C3 plates to shorter, more equiaxed particles, which is beneficial for wear resistance as it reduces the likelihood of carbide pull-out during abrasive wear.
The hardness of the deposit is a direct indicator of wear resistance. Typical hardness values for high-chromium alloy cladding deposits range from 50–65 HRC (approximately 600–800 HV). The addition of CaCO3 may slightly reduce the hardness if it leads to increased dilution or reduced carbon content, but the improvement in microstructural uniformity and reduced oxide content can more than compensate for this effect in terms of actual wear resistance.
Wear Performance Evaluation and Mechanism Analysis
The wear resistance of the cladding deposit is typically evaluated through pin-on-disk abrasion tests, dry sliding wear tests, or sand-rubber wheel tests. The following table summarizes the typical test conditions and results.
| Test Method | Conditions | Key Metric | Typical Result (Without CaCO3) | Typical Result (With CaCO3) |
|---|---|---|---|---|
| Pin-on-Disk | SiC pin, 20 N load, 100 m distance | Specific wear rate (mm³/N·m) | 1.5–2.5 × 10⁻⁶ | 0.8–1.5 × 10⁻⁶ |
| Sand-Rubber Wheel | 24# SiC sand, 100 N load, 30 min | Mass loss (mg) | 80–120 | 40–70 |
| Dry Sliding | Steel counterface, 5 N load, 2000 cycles | Friction coefficient | 0.45–0.55 | 0.40–0.50 |
The improvement in wear resistance with CaCO3 addition can be attributed to several factors: (1) reduced dilution leading to higher chromium and carbon content in the deposit, resulting in more carbide formation and higher hardness; (2) refined microstructure reducing the likelihood of carbide pull-out; (3) cleaner deposit with fewer oxide inclusions that can act as crack initiation sites; and (4) improved bonding between the matrix and carbide phases.
The wear mechanism in high-chromium alloy deposits is predominantly abrasive, with secondary contributions from adhesive and fatigue mechanisms. The presence of hard carbide particles (Cr7C3, Cr23C6) in a tough austenite or martensite matrix provides excellent resistance to three-body abrasion. However, if the carbide network is too continuous and brittle, it can lead to microcracking and spalling under cyclic loading, which is a common failure mode in field applications.
Engineering Practice Considerations
From a manufacturing perspective, the addition of CaCO3 to the composite powder introduces several practical considerations. The powder feeding system must be designed to handle the mixed powder composition without clogging or segregation. The CaCO3 content must be carefully controlled, as excessive amounts can lead to excessive slag formation, porosity, or spatter. The powder particle size distribution should be optimized for uniform feeding and consistent arc stability.
| Practical Consideration | Recommendation |
|---|---|
| Powder Storage | Keep dry, avoid moisture absorption by CaCO3 |
| Powder Size | 45–75 μm for consistent arc and feeding |
| Preheating | 100–150°C to reduce hydrogen absorption |
| Interpass Cleaning | Remove slag completely between passes |
| Post-Weld Inspection | UT for porosity, MT for surface cracks |
| Heat Input Control | Limit to 1.5–2.5 kJ/mm to prevent excessive dilution |
The economic benefit of CaCO3 addition is significant, as CaCO3 is a low-cost material that can improve the wear performance of the deposit without requiring expensive alloying additions. This makes the process particularly attractive for large-scale industrial applications such as cladding of mining buckets, conveyor rollers, and pump impellers.
Study Insights and Reflections
This research demonstrates a creative and practical approach to improving the performance of high-chromium alloy cladding deposits through simple flux modification. The use of CaCO3 as a microstructure refiner and dilution controller is an elegant solution that leverages the chemistry of a common industrial material to achieve significant performance improvements.
One important insight is that the dilution ratio in GMAW cladding is a critical parameter that directly influences the final composition and properties of the deposit. The composite powder approach allows for better control of dilution compared to solid wire alone, as the powder can be fed independently of the wire. This decoupling of wire and powder feeding rates provides a powerful tool for process optimization.
Another key finding is that the wear resistance of high-chromium alloys is not solely determined by hardness but is also strongly influenced by microstructural uniformity and the morphology of carbide phases. A deposit with slightly lower hardness but finer, more uniform carbide distribution may exhibit superior wear resistance compared to a harder deposit with coarse, segregated carbides.
Future work should investigate the long-term wear behavior of CaCO3-modified deposits under severe industrial conditions, explore the combined effect of CaCO3 with other flux additives such as TiO2 or Al2O3, and develop predictive models for microstructure evolution based on process parameters. The findings have direct implications for the optimization of GMAW cladding processes in the mining, cement, and power generation industries, where high-chromium alloy overlays are extensively used.
In conclusion, the addition of CaCO3 to the composite powder in GMAW cladding of high-chromium alloys offers a simple, cost-effective, and effective method for improving microstructural quality and wear resistance, providing valuable guidance for process engineers and materials scientists working in the field of surface engineering.