CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Overlay Welding Electrodes
Literature Overview
This 1997 study by Hong Yongchang and Feng Anhua from East China Institute of Metallurgy (now University of Science and Technology, Anhui), in collaboration with Qing Hua and Huang Ming from Masteel Jiangdong Welding Electrode Factory, represents an important early investigation into the microstructure and mechanical properties of overlay welding electrodes. The study bridges the gap between academic metallurgical research and industrial electrode manufacturing, examining how the electrode coating composition and welding process parameters influence the quality of the deposited cladding layer. Published in the context of the metal mining industry, this work addresses the practical challenges of producing reliable overlay welding consumables for harsh industrial environments.
Core Technical Findings
The researchers investigated multiple electrode coating formulations, examining the effects of coating composition, coating thickness, and welding parameters on the microstructure and mechanical properties of the deposited metal. The following table summarizes the key findings:
| Coating Type | Coating Thickness (mm) | Dilution Rate (%) | Hardness (HV) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|---|---|
| Rutile (TiO2-based) | 2.5–3.0 | 20–25 | 250–300 | 550–620 | 18–22 |
| Cellulosic (Na-cellulose) | 2.0–2.5 | 15–20 | 300–350 | 600–680 | 12–18 |
| Basic (CaO-based) | 3.0–3.5 | 12–18 | 350–420 | 650–750 | 10–15 |
| Composite (mixed) | 2.5–3.0 | 15–22 | 300–380 | 600–700 | 14–20 |
The study found that the electrode coating composition has a profound influence on the weld metal properties. Rutile coatings, while providing excellent arc stability and easy slag removal, tend to produce weld metal with lower hardness and strength. Basic coatings, on the other hand, provide higher alloy retention and produce harder, stronger weld metal, but require more careful welding technique and thorough slag removal.
Microstructural Analysis
The microstructural examination of the deposited metal revealed significant differences between coating types:
- Rutile coating electrodes produced a predominantly ferrite-pearlite microstructure with some retained austenite. The grain size was relatively coarse (ASTM 4–6), and the carbide distribution was relatively uniform.
- Cellulosic coating electrodes produced a martensitic microstructure with some tempered martensite. The hydrogen content in the weld metal was elevated (15–25 mL/100g), which increased the risk of hydrogen-induced cracking.
- Basic coating electrodes produced a tempered martensite microstructure with fine carbide precipitates. The alloying elements (Cr, Mo, Ni) were retained at higher concentrations due to the deoxidizing and alloying properties of the basic flux.
- Composite coating electrodes produced a mixed microstructure that balanced the properties of rutile and basic coatings, offering a compromise between weldability and mechanical performance.
The dilution rate was found to be the primary factor governing the final hardness and strength of the deposited metal. Higher dilution rates (25–30%) resulted in significant dilution of the base metal carbon and alloying elements into the weld pool, reducing the hardness and strength of the deposited layer.
Electrode Design and Manufacturing Considerations
Based on the study's findings, the following electrode design guidelines are recommended:
| Design Parameter | Rutile Electrode | Basic Electrode | Composite Electrode |
|---|---|---|---|
| Optimal coating thickness (mm) | 2.5–3.0 | 3.0–3.5 | 2.5–3.0 |
| Coating density (g/cm³) | 2.8–3.2 | 2.5–2.9 | 2.7–3.0 |
| Coating adhesion strength (N/cm²) | >150 | >180 | >160 |
| Current range (A) | 80–160 | 100–200 | 90–180 |
| Welding position | All positions | Flat/horizontal | All positions |
| Arc voltage (V) | 18–24 | 22–28 | 20–26 |
The study also highlighted the importance of coating adhesion strength in preventing coating detachment during welding, which can lead to arc blow, porosity, and contamination of the weld metal. An adhesion strength below 150 N/cm² was found to be unacceptable for reliable production welding.
Engineering Practice Implications
For the fabrication of bimetal products and pressure vessels using overlay welding electrodes, the selection of the appropriate electrode type is critical. The following decision framework is proposed:
- For low-stress applications where weldability is paramount (e.g., repair welding of carbon steel equipment), rutile coating electrodes are preferred.
- For high-stress applications requiring high hardness and strength (e.g., wear-resistant overlays on mining equipment), basic coating electrodes are recommended.
- For applications requiring a balance of weldability and mechanical properties (e.g., general-purpose overlay welding), composite coating electrodes provide the best compromise.
- For all applications, the dilution rate must be monitored and controlled to ensure the deposited metal meets specification requirements.
Study Insights and Implications
This study, published in the late 1990s, provides a foundational understanding of overlay welding electrode design and performance. The collaboration between academic researchers and industrial manufacturers exemplifies the productive model of applied research. The findings on coating composition effects on dilution rate and weld metal properties remain relevant to current electrode manufacturing and selection practices. For modern cladding operations, the study's emphasis on dilution rate control and coating adhesion strength provides practical guidance that can be directly applied to quality control procedures. The work also highlights the importance of considering the entire welding consumable system — wire composition, coating formulation, and coating application — as an integrated design variable rather than treating each component independently.