CLADDING TECHNOLOGY SHANXI CO., LTDPerformance Comparison of D212 Electrode Overlay Repair on Different Materials of Mining Picks
Literature Overview
This 2016 paper by Zhang Xiangyang and Zhai Xiwei from the School of Materials Science and Engineering, Inner Mongolia University of Technology, investigates the performance of D212 electrode overlay repair on mining picks made of different materials. Mining picks are cutting tools used in coal mining operations, where they are subjected to severe abrasive and impact wear. The overlay repair of worn picks using D212 electrode is a common practice in the mining industry to extend the service life of these tools and reduce replacement costs. The study compares the repair performance on picks made of various materials, including carbon steel, low-alloy steel, and alloy steel, providing guidance for the selection of appropriate base materials for overlay repair.
Core Technical Content
The authors investigated the microstructure, hardness, and wear resistance of D212 overlay layers applied to mining picks made of different base materials. D212 is a high-carbon, high-chromium cast iron electrode that produces overlay layers with a hard, wear-resistant microstructure containing primary chromium carbides and martensite. The overlay layers were applied using shielded metal arc welding (SMAW) with typical parameters, and the resulting overlay layers were characterized using metallographic examination, hardness testing, and wear testing.
The study found that the base material composition and properties significantly influence the quality and performance of the D212 overlay layer. The chemical composition of the base material affects the dilution of the overlay layer, which in turn affects the microstructure and hardness of the overlay layer. Base materials with higher carbon and alloy content tend to produce overlay layers with higher hardness and wear resistance due to reduced dilution, but may also be more susceptible to cracking in the transition zone.
Base Material Comparison
| Base Material | Composition | Hardness (HV) | D212 Overlay Hardness (HV) | Transition Zone Cracking |
|---|---|---|---|---|
| Q235 Carbon Steel | C≤0.2, Mn≤1.4 | 120-180 | 650-750 | Low risk |
| 45 Steel | C≈0.45, Mn≈0.6 | 200-280 | 700-800 | Moderate risk |
| 40Cr Alloy Steel | C≈0.4, Cr≈1.0 | 250-320 | 750-850 | Moderate risk |
| 50Mn Steel | C≈0.5, Mn≈1.2 | 250-320 | 720-820 | Low to Moderate risk |
The microstructural analysis revealed that the D212 overlay layer consists of a hard martensitic matrix with primary chromium carbides (Cr7C3 and Cr23C6) dispersed throughout. The size and distribution of the carbides depend on the cooling rate, which is influenced by the base material properties and the welding parameters. The overlay layers produced on alloy steel base materials exhibited finer carbides and higher hardness compared to those produced on carbon steel base materials, due to the reduced thermal conductivity and slower cooling rate of the alloy steel.
Wear Performance Evaluation
The wear performance of the D212 overlay layers was evaluated using a pin-on-disk wear tester and field trials in mining operations. The laboratory wear testing provided comparative data on the wear resistance of overlay layers applied to different base materials, while the field trials provided practical validation of the laboratory findings under real operating conditions.
The laboratory wear testing showed that the overlay layers applied to alloy steel base materials exhibited slightly higher wear resistance compared to those applied to carbon steel base materials. This difference is attributed to the higher hardness and finer microstructure of the overlay layers on alloy steel base materials. However, the difference in wear resistance was not significant, and all overlay layers demonstrated excellent wear resistance compared to the base materials.
The field trials confirmed the laboratory findings, with the overlay-repaired picks exhibiting significantly extended service life compared to the uncoated picks. The average service life of the overlay-repaired picks was 3-5 times that of the uncoated picks, depending on the operating conditions and the base material. The overlay layers showed good resistance to abrasive wear and impact loading, with minimal spalling or delamination observed during service.
Field Performance Data
| Base Material | Uncoated Pick Life (hours) | Overlay-Repaired Pick Life (hours) | Life Extension Factor |
|---|---|---|---|
| Q235 Carbon Steel | 20-30 | 80-120 | 3-4 |
| 45 Steel | 25-35 | 100-140 | 3-4 |
| 40Cr Alloy Steel | 30-40 | 120-160 | 3-4 |
| 50Mn Steel | 25-35 | 100-140 | 3-4 |
Process Optimization and Quality Control
The study also investigated the effects of welding parameters on the quality and performance of the D212 overlay layer. The welding current, voltage, and travel speed were systematically varied to determine the optimal process window for producing high-quality overlay layers.
The welding current was found to be the most influential parameter, with higher currents producing thicker overlay layers with higher dilution and lower hardness. The optimal welding current range was identified as 80-120 A for a 3.2 mm diameter D212 electrode, which produced overlay layers with a thickness of 2-3 mm and a hardness of 700-800 HV. The travel speed was also critical, with higher speeds producing thinner overlay layers with lower dilution and higher hardness. The optimal travel speed range was 50-80 mm/min, which produced overlay layers with good fusion and consistent hardness.
The interpass temperature was found to have a significant effect on the microstructure and hardness of multi-pass overlay welds. Excessive interpass temperatures above 200 °C promoted grain growth and coarsening of carbide phases, reducing the hardness and wear resistance of the overlay layer. Maintaining the interpass temperature below 150 °C was recommended to preserve the fine microstructure and high hardness of the overlay material.
Optimal Welding Parameters
| Parameter | Recommended Value | Effect on Overlay Layer |
|---|---|---|
| Welding Current | 80-120 A | Controls dilution and thickness |
| Travel Speed | 50-80 mm/min | Controls bead width and penetration |
| Interpass Temperature | Below 150 °C | Preserves fine microstructure |
| Electrode Diameter | 3.2 mm | Standard size for D212 |
| Electrode Drying | 300-350 °C, 1-2 h | Reduces hydrogen content |
Engineering Practice Implications
The findings of this study have direct implications for the maintenance and refurbishment of mining picks in the coal mining industry. The selection of the appropriate base material for overlay repair is critical for achieving optimal performance and service life. Alloy steel base materials, such as 40Cr and 50Mn, are preferred for overlay repair due to their higher hardness and better wear resistance, but carbon steel base materials, such as Q235, can also be successfully repaired with D212 overlay layers.
The study also emphasizes the importance of proper electrode drying and storage to prevent moisture absorption, which can lead to porosity and hydrogen cracking in the overlay layer. The D212 electrode should be dried at 300-350 °C for 1-2 hours before use and stored in a dry environment to maintain low moisture content. The welding operator should follow good practices, including proper electrode handling, consistent travel speed, and avoidance of arc wandering, to ensure the quality of the overlay layer.
Study Reflections and Outlook
This study provides valuable guidance for the selection of base materials and optimization of welding parameters for D212 overlay repair of mining picks. The systematic investigation of the effects of base material composition and welding parameters on the overlay layer properties offers practical insights for engineers and technicians involved in the maintenance of mining equipment. The field trial data confirms the effectiveness of D212 overlay repair in extending the service life of mining picks, making it a cost-effective solution for the mining industry.
Future work should focus on the development of advanced overlay materials with enhanced wear resistance and impact toughness, such as ceramic-reinforced composites and functionally graded materials. The integration of non-destructive testing techniques, such as ultrasonic testing and magnetic particle testing, into the repair process would provide additional quality assurance and enable the detection of subsurface defects that may compromise the integrity of the overlay layer. The application of statistical methods and data analysis algorithms to analyze field performance data would facilitate the optimization of repair procedures and the prediction of service life under different operating conditions.