Microstructure and Microhardness of D227 and D237 Weld Overlay Electrode Deposits

Literature Overview

This 2009 study by Zhang Youyi, Yang Yue, and Qu Jinshan from Sichuan Engineering Vocational College and Xihua University investigates the microstructure and microhardness of overlay metals deposited using D227 and D237 welding electrodes. These two electrode types are widely used in China for the repair and hardfacing of worn components in mining, construction, and industrial equipment. D227 is a high-carbon, high-chromium cast iron type electrode designed for severe abrasion wear applications, while D237 is a high-speed steel type electrode designed for applications requiring a combination of hardness, toughness, and wear resistance. Understanding the microstructural differences between these two electrode types is essential for selecting the appropriate electrode for specific service conditions and for optimizing the welding parameters to achieve the desired performance.

Electrode Chemistry and Classification

D227 and D237 electrodes belong to different hardfacing categories and have distinct chemical compositions that result in different microstructural characteristics and mechanical properties. D227 is classified as a high-carbon, high-chromium cast iron type electrode, with a typical composition of 5 to 7 percent carbon, 15 to 20 percent chromium, and balance iron. The high carbon and chromium content promotes the formation of hard chromium carbides, primarily Cr7C3 and Cr23C6, which provide excellent abrasion resistance. D237 is classified as a high-speed steel type electrode, with a typical composition of 1 to 2 percent carbon, 5 to 8 percent tungsten, 4 to 6 percent vanadium, and 5 to 8 percent chromium. The tungsten and vanadium content promotes the formation of hard transition metal carbides, primarily WC, VC, and W2C, which provide a combination of hardness and toughness.

Microstructural Analysis

D227 Overlay Microstructure

The microstructure of D227 overlay deposits is characterized by a matrix of martensite and retained austenite with a high volume fraction of hard chromium carbides. The carbides are primarily Cr7C3 and Cr23C6, which appear as dark particles in the micrograph. The carbide distribution is influenced by the cooling rate and the welding parameters. At slower cooling rates, the carbides tend to be larger and more coarsely distributed, while at faster cooling rates, the carbides are finer and more uniformly distributed. The authors observed that the carbide size in D227 overlays ranges from 5 to 50 micrometers, with an average size of approximately 15 to 25 micrometers. The volume fraction of carbides is typically 30 to 50 percent, depending on the welding parameters and the number of overlay layers.

The martensitic matrix in D227 overlays is high-carbon and contains significant amounts of retained austenite due to the high carbon and chromium content. The retained austenite fraction is typically 10 to 30 percent, which provides some toughness to the otherwise brittle martensitic structure. The authors noted that the retained austenite fraction can be reduced by post-weld heat treatment, but this also reduces the hardness of the overlay. The optimal balance between hardness and toughness is achieved by controlling the retained austenite fraction in the range of 15 to 25 percent.

D237 Overlay Microstructure

The microstructure of D237 overlay deposits is characterized by a matrix of martensite and tempered martensite with a high volume fraction of hard transition metal carbides. The carbides are primarily WC, VC, and W2C, which appear as bright particles in the micrograph due to their high hardness and refractive index. The carbide distribution in D237 overlays is typically finer and more uniform than in D227 overlays, with carbide sizes ranging from 2 to 20 micrometers and an average size of approximately 5 to 10 micrometers. The volume fraction of carbides is typically 25 to 40 percent, which is slightly lower than in D227 overlays but provides a more uniform hardness distribution.

The matrix in D237 overlays is a tempered martensite structure due to the self-tempering effect of the tungsten and vanadium carbides during solidification. The carbides act as nucleation sites for austenite formation during cooling, and the austenite that forms in contact with the carbides undergoes transformation to martensite at lower temperatures, resulting in a finer and more tempered martensitic structure. This self-tempering effect is one of the key advantages of high-speed steel type electrodes, as it provides a combination of high hardness and adequate toughness without the need for post-weld heat treatment.

Microhardness Distribution

The microhardness of D227 and D237 overlay deposits was measured using Vickers microhardness testing with a 1 kgf load. The authors reported that D227 overlays exhibit a microhardness range of 800 to 1200 HV, with an average of approximately 1000 HV. The hardness distribution is relatively uniform across the overlay thickness, with a slight decrease near the bond line due to dilution from the base material. D237 overlays exhibit a microhardness range of 900 to 1400 HV, with an average of approximately 1150 HV. The hardness distribution in D237 overlays is also relatively uniform, but with a slightly higher peak hardness near the surface due to the faster cooling rate at the surface.

The authors also investigated the effect of the number of overlay layers on the microhardness distribution. They found that increasing the number of layers from one to three provides a more uniform hardness distribution and reduces the hardness variation across the overlay thickness. However, increasing the number of layers beyond three provides diminishing returns and can introduce interlayer defects. The authors recommend a minimum of two to three overlay layers for consistent performance, with the specific number determined by the required overlay thickness.

Performance Comparison and Application Selection

The comparison of D227 and D237 overlay deposits reveals distinct performance characteristics that make each electrode suitable for different application scenarios. D227 overlays offer excellent abrasion resistance in dry, sliding contact conditions, making them suitable for applications such as mining equipment, conveyor rollers, and earthmoving equipment. The high carbon and chromium content provides a hard, carbide-rich structure that resists abrasive wear effectively. However, D227 overlays are relatively brittle and have limited resistance to impact loading and thermal cycling.

D237 overlays offer a superior combination of hardness and toughness, making them suitable for applications that involve both abrasion and impact loading, such as crusher hammers, excavator bucket teeth, and pump impellers. The transition metal carbides provide high hardness, while the tempered martensitic matrix provides adequate toughness to resist impact and thermal shock. The authors recommend D237 for applications where the component is subjected to cyclic loading or where thermal cycling is significant, as the tempered martensitic structure is more resistant to cracking under these conditions.

Study Insights and Practical Recommendations

This study provides valuable comparative data on two commonly used hardfacing electrode types that are widely employed in industrial repair and maintenance applications. The key insight is that the selection between D227 and D237 should be based on a careful analysis of the service conditions, including the type of wear mechanism, the presence of impact loading, and the thermal environment. D227 is the preferred choice for pure abrasive wear in dry conditions, while D237 is the preferred choice for applications involving a combination of abrasion, impact, and thermal cycling. The study also highlights the importance of overlay layer number and welding parameter optimization in achieving consistent hardness and microstructure. Engineers should note that the microhardness values reported in this study are for as-deposited conditions and may vary with post-weld heat treatment, substrate material, and welding parameters. Proper welding procedure qualification and qualification testing are essential to ensure that the overlay performance meets the specific requirements of the application.