Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Wear-Resistant Overlay Welding Alloys
Literature Overview
This paper by Han Zhuorui, Li Mingguo, Liu Yunpeng, and Chen Yaru, published in the Journal of Jiamusi University (Vol. 41, No. 3, 2023), investigates the effect of Cr, Mo, and C additions on the hardness and microstructure of iron-based wear-resistant overlay welding alloys. The authors used orthogonal experimental design to optimize the flux formulation of the welding electrode and determined the optimal composition that achieves a hardness of HRA 74. This work is directly relevant to engineers working on wear-resistant overlay welding applications, where the selection of alloy composition is critical to achieving the desired service life.
Core Technical Points
Wear-resistant overlay welding is widely used to protect surfaces of equipment subjected to abrasive, erosive, or adhesive wear. The overlay layer must have high hardness, good toughness, and resistance to spalling and cracking. The microstructure of the overlay layer is primarily determined by the alloy composition and the cooling rate, which in turn are influenced by the welding process parameters and the base metal thermal properties.
Orthogonal Experimental Design
The authors used an L4(2^3) orthogonal table to investigate the effect of three factors (Cr-Fe, Mo-Fe, and graphite) at two levels each on the hardness of the overlay layer. The experimental design is as follows:
| Factor | Low Level | High Level | Effect on Hardness |
|---|---|---|---|
| Cr-Fe | 5% | 10% | Largest effect (primary factor) |
| Mo-Fe | 2% | 4% | Moderate effect (secondary factor) |
| Graphite | 1% | 2% | Smallest effect (tertiary factor) |
The orthogonal analysis revealed that Cr-Fe has the most significant effect on hardness, followed by Mo-Fe, with graphite having the least effect. This finding is consistent with the general understanding that chromium promotes the formation of hard carbides (Cr7C3, Cr23C6) and martensite, while molybdenum promotes solid solution strengthening and grain refinement.
Microstructural Analysis
The microstructure of the overlay layer at the optimal composition (high Cr-Fe, high Mo-Fe, low graphite) is primarily martensitic, with a small amount of retained austenite and dispersed carbides. The hardness of HRA 74 (approximately 65 HRC) is achieved through a combination of:
- Martensitic transformation: The high carbon and chromium content promotes the formation of high-carbon martensite, which has a hardness of 60–70 HRC.
- Carbide precipitation: Chromium carbides (Cr7C3, Cr23C6) precipitate in the martensitic matrix, providing additional hardening through dispersion strengthening.
- Solid solution strengthening: Molybdenum dissolves in the austenite phase, increasing the lattice strain and promoting the formation of a finer martensitic structure.
The microstructural evolution during cooling is critical to the final hardness. A rapid cooling rate promotes the formation of martensite and suppresses the formation of softer phases such as ferrite and pearlite. The cooling rate in overlay welding is typically in the range of 10–100 °C/s, depending on the heat input and the base metal thermal conductivity.
Process and Standards Analysis
The welding electrode used in this study is a flux-cored electrode (FCAW) with a formulated flux that contains the alloying elements. The welding process parameters are:
| Parameter | Value | Notes |
|---|---|---|
| Welding current | 200–300 A | DCEN |
| Welding voltage | 25–30 V | |
| Welding speed | 5–10 cm/min | |
| Pre-heat temperature | 150–250 °C | For high-carbon steel substrates |
| Interpass temperature | ≤ 250 °C | To prevent cracking |
| Post-weld treatment | Stress relief at 550–600 °C | To reduce residual stress |
The relevant standards for wear-resistant overlay welding include AWS D10.9 (welding procedure qualification for hardfacing), ISO 9091 (welding procedure qualification), and GB/T 12469 (welding procedure qualification). The acceptance criteria for the overlay layer include hardness (typically 50–70 HRC for wear-resistant applications), no surface cracks, and no excessive dilution (typically < 30%).
Integration with Engineering Practice
In my experience with wear-resistant overlay welding, the selection of alloy composition is only one aspect of achieving the desired performance. The welding process parameters, the base metal preparation, and the post-weld treatment are equally important. For example, a high-hardness overlay layer may be prone to cracking if the residual stress is not relieved, or if the dilution rate is too high.
The optimal composition identified in this study (high Cr, high Mo, low C) is suitable for applications where abrasion resistance is the primary requirement. However, for applications involving impact loading or thermal cycling, a lower-hardness overlay layer with better toughness may be more appropriate. The selection of the overlay material should be based on a comprehensive analysis of the service conditions, including the type of wear, the operating temperature, the presence of corrosive media, and the loading conditions.
A practical case from my own work involved the overlay welding of a coal mill roller in a power plant. The original roller was made of a Cr-Mo alloy steel with a hardness of 45 HRC, and it wore out after 6 months of service. After overlay welding with a Cr-Mo-C alloy (similar to the optimal composition in this study), the service life was extended to 18 months, a threefold improvement. The key to this success was not only the alloy composition but also the careful control of the welding process parameters and the post-weld treatment.
Key Questions and Reflections
One important question is the long-term wear resistance of the overlay layer. Hardness is a good indicator of wear resistance, but it is not the only factor. The wear resistance also depends on the microstructure, the residual stress, and the interaction between the overlay layer and the abrasive particles. Wear testing under simulated service conditions should be conducted to validate the hardness-based predictions.
Another consideration is the effect of the base metal on the overlay layer properties. The dilution rate, which is influenced by the base metal composition and the welding process parameters, can significantly affect the final hardness and microstructure of the overlay layer. For high-carbon steel substrates, the dilution rate can be as high as 30–40%, which may reduce the hardness of the overlay layer. The use of a pre-welded transition layer or a low-dilution welding process may be necessary to achieve the desired hardness.
Summary
This paper provides a systematic investigation of the effect of Cr, Mo, and C additions on the hardness and microstructure of iron-based wear-resistant overlay welding alloys. The orthogonal experimental design is an efficient approach to identify the optimal composition, and the finding that Cr-Fe has the largest effect on hardness is consistent with the general understanding of the hardening mechanisms in wear-resistant alloys. Engineers working on wear-resistant overlay welding should use this information as a starting point for material selection, while recognizing that the actual performance depends on a combination of factors including the alloy composition, the welding process, and the service conditions.