Zhuojin Pipe Fitting Co., LtdMicrostructure and High-Temperature Properties of High-Carbon High-Chromium Plasma Arc Overlay Layer
Literature Overview
This 2018 paper by Yang Huiqin and Qu Shengzhi from Shanghai Boiler Works Co., Ltd. and Shanghai Jiao Tong University, published in Boiler Technology (锅炉技术), presents a systematic study of the microstructure, composition, hardness, and high-temperature wear properties of a high-carbon, high-chromium plasma powder overlay welding layer. The research employs optical microscopy, scanning electron microscopy (SEM), and friction testing to characterize the overlay layer under various conditions, including elevated temperatures up to 800°C. This work is directly relevant to the boiler and pressure vessel industry, where high-temperature wear resistance is a critical requirement for components such as burner tubes, air preheater tubes, and slag chute linings.
Materials and Process Parameters
The study investigates a high-C, high-Cr alloy system deposited by plasma arc powder welding. The material design philosophy targets maximum hardness and wear resistance at elevated temperatures through the formation of a high volume fraction of hard carbide phases in a ferritic matrix.
| Parameter | Specification |
|---|---|
| Carbon content | 3.0–4.5 wt% |
| Chromium content | 25–35 wt% |
| Molybdenum content | 3–6 wt% |
| Base substrate | Low-alloy steel or carbon steel |
| Process | Plasma arc powder welding |
| Layer thickness | 1.5–3.0 mm |
| Number of passes | 2–4 |
Microstructural Characterization
The microstructural analysis reveals a complex phase assemblage:
- Ferritic matrix: The base phase providing toughness and structural continuity.
- M23C6 carbides: Large, blocky carbides that form preferentially at grain boundaries and provide moderate wear resistance.
- M12C carbides: Medium-sized carbides distributed within grains, contributing to hardness.
- MC carbides: Fine, spherical carbides that provide the highest hardness and thermal stability.
The relative proportions of these carbide types depend on the cooling rate during the welding process and the post-weld thermal history. The high carbon and chromium contents ensure that the carbon activity is sufficient to form all three carbide types, creating a multi-scale reinforcement architecture.
High-Temperature Hardness Behavior
The hardness measurements as a function of temperature reveal important trends:
| Temperature | Hardness (HV3) | Behavior |
|---|---|---|
| Room temperature | >800 HV3 | Maximum hardness |
| 400°C | >700 HV3 | Moderate decrease |
| 600°C | >600 HV3 | Significant decrease begins |
| 700°C | >500 HV3 | Continued decrease |
| 800°C | >355 HV3 | Still exceeds 355 HV3 |
The hardness decrease between 600°C and 800°C is attributed to:
- Softening of the ferritic matrix due to recovery and recrystallization
- Dissolution of less stable carbide phases (M23C6 and M12C)
- Coarsening of remaining carbides
However, the fact that hardness remains above 355 HV3 even at 800°C is significant, as this exceeds the hardness of most steels at these temperatures. The MC carbides, with their high thermal stability, are the primary contributors to the retained hardness at elevated temperatures.
Friction and Wear Behavior at High Temperature
The friction coefficient measurements at 700°C and 800°C show a characteristic behavior:
- Running-in period: The first 10–20 minutes exhibit fluctuating friction coefficients as the surface conforms and a stable tribolayer develops.
- Steady-state friction: After the running-in period, the friction coefficient stabilizes at approximately 0.6.
- Aging effect: Pre-aging at 700°C before friction testing shortens the running-in period, suggesting that the microstructure reaches a more stable configuration through controlled thermal exposure.
The friction coefficient of 0.6 at high temperature is acceptable for most industrial applications, and the wear mechanism transitions from abrasive wear at lower temperatures to adhesive-abrasive mixed wear at higher temperatures. The hard carbide particles resist penetration by counterface asperities, while the ferritic matrix provides the necessary ductility to prevent catastrophic fracture.
Engineering Application Context
In boiler applications, the high-C high-Cr overlay layer is particularly valuable for:
- Burner throat linings exposed to high-temperature flue gas and refractory particles
- Air preheater tubes experiencing erosion from ash-laden gas streams
- Slag chute linings subject to both abrasion and thermal cycling
- Furnace wall panels in areas of intense radiant heat
The ability to maintain hardness above 355 HV3 at 800°C makes this overlay technology suitable for applications where conventional hardfacing alloys would soften excessively.
Study Reflection
This paper provides valuable quantitative data on the high-temperature performance of high-C high-Cr overlay welding layers, filling an important gap in the engineering literature. The systematic approach — combining microstructural analysis with mechanical property testing at relevant service temperatures — provides a comprehensive understanding of the material behavior. The finding that aging at 700°C shortens the running-in period is particularly interesting from a practical standpoint, as it suggests that a controlled post-weld heat treatment could improve field performance. For boiler engineers, this work demonstrates that plasma arc overlay welding is a viable technology for extending the service life of high-temperature components, and the detailed property data provides a basis for rational material selection and design. The multi-phase carbide architecture, with its combination of M23C6, M12C, and MC phases, represents an elegant materials design solution that leverages the different properties of each carbide type to achieve a balanced combination of hardness, toughness, and thermal stability.