CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and High-Temperature Properties of High-Carbon High-Chromium Plasma Arc Cladding Layer
Literature Overview
This 2018 study by Yang Huiqin and Qu Shengzhi from Shanghai Boiler Works Co., Ltd. and Shanghai Jiao Tong University investigates the microstructure and high-temperature mechanical properties of a high-carbon, high-chromium alloy cladding layer produced by plasma arc welding. The research addresses the needs of the boiler and pressure vessel industry, where components such as superheater tubes, reheater tubes, and economizer tubes are subjected to high-temperature oxidation, corrosion, and abrasive wear from flue gas and ash particles.
Technical Background and Application Context
In coal-fired power boilers, the superheater and reheater tubes operate at temperatures ranging from 500°C to 650°C, depending on the steam conditions. These tubes are exposed to a combination of:
- High-temperature oxidation in the presence of oxygen and sulfur dioxide
- Ash deposition and abrasion from circulating fluidized bed particles
- Corrosive attack from vanadium and sodium compounds in the ash
- Thermal cycling during start-up and shutdown operations
Conventional austenitic stainless steel overlays such as 310 or 310S provide adequate corrosion resistance but may lack sufficient hardness and wear resistance for applications involving abrasive ash. The high-carbon, high-chromium alloy studied in this work is designed to provide a balance of oxidation resistance, wear resistance, and high-temperature strength.
Microstructure Characterization
The cladding layer is produced by plasma arc welding using a wire electrode with the following nominal composition:
| Element | Content (wt%) |
|---|---|
| C | 1.5–2.0 |
| Cr | 22–26 |
| Mo | 3.0–4.0 |
| Fe | Balance |
The microstructure of the as-deposited cladding layer consists of:
- Martensitic matrix: The high carbon and chromium content promote the formation of a martensitic structure upon rapid solidification, providing high hardness and strength.
- Chromium carbides: M₇C₃ and M₂₃C₆ type carbides are distributed along the grain boundaries and within the martensitic laths, providing wear resistance and contributing to precipitation hardening.
- Retained austenite: A small amount of retained austenite (5–10%) is present, which can transform to martensite during service at elevated temperatures, providing additional strengthening.
After post-weld heat treatment at 650°C for 2 hours, the microstructure evolves as follows:
- The martensite partially decomposes to tempered martensite with fine carbide precipitates
- The retained austenite is partially stabilized by the high carbon and chromium content
- The chromium carbides coarsen slightly but remain uniformly distributed
High-Temperature Mechanical Properties
The mechanical properties of the cladding layer are evaluated at room temperature and at elevated temperatures through hardness testing, tensile testing, and oxidation testing:
| Property | As-Deposited | After Heat Treatment |
|---|---|---|
| Hardness at 25°C (HV) | 550–600 | 450–500 |
| Hardness at 600°C (HV) | 350–400 | 380–420 |
| Yield strength at 25°C (MPa) | 900–1000 | 750–850 |
| Yield strength at 600°C (MPa) | 450–500 | 500–550 |
| Oxidation rate at 800°C (mg/cm²·h) | 0.5–0.8 | 0.3–0.5 |
| Thermal cycling resistance (cycles to failure) | 200–300 | 500–800 |
The results show that the heat-treated cladding layer exhibits superior high-temperature hardness and oxidation resistance compared to the as-deposited condition. The improvement in thermal cycling resistance is attributed to the tempering of the martensite, which reduces the residual stress and increases the toughness of the overlay.
Process Parameters and Quality Considerations
The plasma arc welding parameters used in the study are as follows:
| Parameter | Value |
|---|---|
| Plasma current | 180–220 A |
| Arc voltage | 20–25 V |
| Travel speed | 250–400 mm/min |
| Wire feed speed | 2.0–3.0 m/min |
| Shielding gas | Argon + 5% CO₂ |
| Layer thickness per pass | 0.8–1.2 mm |
| Total overlay thickness | 4–6 mm |
| Post-weld heat treatment | 650°C for 2 hours |
The quality of the cladding layer is verified through the following inspections:
- Visual inspection: To detect surface defects such as undercut, porosity, and lack of fusion
- Penetrant testing (PT): To detect surface-breaking cracks
- Ultrasonic testing (UT): To detect subsurface defects and measure overlay thickness
- Hardness testing: To verify the hardness profile across the overlay thickness
- Macrograph examination: To assess the soundness of the weld interface and the uniformity of the overlay
Summary
This study demonstrates that a high-carbon, high-chromium plasma arc cladding layer, when properly heat-treated, provides excellent high-temperature hardness, oxidation resistance, and thermal cycling resistance suitable for boiler tube applications. The combination of martensitic matrix, chromium carbides, and retained austenite provides a synergistic strengthening mechanism that maintains performance at elevated temperatures. This work offers valuable guidance for the selection and optimization of overlay materials for high-temperature pressure vessel components in the power generation industry.