CLADDING TECHNOLOGY SHANXI CO., LTDEffect of High-Chromium Alloy Open-Arc Cladding Parameters on Microstructure and Hardness
Literature Overview
This 2021 study by Zhang Xiaohua, Tan Xiaobo, Zuo Zhijian, and Hong Min, affiliated with Aero Engine Corporation of China and the National Key Discipline Laboratory of Light Alloy Processing Science and Technology at Nanchang Hangkong University, investigates the relationship between open-arc (flux-free) cladding process parameters and the resulting microstructure and hardness of high-chromium alloy overlay layers. Published in Hot Working Technology, the research addresses a critical need in aerospace component manufacturing: extending the service life of turbine engine components through controlled surface hardening via weld overlay.
Core Technical Content
Research Objectives and Materials
The study focuses on high-chromium alloy cladding materials (Cr content typically 20–30 wt%) deposited onto steel substrates using open-arc submerged arc welding without flux, commonly referred to as open-arc SAW or flux-free SAW. The high-chromium composition is selected for its exceptional resistance to oxidation and wear at elevated temperatures, making it suitable for hot-section components in aero engines.
Process Parameters Investigated
The researchers systematically varied the following parameters to determine their influence on the overlay microstructure and hardness:
| Parameter | Levels Tested | Rationale |
|---|---|---|
| Welding current | 200, 250, 300, 350 A | Heat input control |
| Travel speed | 200, 300, 400, 500 mm/min | Cooling rate modulation |
| Wire feed rate | 6, 8, 10, 12 m/min | Deposition rate and dilution |
| Wire diameter | φ1.6 mm, φ2.0 mm | Thermal mass and arc stability |
| Shielding gas flow | 8, 10, 12 L/min | Oxidation prevention |
Microstructural Analysis
The study employs metallographic examination, X-ray diffraction (XRD), and Vickers microhardness testing to characterize the overlay layers. Key findings include:
- Low current, high travel speed (low heat input): Produces fine dendritic microstructure with high volume fraction of primary carbides (M₇C₃ and M₂₃C₆), yielding hardness values of 750–820 HV₀.₂ but increased susceptibility to microcracking due to thermal stresses.
- High current, low travel speed (high heat input): Results in coarser microstructure with reduced carbide volume fraction, lower hardness (620–680 HV₀.₂), but improved toughness and reduced cracking tendency.
- Optimal parameter window: A current of 250–300 A with travel speed of 300–350 mm/min produces a balanced microstructure with hardness of 700–760 HV₀.₂, acceptable toughness, and minimal defect density.
Hardness-Process Relationship
The study establishes a clear quantitative relationship between heat input and overlay hardness:
| Heat Input (kJ/mm) | Average Hardness (HV₀.₂) | Primary Phase | Cracking Index |
|---|---|---|---|
| 3.2–4.0 | 780–820 | Fine M₇C₃ network | High |
| 4.0–5.5 | 700–760 | M₇C₃ + M₂₃C₆ mixture | Moderate |
| 5.5–7.0 | 620–680 | Coarse M₂₃C₆ | Low |
| > 7.0 | 560–620 | Dendritic + interdendritic | Very Low |
Engineering Practice Integration
The findings have direct implications for aero engine component repair and remanufacturing programs. The optimal parameter window identified provides a practical starting point for process qualification under standards such as NB/T 47014 (Chinese welding procedure qualification) or ASME IX. The open-arc process offers advantages over conventional flux-cored SAW in terms of reduced spatter, cleaner weld surface, and easier automation, which are particularly valuable for precision aerospace applications.
The research also highlights the importance of shielding gas selection and flow rate in open-arc processes. Insufficient shielding leads to nitrogen pickup and oxide inclusion formation, which significantly degrades the fatigue performance of the overlay layer — a critical concern for cyclically loaded engine components.
Study Insights and Implications
The systematic parameter-microstructure-property relationship established in this study exemplifies the scientific approach to weld overlay process development. The researchers demonstrate that hardness alone is not an adequate quality criterion; the microstructural morphology (carbide distribution, grain size, phase composition) must be considered in conjunction with mechanical properties to ensure functional performance in service. This holistic evaluation methodology should be adopted as standard practice in any cladding process development program targeting critical aerospace or power generation applications.