CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of Overlay Layers on Turbine Blade Hot Forging Dies
Literature Overview
The paper by Zhai Dajun, Wen Zhongbo, Hou Yong, Tang Daolei, and Li Yanli, published in 2018 in "Thermal Processing Technology," examines the microstructure and mechanical properties of overlay layers applied to hot forging dies used in steam turbine blade manufacturing. This research was supported by the Sichuan Provincial Key R&D Program (2017GZ0145) and the Sichuan Engineering Vocational and Technical College research project (YJ2017KJ-12). The work bridges metallurgical research with practical tool engineering in the power generation sector.
Background and Technical Challenges
Hot forging dies for steam turbine blades are subjected to extreme conditions: temperatures of 900–1100 °C, high contact pressures, cyclic thermal loading, and abrasive contact with hot metal. The die material must simultaneously exhibit high hot hardness, thermal fatigue resistance, wear resistance, and sufficient toughness to withstand impact loading. Surface engineering through cladding or hardfacing is a cost-effective approach to extending die life without replacing the entire die.
The overlay process used in this study was multi-layer hardfacing with wear-resistant alloys, applied to the cavity surfaces of the forging dies. The base die material was typically H13 (4Cr5MoSiV1) hot work tool steel, and the overlay layers were composed of Cr-Co-Ni type or Cr-W type hardfacing alloys.
Overlay Layer Microstructure Analysis
The microstructure of the overlay layers was characterized through optical microscopy, SEM, EDS, and XRD. Key findings included:
| Microstructural Feature | Description | Effect on Properties |
|---|---|---|
| Primary carbides | Cr₇C₃, WC, and mixed carbides | Primary hard phase, controls hardness |
| Matrix | Martensite or austenite | Toughness and thermal stability |
| M-Cr₇C₃ eutectic | Network of carbides in matrix | Wear resistance but potential crack path |
| Columnar grains | Directional solidification | Affects thermal fatigue behavior |
The authors found that the overlay layer microstructure was strongly dependent on the welding process parameters and the number of overlay passes. The first pass exhibited significant dilution from the H13 base metal, resulting in a different microstructure compared to subsequent passes. The dilution rate was approximately 30–40% for the first pass and decreased to 5–15% for subsequent passes.
Mechanical Properties and Performance
The mechanical properties of the overlay layers were evaluated through hardness testing, wear testing, and thermal fatigue testing:
- Hardness: The overlay layer hardness ranged from 60–65 HRC at room temperature, compared to 48–52 HRC for the base H13 steel. After thermal cycling to 900 °C and cooling, the overlay retained 55–58 HRC, demonstrating excellent hot hardness retention.
- Wear resistance: Pin-on-disk wear testing against 1020 steel balls showed that the overlay layers exhibited 3–5 times the wear resistance of the base material.
- Thermal fatigue: After 100 thermal cycles (room temperature to 900 °C), the overlay layer showed minimal cracking, while the base material developed significant surface cracking.
Process Optimization
The authors optimized the welding parameters to achieve the desired overlay properties:
| Parameter | Optimized Value | Rationale |
|---|---|---|
| Welding current | 180–220 A | Adequate penetration without excessive dilution |
| Arc voltage | 22–26 V | Stable arc and consistent bead profile |
| Travel speed | 30–50 cm/min | Controlled heat input |
| Number of passes | 3–4 | Sufficient dilution control |
| Interpass temperature | ≤ 150 °C | Minimize grain growth |
| Preheating | 200–300 °C | Reduce residual stress |
The number of passes was identified as a critical parameter. With only 1–2 passes, the high dilution rate resulted in excessive base metal contamination, reducing the hardness and wear resistance of the overlay. With 3–4 passes, the dilution was sufficiently reduced to achieve the target microstructure and properties.
Engineering Practice and Quality Control
For the application of overlay layers on turbine blade forging dies, the following quality control measures are recommended:
- Surface preparation: The die cavity surface should be machined to Ra ≤ 3.2 μm and cleaned to remove all contaminants.
- Welding procedure qualification: The welding procedure should be qualified per NB/T 47014, with specific attention to the overlay layer thickness and dilution.
- Post-weld heat treatment: Stress relief at 550–650 °C for 2–4 hours should be performed to reduce residual stresses.
- Inspection: Visual inspection for surface quality, MT for surface cracks, and UT for subsurface defects.
- Thickness verification: The overlay thickness should be verified by UT or magnetic thickness gauge, with a minimum thickness of 2 mm and maximum of 6 mm.
Key Reflections and Study Insights
The most important finding of this study is that the overlay layer thickness and dilution rate are the primary factors controlling the performance of the overlay on hot forging dies. This has direct implications for welding procedure design: the number of passes and the first-pass dilution must be carefully controlled to ensure adequate overlay performance.
The study also highlights the importance of thermal fatigue resistance in die applications. While room-temperature hardness is often used as the primary quality metric, the retention of hardness after thermal cycling is equally important for die applications. The overlay layers demonstrated excellent thermal fatigue performance, which directly translates to extended die life in production environments.
For engineers in the power generation sector, this research provides a practical framework for die surface engineering through cladding. The recommended process parameters and quality control measures can be directly applied to die maintenance programs, potentially reducing die replacement frequency by 2–3 times and significantly lowering production costs.