CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of Steam Turbine Blade Hot Forging Die Overlay Layers
Literature Overview
The metallurgical and mechanical characterization of overlay layers on steam turbine blade hot forging dies represents a significant area of research in the die manufacturing and maintenance industry. Hot forging dies operate under extreme conditions, including temperatures exceeding 900 degrees Celsius, cyclic thermal loading, and severe mechanical impact. The overlay layer, typically composed of a cobalt-based or nickel-based alloy, is applied to protect the die surface from thermal fatigue, oxidation, and wear. This study note examines the relationship between the overlay microstructure and the resulting mechanical properties, with particular attention to the effects of welding process parameters and heat treatment.
Overlay Microstructure Characterization
The overlay layers studied were applied using plasma transferred arc (PTA) welding and submerged arc welding (SAW) processes, with the base die material being H13 hot work tool steel (equivalent to AISI H13). The overlay alloy was a cobalt-chromium-tungsten-carbide system, similar to Stellite 21, selected for its excellent hot hardness and thermal shock resistance.
Metallographic examination revealed a complex microstructure consisting of an M7C3 carbide network embedded in a cobalt-rich matrix. The carbide morphology varied significantly between the PTA and SAW deposits. In the PTA overlay, the carbides appeared as fine, uniformly distributed particles with an average size of 2 to 5 micrometers, whereas in the SAW overlay, the carbides were coarser, ranging from 8 to 20 micrometers, and exhibited a dendritic network pattern. This difference is attributed to the significantly higher cooling rate in PTA welding compared to SAW.
| Parameter | PTA Overlay | SAW Overlay |
|---|---|---|
| Average Carbide Size (micrometers) | 2-5 | 8-20 |
| Matrix Grain Size (micrometers) | 15-25 | 40-60 |
| Hardness (HV30) | 420-450 | 380-410 |
| Dilution Ratio (%) | 5-8 | 12-18 |
| Thermal Cycling Resistance (cycles to crack) | 1200-1500 | 800-1000 |
Columnar Grain Structure and Its Implications
The columnar grain structure observed in both overlay types extends from the fusion line to the overlay surface. In the PTA overlay, the columnar grains are finer and more uniform, which contributes to better thermal shock resistance. The finer grain structure provides more grain boundaries, which act as barriers to crack propagation during thermal cycling. In contrast, the coarser columnar grains in the SAW overlay are more susceptible to intergranular cracking, particularly at the grain boundaries where carbide segregation occurs.
The presence of M7C3 carbides is critical for the wear resistance of the overlay. However, excessive carbide formation, particularly in the form of a continuous network at the grain boundaries, can severely reduce the fracture toughness of the overlay. The study found that the fracture toughness of the PTA overlay was approximately 15 percent higher than that of the SAW overlay, which can be attributed to the more favorable carbide distribution and finer grain structure.
Mechanical Property Evaluation
The mechanical properties were evaluated through hardness testing, bending tests, and thermal cycling tests. The hardness values, as shown in the table above, indicate that the PTA overlay achieves higher hardness due to the finer microstructure and lower dilution ratio. However, hardness alone does not fully characterize the performance of the overlay in hot forging applications.
The thermal cycling test, conducted by repeatedly heating the overlay surface to 950 degrees Celsius and cooling it to room temperature in water, revealed significant differences between the two welding processes. The PTA overlay survived approximately 1200 to 1500 cycles before the first surface crack appeared, while the SAW overlay failed after only 800 to 1000 cycles. The crack initiation sites in the SAW overlay were predominantly at the grain boundaries of the coarse columnar grains, confirming the detrimental effect of coarse grain structure on thermal fatigue resistance.
Effect of Heat Treatment
Post-weld heat treatment was found to significantly influence the overlay performance. Forging at 840 degrees Celsius for 2 hours followed by air cooling and tempering at 600 degrees Celsius for 2 hours produced the best combination of hardness and toughness. This heat treatment cycle dissolved the coarse M7C3 carbide network and re-precipitated fine carbides within the matrix, resulting in a more homogeneous microstructure. After heat treatment, the hardness of the SAW overlay increased by approximately 10 percent, and the thermal cycling resistance improved by about 25 percent.
Engineering Practice and Process Optimization
For die manufacturers and maintenance engineers, the key takeaway from this study is that the welding process selection directly influences the service life of the overlay. PTA welding, despite its higher equipment cost, produces superior overlay quality due to the lower dilution ratio and finer microstructure. However, for large-scale die repairs where productivity is a concern, SAW welding with appropriate parameter control can still provide acceptable performance.
The following process recommendations are derived from the study findings:
- Use a low-current, high-speed welding parameter set for the first layer to minimize dilution.
- Apply a backing bar or backing plate to prevent burn-through and ensure full penetration of the first layer.
- Perform interpass temperature control, maintaining the interpass temperature below 250 degrees Celsius to prevent excessive grain growth.
- Apply post-weld heat treatment to optimize the carbide distribution and relieve residual stresses.
- Conduct thorough non-destructive testing, including magnetic particle inspection (MT) and ultrasonic testing (UT), to detect subsurface defects before returning the die to service.
Key Reflections
The most significant insight from this study is the recognition that overlay performance is not solely determined by the alloy composition but is equally dependent on the microstructure, which is governed by the welding process and post-weld treatment. Engineers who focus exclusively on alloy selection without considering the process-induced microstructure may be disappointed by the actual field performance of their overlays. This study reinforces the principle that the welding process is an integral part of the material system, not merely a means of deposition.
Another important reflection is the need for standardized thermal cycling test procedures. The current lack of universally accepted thermal cycling test methods makes it difficult to compare overlay performance across different studies and manufacturers. Establishing a standardized thermal cycling test protocol, similar to the established fatigue test standards, would greatly enhance the reliability and comparability of overlay performance data.
Summary and Outlook
The study on steam turbine blade hot forging die overlay layers demonstrates that the microstructure, particularly the carbide morphology and grain structure, is the primary determinant of overlay performance under thermal cycling conditions. PTA welding produces superior overlays due to lower dilution and finer microstructure, but SAW welding with proper parameter control and post-weld heat treatment can also achieve acceptable results. Engineers should adopt a holistic approach that integrates alloy selection, welding process optimization, and heat treatment to maximize the service life of die overlays in demanding hot forging applications.