Contributing Factors and Control of Formation of Fine-grain Microstructure in Inner and Outer Surfaces of 690 Alloy Tube for Heat Transfer Service

Analyzed are the factors affecting the bright solid solution heat treatment microstructure of the 690 alloy pipe for heat transfer service, including the cold-rolling deformation rate, degreasing quality control, heat-treatment protective atmosphere and original microstructure of the semi-product.The analysis reveals that the difference between the re-crystallization nucleation rate of the inner surface and that of the outer surface acts as the root cause for development of the abnormal fine-grain microstructure on the inner and outer surfaces of the pipe. Accordingly, such fine-grain microstructure of the said 690 alloy pipe as occurred after bright solid solution heat treatment is expected to be eliminated by means of improving the heat treatment atmosphere, increasing the deformation rate and improving degreasing quality, etc.