Advanced Nickel-Based Alloy 00Cr16Ni60Mo16W4 (Hastelloy C-276) Pipe (Tech Center of Shanghai HY Industry Co., Ltd)

Abstract: Aiming at 00Cr16Ni60Mo16W4 alloy, research work on the segregation of alloy elements, precipitation phase and control of pipe microstructure and properties was carried out. The results show that the segregation elements in the alloy are Mo and W, and the main equilibrium precipitation phases are carbide M6C and μ phase; C The content has the most obvious effect on the alloy precipitates, and the control of ultra-low C content is beneficial to the subsequent hot working performance of the alloy. In order to reduce the hardness and obtain a suitable structure, the solution heat treatment temperature of the waste pipe should be higher than 1150℃. 00Cr16Ni60Mo16W4 is a high-end nickel-based alloy, which is difficult to smelt and process. The alloy has excellent corrosion resistance, oxidation resistance and good mechanical properties, and is mainly used in petroleum and petrochemical pipelines, nuclear power equipment and aerospace engines. This paper mainly carries out research work on the segregation of alloy elements, precipitation phase and control of the microstructure and properties of pipes, and provides technical support for the development of steel grades.

1. Experimental materials and methods for Advanced Nickel-Based Alloy 00Cr16Ni60Mo16W4 (Hastelloy C-276)

Alloy chemical composition/mass%

Production process of the pipe is: VIM+ESR+forging tube blank+hot extrusion+cold rolling, heat treatment.

2. Results and Analysis

2.1 Phase diagram analysis The following figure is the alloy phase diagram [1]. It can be seen from the figure that in addition to the matrix γ phase, the main equilibrium precipitation phases of the alloy are carbide M6C and μ phase. The initial melting and final melting temperatures of the alloy are 1360 °C and 1387 °C, respectively. ℃, the solidification range is only 27℃. Carbides start to precipitate at 1082 °C. Although the proportion of carbides is small, they are mostly distributed at the grain boundaries of the alloy, so they have a great influence on the properties of the alloy. Among them, the μ phase is a harmful phase, and a large amount of precipitation will greatly weaken the grain boundary, resulting in cracking during processing. The amount of carbide precipitation is mainly related to the carbon content of the alloy. The carbon content is slightly increased, and the carbide precipitation increases significantly. Therefore, the control of C in alloy smelting is very critical.

2.2 Research on segregation and precipitation
Due to the presence of a large amount of Mo and W elements in the alloy, very serious segregation behavior occurs during the solidification process. If the electroslag ingot shape is not properly controlled, large and harmful precipitates that cannot be eliminated at a later stage will be produced. The figure below shows the morphology and composition of some typical precipitates due to segregation in electroslag ingots.
The electroslag ingot mainly has the segregation of Mo and W, among which Mo is a positive segregation element and W is a negative segregation element. Identifying from the morphology of the precipitated phase, there are mainly MC and M6C phases along the crystal chain, and a certain amount of μ phase, which is a harmful phase. The reflow site can cause voids that seriously affect hot workability.

2.3 Research on the microstructure and properties of the pipe The hot extrusion process is used to obtain the waste pipe. Since the alloy contains a large amount of solid solution strengthening elements, the waste pipe needs to be heat treated before the subsequent cold rolling production in order to obtain a uniform structure and suitable hardness. When the temperature is lower than 1080 °C, the hardness does not decrease significantly, and the corresponding microstructure does not undergo static recrystallization and is in the state of extruded fine grains. When the temperature is higher than 1080 °C and lower than 1150 °C, although the hardness decreases significantly, the structure is in a mixed crystal state. When the temperature is higher than 1150 °C, the structure is completely statically recrystallized, the hardness decreases again, and the structure is uniform. The heat treatment of the waste pipe in this temperature range is beneficial to the subsequent cold rolling production.

Multi-pass cold rolling-heat treatment is performed on the waste pipe, and finally a small-diameter finished pipe is obtained. The mechanical properties of the pipe are tested to meet the standard requirements.
Mechanical properties of Advanced Nickel-Based Alloy 00Cr16Ni60Mo16W4 (Hastelloy C-276):

3. Conclusion

1. The segregation elements in the alloy are Mo and W, and the main equilibrium precipitation phases are carbide M6C and μ phase

2. C content has the most obvious effect on the alloy precipitates, and the realization of ultra-low C content control is beneficial to the subsequent hot working properties of the alloy.

3. In order to reduce the hardness and obtain a suitable structure, the solution heat treatment temperature of the waste pipe should be higher than 1150℃.

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