Study on the Manufacturing Process of Large Super Alloy X-750 Forgings for Nuclear Equipment

Abstract

X-750 is a nickel-chromium based super alloy with high strength, high toughness, excellent creep resistance and good stress relaxation resistance at high temperature. It can be used in the manufacture of holddown springs for reactor vessel internals (RVI). Because of high content of alloying elements and the large size of ingot for the RVI holddown spring, observations by optical microscopy (OM) and scanning electron microscopy (SEM) showed that there was obvious dendrite segregation and serious carbonitride net in the as-cast electroslag remelting (ESR) ingot. Before forging, homogenization treatment was carried out for the ESR ingot to reduce element segregation, and promote the secondary carbonitrides dissolve into the alloy, which could significantly improve the malleability of the billet and reduce forging cracks. The homogenization process adopted in this study was annealing at 1220°C for 70 hours. In addition, the final forging temperature also played a key role in the control of formability of the billet and grain size of the forging. The carbonitrides or grain size of both the as-cast and forged samples annealed at 900°C∼1200°C for 10h were observed, and also charpy impact tests were carried out on the forged and annealed samples. The results showed that the final forging temperature for upsetting and drawing process should be no less than 1100°C, and the final forging temperature for forming process could be reduced to 1000°C. After solution and aging treatment, the comprehensive mechanical properties of the forging were excellent, with KV impact value at room temperature as high as 92J, yield strength of tensile test at 350°C up to 696MPa, and tensile strength up to 1019MPa.