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TISCO’s high purity stainless steel used for the world’s largest one-piece ring forging

By Chen xin, Yin Wei and Wang Yongze UpDate 2019-03-19

On March 12th, the world’s largest seamless circular forging in size and weight has been successfully fabricated with 316H high-purity stainless steel from TISCO. It will be used for the 4th-generation nuclear power set in China for the first time---to make the support ring for core component of the pilot reactor for 60 million kilowatts fast neutron reactor (fast reactor), located in Xiapu, Fujian province, in southeast China. At the moment, TISCO has completed the material supply and it is the only one producer of the stainless steel satisfying all technical requirements of the forging process in China.

Fast reactor is the second step of China’s “thermal reactor---fast reactor---fusion reactor” strategy for nuclear power development in China. It is the primary option for the 4th-generation nuclear power units and capable of improving the utilization of nuclear fuel source. As the “backbone” for reactor vessel, the giant forging is 15.6 meters in diameter and 150 tons in weight. It is required to structurally bear a weight of 7000 tons , a temperature of 650 and operate continuously for 4 decades. In the past, the forging of this sort at home and abroad was generally fabricated with multiple billets by assembling and welding, causing weakness in material microstructure at welds, a potential risk for reactor’s operation. Currently, a new technical process, initiated by the Institute of Metal Research of Chinese Academy of Science, has been used to make huge forging, by which a 100-ton raw material blank is forged in an overlapping fashion with 58 pieces of high-purity 316H continuous casting (CC) billets. This method, a “small-to-big” process, has cracked the inherent metallurgical defects arising from traditional “big-to-big” process.

The harsh operating conditions and the new process have brought unprecedented challenges to the CC billets in terms of chemical compositions and homogeneity. In the process of the material D&R, TISCO, working together with CIAE and the Institute of Metal Research of Chinese Academy of Science, took the related experiments and production as a top priority and went all out to solve technological difficulties. As a result, the company has made breakthroughs in smelting and CC processes, bringing the material indicators such as high temperature resistance, corrosion resistance, purity of steel, homogeneity of microstructure, dimension precision to a brand new level. It has also grasped technologies to produce 316H stainless steel plate, CC billet, and electro-slag ingot, the materials for key parts of fast reactor, and is capable of producing in mass.

As one of earliest participants in developing and producing material for nuclear power, TISCO was the first in China to produce high temperature, high pressure and corrosion resistant stainless steel dedicated to nuclear power, carbon steel, high grade cold-rolled silicon steel with thick coating. It is the only one stainless steel producers capable of supplying full-series of stainless steel material for the 3rd-generation nuclear power units, including 304, 304H for CAP 1000 and CAP 1400, 304L nitrogen-controlled stainless steel plate of ultra thickness and width, high strength stainless steel cladding plate, duplex stainless steel for module, wide cold-rolled stainless steel strip for the components in the reactor of home-made “Hualong 1”. In addition, the company has also successfully developed stainless steel materials for key parts of the international thermonuclear fusion reactor (ITRE) project. Up to now, TISCO has supplied more than 50,000 tons of steel for nuclear power projects, including 45,000 tons of stainless steel.