Manufacturing technology and material properties of high nitrogen austenitic stainless steel forgings for ITER TF coil cases

ITER TFコイル容器用高窒素添加強化型オーステナイト系ステンレス鍛鋼品の製造技術及び材料特性について

押川 巧*; 船越 義彦*; 今岡 宏志*; 吉川 耕平*; 真有 康孝*; 井口 将秀; 櫻井 武尊; 中平 昌隆; 小泉 徳潔; 中嶋 秀夫

Oshikawa, Takumi*; Funakoshi, Yoshihiko*; Imaoka, Hiroshi*; Yoshikawa, Kohei*; Maari, Yasutaka*; Iguchi, Masahide; Sakurai, Takeru; Nakahira, Masataka; Koizumi, Norikiyo; Nakajima, Hideo

ITERは核融合発電を検証するために建設が進められている実験炉である。日本が調達責任を有しているトロイダル磁場コイル(TFC)は、高さ約17m、幅約9mのD型形状の溶接鋼構造体であり、重要なITER構成部品の一つである。ITERの運転温度である4Kにおいて、TFCの超伝導部に生じる電磁力を支えるためにTFC容器は強化型オーステナイト系ステンレス鋼を使用する。また、高剛性を実現するために600mmを超える板厚を有し、かつ複雑な三次元形状を呈している部材もある。鍛造後の機械加工量を最小化するために、最終形状に極力近づけた仕上げ形状に鍛造する必要がある。しかし、このような鍛造プロセスを適用して極厚複雑形状部材を製造した実績はないため、二種類の極厚複雑形状材料の実規模試作を行い、自由鍛造による製造プロセスの検証、超音波探傷試験,冶金試験,常温及び4Kでの機械特性試験を実施した。その結果、自由鍛造プロセスを用いた鍛造によって最終形状に近い鍛造仕上げ形状を実現できること、及びこれらの材料がITER要求値を上回る材料特性を有していることを確認した。

ITER is a large-scale experiment that aims to demonstrate that it is possible to produce commercial energy from fusion. ITER Toroidal Field Coil Case (hereinafter referred to as "ITER TFCC") is one of the important components of ITER. The ITER TFCC materials are made of high nitrogen austenitic stainless steel and having various configurations. The ITER TFCC material which manufactured by JCFC has a complex configuration with heaver thickness than other materials. It is difficult to form near net shape to delivery configuration by ordinary open die forging method such as upset and stretching, because the ITER TFCC materials manufactured by JCFC have a complex configuration. Therefore ingot weight and lead time of machining increase when ITER TFCC materials are forged by ordinary open die forging method. Moreover, in order to get good attenuation at Ultrasonic examination, it is necessarily to make fine and uniform grain of the material. However, it is impossible to control grain size of austenitic stainless steel by heat treatment. The grain becomes fine and uniform by only forging process with suitable condition. Therefore, JCFC has studied suitable forging method to become near net shape to delivery configuration and also to get fine grain of center of the material. Based on these result, ITER TFCC materials were manufactured. This innovative forging process led to reduce the weight of ingot compared with general forging. And it had good Ultrasonic attenuation. It was confirmed that the results of material test and nondestructive examination satisfied the requirements of Japan domestic agency (hereinafter referred to as "JADA"). Moreover, the test coupons were taken from center of thick part of product and used for various tests. As the result of tests, it was confirmed that results of material test satisfied the requirements of JADA. It is clear that this innovative forging method is very suitable process for manufacturing of ITER TFCC materials.