Characterization of Nitronic-40 Stainless Steel Shells

The construction of magnets with ever higher magnetic fields requires ever stronger and tougher reinforcement materials. Nitronic stainless steels are sometimes used for this purpose. Avoiding the sensitization temperature of these steels during fabrication is of critical importance because excessive exposure to this temperature may make them sensitive to corrosion. Even though these steels have exceptionally high mechanical strength and satisfactory toughness for use at room temperatures, low cryogenic temperatures may reduce their toughness to the degree that they are rendered unsatisfactory as reinforcement materials. This low toughness at cryogenic temperature in Nitronic steels often stems from the microstructure established during fabrication, even when the heat treatment profile follows the required protocol. Even materials that have not previously been exposed to sensitization temperatures during fabrication may later be exposed during the construction of magnets. Consequently, quality control of reinforcement materials must include both toughness tests and microstructure analyses. This paper reports recent studies of the effect of component volume and microstructure on cryogenic temperature toughness in Nitronic-type stainless steels. By comparing four different shells, we established that the presence of grain boundary particles caused lower impact fracture energy but had no significant impact on mechanical strength.