Evaluation of Thermal Aging Embrittlement of Austenitic Stainless Steels JN 1, JJ 1 and JK 2 by Cryogenic Small Punch Tests

The small punch (SP) tests were conducted at 77 K and 4 K using austenitic stainless steels JN 1, JJ 1 and JK 2, which have been recently developed for use as a cryogenic structural material. The tests were applied to the steels aged at 923-1 073 K to investigate changes in their toughness at cryogenic temperature due to thermal aging. The area under the load-deflection curve up to the macro-crack initiation was defined as SP energy to evaluate the fracture toughness of the steels. The experimental results showed that the SP energy of JN 1 decreased monotonously with an increase in aging temperature. On the other hand, the SP energy of JK 2 showed no significant change even after the thermal aging at 923 K and 973 K and decreased after the thermal aging at 1 023 K. Metallogra-phic examination revealed that this difference in decrease of SP energy due to thermal aging was caused by the variation in amount and distribution of grain boundary M₂₃C₆ carbides. As a consequence, JN 1 was more susceptible to thermal aging embrittlement than the other steels, and high resistance of JK 2 to embrittlement was attributable to the depression of precipitation and coarsening of grain boundary precipitates.