TOUGHENING OF 18-PERCENT NI MARAGING-STEEL BY THE COMBINED ADDITION OF NB AND B

Effects of Nb content and solution treatment temperature on the tensil e strength and fracture toughness were examined in 18% Ni maraging ste el containing 0.0012 similar to 0.0013% B. When Nb was added as a bori de former element, the recovery process of reverse-transformed austeni te was retarded. This retardation of recovery led to the suppression o f recrystallization and further to the rising in recrystallization tem perature of reverse-transformed austenite. The recrystallization tempe rature was raised with increasing the Nb contents. When the addition o f Nb was more than 0.16 mass%, however, the fracture toughness was red uced markedly. This decrease of fracture toughness was ascribed to the thermal embrittlement which was caused by growth of NbB2 type borides . In conclusion, when Nb nas added by an amount of 0.05 to 0.07 mass%, both strengthening and toughening can be realized without any thermal embrittlement, by means of the solution treatment in a wide temperatu re range where the steels are in a state of unrecrystallized austenite .