Strength and Toughness of 9Ni-Cr Steels in the Temperature Range from 77 to 4.2K

The temperature dependence of the strength and toughness was measured for quench-and-tempered 9%Ni steels containing 0.5% Cr and 1.0% Cr which were quenched with the cooling rate simulated to heavy plate, and a commercial 9%Ni steel in the temperature range from 77 to 4.2 K. The yield strength of the three steels increased monotonically and reached the same value with decreasing temperature down to 20 K. Below 20 K, anomal change in yield strength was observed. The Charpy impact tests showed a decrease in the absorption energy and an increase in the brittle area on fracture surface in 9Ni-0.5Cr and 9Ni-1.0Cr steels at the temperature between 77 and 40 K, but below 40 K, only a slight change in the absorption energy and no apparent ductile-brittle transition temperature. The commercial 9%Ni steel showed a full ductile fracture mode in the whole temperature range tested. The fracture toughness test (J ingegral test) of the three steels showed the clear ductile and brittle transition and a low energy brittle fracture below 30 K. The nil-ductility temperature and the ductile and brittle transition temperature are almost the same among three steels, but KIC(J) values are different. The 9Ni-1.0Cr steel showed a relatively high K IC (J) value especially in the brittle fracture range.