Study of the Microstructures and Toughness of Steels with Precipitation-Hardening Due to Cu and NiAl

Abstract

Age-hardening behavior, especially related to the influence of matrix microstructures, and the resulting changes of toughness, was investigated in low-carbon Ni-containing steels with the addition of Cu and Al. Main results are as follows:
a. Age-hardening was most prominent when the matrix microstructure was bainitic.
b. Precipitates that contribute to age-hardening was concluded to be NiAl and Cu. Grobular NiAl under 100 A in diameter precipitated at dense dislocations in bainite at peak hardness.
c. In 0.15C-1.5Ni-0.5Mo steels, the amount of age-hardening increased, but the toughness was greatly impaired, with increase of the amount of Cu or Al.
d. Age-hardening and corresponding change in toughness with the addition of Cu and Al were studied in various low-carbon bainitic steels containing Ni. Among these, low carbon 6Ni-4Cr-1.5Mo-Cu-Al steels maintained fairly good toughness up to the strength level of 200kg/mm², as compared with maraging type of steels.