The CF-3CM cast stainless alloys containing 0%-25% δ-ferrite volume fraction were achieved only by varying the chemical composition in the range of the specification, and the influence of δ-ferrite volume fraction on the low temperature mechanical properties has been studied.
The initial fracture toughness values ($K_{IC}$) of CF-3M castings containing 0.2 to 24.5 pct δ-ferrite ranged from 74 to 293 MPa·m on the basis of estimates from J-integral data. With increase of δ-ferrite volume fraction, the fracture toughness was more sensitively decreased than the CVN energy. The slow decrease oif CVN energy seems to be resulted from grain-size refinement effect. But the steep decrease of the fracture toughness value $K_{IC}$ could not be explained.
Martensite transformation induced high elongation was shown even on the specimen having 24.5% δ-ferrite tensile tested at -196℃.
Both of the tensile and yield strength behave nearly constant below about 5% and over about 15% δ-ferrite, and inbetween they increased significantly. A proposed strengthening model of the summation of 'rule of mixture' and 'grain size refinement effect' was tried to explain such strengthening.