Abstract
1. An increase in nickel content from 3.4 to 9.1% leads to an increase in the tendency of low-carbon steel toward the formation of lath martensite and in the volume share of residual (primary) austenite. After water quenching, the structure of ON3 steel is ferritic-martensitic and of ON6 and ON9 steels martensitic, while after air cooling a ferritic structure is formed in ON3 and ON6 steels and a ferritic-martensitic one in ON9 steel. 2. With an increase in nickel content in the steel from 3.4 to 9.1% the size of the pits in ductile fracture increases, and to a greater degree in the air-cooled steel. 3. After cooling from austenite-area temperatures, residual austenite is observed in the investigated steels both at the boundaries of the martensite laths and in the form of individual islands within the α-ferrite grains, and after tempering in the form of round precipitates at the boundaries and within the α-ferrite grains. 4. As the result of tempering of the nickel steels stable (secondary) austenite is formed in their structure, which leads to an increase in cold resistance and specific work for propagation of a ductile crack.
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