Abstract
Influence of nitrogen on microstructures, mechanical properties and acid-resistivities of 18%Cr-10%Mn stainless steels has been studied. The main results obtained are as follows:
(1) The optimum temperature of solution treatment is found to be 1050°C for 0.14 N (the steel containing 0.14% nitrogen), 1100°C for 0.52 N, 1150°C for 0.77 N and 1200°C for 0.96 N.
(2) With increasing nitrogen content, the microstructure of the steel solution-quenched changes from ferrite in the low nitrogen steel to ferrite plus austenite in the intermediate nitrogen steel, and finally to complete austenite in the high nitrogen steel containing more than 0.5%N.
(3) Hardness and strength increase markedly in proportion to the increase in nitrogen content, but the maximum elongation value is obtained at about 0.5%N. Consequently, the high nitrogen steel, such as 0.96%N steel, indicates excellent mechanical properties, that is, tensile strength of 107 kg/mm2, proof stress of 77 kg/mm2, and elongation of about 40%.
(4) In the steel, containing high concentration of nitrogen more than 0.5%, tempered at about 700°C or more after solution-quenched, the lamella phase Cr2N precipitates by a grain boundary reaction, by which the strength increases a little and the impact value is remarkably reduced.
(5) Corrosion resistivity of the steel in 1%HCL solution and in 5%H2SO4 solution is far inferior to that of commercial 18Cr-8Ni stainless steel, but shows nearly the same value as commercial 18Cr-8Ni stainless steel in 63%HNO3 solution, and from the above experiment it was found that the corrosion resistivity does not depend upon nitrogen concentration.
