Abstract
An experimental study was made on the influence of nitrogen content up to about 0·7% on the properties of 18%Cr-7%Ni stainless steels prepared by the high-pressure melting process in nitrogen atmosphere. Main results obtained were as follows.
1. It was necessary to heat for 1/2h at 1200°C for the solution-treatment of the steels with high nitrogen concentration. Stability of austenite to martensite transformation was markedly increased with an increasing nitrogen concentration in the steels; both Ms and Md were lowered considerably with an increasing nitrogen content. The Ms of the lowest nitrogen steel 1Mn-0·04N was found to exist at about 60°C, and the steel solution-quenched was found to contain a considerable amount of isothermal component of austenite-martensite transformation at room temperature.
2. During ageing at temperatures between 700°C and 900°C, peculiar lamellar precipitates consisting of chromium nitride and austenite depleted in both chromium and nitrogen appeared around the austenite grain boundaries due to the so-called grain boundary reaction.
It was often found that the depleted austenite, formed by the reaction described above, transforms into martensite during cooling from the ageing temperatures.
3. The strength increased markedly with an increasing nitrogen concentration, but in the steel with low nitrogen content higher strength was often obtained than in the steels of medium nitrogen content due to the martensitization of unstable austenite in the former steels either by subzero-treatment, by cold-rolling or even by plastic deformation during tensile tests. A good combination of tensile strength and elongation or reduction of area, however, was found to be obtained especially in the steels with high nitrogen concentration.
4. It was found from the high-temperature tensile creep-rupture test at 700°C that the highest value is obtained at the nitrogen concentration around 0·32%, while the creep-resistance is deteriorated by the higher nitrogen concentration in the steels.
