On the High-Nitrogen 25/0Cr-28% Ni and 25%Cr-20%Ni-80%Co Heat-Resisting Steels

Studies on high-nitrogen 25%Cr austenitic heat-resisting steels-I

Abstract

The effect of nitrogen addition to 25%Cr-28%Ni austenitic heat-resisting steels prepared by the high-pressure melting process in nitrogen atmosphere on both microstructures and mechanical properties at room- and high-temperatures was studied. The influence of replacing a part of nickel by cobalt in those steels was also examined. Main results obtained were as follows:
(1) It was necessary to heat for 1h at 1250.. for the solution treatment of the steels with high nitrogen content.
(2) The recrystallization temperature estimated from the hardness change during tempering of cold rolled specimens was found to be higher by about 100.. in the 0.54%N steel than in the 0.05%N steel, and the temperature of the former steel decreased with the replacement of a part of nickel by cobalt.
(3) It was observed from microstructural changes during ageing at high temperatures of the steels with high nitrogen concentration that the precipitation of nitride Cr₂N from the austenite proceeds in the following two different modes: one so-called general precipitation and the other a grain boundary reaction. The latter mode of reaction, which proceeds rapidly by consuming the nitride particles formerly precipitated by the mode of the general precipitaion, was found to be brought to a stop when the reaction covers about a half of the area under the microscope of the solution treated steels, whereas it was found to proceed until the reaction covers almost all of the structure in the cold-rolled steels.
(4) Proof stress and tensile strength of the steels at room-temperature increased markedly with an increasing concentration of nitrogen without any loss in ductility. At high-temperatures, the tensile strength increased with an increasing concentration of nitrogen, the ductility being deteriorated considerably.
(5) It was found from the tensile creep-rupture test at 700.. that the increase in creeprupture strength of the steels with an addition of nitrogen only was not so large as in the high-nitrogen 316L type steels, and that the creep-rupture properties of such steels were somewhat improved by cold-working, hot-cold working or ageing treatment at 800.. of the solution treated steels. It is expected that those properties may be improved considerably by a simultaneous addition of nitrogen and molybdenum, or nitrogen and niobium.