On the Effect of Nitrogen-Absorption Treatment on Properties of Low Ni-18Cr Stainless Steels.

Abstract

Studies have been made on the mode of the formation of a nitrogen-bearing austenite in 18%Cr-Ni-Fe alloys containing Ni up to 6% and Mo up to 3% by the authors' nitrogen-absorption method and on the stability of the formed austenite at the plastic working.Mechanical properties and corrosive resistivities of those alloys have also been investigated, the results obtained being as follows.
(1) Austenite containing about 0.5%N is formed to a considerable depth in the surface zone of 18%Cr-Ni-Fe alloys by nitrogen-absorption at 1250°C for 32 hours.The depth of the nitrogen-bearing austenite zone from the surface increases with an increase in the Ni content of the alloys and with a longer time of nitrogen-absorption.By nitrogen-absorption treatment at 1250°C for 32 hours, the specimens having 4.0mm thickness and containing 2%or more of Ni without Mo have been found to turn completely austenitic to the core of the plate.
(2) When the Ni content of the alloys is 2% or less, the nitrogen-bearing austenite decomposes into martensite on cooling to room temperature, while one bearing 3% or more of Ni is retained as austenite without being decomposed by water quenching from high temperatures. However, some part of the austenite of the alloy containing 4% Ni and free from Mo decomposes into martensite on subzero-cooling in the liquid-nitrogen bath.Nitrogen-bearing austenite containing 1% Mo and 4% Ni or more of 5% Ni is not decomposed into martensite even by the cooling to the temperature of liquid-nitrogen.
(3) When the alloys containing 4% or more of Ni are rolled at room teperature, the nitrogen-bearing austenite is considerably hardened due to the martensitic transformation of a part of the austenite, while in the alloys containing 5 to 6% Ni and 1 to 3%Mo, the nitrogen-bearing austenite is not decomposed into martensite even by the 30% rolling at room temperature.
(4) Tensile properties of the nitrogen-absorbed alloys containing 4% or more of Ni and up to 3% Mo are δB 95-84kg/mm₂, δs 68-61kg/mm₂, δ45-68%.To make the alloys stronger, it is adequate to cold roll the nitrogen-absorbed alloys by 30% at room temperature, the final tensile-properties of them being δB 115-135 kg/mm₂, δs 113-128 kg/mm₂ and 5 21-40%.
(5) Corrosive resistivities of the alloys to the aqueous boiling solution of 5% H₂SO₄ or 1% HCl are improved markedly by the nitrogen-absorption treatment; especially those alloys are characterized by the strong resistivity against HCI solution.The corrosive resistivity to boiling HC1 solution of the nitrogen-absorbed alloys containing 1% Mo and 5% Ni is conspicuously superior to that of SUS 27 stainless steel water-quenched from 1050°C after held at this temperature for 30 minutes.