Occasionally in the application of the austenitic chromium-nickel steels to corrosive conditions, failures have occurred by cracking without serious general over-all attack of the metal. As pointed out by Hoyt and Scheil(1), and by Scheil, et al.(2) as well as by Hodge and Miller(3), the stress-corrosion failures that have occurred have been limited in number, and have taken place only when the steels were exposed to certain corrodents. These investigators have stated that while stress-corrosion cracking can be intergranular in nature and originate at the grain boundaries of the austenitic chromium-nickel steels, it can also take place in transgranular fashion. They have shown that the cracking may be either initiated at the grain boundaries and may propagate along grain boundaries for some distance and then suddenly extend across grains, or it may begin in a transgranular fashion and suddenly proceed along grain boundaries until the cracking stops. Their data further show that when austenitic stainless steel is subject to intergranular attack, stress will concentrate and cause cracking in service.
The influence of columbium on the impact properties of 18-8 steels has been studied in steels containing 8 to 16 per cent nickel and 0 to 3 per cent columbium. The addition of more than 2 per cent columbium decreases the toughness of the steel in the annealed condition due to the formation of M6C carbide and the compound Fe3Cb2. Long heating at 650 and 870 C. (1200 and 1600 F.) has no significant influence on impact toughness of steels containing up to 2 per cent columbium, but it is more detrimental to higher columbium steels. Positive X-ray identification of sigma phase was obtained after long periods at 650 C. (1200 F.) over the range 8 to 16 per cent nickel and 0.75 to 3 per cent columbium. However, the steels do not form large amounts of sigma even though strained at elevated temperatures. Cold working, followed by low-temperature annealing, accelerates the rate of precipitation of sigma phase and favors particle growth, whereas, high-temperature annealing inhibits precipitation and particle growth. The precipitation of sigma phase in a critical particle size is believed to have a beneficial effect on creep strength.
SYNOPSISThe demand in recent years for lightweight high-strength structures, especially in the transportation industry, has created considerable interest in the corrosion-resisting chromium-nickel and chromium-manganese-nickel steels. These steels are austenitic in character, and their strength can be most economically increased by application of cold-work, which, together with composition, determines their ultimate mechanical properties. Data have been published on the cold-rolled chromium-nickel steels, but practically no information is available on the austenitic type of chromium-manganese-nickel steels. The present paper gives data on these latter steels. The effects of various degrees •of cold-work on steels of different compositions are presented in •order to describe those steels that have high strength associated with good ductility, which is important from the standpoint of fabrication of the sections required for lightweight high-strength construction.The paper gives information on the stress-strain properties of the steels in both the longitudinal and transverse directions to rolling and shows the improvement obtained in these properties by application of the low temperature (200° to 300 °C.) stressrelieving heat-treatment. It further shows that the 17 per cent chromium-7 per cent nickel steels, and the 17 per cent chromium-5.50 per cent manganese-4.50 per cent nickel steels have better tension and compression properties longitudinal to the direction of rolling than do the 18 per cent chromium-8 per cent nickel steels, particularly when the steels are cold-rolled to a tensile strength exceeding about 150,000 lbs. per sq.in. All these steels have better compressive properties transverse to the direction of rolling than longitudinal to the direction of rolling, but this difference is less marked in the 17 per cent chromium-7 per cent nickel steels and the 17 per cent chromium-5.50 per cent manganese-4.50 per cent nickel steels than in the 18 per cent chro^-mium-8 per cent nickel steels. An attempt has been made to present the data on the steels so they will be of greatest value to the designer of lightweight high-strength structures.
Comparative intergranular corrosion tests have been made on five types of 18-8 steel containing 0.06 and 0.03 per cent maximum carbon, including the molybdenum and molybdenum-columbium variations, to gain a better understanding of their corrosion behavior. The data show that the increased corrosion rate noted in boiling 65 per cent nitric acid must be ascribed to a phase formation other than chromium carbide precipitation when molybdenum is present in steels containing about 0.03 per cent carbon or sufficient columbium to combine with all the carbon. A study of steels containing 0.01 per cent maximum carbon indicates that the phase is sigma. The phase has been found to be most damaging to corrosion during its formative period when it exists as an envelope of fine particles around the austenite grains. Although the steel in this condition is subject to intergranular attack by strong nitric acid, it is resistant to such media as boiling acidified copper sulfate, mixed nitric-hydrofluoric acids, and air-free sulfuric acid. Prolonged heating at 750 C., which coalesces the particles, restores the resistance of the steel to intergranular attack in nitric acid. Of the two causes of intergranular corrosion noted in the molybdenum-bearing 18-8 steels, the precipitation of chromium carbide appears to be the more dangerous. It has produced susceptibility to intergranular attack in both reducing acids and oxidizing acids, whereas the effect of the intermetallic compound has been observed only in nitric acid. The nitric acid test reveals both types of intergranular attack. To distinguish between these two causes in assessing the molybdenum steels for service, the nitric acid test should be supplemented with at least one other intergranular corrosion test to insure that a high corrosion rate in nitric acid does not prejudice the use of the steels in reducing acid media, if the attack is due to the formation of the sigma phase.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.