Авіаційно-космічна техніка та технологія (Aug 2020)
MAGNETOMETRIC METHOD FOR INVESTIGATION THE EFFECT OF CARBON AND NITROGEN ON THE CORROSION RESISTANCE OF AUSTENITIC CHROMIUM-NICKEL STEELS
Abstract
Extensive investigations of the corrosion resistance of austenitic chromium-nickel steels (with stable and unstable austenite) do not reveal the full nature of the phenomena studied. By the magnetometric method, using the specific magnetic susceptibility c0 of austenite and low content Рd d-ferrite (0…0.5 %), the corrosion properties of steels were studied. These parameters are determined by the chemical composition of steel, heat treatment and deformation. The pitting corrosion rate correlates with these parameters. In the scientific literature, the results of studies of the corrosion resistance of metals with variation over a wide range of carbon and nitrogen contents are given. In this work, the influence of these elements in five swimming trunks of each steel grade AISI 304 and AISI 321 is investigated. In steels AISI 304 and AISI 321, with an increase in c0 and Рd (under the condition low content of d-ferrite), the pitting corrosion rate К decreases. The values of c0 and Рd from the total content Q (wt.%) of all alloying elements for each melt are determined. The dependences of the corrosion rate K on Q for each melt of AISI 304 and AISI 321 steels are obtained. Various forms of К(Q) dependency curves are caused by a lower content of nickel, silicon and a higher content of chromium, manganese, carbon, nitrogen in AISI 304 steel. Also in AISI 321 steel, titanium is present. The dependences of К, c0 and Рd on the content of carbon and nitrogen, in the presence of all other alloying elements, are determined. Various (presence of a maximum or minimum) parabolic dependencies of К, c0 and Рd on the carbon and nitrogen contents were revealed. For melts of steel AISI 304, of the effect carbon and nitrogen on corrosion resistance are the opposite of the sum of all other alloying elements. For melts of steel AISI 321, the carbon present makes a smaller contribution to corrosion resistance compared to nitrogen. For AISI 321 steel melts, an ambiguous character of the dependence of the pitting corrosion rate K on the total content Q of all other alloying elements was found. Two values of Q (a parabolic dependence of K(Q)) can correspond to one value of K. The critical points (the content of alloying elements), before and after which the parameters c0, Pd can increase or decrease, are determined. As a result, the paramagnetic state of austenite changes, which correlates with a change in the corrosion properties of steel.
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