In the 1st report (Tetsu to Hagané, 39, Nov. 1953, No. 11, p. 1266-1270) we described that 27Cr-5Ni-1Mo-1Cu stainless steel had good acid resistance when heat treated at the temperature range from 700° to 1100°C and that content of 10% Ni in such steels helped the formation of sigma phase, so woresened the acid resistance.
Here, we researched on the effect of decreasing the Cr content on the acid resistance in such high-Cr steels containing 5%Ni, 1%Mo & 1%Cu by the same experimental method as in the lst report. At the same time, research was made on the effect of heat treatment on the acid resistance of several austenite stainless steels which were made in Japan and foreign countries.
25Cr-5Ni-1Mo-1Cu steel had a tolerable good acid resistance when heat-treated at the temperature range from 700° to 900°C, but at 1000° and 1100°C the acid resistance worenened presenting a large γ-phase accompanying a grain growth.
In 23Cr- & 20Cr-5Ni-1Mo-1Cu steel, the acid resistance woresened initially and more at the carbide-precipitating temperature, that is, at 700° and 800°C; but at 900°, 1000°C and 1100°C the acid resistance was better by re-dissolution of carbide. Accordingly, it was presumed that lower limit of Cr content on such high-Cr steels was 25% as far as the acid resistivity is concerned.
When these steels were heated for 200 hours at 800°C, even 25Cr steel formed no sigma phase but the aggregate of α and sigma. On account of this phenomena, the acid resistance worsened, but the weight loss by corrosion showed less than 20g/m
2/hr which 27Cr steel showed in the same condition. 23Cr and 20Cr steel did not form sigma or aggregate, thus the hardness and weight loss by corrosion did not vary.
The acid resistance of austenite stainless steels containing both Cu and Mo showed nearly no change by heat treatment. The weight loss by corrosion of the steels which did not contain both Mo & Cu or which contained Mo alone increased remarkably when heat-treated at the carbide-precipitation temperature.
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