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The toughness of several vacuum-melted Fe-25Cr and Fe-18Cr-2Mo ferritic stainless steels containing between 40 and 1000 ppm combined carbon and nitrogen has been investigated in the water-quenched condition. The toughness of these alloys has been assessed using notched-bar impact tests. For a given interstitial content, the ductile-to-brittle transition temperature and the impact shelf energy were not influenced by the presence of 2 percent molybdenum or the change in chromium content from 18 to 25 percent. In order to achieve a low transition temperature and high shelf energy, the combined carbon and nitrogen (C + N) content should be maintained below 150 ppm, in which case the interstitial amount has no effect. Exceeding this threshold level results in a marked increase in transition temperature and decrease in shelf energy. At higher interstitial levels [∼600 ppm (C + N)] further increases are not as significant. Addition of nickel resulted in a decrease in transition temperature, but this was accompanied by a decrease in shelf energy when compared with plain chromium steels with the same interstitial content. The addition of titanium, however, caused an improvement in both transition temperature and impact shelf energy.
It is shown that increasing the interstitial content of these ferritic stainless steels is associated with an increase in the amount of second phase present, particularly at the grain boundaries. Such particles enhance cleavage fracture by reducing the surface energy. The tendency toward a more brittle condition with increased interstitial content and larger grain size is explained in terms of the Cottrell theory for brittle fracture.
ferritic stainless steels, molybdenum additions, titanium additions, nickel addition, interstitials, second-phase content, grain boundaries, fracture toughness, ductile-to-brittle transition temperature, Cottrell theory for brittle fracture
Professor, University of Waterloo, Waterloo, Ont.
Marketing Manager, Tubular Products, Research Center,