Nitrogen solution and titanium nitride precipitation in liquid Fe-Cr-Ni alloys
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CONTROLLING nitrogen contents at an optimum l e v e l is important for developing desirable properties in alloy steels. An appropriate l e v e l of nitrogen often shows a beneficial effect in stabilizing austenite and strengthening these steels. In other instances, nitrogen is r e g a r d e d as a harmful impurity, so that its content is maintained at a low l e v e l or it is stabilized by addition of strong nitride-forming elements such as titanium. In the latter case, precipitation of nitride or carbo-nitride in processing and heat treatment of s t e e l reduces soluble nitrogen in the m a t r i x and removes its adverse effects. Because fine nitride or carbo-nitride precipitates often improve the mechanical properties of steels, intentional additions of nitrogen and a nitride forming element or elements are sometimes employed t o obtain these precipitation effects. A considerable amount of work has been c a r r i e d out on the solubility of nitrogen in pure iron and dilute iron alloys. The published data on nitrogen solubility in dilute iron alloys have been reviewed by Pehlke and Elliott.1 Humbert and Elliott2 measured the nitrogen solubility in binary and ternary alloys of the entire F e - C r - N i system. Also, the nitrogen solubility in liquid F e - N i or F e - C r alloys has been measured by Pehlke and Elliott,1 Saito, e t al, 3 Schenck, e t al, 4 Blossey and Pehlke, 5 Wada, e t a l ,6 and Kojima, e t a l .7 T h e s e studies cover the concentration r a n g e from dilute solution to pure Ni or Cr. However, few m e a s u r e ments have been made of nitrogen solubility in multicomponent iron alloys with relatively high alloy contents.8-1° Gunji, e t a111 measured the nitrogen solubility in liquid multicomponent alloys by the quenching method in the composition r a n g e up to 25 wt pct Cr and 15 wt pct Ni. Titanium nitride precipitation in liquid pure iron has been investigated by E v a n s and Pehlke, 12 and Rao and Parlee.13 F r a g e , e t a114 proposed an equation represent-
ing the solubility product of titanium nitride as a function of titanium concentration. However, direct m e a s urements of the solubility limits for titanium nitride precipitation have not been made in liquid binary or ternary F e - N i - C r alloys. The object of the present work was to determine the effects of alloying elements and temperature on nitrogen solubility and the solubility product of titanium nit r i d e over a wide concentration r a n g e in binary or ternary F e - C r - N i alloys. Experiments were c a r r i e d out in the temperature r a n g e 1450 t o 1800°C. EXPERIMENTAL METHOD Apparatus The Sieverts' method was used for measuring nitrogen solubility and nitride formation in this workP ,8,9,1°,12 The apparatus is shown schematically in Fig. 1. Materials The purities and sources of the metallic materials used in this work are summarized in Table I. b
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HARUE WADAis Research Associate and ROBERT D. PEHLKE is Professor and Chairman, Department of Materials a
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