Formation of Fe 3 N, Fe 4 N and Fe 16 N 2 on the surface of iron
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EXPERIMENTAL
PROCEDURE
The starting material used in this study was electrolytic i r o n (99.9 pet) m e l t e d in vacuo; c h e m i c a l c o m p o s i t i o n is shown in Table I. P r i o r to the s t r a i n a n n e a l i n g t r e a t m e n t , cold r o l l e d s p e c i m e n s w e r e p r e a n h e a l e d in d r y hydrogen gas at 800~ for 5 h and quenched into ice w a t e r to obtain homogeneous, fine Y. 1NOKUTIand N. NISHIDAare Senior Research Associates, and N. OHASHI is Manager of Sheet Section, Research Laboratories, Kawasaki Steel Corporation, Chiba 280, Japan. Y. Inokuti is a Visiting Research Fellow at the Applied Sciences Laboratories, University of Sussex, Falmer, Brighton, BN1 9QT, U.K. Manuscript submitted April 16, 1974. METALLURGICAL TRANSACTIONS A
Table I. Chemical Composition of Vacuum Melted Iron, Wt Pct
C
Si
Mn
P
S
A1
N
O
Pure Iron 0.002 0.008 0.002 0.002 0.002 0.002 0.0012 0.0038 (Vacuum Melt)
grained structures. After this, optimum strain (about 3 to 4 pct) which was previously determined as necessary for grain growth, was introduced by stretching the specimens at a crosshead speed of 0.2 mm/min in an Intron type tensile testing machine. Both prequenching and slow straining were necessary to obtain large grains free from very small island grains, because the occurrence of Lfiders bands which cause an inhomogeneous deformation during straining must be prohibited. The specimens were then finally annealed in dry hydrogen gas at 850~ for 5 h. The final grain sizes were 5 to 30 mm in diam. The surfaces of the specimens were mechanically polished with emery papers up to # 1200 and then were chemically polished in HF (3 pct)+ HaOa solution, rinsed in water and alcohol and dried by hot air. Nitriding treatment of the polished specimens was performed with the annealing cycle in Fig. 1. The surfaces of specimens were cleaned in flowing pure dry hydrogen gas for 2 h after heating up to each annealing temperature between 350 and 700~ and then nitrided in flowing H~ +NII3 (10 to 90 pct) gases for 5 to 40 rain. Subsequently the specimens were quenched to room temperature. The structures of nitrides formed on the surfaces of the specimens were determined from X-ray diffraction. The morphologies of nitrides were observed through an optical, differential interference microscope and scanning electron microscope (JSM-U3 type). Change in the nitride morphologies due to vacuum reannealing after nitriding was also observed through high-temperature optical microscope at a heating rate of 60~ in a vacuum of 2 ~,~. !. " ,.:~,.
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e n t a t i o n among (321)~ d i r e c t i o n s on the i r o n s u r f a c e , and to f o r m a " n e e d l e = l l k e " morphology. The t h r e e kinds of p r e c i p i t a t
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