The effect of step quenching on the microstructure and fracture toughness of aisi 4340 steel
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I ~ - E C E N T s t u d i e s by m a n y r e s e a r c h e r s have shown that by u s i n g a 1473 K a u s t e n i t i z i n g t e m p e r a t u r e , a m a r k e d e n h a n c e m e n t in f r a c t u r e t o u g h n e s s can be obtained for s e v e r a l high s t r e n g t h , low alloy s t e e l s in the as q u e n c h e d or quenched and t e m p e r e d c o n d i t i o n s . 1"4 T h i s e n h a n c e m e n t has b e e n o b s e r v e d without arty app r e c i a b l e l o s s of s t r e n g t h and has b e e n a t t r i b u t e d to the p r e s e n c e of thin f i l m s of r e t a i n e d a u s t e n i t e b e t w e e n m a r t e n s i t e laths, s e g r e g a t i o n effects, d i s s o l u tion of c a r b i d e s , and so forth. C u r s o r y s t a t e m e n t s have also b e e n made about other f a c t o r s such as a m o u n t of t w i n n i n g and m a r t e n s i t e lath and packet size which may a l s o play i m p o r t a n t r o l e s in d e t e r m i n i n g the f r a c t u r e t o u g h n e s s p r o p e r t i e s of high s t r e n g t h s t e e l s . The p u r p o s e of this study has b e e n to e x a m i n e the r o l e of twins in g o v e r n i n g the f r a c t u r e p r o p e r t i e s of these a l l o y s .
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The p r e s e n t i n v e s t i g a t i o n used a i r c r a f t q u a l i t y AISI 4340 s t e e l r e c e i v e d as fully a n n e a l e d r o l l e d plate. C h e m i c a l c o m p o s i t i o n of the above s t e e l is given in T a b l e I. S t a n d a r d ASTM c o m p a c t t e n s i o n s p e c i m e n s 0.625 in. (1.59 cm) thick w e r e m a c h i n e d , heat t r e a t e d and then t e s t e d in a c c o r d a n c e with ASTM s p e c i f i c a t i o n E 3 9 9 - 7 2 . F a t i g u e p r e c r a c k i n g was done on a closed loop I n s t r o n t e s t s y s t e m at 6 Hz. F r a c t u r e toughness t e s t i n g was c a r r i e d out at a c r o s s - h e a d speed of 0.04 i n . / m i n (0.i c m / m i n ) . M i c r o s t r u c t u r a l a n a l y s i s c o n s i s t e d of optical, t r a n s m i s s i o n , and s c a n n i n g e l e c t r o n m i c r o s c o p y . E x t e n s i v e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y was e m p l o y e d to c h a r a c t e r i z e and identify the p r e s e n c e of r e t a i n e d a u s t e n i t e , twins, " b l o c k y m a r t e n s i t e " , arid so forth. T h i n foils for T E M work w e r e taken f r o m f r a c t u r e d Klc s p e c i m e n s . Sections 0.015 to 0.020 in. (0.4 to 0.5 mm) thick w e r e cut u s i n g a 0.030 in. (0.76 mm) thick s i l i c o n c a r b i d e a b r a s i v e wheel with the s p e c i m e n flooded with coolant to avoid heating. A s e r i e s of 120, KHALID H. KHAN and WILLIAME. WOODare Research Associate-Lecturer and Associate Professor respectively, Dept. of Materials Science, Oregon Graduate Center, Beaverton, OR 97005. Manuscript submitted September 8, 1977. METALLURGICALTRANSACTIONSA
240, 400 and 600 g r i t size s i l i c o n c a r b i d e p a p e r s w e r e used t
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