Microstructural effects on fatigue crack growth in a low carbon steel
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T h e c o n n e c t i v i t y o f t h e m a r t e n s i t i c s t r u c t u r e is def i n e d by t h e p a r a m e t e r ~, w h e r e ~) = N g / N g + N b , a n d N g i s t h e a v e r a g e n u m b e r of i n t e r s e c t i o n s w i t h b o u n d a r i e s of t h e m a r t e n s i t i c s t r u c t u r e p e r u n i t o f l e n g t h , m m -1, a n d N b i s the a v e r a g e n u m b e r o f i n t e r s e c t i o n s w i t h the g r a i n b o u n d a r i e s of t h e f e r r i t e p e r u n i t l e n g t h ( e x c l u d i n g t h e l e n g t h s h a r e d by t h e m a r t e n s i t i c s t r u c t u r e ) , m m -~. In T a b l e I I i t i s s e e n that t h e p r i n c i p a l d i f f e r e n c e b e t w e e n t h e s e two m i c r o s t r u c t u r e s is t h e d i s t r i b u t i o n o f p h a s e s . I n a l l o t h e r r e s p e c t s they a r e q u i t e s i m i l a r . T h e tensile properties, however, r e f l e c t t h i s d i f f e r e n c e in m o r p h o l o g y as s h o w n in Fig. 3. H e a t t r e a t m e n t B r e s u l t s in h i g h e r s t r e n g t h but w i t h l o s s i n d u c t i l i t y c o m p a r e d t o h e a t t r e a t m e n t A . A t a b u l a t i o n o f the m e c h a n i c a l p r o p e r t i e s f o l l o w s . T h e fatigue t e s t s were c a r r i e d out in a n I n s t r o n (CGS) c l o s e d - l o o p s e r v o - h y d r a u l i c t e s t i n g m a c h i n e u n d e r l o a d c o n t r o l . T h e f r e q u e n c y of t e s t i n g w a s b e -
MATERIAL, SPECIMENS AND T E S T S
Step 1-Homogenize at 1473K for one h , furnace cool t o room temperature. Step 2-Reheat t o 1223K for five h t o obtain a desired austenite grain size. Step 3-Furnace cool from 1223t o 1033K, a temperature between theA3 and A1 temperatures, and hold for one hour t o obtain a two-phase mixture of ferrite and austenite. Step 4-Quench in ice water t o transform the austenite to martensite. Step 5-Temper at 473K for one h t o relieve internal stresses due t o quenching.
T a b l e I. Heat T r e a t i n g Procedures
Heat Treatment A T h e m a t e r i a l c h o s e n f o r t h i s i n v e s t i g a t i o n w a s a 13 m m t h i c k p l a t e o f A I S I - S A E 1018 s t e e l , t h e n o m i n a l c o m p o s i t i o n of w h i c h i s C : 0.15 t o 0 . 2 0 , M n : 0.60 t o 0 . 9 0 , P : 0 ; 0 4 0 m a x , S: 0 . 0 5 0 m a x . T h e s p e c i m e n s f o r the f a t i g u e c r a c k g r o w t h t e s t s w e r e o f the c o m p a c t t e n s i o n t y p e , w i t h t h e t o t a l h e i g h t e q u a l t o 68.6 m m , t h e e f f e c t i v e w i d t h e q u a l t o 57.2 m m , a n d t h e t h i c k n e s s e q u a l t o 6.3 m m . T h e s t r e s s intensity factor a s a function o f l o a d a n d c r a c k l e n g t h f o r t h i s s p e c i m e n w a s c a l c u l a t e d in a c c o r d w i t h t h e e x p r e s s i o n g i v e n in A S T M specification E399. I n o r d e r t o d e v e l o p the d e s i r e d d u p l e x m i c r o s t r u c t u r e s t h e h e a t t r e a t i n g p r o c e d u r e s g i v e n i
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