The effect of austenitizing temperature upon the microstructure and mechanical properties of experimental Fe/Cr/C steels

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r e c e n t y e a r s it has b e e n r e p o r t e d 1-13 that the f r a c t u r e toughness, Klc, of low alloy high s t r e n g t h s t e e l s may be i n c r e a s e d by as much as a factor of two with no loss in s t r e n g t h , by a u s t e n i t i z i n g at t e m p e r a t u r e s g r e a t e r than 1100~ p r i o r to quenching. The advantage of the i n c r e a s e d toughness is to i n c r e a s e the c r i t i c a l flaw size (that causes u n s t a b l e c r a c k propagation) by a s i m p l e change in heat t r e a t m e n t . Although a c o r r e sponding i n c r e a s e in Charpy impact e n e r g y might be expected, r e s u l t s for 4340,14,15 F e / C r / C , 9 En25, ~6 300M, 1~ and a low carbon C r - N i - M n 17 s t e e l have shown e i t h e r no i n c r e a s e or a d e c r e a s e in i m p a c t e n e r g y , or an i n c r e a s e in t r a n s i t i o n t e m p e r a t u r e ~7 a f t e r a u s t e n i tizing at high t e m p e r a t u r e s in both the a s - q u e n c h e d and quenched and t e m p e r e d (.. D

2 u_ 2 o

-5_ u

900

I I000 Aus~enilizinq

I I I]O0 1200 Temperoture, 6C

I 1500

Fig. 17-Plot of critical strain to failure, e/-, vs austenitizing temperature for the 0.3 pct C and 0.2 pct Ti alloys.

d a t a obtained in t o u g h n e s s t e s t s is e q u a l to a%41 o r f o l lows the s a m e t r e n d s a s the p l a n e s t r a i n d u c t i l i t y 1%41 o r the s t r a i n a t f a i l u r e f o r the o u t e r f i b e r of a bend s p e c i m e n 39 Thus, t h e r e a r e two i m p o r t a n t p a r a m e t e r s to be cons i d e r e d in e x p l a i n i n g the v a r i a t i o n of m e c h a n i c a l p r o p e r t i e s with a u s t e n i t i z i n g t e m p e r a t u r e , viz, notch r o o t r a d i u s and f a i l u r e s t r e s s / s t r a i n . F o r a m a r t e n s i t i c s t r u c t u r a l s t e e l if t h e r e a r e no s i g n i f i c a n t v a r i a tions in s u b s t r u c t u r e , amount of a u t o t e m p e r i n g o r amount of r e t a i n e d a u s t e n i t e then the f a i l u r e s t r e s s / s t r a i n can be r e l a t e d to the v o l u m e f r a c t i o n of und i s s o l v e d c o a r s e p a r t i c l e s and the lath, p a c k e t and g r a i n s i z e s . In the p r e s e n t i n v e s t i g a t i o n , no changes in a v e r a g e lath widths was o b s e r v e d in any of the a l l o y s t e e l s i n v e s t i g a t e d when the a u s t e n i t i z i n g t e m p e r a t u r e is i n c r e a s e d f r o m 870 to 1200~ (Table I). It was r e p o r t e d 17,42 that the d e f o r m a t i o n b e h a v i o r of lath m a r t e n s i t i c s t e e l s is highly s e n s i t i v e to the l a t h width and p a c k e t d i a m e t e r . A c c u m u l a t e d e v i d e n c e s u g g e s t s 42,4a that a d e c r e a s e in e i t h e r of t h e s e d i m e n s i o n s i n c r e a s e s the f a i l u r e s t r e s s / s t r a i n . S i m i l a r l y , the f a i l u r e s t r e s s / s t r a i n can a l s o be i n c r e a s e d by r e m o v ing the s t r e s s /

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