Effect of thickness and orientation on fatigue crack growth rate in 4340 steel
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t i m a t e i n t e r e s t is in fatigue c r a c k growth in heavy s e c tions, where cracks p r o g r e s s under approximately p l a n e s t r a i n c o n d i t i o n s , the effect of s t r e s s state on c r a c k growth r a t e was evaluated by t e s t i n g s p e c i m e n s of v a r i o u s t h i c k n e s s e s . MATERIALS AND PROCEDURES The b a s e m a t e r i a l was 2 in. thick c o m m e r c i a l hot r o l l e d 4340 s t e e l plate with the following c h e m i s t r y : C
P
S
Mn
Mo
Ni
Cr
0.40
0.025
0.023
0.72
0.25
1.71
0.80
All the t e s t s p e c i m e n s r e c e i v e d the following c o m mon heat t r e a t m e n t as rough m a c h i n e d b l a n k s , a p p r o x i m a t e l y 84 by 2 by -~ in.: 1550 ~
1 h r - - A u s t e n i t i z e - - w a t e r quench
l l00~
2 hr--Temper---water quench
Fig. 1 shows the t e m p e r e d m a r t e n s i t e m i c r o s t r u c t u r e t y p i c a l of all o r i e n t a t i o n s and t h i c k n e s s e s . The i n c l u s i o n s w e r e p r e d o m i n a n t l y s u l f i d e s , Fig. 2. The m e c h a n i c a l p r o p e r t i e s of the b a s e m a t e r i a l w e r e e v a l u a t e d by t e n s i l e and C h a r p y V - n o t c h t e s t s . In the l a t t e r , the n o t c h e s were o r i e n t e d i d e n t i c a l l y to those in the fatigue s p e c i m e n s . All t e s t s w e r e p e r f o r m e d at r o o m t e m p e r a t u r e . The data obtained w e r e a s follows: TS, psi CrackArrester 164,000 Crack Divider 175,000 Short Transverse 162,000
YS, psi 157,000 163,000 155,000
El, Pct
RA, Pct
Cv, ft-lb
16 14 10
48 42 25
22 24 8
To evaluate the effects of o r i e n t a t i o n , t e s t s p e c i m e n s w e r e taken in t h r e e o r i e n t a t i o n s as shown in Fig. 3. C o n s t a n t load fatigue t e s t s w e r e r u n at 30 cps on s i n g l e edge notch (SEN) s p e c i m e n s , with the s p e c i m e n d i m e n 1 i 1 s i o n s , 8 by 1.8 in. by t h i c k n e s s (t), w h e r e t - 18, 8, 4, 1 and -~ in. B e c a u s e of the l i m i t e d t h i c k n e s s of the t e s t plate, it was n e c e s s a r y to e l e c t r o n b e a m weld e x t e n s i o n t a b s onto the s h o r t t r a n s v e r s e s p e c i m e n s . T h i s was done b e f o r e heat t r e a t i n g , and did not affect the r e s u l t s . To g e n e r a t e the c r a c k growth data, two s p e c i m e n s w e r e r u n at different n o m i n a l s t r e s s l e v e l s for each t h i c k n e s s - o r i e n t a t i o n c o m b i n a t i o n , Table I. C r a c k length (a) was m o n i t o r e d with a X60 m i c r o m e t e r t e l e s c o p e a s a function of c y c l e s (N). C r a c k growth VOLUME 3, AUGUST 1972-2119
Fig. 1--Typical martensitic microstructure for all orientations and thicknesses (Nital--magnification 755 times).
Fig. 3--Relationships of orientations studied to the rolling direction and plane.
Table I.
Crack Arrester NominalStress,psi J in. ~in. 88 89
Fig. 2--Typical sulfide inclusions (as polished--magnification 780 times).
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