Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature
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Here a study is made of the p r o c e s s e s r e s p o n s i b l e for fatigue c r a c k propagation in two austenitic s t a i n l e s s s t e e l s , a 20 pct C r / 2 5 pet Ni/Nb s t a b i l i z e d and a type 316 s t a i n l e s s s t e e l , at e l e v a t e d t e m p e r a t u r e by o b s e r v a t i o n s of the c r a c k growth c h a r a c t e r i s t i c s of a r t i f i c i a l l y - i n d u c e d notches. Fatigue c r a c k growth studies of the room t e m p e r a t u r e behavior of the 20/ 25/Nb s t e e l is included for c o m p a r i s o n between a m bient and elevated t e m p e r a t u r e b e h a v i o r . By r e f e r ence to p r e v i o u s l y published 9 and r e c e n t l y obtained fatigue life data for smooth unnotched s p e c i m e n s , t e s t e d under identical conditions to the c r a c k growth studies, an a t t e m p t is made to r a t i o n a l i z e the r o l e s of c r a c k initiation and propagation in the fatigue p r o c e s s , and indicate the r e l e v a n c e of c r a c k propagation data in p r e d i c t i n g fatigue life data. 1. MATERIALS AND TESTING PROCEDURE Specimens were machined f r o m b a r stock of 2 0 / 2 5 / Nb s t a i n l e s s s t e e l and type 316 s t a i n l e s s s t e e l . The compositions of these a r e l i s t e d in Table I. Two types of s p e c i m e n were employed. F o r the total endurance t e s t s , s p e c i m e n s had a 6.35 mm diam p a r a l l e l gage length, whilst for c r a c k propagation studies s p e c i m e n s with a 5.0 mm s q u a r e section gage length w e r e utilized. P r i o r to testing, the 20/25/Nb s t a i n l e s s s t e e l s p e c i mens were heat t r e a t e d in vacuum for 1 h at 1000~ and, for the endurance s p e c i m e n s , e l e c t r o p o l i s h e d . The type 316 s t a i n l e s s s t e e l s p e c i m e n s were t e s t e d in the a s - r e c e i v e d condition, with finely ground, but not e l e c t r o p o l i s h e d s u r f a c e s . A s p a r k machined notch 0.025 mm wide and 0.050 deep was introduced into one face of the c r a c k propagation s p e c i m e n . The s p e c i m e n face, adjacent to this notched face, was then polished with e m e r g y p a p e r s until a highly r e f l e c t i n g surface, suitable for optical m e a s u r e m e n t of c r a c k length, was achieved. The m a j o r i t y of the t e s t s were p e r f o r m e d in a m o d i fied Instron t e s t i n g machine, ~~ a Schenck s e r v o h y d r a u l i c VOLUME 6A, JULY 1975-1367
Table I. Composition of Steels Used
Type 316 steel 20/25/Nb steel
(Wt Pct)
C
Si
Mn
S
P
Ni
Cr
Mo
Nb
0.04 0,035
0.48 0.65
1,57 0.68
0.026 0.01
0.032 0.007
12.8 25.1
17.8 20,5
2.57 -
.0,67
t e s t i n g m a c h i n e was u s e d f o r the o t h e r s . All but a few t e s t s w e r e c a r r i e d out under s t r a i n cycling, p u s h - p u l l c o n d i t i o n s . Control was b y d i a m e t r a l e x t e n s o m e t e r s , ~ t w h i l s t the load on the s p e c i m e n s was s i m u l t a n e o u s l y m o n i t o r e d . The r
