Jumps in the creep curve of austenitic stainless steels
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TITO LUIZ DA SILVELRA AND SERGIO NEVES MONTEIRO
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P e r t u r b a t i o n s in the c r e e p c u r v e s of m e t a l s and a l l o y s have been r e p o r t e d as e a r l y as 1910.1 Working with copper w i r e s Andrade 1 noted that " . . . it showed sudden s l i p s at i r r e g u l a r i n t e r v a l s , which, p e r h a p s , may be c a l l e d copper quakes as being analogous to the g e o l o g i c a l slipping supposed to r e s u l t in e a r t h q u a k e s " F u r t h e r m o r e he mentioned that " T h e s e were not due to the r e c o r d i n g a p p a r a t u s " . F i g u r e 1 is a h i s t o r i c a l i l l u s t r a t i o n of what was probably the f i r s t r e c o r d of this phenomenon. Lubahn and F e l g a r 2 a l s o r e p o r t e d sudden extensions in c r e e p t e s t s of aluminum alloy in the t e m p e r a t u r e range where the s t r e s s - s t r a i n curve is s e r r a t e d . They noted that an e x t r e m e example of these sudden extensions was encountered in p a r t i a l l y aged m a t e r i a l s . M o r e o v e r , they suggested that this was a " y i e l d p o i n t " type of d e f o r m a t i o n r a t h e r than a g r a d u a l type of d e f o r m a t i o n . Recently, Sikka e t al, 3 r e p o r t e d s t r a i n b u r s t s in c r e e p s p e c i m e n s of Type 316, c a s t s t a i n l e s s s t e e l . The c r e e p c u r v e s at 538, 593, and 649~ showed instantaneous i n c r e a s e s in s t r a i n , as many as 3 p e r curve, and as l a r g e as 1.7 pct in the amount of deformation introduced p e r burst. The explanation given to the s t r a i n b u r s t s by Sikka e t al 3 r e l a t e s to the d i f f e r e n t i a l concentration of c r e e p strengthening e l e m e n t s in i n t e r d e n d r i t i c r e g i o n s as c o m p a r e d to dendrite a r m s . T h e i r suggestion is that the extent and distribution of these low and high conc e n t r a t i o n regions c a u s e s the o b s e r v e d phenomenon. In r e c e n t studies of c r e e p in austenitie s t a i n l e s s s t e e l s , we have a l s o o b s e r v e d sudden extension in the deformation vs time curve which we t e r m e d " j u m p s " . 4 However in our ease the s t e e l w i r e s were r e c e i v e d in as r o i l e d condition which c o n t r a d i c t s Sikka e t al 3 r e m a r k that a s i m i l a r phenomenon had not been obs e r v e d in wrought m a t e r i a l , being t y p i c a l of e a s t m a t e r i a l only. This communication p r e s e n t s our p r e l i m i n a r y r e suits on the " j u m p s " in the c r e e p curve of r o l l e d w i r e s of s e v e r a l austenitic s t a i n l e s s s t e e l s . One batch of type 304 and two batches of Type 316 s t a i n l e s s s t e e l were c r e e p t e s t e d in the range 550 to 800~ The c h e m i c a l a n a l y s e s of the different s t e e l used a r e given in Table I. Following a c o m m e r c i a l annealing, c r e e p t e s t s were c a r r i e d out at constant load on a WPM machine with continuous r
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