Measurement of the mechanical state of a low alloy steel at elevated temperature
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tion, heat t r e a t m e n t and r o o m t e m p e r a t u r e p r o p e r t i e s a r e given in Table I. This work is of p r a c t i c a l v a l u e in two r e s p e c t s : (1) If the s t r e s s - s t r a i n r a t e r e s p o n s e uniquely c h a r a c t e r i z e s the m e c h a n i c a l state, it m a y have d i r e c t b e a r i n g on p r o b l e m s a s s o c i a t e d with p r e d i c t i n g d e f o r m a t i o n r e s p o n s e u n d e r one s e t of t e s t conditions a f t e r p r i o r e x p o s u r e u n d e r d i f f e r e n t conditions. In p r a c t i c e , this t r a n s l a t e s to the p r o b l e m of p r e d i c t i n g m e c h a n i cal b e h a v i o r after p r i o r s e r v i c e e x p o s u r e . (2) If a unique s t r e s s - s t r a i n r a t e c u r v e is obtained, it should be p o s s i b l e to p r e d i c t b e h a v i o r beyond the s t r e s s r e g i m e c o v e r e d b y the data. In p a r t i c u l a r , it m a y be p o s s i b l e to p r e d i c t the r e s p o n s e at a lower temperature.
EXPERIMENTAL
PROCEDURE
All the previously reported results were obtained by measuring the load response as a function of time after stopping the machine crosshead. The inelastic strain was computed from these measurements in conjunction with the measured elasticity of the specimen and machine. Preliminary attempts to use the same technique at higher temperatures in a standard
Table I. Composition, Heat Treatment, and Mechanical Properties of Test Steel
Chemical Composition C
Mn
P
S
Si
Ni
Cr
V
Mo
0.030
0.92
0.030
0.031
0.34
0.08
1.16
0.23
1.22
Heat Treatment Austenitize: 1010~ 27 h, air cool 954~ 27 h, air cool Temper and Stress relief: 675~C, 36 h, furnace cool 620~ 36 h, furnace cool Tensile Strength Mn/m2
0.2 Pct Yield Strength Mn/rn2
Elongation Pet
Reduction of Area Pct
772
595
18.5
47.2
Manuscript submitted November 5, 1974. METALLURGICAL TRANSACTIONS A
VOLUME 6A, SEPTEMBER 1975-1693
tensile machine indicated significant discrepancies b e t w e e n the c o m p u t e d s t r a i n and that m e a s u r e d on s p e c i m e n s a f t e r t e s t . It has, in fact, b e e n shown r e c e n t l y ~~that m a c h i n e s t i f f n e s s often d o e s not r e m a i n c o n s t a n t d u r i n g a t e s t . This effect s e e m s to be a s s o c i a t e d with the r e l a t i v e m o t i o n of m o v i n g p a r t s of a m a c h i n e p a r t i c u l a r l y in the u n i v e r s a l j o i n t s , and m a y be e x a c e r b a t e d at high t e m p e r a t u r e . It was, t h e r e f o r e , c o n s i d e r e d n e c e s s a r y to r e c o r d the s p e c i m e n s t r a i n d i r e c t l y in the p r e s e n t e x p e r i m e n t s to e l i m i nate the need to a c c o u n t for m a c h i n e s t i f f n e s s . The e q u i p m e n t used was a m o d i f i e d c r e e p m a c h i n e and all t e s t i n g was in a i r . A t h r e e zone f u r n a c e d e s i g n e d for long t i m e s t a b i l i t y p r o v i d e d t e m p e r a t u r e c o n t r o l within :~0.
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