On the strength-differential phenomenon in hardened steel
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r e c e n t l y b e e n e s t a b l i s h e d ~ that m a r t e n s i t i c s t e e l s a r e s i g n i f i c a n t l y s t r o n g e r in u n i a x i a l c o m p r e s sion than in uniaxial t e n s i o n , and the p h e n o m e n o n has now b e c o m e known as the s t r e n g t h - d i f f e r e n t i a l effect, or m o r e s i m p l y the S-D effect. P r e v i o u s i n d i c a t i o n of this a n o m a l o u s b e h a v i o r s e e m s to have been i g n o r e d as a s h o r t c o m i n g of the c o m p r e s s i v e test. H o w e v e r , the evidence for the S-D effect is now quite c o n v i n c i n g , ~ and so the p h e n o m e n o n m u s t be e x a m i n e d s e r i o u s l y in the light of its t h e o r e t i c a l and p r a c t i c a l i m p o r t a n c e . This p a p e r p r e s e n t s an e x p l a n a t i o n for at l e a s t p a r t of the s t r e n g t h d i f f e r e n t i a l . THE S T R E N G T H - D I F F E R E N T I A L PHENOMENON Thus f a r , the S-D effect has b e e n found p r i m a r i l y in m a r t e n s i t i c s t e e l s , whether a s - h a r d e n e d or t e m pered.1 The s t r e n g t h d i f f e r e n c e b e t w e e n the c o m p r e s sive and t e n s i l e flow s t r e s s , at a given p l a s t i c s t r a i n , d e c r e a s e s with i n c r e a s i n g t e m p e r i n g t e m p e r a t u r e , but does not v a n i s h until a r e l a t i v e l y high t e m p e r i n g t e m p e r a t u r e (>650~ is reached. The m a g n i t u d e of this s t r e n g t h d i f f e r e n c e i n c r e a s e s with the c a r b o n c o n t e n t a c c o r d i n g to the data of L e s l i e and Sober f o r AISI 4300 s t e e l s in the b r i n e - q u e n c h e d condition: ao.2(compression) = [95 + 540(wt pat C)] ksi
[1]
~o.2(tension) = [95 + 350(wt pat C)] ksi
[2]
A(TS_D = 190(wt pct C) ksi
[3]
According to Eq. [3], the S-D i n c r e m e n t is about 57 ksi for a 0.3 wt pet C s t e e l , which is an i n c r e a s e of n e a r l y 30 pct* over the t e n s i l e yield s t r e s s . Other *Morerecently,G. C. Rauchand W.C. Leslie(privatecommunication)have encountereduncertaintiesin the determinationof the magnitudeof the S-Deffect in asqtuenchedmartensites.Thesevariationsprobablystemfromspecimen-size effects in heat treatmentand testing.It is likelythat the tree strengthdifferentials are-~ th ~ those reportedby Leslieand Sober.l t y p i c a l i n c r e m e n t s a r e about 10 pct. 2 By the s a m e token, the s t r e n g t h d i f f e r e n t i a l b e c o m e s n i l as the c a r b o n content a p p r o a c h e s z e r o . In fact, no S-D effect has b e e n detected in a s - q u e n c h e d f e r r i t e s (maxi m u m c a r b o n = 0.025 wt pct). 3 J. P. HIRTH is associated with the MetallurgicalEngineeringDepartment, Ohio State University, Columbus, Ohio. MORRIS COHEN is associated with the Department of Metallurgyand Materials Science, Massachusetts Institute of Technology, Cambridge, Mass. Manuscript submitted August 5, 1969. METALLURGICALTRANSACTIONS
The S-D p h e n o m e n o n is
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