Effect of hydrogen on fracture of U-notched bend specimens of quenched and tempered AISI 4340 steel
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IN an effort t o elucidate the role of hydrogen in degrading the properties of steel, blunt U-notch bend tests have been performed on a low strength, spheroidized AISI 1095 steel.~,2 The tests were made a f t e r precharging or dynamically charging specimens electrolytically with hydrogen. The results indicated m a r k edly enhanced degradation of mechanical properties compared t o other testing methods. The r e a s o n for this enhancement in the precharged case was the l a r g e plastic s t r a i n gradient at the surface under planestrain conditions, with the presence of hydrogen c a u s ing the onset of plastic instability at reduced strains, in turn producing void formation and ultimate mode II fracture along characteristic slip t r a c e s . Under dyn a m i c charging conditions, mode I c r a c k s f o r m e d at even lower strains. In both c a s e s cracking initiated at the surface and hence was unaffected by the int e r n a l s t r e s s concentrations produced by the notches of varying radius of curvature. The present studies were undertaken t o determine whether hydrogen would have s i m i l a r effects for a high strength s t e e l tested in the same manner. AISI 4340 was selected because it is a commonly used high strength, low alloy s t e e l and because it has been extensively studied, b o t h for hydrogen embrittlement s'el and for notch-effects in the absence of hydrogen. 22"e4 Most of these studies involved tensile tests of smooth or notched round or sheet specimens. In the case of smooth specimens hydrogen was found t o have little effect on yield strength, t o r e d u c e ductility (but not to zero R ~ . or pct elongation), t o r e d u c e fracture strength, and t o cause delayed failure (static fatigue). 4-%9'~°'12'~s,~7 In all c a s e s the degradation of properties was g r e a t e r with increased strength l e v e l of the s t e e l and with increased amounts of hydrogen in the steel. Notch tensile tests revealed a d e c r e a s e in fracture s t r e s s with increasing notch curvature for sharp notches, a reduction in ductility, and a g a i n a T. D. LEE, formerly with the Ohio State University is now Metallurgist, KoreanInstituteof Science and Technology, Seoul, Korea, T. GOLDENBERG, formerly withthe Ohio State University, is now Metallurgist, Bell Telephone Laboratory, Allentown, PA and J. P. HIRTH is Professor, Ohio State University, Columbus, OH43210. Manuscript submittedOctober 11, 1978. METALLURGICAL TRANSACTIONS A
g r e a t e r degradation the h i g h e r the strength level. 7"°,21 Significantly, with respect t o the present work, duetility as measured by R.A. was not reduced t o zero even upon testing at - 196°C unless the charging was so s e v e r e as t o produce blistering. 9 Measurements 11'~4-~9 of c r a c k growth velocity v s K I , the s t r e s s intensity factor, showed that the threshold Kth dec r e a s e d with increasing hydrogen potential and strength l e v e l of the steel. These tests mainly involved the electrolytic introduction of hydrogen but in twox4,~5 hydrogen w
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