Temper embrittlement in a 3-pct Cr-Mo turbine disc steel
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S e p t e m b e r 1969 an L P r o t o r in one of the 82 MW t u r b i n e s at Hinkley P o i n t " A " Power Station failed catastrophically during routine overspeed testing. T h e r e s u l t i n g investigation, which has b e e n d e s c r i b e d e l s e w h e r e , 1,~ showed the c a u s e of f a i l u r e to be the d i s i n t e g r a t i o n of two of the s h r i n k - f i t t e d t u r b i n e d i s c s by b r i t t l e f r a c t u r e s o r i g i n a t i n g in the keyways i n the d i s c b o r e s , followed by d e t a c h m e n t of a t h i r d d i s c and the d i s r u p t i o n of the t u r b i n e c a s i n g and c o n d e n s e r . The b r i t t l e f r a c t u r e s e m a n a t i n g f r o m the keyways w e r e i n i t i a t e d by p r e e x i s t i n g , i n t e r g r a n u l a r , s t r e s s c o r r o s i o n c r a c k s f o r m e d by e x p o s u r e of m a t e r i a l s u b j e c t to high t e n s i l e s h r i n k a g e s t r e s s e s to wet s t e a m in s e r v i c e . T h e L P d i s c forgings, which w e r e of 3 pct C r - M o s t e e l p r o d u c e d by the acid open h e a r t h (AOH) and b a s i c e l e c t r i c a r c (BEA) p r o c e s s e s , were slowly cooled a f t e r t e m p e r i n g to avoid high r e s i d u a l s t r e s s e s in the t h i c k s e c t i o n s . However, this slow cooling r e s u l t e d in a t e m p e r e m b r i t t l e d condition, the d e g r e e of e m b r i t t l e m e n t b e i n g most s e v e r e in the AOH m a t e r i a l f r o m which all of the failed d i s c s were m a d e . T e m p e r embrittlement in critical components such as L P t u r b i n e d i s c s is i m p o r t a n t f r o m two s t a n d p o i n t s : t h e r e is evidence that the f r a c t u r e t o u g h n e s s is r e d u c e d and the s u s c e p t i b i l i t y to s t r e s s c o r r o s i o n c r a c k i n g is i n c r e a s e d . 2 T h e r e f o r e , as well as r e d u c ing the c r i t i c a l defect size for fast f r a c t u r e , t e m p e r e m b r i t t l e m e n t a l s o i n c r e a s e s the p r o b a b i l i t y of f o r m ing such a defect in s e r v i c e , t h e r e b y e n h a n c i n g the r i s k of f a i l u r e . A s u r v e y of the l i t e r a t u r e r e v e a l e d little i n f o r m a t i o n on the k i n e t i c s of t e m p e r e m b r i t t l e m e n t in 3 pct Cr-Mo s t e e l . C o n s e q u e n t l y the p r e s e n t work sought to e x a m i n e the k i n e t i c s of e m b r i t t l e m e n t u n d e r i s o t h e r m a l and continuous cooling conditions and to explore the f e a s i b i l i t y of r e h a b i l i t a t ing e m b r i t t l e d d i s c s for f u r t h e r s e r v i c e in a tougher condition by the use of r e t e m p e r i n g t r e a t m e n t s .
B. L. KING and G. WIGMOREare Research Officers, Central Electricity Generating Board, U. K. B. L. King is at Materials Division, Central Electricity Research Laboratories, Leatherhead, Surrey, England, while G. Wigmoreis at South Western Region Scientific Services Department, Bristol,
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