The influence of intercritical heat treatment on the temper embrittlement susceptibility of an Sb-Doped Ni-Cr steel
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In a p r e v i o u s r e p o r t ~ it was shown that the s u s ceptibility to t e m p e r e m b r i t t l e m e n t in a N i - C r s t e e l doped with 0.06 pct P could be a l m o s t e l i m i n a t e d by use of an i n t e r c r i t i c a l heat t r e a t m e n t (IHT). This t r e a t m e n t involves h e a t i n g the s t e e l to below the As t e m p e r a t u r e ( i . e . , in the ~ + y region) p r i o r to t e m p e r i n g . In the p r e v i o u s study 1 the effects of convent i o n a l heat t r e a t m e n t (CHT) involving t h r e e a u s t e n i t i z ing t e m p e r a t u r e s (1200, 1050, and 850~ to produce a r a n g e of p r i o r a u s t e n i t i c g r a i n s i z e s were c o m p a r e d with those of IHT, in which the s a m e p r i o r a u s t e n i t i z ing t r e a t m e n t s were used. (The IHT at 745~ followed the a u s t e n i t i z i n g t r e a t m e n t s . ) The CHT s a m p l e s were t e m p e r e d at 625~ for 1 h, and the IHT s a m p l e s at 560~ for 2 h, to give the s a m e h a r d n e s s (~DPH 300) for each. T e m p e r e m b r i t t l e m e n t was then c a r r i e d out by aging at 480~ for up to 3000 h. It was found that the p r i n c i p a l effect of IHT was to r e d u c e the effective g r a i n size along the f r a c t u r e path. T h i s path s t i l l lay roughly along the p r i o r a u s t e n i t i c g r a i n b o u n d a r i e s ; however, s m a l l a u s t e n i t e g r a i n s had f o r m e d in these b o u n d a r i e s d u r i n g IHT, and the f r a c t u r e o c c u r r e d along the b o u n d a r i e s of t h e s e s m a l l p r i o r a u s t e n i t e g r a i n s . T h i s r e s u l t e d in a much m o r e t o r t u o u s f r a c t u r e path and m e a n t that the i n i t i a l c r a c k s i z e was much s m a l l e r (since it was p r e s u m a b l y e q u i v a l e n t to the a r e a of one i n t e r g r a n u l a r facet). The IHT also r e d u c e d the a m o u n t of P which s e g r e g a t e d to the g r a i n b o u n d a r i e s d u r i n g aging, but this a p p e a r e d to be of s e c o n d a r y i m p o r t a n c e , c o m p a r e d with the g r a i n s i z e effect. A p a r a l l e l study was c a r r i e d out on a s i m i l a r s t e e l doped with 0.06 pct Sb. The e x p e r i m e n t a l p r o c e d u r e s were i d e n t i c a l to those d e s c r i b e d e a r l i e r , ~ except for the etching p r o c e d u r e . In the P - d o p e d s t e e l it was found that r e g i o n s e n r i c h e d with P (e.g., g r a i n bounda r i e s ) were s u s c e p t i b l e to d i s s o l u t i o n at r o o m t e m p e r a t u r e in an etchant of s a t u r a t e d aqueous p i c r i c acid c o n t a i n i n g 1 g/100 m l of s o d i u m t r i d e c y l b e n z e n e s u l fonate as a wetting agent. In the p r e s e n t Sb-doped s t e e l this etchant, when applied at r o o m t e m p e r a t u r e , did not attack S b - e n r i c h e d g r a i n b o u n d a r i e s . Howe v e r , it was found that p r e f e r e n t i a l attack coul
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