The high temperature transformation of high speed steel
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Low t e m p e r e d h a r d n e s s e s on high speed s t e e l s which had b e e n h a r d e n e d in vacuum or a t m o s p h e r e f u r n a c e s and gas quenched led to a s e a r c h for a r e a c t i o n o c c u r r i n g above 1400~ and probably initiated as a r e s u l t of the s l o w e r cooling rate of the gas quench. A s t a n d a r d T T T e v a l uation was c a r r i e d out on M1, M2 high carbon~ M3, and M7 high speed steels~ quenching f r o m c o n v e n t i o n a l h a r d e n i n g t e m p e r a t u r e s , into salt baths m a i n t a i n e d at 2000 ~ to 1500~ Examination of these s p e c i m e n s indicated the p r e s e n c e of a r e a c t i o n o c c u r r i n g at a l l t e m p e r a t u r e s i n v e s t i g a t e d but having its m a x i m u m r a t e at 1700 ~ to 1800~ The r e a c t i o n involved p r e c i p i tation of an MoC/Mo2C c a r b i d e at g r a i n b o u n d a r i e s and within the grains~ with the a m o u n t of p r e c i p i t a t e i n c r e a s i n g with t i m e . The p r e c i p i t a t i o n of MoC/MozC is d i r e c t l y r e l a t e d to the loss in t e m p e r e d h a r d n e s s and a p p a r e n t l y c a u s e s this effect by 1) r e d u c i n g p r e c i p i t a t i o n h a r d e n i n g on t e m p e r i n g , 2) d r a s t i c a l l y l o w e r i n g r e t a i n e d a u s t e n i t e c o n t e n t s , and 3) r e d u c i n g a s - q u e n c h e d hardnesses.
INVESTIGATIONS of the heat t r e a t m e n t of high speed s t e e l m a n y y e a r s ago showed that a i r cooling of l a r g e s e c t i o n s from the h a r d e n i n g heat r e s u l t e d in a slightly lower t e m p e r e d h a r d n e s s than oil or salt quenching the s a m e s e c t i o n s . Despite the o r i g i n a l , quite i n t e n s e s c i e n t i f i c i n t e r e s t in the heat t r e a t m e n t of these s t e e l s , this p h e n o m e n o n s e e m s to have b e e n touched on but lightly. So little i n t e r e s t was developed in fact, that it s e e m s doubtful if the p h e n o m e n o n i s even well known to p r e s e n t day e n g i n e e r i n g p e r s o n n e l . Undoubtedly~ the lack of i n t e r e s t was connected with the r a p i d dev e l o p m e n t of salt bath h a r d e n i n g which effectively e l i m i n a t e d the p r o b l e m from p r a c t i c a l c o n s i d e r a t i o n . With the i n t r o d u c t i o n of a t m o s p h e r e and v a c u u m heat t r e a t i n g f u r n a c e s , however~ where cooling f r o m the h a r d e n i n g heat is a c c o m p l i s h e d by a f a n - a g i t a t e d i n e r t gas~ we have once m o r e e n c o u n t e r e d the condition where these l a r g e r s e c t i o n s do not develop m a x i m u m h a r d n e s s after t e m p e r i n g . Since these new h a r d e n i n g methods offer m a n y a d v a n t a g e s with r e g a r d to s u r f a c e c l e a n l i n e s s and lack of e x p e n s i v e p o s t - h a r d e n i n g c l e a n i n g o p e r a t i o n s , an e v a l u a t i o n of the f a c t o r s conn e c t i n g the t
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