Preliminary thermomechanical treatment of D6AC steel
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a recent review of the processing of steel by thermomechanical treatment of steel in the Soviet Union,* it was established that the Soviets have developed a new strengthening technique referred to as preliminary thermomechanical treatment, P T M T . This new thermomechanical process has the unique characteristic of performing the deformation prior to austenitizing as shown in the schematic isothermal transformation diagram, Fig. 1. Following the deformation, a preaustenitization, low temperature heat treatment is sometimes required for a successful response to the processing. After this, rapid rate heating must be used to austenitize the steel. To be a useful process, the heating rate must be rapid enough to avoid recrystallization prior to the transformation to austenite. The initialdevelopment of this process took place in the early 1960's by Bernshtein and coworkers 2-4 at the M o s c o w Institute of Steel and Alloys. It was established that steels were stronger following austenitization and quenching if they were 1) plastically deformed prior to the austenitization treatment, and 2) rapidly heated to austenite following this prior deformation. F r o m this observation it was hypothesized that the imperfections (dislocations) created by the deformation could be retained in going through the ferrite + carbide a u s t e n i t e ~ m a r t e n s i t e t r a n s f o r m a t i o n s . Following p r o c e s s i n g by PTMT, the enhanced m e c h a n i c a l p r o p e r t i e s can be r e p r o d u c e d through s e v e r a l f e r r i t e + c a r b i d e s (or m a r t e n s i t e ) ~ a u s t e n i t e - - m a r t e n s i t e c y c l e s , s'6 Since the strengthening is r e v e r s i b l e , the defects producing the strengthening a r e p r e s u m a b l y also r e t a i n e d throughout the v a r i o u s phase t r a n s f o r m a tions. A number of Soviet r e s e a r c h e r s have concluded that this retention behavior e x p l a i n s t h e i r e x p e r i m e n t a l o b s e r v a t i o n s , s'7-14 In s u m m a r y , these r e s e a r c h e r s find that p r e l i m i n a r y t h e r m o m e c h a n i c a l t r e a t m e n t of s t e e l s s u p e r i m p o s e s work hardening on the n o r m a l strengthening produced by the m a r t e n s i t e t r a n s f o r m a tion. The r e s e a r c h d e s c r i b e d h e r e was d i r e c t e d at e v a l u ating p r e l i m i n a r y t h e r m o m e c h a n i c a l t r e a t m e n t in one T. J. KOPPENAALis Supervisor,Physical Metallurgy,Aeronutronic Division,Philco-Ford Corp., Newport Beach, Calif. Manuscript submitted April 2, 1970. METALLURGICALTRANSACTIONS
p a r t i c u l a r s t e e l , D6AC. The p r i m a r y objective was to investigate some of the v a r i a b l e s known to effect the r e s p o n s e of this p r o c e s s . In addition, the g e n e r a l Soviet explanation for the strengthening was examined. PROCEDURE The e x p e r i m e n t a l work was done with consumable e l e c t r o d e vacuum m e l t e d D6AC steel. The selection was b a s e d upon two f
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