Continuous cooling transformation kinetics of thermomechanically worked low-carbon austenite
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Continuous cooling transformation diagrams were determined f o r molybdenum-boronsteels containing 0.24, 0.4, and 0.66 pct Mo with 0.1 pct C, and also 0.4 pct Mo with 0.2 pct C, after thermomechanically working by compressive deformation to 12, 25, and 50 pct reduction at 830~ (1525~ as well as for the steels in the undeformed condition. In undeformed specimens, higher carbon or molybdenumdecreased the limiting cooling rate for the avoidance of polygonalferrite formation. The same was true for deformed specimens, although increased deformation raised the limiting cooling rates of all compositions. The limiting cooling rate for polygonalferrite formation increased exponentiallywith austenite deformation, as measured by true strain. Thermomechanical working also raised bainite start temperatures at fast cooling rates and caused small increases in martensite start temperatures.
I N the past few y e a r s n u m e r o u s e f f o r t s have been dir e c t e d toward the d e v e l o p m e n t of t h e r m o m e c h a n i c a l p r o c e s s i n g s c h e d u l e s for m e t a l s , p a r t i c u l a r l y s t e e l s . S e v e r a l a u t h o r s have r e v i e w e d the p r o g r e s s that has been made in this a r e a J -3 In g e n e r a l , the t e r m s " t h e r momeehanical working" or " t h e r m o m e c h a n i c a l treatm e n t " , a s applied to s t e e l s i m p l y s t r a i n h a r d e n i n g of the a u s t e n i t e by d e f o r m a t i o n that t a k e s place e i t h e r p r i o r to o r during the ;r - - a t r a n s f o r m a t i o n , A p r i m a r y complication that a r i s e s , in the d e v e l o p m e n t of such t h e r m o m e c h a n i c a l w o r k i n g s c h e d u l e s f o r s t e e l s , is that the k i n e t i c s of the ~ - - c~ t r a n s f o r m a t i o n a r e a c c e l e r a t e d by s t r a i n h ar d en i n g the a u s t e n i t e . A r e v i e w of s o m e of the e f f o r t s to q u a n t i t a t i v e l y e v a l u a t e the change in t r a n s f o r m a t i o n k i n e t i c s was p r e s e n t e d in an e a r l i e r p a p e r . 4 A l l t h e s e s t u d i e s w e r e conducted u n d e r i s o t h e r m a l c o n d i t i o n s . F o r a p p l ic a t io n to m o s t p r a c t i c a l p r o c e s s e s , continuous cooling t r a n s f o r m a t i o n (CCT) data a r e n eed ed . It was the p u r p o s e of this i n v e s t i g a t i o n to d e t e r m i n e the e f f e c t of p r i o r d e f o r m a t i o n of the a u s tenite on the k i n e t i c s of t r a n s f o r m a t i o n during the c o n tinuous cooling of l o w - c a r b o n m o l y b d e n u m - b o r o n s t e e l , and to o b s e r v e the influence of v a r i a t i o n s of c a r b o n and molybdenum contents on this effect on k i n e t i c s . T h e m o l y b d e n u m - b o r o n s t e e l s that w e r e c h o s e n f o r this study w e r e i n t r o d u c e d by B a r d g e t t and R e e v e , 3 who o b s e r v e d that this unique c o m b i n a t i o n of a l l o y i n g e l e m e n t s r e s u l t e d in a s
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