Laser melting and heat treatment of m2 tool steel: A microstructural characterization
- PDF / 3,010,200 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 195 Views
. INTRODUCTION I T has recently been s h o w n that l a s e r s u r f a c e m e l t ing p r o d u c e s a highly refined a n d homogeneous m i c r o s t r u c t u r e in n o n f e r r o u s a l l o y s , i n c l u d i n g t h e n i c k e l base superalloys. 1 S i n c e t h e technique naturally l e n d s i t s e l f t o the t r e a t m e n t o f m a c h i n e t o o l c u t t i n g e d g e s S t r u t t e t al2'3 i n v e s t i g a t e d the c h a r a c t e r i s t i c s of l a s e r s u r f a c e m e l t e d t o o l steels, specifically M 2 a n d 440-C. In this investigation the specimen s u r f a c e underwent s e v e r a l m e l t i n g a n d s o l i d i f i c a t i o n c y c l e s by r e p e a t e d l y s c a n n i n g w i t h a c o n t i n u o u s CO2 l a s e r , t h e c o n d i t i o n s w e r e s u c h t h a t the s o l i d i f i c a t i o n c o o l i n g r a t e w a s e s t i m a t e d by a h e a t t r a n s f e r a n a l y s i s4 t o be ~ 5 × 105°C • s - I . W h e n t h e l a s e r m e l t i n g m e t h o d was a p p l i e d to t h e t o o l s t e e l M 2 t h e r e w a s n e a r l y c o m p l e t e c a r b i d e diss o l u t i o n a n d r e t e n t i o n of a s u b s t a n t i a l a m o u n t of 5f e r r i t e i n a d d i t i o n t o a u s t e n i t e . I n c o n t r a s t t o the r a t h e r c o m p l e x m i c r o s t r u c t u r e in M 2 , l a s e r m e l t i n g produced a s i m p l e coupled growth 5-ferrite/CrTCa earbide e u t e c t i c s t r u c t u r e i n 4 4 0 - C . A n o t h e r f i n d i n g i s that b o t h M 2 a n d 4 4 0 - C h a r d e n e d t o a d e g r e e u n a t t a i n able in c o n v e n t i o n a l p r a c t i c e f o l l o w i n g subsequent h e a t t r e a t m e n t . T h i s f e a t u r e is i n t r i g u i n g s i n c e t h e r e i s no m i c r o s t r u c t u r a l e v i d e n c e o f m a r t e n s i f e , h e n c e the s u b s t a n t i a I h a r d n e s s a p p e a r s t o a r i s e f r o m a distribution of fine c a r b i d e s precipitated upon s u b s e q u e n t heat treatment. T h e p r e s e n t study w a s c a r r i e d out t o f u r t h e r e l u c i d a t e the i n i t i a l f i n d i n g s o n l a s e r m e l t e d M2 t o o l s t e e l a n d d e t e r m i n e m i c r o s t r u e t u r a l c h a n g e s p r o d u c e d by subsequent h e a t t r e a t m e n t . Specific a t t e n t i o n is dev o t e d t o t h e i d e n t i f i c a t i o n of c a r b i d e p a r t i c l e s a n d t h e i r distribution.
p o s i t i o n of the a l l o y i s g i v e n i n T a b l e I . T h e s p e c i m e n s u r f a c e was s c a n n e d w i t h a c o n t i n u o u s l a s e r such t h a t e a c h m e l t e d r e g i o n was m e l t e d a n d solidified o n l y o n c e , f u r t h e r d e t a i l s a r e g i v e n e l s e w h e r e .2'3 A f t e r s u r f a c e melting t h e specimens were cross-sectioned a n d o b s e r v e d by o p t i c a l a n d e l e c t r o n m i c r o s c o p y ; F i g . 1 s h o w s a t y p i c a l c r o s s - s e c t i o n a l v i e w of a m e l t r e gion. V a r i o
Data Loading...
