The formation of a solf layer in borided hot work die steels
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1. J. Kaiser:Arch. Eisenhuettenw., 1953, vol. 24, pp. 43-45. 2. J. D. Eshelby,F. C. Frank, and F. R. N. Nabarro:Phil.Mag., 1951, vol.42, p. 351. 3. J. P. Hirth and J. Lothe: Theory of Dislocations, p. 683, McGraw-HillBook Co., New York, 1968. 4. Ibid, p. 700. 5. R. B. Nicholsonand J. Nutting:Phil.Mag., 1958,vol. 3, p. 531.
The Formation of a Soft Layer in Borided Hot Work Die Steels H. C. FIEDLER
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s t e e l s , AISI H l l , H12, and H13, d e v e l o p e d a l a y e r b e t w e e n the h a r d b o r i d e c o a t i n g and the c o r e that was s u b s t a n t i a l l y s o f t e r than the c o r e . T h i s soft l a y e r had b e e n o b s e r v e d in b o r i d e d H l l by Kunst and S c h a a b e r , ~2 but no explanation f o r its o c c u r r e n c e was given. T h e s e s t e e l s n o m i n a l l y contain about 0.4 pet C, 0.3 pet Mn, 1 pct Si, 5 pet C r , and 1.4 pet Mo. H13 a l s o c o n t a i n s 1 pet V while H l l and H12 contain 0.4 pct; the l a t t e r a l l o y a l s o c o n t a i n s 1.3 pet W. The i n v e s t i g a t i o n to be r e p o r t e d was u n d e r t a k e n to u n d e r s t a n d the r e a s o n f o r the f o r m a t i o n of the soft phase and to find m e a n s of p r e v e n t i n g its o c c u r r e n c e . The b o r i d i n g was done e l e c t r o c h e m i c a l l y in m o l t e n s a l t at 900~ the s a m p l e b e i n g the cathode and e l e m e n t a l b o r o n , contained in a c o p p e r b a s k e t , the anode. A fused salt m i x t u r e of 20 m o l e pet K F , 30 m o l e pet N a F , and 50 m o l e pet L i F was the e l e c t r o l y t e . 0.2 m o l e pet BF2 was d i s s o l v e d into the m o l t e n salt. The bath was contained in a Monel v e s s e l and an a t m o s p h e r e of 90 pet N2 and 10 pct H2 c o v e r e d the salt at all t i m e s . B o r o n was d e p o s i t e d onto the s u r f a c e of the w o r k p i e c e by the e l e c t r o l y t i c d i s c h a r g e of b o r o n ions f r o m the solution. The d e p o s i t i o n r a t e , which was c o n t r o l l e d by the c u r r e n t d e n s i t y , was m a i n t a i n e d below the r a t e at which b o r o n was a b s o r b e d by the s t e e l so that no e l e m e n t a l b o r o n f o r m e d on the s u r f a c e . In the runs to be r e p o r t e d , the c e l l o p e r a t e d between 88 and 100 pet c u r r e n t e f f i c i e n c y , and b e t w e e n 2.4 and 3.0 m g p e r sq c m of b o r o n w e r e d e p o s i t e d . The h a r d e n i n g heat t r e a t m e n t following the b o r i d i n g c o n s i s t e d of heating the s a m p l e f o r 1 h r at 1000~ in a q u a r t z tube through which a r g o n was p a s s e d , r e m o v ing the tube f r o m the f u r n a c e , and cooling the s a m p l e to r o o m t e m p e r a t u r e in the argon a t m o s p h e r e . The s a m p l e s w e r e t e m p e r e d f o r 1 h r at 540~ following the same procedure. A c r o s s - s e c t i o n of b o r i d e d and heat t r e a t e d H13 is shown in F i g . 1. The b o r i d e l a y e r c o n s i s t e d of Fe2B; p r i
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