Measurement of Case Depth By DALE J. WRIGHT, General Superinlendent Heat Treat Div., Caterpillar Tractor Co. Peoria, ill. N U M E R O U S methods are employed through- out industry to measure the depth of carburized cases on steel, largely because there is apparently no standardized procedure which is acceptable t o a l l . A r t i c l e s h a v e been published describing various "approved" or "standard" procedures, but generally each company or in- dustry has preferred t o a p p l y i t s o w n particular technique based on its own interpretation of case depth and the a v a i l a b i l i t y o f e i t h e r shop or laboratory equipment. Until approximately seven years ago, the heat t r e a t i n g d i v i s i o n o f Caterpillar Tractor Co. encountered inconsistencies in case depths on test bars which were not always explainable. Each such instance i n v o l v e d c o n s i d e r a b l e extra expense because of additional testing and destruction of piece parts. An analysis of the problem revealed that very often the fault was in the manner of determining case depth rather than i n t h e carburizing operation. A t t h a t time, therefore, we set out to define the most applic- able and economical manner of checking this metallurgical q u a l i t y a n d standardizing accord- ingly. The objectives which w o u l d h a v e to be met f o r o u r u s e were as follows: 1. Determinations must be accurate and repetitive. The method must eliminate differ- ences of judgment between people making the determination, something we found to be the greatest problem in our previous practices. 2. Determinations must be made quickly so that "in process" heats can be checked at any time without delay. 3. The procedure must be simple so as not to demand h i g h l y s k i l l e d t e c h n i c i a n s , a require- ment particularly important during week-end o r h o l i d a y operation. 4. Equipment should be relatively inexpen- sive and durable, and cost of individual deter- minations be as low as possible. Importance of the latter consideration is attested by the fact that we average 250 determinations a day. 5. The method used should represent a measure of the control of the carburizing process, and not attempt to evaluate other variables such as m a t e r i a l o r d e s i g n of the piece. 6. The determination should indicate a case depth which would find acceptance under a broad interpretation of the term "effective" depth of case. A number of approaches were tried during the investigation for a method which would meet these requirements. Thus, test bars of carbon and alloy steels were etched or heat tinted to resolve the carburized case for its measurement with a special scale. Brinell or equivalent low-power magnifications in con- junction with appropriately calibrated lineal divisions were used t o m e a s u r e the case on frac- tured o r p o l i s h e d and etched transverse sections. Two other ideas studied were hardness tests on step-ground and tapered bars and transverse sections; and the microscopic examination of carburized, and of carburized and hardened bars which had been tempered at 1100° F. in cast iron borings. In keeping with our stated objectives, we decided on a combination of the first two methods, regulated so as to control the known variables and made to conform easily with all of our needs. A f t e r m a n y years o f i t s u s e we are well pleased with t h e r e s u l t s , and confident that any troubles we may encounter are not due to the method of checking case depth. The following describes the procedure we employ, which is actually very simple to apply after a brief initial instruction. METAL PROGRESS; PAGE 86
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M e a s u r e m e n t o f C a s e D e p t h
B y D A L E J . W R I G H T , G e n e r a l S u p e r i n l e n d e n t H e a t T r e a t D i v . , C a t e r p i l l a r T r a c t o r C o .
P e o r i a , i l l .
NU M E R O U S m e t h o d s a r e e m p l o y e d t h r o u g h o u t i n d u s t r y t o m e a s u r e t h e d e p t h o f
c a r b u r i z e d c a s e s o n s t e e l , l a r g e l y b e c a u s e t h e r e i s a p p a r e n t l y n o s t a n d a r d i z e d p r o c e d u r e w h i c h i s a c c e p t a b l e t o a l l . A r t i c l e s h a v e b e e n p u b l i s h e d d e s c r i b i n g v a r i o u s " a p p r o v e d " o r " s t a n d a r d " p r o c e d u r e s , b u t g e n e r a l l y e a c h c o m p a n y o r i n d u s t r y h a s p r e f e r r e d t o a p p l y i t s o w n p a r t i c u l a r t e c h n i q u e b a s e d o n i t s o w n i n t e r p r e t a t i o n o f c a s e d e p t h a n d t h e a v a i l a b i l i t y o f e i t h e r s h o p o r l a b o r a t o r y e q u i p m e n t .
U n t i l a p p r o x i m a t e l y s e v e n y e a r s a g o , t h e h e a t t r e a t i n g d i v i s i o n o f C a t e r p i l l a r T r a c t o r C o . e n c o u n t e r e d i n c o n s i s t e n c i e s i n c a s e d e p t h s o n t e s t b a r s w h i c h w e r e n o t a l w a y s e x p l a i n a b l e . E a c h s u c h i n s t a n c e i n v o l v e d c o n s i d e r a b l e e x t r a e x p e n s e b e c a u s e o f a d d i t i o n a l t e s t i n g a n d d e s t r u c t i o n o f p i e c e p a r t s . A n a n a l y s i s o f t h e p r o b l e m r e v e a l e d t h a t v e r y o f t e n t h e f a u l t w a s i n t h e m a n n e r o f d e t e r m i n i n g c a s e d e p t h r a t h e r t h a n i n t h e c a r b u r i z i n g o p e r a t i o n . A t t h a t t i m e , t h e r e f o r e , w e s e t o u t t o d e f i n e t h e m o s t a p p l i c a b l e a n d e c o n o m i c a l m a n n e r o f c h e c k i n g t h i s m e t a l l u r g i c a l q u a l i t y a n d s t a n d a r d i z i n g a c c o r d i n g l y . T h e o b j e c t i v e s w h i c h w o u l d h a v e t o b e m e t f o r o u r u s e w e r e a s f o l l o w s :
1 . D e t e r m i n a t i o n s m u s t b e a c c u r a t e a n d r e p e t i t i v e . T h e m e t h o d m u s t e l i m i n a t e d i f f e r e n c e s o f j u d g m e n t b e t w e e n p e o p l e m a k i n g t h e d e t e r m i n a t i o n , s o m e t h i n g w e f o u n d t o b e t h e g r e a t e s t p r o b l e m i n o u r p r e v i o u s p r a c t i c e s .
2 . D e t e r m i n a t i o n s m u s t b e m a d e q u i c k l y s o t h a t " i n p r o c e s s " h e a t s c a n b e c h e c k e d a t a n y t i m e w i t h o u t d e l a y .
3 . T h e p r o c e d u r e m u s t b e s i m p l e s o a s n o t t o d e m a n d h i g h l y s k i l l e d t e c h n i c i a n s , a r e q u i r e m e n t p a r t i c u l a r l y i m p o r t a n t d u r i n g w e e k - e n d o r h o l i d a y o p e r a t i o n .
4 . E q u i p m e n t s h o u l d b e r e l a t i v e l y i n e x p e n
s i v e a n d d u r a b l e , a n d c o s t o f i n d i v i d u a l d e t e r m i n a t i o n s b e a s l o w a s p o s s i b l e . I m p o r t a n c e o f t h e l a t t e r c o n s i d e r a t i o n i s a t t e s t e d b y t h e f a c t t h a t w e a v e r a g e 2 5 0 d e t e r m i n a t i o n s a d a y .
5 . T h e m e t h o d u s e d s h o u l d r e p r e s e n t a m e a s u r e o f t h e c o n t r o l o f t h e c a r b u r i z i n g p r o c e s s , a n d n o t a t t e m p t t o e v a l u a t e o t h e r v a r i a b l e s s u c h a s m a t e r i a l o r d e s i g n o f t h e p i e c e .
6 . T h e d e t e r m i n a t i o n s h o u l d i n d i c a t e a c a s e d e p t h w h i c h w o u l d find a c c e p t a n c e u n d e r a b r o a d i n t e r p r e t a t i o n o f t h e t e r m " e f f e c t i v e " d e p t h o f c a s e .
A n u m b e r o f a p p r o a c h e s w e r e t r i e d d u r i n g t h e i n v e s t i g a t i o n f o r a m e t h o d w h i c h w o u l d m e e t t h e s e r e q u i r e m e n t s . T h u s , t e s t b a r s o f c a r b o n a n d a l l o y s t e e l s w e r e e t c h e d o r h e a t t i n t e d t o r e s o l v e t h e c a r b u r i z e d c a s e f o r i t s m e a s u r e m e n t w i t h a s p e c i a l s c a l e . B r i n e l l o r e q u i v a l e n t l o w - p o w e r m a g n i f i c a t i o n s i n c o n j u n c t i o n w i t h a p p r o p r i a t e l y c a l i b r a t e d l i n e a l d i v i s i o n s w e r e u s e d t o m e a s u r e t h e c a s e o n f r a c t u r e d o r p o l i s h e d a n d e t c h e d t r a n s v e r s e s e c t i o n s . T w o o t h e r i d e a s s t u d i e d w e r e h a r d n e s s t e s t s o n s t e p - g r o u n d a n d t a p e r e d b a r s a n d t r a n s v e r s e s e c t i o n s ; a n d t h e m i c r o s c o p i c e x a m i n a t i o n o f c a r b u r i z e d , a n d o f c a r b u r i z e d a n d h a r d e n e d b a r s w h i c h h a d b e e n t e m p e r e d a t 1100° F . i n c a s t i r o n b o r i n g s .
I n k e e p i n g w i t h o u r s t a t e d o b j e c t i v e s , w e d e c i d e d o n a c o m b i n a t i o n o f t h e first t w o m e t h o d s , r e g u l a t e d s o a s t o c o n t r o l t h e k n o w n v a r i a b l e s a n d m a d e t o c o n f o r m e a s i l y w i t h a l l o f o u r n e e d s . A f t e r m a n y y e a r s o f i t s u s e w e a r e w e l l p l e a s e d w i t h t h e r e s u l t s , a n d c o n f i d e n t t h a t a n y t r o u b l e s w e m a y e n c o u n t e r a r e n o t d u e t o t h e m e t h o d o f c h e c k i n g c a s e d e p t h .
T h e f o l l o w i n g d e s c r i b e s t h e p r o c e d u r e w e e m p l o y , w h i c h i s a c t u a l l y v e r y s i m p l e t o a p p l y a f t e r a b r i e f i n i t i a l i n s t r u c t i o n .
M E T A L P R O G R E S S ; P A G E 8 6
D e s c r i p t i o n o f T e s t B a r s — T e s t b a r s a r e p r o c u r e d f r o m a m i l l h e a t o f k n o w n c h a r a c t e r i s t i c s a n d i n s u f f i c i e n t q u a n t i t y t o l a s t a n u m b e r o f y e a r s . T h e c o l d finished b a r s a r e 7 / 1 6 i n . s q u a r e a n d 3/4 i n . l o n g , a n d a h o l e i s d r i l l e d n e a r o n e e n d o n t h o s e w h i c h r e q u i r e s u s p e n s i o n i n t h e f u r n a c e . C h e m i c a l c o m p o s i t i o n o f t h e b a r s i s : 0 . 1 7 t o 0 . 2 0 C , 0 . 8 0 t o 0 . 9 0 M n , 0 . 1 7 t o 0 . 2 2 S i , 0 . 0 4 0 S , 0 . 0 2 0 P , 0 . 1 2 N i , 0 . 0 8 C r , 0 . 0 2 M o , 0 . 0 5 C u , 0 . 1 5 t o t a l N i p l u s C u ; a l l b u t t h e first t h r e e a r e m a x i m u m v a l u e s .
T h e A . S . T . M . g r a i n s i z e i s N o . 1 t o 4 , w i t h a m a x i m u m s p r e a d o f t w o n u m b e r s . T h e t e s t b a r s a r e r u n w i t h t h e p r o d u c t i o n p a r t s i n e v e r y f u r n a c e c h a r g e a n d a t r e g u l a r f r e q u e n t i n t e r v a l s o n c o n t i n u o u s f u r n a c e s . T e s t b a r s w h i c h h a v e b e e n c h e c k e d f o r c a s e d e p t h i n t h e m a n n e r t o b e d e s c r i b e d a r e u s e d t o a c c e p t o r r e j e c t p r o d u c t i o n m a t e r i a l w i t h w h i c h t h e y h a v e b e e n c a r -b u r i z e d . U s e o f t e s t b a r s o f u n i f o r m c a r b u r i z i n g c h a r a c t e r i s t i c s i s n e c e s s a r y i n o r d e r t o i s o l a t e t h e v a r i a b l e b e i n g c h e c k e d t o t h a t o f t h e c a r b u r i z i n g r o u t i n e i t s e l f .
H a r d e n i n g o f T e s t B a r s — A f t e r c o o l i n g f r o m c a r b u r i z i n g t o a t l e a s t a " b l a c k " h e a t , b a r s
F i g . 1 — I n s t r u m e n t D e s i g n e d at C a t e r p i l l a r T r a c tor C o . f o r A c c u r a t e M e a s u r e m e n t of C a s e D e p t h . P r i n c i p a l p a r t s c o n s i s t of a n i l l u m i n a t e d B a u s c h a n d L a m b i O X B r i n e l l m i c r o s c o p e a n d a d i a l i n d i c a t o r m o u n t e d o n a m e t a l base. T h e case d e p t h is r e a d o n t h e d i a l i n d i c a t o r
i n s t r u m e n t E l i m i n a t e s H u m a n E l e m e n t
a r e l o a d e d i n t o a s m a l l H e v i - D u t y e l e c t r i c f u r n a c e e q u i p p e d w i t h a c i r c u l a t i n g f a n . U p t o 2 0 p i e c e s a r e l o a d e d a t o n e t i m e i n a n a l l o y p a n . T h i s f u r n a c e i s u s e d f o r t h i s w o r k o n l y a n d i s m a i n t a i n e d a t 1440° F . ± 5° F . a t a l l t i m e s . H e a t i n g t i m e i s h e l d t o 2 2 m i n . ± 3 0 s e c , a n d n o p r o t e c t i v e a t m o s p h e r e i s u s e d . T e s t b a r s a r e t h e n q u e n c h e d u n t i l c o l d i n a f u l l flow o f w a t e r a t 75° F . ± 5° F .
F r a c t u r i n g a n d E t c h i n g — T h e b a r s a r e b r o k e n t r a n s v e r s e l y a t a p p r o x i m a t e l y t h e i r m i d p o i n t a n d e t c h e d i n 10% n i t a l f o r 8 s e c . C a r e m u s t b e e x e r c i s e d t o u s e f r e s h n i t a l w h i c h h a s n o t b e e n e x p o s e d t o a i r f o r m o r e t h a n 2 h r . T h e e t c h e d f r a c t u r e i s r i n s e d i n w a t e r a n d d r i e d w i t h c o m p r e s s e d a i r , r e s u l t i n g i n a c l e a r , d e f i n i t e l i n e o f d e m a r c a t i o n b e t w e e n c a s e a n d c o r e .
R e a d i n g C a s e D e p t h — T h e i n s t r u m e n t w e d e v e l o p e d f o r d e t e r m i n i n g d e p t h i s o f a s i m p l e , i n e x p e n s i v e d e s i g n , y e t i t h a s e l i m i n a t e d t h e d i f f e r e n c e b e t w e e n i n d i v i d u a l s m a k i n g t h e d e t e r m i n a t i o n a n d t h e n a t u r a l i n c l i n a t i o n o f p e o p l e t o " i n t e r p r e t " t h e c a s e m e a s u r e m e n t a s t h e y m a y h a p p e n t o s e e fit. T h e p r i n c i p a l p a r t s o f t h i s i n s t r u m e n t a r e a n i l l u m i n a t e d B a u s c h & L o m b B r i n e l l m i c r o s c o p e ( m a g n i f i c a t i o n 1 0 X ) ,
F i g . 2 — R e a r V i e w of M e a s u r i n g I n s t r u m e n t W i t h a T e s t B a r i n P l a c e
F i g . 3 - E n d V i e w of T e s t B a r A f t e r B e i n g F r a c t u r e d a n d E t c h e d . T h e case d e p t h i s c l e a r l y def i n e d by t h e d a r k e r p o r t i o n ; free f e r r i t e i s i n d i c a t e d by t h e l i g h t .spots i n t h e center p o r t i o n . l O X
a n d a d i a l i n d i c a t o r m o u n t e d o n a m e t a l b a s e . W h e n t h e t e s t b a r i s p l a c e d i n t h e i n s t r u m e n t , t h e h a i r l i n e o f t h e m i c r o s c o p e i s z e r o e d t o i t s e d g e . T h e b a r i s m o v e d s o t h a t t h e h a i r l i n e c o i n c i d e s w i t h t h e first f r e e f e r r i t e . T h e c a s e d e p t h i s t h e n r e a d d i r e c t l y o n t h e d i a l i n d i c a t o r .
F r o m o u r i n v e s t i g a t i o n a l w o r k i t w a s f o u n d t h a t a r e a d i n g t o t h e first g r a i n o f f r e e f e r r i t e w o u l d c o i n c i d e c l o s e l y w i t h a 0 .40% c a r b o n c o n
t e n t a n d a h a r d n e s s o f R o c k w e l l C - 5 0 . O n t h i s i n s t r u m e n t a c c u r a c y o f r e p e t i t i v e r e a d i n g s , b e t w e e n i n d i v i d u a l s a s w e l l a s b e t w e e n d i f f e r e n t r e a d i n g s b y t h e s a m e i n d i v i d u a l , i s w i t h i n 0 . 0 0 1 i n .
F i g u r e 3 ( r e p r o d u c e d a t t h e s a m e m a g n i f i c a t i o n t h e o p e r a t o r r e c e i v e s i n t h e i n s t r u m e n t ) c l e a r l y s h o w s t h e d a r k e r c a s e d e p t h .
I t i s r e c o g n i z e d t h a t t h e m e t h o d s d e s c r i b e d i n t h i s a r t i c l e c o u l d n o t b e a p p l i e d t o a l l s h o p s a s a m e a n s o f p r o d u c t i o n c o n t r o l , b e c a u s e s o m e e q u i p m e n t i s n e c e s s a r y , a l t h o u g h i t i s b y n o m e a n s e l a b o r a t e o r e x p e n s i v e . H o w e v e r , i t i s v w t h s o m e h o p e o f s t a n d a r d i z a t i o n t h a t t h i s d e s c r i p t i o n h a s b e e n p r e p a r e d , s i n c e t h e r e c e r t a i n l y i s a n e e d i n
• i n d u s t r y f o r c l o s e r m e a s u r e m e n t a s w e l l a s u n i f o r m i t y
i n m e t h o d s e m p l o y e d . P e r h a p s v a r i a t i o n s f r o m t h e f o r e g o i n g w o u l d a t l e a s t p e r m i t u s a l l t o " s p e a k t h e s a m e l a n g u a g e " a n d m a k e c o m p a r i s o n s f r o m t h e s a m e b a s i c b e g i n n i n g . I f t h e r e w e r e e n o u g h i n t e r e s t i n s t a n d a r d i z a t i o n , i t w o u l d n o t b e d i f f i c u l t t o m a k e t e s t b a r s a n d r e a d i n g i n s t r u m e n t s a v a i l a b l e , j u s t a s s t a n d a r d m e t h o d s a n d e q u i p m e n t f o r h a r d n e s s m e a s u r e m e n t s h a v e b e e n a v a i l a b l e f o r m a n y y e a r s . ^
Current R u s s i a n Metallurgical IN C O N T I N U A T I O N o f t h e s u r v c y o f c o n t e m p o
r a r y R u s s i a n m e t a l l u r g y , p r e s e n t e d i n t h e A u g u s t 1 9 5 1 a n d M a r c h 1 9 5 3 i s s u e s o f M e t a l P r o g r e s s , five a d d i t i o n a l b o o k s p u b l i s h e d i n t h e S o v i e t U n i o n i n t h e p e r i o d 1 9 4 7 t o 1 9 5 1 a r e h e r e a p p r a i s e d . T h e first t h r e e a r e o f s p e c i a l i n t e r e s t b e c a u s e t h e y g i v e a c o m p r e h e n s i v e a n d d e t a i l e d d e s c r i p t i o n o f b o t h f e r r o u s a n d n o n f e r r o u s m e t a l s a n d a l l o y s i n p r o d u c t i o n a s o f 1 9 5 0 - 5 1 . T h e o t h e r t w o a r e c o n c e r n e d w i t h n o n f e r r o u s i n d u s t r i e s , o n e e x t e n s i v e l y d e s c r i b i n g n i c k e l a n d
c o p p e r ( a f e w c r i t i c a l i s s u e s e x c e p t e d ) a n d t h e o t h e r d a b b l i n g w i t h a n u m b e r o f t h e s o - c a l l e d " r a r e m e t a l s " , i n c l u d i n g m o l y b d e n u m , t i t a n i u m a n d t h e t r a n s u r a n i c s .
S O V I E T M E T A L S A N D A L L O Y S
" I n t r o d u c t i o n t o M e t a l l u r g y , " b y G . I . P o g o -d i n - A l e k s e e v , Y u . A . G e l l e r a n d A . G . R a k h -s h t a d t ; G o v e r n m e n t a l P u b l i c a t i o n f o r t h e W a r I n d u s t r y , M o s c o w , 1 9 5 0 , 4 5 5 p .
M E T A L P R O G R E S S ; P A G E 88
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