- Email: [email protected]
Boron nitride improves machinability for PM valve guides
PM F~IGUe endurlnca limits (MPa) 300
TECHNOLOGY
i
Tensile elongation (%) 8
250
20O
150
IO0
50
0
ZMe,~
oaal Iron
•
Ductile east Iron
0
Fatigue ms Elongation
FIGURE 2: Fatigue endurance and ductility comparison.
tion, density, and p r o c e s s i n g conditions. Tests for permissi-
ble inclusion size and indentation damage were m a d e by t h e
TRENDS
unusual routes of pressing the d e f e c t s i n t o t h e s u r f a c e s of finished parts. These t e s t s provided useful d a t a on t h e influence of inclusion size on fatigue s t r e n g t h as well as guidance on h a n d l i n g of t h e p o w d e r raw material. Finally, verification of t h e PM MBCs was completed w i t h t h e pilot p r o d u c t i o n of 10 000 sets of road testing. Main bearing caps r e p r e s e n t the latest in a series of successful conversions to PM of high p e r f o r m a n c e automotive components, d e m o n s t r a t i n g the adv a n t a g e s t h a t can be gained from PM t e c h n o l o g y t h r o u g h painstaking dedication to the task. J.M.C.
rltl
71
N "A'S'H'V'I'L'L'E
Boron nitride improves machinability for PM valve guides n a n o t h e r e x a m p l e where i l several competing factors converge to influence the success of a powder metallurgy (PM) conversion in an automotive component, Victoria Noble of E a t o n C o r p r e p o r t e d a t PM2Tech'93, Nashville, 16-19 May, on progress in the optimizing of valve guides. Co-authors of the p a p e r s were Sandy Schaefer, also of Eaton Corp, and Joel Poirier of QMP. On the advantage side, PM valve guides can provide reduced wear c o m p a r e d with east iron. This h a s b e c o m e m o r e significant with the drive towards reduced automotive emissions -- reducing valve guide wear can have an i m p o r t a n t impact on oil retention. Noble showed illustrations of the types of severe wear problem encount e r e d w i t h cast iron guides: scuffing wear of valve stems, adhesion of valve-guide material wear debris to the valve stem, and even fatigue failure of valve s t e m s . B e c a u s e of i m p r o v e d w e a r p e r f o r m a n c e , PM Valve guides can play a role in meeting new emission regulations, as
well as 160 000 km durability requirements. On the o t h e r hand, concern about the machinability of PM versus cast iron was said to have m a d e engine manufacturers reluctant to make the switch to PM valve guides, d e s p i t e the improved performance characteristics. Machinability impacts cylinder-head manufacturing costs t h r o u g h its effect on tool life and hence production downtime. One engine m a n u f a c t u r e r was q u o t e d as having set a target of 10 000 valve guide reaming operations between tool changes. Noble referred to Taguchi e x p e r i m e n t a t i o n d e s i g n e d to maximize machinability of PM valve guides w h i c h i n d i c a t e d t h a t the type of iron powder employed in t h e c o m p o s i t i o n had a major impact on tool life. Machinability t e s t s on PM valve guides m a d e with machinable g r a d e p o w d e r s s h o w e d some interesting results. A machinable grade of iron powder containing pre-alloyed manganese sulphide was found to be superior to iron powder with
MnS introduced as a blended additive. However, the powder w i t h p r e a l l o y e d MnS s t i l l s h o w e d limited m a c h i n a b i l i t y w h e n t e s t e d w i t h an e n g i n e manufacturer's preferred reamer type and machining conditions (feeds and s p e e d s ) . F u r t h e r t e s t s w i t h a sulphur-free machinable grade ( A t o m e t 29M, containing adm i x e d hexagonal boron nitride p o w d e r as t h e m a c h i n a b i l i t y additive) showed a 25% imp r o v e m e n t in m a c h i n a b i l i t y . Other advantages of the boron nitride additive are t h a t it is stable, non-toxic, has a high melting point and does not r e a c t w i t h t h e iron p o w d e r during sintering. Wear and scuff r e s i s t a n c e of PM valve guides m a d e with Atomet 29M were verified in engine tests and found to be m u c h superior to cast iron. F u r t h e r m o r e it was found t h a t machinability of t h e PM c o m p o n e n t s could be improved and could a p p r o a c h t h a t of cast iron by optimizing t h e machining set-up at the manufacturer's facility. J.M.C.
MPR July/August 1993 19
TECHNOLOGY
i
Tensile elongation (%) 8
250
20O
150
IO0
50
0
ZMe,~
oaal Iron
•
Ductile east Iron
0
Fatigue ms Elongation
FIGURE 2: Fatigue endurance and ductility comparison.
tion, density, and p r o c e s s i n g conditions. Tests for permissi-
ble inclusion size and indentation damage were m a d e by t h e
TRENDS
unusual routes of pressing the d e f e c t s i n t o t h e s u r f a c e s of finished parts. These t e s t s provided useful d a t a on t h e influence of inclusion size on fatigue s t r e n g t h as well as guidance on h a n d l i n g of t h e p o w d e r raw material. Finally, verification of t h e PM MBCs was completed w i t h t h e pilot p r o d u c t i o n of 10 000 sets of road testing. Main bearing caps r e p r e s e n t the latest in a series of successful conversions to PM of high p e r f o r m a n c e automotive components, d e m o n s t r a t i n g the adv a n t a g e s t h a t can be gained from PM t e c h n o l o g y t h r o u g h painstaking dedication to the task. J.M.C.
rltl
71
N "A'S'H'V'I'L'L'E
Boron nitride improves machinability for PM valve guides n a n o t h e r e x a m p l e where i l several competing factors converge to influence the success of a powder metallurgy (PM) conversion in an automotive component, Victoria Noble of E a t o n C o r p r e p o r t e d a t PM2Tech'93, Nashville, 16-19 May, on progress in the optimizing of valve guides. Co-authors of the p a p e r s were Sandy Schaefer, also of Eaton Corp, and Joel Poirier of QMP. On the advantage side, PM valve guides can provide reduced wear c o m p a r e d with east iron. This h a s b e c o m e m o r e significant with the drive towards reduced automotive emissions -- reducing valve guide wear can have an i m p o r t a n t impact on oil retention. Noble showed illustrations of the types of severe wear problem encount e r e d w i t h cast iron guides: scuffing wear of valve stems, adhesion of valve-guide material wear debris to the valve stem, and even fatigue failure of valve s t e m s . B e c a u s e of i m p r o v e d w e a r p e r f o r m a n c e , PM Valve guides can play a role in meeting new emission regulations, as
well as 160 000 km durability requirements. On the o t h e r hand, concern about the machinability of PM versus cast iron was said to have m a d e engine manufacturers reluctant to make the switch to PM valve guides, d e s p i t e the improved performance characteristics. Machinability impacts cylinder-head manufacturing costs t h r o u g h its effect on tool life and hence production downtime. One engine m a n u f a c t u r e r was q u o t e d as having set a target of 10 000 valve guide reaming operations between tool changes. Noble referred to Taguchi e x p e r i m e n t a t i o n d e s i g n e d to maximize machinability of PM valve guides w h i c h i n d i c a t e d t h a t the type of iron powder employed in t h e c o m p o s i t i o n had a major impact on tool life. Machinability t e s t s on PM valve guides m a d e with machinable g r a d e p o w d e r s s h o w e d some interesting results. A machinable grade of iron powder containing pre-alloyed manganese sulphide was found to be superior to iron powder with
MnS introduced as a blended additive. However, the powder w i t h p r e a l l o y e d MnS s t i l l s h o w e d limited m a c h i n a b i l i t y w h e n t e s t e d w i t h an e n g i n e manufacturer's preferred reamer type and machining conditions (feeds and s p e e d s ) . F u r t h e r t e s t s w i t h a sulphur-free machinable grade ( A t o m e t 29M, containing adm i x e d hexagonal boron nitride p o w d e r as t h e m a c h i n a b i l i t y additive) showed a 25% imp r o v e m e n t in m a c h i n a b i l i t y . Other advantages of the boron nitride additive are t h a t it is stable, non-toxic, has a high melting point and does not r e a c t w i t h t h e iron p o w d e r during sintering. Wear and scuff r e s i s t a n c e of PM valve guides m a d e with Atomet 29M were verified in engine tests and found to be m u c h superior to cast iron. F u r t h e r m o r e it was found t h a t machinability of t h e PM c o m p o n e n t s could be improved and could a p p r o a c h t h a t of cast iron by optimizing t h e machining set-up at the manufacturer's facility. J.M.C.
MPR July/August 1993 19