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Getting rid of lubricants vital for PM part quality
PM
TECHNOLOGY
TRENDS
Getting rid of lubricants vital for PM part quality ubricants are necessary evils within the PM process. It is not possible to p r e s s high density p a r t s without the use of lubricants. But if the lubricant is not removed during the sintering process it can lead to a degradation of properties in the final p a r t delubing is also a costly and complex process. The best conditions for sintering the c o m p o n e n t are not necessarily the best for removing the lubricant, therefore e x t r a energy is required to run a s e p a r a t e delubing cycle. During a session on lubric a n t s for p o w d e r m e t a l l u r g y (PM) held during the PM2Tec93 conference in Nashville, Tennessee, Herb Nayer of Airco told delegates t h a t the lubricant m u s t come out of the p a r t s during the early p a r t of the sintering cycle. If it remains in place it prevents wetting of t h e p a r t i c l e s a n d t h u s gives p o o r bonding and p r o d u c e s a low s t r e n g t h part, he said. But if the lubricant is not removed properly a range of problems can occur:
residue in the form of soot. To avoid sooting the t h e r m a l decomposition of the lubricant m u s t be minimized, this can be done by inducing a chemical reaction which p r o d u c e s a gaseous by-product. Effective d e l u b i n g can be c a r r i e d o u t in t h e following manner, says Nayer:
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external and internal sooting of the parts; d e p o s i t s in t h e p r e - h e a t and hot zones of the furnace; blistering; c o m p o n e n t s w i t h a low tensile strength; or c o m p o n e n t s with an excessive variation in strength.
A variety of factors cause sooting, for instance the size, s h a p e and density of the p a r t s can all have an effect on the rate of lubricant removal as well as the ratio of length of p r e - h e a t zone to hot zone and the type of lubricant used. The a t m o s p h e r e p r e s e n t in t h e furnace is also a factor. A dry a t m o s p h e r e is desirable for most of the processes involved in sintering PM parts. However, it is d e t r i m e n t a l to d e l u b i n g a n d leaves a solid
14 MPR February 1994
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Bring the entire p a r t to a t e m p e r a t u r e of between 425-540°C relatively quickly, but not too quickly otherwise r u p t u r e may occur; Allow the lubricant to come out of the p a r t and vaporize. If possible the lubricant s h o u l d c o m e o u t in its o r i g i n a l m o l e c u l a r form, this prevents internal sooting but will only occur if the t e m p e r a t u r e is kept low;, The vapours m u s t t h e n be swept to the front of the furnace -- not into the hot zone; Along the way the vapours should be oxidized -- an e x o t h e r m i c a t m o s p h e r e is best for achieving this; Finally, t h e vapours should be burnt-off at the door of the furnace.
It is essential t h a t the t e m p e r a ture should not rise above 540°C until all the lubricant h a s b e e n r e m o v e d from t h e part, Nayer emphasized. A n o t h e r factor t h a t m u s t be considered is the variation in behaviour between different lubricants. The two m o s t commonly used lubricants in the PM i n d u s t r y for conventional compaction of ferrous materials are zinc s t e a r a t e and acrawax. Lithium stearate and o t h e r lubricants are preferred for nonferrous and stainless steels. Zinc stearate is considered to have good compaction and die ejection properties. However, it is a relatively difficult material to r e m o v e f r o m a g r e e n compact prior to sintering. This
is because of the metallic comp o n e n t t h a t is not p r e s e n t in acrawax. Researchers at Airco found t h a t t h e r e are two distinct reactions occuring during t h e h e a t up a n d removal of t h e soap c o m p o n e n t of zinc stearate. Hydrogen i n c r e a s e s t h e total a m o u n t of lubricant t h a t is removed, but also increases t h e t e m p e r a t u r e r e q u i r e d to achieve m a x i m u m removal. The Airco team also found t h a t the p r e s e n c e of water, in t h e absence of hydrogen, d o e s n ' t effect the removal of zinc stearate. For a given level of hydrogen, increassing a m o u n t s of w a t e r r e d u c e t h e t o t a l a m o u n t of lubricant removed indicating the H2/H20 ratio is critical. Acrawax, while being less d e s i r a b l e from a c o m p a c t i o n standpoint, has much better removal characteristics. Nayer a n d his c o l l e a g u e s at Airco have found t h a t acrawax is very insensitive to a t m o s p h e r e composition, with temperature being t h e p r i m a r y p a r a m e t e r dictating the rate of removal. There is essentially zero residue r e m a i n i n g a t 550°C for all a t m o s p h e r e conditions. Four commercial delube assist systems exist on the North A m e r i c a n m a r k e t . These are Drever's rapid burn off system (RBO); Airco's Wet N2 system, Sinterite's accelerated delube s y s t e m (ADS) a n d A b b o t t ' s quick delube process (QDP). Challenges to be solved by furnace and a t m o s p h e r e manufacturers in the future include finding a way of reducing t h e time required for effective delubing. Nayer suggested t h a t this will involve re-evaluating t h e chemistry of PM systems a n d balancing t h e n e e d s of compacting w i t h t h e m e c h a n i c s o f delubing. If internal oxidation could be induced in-situ this would help reduce t h e problem, he said. Methods of quick determination of i m p r o p e r delubing m u s t also be developed and monitoring via sensors during delubing should be improved. The d e v e l o p m e n t of environmentally cleaner and better lubricants will also play a part. Nayer also suggested t h a t powder producers should look at technology to coat p o w d e r s with a lubricant, reducing t h e overall lubricant required and placing it only w h e r e needed. •
TECHNOLOGY
TRENDS
Getting rid of lubricants vital for PM part quality ubricants are necessary evils within the PM process. It is not possible to p r e s s high density p a r t s without the use of lubricants. But if the lubricant is not removed during the sintering process it can lead to a degradation of properties in the final p a r t delubing is also a costly and complex process. The best conditions for sintering the c o m p o n e n t are not necessarily the best for removing the lubricant, therefore e x t r a energy is required to run a s e p a r a t e delubing cycle. During a session on lubric a n t s for p o w d e r m e t a l l u r g y (PM) held during the PM2Tec93 conference in Nashville, Tennessee, Herb Nayer of Airco told delegates t h a t the lubricant m u s t come out of the p a r t s during the early p a r t of the sintering cycle. If it remains in place it prevents wetting of t h e p a r t i c l e s a n d t h u s gives p o o r bonding and p r o d u c e s a low s t r e n g t h part, he said. But if the lubricant is not removed properly a range of problems can occur:
residue in the form of soot. To avoid sooting the t h e r m a l decomposition of the lubricant m u s t be minimized, this can be done by inducing a chemical reaction which p r o d u c e s a gaseous by-product. Effective d e l u b i n g can be c a r r i e d o u t in t h e following manner, says Nayer:
•
•
L
•
• • •
external and internal sooting of the parts; d e p o s i t s in t h e p r e - h e a t and hot zones of the furnace; blistering; c o m p o n e n t s w i t h a low tensile strength; or c o m p o n e n t s with an excessive variation in strength.
A variety of factors cause sooting, for instance the size, s h a p e and density of the p a r t s can all have an effect on the rate of lubricant removal as well as the ratio of length of p r e - h e a t zone to hot zone and the type of lubricant used. The a t m o s p h e r e p r e s e n t in t h e furnace is also a factor. A dry a t m o s p h e r e is desirable for most of the processes involved in sintering PM parts. However, it is d e t r i m e n t a l to d e l u b i n g a n d leaves a solid
14 MPR February 1994
•
•
•
•
Bring the entire p a r t to a t e m p e r a t u r e of between 425-540°C relatively quickly, but not too quickly otherwise r u p t u r e may occur; Allow the lubricant to come out of the p a r t and vaporize. If possible the lubricant s h o u l d c o m e o u t in its o r i g i n a l m o l e c u l a r form, this prevents internal sooting but will only occur if the t e m p e r a t u r e is kept low;, The vapours m u s t t h e n be swept to the front of the furnace -- not into the hot zone; Along the way the vapours should be oxidized -- an e x o t h e r m i c a t m o s p h e r e is best for achieving this; Finally, t h e vapours should be burnt-off at the door of the furnace.
It is essential t h a t the t e m p e r a ture should not rise above 540°C until all the lubricant h a s b e e n r e m o v e d from t h e part, Nayer emphasized. A n o t h e r factor t h a t m u s t be considered is the variation in behaviour between different lubricants. The two m o s t commonly used lubricants in the PM i n d u s t r y for conventional compaction of ferrous materials are zinc s t e a r a t e and acrawax. Lithium stearate and o t h e r lubricants are preferred for nonferrous and stainless steels. Zinc stearate is considered to have good compaction and die ejection properties. However, it is a relatively difficult material to r e m o v e f r o m a g r e e n compact prior to sintering. This
is because of the metallic comp o n e n t t h a t is not p r e s e n t in acrawax. Researchers at Airco found t h a t t h e r e are two distinct reactions occuring during t h e h e a t up a n d removal of t h e soap c o m p o n e n t of zinc stearate. Hydrogen i n c r e a s e s t h e total a m o u n t of lubricant t h a t is removed, but also increases t h e t e m p e r a t u r e r e q u i r e d to achieve m a x i m u m removal. The Airco team also found t h a t the p r e s e n c e of water, in t h e absence of hydrogen, d o e s n ' t effect the removal of zinc stearate. For a given level of hydrogen, increassing a m o u n t s of w a t e r r e d u c e t h e t o t a l a m o u n t of lubricant removed indicating the H2/H20 ratio is critical. Acrawax, while being less d e s i r a b l e from a c o m p a c t i o n standpoint, has much better removal characteristics. Nayer a n d his c o l l e a g u e s at Airco have found t h a t acrawax is very insensitive to a t m o s p h e r e composition, with temperature being t h e p r i m a r y p a r a m e t e r dictating the rate of removal. There is essentially zero residue r e m a i n i n g a t 550°C for all a t m o s p h e r e conditions. Four commercial delube assist systems exist on the North A m e r i c a n m a r k e t . These are Drever's rapid burn off system (RBO); Airco's Wet N2 system, Sinterite's accelerated delube s y s t e m (ADS) a n d A b b o t t ' s quick delube process (QDP). Challenges to be solved by furnace and a t m o s p h e r e manufacturers in the future include finding a way of reducing t h e time required for effective delubing. Nayer suggested t h a t this will involve re-evaluating t h e chemistry of PM systems a n d balancing t h e n e e d s of compacting w i t h t h e m e c h a n i c s o f delubing. If internal oxidation could be induced in-situ this would help reduce t h e problem, he said. Methods of quick determination of i m p r o p e r delubing m u s t also be developed and monitoring via sensors during delubing should be improved. The d e v e l o p m e n t of environmentally cleaner and better lubricants will also play a part. Nayer also suggested t h a t powder producers should look at technology to coat p o w d e r s with a lubricant, reducing t h e overall lubricant required and placing it only w h e r e needed. •