- Email: [email protected]
Advances in hot isostatic pressing
A d v a n c e s in hot
isostatic p r e s s i n g H I P is t o d a y a well e s t a b l i s h e d
techniq.e
for p r o d . c t i o n
of
advanced materials of all types. Typical applications are near-net-shaped products from metal powders and net-shaped high performance ceramics. The new generation of ABB H I P p r e s s e s a r e m o r e flexible a n d reliable than existing equipment and produce parts at l o w e r c o s t a s e f f i c i e n c y is increased, says A n d e r s Tr~ff. I n s t e a d o f diversifying the prod u c t range into a n u m b e r of s p e c i a l i z e d p r e s s types, the Swedish c o m p a n y has chosen to c o n c e n t r a t e on m a k i n g t w o types of production s y s t e m s as v e r s a t i l e as p o s s i b l e . One is b a s e d on the m o l y b d e n u m furnace, with the A B B p r o p r i e t a r y u n i f o r m rapid cooling, a n d is u s e d f o r all t y p e s o f m e t a l
processing. The o t h e r is b a s e d
FIGURE 1: Steam chest for r a d i a l turbine m a d e from powder. A l l internal cavities are made in the
HIPprocess;
on the graphite furnace, also e q u i p p e d with the A B B proprietary uniform rapid cooling, a n d used for cemented carbides and ceramics, ~
--
only outer
(Courtesy of ABB P o w d e r m e t AB).
MPR
April
1992
FIGURE 2: Production HIP process for 2200°C under pressure and 2000°C under vacuum using the ABB p r o p r i e t a r y h i g h t e m p e r a t u r e m e a s u r i n g system.
~ "l "
he high productivity molybdenum furnace uses ABB's u n i f o r m r a p i d cooling system. The f u r n a c e h a s high i n s t a l l e d p o w e r a n d a very efficient h e a t shield of t h e closed bell type for fast h e a t up a n d a c c u r a t e t e m p e r a t u r e control. These features combined with the controlled uniform rapid cooling assure processing cycles of 6-8 hours. H e a t i n g a n d cooling r a t e s c a n be p r o g r a m m e d . In o r d i n a r y processing, where only a short turn-around t i m e is required, cooling r a t e s a r e in t h e o r d e r of 10-60°C/min. It is p o s s i b l e however to e n h a n c e t h e q u a l i t y of t h e p r o d u c t or to
c a r r y o u t densific~ation a n d h e a t t r e a t m e n t in one s t o p if t h e cooling r a t e is increased. This is a p p l i c a b l e to N i - b a s e d superalloys, castings, a n d steel p a r t s . Cooling r a t e s can t h e n be up to 500°C/min. When densification and heat treatment are combined, the saving in p r o c e s s i n g cost is 50% o r m o r e compared with conventional processing (Figure 3). In o r d e r to achieve t h e s e cooling r a t e s a n d still m a i n t a i n a r e a s o n a b l e t e m p e r a t u r e s p a n in t h e furnace, t h e gas is p u m p e d in two l o o p s i n s i d e t h e p r e s s system. In t h e o u t e r l o o p t h e gas is cooled, t h u s r e m o v i n g h e a t from t h e system. The i n n e r loop stirs t h e gas inside t h e furnace a n d cool gas is m i x e d into t h e flow..Typical s p a n from t h e t o p to t h e b o t t o m of a p r o d u c t i o n system is 50°C.
surfaces are machined,
40
I
0026-0657/92/$3.50 @1992, ElsevierScience Publishers Lid
The w s t e m can also be used as a high p r e s s u r e gas quencher. The High Pressure Gas Q u e n c h e r unit o p e r a t e s with a gas p r e s s u r e of n o r m a l l y 100 to 200 MPa, which r e s u l t s in an e x t r e m e l y high h e a t t r a n s f e r from the p a r t to t h e gas. The high a n d u n i f o r m h e a t t r a n s f e r achieved due to the high d e n s i t y of t h e gas e n s u r e s t h a t tile p a r t will closely follow t h e gas t e m p e r a t u r e " The high h e a t t r a n s f e r resulting from t h e high d e n s i t y also m a k e s it p o s s i b l e to have a low gas flow speed, a n d c o n s e q u e n t l y t h e w o r k s p a c e can be p a c k e d as d e n s e l y as p o s s i b l e w i t h o u t having to
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Since t h e gas is i n e r t t h e r e is no p r o b l e m w i t h s u r f a c e re.actions. T h i s m a k e s it e c o n o m i c a l to h e a t t r e a t p r e c i s i o n cast Nib a s e d parts, high s p e e d steel tools, etc, in a High Pressure Gas Quencher.
:. . . . . . . .
ABB can m a k e t h e r m a l analysis calculations s i m u l a t i n g the cooling b e h a v i o u r for v a r i o u s parts. T h a t t h e a c t u a l t e m p e r a t u r e s of t h e p a r t s d u r i n g cooling follow t h e t h e o r e t i c a l c a l c u l a t i o n s closely has been verified by tests. With this t h e r m a l analysis, using finite e l e m e n t m e t h o d (FEM) technique, it is p o s s i b l e to d e t e r m i n e w h a t cooling r a t e s a r e n e e d e d in the p r e s s in o r d e r to achieve t h e r e q u i r e d p r o p e r t i e s in t h e p a r t to be processed, ABB can design t h e charging device in such a w a y t h a t t h e h e a t t r a n s f e r can be varied over t h e part. Thicker s e c t i o n s will have a h i g h e r h e a t t r a n s f e r t h a n t h i n s e c t i o n s in o r d e r to achieve uniform cooling t h r o u g h o u t ~he p a r t (Figure 4). The f e a t u r e s can be s u m m a r i z e d as follows: t h e cooling r a t e s in t h e p r e s s n e e d e d for a successful result can be c a l c u l a t e d using t h e ABB FEM p r o g r a m ; t h e close c o u p l i n g between gas t e m p e r a t u r e a n d p a r t surface a s s u r e s t h a t t h e w h o l e p a r t follows t h e p r o g r a m m e d c o o l i n g r a t e . T h e r e f o r e t h e r i s k of t h e r m a l l y i n d u c e d d i s t o r t i o n is minimal; very high a n d a c c u r a t e l y c o n t r o l l e d cooling r a t e s can be achieved comparable
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FIGURE 3: HIP cycle for
combined rejuvenation
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and heat treatment of turbine blades. The total
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When p r o d u c i n g PM p a r t s w i t h h o t i s o s t a t i e p r e s s i n g (HIP), it is p o s s i b l e to save m o n e y by using a F E M - b a s e d p r o g r a m to determ i n e t h e s h r i n k a g e of t h e powder-filled c a p s u l e d u r i n g processing. F o r a known p r o d u c t it is p o s s i b l e to d e t e r m i n e t h e c a p s u l e s h a p e with t h i s calculation. This saves b o t h e x p e n s i v e m a t e r i a l a n d t i m e as no trial r u n s or large safety m a r g i n s a r e t h e n n e e d e d to p r o d u c e t h e part. N o r m a l l y FEM c a l c u l a t i o n s a r e c a r r i e d o u t in large c o m p u t e r s . However, t h i s m o d e l is s i m p l i f i e d so t h a t c a l c u l a t i o n s can be e x e c u t e d on a n o r m a l p e r s o n a l c o m p u t e r (PC) a n d still a t t a i n a sufficiently a c c u r a t e
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c r e a t e s c o m p r e s s i v e s t r e s s e s in t h e p a r t during the treatment.
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120°Clminute.
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cooling rate is
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with s a l t b a t h a n d oil quenching; a n d - - t h e r m a l l y i n d u c e d ( A r ) p o r o s i t y in p o w d e r m e t a l l u r g y (PM) p a r t s a n d cracking of difficult materials are a v o i d e d as t h e h e a t t r e a t m e n t and, if called for, a n n e a l i n g a r e c a r r i e d o u t u n d e r very high gas p r e s s u r e , w h i c h
weight of the load is 100 kg. The programmed
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FIGURE 4: Computer modelling of cooling of turbine blades at 0.6 MPa and 200 MPa p r e s s u r e respectively, and the coupling b e t w e e n gas t e m p e r a t u r e and part t e m p e r a t u r e at each
pressure.
MPR April 1992 41
~i~i~i~!~i~i~7!~ii~P!!Mi~i~i~iP~iEc~AiiL~ii~F~EiAT~RE!i~7~!~i~i~i~i!~i~!!i~
FIGURE 5: FEM calculated s h r i n k a g e of arotation-symmetrical p o w d e r filled thickw a l l e d capsule c o n t a i n i n g core (Courtesy of VILS, GIN).
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radie Filled capsule
Pressed capsule
[ ] Powder
[ ] Core
[ ] Thick-walled capsule
e n d result. The m e t h o d of a c q u i r i n g i n p u t d a t a to t h e c a l c u l a t i o n p r o g r a m is s t r a i g h t f o r w a r d a n d fairly s i m p l e (Figure 5).
Versatile graphite furnace system IF1
.........
Processing of c e r a m i c s a n d WC-Co involves moulding, debinding, s i n t e r i n g and, possibly, high p r e s s u r e t r e a t m e n t before t h e final machining. A sinter-HIP system c o m b i n i n g debinding, s i n t e r i n g a n d p r e s s u r e t r e a t m e n t w o r k s w i t h low p r e s s u r e , t y p i c a l l y 5 to 10 MPa, a n d r e q u i r e s a c o m p l e x d e s i g n a n d c o n t r o l system. The s i n t e r / h i g h pressure HIP on t h e o t h e r h a n d is a relatively s i m p l e a n d well e s t a b l i s h e d process. F o r fine grained, low c o b a l t c o n t e n t WC-Co, p o s t - H I P p r o c e s s i n g also yields t h e b e s t m a t e r i a l q u a l i t y (1). The d e b i n d i n g t i m e s for c e r a m i c m a t e r i als a r e very long, a n d it is n o t e c o n o m i c a l l y feasible to r u n t h e whole cycle in one lot of e q u i p m e n t . ABB can offer an efficient, hotw a l l e d high t e m p e r a t u r e d e b i n d i n g furnace in c o m b i n a t i o n w i t h a sinter-HIP u n i t t h a t can p e r f o r m v a c u u m sintering, low p r e s s u r e i
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FIGURE 6: A B B c o m p u t e r
controlled debinding f u r n a c e with 120 litre u s e f u l volume,
42 MPR April 1992
.-,
sintering a n d high p r e s s u r e densification. The d e b i n d i n g furnace can o p e r a t e u p to 750°C w i t h v a c u u m or a gas flow. (Choice of v a c u u m or a gas f l o w d e p e n d s on t h e b i n d e r m a t e r i a l u s e d ) . The t e m p e r a t u r e is cont r o l l e d v i a a t h e r m o c o u p l e i n s i d e t h e hotwalled chamber and the whole debinding cycle is c a r r i e d o u t a u t o m a t i c a l l y t h r o u g h t h e PC controller. The h o t wall m a k e s t h e t r a n s p o r t a t i o n of b i n d e r o u t of t h e furnace m o r e efficient ( F i g u r e 6). The HIP system for c e r a m i c s i n t e r HIP cycles h a s a c a p a b i l i t y of 2000°C u n d e r v a c u u m -- typically 0.1 -- a n d 220°C u n d e r p r e s s u r e . The p r e s s u r e can be r a i s e d u p to 200 MPa. A r g o n gas or n i t r o g e n gas is u s e d as p r e s s u r e m e d i u m . The m a i n f e a t u r e s of t h e high t e m p e r a t u r e , high p r e s s u r e sinterHIP systems a r e t h e t e m p e r a t u r e a c c u r a c y in t h e w h o l e p r e s s u r e range, t h e fast a n d c o n t r o l l a b l e h e a t i n g a n d cooling rates, a n d the reliable temperature measurement system. The ABB HIPer h a s a p a t e n t e d insulation h o o d w i t h a m i n i m u m of t h e r m a l m a s s for fast cycling. A w o r k l o a d p o t s e p a r a t e s t h e p a r t s being p r e s s e d from t h e s u r r o u n d ing h e a t e r a r r a n g e m e n t . This h a s several a d v a n t a g e s . The furnace is easy to l o a d a n d u n l o a d w i t h o u t risk of d a m a g i n g t h e h e a t e r elements. The furnace a t m o s p h e r e is often c o n t a m i n a t e d by d i r t from t h e w o r k l o a d . T h a t d i r t is c o n t a i n e d in t h e p o t as it c o n d e n s e s on t h e walls w h e n t h e system cools down, t h u s i n c r e a s i n g t h e life of t h e h e a t e r s by a f a c t o r two to three. Process gas can b e f l u s h e d in a c o n t r o l l e d m a n n e r t h r o u g h t h e w o r k l o a d d u r i n g p r e h e a t i n g to r e m o v e m o i s t u r e or w o r k l o a d gases, a n d d u r i n g t h e cool down p h a s e to s p e e d u p cooling rates. The furnace is e q u i p p e d w i t h t h e ABB p r o p r i e t a r y u n i f o r m r a p i d cooling, m a k i n g it p o s s i b l e to r u n a cycle every shift. The f u r n a c e c a n a l s o be e q u i p p e d w i t h a d i l a t o m e t e r for o p t i m i z a t i o n of t h e sintering cycle. F o r very special high t e m p e r a t u r e m a t e r i a l s , ABB h a s d e v e l o p e d a s i n t e r HIP
system capable o f 2000°C in v a c u u m a n d 3000°C u n d e r p r e s s u r e (Figure 7). Both this furnace a n d the p r o d u c t i o n furnaces working with t e m p e r a t u r e s above 1800°C t o d a y u s e t h e ABB p r o p r i e t a r y High T e m p e r a t u r e Measuring System (HTMS). Normal t u n g s t e n - r h e n i u m thermocouple have a very short, life at high t e m p e r a t u r e s when the atmosphere contains carbon m o n o x i d e . I n s u l a t o r s m a d e from b o r o n n i t r i d e will decompose quickly w h e n the t e m p e r a t u r e e x c e e d s 1800°C, a n d free boron reacts to the m e t a l wire. All this results in b r i t t l e n e s s of the wire a n d drift in electromotive force. The drift can be high as 100°C after one high t e m p e r a t u r e cycle a n d t h e t h e r m o c o u p l c h a s to be c h a n g e d frequently. There is also a great risk t h a t w h o l e w o r k l o a d s will be l o s t d u e to t h e r m o c o u p l e failure or misreading. The ABB HTMS is superior to all other systems on the m a r k e t in t h a t it registers t e m p e r a t u r e s up to 3000°C at well defined places a n d can work u n d e r vacuum, protective gas a n d n i t r o g e n gas a n d is n o t sensitive to electrical noise or arcing. This is a c c o m p l i s h e d by o n l y u s i n g a n all m e c h a n i c a l system of graphite p a r t s in the hot zone. The c o n t i n u o u s l y m e a s u r e d signal is converted to a n electrical signal in the l o w e r p a r t of t h e press. A c o m p u t e r receiving the t e m p e r a t u r e signal a n d a pressure signal linearizes the values to a n i n p u t acceptable to a s t a n d a r d controller. ABB has s u p p l i e d several HTMS systems. Two are installed in p r o d u c t i o n u n i t s which have r u n a s u b s t a n t i a l n u m b e r of cycles. Control m e a s u r e m e n t s with t h e r m o c o u p l e have been m a d e at regular intervals to 1700-1800°C. The system has b e e n exposed to several h u n d r e d s of h o u r s at t e m p e r a t u r e s above 1800c'C. Over this period of time there has been no drift in the o u t p u t signal from the system. The calculated payback is less t h a n six m o n t h s c o m p a r i n g solely with the cost of replacing thermocouples. For p r o d u c t i o n r u n s at t e m p e r a t u r e s from 1800°C a n d u p w a r d s the cost of replacing t h e r m o c o u p l e is of the order of 25 to 35% of the total cycle cost calculated for one-shift operation, a cost which is saved using ABB's HTMS system. On top of this you have the security of 100% r e p e a t a b i l i t y for every cycle a n d no risk of losing a n expensive workload.
Control system -~. . . . . . . . . .
...........
FIGURE 7: World's first standard ABB Mini-HIPer
(WPYC) are used. The WVI'C will a d j u s t the s e t p o i n t of the controller to heat the whole load quickly to process t e m p e r a t u r e a n d will t h e n t u n e the t e m p e r a t u r e to assure
press for 3000°C.
difference d u r i n g the hold time. The second control loop uses the p e r m a n e n t l y i n s t a l l e d t h e r m o c o u p l e a d j a c e n t to the heater, giving low time c o n s t a n t a n d i n d e p e n d e n c e from workload size. This loop is c o n s e q u e n t l y quick to respond, accurate a n d free from overshoots. The system can calculate the cooling power required to cool a workload a t a given rate a n d control a n d cooling flows for accurate ramping. The system can also be used for real time control u s i n g WVFC in c o m b i n a t i o n with a c a l c u l a t i o n p r o g r a m for t h o r o u g h h e a t i n g in order to optimize cycle times, a n d the PC can be used for FEM calculations of capsule modelling. The ABB HIPer is reliable a n d safe e q u i p m e n t which yields a low cost per p a r t processed due to its short processing t i m e s and dependable measuring and control system. The possibility to optimize sintering cycles with d i l a t o m e t e r s a n d to calculate r e q u i r e d cooling r a t e s a n d capsule s h a p e s offers a d d i t i o n a l scope for cost savings. All this gives the u s e r a powerful, cost efficient a n d reliable p r o d u c t i o n tool for processing of m e t a l s a n d ceramics.
Acknowledgements: I wish to t h a n k all the colleagues who provided i n v a l u a b l e suggestions a n d ideas for this paper.
The new control system is based on the ABB Master i n d u s t r i a l controller a n d a n IBM PC for m a n / m a c h i n e c o m m u n i c a t i o n s . The References system is designed for easy use a n d fail . . . . . . . . . . safe operation. The f u r n a c e c a n o p e r a t e in cascade (1) B. North, et al, Metal P o w d e r Report c o n t r o l if w o r k p i e c e t h e r m o c o u p l e s December 1991, p40-45.
MPR April 1992 43
isostatic p r e s s i n g H I P is t o d a y a well e s t a b l i s h e d
techniq.e
for p r o d . c t i o n
of
advanced materials of all types. Typical applications are near-net-shaped products from metal powders and net-shaped high performance ceramics. The new generation of ABB H I P p r e s s e s a r e m o r e flexible a n d reliable than existing equipment and produce parts at l o w e r c o s t a s e f f i c i e n c y is increased, says A n d e r s Tr~ff. I n s t e a d o f diversifying the prod u c t range into a n u m b e r of s p e c i a l i z e d p r e s s types, the Swedish c o m p a n y has chosen to c o n c e n t r a t e on m a k i n g t w o types of production s y s t e m s as v e r s a t i l e as p o s s i b l e . One is b a s e d on the m o l y b d e n u m furnace, with the A B B p r o p r i e t a r y u n i f o r m rapid cooling, a n d is u s e d f o r all t y p e s o f m e t a l
processing. The o t h e r is b a s e d
FIGURE 1: Steam chest for r a d i a l turbine m a d e from powder. A l l internal cavities are made in the
HIPprocess;
on the graphite furnace, also e q u i p p e d with the A B B proprietary uniform rapid cooling, a n d used for cemented carbides and ceramics, ~
--
only outer
(Courtesy of ABB P o w d e r m e t AB).
MPR
April
1992
FIGURE 2: Production HIP process for 2200°C under pressure and 2000°C under vacuum using the ABB p r o p r i e t a r y h i g h t e m p e r a t u r e m e a s u r i n g system.
~ "l "
he high productivity molybdenum furnace uses ABB's u n i f o r m r a p i d cooling system. The f u r n a c e h a s high i n s t a l l e d p o w e r a n d a very efficient h e a t shield of t h e closed bell type for fast h e a t up a n d a c c u r a t e t e m p e r a t u r e control. These features combined with the controlled uniform rapid cooling assure processing cycles of 6-8 hours. H e a t i n g a n d cooling r a t e s c a n be p r o g r a m m e d . In o r d i n a r y processing, where only a short turn-around t i m e is required, cooling r a t e s a r e in t h e o r d e r of 10-60°C/min. It is p o s s i b l e however to e n h a n c e t h e q u a l i t y of t h e p r o d u c t or to
c a r r y o u t densific~ation a n d h e a t t r e a t m e n t in one s t o p if t h e cooling r a t e is increased. This is a p p l i c a b l e to N i - b a s e d superalloys, castings, a n d steel p a r t s . Cooling r a t e s can t h e n be up to 500°C/min. When densification and heat treatment are combined, the saving in p r o c e s s i n g cost is 50% o r m o r e compared with conventional processing (Figure 3). In o r d e r to achieve t h e s e cooling r a t e s a n d still m a i n t a i n a r e a s o n a b l e t e m p e r a t u r e s p a n in t h e furnace, t h e gas is p u m p e d in two l o o p s i n s i d e t h e p r e s s system. In t h e o u t e r l o o p t h e gas is cooled, t h u s r e m o v i n g h e a t from t h e system. The i n n e r loop stirs t h e gas inside t h e furnace a n d cool gas is m i x e d into t h e flow..Typical s p a n from t h e t o p to t h e b o t t o m of a p r o d u c t i o n system is 50°C.
surfaces are machined,
40
I
0026-0657/92/$3.50 @1992, ElsevierScience Publishers Lid
The w s t e m can also be used as a high p r e s s u r e gas quencher. The High Pressure Gas Q u e n c h e r unit o p e r a t e s with a gas p r e s s u r e of n o r m a l l y 100 to 200 MPa, which r e s u l t s in an e x t r e m e l y high h e a t t r a n s f e r from the p a r t to t h e gas. The high a n d u n i f o r m h e a t t r a n s f e r achieved due to the high d e n s i t y of t h e gas e n s u r e s t h a t tile p a r t will closely follow t h e gas t e m p e r a t u r e " The high h e a t t r a n s f e r resulting from t h e high d e n s i t y also m a k e s it p o s s i b l e to have a low gas flow speed, a n d c o n s e q u e n t l y t h e w o r k s p a c e can be p a c k e d as d e n s e l y as p o s s i b l e w i t h o u t having to
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Since t h e gas is i n e r t t h e r e is no p r o b l e m w i t h s u r f a c e re.actions. T h i s m a k e s it e c o n o m i c a l to h e a t t r e a t p r e c i s i o n cast Nib a s e d parts, high s p e e d steel tools, etc, in a High Pressure Gas Quencher.
:. . . . . . . .
ABB can m a k e t h e r m a l analysis calculations s i m u l a t i n g the cooling b e h a v i o u r for v a r i o u s parts. T h a t t h e a c t u a l t e m p e r a t u r e s of t h e p a r t s d u r i n g cooling follow t h e t h e o r e t i c a l c a l c u l a t i o n s closely has been verified by tests. With this t h e r m a l analysis, using finite e l e m e n t m e t h o d (FEM) technique, it is p o s s i b l e to d e t e r m i n e w h a t cooling r a t e s a r e n e e d e d in the p r e s s in o r d e r to achieve t h e r e q u i r e d p r o p e r t i e s in t h e p a r t to be processed, ABB can design t h e charging device in such a w a y t h a t t h e h e a t t r a n s f e r can be varied over t h e part. Thicker s e c t i o n s will have a h i g h e r h e a t t r a n s f e r t h a n t h i n s e c t i o n s in o r d e r to achieve uniform cooling t h r o u g h o u t ~he p a r t (Figure 4). The f e a t u r e s can be s u m m a r i z e d as follows: t h e cooling r a t e s in t h e p r e s s n e e d e d for a successful result can be c a l c u l a t e d using t h e ABB FEM p r o g r a m ; t h e close c o u p l i n g between gas t e m p e r a t u r e a n d p a r t surface a s s u r e s t h a t t h e w h o l e p a r t follows t h e p r o g r a m m e d c o o l i n g r a t e . T h e r e f o r e t h e r i s k of t h e r m a l l y i n d u c e d d i s t o r t i o n is minimal; very high a n d a c c u r a t e l y c o n t r o l l e d cooling r a t e s can be achieved comparable
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and heat treatment of turbine blades. The total
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When p r o d u c i n g PM p a r t s w i t h h o t i s o s t a t i e p r e s s i n g (HIP), it is p o s s i b l e to save m o n e y by using a F E M - b a s e d p r o g r a m to determ i n e t h e s h r i n k a g e of t h e powder-filled c a p s u l e d u r i n g processing. F o r a known p r o d u c t it is p o s s i b l e to d e t e r m i n e t h e c a p s u l e s h a p e with t h i s calculation. This saves b o t h e x p e n s i v e m a t e r i a l a n d t i m e as no trial r u n s or large safety m a r g i n s a r e t h e n n e e d e d to p r o d u c e t h e part. N o r m a l l y FEM c a l c u l a t i o n s a r e c a r r i e d o u t in large c o m p u t e r s . However, t h i s m o d e l is s i m p l i f i e d so t h a t c a l c u l a t i o n s can be e x e c u t e d on a n o r m a l p e r s o n a l c o m p u t e r (PC) a n d still a t t a i n a sufficiently a c c u r a t e
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c r e a t e s c o m p r e s s i v e s t r e s s e s in t h e p a r t during the treatment.
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with s a l t b a t h a n d oil quenching; a n d - - t h e r m a l l y i n d u c e d ( A r ) p o r o s i t y in p o w d e r m e t a l l u r g y (PM) p a r t s a n d cracking of difficult materials are a v o i d e d as t h e h e a t t r e a t m e n t and, if called for, a n n e a l i n g a r e c a r r i e d o u t u n d e r very high gas p r e s s u r e , w h i c h
weight of the load is 100 kg. The programmed
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FIGURE 4: Computer modelling of cooling of turbine blades at 0.6 MPa and 200 MPa p r e s s u r e respectively, and the coupling b e t w e e n gas t e m p e r a t u r e and part t e m p e r a t u r e at each
pressure.
MPR April 1992 41
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FIGURE 5: FEM calculated s h r i n k a g e of arotation-symmetrical p o w d e r filled thickw a l l e d capsule c o n t a i n i n g core (Courtesy of VILS, GIN).
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radie Filled capsule
Pressed capsule
[ ] Powder
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[ ] Thick-walled capsule
e n d result. The m e t h o d of a c q u i r i n g i n p u t d a t a to t h e c a l c u l a t i o n p r o g r a m is s t r a i g h t f o r w a r d a n d fairly s i m p l e (Figure 5).
Versatile graphite furnace system IF1
.........
Processing of c e r a m i c s a n d WC-Co involves moulding, debinding, s i n t e r i n g and, possibly, high p r e s s u r e t r e a t m e n t before t h e final machining. A sinter-HIP system c o m b i n i n g debinding, s i n t e r i n g a n d p r e s s u r e t r e a t m e n t w o r k s w i t h low p r e s s u r e , t y p i c a l l y 5 to 10 MPa, a n d r e q u i r e s a c o m p l e x d e s i g n a n d c o n t r o l system. The s i n t e r / h i g h pressure HIP on t h e o t h e r h a n d is a relatively s i m p l e a n d well e s t a b l i s h e d process. F o r fine grained, low c o b a l t c o n t e n t WC-Co, p o s t - H I P p r o c e s s i n g also yields t h e b e s t m a t e r i a l q u a l i t y (1). The d e b i n d i n g t i m e s for c e r a m i c m a t e r i als a r e very long, a n d it is n o t e c o n o m i c a l l y feasible to r u n t h e whole cycle in one lot of e q u i p m e n t . ABB can offer an efficient, hotw a l l e d high t e m p e r a t u r e d e b i n d i n g furnace in c o m b i n a t i o n w i t h a sinter-HIP u n i t t h a t can p e r f o r m v a c u u m sintering, low p r e s s u r e i
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FIGURE 6: A B B c o m p u t e r
controlled debinding f u r n a c e with 120 litre u s e f u l volume,
42 MPR April 1992
.-,
sintering a n d high p r e s s u r e densification. The d e b i n d i n g furnace can o p e r a t e u p to 750°C w i t h v a c u u m or a gas flow. (Choice of v a c u u m or a gas f l o w d e p e n d s on t h e b i n d e r m a t e r i a l u s e d ) . The t e m p e r a t u r e is cont r o l l e d v i a a t h e r m o c o u p l e i n s i d e t h e hotwalled chamber and the whole debinding cycle is c a r r i e d o u t a u t o m a t i c a l l y t h r o u g h t h e PC controller. The h o t wall m a k e s t h e t r a n s p o r t a t i o n of b i n d e r o u t of t h e furnace m o r e efficient ( F i g u r e 6). The HIP system for c e r a m i c s i n t e r HIP cycles h a s a c a p a b i l i t y of 2000°C u n d e r v a c u u m -- typically 0.1 -- a n d 220°C u n d e r p r e s s u r e . The p r e s s u r e can be r a i s e d u p to 200 MPa. A r g o n gas or n i t r o g e n gas is u s e d as p r e s s u r e m e d i u m . The m a i n f e a t u r e s of t h e high t e m p e r a t u r e , high p r e s s u r e sinterHIP systems a r e t h e t e m p e r a t u r e a c c u r a c y in t h e w h o l e p r e s s u r e range, t h e fast a n d c o n t r o l l a b l e h e a t i n g a n d cooling rates, a n d the reliable temperature measurement system. The ABB HIPer h a s a p a t e n t e d insulation h o o d w i t h a m i n i m u m of t h e r m a l m a s s for fast cycling. A w o r k l o a d p o t s e p a r a t e s t h e p a r t s being p r e s s e d from t h e s u r r o u n d ing h e a t e r a r r a n g e m e n t . This h a s several a d v a n t a g e s . The furnace is easy to l o a d a n d u n l o a d w i t h o u t risk of d a m a g i n g t h e h e a t e r elements. The furnace a t m o s p h e r e is often c o n t a m i n a t e d by d i r t from t h e w o r k l o a d . T h a t d i r t is c o n t a i n e d in t h e p o t as it c o n d e n s e s on t h e walls w h e n t h e system cools down, t h u s i n c r e a s i n g t h e life of t h e h e a t e r s by a f a c t o r two to three. Process gas can b e f l u s h e d in a c o n t r o l l e d m a n n e r t h r o u g h t h e w o r k l o a d d u r i n g p r e h e a t i n g to r e m o v e m o i s t u r e or w o r k l o a d gases, a n d d u r i n g t h e cool down p h a s e to s p e e d u p cooling rates. The furnace is e q u i p p e d w i t h t h e ABB p r o p r i e t a r y u n i f o r m r a p i d cooling, m a k i n g it p o s s i b l e to r u n a cycle every shift. The f u r n a c e c a n a l s o be e q u i p p e d w i t h a d i l a t o m e t e r for o p t i m i z a t i o n of t h e sintering cycle. F o r very special high t e m p e r a t u r e m a t e r i a l s , ABB h a s d e v e l o p e d a s i n t e r HIP
system capable o f 2000°C in v a c u u m a n d 3000°C u n d e r p r e s s u r e (Figure 7). Both this furnace a n d the p r o d u c t i o n furnaces working with t e m p e r a t u r e s above 1800°C t o d a y u s e t h e ABB p r o p r i e t a r y High T e m p e r a t u r e Measuring System (HTMS). Normal t u n g s t e n - r h e n i u m thermocouple have a very short, life at high t e m p e r a t u r e s when the atmosphere contains carbon m o n o x i d e . I n s u l a t o r s m a d e from b o r o n n i t r i d e will decompose quickly w h e n the t e m p e r a t u r e e x c e e d s 1800°C, a n d free boron reacts to the m e t a l wire. All this results in b r i t t l e n e s s of the wire a n d drift in electromotive force. The drift can be high as 100°C after one high t e m p e r a t u r e cycle a n d t h e t h e r m o c o u p l c h a s to be c h a n g e d frequently. There is also a great risk t h a t w h o l e w o r k l o a d s will be l o s t d u e to t h e r m o c o u p l e failure or misreading. The ABB HTMS is superior to all other systems on the m a r k e t in t h a t it registers t e m p e r a t u r e s up to 3000°C at well defined places a n d can work u n d e r vacuum, protective gas a n d n i t r o g e n gas a n d is n o t sensitive to electrical noise or arcing. This is a c c o m p l i s h e d by o n l y u s i n g a n all m e c h a n i c a l system of graphite p a r t s in the hot zone. The c o n t i n u o u s l y m e a s u r e d signal is converted to a n electrical signal in the l o w e r p a r t of t h e press. A c o m p u t e r receiving the t e m p e r a t u r e signal a n d a pressure signal linearizes the values to a n i n p u t acceptable to a s t a n d a r d controller. ABB has s u p p l i e d several HTMS systems. Two are installed in p r o d u c t i o n u n i t s which have r u n a s u b s t a n t i a l n u m b e r of cycles. Control m e a s u r e m e n t s with t h e r m o c o u p l e have been m a d e at regular intervals to 1700-1800°C. The system has b e e n exposed to several h u n d r e d s of h o u r s at t e m p e r a t u r e s above 1800c'C. Over this period of time there has been no drift in the o u t p u t signal from the system. The calculated payback is less t h a n six m o n t h s c o m p a r i n g solely with the cost of replacing thermocouples. For p r o d u c t i o n r u n s at t e m p e r a t u r e s from 1800°C a n d u p w a r d s the cost of replacing t h e r m o c o u p l e is of the order of 25 to 35% of the total cycle cost calculated for one-shift operation, a cost which is saved using ABB's HTMS system. On top of this you have the security of 100% r e p e a t a b i l i t y for every cycle a n d no risk of losing a n expensive workload.
Control system -~. . . . . . . . . .
...........
FIGURE 7: World's first standard ABB Mini-HIPer
(WPYC) are used. The WVI'C will a d j u s t the s e t p o i n t of the controller to heat the whole load quickly to process t e m p e r a t u r e a n d will t h e n t u n e the t e m p e r a t u r e to assure
press for 3000°C.
difference d u r i n g the hold time. The second control loop uses the p e r m a n e n t l y i n s t a l l e d t h e r m o c o u p l e a d j a c e n t to the heater, giving low time c o n s t a n t a n d i n d e p e n d e n c e from workload size. This loop is c o n s e q u e n t l y quick to respond, accurate a n d free from overshoots. The system can calculate the cooling power required to cool a workload a t a given rate a n d control a n d cooling flows for accurate ramping. The system can also be used for real time control u s i n g WVFC in c o m b i n a t i o n with a c a l c u l a t i o n p r o g r a m for t h o r o u g h h e a t i n g in order to optimize cycle times, a n d the PC can be used for FEM calculations of capsule modelling. The ABB HIPer is reliable a n d safe e q u i p m e n t which yields a low cost per p a r t processed due to its short processing t i m e s and dependable measuring and control system. The possibility to optimize sintering cycles with d i l a t o m e t e r s a n d to calculate r e q u i r e d cooling r a t e s a n d capsule s h a p e s offers a d d i t i o n a l scope for cost savings. All this gives the u s e r a powerful, cost efficient a n d reliable p r o d u c t i o n tool for processing of m e t a l s a n d ceramics.
Acknowledgements: I wish to t h a n k all the colleagues who provided i n v a l u a b l e suggestions a n d ideas for this paper.
The new control system is based on the ABB Master i n d u s t r i a l controller a n d a n IBM PC for m a n / m a c h i n e c o m m u n i c a t i o n s . The References system is designed for easy use a n d fail . . . . . . . . . . safe operation. The f u r n a c e c a n o p e r a t e in cascade (1) B. North, et al, Metal P o w d e r Report c o n t r o l if w o r k p i e c e t h e r m o c o u p l e s December 1991, p40-45.
MPR April 1992 43