FeCrAl alloy shows good hot strength

PM

SPECIAL

FEATURE

FeCrAI alloy shows good hot strength A P M is an FeCrAI alloy that retains its form when used in high-power electrical heating elements. The developer of this PM material, Kanthal of HaUstah a m m a r , Sweden, introduced the m e t a l in 1 9 8 9 . Since then its use in non-electrical applications has been increasing. K a n t h a l ' s Roger B e r g l u n d describes the production of the alloy and the properties of this material that m a k e it suitable for use at high temperatures. I !

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he first FeCrA1 alloy for resistance electrical heating was introduced by Kanthal in 1931. The composition, 20% Cr, 5.5% A1, balance Fe, has remained basically the same over the years. However, considerable improvements in performance have been achieved by optimizing trace element additions and also by using better production techniques. Today Kanthal is the world's largest m a n u f a c t u r e r of heating alloys and the FeCrA1 alloy group is the main contributor to the company's success. APM has a better hot strength than FeCrA1 alloys produced by conventional routes. Its stability of form has made easier the design of heating elements. A number of non-electric applications, new for FeCrA1 alloys, take more than half of the APM produced today. The volume produced of this alloy has been growing continuously

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TABLE 1: Some physical properties of Kanthal APM. The alloy is ferritic at all temperatures.

Chemical composition AI

5.8%

Cr Fe

21% Balance

A p p r o x i m a t e melting point

1500 ° C

Emissivity, ~, fully o x i d i z e d

0.70 7.10 g/cm 3

Density

Temperature

Electrical resistivity (~,mm'tm)

20°C 600°C 800°C 1000°C 1200°C 1400°C

1.45 1.48 1.49 1.51 1.51 1.52

46 MPR October 1992

Thermal expansion from 20°C (l/K) 13 x 14 x 15 x 16 x

10"s 10"6 10 "6 10-s

Thermal Specific heat conductivity capacity (WlmK) (kJIkgK) 10.8 19.4 21.9 25.5 28.7 34.9

0.46 0.73 0.72 0.72 0.74 0.80

since its introduction in 1989. Conventional FeCrA1 alloys are characterized by excellent high-temperature oxidation and corrosion resistance but they also have low creep strength. To avoid deformation during operation FeCrAl heating elements often need mechanical support. These s u p p o r t s may take the form of ceramic pieces but they can be eliminated altogether by giving the elements themselves sufficient stability of form. Table 1 shows some of the physical properties of APM, a dispersion strengthened FeCrA1 alloy produce by a powder metallurgy (PM) route. Today APM is well established as a peak performance resistance-heating alloy used for furnace t e m p e r a t u r e s up to 1400°C. It is also used as a construction material in a large and growing number of applications where its unique combination of oxidation properties and form stability is useful.

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By consolidating an FeCrA1 powder to full density, an even d i s t r i b u t i o n of small nonmetallic dispersoids (typical diameter 0.1 ~m), formed during the rapid solidification, is preserved in the structure. These dispersoids reduce the mobility of grain boundaries and dislocations (Figure 1). APM is normally delivered in a fine grained a n n e a l e d state. I n t e r a c t i o n between disperoids and dislocations give the alloy a b e t t e r creep strength than similar alloys produced by a conventional route. When t h e PM m a t e r i a l is h e a t e d , d i s p e r o i d s will p r e v e n t n o r m a l g r a i n growth. This m e c h a n i s m e n a b l e s a socalled secondary recrystallization. After a time at high t e m p e r a t u r e some grains suddenly start to grow very rapidly. The result is an extremely large grained structure. After this transformation the creep strength rises even further. The grain size transformation drastically increases the creep s t r e n g t h and form stability of the alloy. In many cases it can be observed as a s u d d e n d r o p in the deformation rate. The transformation normally starts after a time in service. In some cases a heat t r e a t m e n t is made on an unloaded product, before installation, to

PM

SPECIAL

FEATURE

get large grains right from the start. This t r e a t m e n t avoids d e f o r m a t i o n d u r i n g an initial fine grained period.

Creep properties To get the necessary m e c h a n i c a l d a t a for the new PM alloy K a n t h a l has invested in a n advanced creep t e s t i n g laboratory. Here new furnaces a n d s a m p l e designs e n a b l e accurate creep tests to be m a d e at t e m p e r a t u r e s up to 1400°C. Figure 3 shows some of the results of t e s t i n g in this laboratory. Creep tests with r u p t u r e t i m e s longer t h a n 5000 h o u r s a r e r u n n i n g a t t h e m o m e n t . Designers who use 5 or 10 years calculated lifetime for t h e i r c o n s t r u c t i o n still have to be patient. For rough estimations the p r e s e n t curves can be e x t r a p o lated, b u t t h e n the e s t i m a t e d life from the p o i n t of view of o x i d a t i o n has also to be considered.

FIGURE 1: TEM micrograph (24 000 x) showing disperoids and dislocations in APM.

Oxidation properties Because of their high a l u m i n u m content, FeCrA1 alloys will form. a t h i n surface layer of ¢(-alumina when heated in most atmospheres c o n t a i n i n g traces of oxygen. This oxide layer works as an excellent diffusion barrier, p r o t e c t i n g the metal from further reaction with the atmosphere. The a l u m i n a scale is very t h i n a n d a d h e r e n t to the metal. It will not fall off as easily as the c h r o m i u m oxide scale formed on NiCr a n d FeNiCr alloys. Because FeCrA1 alloys have a higher m e l t i n g p o i n t a n d more protective oxide, they can o p e r a t e at t e m p e r a t u r e s up to 1400°C c o m p a r e d to a b o u t 1200°C for the best NiCr alloys. The a l u m i n u m in the alloy is gradually c o n s u m e d by forming a n d m a i n t a i n i n g the a l u m i n a scale. When the a l u m i n i u m c o n t e n t reaches a critically low value, cracks in the a l u m i n a scale can no longer be repaired. A c a t a s t r o p h i c oxidation, leading to failure, will s t a r t . The r a t e of t h e a l u m i n u m c o n s u m p t i o n and the critical a l u m i n i u m c o n t e n t are both f u n c t i o n s of a n u m b e r of variables such as t e m p e r a t u r e , d i m e n s i o n , a t m o s p h e r e a n d t h e r m a l cycling. The a l u m i n a scale works perfectly well in most protective a t m o s p h e r e s . C a r b u r i z i n g a n d s u l p h u r o u s e n v i r o n m e n t s are u s u a l l y h a r m l e s s to FeCrAI alloys b u t can be very corrosive to NiCr a n d FeNiCr alloys. In such cases a d r a m a t i c increase in life is u s u a l l y observed if Ni m a t e r i a l is replaced by a n FeCrA1 material. In the case of very low oxygen pressures, such as in v a c u u m or in p u r e argon, the f o r m a t i o n rate of the oxide can be slow. W i t h o u t the oxide, a l u m i n i u m a n d chro-

// Conventional

FeCrAI

Creep rate (l/s) FIGURE 2: Creep test made on 4.00 mm diameter wire. Secondary creep rate versus stress for PM FeCrAI (Kanthal APM) and a similar alloy produced in the conventional way.

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Time to rupture (h) FIGURE 3: Creep rupture strength versus time for APM, tested on 4.00 mm diameter wire.

MPR October 1992 47

PM

SPECIAL

FEATURE

FIGURE 4: Longitudinal section of a partially transformed 3 mm thick tube wall. One large grain has grown from the surface (top half of picture) through half the wall thickness. The original grain size can still be seen in the lower half.

m i u m can vaporize from the m e t a l surface. To avoid this, a n oxide scale m u s t be formed a n d m a i n t a i n e d t h r o u g h pre- a n d r e - o x i d a t i o n in air. In N2 c o n t a i n i n g H2, often 5% to 10%, or in cracked a m m o n i a , a d e w p o i n t above -25°C is e n o u g h to e n s u r e the safe formation a n d m a i n t e n a n c e of a protective oxide scale. At lower d e w p o i n t s pre- a n d reo x i d a t i o n in air are r e c o m m e n d e d . In some a p p l i c a t i o n s such as fluidized bed furnaces, a hot s t r e a m of gas c o n t a i n i n g solid particles hit the m e t a l surface. The c o m b i n a t i o n of m e c h a n i c a l erosion a n d the corrosive reaction with the gas can be very aggressive to the metal. Recent tests on APM have given positive results because of good a d h e r e n c e a n d h a r d n e s s of t h e a l u m i n a scale.

Applications :[J[[

FIGURE 5: Tubothal is a bundle rod heating system with Kanthal APM used in the heating wire, centre rod and the tube. The heater is available in five different diameters from 68-170 mm. The length is variable.

FIGURE 6: A 10 kW electric ladle preheater made of a direct heated PM tube of diameter 26 mm.

48 MPR October 1992

For a n u m b e r of r e a s o n s APM r a p i d l y b e c a m e a well e s t a b l i s h e d resistance heating alloy. Its good hot s t r e n g t h m a d e it p o s s i b l e to i n c r e a s e t h e p o w e r of t h e h e a t i n g e l e m e n t s t h a t it was u s e d i n w i t h o u t r u n n i n g i n t o d e f o r m a t i o n problems. It was possible to e l i m i n a t e e x p e n sive systems for s u p p o r t i n g the elements. Totally n e w or simplified e l e m e n t designs could be produced. The spiral e l e m e n t h a n g i n g on a ceramic t u b e is c o m m o n in i n d u s t r i a l furnaces. The m a x i m u m power from these e l e m e n t s is limited by e l e c t r o m a g n e t i c forces at the coil ends. These forces t e n d to cause b u n c h i n g of t h e e n d t u r n s . T e s t s a n d p r a c t i c a l e x p e r i e n c e have shown t h a t coils m a d e of APM can w i t h s t a n d higher power in these a p p l i c a t i o n s t h a n a n y c o n v e n t i o n a l alloy. The s e m i c o n d u c t o r i n d u s t r y uses diffusion furnaces for processing silicon wafers. Here a c c u r a c y a n d even d i s t r i b u t i o n of t e m p e r a t u r e are extremely i m p o r t a n t . Def o r m a t i o n of the e l e m e n t in these furnaces is n o t acceptable. APM m a k e s it possible to b u i l d bigger furnaces a n d use fewer ceramic APM is also found in p a r t s to hold the h e a t e r in place. APM is a l s o f o u n d in b u n d l e r o d elements. These consist of a n u m b e r of Us h a p e d wire h a i r p i n s c o n n e c t e d in series a n d held in place by ceramic discs placed on a s u p p o r t i n g c e n t r e rod. In a v e r t i c a l p o s i t i o n the h e a t e r is self s u p p o r t i n g b u t it is u s u a l l y p r o t e c t e d by a tube. For horizontal m o u n t i n g the h e a t e r rests inside a tube. C o n v e n t i o n a l FeCrAl alloys have been tested in b u n d l e rod e l e m e n t s b u t they fail because they deform and cause short circuits. With APM this type of e l e m e n t does n o t deform. The T u b o t h a l b u n d l e rod

PM

FIGURE 7: Kanthal APM radiant tubes used with different heat sources. Left: U-shaped gas fired. Right, from the top: single end gas fired, SiC (electric heater), Kanthal Super element (MoSi 2, metallic coil on ceramic core and Tubothal element.

h e a t i n g system (Figure 5) has become very p o p u l a r because of its low weight a n d high h e a t i n g power. Some of the biggest e l e m e n t s built to date were recently installed in a tiltable m e l t i n g a n d h o l d i n g f u r n a c e in a n alumin u m foundry. An old system of oil b u r n e r s was replaced by n i n e T u b o t h a l e l e m e n t s of 95 kW each. The 5.2 m long elements, of d i a m e t e r 170 m m , w e r e h o r i z o n t a l l y m o u n t e d in 6.3 m long tubes.

FIGURE 8: Examples of application of APM radiant tubes: continuous annealing of wire.

SPECIAL

FEATURE

FIGURE 9: Two different types of gas burner nozzles made of Kanthal APM.

An interesting, b u t n o t as yet c o m m o n , h e a t e r design is the directly heated tube. This is used directly as a n electric r e s i s t a n c e h e a t i n g element. Tubes have a high surface to weight ratio a n d a good form stability. This m a k e s these types of e l e m e n t s light, fast a n d robust. Figure 6 shows an e x a m p l e of a directly heated t u b e element. It is a self s u p p o r t i n g e l e m e n t for p r e h e a t i n g of ladles in a steel foundry. The 10 kW conic h e a t e r coil is m a d e of a 2 m long APM t u b e of d i a m e t e r 26mm. Figure 6 also illustrates the good formability of the tubes. K a n t h a l APM has the o x i d a t i o n properties of a r e s i s t a n c e h e a t i n g m a t e r i a l a n d the s t r e n g t h of a c o n s t r u c t i o n material. Today more t h a n half of the volume of p a r t s m a d e from this alloy are used in non-electric a p p l i c a t i o n s in furnaces. The n u m b e r of such a p p l i c a t i o n s is growing all the time. The biggest a p p l i c a t i o n for APM today is in r a d i a n t t u b e s in i n d u s t r i a l furnaces. These are t u b e s in which gas b u r n e r s or electrical h e a t i n g e l e m e n t s can be used as t h e h e a t source. The t u b e s s h i e l d t h e h e a t i n g source from the f u r n a c e a t m o sphere (Figures 7 a n d 8). The h i g h e s t r e c o m m e n d e d o p e r a t i n g t e m p e r a t u r e for APM r a d i a n t t u b e s is 1300°C. This is a b o u t 150°C higher t h a n any NiCr t u b e can w i t h s t a n d . In the case of conversions to APM tubes, c u s t o m e r s can use this m a r g i n either to increase t h e i r f u r n a c e p o w e r a n d p r o d u c t i v i t y , or to reduce their m a i n t e n a n c e costs by increasing t u b e life. K a n t h a l APM has also been tested in nozzles in gas b u r n e r s (Figure 9). The alloy can be m a c h i n e d to complicated shapes. The a l u m i n a oxide in APM gives these nozzles p r o t e c t i o n a g a i n s t chemical corrosion a n d good p r o t e c t i o n a g a i n s t erosion of particles in the gas. Tests results so far look very promising.

MPR October 1992 49