RESISTANCE-HEATING ELEMENT

Abstract: An electrical resistance element consists mainly of a silicide phase according to the formula Mo(Si1-xAlx)2, which phase forms alumina on the surface of the element. The element is intended to be used in connection with sintering of metal powder. Because of its surprisingly good ability to form an alumina layer on the surface, this material has shown to be very resistant against corrosion at high termperatures over 1500 DEG C in both reducing and oxidizing atmosphere.

Description: RESISTANCE-HEATING ELEMENT The present invention relates to a new electrical resistance-heating element of molybdenum suicide type intended to use in connection with sintering of metal powder. Resistance-heating elements of the above-mentioned kind are existing since the 1950ies for instance with the trademark Kanthal Super. These consist usually mainly of a metal like phase of MoSi 2 , alternatively of Mo x Wι_ x Si and also an oxide phase of type aluminum silicate. Similar types of material can be used at element temperatures up to 1900°C in oxidizing atmosphere. What makes this high working temperature possible is, besides the high melting point of the material (over 2000°C), the oxide layer of SiO 2 , which will be formed and rapidly makes the basic material passive against accelerating oxidation and by this makes a long service time for the heating element possible. This outer layer gives a lasting protection in several furnace- and heat treating atmospheres, as air, oxygen, nitrogen/ hydrogen gas, cracked ammonia, and others. Conditions that often limit the use in these atmospheres are, when a high temperature of the element is prevailing simultaneous with a too low potential of oxygen, alternatively dew point, in the atmosphere. What happens if the critical proportions between dew point - temperature of the element is exceeded is, that the SiO 2 -layer becomes unstable and by that after a certain time does not give any protection of the base material. For instance in hydrogen gas this occurs at an element temperature of 1300°C when the dew point is lower than about -30°C. To keep the SiO 2 -layer remaining stable at a temperature of the element of 1450°C, a dew point over +20°C is required, i.e. a hydrogen gas containing more than 2.3 percent by volume. The stability properties of the SiO 2 -layer make up a restriction of the usage of the element in certain connections. An application example where such limitation manifests itself is at sintering of metal powder in order to produce stainless steel. Components of the stainless steel grade AISI316L are above all produced by pressing of powder, alternatively by injection moulding of metal powder. After evaporation of the binding agent at a low temperature often a final sintering in the range of temperature between 1300-1360°C in reduced atmosphere is required. The reducing gas can be pure hydrogen gas with a dew point at -40°C to -60°C, corresponding about 0.01 and 0.001 percent by volume of water respectively. The low dew point has to be obtained in order to reduce metal oxides during sintering process and by that resulting in that a material with high density and good mechanical properties. In such an application example an element temperature between 1400 and 1550°C should be required, dependent on the element shape and furnace design. Under those conditions the SiO 2 -layer is consequently not stable on heating elements based on MoSi . Heating elements which are used today in many furnaces for sintering of metal powder in the temperature range over 1250-1300°C are manufactured of above all molybdenum, but also of tungsten. A limitation of this material is, besides its relatively high total cost in furnaces, the requirement to hold the elements under all circumstances over 400°C in an atmosphere deficient in oxygen to avoid, that detrimental oxidation of the pure molybdenum metal occurs. For example at leakage of furnaces or other breakdowns those elements can consequently be damaged. The alternative materials, which exist for electrical resistance heating under these conditions, are alloys and intermetallic compounds as FeCrAl, NiCr and MoSi 2 (e.g. Kanthal Super as above). The limitations of MoSi 2 -material were described above. FeCrAl and NiCr form oxides of Al 2 O 3 and Cr 2 O 3 respectively on the surface under use in air. In reducing atmosphere, as dry hydrogen gas, the range of temperature under use is limited to about 1400°C for FeCrAl and 1250°C for for example NiCr of the trademark Nicrothal 80 respectively. In case of NiCr-alloys the Cr 2 O 3 is not stable above this temperature. In case of FeCrAl the Al O -layer certainly remains stable, but the service time of the material at this temperature is limited by the closely allied smelting temperature of about 1500°C. Thus, if the FeCrAl should be used for sintering of 316L, the requirements for high element temperatures would lead to very limited service times. It would be desirable to use a material that combines the possibility to form alumina on the surface with a melting temperature considerable higher than 1500°C and by that, if necessary alternating could be used in reducing and oxidizing atmosphere. Besides, then the disadvantages of the molybdenum elements could be eliminated since the elements not always have to be used in an atmosphere deficient in oxygen. It has suφrisingly shown, that by alloying MoSi with Al in a sufficient quantity and also an alumina phase, a phase of molybdenum alumina suicide, Mo(Siι- x Al x ) 2 is obtained, which is stable in dry hydrogen gas at high temperatures. It has for example shown under a corrosion test in hydrogen gas at 1450°C, that the compound material MoSiι .6 Alo .4 /Al 2 O 3 does not show any corrosion after 200 hours, but an insignificant weight increase of 0.2%, which replies to an oxidation of aluminum in the aluminoslilicide into Al 2 O 3 . In a comparing investigation with Kanthal Super 1800, consisting of MoSi and about 20 percent by volume of aluminum silicate it has shown, that the weight was reduced with about 30% under the same conditions. In this case besides the SiO -layer both the MoSi 2 -phase, Mo 5 Si 3 and Mo 3 Si and also the aluminum silicate containing binding agent were reduced. It is reasonable to assume that even other compositions can be used to obtain similar results. For instance it has shown, that MoSi]. 75 Alo .25 bAl 2 O 3 forms Al 2 O 3 under oxidation in air at 1200°C. At values for x in the range of 0.1-0.6 the hexagonal, so called C-40-phase of Mo(Si,Al) , is stable. It is reasonable to assume that the present invention even could be applied with those compositions. The value of x should amount to 0.10-0.60, preferably 0.20-0.55. It is also beyond all doubt, that molybdenum could be substituted by tungsten to Mθι. y W y Siι. x Al x under maintenance of the desirable properties for sintering of metal powder. Herewith y should amount to a numerical value in the range of 0-0.4, preferably 0.05-0.20. Substitution of Mo with W can be done with maintained crystal structure, Cl 1, and thereby increase the service temperature of the heating element with the composition Mθι- y W y Si 2 . This is applied for instance for heating elements with the trademark Kantahl Super 1900, which in similarity with Kanthal Super 1800 form SiO 2 on the surface. Analogous the C-40- phase will be formed even at alloying of the aluminum at the substitution of molybdenum with tungsten according to Moi_ y W y Sii. x Al x . The remaining phases, which can be formed at high x-values, in the system Mo-Si-Al are for example aluminides of molybdenum, which appears from the phase diagram Figure 1 , valid at 1823 K. Under experimental work in connection with the present invention it was proved suitable, that the suicide phase amounts to between 65 and 95 percent by weight of the total weight of the resistance element, preferably between 75 and 85 percent by weight. As mentioned above, the resistance element contains besides the suicide phase alumina too. The balance can consist of SiO 2 , suitably between 0 and 1 percent by weight. The above-described invention could also be used in other sintering applications, where low oxygen potential, alternatively a low dew point, is required. This is sometimes the case at sintering of powder of tungsten heavy alloy, certain types of alloys of titanium and intermetallic compounds, and also of low alloyed steels. For instance the heavy alloy W-Cu have been sintered in hydrogen at 1400°C with a dew point at -36°C. The above mentioned formed alumina is expected to be stable up to about 1595°C, which is the eutectic temperature in the system SiO 2 -3Al 2 O 3 -2SiO 2 (mullite). This oxide is expected to have different corrosion properties than the pure alumina. Up to at least the temperature of the element of around 1595°C could thereby this material constitute a general alternative to the heating elements of molybdenum-type.

Claims: Claims 1. An electrical resistance element mainly consisting of a silicide phase according to the formula Mo(Siι- x Al x ) 2> which phase forms alumina on the surface of the element, whereby the element is intended to be used in connection with sintering of metal powder. 2. An electrical resistance element according to claim 1, characterized in, that Mo is partly substituted by W for forming a silicide phase according to the formula Moi_ y W y (Sii- x Al x ) 2 , which phase forms alumina on the surface of the element. 3. An electrical resistance element according to claim 1, characterized in, that x amounts to between 0.10 and 0.60, preferably between 0.20 and 0.55. 4. An electrical resistance element according to claim 2, characterized in, that y amounts to between 0 and 0.40, preferably between 0.05 and 0.20. 5. An electrical resistance element according to any of preceding claims, characterized in, that the silicide phase amounts to between 65 and 95 percent by weight of the total weight of the resistance element, preferably between 75 and 85 percent by weight. 6. An electrical resistance element according to any of the preceding claims, characterized in, that the element contains besides the mentioned silicide phase 0 to 40 percent by volume of AlO 3 , preferably 10 to 20 percent by volume. 7. An electrical resistance element according to any of the preceding claims, characterized in, that the element contains a balance of SiO 2 besides the mentioned silicide phase and prospective Al 2 O 3 . 8. A metal powder, which will be sintered by help of an electrical resistance element according to any of the preceding claims, characterized in, that it is of type iron-based alloys. 9. Metal powder according to claim 8, characterized in, that the alloy is a stainless steel. 10. Metal powder according to claim 8, characterized in, that the designation of the alloy is AISI316L. 11. Method of sintering a metal powder, whereby as a source of heat a heating element containing molybdenum silicide is used, characterized in, that the heating element is in addition to Mo and Si also alloyed with Al and AlO 3 in quantities, optimized such, that the heating element forms a phase of molybdenum aluminum silicide of type Mo(Siι. x Al x ) 2 , which is stable at the sintering temperature and where x amounts to between 0.1 and 0.6. 12. Method according to claim 11, characterized in, that Mo is partly substituted by W in order to form a material Mθι. y W y (Siι. x Al x ) . 13. Method according to any of claims 10-12, characterized in, that x amounts to a value between 0.20 and 0.55. 14. Method according to any of claims 10-12, characterized in, that y amounts to a value between 0 and 0.40. 15. Method according to any of claims 10-14, characterized in, that it applies at sintering of iron-based powder, preferably stainless steel. 16. Method according to any of claims 10-15, c h a r a c t e r i z e d in, that the heating element contains above the silicide phase even Al 2 O in an amount between 10 and 20 percent by volume.

Inventors: SUNDBERG MATS, SUNDBERG, MATS
Applicants: SANDVIK AB
Application year: 2000
Main classification: C22C29
IPC codes: B22F3, C04B35, C22C33, H05B3