New process for manufacturing a chromium alloyed molybdenum silicide portion of a heating element

Abstract

A process of manufacturing a chromium alloyed molybdenum silicide portion of a heating element comprising the steps of: forming a mixture of a chromium powder and a silicon powder; reacting the mixture to a reaction product in an inert atmosphere at a temperature of at least 1100° C. but not more than 1580° C.; converting the reaction product to a powder comprising CrSi.sub.2; forming a powder ceramic composition by mixing the powder comprising CrSi.sub.2 with a MoSi.sub.2 powder and optionally with an extrusion aid; forming the portion of the heating element; and sintering the portion of the heating element in a temperature of from about 1450° C. to about 1700° C.; characterized in that the chromium powder and the silicon powder are provided separately to the mixture.

Claims

1. A process of manufacturing a chromium alloyed molybdenum silicide portion of a heating element comprising the steps of: forming a mixture of a chromium powder and a silicon powder; reacting the mixture to a reaction product in an inert atmosphere at a temperature of at least 1100° C. but not more than 1580° C.; converting the reaction product to a powder comprising CrSi.sub.2; forming a powder ceramic composition by mixing the powder comprising CrSi.sub.2 with a MoSi.sub.2 powder and optionally with an extrusion aid; forming the portion of the heating element; and sintering the portion of the heating element in a temperature of from about 1450° C. to about 1700° C.;  wherein the chromium powder and the silicon powder are provided separately to the mixture.

2. The process according to claim 1, wherein the forming step is extrusion of powder ceramic composition to the portion of the heating element.

3. The process according to claim 1, wherein the forming step comprises subjecting the ceramic powder composition to an isostatic forming pressing process wherein said process comprises the step of: adding the ceramic powder composition to a capsule which has at least part of the shape of the desired object; and subjecting said capsule to a predetermined pressure at a predetermined temperature for a predetermined time.

4. The process according to claim 1, wherein the portion of the heating element is optionally pre-sintered before sintering.

5. The process according to claim 4, wherein the pre-sintering step is performed in inert atmosphere at a temperature of from about 1400° C. to about 1600° C.

6. The process according to claim 1, wherein the extrusion aid is an inorganic clay.

7. The process according to claim 6, wherein the inorganic clay is an aluminum silicate clay, such as bentonite clay.

8. The process according to claim 1, wherein the sintered portion of the heating element comprises more than 90 weight % Mo.sub.1-xCr.sub.xSi.sub.2 and the balance is extrusion aid and unavoidable impurities.

9. The process according to claim 8, wherein x is between 0.08 to 0.15.

10. A heating element comprising the chromium alloyed molybdenum silicide portion according to claim 1.

11. A heating element comprising more than one of the chromium alloyed molybdenum silicide portion according to claim 1.

12. The heating element according to claim 10, wherein the whole heating element is composed of the chromium alloyed molybdenum silicide portion.

13. Use of the process according to claim 1 for manufacturing a heating element.

14. A heating element comprising the chromium alloyed molybdenum silicide portion manufactured according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 shows the weight change during air exposure at 450° C.;

[0030] The disclosure is further illustrated by the following non-limiting illustrative examples.

EXAMPLE

[0031] A chromium powder and a silicon powder were mixed, and the mixture was reacted in argon atmosphere to form CrSi.sub.2 and analyzed by x-ray diffraction (XRD). The obtained powder, which was obtained through milling, contained mostly CrSi.sub.2, a small fraction of CrSi and indications of elemental Si and Cr. A CrSi.sub.2powder and was then mixed with a production charge of MoSi.sub.2 powder according to specified stoichiometric compositions, and 4 to 6 wt % Bentolite-L was used as binder phase (extrusion aid) and petrol in a ball mill. The ceramic paste was extruded into 6 mm diameter rods, which were subsequently dried and pre-sintered and then sintered in inert atmosphere, using for example as hydrogen or argon, in temperatures of 1000° C. to 1520° C. until a dense material is obtained. The resulting material contained Mo.sub.1-xCr.sub.xSi.sub.2, where x=0.08, 0.12, 0.14 and 0.16, equivalent to 2.7, 4.1, 4.9 and 5.4 at % Cr, respectively. A reference material made from purchased CrSi.sub.2 powder, with a final composition of x=0.13, equivalent of 4.3 at % Cr was also prepared. Samples of each composition were ground to remove the protective SiO.sub.2 scale that was formed during final sintering. Samples were placed individually on alumina sample holders to collect potential oxidation products and include them in the weight measurements. The samples were placed in laboratory air in an electrical furnace heated to 450° C. employing FeCrAl heating elements and utilized with ceramic fiber insulation. Sample and holder were weighted to monitor individual weight changes as function of exposure time. For the two reference materials, MoSi.sub.2 reference and Cr-alloyed MoSi.sub.2 reference, data were taken from other pest tests. The resulting data is presented in the figure below:

[0032] FIG. 1 shows that all Cr-alloyed materials performed substantially better than the reference MoSi.sub.2-based material KS1700 and better in comparison to previous experience of Cr-alloyed material. Cr.sub.0.14 equivalent to 4.9 at % Cr (orange) appeared to have an ideal composition with respect to resistance against pesting.

[0033] The material was also evaluated by SEM-EDS with respect to phase distribution and oxide thickness. In comparison to the reference Cr.sub.0.15 material the distribution of Cr seems to be slightly different. In the case of reference Cr.sub.0.15, which was made from elemental Mo, Si and Cr, Cr was concentrated to certain areas unevenly distributed in the cross section of the material. In the case of material alloyed by sintering MoSi.sub.2 and CrSi.sub.2, Cr was more homogeneously distributed along the grain boundaries of MoSi.sub.2, either as tetragonal (Mo,Cr).sub.5Si.sub.3 (D8.sub.m) phase or as hexagonal (Mo,Cr)Si.sub.2 (C40) phase. Hence, EDS showed that Cr distribution was concentrated to certain grain boundaries in case of reference Cr.sub.0.15 made from elemental powders while homogeneous distribution of Cr along grain boundaries of MoSi.sub.2 was found in Cr.sub.0.14 made from CrSi.sub.2 and MoSi.sub.2 powders.