HEAT-INSULATING COMPONENT AND A METHOD OF MAKING SAME

Abstract: A heat-insulating component consists of a porous body obtained by moulding and sintering an iron-base powder having an admixture of 2-10% by weight of sili­ con, preferably 4-8% by weight. A method of making the heat-insulating component includes the steps of preparing an iron-base powder mixture with an admix­ ture of 2-10% by weight of silicon, preferably 4-8% by weight; optionally supplying the powder with fur­ ther admixtures in the form of 3-12% by weight of manganese, preferably 5-10% by weight, up to 25% by weight of chromium, up to 15% by weight of nickel, up to 2.5% by weight of molybdenum and up to 2% by weight of carbon, moulding the powder mixture into a body of desired shape, and sintering the body for obtaining a porous component having a coefficient of thermal conductivity below about 12 W/m°K, prefer­ ably below 7 W/m°k.

Description: The present invention relates to a heat-insulating component and a method of making same. The invention also relates to a method of lowering the thermal conduc­ tivity of a component obtained from an iron-base powder mixture by moulding and sintering. Substantial efforts have been made over the years to develop ceramic materials which are suitable for use in internal combustion engines. Although these ef­ forts have met with some success, the ceramic materials, by being relatively brittle, have however caused a num­ ber of problems reducing their usefulness. Also, diffi­ culties in durably joining the ceramic material to metal are encountered since the materials used normally have different coefficients of heat expansion. Similarly, the ceramic materials are difficult or impossible to use if after-treatment is necessitated by shape or de­ mands on tolerance. The need of being able to prevent heat from being conducted out to the engine block of an internal combus­ tion engine has increased with the demand for exhaust emission control, like the demand for an increase of the efficiency of a diesel engine, e.g. by controlling the thermal losses. The object of the invention therefore is to develop a product having a low thermal conductivity, more speci­ fically a coefficient of thermal conductivity below about 12 W/m°K, and most preferably below about 7 W/m°K, in combination with toughness, strength, machinability, freedom of choice in respect of manufacturing method, and a coefficient of heat expansion allowing joining the product to metal in a simple and durable manner. It has been found quite surprisingly that this is feasible starting from a metallic powder. It is not to be expected that metals without the addition of oriented ceramic flakes may be used for heat-insulating purposes. From British patent specifica­ tion GB-2,l24,658 it is thus known to use 10-30% by weight of oriented ceramic flakes in a stainless alloy for manufacturing brake components with directional heat transmission. By adding silicon and possibly manganese to a pow­ der or to a melt for atomization, consisting of pure iron or iron-nickel, and thereafter manufacturing porous sintered bodies, it was however found that it was possible to adjust the heat-insulating properties to values equi­ valent to those obtained with zirconium oxide. Silicon strongly affects the thermal conductivity and the amount of silicon should be between 2 and 10% by weight and preferably between 4 and 8% by weight. If the amount of silicon becomes excessive, the liquid phase also becomes excessive, entailing that the powder body will collapse upon sintering and the porosity will decrease dramatically. The addition of manganese primarily affects the workability of the sintered body but also, to some extent, the thermal conductivity. It has been found that if manganese is to be added, the amount should be between 3 and 12% by weight and preferably between 5 and 10% by weight. If there is a demand for high corrosion resistance, chromium may also be added. The amount of chromium must not exceed 25% by weight since with larger amounts, a compact will not hold together after compaction. A chromium amount of about 21% has been particularly suit­ able. For increased strength of the sintered body, nickel may be added in an amount of up to 15% by weight. Also other alloying materials, such as molybdenum and carbon, may be added without noticeably deteriorating the inventive effect. Powder mixtures may be preferable, giving in­ creased flexibility in the choice of alloying additives and are sometime necessary for achieving the required compressibility. For certain components and methods of manufacture, it has however been found more appro­ priate to use prealloyed atomized powder. To sum up, the present invention requires no ceramic flakes or in any way oriented particles, but the excellent heat-insulating properties are achieved by producing thermal barriers by structural transition, primarily by means of silicon but also by means of manganese. This entails e.g. that the components ac­ cording to the invention, as opposed to those disclosed in GB-2,124,658, can be manufactured by all techniques currently used within the powder metallurgy, with or without additives for pore formation in dependence upon the desired insulating capacity and the required accuracy of the finished component. The invention will now be exemplified in more detail in the non-limitative Examples given below. EXAMPLE 1 Three metal powders A, B and C of the following compositions were prepared. A: 100.0% pure iron powder B: 97.5% Fe + 2.5% Si C: 90.0% Fe + 7.5% Mn + 2.5% Si From these three powders, specimens were compacted at a compacting pressure of 400 MPa. The specimens were sintered at 1250°C for 1 h in hydrogen gas atmos­ phere. Since the thermal conductivity is directly depen­ dent on the porosity of the material, the compacting pressure was so adjusted that the specimens of the three different powders all had a porosity of 25% by volume after sintering. The coefficient of thermal conductivity was then determined and the following results were obtained. EXAMPLE 2 Four metal powders D, E, F and G of the following compositions were prepared. D: 85% Fe + 15% Cr E: 80% Fe + 15% Cr + 5% Si F: 75% Fe + 15% Cr + 5% Si + 5% Mn G: 70% Fe + 15% Cr + 5% Si + 10% Ni + 0.8% C As in Example 1, specimens were manufactured having a porosity of 25% by volume after sintering. The coefficient of thermal conductivity for the different materials was determined as well as the co­ efficient of heat expansion and tensile strength (R m ), giving the following results. It appears from the above Table that powder F yields a material in which it has been possible, most surpris­ ingly, to combine a very low thermal conductivity with a coefficient of heat expansion which closely conforms to e.g. cast iron, and a satisfactory mechanical strength. EXAMPLE 3 Two metal powders H and I of the following composi­ tions were prepared. H: 70% Fe + 10% Ni + 18% Cr + 2% Mo I: 62% Fe + 10% Ni + 18% Cr + 2% Mo + 8% Si As in the earlier Examples, specimens were prepar­ ed having a porosity of 25% by volume, whereupon thermal conductivity, coefficient of heat expansion and tensile strength were determined. The following results were obtained. These results show that the thermal conductivity, without altering the tensile strength, can be consider­ ably reduced by alloying a stainless powder with sili­ con or silicon and manganese. In order to check that the thermal barrier is not adversely affected by different methods of manu­ facture, specimens according to Examples 1, 2 and 3 were prepared by extrusion, injection moulding and isostatic compacting. After sintering and correction for a slightly varying pore volume, it was found that different methods of manufacture, using Examples 1, 2 and 3, give a fully comparable coefficient of thermal conductivity. In order to further elucidate the effect of a variation of the amount of silicon, manganese and chromium on the coefficient of thermal conductivity, specimens were prepared as described above on the basis of metal powder with varying amounts of one of these alloying materials. EXAMPLE 4 Four metal powders J, K, L and M were prepared having a constant amount of manganese and chromium and a varying amount of silicon, as stated below. J: 80% Fe + 10% Mn + 10% Cr + 0% Si K: 78% Fe + 10% Mn + 10% Cr + 2% Si L: 75% Fe + 10% Mn + 10% Cr + 5% Si M: 70% Fe + 10% Mn + 10% Cr + 10% Si The thermal conductivity of the specimens manufac­ tured from these mixtures was determined and the follow­ ing results were obtained. Material M exhibited a considerably reduced poro­ sity as a consequence of an excessive liquid phase. Thus, the thermal conductivity decreases considerably with an increasing amount of silicon up to about 10% silicon. EXAMPLE 5 Four metal powders N, O, P and Q were prepared having a constant amount of silicon and manganese and a varying amount of chromium, as stated below. N: 80% Fe + 5% Si + 5% Mn + 10% Cr O: 75% Fe + 5% Si + 5% Mn + 15% Cr P: 70% Fe + 5% Si + 5% Mn + 20% Cr Q: 65% Fe + 5% Si + 5% Mn + 25% Cr The thermal conductivity of the specimens manu­ factured from these mixtures was determined and the following results were obtained. Material Q exhibited poor green strength and did not hold together after compacting and, therefore, could not be sintered. A certain minor reduction of the thermal conductivity with an increasing amount of chromium was thus found. EXAMPLE 6 Three metal powders R, S and T of the following compositions were prepared. R: 80% Fe + 5% Si + 15% Cr + 0% Mn S: 75% Fe + 5% Si + 15% Cr + 5% Mn T: 75% Fe + 5% Si + 10% Cr + 10% Mn The thermal conductivity of the specimens manufac­ tured from these mixtures was determined and the fol­ lowing results were obtained. Also in this case, there was a slight reduction of the thermal conductivity with an increasing amount of manganese.

Claims: 1. A heat-insulating component, charac­ terized in that it consists of a porous body obtained by moulding and sintering an iron-base powder having an admixture of 2-10% by weight of silicon, preferably 4-8% by weight. 2. Heat-insulating component as claimed in claim 1, characterized in that the powder has a further admixture of 3-12% by weight of manganese, preferably 5-10% by weight. 3. Heat-insulating component as claimed in claim 1 or 2, characterized in that the powder has a further admixture of less than 25% by weight of chromium, preferably about 21% by weight. 4. Heat-insulating component as claimed in claim 2 or 3, characterized in that the powder has a further admixture of up to 15% of nickel. 5. Heat-insulating component as claimed in any one of claims 2-4, characterized in that the powder has a further admixture of up to 2.5% by weight of molybdenum. 6. Heat-insulating component as claimed in any one of claims 2-5, characterized in that the powder has a further admixture of up to 2% by weight of carbon. 7. A method of making a heat-insulating component, characterized by the steps of preparing an iron-base powder mixture with an admixture of 2-10% by weight of silicon, preferably 4-8% by weight; op­ tionally adding to the powder further admixtures in the form of 3-12% by weight of manganese, preferably 5-10% by weight, up to 25% by weight of chromium, up to 15% by weight of nickel, up to 2.5% by weight of molybdenum and up to 2% by weight of carbon; moulding the powder mixture into a body of desired shape, and sintering the body for obtaining a porous component having a coefficient of thermal conductivity below about 12 W/m°K, preferably below 7 W/m°K. 8. A method of lowering the thermal conductivity of a component obtained from an iron-base powder mix­ ture by moulding and sintering, character­ ized in that the powder mixture is supplied with an admixture of 2-10% by weight of silicon, preferably 4-8% by weight. 9. Method as claimed in claim 8, charac­ terized in that the powder mixture is supplied with a further admixture of one or more of the group consisting of manganese, chromium, nickel, molybdenum and carbon.

Inventors: ENGSTROEM, ULF, MUSTONEN, OLAVI, Engström, Ulf
Applicants: HOEGANAES AB
Application year: 1987
Main classification: B22F3
IPC codes: C22C38, C22C33