HEAT RESISTANT STRUCTURE AND METHOD OF MANUFACTURE THEREOF

Abstract: This invention relates to a heat resistant structure used in an atmosphere of high temperature, for example, a catalytic converter for purifying exhausted gas from an automobile engine and the like, to which catalyst as a carrier is stuck, and also relates to a method of manufacture thereof. The structure (1) is composed of stainless steel corrugated sheets (2) and flat sheets (4) which are alternately joined to each other through the brazing material (3), (7), and (8) so as to form a honeycomb structure of a roll-like or laminated block type. This invention can easily eliminate such problems as segregation of Cr, stress corrosion cracking, grain attack cracking grain separation and cracks, that is to say, insufficient resistance to acidification and low durability, which are liable to occur in the vicinity of the joint surfaces of the sheets when used in the high temperature atmosphere, and have hitherto been desired to be solved. Technical characteristics of the structure (1) of the invention reside in the use of Ni group brazing material including Si and B, distribution of a certain fixed quantity of general Ni group brazing material to required parts, or the use of Fe group brazing material.

Description: The present invention relates to a heat resisting structure to be used at a high environmental temperature. More particularly, the present invention relates to a heat resisting structure wherein stainless steel corrugated sheets, flat sheets, etc. are joined together with brazing filler metal to form a honeycomb-like structure and it is used, for example, for a catalytic converter to treat exhaust gas of an automobile engine or the like as a catalyst holding body to which catalyst is made adhered. A heat resisting structure wherein stainless steel corrugated sheets having alternating ridges and grooves formed by folding the sheet continuously and flat stainless steel sheets, etc. are joined together with nickel(Ni)-base brazing filler metal to form honeycomb-like structure is conventionally used. In such a conventional heat resisting structure, stainless steel corrugated sheets, flat sheets, etc. which are highly resistant to corrosion and heat are used as base metal and nickel (Ni)-base brazing filler metal which is also highly resistant to corrosion and heat is used to withstand a high environmental temperature. The heat resisting structure is formed into a roll-like form by rolling up the corrugated sheet, flat sheet, etc. and joining together with nickel (Ni)-base brazing filler metal or formed into a laminated block by stacking the materials into many layers and joining together, and thus formed honeycomb-like structure is utilized as a catalyst holding body and for other various uses. Now, relating to the corrugated sheet, flat sheet, brazing filler metal, etc. of the heat resisting structure, prior art will be examined in detail. First, as to the corrugated sheet, flat sheet, etc. constituting the base metal, those of austenitic stainless steel are used. Recently, those of ferritic stainless steel also are in use. As to the brazing filler metal, nickel (Ni)-base brazing filler metal is used generally by applying it to the entire surface. The brazing filler metal is provided over the entire surface between the corrugated sheet, flat sheet, etc. constituting the base metal by, for example, coating the entire surface, placing foil-like brazing filler metal between the sheets over the entire surface, or using a brazing sheet coated with brazing filler metal over the entire surfaces. Composition of brazing filler metal currently in use is, by weight, 0.1% C maximum, 7.0 to 8.0% silicon Si, 18.0% to 19.0% chromium Cr, 1.0 to 1.5% boron B, and the remainder is nickel Ni. However, the following problems have been pointed out concerning the above-mentioned conventional heat resisting structure. First, the heat resisting structure employing austenitic stainless steel corrugated sheets, flat sheets, etc. is excellent in oxidation resistance as well as high-temperature strength to withstand a high environmental temperature during the use. However, this heat resisting structure, when stress is applied during use at a high environmental temperature, stress corrosion cracks sometimes occur in the corrugated sheets, flat sheets, etc. constituting the base metal, in some cases carbide is precipitated on grain boundaries of the austenitic portion of the base metal, where intergranular corrosion cracking occurs, and thus durability of the structure at a high environmental temperature is affected. In addition, there is a problem in cost due to expensiveness of the material. To solve the problems in the heat resisting structure of austenitic stainless steel mentioned above, heat resisting structure wherein ferritic stainless steel corrugated sheets, flat sheets, etc. are used as the base metal has been developed. Secondly, in the heat resisting structure employing stainless steel, particularly ferritic stainless steel corrugated sheets, flat sheets, etc., the state of elements of the alloy, so-called phase, of the corrugated sheets, flat sheets, etc. which are the base metal is different from that of nickel(Ni)-base brazing filler metal. Therefore, when nickel(Ni)-base brazing filler metal diffuses into the base metal owing to brazing for joining, the phase tends to change in the vicinity of the boundary where the base metal is joined, causing phase transition. (For example, transition to gamma phase. Hereinafter, the same applies.) Therefore, the use at a high environmental temperature is sometimes affected adversely. More specifically, when such conventional heat resisting structure is used at a high environmental temperature, chromium Cr contained in nickel(Ni)-base brazing filler metal is segregated in the vicinity of the boundary where the base metal of, for example, ferritic stainless steel is joined by brazing, making the chromium Cr concentration very high at the portion of the metal phase, and the portion of low chromium Cr concentration is prone to oxidation. Further, when the conventional heat resisting structure is used at a high environmental temperature, carbon C contained in the nickel(Ni)-base brazing filler metal is sometimes precipitated as carbide on the grain boundaries in the vicinity of the boundary where the base metal is joined by brazing. The precipitated material reduces the strength lower than the toughness. Therefore, stress corrosion cracking, intergranular corrosion cracking, intergranular separation, etc. tend to occur in the austenitized portion in a similar manner to that mentioned above. (For example, transition to gamma phase. Hereinafter, the same applies.) Thirdly, because the phase of the stainless steel, for example, ferritic stainless steel corrugated sheets, flat sheets, etc. is different from the phase of nickel(Ni)-base brazing filler metal, there is a difference in coefficient of thermal expansion between the two. Consequently, repeated use of the heat resisting structure sometimes generates stress cracks between them. Particularly, since there is a great difference in the coefficient of thermal expansion between the austenitized portion and the base metal, cracks tend to occur there as mentioned above. Fourthly, due to above-mentioned reasons, a problem that conventional heat resisting structure is poor in oxidation resistance and durability when uses at a high environmental temperature has been pointed out. Furthermore, in such heat resisting structure, brazing filler metal which causes the above-mentioned problem is arranged in large quantities over the entire surface between the corrugated sheets and flat sheets constituting the base metal, therefore oxidation rate is large, oxidation resistance is poor, and durability is also poor. Fifthly, when such conventional heat resisting structure is used, for example, for a catalytic converter to treat exhaust gas of an automobile engine as a catalyst holding body to which catalyst is made adhered, a problem has been pointed out in treatment efficiency of the catalyst. In detail, since brazing filler metal made of nickel(Ni)-base brazing filler metal is used in quantities over the entire surface as described previously, adhesion of alumina wash-coat to be applied as primer between the corrugated sheets, flat sheets, etc. constituting the base metal and the catalyst to the base metal is made poor, and as a result, adhesion of the catalyst to the base metal is sometimes impaired. Even if the catalyst adheres normally, the catalyst dissolves into the brazing filler metal as solid solution, thereby lowering the catalyst density at the surface of the base metal. As described above, when the conventional heat resisting structure is used as a catalyst holding body, the catalyst is hard to adhere and tends to dissolve into the brazing filler metal as solid solution, therefore, treatment efficiency of the catalyst becomes low and exhaust gas treatment efficiency becomes poor. Such problems have been pointed out concerning the conventional heat resisting structure . FR-A-2577616 discloses a heat resisting structure of a honeycomb-like structure formed by alternately joining a corrugated stainless steel sheet, having alternating ridges and grooves formed by folding the sheet continuously, and a flat or corrugated stainless steel sheet by brazing material, and, according to the present invention, such a structure is characterised in that the brazing material is a nickel-based material substantially free of C and Cr and containing 4.0 to 8.0 by weight of Si and 2.0 to 4.5% by weight of B. This heat resisting structure may be made in a roll-like form wherein a belt-like corrugated sheet and flat sheet are alternately overlapped, rolled up, and joined. Alternatively the heat resisting structure may be made in a laminated block-like form wherein corrugated sheets of predetermined length and flat sheets are alternately and sequentially overlapped in many layers and joined. Further, this heat resisting structure, for example, is made in a roll-like form using a belt-like corrugated sheet only or made in the laminated block form using corrugated sheets only as the case may be. The invention provides various advantages. Thus, when this heat resisting structure is used at a high environmental temperature, occurrence of segregation at the vicinity of the joint boundary of the corrugated sheet, flat sheet, etc. constituting the base metal joined with brazing filler metal is reduced and consequently generation of a high chromium Cr concentration portion is reduced, as a result, occurrence of a problem that a low chromium Cr concentration portion is prone to oxidation is prevented. In addition, in this heat resisting structure, the effect of carbide precipitated in the vicinity of the joint boundary on the grain boundaries is reduced, thereby stress corrosion cracking, intergranular corrosion cracking, intergranular separation, etc. are reduced. Secondly, since adverse effect of austenitization in the vicinity of the joint boundary is reduced, cracking in the vicinity of the joint boundary is avoided. Accordingly, in spite of the difference in the composition between the corrugated, flat sheets, etc. and the brazing filler metal, and difference in the coefficient of thermal expansion between them, occurrence of cracks in the vicinity of the join boundary is almost avoided. Thirdly, considering the points mentioned above, this heat resisting structure is small in the oxidation rate and has improved oxidation resistance and excellent durability. Fourthly, this heat resisting structure is advantageous in the aspect of cost, because it can be manufactured with ease with least man-hours. For example, arranging the brazing filler metal and its setting are easy. In the accompanying drawings:- Figs. 1 and 2 show preferred embodiments of the heat resisting structure in accordance with the present invention, in which Fig. 1 is a perspective view of the roll-form structure and Fig. 2 is a perspective view of the laminated block-form structure. Fig. 3 is a graph showing the comparison of the oxidation rate between the heat resisting structure of the present invention and conventional heat resisting structure. Referring first to Figs. 1, 2 and 3, the heat resisting structure 1 shown in these drawings is of a honeycomb-like structure wherein a belt-like stainless steel corrugated sheet 2, etc. having alternating ridges and grooves formed by folding the sheet continuously is joined alternately via brazing filler metal 3. First, referring to Fig. 1, as a base metal of the heat resisting structure 1, one belt-like corrugated sheet 2 having alternating ridges and grooves formed by folding the sheet continuously and made of stainless steel foil, and one belt-like flat sheet 4 of stainless steel foil are used. In Fig. 1, one long sheet of such corrugated sheet 2 and one long sheet of flat sheet 4 are used. The corrugated sheet 2 is formed into a large number of straight ridges and grooves of predetermined pitch and height continuously and in parallel by folding the sheet by a corrugating machine, press, etc. The base metal of the heat resisting structure 1 may be such that more than one corrugated sheets 5 and flat sheets 4 may be used disregarding the example of Fig. 1. Further, use of one or more corrugated sheets, without using the flat sheet 4, may be feasible. In the embodiment shown in Fig. 1, the shape of the heat resisting structure 1 is a roll-form of a circular cylinder, elliptical cylinder, etc. wherein one corrugated sheet 2 and one flat sheet 4 are alternately lapped, rolled up, and joined starting at the center via the brazing filler metal 3. The shape of the heat resisting structure may be such, different from the embodiment shown in Fig. 1, that more than one plate-like corrugated sheets 2 of a predetermined length, flat sheets 4, etc. constituting the base metal are alternately and sequentially stacked in many layers and joined via brazing filler metal 3 to make a laminated block-like form as shown in Fig. 2. And the heat resisting structure 1 is a honeycomb-like construction. That is, the base metal, i.e., in the examples of Figs. 1 and 2, the corrugated sheets 2 and flat sheets 4 form cell walls and a honeycomb-like structure which is a plane assembly of many hollow column-like cells 5 of a variety of shapes including a triangle, semihexagon, trapezoid, or the like. Accordingly, this heat resisting structure 1 has an excellent specific strength, light weight, and high rigidity and strength. It is also known to have a good flow straightening effect, easy-to-form feature, and low cost. Further, the surface area per unit volume is large, in other words, the surface area of cell walls formed by the corrugated sheets 2, flat sheets 4, etc., is large, and in addition the high temperature strength is large since the corrugated sheets 2 and flat sheets 4 are made of stainless steel which is highly resistant to heat and the brazing filler metal 3 is made of nickel (Ni)-base brazing filler metal which is also highly resistant to heat. Due to such advantages, this heat resisting structure 1 is, for example, used for a catalytic converter to treat exhaust gas of an automobile engine, etc., as a catalyst holding body and catalyst is made adhered to the surface of the corrugated sheets 2 and flat sheets 4 which form the cell walls. In such a heat resisting structure 1, nickel(Ni)-base brazing filler metal containing silicon Si and boron B is used as the brazing filler metal 3 for joining. More specifically, nickel(Ni)-base brazing filler metal composed mainly, by weight, of 4.0 to 8.0% silicon Si, 2.0 to 4.5% boron B, and the remainder nickel Ni is used as the brazing filler metal 3. Carbon C and chromium Cr are not almost contained in the main components. Such brazing filler metal 3 is provided at places in the joint area between the corrugated sheets 2 and flat sheets 4 which constitute the base metal, and melts when heated and brazes the joint area when hardens. In Fig. 1, reference number 6 designates a case which is an outer cylinder for holding an inserted roll-form heat resisting structure 1. The heat resisting structures 1 shown in Figs. 1 and 2 are constructed in the above-mentioned manner and used, for example, as a catalyst holding body of a catalytic converter of an automobile engine or the like. Exhaust gas of an automobile engine or the like which contains harmful substances and of a high temperature is treated by reaction of the catalyst made adhered to the corrugated sheet 2, flat sheet 4, etc. which form the cell walls while it is passing through each cell 5 of honeycomb-like structure of the heat resisting structure 1. This heat resisting structure 1 uses nickel(Ni)-base brazing filler metal containing silicon Si and boron B as the brazing filler metal for joining stainless steel corrugated sheets 2 and flat sheets 4. Therefore, advantages described in (1), (2), (3) and (4) below are obtained. (1) The brazing filler metal 3 contains almost no chrome Cr. Accordingly, even when this heat resisting structure 1 is used at a high environmental temperature, segregation (chromium Cr concentration becomes very high partially) in the vicinity of the brazing joint boundary of the stainless steel corrugated sheets 2, flat sheets 4, etc. constituting the base metal does not occur, thereby eliminating the possibility that the low chromium portion becomes prone to oxidation, and oxidation rate of this heat resisting structure 1 is small. Fig. 3 is a graph showing the comparison of oxidation rate between a heat resisting structure of prior art and the heat resisting structure 1 of the present invention. More specifically, Fig. 3 shows a cycle test result wherein one cycle was 30-min period, the heat resisting structure was heated to a high temperature of 1,000°C, and then the structure was air-cooled for 10 minutes, again one cycle of high temperature heating was performed, thereafter this procedure was repeated many times, and each time increase of oxide was measured under such test conditions. As shown in Fig. 3, the heat resisting structure 1 of the present invention which uses the brazing-filler metal 3 containing no chromium Cr is far low in the oxidation rate in comparison with a heat resisting structure of prior art which uses the brazing filler metal containing chromium Cr. This fact does not change even when the number of cycles is increased. This brazing filler metal 3 contains almost no carbon C. Accordingly, when the heat resisting structure 1 is used at a high environmental temperature, adverse effect due to precipitation of carbide on the grain boundary in the vicinity of brazing joint boundary of the stainless steel corrugated sheets 2, flat sheets 4, etc. constituting the base metal, does not exist. Since no carbide is precipitated, there is no possibility of formation of precipitation phase of shard portion and other soft portion, and accordingly occurrence of stress corrosion cracking, intergranular corrosion cracking, intergranular separation, etc. at this portion are prevented. (2) The heat resisting structure 1 of the present invention is small in the oxidation rate, high in oxidation resistance, and excellent in durability. (3) These advantages are realized without reducing the quantity of the brazing filler metal 3. In other words, the heat resisting structure 1 of the present invention is not designed to promote oxidation resistance and durability by reducing the quantity of the brazing filler metal 3 to be used. Arranging the brazing filler metal 3 is simple and easy because positioning of the brazing filler metal 3 in the corrugated sheets 2, flat sheets 4, etc. constituting the base metal for forming the heat resisting structure 1 of the present invention requires no exact control, and consequently required production man-hours are relatively small. (4) The heat resisting structure 1 of the present invention is excellent in strength since the brazing joint portion is sufficiently secured as a whole. Particularly, when the flat sheets 4 are not used but the corrugated sheets 2 alone are used as the base metal of the heat resisting structure 1 without following the embodiments shown in Figs. 1 and 2, the advantage in terms of strength is significant. That is, in the case of such construction, the brazing joint area is small and therefore the strength is uncertain; if the brazing joint area is reduced by reducing the quantity of the brazing filler metal to be used despite this fact, the strength decreases remarkably. On the contrary, however, in the heat resisting structure 1 of the present invention, the brazing joint area is sufficiently secured even in such construction, thereby an advantage in the aspect of strength is maintained. As have been described so far, the heat resisting structure in accordance with the present invention is suitable for use at a high environmental temperature, and actually it is used for a variety of uses. For example, it is used for a catalytic converter for treatment of exhaust gas of an automobile engine as a catalyst holding body, to which the catalyst is made adhered.

Claims: A heat resisting structure (1) of a honeycomb-like structure formed by alternately joining a corrugated stainless steel sheet, (2) having alternating ridges and grooves formed by folding the sheet continuously, and a flat or corrugated stainless steel sheet (4) by brazing material (3); characterised in that the brazing material is a nickel-based material substantially free of C and Cr and containing 4.0 to 8.0% by weight of Si and 2.0 to 4.5% by weight of B. A structure as claimed in claim 1, wherein a belt-like corrugated sheet (2) and a flat sheet (4) are alternately overlapped, rolled up, and joined to make a roll-like form. A structure as claimed in claim 1, wherein plate-like corrugated sheets (2) and flat sheets (4) of a predetermined length are alternately and sequentially stacked in many layers and joined to make a laminated block-like form.

Inventors: MATSUOKA KATSUNORI, TOYODA TETSURO SHOWA AIRCRAFT, MATSUOKA, KATSUNORI, TOYODA, TETSURO SHOWA AIRCRAFT INDUSTRY CO., LTD.
Applicants: SHOWA AIRCRAFT IND, SHOWA AIRCRAFT INDUSTRY CO., LTD.
Application year: 1990
Main classification: B32B3
IPC codes: B21D47, B32B3, F01N3