BRAZED OBJECT AND PROCESS FOR BRAZING TWO OR MORE PARTS

Abstract

The invention provides a process for brazing two or three parts. A braze with a composition consisting of Ni.sub.resCr.sub.aB.sub.bP.sub.cSi.sub.d with 20 atomic percent<a<22 atomic percent; 1.2 atomic percentb3.6 percent; 12.5 atomic percentc14.5 atomic percent; 0 atomic percentd<1.5 atomic percent; incidental impurities0.5 atomic percent; and residual Ni is inserted between two or more parts to be joined to form a joint, the parts to be joined having a higher melting temperature than the braze. The joint is heated to a temperature of between 1020 C. and 1070 C. and cooled to form a brazed joint between the parts.

Claims

1. A brazed object, comprising a first part of the object being connected fast to a second part by a solder seam, the solder seam comprising a braze produced with a composition consisting of
Ni.sub.resCr.sub.aB.sub.bP.sub.cSi.sub.d with 20 atomic percent<a<22 atomic percent; 1.2 atomic percentb3.6 atomic percent; 12.5 atomic percentc14.5 atomic percent; 0.5 atomic percentd1.5 atomic percent; incidental impurities0.5 atomic percent; and residual Ni, wherein the loss of solder seam mass after ageing for 1000 hours at 70 C. in a corrosion medium with a pH value<2 and SO.sub.4.sup.2 NO.sub.3.sup. Cl.sup. ions is less than 0.08%.

2. The brazed object in accordance with claim 1, wherein the solder seam comprises intermetallic phases comprising Cr and P and/or B which have a size d of 0 m<d3 m.

3. The brazed object in accordance with claim 2, wherein the size d of the intermetallic phases is 0.5 md2 m.

4. The brazed object in accordance with claim 1, wherein the solder seam has a thickness of greater than 15 m.

5. The brazed object in accordance with claim 1, wherein the solder seam is produced at a temperature of 1020 C. to 1070 C.

6. The brazed object in accordance with claim 1, wherein the solder seams have tensile strength that is greater than 200 MPa.

7. The brazed object in accordance with claim 1, wherein the first part and the second part each consist of a chromium-containing stainless steel.

8. The brazed object in accordance with claim 5, wherein the brazed object is a heat exchanger or an exhaust gas recirculation cooler or a metallic particle filter.

9. The brazed object in accordance with claim 7, wherein the chromium-containing stainless steel comprises an austenitic stainless steel or Ni alloy or a Co alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments are explained in greater detail below with reference to the drawings.

[0037] FIG. 1 illustrates a brazed object in accordance with a first embodiment.

[0038] FIG. 2 illustrates a diagram representing the relationship between corrosion resistance and soldering temperature in brazed objects in accordance with a second embodiment.

[0039] FIG. 3A illustrates a micrograph of an object brazed at a temperature of 1000 C., FIG. 3B illustrates a micrograph of an object brazed at a temperature of 1050 C., FIG. 3C illustrates a micrograph of an object brazed at a temperature of 1100 C. and FIG. 3D illustrates a micrograph of an object brazed at a temperature of 1150 C. after corrosion testing in accordance with a third embodiment.

[0040] FIG. 4 illustrates a diagram representing the relationship between corrosion resistance and soldering temperature in brazed objects in accordance with a fourth embodiment.

[0041] FIG. 5 illustrates a micrograph of a solder seam of a brazed object in accordance with a fifth embodiment.

[0042] FIG. 6 illustrates a micrograph of a solder seam of a brazed object in accordance with a sixth embodiment.

[0043] FIGS. 7A and 7B illustrate micrographs of a solder seam of a brazed object in accordance with a seventh embodiment.

[0044] FIG. 8 illustrates a diagram representing the relationship between tensile strength and soldering temperature in brazed objects in accordance with an eighth embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0045] FIG. 1 illustrates a schematic representation of a brazed object 1 in accordance with a first embodiment.

[0046] The object 1 has a first part 2 and a second part 3 which are joined by fusion by a solder seam 4. In this embodiment the parts 2, 3 are made of stainless steel. In further embodiments the parts are made of an austenitic stainless steel or a Ni alloy or a Co alloy.

[0047] The solder seam 4 is produced with a braze with a composition consisting of Ni.sub.resCr.sub.aB.sub.bP.sub.cSi.sub.d with 20 atomic percent<a<22 atomic percent; 1.2 atomic percentb3.6 atomic percent; 12.5 atomic percentc14.5 atomic percent; 0 atomic percentd1.5 atomic percent; incidental impurities0.5 atomic percent; and residual Ni. However, the overall composition of the solder seam 4 cannot correspond to the composition of the braze if the braze has reacted with elements of the parts 2, 3 or if elements from the parts 2, 3 have migrated to the braze during the brazing process and formed phases with the components of the braze.

[0048] The solder seam 4 has intermetallic phases 5 which comprise Cr, in particular at least 80% by weight Cr, and P and/or B and have a size d of 0 m<d3 m. The size of these intermetallic phases 5 can be measured in a ground solder seam by means of analysis using an optical or scanning electron microscope. The size of the individual grains of the intermetallic phases 5 can vary but lies within this range. The intermetallic phases comprising Cr and P and/or B are distributed across the entire thickness of the solder seam 4.

[0049] The brazed object 1 is produced with a braze which has a composition consisting of Ni.sub.resCr.sub.aB.sub.bP.sub.cSi.sub.d with 20 atomic percent<a<22 atomic percent; 1.2 atomic percentb3.6 percent; 12.5 atomic percentc14.5 atomic percent; 0 atomic percentd1.5 atomic percent; incidental impurities0.5 atomic percent; and residual Ni. The braze is provided in the form of an amorphous ductile foil and inserted between the first part 2 and the second part 3, thereby creating a joint from the first part 2, the brazing foil and the second part 3. The parts 2, 3 to be joined have a higher melting temperature than the braze.

[0050] The joint is heated to a soldering temperature of between 1020 C. and 1070 C., preferably 1030 C. and 1060 C., in a hydrogenous atmosphere and then cooled to form a brazed joint between the parts 2, 3, thereby connecting the first part 2 to the second part 3 by a solder seam 4.

[0051] A soldering temperature within the range of 1020 C. to 1070 C. permits the reliable production of a brazed object 1 with a corrosion-resistant and mechanically stable solder seam 4. Furthermore, this solder seam 4 can have intermetallic phases 5 containing Cr and P and/or B and a size d of 0 m<d3 m. In particular, the solder seam 4 has good corrosion resistance in aggressive media such as acid media.

EXAMPLE 1

[0052] First, a Ni-based brazing alloy with the composition NiCr21P8Si0.5B0.5 (% by weight) is produced as an amorphous soldering foil with a thickness of 30 m using rapid solidification technology. This brazing foil is used to solder samples of stainless steel (in particular stainless steel 316L, 1.4404) in which a base plate is joined to two pipe sections at soldering temperatures of 1000 C., 1050 C., 1100 C. and 1150 C. in a vacuum for a soldering time of 15 minutes.

[0053] These samples are then aged in a corrosion medium with a pH value <2 and SO.sub.4.sup.2 NO.sub.3.sup. Cl.sup. ions at 70 C. for a total period of 1000 h. The change in mass of the samples is recorded at 200 h intervals.

[0054] FIG. 2 illustrates the loss in mass of stainless steel samples joined with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight) at different soldering temperatures of 1000 C., 1050 C., 1100 C. and 1150 C. in relation to ageing time. The brazed samples joined at 1150 C. and 1100 C. illustrate a significantly greater loss in mass, which is synonymous with markedly greater corrosion, than the samples brazed at 1000 C. and 1050 C. The samples joined at the higher soldering temperatures of 1100 C. and 1150 C. also illustrate a greater rise in curve after 1000 h ageing, suggesting that corrosion is further advanced.

[0055] Better corrosion resistance corresponding to the lowest loss in solder sample mass is observed at soldering temperatures of 1050 C. and 1000 C.

EXAMPLE 2

[0056] First a Ni-based brazing alloy with the composition NiCr21P8Si0.5B0.5 (% by weight) is produced as an amorphous soldering foil with a thickness of 30 m using rapid solidification technology. This brazing foil is used to solder samples of stainless steel (in particular stainless steel 316L, 1.4404) in which a base plate is joined to two pipe sections at soldering temperatures of 1000 C., 1050 C., 1100 C. and 1150 C. in a vacuum.

[0057] A corrosion test is then carried out. Prior to ageing the samples are cut up to give the corrosion medium as great a contact surface as possible in the area of the solder seams. Ageing then takes place in a corrosion medium with a pH value of <2 and SO.sub.4.sup.2 NO.sub.3.sup. Cl.sup. ions at 70 C. over a total period of 1000 h. Following ageing the brazed stainless steel samples are prepared metallographically to evaluate the corrosion of the solder seams.

[0058] FIG. 3 illustrates a metallographic evaluation of the stainless steel samples brazed in a vacuum at various soldering temperatures produced with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight) after ageing in the corrosion medium for 1000 hours.

[0059] FIGS. 3A to 3D illustrate metallographic specimens from the soldering seams joined at soldering temperatures of 1000 C. (FIG. 3A), 1050 C. (FIG. 3B), 1100 C. (FIG. 3C) and 1150 C. (FIG. 3D). It is clear that the samples brazed at 1100 C. and 1150 C. in particular have undergone massive corrosion as evidenced by the black areas on the specimens. These black areas are areas of the solder seam dissolved by corrosion. Large areas of the solder seam have been significantlyat 1150 C. soldering temperatureand even completely dissolved by the corrosion medium. The joint is no longer mechanically stable or tight.

[0060] In the sample brazed at 1050 C. the solder seam illustrates only local corrosion as indicated by the black areas in the micrograph. In the sample brazed at 1000 C. no significant area of corrosion can be seen. Better corrosion resistance ensuring a stable, tight soldered joint over the entire period of use can be achieved at a soldering temperature <1100 C.

EXAMPLE 3

[0061] First a Ni-based brazing alloy with the composition NiCr21P8Si0.5B0.5 (% by weight) is produced as an amorphous soldering foil with a thickness of 35 m using rapid solidification technology. This brazing foil is then used to solder samples of stainless steel (in particular stainless steel 3104; 1.4404) at soldering temperatures of 1000 C., 1050 C. and 1100 C. in a continuous furnace under hydrogen for a soldering time of 10 minutes. Parts of these solder samples are aged in a corrosion medium with a pH value <2 and SO.sub.4.sup.2 NO.sup.3 Cl.sup. ions at 70 C. for a total period of 1000 hours. The change in mass of the samples is recorded at 200 h intervals.

[0062] FIG. 4 illustrates the loss in mass of the stainless steel samples joined with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight) at the various soldering temperatures in relation to ageing time. An increased loss in mass is an indicator that the soldered joint is damaged and the long-term stability of the soldered joint is thus no longer ensured. The brazed samples joined at 1100 C. illustrate a significantly greater loss in massconsistent with significantly more marked corrosionthan the samples brazed at temperatures of below 1100 C. Better corrosion resistance corresponding to the lowest loss in soldering sample mass is once again achieved at soldering temperatures of 1050 C. and 1000 C.

[0063] It is thus established that joint formation/microstructure within the solder seam is influenced by soldering temperature.

[0064] FIG. 5 illustrates the microstructure/phase formation of a brazed stainless steel sample produced with a braze foil with the composition NiCr21P8Si0.5B0.5 (% by weight), the object having been soldered for 10 minutes at 1000 C. under hydrogen in a continuous furnace.

[0065] The microstructure within the solder seams with NiCrP and NiCrSiP brazes is characterized by the marked formation of intermetallic phases or brittle phases. While with NiCrBSi brazes silicidic and boridic brittle phases occur only in the center of the solder seam with wide solder gaps, with NiCrPSiB solders the entire solder seam is generally run through by various intermetallic phosphoridic phases as can be seen in FIG. 5.

[0066] One reason for the improved corrosion resistance of the samples soldered at temperatures of less than 1100 C. could lie in the formation of intermetallic phases with Cr and B and/or P, in particular a high chromium-containing phase which contains approx. 80% chromium and phosphorus and boron in addition to a metal content (Ni, Fe) of <10% by weight.

[0067] This phase clearly binds large amounts of chromium to a CrB/P compound. These relatively large amounts of bound chromium are therefore no longer available to improve corrosion resistance. In particular, the areas of the joint adjacent to these CrB/P phases could be significantly chromium-impoverished, thereby significantly weakening the corrosion resistance of these areas and making them susceptible to greater corrosion.

[0068] FIG. 6 illustrates the microstructure/phase formation of a brazed stainless steel sample produced with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight). This sample was brazed for 10 minutes at 1000 C. under hydrogen in a continuous furnace. The solder seam has finely distributed CrPB brittle phases with a size of 1-2 m.

[0069] FIG. 7A and FIG. 7B illustrate the microstructure/phase formation of the solder seam of a stainless steel sample joined with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight). The sample was soldered for 10 minutes at 1050 C. under hydrogen in a continuous furnace.

[0070] FIG. 7A illustrates distributed CrPB brittle phases some of which are arranged in agglomerations with a size of between 3-6 m. FIG. 7B illustrates a detailed view of an agglomeration of straight-edged CrB/P brittle phases, some of which are rectangular, with a size between 3-6 m. FIG. 7A and FIG. 7B also illustrate these CrB/P phases which are significant in terms of corrosion as straight-edged and often rectangular structures, some of which are also arranged in agglomerations.

[0071] As the soldering temperature increases, this CrPB phase appears to become coarser and to occupy a greater volume of the solder seam. Thus, for example, the typical size of these CrB/P brittle phases increases from 1-3 m at a soldering temperature of 1000 C. (FIGS. 6) to 3-6 m at 1050 C. (FIG. 7A and FIG. 7B). The increasing volume of this phase in conjunction with the coarser aspect leads to a greater percentage of bound chromium within the solder seam. This is associated with poorer corrosion resistance. For better corrosion resistance it would appear advantageous for this CrPB phase to be as small as possible and not to exceed a size of approx. 3-6 m.

EXAMPLE 4

[0072] A static tensile test is carried out to determine the mechanical strength of the soldered joints. The type of sample chosen is a butt-soldered tensile sample (DIN EN 12797:200 type 3) made of steel 316/1.4404. The samples are butt-soldered with a brazing foil with the composition NiCr21P8Si0.5B0.5 (% by weight) at different soldering temperatures with a soldering time of 30 minutes.

[0073] FIG. 8 represents in graphic form the measured tensile strength of these joints soldered at different soldering temperatures.

[0074] A soldering temperature of 1000 C. results in a tensile strength of less than 25 MPa which is insufficient for many technical applications. At soldering temperatures of 1030 C., 1090 C. and 1150 C. a tensile strength of above 200 MPa is achieved, with relatively stable values being achieved in this temperature range. Consequently, it is possible to ensure a long-term mechanically stable and tight connection if brazing is carried out with this composition at a temperature above 1020 C. or 1030 C.

[0075] The invention having been thus described with reference to certain specific embodiments and examples thereof, it will be understood that this is illustrative, and not limiting, of the appended claims.