LOW MELTING IRON BASED BRAZE FILLER METALS FOR HEAT EXCHANGER APPLICATIONS

Abstract

Iron-based braze filler alloys having unexpectedly narrow melting temperature ranges, low solidus and low liquidus temperatures, as determined by Differential Scanning calorimetry (DSC), while exhibiting high temperature corrosion resistance, good wetting, and spreading, without deleterious significant boride formation into the base metal, and that can be brazed below 1,100 C contains a) nickel in an amount of from 0% to 35% by weight, b) chromium in an amount of from 0% to 25% by weight, c) silicon in an amount of from 4% to 9% by weight, d) phosphorous in an amount of from 5% to 11% by weight, e) boron in an amount of from 0% to 1% by weight, and f) the balance being iron, the percentages of a) to f) adding up to 100% by weight. The braze filler alloys or metals have sufficient high temperature corrosion resistance to withstand high temperature conditions of Exhaust Gas Recirculation Coolers.

Claims

1. An iron-based braze filler alloy comprising: a) nickel in an amount of from 0 wt % to 35 wt, b) chromium in an amount of from 0 wt % to 25 wt %, c) silicon in an amount of from 4% wt % to 9% wt %, d) phosphorous in an amount of from 5 wt % to 11 wt %, e) boron in an amount of from 0 wt % to 1 wt %, and f) the balance being iron, the percentages of a) to f) adding up to 100 wt %, and wherein the total amount of iron, nickel, and chromium is from 84 wt % to 90 wt, the ratio of a/(a+f) is from 0 to 0.5, and the ratio of b/(a+b+f) is from 0 to 0.33, wherein the iron-based braze tiller alloy has a brazing temperature of less than 1,100° C., and wherein the iron-based braze filler alloy has at least one of: a solidus temperature which is less than or equal to 1,030° C., a liquidus temperature which is less than or equal to 1,075° C., or a melting range where the difference between the solidus temperature and the liquidus temperature is less than 85° C.

2. The iron-based braze filler alloy as claimed in claim 1 which is a ternary alloy FeSiP wherein the amount of iron is from 84 wt % to 90 wt %, the percentages of [a)+c)+d)] adding up to 100 wt %, and said melting range is less than or equal to 25° C.

3. The iron-based braze filler alloy as claimed in claim 1 wherein the amount of nickel is from 25 wt % to 35 wt %, the percentages of a) to f) adding up to 100 wt %.

4. The iron-based braze filler alloy as claimed in claim 1, wherein the amount of chromium is from 18 wt % to 25 wt %, the percentages of a) to f) adding up to 100 wt %.

5. The iron-based braze filler alloy as claimed in claim 1, wherein the amount of boron is greater than 0 wt % but less than 1 wt %, the percentages of a) to f) adding up to 100 wt %.

6. The iron-based braze filler alloy as claimed in claim 5 wherein the amount of boron is from 0.1 wt % to 0.5 wt %, the percentages of a) to f) adding up to 100 wt %.

7. The iron-based braze filler alloy as claimed in claim 1 wherein: a) the nickel is in an amount of from 25 wt % to 35 wt %, b) the chromium is in an amount of from 18 wt % to 25 wt %, c) the silicon is in an amount of from 4 wt % to 9 wt %, d) the phosphorous is in an amount of from 5 wt % to 11 wt %, and e) the boron is in an amount of from 0.1 wt % to 0.5 wt % and f) the balance is iron.

8. The iron-based braze filler alloy as claimed in claim 1 wherein: a) the nickel is in an amount of from 28 wt % to 33 wt %, b) the chromium is in an amount of from 18 wt % to 22 wt %, c) the silicon is in an amount of from 4.5 wt % to 6 wt %, d) the phosphorous is in an amount of from 6 wt % to 10 wt %, and e) the boron is in an amount of from 0.1 wt % to 0.5 wt % and f) the balance is iron.

9. The iron-based braze filler alloy as claimed in claim 1, wherein the boron is in an amount of from 0.3 wt % to 0.4 wt %.

10. The iron-based braze filler alloy as claimed in claim 1, wherein the iron content is 29 wt % 40 wt %.

11. The iron-based braze filler alloy as claimed in claim 1, wherein the solidus temperature is less than or equal to 1,000° C.

12. The iron-based braze filler alloy as claimed in claim 6 wherein the solidus temperature is less than or equal to 975° C.

13. The iron-based braze filler alloy as claimed in claim 1, wherein the liquidus temperature is less than 1,050° C.

14. The iron-based braze filler alloy as claimed in claim 1, wherein the difference between the solidus temperature and the liquidus temperature is less than 50° C.

15. The iron-based braze filler alloy as claimed in claim 1 having a brazing temperature of less than 1,060° C.

16. The iron-based braze filler alloy as claimed in claim 1, which is in the form of a powder, amorphous foil, atomized powder, paste, tape, or sintered preform.

17. A powder spray coating comprising the iron-based braze filler alloy as claimed in claim 1 and a binder.

18. A heat exchanger comprising an iron-based braze filler alloy as claimed in claim 1.

19. The heat exchanger as claimed in claim 18, which is an Exhaust Gas Recirculation Cooler (EGR cooler) that aids in reducing nitrogen oxide emissions (NOx) for internal combustion engines.

20. A method for producing or repairing a heat exchanger comprising brazing the exchanger with an iron-based braze filler alloy as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The present invention is further illustrated by the accompanying drawings wherein:

[0075] FIG. 1 is a Differential Scanning calorimetry curve exhibiting a single peak in a heating and cooling cycle illustrating a near true eutectic melting behavior, with a narrow melting range of 19° C., and the solidus temperature and liquidus temperature for a ternary 86.2Fe-5.1Si-8.7P iron-based braze filler alloy of Example 1 of the present invention.

[0076] FIG. 2 is a Differential Scanning calorimetry curve exhibiting double peaks in a heating and cooling cycle illustrating a wide melting range of 102° C., and the solidus temperature and liquidus temperature of a filler metal with B and Mo, (50Fe-20Ni-12Cr-3Cu-3Mo-7P-4Si-1B), an iron-based braze filler alloy of Hong et al of Comparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

[0077] An alloy starts to melt at one temperature called the solidus, and is not completely melted until it reaches a second higher temperature, the liquidus. As used herein the solidus is the highest temperature at which an alloy is solid—where melting begins. As used herein, the liquidus is the temperature at which an alloy is completely melted. At temperatures between the solidus and liquidus the alloy is part solid, part liquid. As used herein, the difference between the solidus and liquidus is called the melting range. As used herein, the brazing temperature is the temperature at which the iron-based braze filler alloy is used to form a braze joint. It is preferably a temperature which is at or above the liquidus, but it is below the melting point of the base metal to which it is applied. The brazing temperature is preferably 25° C. to 50° C. higher than the liquidus temperature of the iron-based braze filler alloy.

[0078] The melting range is a useful gauge of how quickly the alloy melts. Alloys with narrow melting ranges flow more quickly and when melting at lower temperatures, provide quicker brazing times and increased production. Narrow melting range alloys generally allow base metal components to have fairly tight clearances, for example 0.002″.

[0079] Filler alloys which have a wide melting range, which provides a wider temperature range between the solidus and liquidus where the filler metal is part liquid and part solid, may be suitable for filling wider clearances, or “capping” a finished joint. However, while helpful in bridging gaps, slowly heating a wide melting range alloy can lead to an occurrence called liquation. Long heating cycles may cause some element separation where the lower melting constituents separate and flow first, leaving the higher melting components behind. Liquation is often an issue in furnace brazing because extended heating time required to get parts to brazing temperature may promote liquation. A filler metal with a narrow melting range is preferred for this application. Even alloys with wide melting ranges, will melt quickly if they are applied at, or near, the liquidus, which is the temperature where the alloy is completely melted. The best capillary action and strongest brazed connections require close clearance between base metal parts. Accordingly, maintaining recommended clearance and brazing close to the liquidus temperature is preferred.

[0080] The solidus temperature, liquidus temperature, and melting range of the iron-based alloys are determined herein by Differential Scanning calorimetry (DSC) in accordance with the NIST practice guide, Boettinger, W. J. et al, “DTA and Heat-flux DSC Measurements of Alloy Melting and Freezing” National Institute of Standards and Technology, special Publication 960-15, November 2006, the disclosure of which is herein incorporated by reference in its entirety. In making the determinations the individual metallic powders are mixed and melted to form an alloy, the resulting alloy is solidified, the solidified alloy is ground to form a powdered alloy, and then the powdered alloy is subjected to the DSC analysis. The liquidus and solidus temperatures are determined by the profiles of the second heatings, which provides for better conformity of the alloy to the shape of the crucible, and more accurate determinations as indicated, for example, at page 12 of the NIST practice guide. The DSC analysis is performed using a STA-449 DSC of Netzsch (Proteus Software) with a 10° C./min heating rate from 700° C. to 1,100° C., or to a higher temperature as needed to exceed the liquidus temperature. From room temperature to 700° C., the differential scanning calorimeter heats at its faster programmed rate which usually takes about 20 minutes or about 35° C./min. The cooling rate employed for the DSC analysis from above the liquidus temperature back down to room temperature is also at 10° C./min, but other cooling rates may be used.

[0081] The present invention provides iron based braze filler metals or alloys that have low melting points and can be brazed below 1,100° C. They do not contain high amounts of boron which can cause erosion of base metals. The braze filler metals have sufficient high temperature corrosion resistance to withstand high temperature conditions of Exhaust Gas Recirculation Coolers (EGR coolers) which are devices that aid in reducing nitrogen oxide emissions (NOx) for internal combustion engines. The braze filler metals or alloys may be employed for brazing of catalytic converters for automobiles, heat exchangers, and other devices where, for example, brazing of thin base metals is needed.

[0082] In embodiments of the invention, iron-based braze filler metals or alloys are provided which are at or very close to the true eutectic point of the Fe—Si—P ternary system, which is the temperature at which the melting and solidification occur at a single temperature a for a pure element or compound, rather than over a range. The true ternary eutectic point of the Fe—Si—P system is difficult to determine because it must be determined using equilibrium conditions which can take days of testing to reach. In an aspect of the invention, after determining the lowest melting ternary eutectic point in the Fe—Si—P system, or as close to it as reasonably possible, as evidenced, for example, by a single peak in the DSC curve or a very narrow melting range, compositional adjustments are made with controlled additions of nickel and chromium to partly replace iron to gain high temperature corrosion resistance without any substantial increase of the melting point.

[0083] The silicon reduces the melting temperatures, and it cannot be readily diffused into the base metal as is boron. However, if too much silicon is included, it may increase brittleness and increase the liquidus temperature. The phosphorus increases wetting and flow behavior, but too much may increase brittleness, and weakness. The chromium improves corrosion resistance and increases melting temperatures, but the nickel decreases the melting temperatures. The nickel also improves both mechanical strength and corrosion resistance, with substantially lowering of the solidus and liquidus temperatures to achieve low brazing temperatures and strong bonding to the base metal, which is particularly important in thin-walled heat exchanger brazing operations and applications. Micro-alloying with small amounts of boron enables further improvement in brazeability and melting points of the iron-based braze filler metals or alloys without the deleterious effect of significant boride formation into the base metal.

[0084] Reducing the solidus temperature and the liquidus temperature to narrow the melting range of the iron-based braze filler metals or alloys provides compositions which behave more like a eutectic composition where there is no difference between the solidus and the liquidus temperatures. The narrowed down melting range provides alloys with brazing temperatures of less than 1,100° C., preferably less than 1,060° C., most preferably less than 1,050° C., with good wetting and spreading capabilities. In embodiments of the invention the iron-based braze filler metals or alloys exhibit narrow melting temperature ranges of less than 85° C., preferably less than or equal to 50° C., more preferably less than or equal to 25° C., and/or low solidus temperatures of less than or equal to 1,030° C., preferably less than or equal to 1,000° C., more preferably less than or equal to 975° C., and/or low liquidus temperatures of less than or equal to 1,075° C., preferably less than or equal to 1,050° C., even if two phases or two peaks are present, as determined by Differential Scanning calorimetry (DSC).

[0085] It not necessary to limit the chromium content, and to compensate with the addition of Cu, Mo, Ti, or rare earth elements to increase corrosion resistance, improve bonding strength, or obtain joints with high ductility. While copper may reduce melting temperatures slightly, molybdenum is a refractory metal which substantially increases melting points.

[0086] The iron-based braze filler alloy or metals of the present invention comprise: [0087] a) nickel in an amount of from 0 wt % to 35 wt %, generally at least 10 wt %, for example from 25 wt % to 35 wt %, preferably from 28 wt % to 33 wt %, more preferably from 29 wt % to 32 wt %, most preferably from 29 wt % to 31 wt %, [0088] b) chromium in an amount of from 0 wt % to 25 wt %, generally at least 10 wt %, for example from 18 wt % to 25 wt %, preferably from 18 wt % to 23 wt %, more preferably from 18 wt % to 22 wt %, for example, from 19 wt % to 21 wt %, [0089] c) silicon in an amount of from 4 wt % to 9 wt %, for example from 4 wt % to 6 wt %, preferably from 4.5 wt % to 6 wt %, more preferably from 5 wt % to 6 wt %, [0090] d) phosphorous in an amount of from 5 wt % to 11 wt %, preferably from 5 wt % to 10 wt %, more preferably from 6 wt % to 10 wt %, [0091] e) boron in an amount of from 0 wt % to 1 wt %, preferably greater than 0 wt % but less than 1 wt %, for example from 0.1 wt % to 0.8 wt %, preferably from 0.1 wt % to 0.5 wt %, more preferably from 0.3 wt % to 0.5 wt %, for example from 0.3 wt % to 0.4 wt %, and [0092] f) the balance being iron, for example from 29 wt % to 60 wt %, preferably from 29 wt % to 40 wt %, more preferably from 29 wt % to 35 wt %, most preferably from 29 wt % to 33 wt %,
the percentages of a) to f) adding up to 100 wt %. The total amount of iron, nickel, and chromium is from 84% to 90 wt %, the ratio of a/(a+f) is from 0 to 0.5, for example from 0.2 to 0.5, preferably from 0.3 to 0.5, more preferably from 0.4 to 0.5, and the ratio of b/(a+b+f) is from 0 to 0.33, preferably from 0.1 to 0.3, more preferably from 0.15 to 0.3, for example from 0.20 to 0.26. The weight percentages are based upon the weight of the iron-based filler alloy.

[0093] In aspects of the invention where the iron-based filler alloy is a ternary system of iron, silicon, and phosphorous, the iron content ranges from 84 wt % to 90 wt %, the ratio of a/(a+f) is 0, and the ratio of b/(a+b+f) is also 0. The ternary alloy has a very narrow melting range, for example, less than or equal to 25° C., approaching the melting behavior of a eutectic composition where the solidus and the liquidus temperatures are the same.

[0094] In aspects of the invention, the iron-based braze filler alloy has solidus temperatures of less than 975° C. and liquidus temperatures of less than 1,050° C. when: [0095] a) the nickel is in an amount of from 25 wt % to 35 wt %, [0096] b) the chromium is in an amount of from 18 wt % to 25 wt %, [0097] c) the silicon is in an amount of from 4 wt % to 9 wt %, [0098] d) the phosphorous is in an amount of from 5 wt % to 11 wt %, [0099] e) the boron is in an amount of from 0.1 wt % to 0.5 wt %, and [0100] f) the balance being iron,
the percentages of a) to f) adding up to 100 wt %.

[0101] In embodiments of the invention, the iron-based braze filler alloy or metal may be manufactured in the form of a powder, an amorphous foil, an atomized powder, a paste based on the powder, a tape based on the powder, sintered preforms, a powder spray coating with a binder, or a screen printing paste. The iron-based braze filler alloy or metal may be applied by spraying, or by screen printing.

[0102] In an additional aspect of the invention, a method is provided for producing or repairing a heat exchanger by brazing the exchanger with the iron-based braze filler alloy at a temperature of less than 1,100° C., preferably less than 1,060° C., more preferably less than 1,050° C.

[0103] The iron-based braze filler alloy or metal may be made using conventional methods for producing braze filler alloys or metals. For example, as conventional in the art, all of the elements or metals in the correct proportions may be mixed together and melted to form a chemically homogenous alloy which is atomized into a chemically homogeneous alloy powder. The particle size of the iron-based braze filler alloy or metal may depend upon the brazing method employed. Conventional particle size distributions conventionally employed with a given brazing method may be used with the iron-based braze filler alloy or metal of the present invention.

[0104] The base metal which is brazed with the iron-based braze filler alloy or metal may be any known or conventional material or article in need of brazing. Non-limiting examples of the base metal include alloys, or superalloys used in the manufacture of heat exchangers, Exhaust Gas Recirculation Coolers (EGR coolers), and other high temperature devices. Other non-limiting examples of known and conventional base metals which may be brazed with the iron-based braze filler alloy or metals of the present invention include carbon steel and low alloy steels, nickel and nickel alloys, stainless steel, and tool steels.

[0105] The present invention is further illustrated by the following non-limiting examples where all parts, percentages, proportions, and ratios are by weight, all temperatures are in ° C., and all pressures are atmospheric unless otherwise indicated:

EXAMPLES

[0106] Examples 1-12 relate to iron-based braze filler alloys or metals of the present invention based upon a ternary Fe—Si—P system, with additions of Ni alone, Ni and Cr alone, and Ni and Cr and B, alone. Cu and Mo are not employed as they are in Hong, Li et al, “The effect of iron-based filler metal element on the properties of brazed stainless steel joints for EGR cooler application,” Welding in the World (2019) 63:263-275, published online Dec. 14, 2018. Comparative Examples 2-5 relate to iron-based braze filler metals of Hong et al which are Fe—Ni—Cr—Cu—Mo—P—Si alloys with or without B. Comparative Example 1 relates to Amdry 805 which is discussed in Hong et al, and is an Fe—Ni—Cr—Si—P iron-based braze filler alloy which does not contain Cu or Mo, and does not contain B, all of which are indicated in Hong et al as critical for a narrow melting range with a single peak, and for enabling brazing at a temperature of 1,050° C. The compositions of iron-based braze filler alloys or metals of the present invention and comparative iron-based braze filler alloys or metals with their solidus temperature, liquidus temperature and melting range, all determined by DSC in the same manner using the STA 449(DSC) of Netzsch, using a heating rate and a cooling rate of 10° C./min are shown in Table 1:

TABLE-US-00001 TABLE 1 Melting temperature of low melting Fe(Ni, Cr)—Si—P—B alloys Melting Composition(wt %) M.P.(° C.) * Range Example No. Fe Ni Cr B P Si Cu Mo Solidus Liquidus (° C.) (1) 86.2 — — — 8.7 5.1 — — 1024 1043 19 (2) 55.1 31.3 — — 8.6 5.0 — — 934 1007 73 (3) 33.6 31.8 20.8 — 8.7 5.1 — — 1001 1042 41 (4) 31.7 32.1 21.1 — 9.4 5.7 — — 1002 1027 25 (5) 31.8 31.7 21.2 0.1 9.5 5.7 — — 974 1022 48 (6) 32.1 30.8 21.4 0.3 9.6 5.8 — — 971 1008 37 (7) 32.4 30.1 21.5 0.5 9.6 5.8 — — 963 1033 70 (8) 38.4 30.6 20.4 0.1 6.1 4.4 — — 975 1044 69 (9) 38.8 29.7 20.6 0.3 6.1 4.5 — — 965 1044 79 (10)  39.1 29.0 20.8 0.5 6.2 4.5 — — 967 1038 71 (11)  36.4 31.3 20.5 — 7.3 4.4 — — 1002 1059 57 (12)  37.2 29.3 21.0 0.5 7.5 4.5 — — 976 1029 53 Comparative 1 41 17.5 29 — 6.5 6 — — 1055 1110 55 Amdry 805 Comparative 2 50 20 12 1 7 4 3 3 905 1007 102 Hong et al Comparative 3 50.25 20 12 0.75 7 4 3 3 902 1013 111 Hong et al Comparative 4 50.75 20 12 0.25 7 4 3 3 970 1034 64 Hong et al (BJUT-Fe) Comparative 5 51 20 12 0 7 4 3 3 990 1046 56 Hong et al

[0107] Example 1 is a ternary 86.2Fe-5.1Si-8.7P iron-based braze filler alloy of the present invention. As shown in FIG. 1, the Differential Scanning calorimetry curve for the ternary alloy of Example 1 exhibits a single peak in a heating and cooling cycle indicating a near true eutectic melting behavior, with a narrow melting range of 19° C., and a solidus temperature of 1,024° C. and a liquidus temperature of 1,043° C. FIG. 2 (Prior Art) is a Differential Scanning calorimetry curve exhibiting double peaks in a heating and cooling cycle illustrating a wide melting range of 102° C., with a solidus temperature of 905° C. and a liquidus temperature of 1,007° C. for a filler metal with Cu and Mo, and B (50Fe-20Ni-12Cr-3Cu-3Mo-7P-4Si-1B), an iron-based braze filler metal of Hong et al of Comparative Example 2.

[0108] The data listed in Table 1 show that the iron-based braze filler alloys of the present invention, Examples 1-12 exhibit: a) unexpectedly low solidus temperatures of less than 1,030° C., ranging from 934° C. to 1,024° C., b) unexpectedly low liquidus temperatures of less than 1,050° C., ranging from 1,007° C. to 1,043° C., c) unexpectedly low melting ranges of less than 85° C., the melting ranges ranging from 19° C. for Example 1 to 79° C. for Example 9, and d) unexpectedly low brazing temperatures of less than 1,100° C., with no or very small amounts of boron, and without the need for copper or molybdenum as in Comparative Examples 2-5.

[0109] Also, substantially higher amounts of nickel ranging from 29.0 to 32.1 wt % in Examples 2 through 12, compared to the 20% by weight in Comparative Examples 2-5 and 17.5% by weight in Comparative Example 1 provides both improved mechanical strength and corrosion resistance, with substantial lowering of the solidus and liquidus temperatures to achieve low brazing temperatures and strong bonding to the base metal, which is particularly important in thin-walled heat exchanger brazing operations and applications. The substantially higher amounts of chromium ranging from 20.4% by weight to 21.4% by weight in Examples 3 through 12, compared to the 12% by weight in Comparative Examples 2-5 provides improved corrosion resistance and increases melting temperatures, but the nickel decreases the melting temperatures.

[0110] Also, where no boron, copper or molybdenum are employed, as in Examples 1-4 and 11: a) the solidus temperature ranges from 934° C. to 1,024° C. whereas in Comparative Example 1 (Amdry 805), the solidus temperature of 1,055° C. is at least 31° C. higher, and b) the liquidus temperature ranges from 1,007° C. to 1,059° C. whereas in Comparative Example 1 (Amdry 805), the liquidus temperature of 1,110° C. is at least 51° C. higher which would indicate the need for a brazing temperature which is at least 51° C. higher. Where boron is employed, but copper and molybdenum are not employed, as in Examples 5-10 and 12: a) the solidus temperature ranges from 963° C. to 976° C. whereas in Comparative Example 1 (Amdry 805), the solidus temperature of 1,055° C. is at least 79° C. higher, and b) the liquidus temperature ranges from 1,022° C. to 1,044° C. whereas in Comparative Example 1 (Amdry 805), the liquidus temperature of 1,110° C. is at least 66° C. higher which would indicate the need for a brazing temperature which is at least 66° C. higher.

[0111] Further, at least because the invention is disclosed herein in a manner that enables one to make and use it, by virtue of the disclosure of particular exemplary embodiments, such as for simplicity or efficiency, for example, the invention can be practiced in the absence of any step, additional element or additional structure that is not specifically disclosed herein.

[0112] It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.