Iron-chromium based brazing filler metal

Abstract

A brazing filler metal with excellent wetting behavior on stainless steel base material is provided. The brazing filler metal produces a brazed joint with high strength and good corrosion resistance. The brazing filler metal is suitable for brazing stainless steel and other materials where corrosion resistance and high strength is required. Typical examples of applications are heat exchangers and catalytic converters. The iron-chromium based brazing filler metal powder comprises: 11-35 wt % chromium, 0-30 wt % nickel, 2-20 wt % copper, 2-10 wt % silicon, 4-10 wt % phosphorous, 0-10 wt % manganese, and at least 20 wt % iron, and if Si is equal to or less than 6 wt % then P should be above 8 wt %, and if P is less or equal to 8 wt % then Si should be above 6 wt %.

Claims

1. An iron-chromium based brazing filler metal powder suitable for brazing of stainless steel base material comprising: 11-35 wt % chromium, 0-30 wt % nickel, 5-20 wt % copper, 2-10 wt % silicon, 4-10 wt % phosphorous, 0-10 wt % manganese, and trace elements in amounts less than 1 wt % balanced with at least 20 wt % iron, and if silicon is equal to or less than 6 wt % then phosphorous should be above 8 wt %, and if phosphorous is less or equal to 8 wt % then silicon should be above 6 wt %, wherein the iron based brazing filler metal powder has a mean particle size of 10-100 m.

2. The brazing filler metal powder according to claim 1, wherein the silicon content is above 6 wt % and below or equal to 10 wt %.

3. The brazing filler metal powder according to claim 1, wherein the phosphorous content is above 6 wt % and below or equal to 10 wt %.

4. The brazing filler metal powder according to claim 1, wherein the total amount of silicon and phosphorous is below or equal to 20 wt %.

5. The brazing filler metal powder according to claim 1, wherein the nickel content is between 10 and 20 wt %.

6. The brazing filler metal powder according to claim 1, wherein the copper content is between 5 and 15 wt %.

7. The brazing filler metal powder according to claim 1, wherein the manganese content is below 7 wt %.

8. The brazing filler metal powder according to claim 1, wherein the chromium content is between 20 and 30 wt %.

9. The brazing filler metal powder according to claim 1, wherein the chromium content is between 11 and 20 wt %.

10. The brazing filler metal powder according to claim 1, wherein the iron based brazing filler metal is converted to paste, tape, or foil by conventional methods.

11. The brazing filler metal powder according to claim 1, wherein the phosphorous content is above 8 wt % and below or equal to 10 wt %.

12. The brazing filler metal powder according to claim 1, wherein the nickel content is 0 and 20 wt %, wherein the phosphorous content is above 8 wt % and below or equal to 10 wt %.

13. An iron-chromium based brazing filler metal powder suitable for brazing of stainless steel base material comprising: 11-35 wt % chromium, 0-20 wt % nickel, 5-20 wt % copper, 2-10 wt % silicon, 4-10 wt % phosphorous, 0-10 wt % manganese, and trace elements in amounts less than 1 wt % balanced with at least 20 wt % iron, and if silicon is equal to or less than 6 wt % then phosphorous should be above 8 wt %, and if phosphorous is less or equal to 8 wt % then silicon should be above 6 wt %.

14. An iron-chromium based brazing filler metal powder suitable for brazing of stainless steel base material comprising: 11-35 wt % chromium, 0-30 wt % nickel, 5-20 wt % copper, 2-10 wt % silicon, 8-10 wt % phosphorous, 0-10 wt % manganese, and trace elements in amounts less than 1 wt % balanced with at least 20 wt % iron.

15. An iron-chromium based brazing filler metal powder suitable for brazing of stainless steel base material comprising: 11-35 wt % chromium, 0-30 wt % nickel, 5-20 wt % copper, 2-10 wt % silicon, 4-10 wt % phosphorous, 0-10 wt % manganese, and trace elements in amounts less than 1 wt % balanced with at least 20 wt % iron, and if silicon is equal to or less than 6 wt % then phosphorous should be above 8 wt %, and if phosphorous is less or equal to 8 wt % then silicon should be above 6 wt %, wherein the iron based brazing filler metal is converted to paste, tape, or foil by conventional methods.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) This invention relates to an iron-chromium based brazing filler metal with excellent wetting on stainless steel. The brazing filler metal produces high strength brazed joints with good corrosion resistance and is significantly lower in cost compared to nickel based brazing filler metals. This brazing filler metal is suitable for brazing different types of heat exchangers and catalytic converters at a significantly lower cost than conventional nickel based brazing filler metals.

(2) The typical use for this brazing filler metal is in high temperature applications operating in corrosive environments. These applications can be different types of heat exchangers (plate or pipe) that are used in automotive applications, for example exhaust gas recirculation. Other examples are catalytic converters of different types.

(3) The composition of the brazing filler metal according to this invention is

(4) Iron at a content of at least 20 wt %

(5) Copper about 2-20 wt %, preferably 5-15 wt %

(6) Chromium about 11-35 wt %, preferably 20-30 wt %

(7) Nickel about 0-30 wt %, preferably 10-20 wt %

(8) Silicon about 2-10 wt %

(9) Phosphorous about 4-10 wt %

(10) and if Si is equal or less than 6 wt % then P should be above 8 wt %

(11) and if P is less or equal to 8 wt % then Si should be above 6 wt %.

(12) This means that either the content of Si is above 6 wt % or the content of P is above 8 wt % or both i.e. the content of Si is above 6 wt % and the content of P is above 8 wt %.

(13) According to one embodiment Si should be above 6 wt % and up to 10 wt % and P between 6 wt % and 10 wt %. According to a second embodiment Si should be above 6 and up to 10 wt % and P between 8 wt % and 10 wt %.

(14) Other components than those listed can be present.

(15) The brazing filler metal may optionally contain manganese up to 10 wt %, normally less than 7 wt %.

(16) It is recognized that it can be advantageous for the composition of the main components of the brazing filler material to be similar to the composition of the stainless steel base material. Examples of stainless steel grades are 316L having a typical composition of Fe-17 Cr-13,5 Ni-2,2 Mo and 304L, having a typical composition of Fe-18,8 Cr-11,2 Ni. All stainless steel contain by definition a minimum of 11% chromium and few stainless steels contains more then 30% chromium. Chromium content above 11% is required for the formation of the protective chromium oxide layer which gives the steel its corrosion resistant characteristics. The higher chromium content the better corrosion resistance but contents above 35% may cause decrease in the joint strength. Thus the chromium content should be between 11 and 35 wt %, preferably 20-30 wt %.

(17) To reduce the melting point of the alloy, melting point depressants are added. It is well known that silicon, boron and phosphorous each are effective melting point depressants. Studying the phase diagram for FeP it is found that the system has a melting point minimum of 1100 C. at approx 10 wt % phosphorous. The FeSi system has a melting point of 1380 C. at 10 wt % Si and a melting point minimum of approx. 1210 C. at approx. 19 wt % Si. Contents of phosphorous and silicon above 10 wt % each is not desirable since the risk for brittle phase formation is too high. In U.S. Pat. Nos. 6,696,017 and 6,203,754 it is mentioned that the Si+P content should be kept between 9 and 11.5 wt % In case of the alloy having a total content of Si and P higher than 11.5 wt % the alloy becomes brittle and the strength decreases.

(18) Surprisingly it was found that copper reduces the diffusion of silicon and phosphorous into the base material during the brazing operation. The precipitation of phosphorous at the grain boundary in the base material is also prevented which means that the embrittlement of the base material is also avoided. Further, this means that a higher total amount of Si and P, in combination with Cu may be used in order to increase the strength of the brazed joint. Where high strength is required it is therefore preferred to keep the phosphorous and silicon content high. The total amount of Si and P in a Cu containing brazing material may therefore be up to 20 wt %.

(19) Therefore when Si is equal to or less than 6 wt %, P should be above 8 wt % and if P is less or equal to 8 wt %, Si should be above 6 wt %. Also, Si+P have to be above 10 wt % and normally Si+P will be above 14 wt %.

(20) It was also unexpectedly found that the presence of copper has a positive effect on the corrosion resistance of the brazed joints in 10% H.sub.2SO.sub.4. It is believed that 2 wt % copper is needed to gain the positive effect of copper. The copper content of the brazing filler metal covered by this invention should be kept below 20 wt % in order not to differ too much in chemistry from the base material to be brazed. Thus the copper content should be between 2 and 20 wt %, preferably 5-15 wt %.

(21) The FeB system has a melting point minimum of 1174 C. at approx 4 wt % boron. However boron has the disadvantage to cause embrittlement of the brazed component. Boron is an interstitial and because of its small diameter it can quickly diffuse into the lattice of the base material and form the brittle CrB phase. Due to the diffusion of boron the re-melting temperature of the alloy is increased which in some cases is a desirable effect. U.S. Pat. No. 4,444,587 describes how manganese can be a good substitute for boron since manganese also depresses the melting point. 10-30 wt % manganese together with silicon and carbon will in the iron-based system lower the melting temperature with over 200 C. Secondly, manganese will almost completely vaporize during the brazing cycle which will allow rising of the re-melting temperature but without the risk of forming any brittle phases like CrB.

(22) Nickel stabilises austenite which enhances the oxidation resistance of the alloy. Nickel also increases the toughness of the brazed joint. Looking at the tertiary phase diagram for CrFeNi it can be seen that nickel also has a melting point depressing effect. With 30 wt % Cr and 20 wt % Ni the melting point of the CrFeNi system is approx 1470 C. according to ASM specialty hand book Stainless Steel. The nickel content of the brazing filler metal related to this invention should be kept below 30 wt % to minimize the cost of the brazing filler metal.

(23) The brazing filler metal according to this invention is in the form of powder and can be produced by either gas or water atomization. The brazing filler metal can be used in the form of powder or converted to a paste, tape, foil or other forms by conventional methods. Depending on the application technique different particle size distribution is needed but the mean particle size of the brazing filler metal powder is 10-100 m.

(24) The brazing filler metal is suitable for vacuum furnace brazing using vacuum (<10.sup.3 Torr). The brazing filler metal has a melting point below 1100 C. and produce joints at a brazing temperature of 1120 C. having high strength and good corrosion resistance without any observed grain growth.

(25) The brazing filler metal in the form of paste, tape, foil or other forms is placed at the gap or in the gap between the surfaces of the base material which are to be joined. During heating the brazing filler metal melts and by capillary forces the melted brazing filler metal wets the surface of the base material and flows into the gap. During cooling it forms a solid brazed joint. Because the brazing filler metal is acting on capillary forces the wetting of the brazing filler metal on the base material to be brazed is crucial. The brazing filler metal covered by this invention has excellent wetting on stainless steel base material. The brazing filler metal also has good gap width tolerance and is able to braze gaps above 500 m.

(26) The joints brazed with the brazing filler metal according to this invention have a microstructure consisting of a homogenous mix of CrP rich phases and NiFeSiCu rich phases. Surprisingly it was found that the diffusion of silicon and phosphorous was limited by the presence of copper in the brazing filler metal. The precipitation of phosphorus at the grain boundaries in the base material was also prevented by the presence of Cu. The brazing filler metals without copper had a wider diffusion zone in the base material and there was also precipitation of phosphorous at the grain boundaries which may cause embrittlement of the base material.

EXAMPLES

(27) As reference materials two iron brazing filler metals were used; Fe24Cr20Ni10Cu7P5Si5Mn (called Ref 1) and Fe29Cr18Ni7Si6P (called Ref 2).

(28) Ref 1 is an iron based brazing filler metal, BrazeLet F300, produced by Hgans AB.

(29) Ref 2 is an iron based brazing filler metal described in patent application US2008006676.

(30) Further, three different brazing filler metals according to the invention were prepared by water atomization.

(31) Table 1 shows the actual composition of the produced brazing filler metals. The amount of each component is given in weight percent. The expression bal (balance) means that the remaining material in the melt consists of Fe. According to the invention, the filler metal powder comprises at least 20 wt % Fe, and the remaining components are adjusted within the indicated limits in order to add up to 100 wt %. Trace elements are results of inevitable impurities caused by the method of production and trace elements are present in such a small amount that they do not influence the properties of the brazing filler material. Trace elements are normally present in a total amount smaller than 1 wt %.

(32) A first criteria to be satisfied for the brazing filler material is that the braze temperature should preferably be 1100 C. or lower. It can be seen in table 1 that the temperature at which the brazing filler metal melts and brazes is affected by copper. Ref 2 does not melt at 1100 C.

(33) The methods used for testing the properties are as follows:

(34) 1) Wetting Test.

(35) The brazing filler metal (0.2 grams) was placed in the shape of a circle having a diameter of 9 mm at the centre of a stainless steel substrate. The area covered by the powder, the initial powder area (A.sub.i) was 63.6 mm.sup.2. The substrates with the brazing filler metal were then heated at 1100 C. for 30 min in vacuum of 10.sup.4 Torr. The wetting was determined in terms of spreading ration defined as: S=A.sub.m/A.sub.i

(36) where A.sub.m is the area of the melted filler metal and A.sub.i is the initial powder area.

(37) From table 2 it can be seen that the brazing filler metals with high copper (Ref 1) has very good wetting. The brazing filler metal covered by this invention (Alloy 1, 2 and 3) have moderate wetting.

(38) 2) Metallographic Examination

(39) The brazing filler metal was converted to a paste by mixing the metal powder with a binder. 304 stainless steel was used as base material. T-specimens, according to FIG. 1 were brazed at 1100 C. for 30 min in vacuum of 10.sup.4 Torr. After brazing the T-specimens were cross-sectioned. The cross section area of the brazed joint was investigated in Light Optical Microscope. A good brazed joint is identified as a pore and crack free joint with a homogenous microstructure.

(40) As seen in table 2 the brazing filler metal alloy according to this invention (Alloy 1, 2 and 3) forms a homogenous microstructure with limited diffusion of elements into the base material and no precipitation of phosphorous at the grain boundaries. Significantly more diffusion into the base material was found when using iron based brazing filler metal without copper (Ref 2)

(41) 3) Joint Strength.

(42) Joint strength was tested using procedures similar to those recommended in ANSI/AWS C3.2M/C3.2.2001 for the lap type joint configuration with 100 m parallel clearance (see FIG. 2). Before brazing the brazing filler metal was converted to a paste by mixing the brazing filler metal with a binder. The joint strength specimens with the applied paste were then heated to 1100 C. for 30 min in vacuum of 10.sup.4 Torr.

(43) From table 2 it can be seen that the highest strength is obtained for the alloys with the highest Si+P content; Alloy 1, 3. Even Alloy 2 also has significantly higher strength than Ref 1 with less Si+P. This proves, quite contrary to what was believed that high silicon and phosphorous contents in combination with Cu results in high strength.

(44) 4) Corrosion Tests

(45) The corrosion resistance was investigated by placing brazed T-specimens into beakers containing corrosion media for one week. The specimens were thereafter inspected for signs of corrosion. A total of 5 brazed specimens were produced: one for each alloy. The corrosion media used were water solutions of 10% by weight of H.sub.2SO.sub.4.

(46) The results are found in table 2. Alloy 1-3 and Ref 2 had signs of corrosion attack but Ref 2 was most affected. Comparing Ref 2 to the other iron based alloys proves the positive effect of Cu on the corrosion resistance in an iron-chromium based brazing material. Comparing the alloys covered by this invention (Alloy 1, 2 and 3) and Ref 1 it shows the benefits of higher chromium content when it comes to corrosion resistance.

(47) TABLE-US-00001 TABLE 1 Chemistry and melting temperature of the tested brazing filler metals. Alloy Fe Cu Cr P Si Ni Mn Si + P Melting at 1100 C. 1 Inv Bal 5.3 15.3 7.4 9.4 11 0 16.8 Yes 2 Inv Bal 5.1 15.2 9.2 4.9 10.1 0 14.1 Yes 3 Inv Bal 5.1 15.1 8.5 7.1 9.9 0 15.6 Yes Ref 1 Ref Bal 10 24 7 5 10 5 12 Yes Ref 2 Ref Bal 0 28 7 6 18 0 13 No

(48) TABLE-US-00002 TABLE 2 Results from wetting test, metallographic examination, joint strength test and corrosion tests. Joint Corrosion Diffusion of Spreading strength on brazed elements and Alloy ratio (N/mm.sup.2) joints precipitation of P 1 Inv 15 105 Some Limited diff, no precip. 2 Inv 20 88 Some Limited diff, some precip. 3 Inv 23 110 Some Limited diff, no precip. Ref 1 Comp 40 77 None No diff, no precip. Ref 2 Comp 15 93 Severe Diffusion, no precip.