WO2016193383A1

WO2016193383A1 - Brazing method for brazing articles, a brazed heat exchanger and a brazing alloy

Info

Publication number: WO2016193383A1
Application number: PCT/EP2016/062517
Authority: WO
Inventors: Tomas Dahlberg; Niclas BORNEGÅRD
Original assignee: Swep International Ab
Priority date: 2015-06-03
Filing date: 2016-06-02
Publication date: 2016-12-08
Also published as: SE1550718A1; SE539565C2

Abstract

A brazing method for brazing article of stainless steel comprises the steps of: Providing a brazing paste comprising a metal powder comprising: Trace amounts of C and S; 2-2,5% Mo; 12-13% Ni; 17,5-18,5% Cr; 6-8.3% Si; 4.5-5.5% Mn; all percentages being given by weight, balance being iron (Fe). The metal powder has a particle size less than 106 μm, wherein the metal powder is made into a paste by addition of binder and solvents. The paste is applied on or close to contact points between the stainless steel articles to be brazed, whereafter the articles are placed in a furnace. The furnace is heated to a temperature lower than the temperature at which the metal powder of the brazing paste is completely melted, whereafter the stainless steel articles are allowed to set by lowering the temperature.

Description

Brazing method for brazing articles, a brazed heat exchanger and a brazing alloy

Technical field

The present disclosure relates to a brazing method for brazing articles of stainless steel. The present disclosure also relates to a brazed plate heat exchanger. The present disclosure further relates to a brazing alloy. The present disclosure also relates to a brazing material. Background art

The purpose of the present disclosure is to evaluate the possibility to join AISI 316 stainless steel base material with boron free fillers. Boron is known to combine with the Chromium in stainless steel and generate Chromium borides. The formation and precipitation of Chromium borides reduce the mechanical strength as well as the corrosion properties of the stainless steel base metal.

Summary of the invention

Joining tests have been conducted with 316 base materials and different blends of pure 316L PM and boron free F312 fillers. The Boron free F312 fillers have, according to DTA-TGA (Differential Thermal Analysis - Thermo Gravimetry Analysis) measurements, an approximate solidus temperature, i.e.. melting temperature, of 1250 °C. The heat treating temperatures in this study are conducted at temperatures equal to or lower than 1250 °C. Thus, the joining process can be seen as activated diffusion bonding or brazing with partly melted fillers, depending of the Si-content in the fillers in this study.

It has been shown that it is possible to braze articles of stainless steel 316, more specifically plate heat exchangers with a boron free, iron based brazing material comprising Si and Mn as melting point depressants. Moreover, it has been found that the article of stainless steel may be joined in temperatures lower than the temperature at which the brazing material is completely melted. Initial tests show, however, that brazing joints obtained at temperatures below the melting point of the brazing material, although sufficiently strong, are not fluid tight, probably due to micropores. Therefore, in the case of brazed plate heat exchangers, the circumferential skirt and the areas surrounding the port openings may be brazed with a brazing material having a melting point lower than the brazing temperature (NB - this brazing material may comprise more silicone or even boron as melting point depressant), while the heat exchanging areas may be brazed with a brazing material according to the invention.

Moreover, it has been found that the particle size of the brazing material is crucial for achieving the best results. Smaller particle sizes give stronger joints. Moreover, tests have shown that it is possible to join articles of stainless steel by applying small-grain particles of pure stainless steel between the surfaces to be joined. These joints do, however, not yet provide sufficient reliability and strength, but results indicate that better results may be achieved by using even higher temperatures and smaller grains.

Boron free brazing material showing excellent binding properties between articles of stainless steel when brazed at a temperature of 1250 degrees C comprises a metal content comprising:

Trace amounts of C and S;

2.3% Mo;

12,3% i;

17,9% Cr;

6.4% Si;

4.8% Mn;

Balance being iron (Fe), the metal content of the brazing material being in powder form and having a particle size less than 106 μιη. The powdered metal content is made into a paste by addition of 7% binder and solvents. All percentages are given by weight. The metal content of the brazing material according to the above may be manufactured by mixing an alloy powder comprising:

2,3% Mo;

12,9% i;

18,4% Cr;

8,3% Si;

5,9% Mn,

Balance being iron (Fe), with stainless steel powder in the ratio 75/25. Concerning particle size, provisional results have shown that the properties of a blend of grain sizes of the metal powders comprised in the brazing material are characterised by the smallest particle size in terms of larger contact area per weight unit, significant pore size of the resulting joint, and the resulting creep characteristics. This means that it is possible to attain the characteristics of a small grain material even for a blend of coarse (large size) particles and small size particles. Since brazing alloys having a coarse grit (i.e. large particles) are significantly less expensive than fine particles, this is beneficial from a cost standpoint.

Brief description of the drawings

Figure 1 : DTA-TGA measurements for 1649135

Figure 2: DTA-TGA measurements for G577 and a brazing material containing boron.

Figure 3: Test samples used in tensile testing of a braze of the present disclosure.

Figure 4: A fixture for Innovation Disc Samples (IDS) used in tensile testing.

Figures 5 - 8: Diagrams showing furnace temperature and pressure of various brazing cycles used in experiments according to the present disclosure.

Figure 9: A diagram showing rupture strength of samples produced at various brazing temperatures.

Figure 10: A diagram showing Time-Temperature effect on rupture force on G577. Figures 1 1 - 13: Diagrams showing a comparison of rupture force for different filler of the present disclosure.

Figures 14 - 20: Micrographs showing brazing joints of the present disclosure. Figures 22 - 30: Rupture photographs of strength tested joints of the present disclosure. Figures 31, 32: Diagrams showing Force vs Elongation of tensile samples of the present disclosure.

Figure 33 : A diagram showing burst pressures of various samples of the present disclosure.

Figure 34a - 34d: Diagrams showing joint strengths samples of the present disclosure. Detailed description of embodiments.

In the following embodiments of the present disclosure will be described with reference to tests and to figures 1 - 34.

Fillers

The tested alloy, below called G577, comprises trace amounts of C and S, 2.3% Mo, 12.9% Ni, 18,4% Cr, 8.3% Si, 5,6 % Mn, balance being Iron (Fe). To evaluate fillers with different Silicon contents, different blends of alloy G577 powder were mixed with pure 316L sinter powders (PM) into a paste. Alloy G577 was diluted with 316L PM powders in ratios 75/25 and 50/50. Also, fillers comprising 316L PM powders and binder only were tested, showing surprisingly good results. Moreover, the bonding properties with respect to particle size distribution were also evaluated. Two different sievings of Alloy G577, i.e. >45 μιη and <106 μιη, were mixed with the 316L PM of <22 μιη in the ratios 75/25 and 50/50 respectively. The properties of the different filler pastes are presented in table l.The chemical compositions for the F313/316L blends in table 1 are calculated values. Braze Paste Alloy_ Particle Metal C S Mo Ni Cr Si Mn T sol T liq filler batch Lot No Size_ Cont

No μιη wt%

F313-B-D- 204 1649135 <106 93 2.3 13.1 19.1 7.3 5.0 1280 1340 9302

F313P- 15 009 G577 <45 91 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 1315 91XX

F313- 15 010 G577 <45 91 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 1315 91XX

316LP- 15 003B G14637 <25 92 0.025 0.007 2.1 10.4 16.5 0.6 1.4

92XX /645

F313- 15 011 G577 <106 90 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 1315 90XX

F313/316L- 15 015 A 50/50 <45/25 91 0.027 0.007 2.2 11.7 17.5 4.4 3.6

91XX

F313/316L- 15 016B 72/25 <106/22 90 0.027 0.007 2.3 12.3 17.9 6.4 4.8

90XX

F313/316L- 15 015B 75/25 <45/22 91 0.027 0.007 2.3 12.3 17.9 6.4 4.8

91XX

F313/316L- 15 016A 50/50 <106/22 90 0.027 0.007 2.2 11.7 17.5 4.4 3.6

91XX

In figure 1, DTA/TGA measurements for the first alloy of Table 1 is shown. As can be seen, there is a melting onset of the alloy at about 1280 degrees C, and a second peak at about 1340 degrees C. The onset temperature is the temperature where the alloy starts to melt, and the 1340 degrees C peak represents roughly the temperature where the alloy is completely melted.

In Figure 2, DTA-TGA measurements for two different brazing materials, the G577 (line 1) and a brazing material being equal to G577 but with an additional content of boron (line 2) are shown. As can be seen, the melting temperature of the Boron containing brazing material is significantly lower than for the G577 brazing filler - 1 180 degrees C for the brazing material containing boron and 1310 degrees C for G577, respectively. Vertical axis shows DSC/(mW/mg).

Tests of mechanical strength

The mechanical strength of the braze was evaluated by tensile testing of "Innovation disc samples (IDS)", i.e. test samples in the form of small discs provided with a pressed pattern resembling the herringbone pattern provided on heat exchanger plates in order to provide contact points between neighbouring heat exchanger plates while holding the plates on a distance from one another. The samples were coated with brazing material using a stencil, see figure 3. The stencil is identical to a stencil used for actual coating of heat exchanger plates with brazing material. The chevron angle, i.e. crossing angle, of the neighbouring IDS:es is 45°. Although the same pattern is used for the paste application and measures are taken to select samples with similar amounts of paste, the amount of filler varies from sample to sample. The variation in paste amount results in different cross sections of the braze joints and thereby differences in "rupture force". The gap clearance between the beams of the two plates may also vary.

Figure 3 shows sample 3 with <106 μιη PSD and 6% Si (left). Sample 3 with <106 μιη PSD and 4% Si (right). Tests concerning actual heat exchangers

4 heat exchangers were initially manufactured, where the heat transfer area, including port regions, wherein a paste comprising G577 alloy was applied to areas surrounding port openings and the heat exchange area, i.e. the area provided with herringbone pattern. Flanks were robot dispensed with a paste having low melting point by addition of boron with the standard dispensing program. All units leaked in the port area. It was therefore concluded that G577 brazing material did not provide "fluid tight" joints. This is probably due to inherent cavities in the brazing joint, which will be described later. The robot dispensing program was therefore modified so as to apply paste containing brazing material comprising boron and having a melting temperature lower than the brazing temperature on flanks as well as in the ports to be sealed.

Brazing cycles

"Boron free" fillers should theoretically not gain anything from a diffusion step during brazing. Contrary, by minimising time at elevated temperatures, grain growth could be reduced and thereby increasing the base material strength. Different brazing cycles have therefore been tested. See Figures 5 to 6, wherein temperature and pressure of the brazing furnace is shown as a function of time in HH:MM. It should be noted that the following results, if not otherwise stated, were obtained with a brazing cycle according to Figure. 6. Figure 5 shows furnace temperature of 1 160 °C; 120 min. Pink line (1) represents furnace pressure.

Figure 6 shows furnace temperature of 1250°C; 120 min + 1100°C; 180 min. Pink line (1) represents furnace pressure.

Figure 7 shows furnace temperature of 1250°C; 120 min. Pink line (1) represents pressure.

Figure 8 shows furnace temperature of 1250°C; 90 min + 1 150°C; 60 min. Pink line (1) shows furnace pressure.

Results

Figure 9 shows the tensile strength of G577 applied on SS316 IDS having a plate thickness of 0,3 mm for different brazing temperatures. The best results are achieved for a brazing temperature of 1250 degrees C (brazing cycle according to Fig. 6).

Figure 10 shows Time-Temperature effect on mechanical strength for G577 having a grain size of less than 106 μιη. Please note that there is a trend indicating that shorter time at the highest temperature yields a stronger joint. This might be due to the fact that shorter time periods at the elevated temperatures reduces grain growth of the base material.

Figure 11 shows comparison of tensile strength for 0,3 mm coil thickness with different fillers, heat treated at 1250 C according to the brazing cycle of Fig. 6. "Genii" is a brazing material identical to G577, except for the addition of 1,1% B. 316L PM contains 0,4% Si. Cu-foil is heat treated at 1130 C. The brazing materials containing 4% and 6% Si were obtained by mixing G577 with stainless steel powder in the relations 50/50 and 75/25, respectively. Please note that the small-grain (less than 45 μιη) filler with 6% Si gives the same strength as the "Genii" filler containing boron, but the variance between strongest and weakest joint is significantly smaller, which is beneficial.

Figure 12 shows tensile strength 0,4 mm coil thickness with different fillers, heat treated at 1250 °C according to the brazing cycle of Fig. 6. Cu-foil is heat treated at 1 130 °C. Please note that also for the 0.4 mm coil, the best result concerning variance of the strength is obtained with small-grain braze filler containing 6% Si.

Figure 13 shows tensile strength 0,5 mm coil thickness with different fillers, heat treated at 1250 °C. Cu-foil is heat treated at 1 130 °C. Most consistent results achieved with small-grain braze filler containing 6% Si.

Tensile strength testing

In figures 14 - 20 micrographs of cut-up brazing joints are shown. If not otherwise stated, the samples have been heat-treated according to the brazing cycle of Fig. 6.

Figure 14 shows brazing filler comprising 8% Si, grain size less than 106 μιη. Heat treated at 1250 °C according to Fig.6. Figure 15 shows brazing filler comprising 8% Si, <45 μιη grain size (top) . <106 μιη grain size (bottom).

Figure 16 shows brazing filler comprising pure 316L powder having a grain size of 22 μιη. Please note that the structure of the base material seems totally unaffected by the brazing material (no entrainment) and that the joint is porous. The joint is, however, surprisingly strong.

Figure 17 shows brazing filler comprising 4% Si, grain size <106 μιη. Please note that the base material seems unaffected by the brazing material and that the pores are significantly larger than for the joint obtained by the 22 μιη pure stainless steel powder brazing filler. Figure 18 shows brazing joint with a brazing material comprising 6% Si (i.e. a mixture of 75% G577 with a grain size ofl06 μιη and 25% pure stainless steel powder having a grain size of 22 μιη). Please note that the brazing material has entrained well into the base material, however without eroding the same. The pore size of the brazed joint is, however, rather large.

Figure 19 shows 4% Si. <45 μιη Figure 20 shows a joint obtained by small-grain (less than 45 μιη) braze filler with 6% Si. Satisfactory entrainment of brazing filler material into base material, excellent erosion properties and significantly smaller pore size than with 160 μιη braze filler having the same percentage of Si. Rupture samples

Figures 21 - 30 shows some rupture photographs of strength tested joints are shown. Please note that for almost all of the samples, the rupture occurs in the base material, not the brazing joint itself. It can hence be concluded that most of the brazing materials (except, maybe the brazing materials comprising only 4% Si) are "strong enough" - it is of no use providing a brazing material that gives a stronger joint than the material it is supposed to join, since the resulting strength of the system brazing joint - base material will not be stronger than its weakest link, i.e. the base material.

Figures 21 - 23 shows filler G577.

Figure 24 shows filler #15_016B (see table 1), i.e. a filler comprising 6% Si

Figure 25 shows filler #15_016A (see table 1), i.e. a filler comprising 4% Si.

Figure 26 shows Genii filler.

Figure 27 shows 0,5 mm coil thickness heat treated with Cu filler material (only for comparison. Heat treated at 1 160 degrees C). Figure 28 shows brazing material comprising 4% Si, grain size of 106 μιη. 0,5 mm coil thickness. As can be seen, in this test, the brazing material was the weak link for three out of four joints. It should be borne in mind, however, that this test was made for the weakest brazing material and the strongest base material.

Figure 29 shows brazing material comprising 6% Si, 0,5 mm coil thickness, grain size 106 μιη PSD. All ruptures in brazing material.

Figure 30 shows brazing material comprising 6% Si, 45 μιη grain size. All ruptures occurred in base material.

Ductility of joints.

It is a well known problem of brazed heat exchanger brazed with the prior art braze fillers comprising boron that they tend to be less ductile than copper brazed heat exchangers. This problem is, however, overcome according to the present invention. In Fig. 31, the force vs. elongation for different brazing joints are shown. One brazing material stands out from the rest, namely the prior art brazing joint comprising boron. As can be seen, the force required for elongating the brazing joint with this material is significantly larger than for all the other brazing material, including copper, i.e. it is less ductile than copper. All brazing materials according to the invention are, however, about as ductile as the copper brazings.

Figure 31 shows Force vs Elongation for "best off tensile samples and 0,4 mm coil thickness, BT 1250 °C. References a - f denotes the following:

"a" is 2: Cu-foil, 0.4 coil; "b" is 3 : <45my 4%Si; "c" is 2 :< 45my 6%Si; "d" is 1 : <106my 4%Si; "e" is l :<106my 6%Si; "f ' is 3: GIL

Figure 32 shows Force vs Elongation for "best off tensile samples and 0.3 mm coil thickness, BT 1250 °C. References a - i denotes the following: "a" is 1 : Genii; "b" is 5: 316L; "c" is 3: < 106my 7%Si; "d" is 3 : <106my 6%Si; "e" is 2: <106my 4%Si; "f ' is 2: < 45my 8%Si; "g" is 3 : <45my 6%Si; "h" is 1 : <45my 4%Si; "i" is 1 : Cu-foil. Burst pressures of prototype units

Burst tests have also bee performed for actual heat exchangers. The tested exchangers comprise 18 plates. In figure 33, some burst pressure for an identical heat exchanger brazed with 106 μιη braze filler comprising 4%, 6% and 8% Si, respectively, a 45 μιη braze filler comprising 4% Si and a prior art braze filler comprising boron ("Genii") are shown. As can be seen, the tested braze fillers comprising 6 and 8 % Si show good results concerning burst pressure, whereas the braze fillers comprising 4% do not.

In Figure 34a - 34b, comparative measurements are shown for brazing materials according to the invention having 4, 6 and 8% Si, respectively, for plate thicknesses of 0.3, 0.4 and 0.5 mm, respectively. A copper braze is shown for comparison. Figure 34a shows the 4% Si, Figure 34b shows the 6% Si, figure 34c shows the 7-8 % Si and figure 34d shows the copper brazed joint (copper foil). As can be seen, the copper brazing joints have comparable strength for 0.3 mm plate thickness, but the brazing joints according to the invention are significantly stronger for thicker plates.

Figure 33) Burst pressures for B5T-18 units

Figure 34 shows joint strength plots for iron brazed brazing materials containing 4, 6 and 8% Si, respectively, for different base material thicknesses. A copper brazed joint is shown for comparison. Please note that the iron based brazing joints are significantly stronger than copper brazed joints for plate thicknesses of 0,4 and 0,5 mm.

Discussion

Effect of Time-Temperature on sample strength

The IDS samples heat treated at 1 160 °C and 1210 °C, have lower mechanical strength compared to IDS heat treated at 1250 °C, figure 9. Low strength is also obtained for the pure 316L PM, heat treated at 1250 °C, figure 9. Although diffusion bonding is obtained between the different particles of the 316L PM as well as between the 316L PM and the base material, figure 14, the high porosity of the joint results in low mechanical strength.

Figure 10 shows the effect of different "heat loads" on the same filler and base material. The three sub groups have been heat treated at 1250 °C. The sub-groupe indicating the lowest strength have been subjected to the highest "heat load", i.e. 2 hours at 1250 °C + 3 hours at 1100 °C. The sub-groupe only subjected to 1,5 hours at 1250 °C + 1 hour at 1 150 °C give higher values. This may be due to stronger base material.

Joint strength for 0,3 mm coil thickness

For the IDS-316 in 0,3 mm coil thickness, heat treated at 1250 °C, the rupture always occur in the base material in the vicinity of the joint, figures 13-15. Higher mechanical strength is obtained for these samples compared to samples heat treated at lower temperatures, figure 9. Moreover, no significant difference in strength can be seen between fillers containing 4% Si, 6% Si or 8% Si, when subjected to the same "heat load". No difference can be seen due to the particle size distribution of the filler either. See figure 1 1.

Joint strength for 0,4 mm coil thickness

For the IDS-316 in 0,4 mm coil thickness, heat treated at 1250 °C, the rupture also occurs in the base material in the vicinity of the joint, figure 19-21. Higher mechanical strength is obtained for these samples compared to 0,3 mm coil thickness. No significant difference can be seen between 4% and 6% Si. No significant difference can be seen for <45 μιη particle size compared to < 106 μιη particle size.

When the rupture occurs in the base material, the integrity of the joint is satisfactory. When the samples have been subjected to similar "heat loads", the variation in joint strength between the different fillers, as well as between the samples with the same fillers, is probably due to variations in the amount of applied paste. More paste results in larger joints and bigger cross section areas. Figure 3 shows two samples after capillary dispensing, where the left sample gave 4085N and the right sample 4896N. As can be seen, the amount of applied paste is lower on the sample to the left, despite being applied at the same occasion. The fracture area can not be measured accurately after tensile testing, since the rupture runs through the base material, as can be seen from figure 22 and 23.

The Genlll samples with < 6% Si have longer elongation prior to rupture compared to the Genii samples and "boron free" samples with 7-8% Si. This indicates a higher ductility for < 6% Si samples, figures 25-26.

Erosion versus Si-content

The "Boron free" filler with 7,3% Si, have an approximate solidus temperature (Tsol) of 1280 °C, figure 1. The Tsol for the filler with 8,3% Si is approx 1250 °C, figure 2. However, metallographic investigations of CP-1501 14-06 and CP-150223-01 still showed a substantial dissolution of the base metal at 1250 °C, figures 13 andl4, despite heat treating temperatures being lower than measured Tsol of the fillers. As can be seen from figures 13 to 19, the erosion rate decreases with decreasing Si-content. Slight dissolution of the base metal can be seen for 6% Si at 1250 °C. Hardly any erosion for 4%Si.

Claims

1. A brazing method for brazing articles of stainless steel, comprising the steps of: - providing a brazing paste comprising a metal powder comprising:

trace amounts of C and S;

2-2.5% Mo;

12-13% i;

17.5-18.5% Cr;

6-8.3% Si;

4.5-5.5% Mn;

all percentages being given by weight,

balance being iron (Fe), the metal powder having a particle size less than 106 μιη, wherein the metal powder is made into a paste by addition of binder and solvents; - applying the paste on or close to contact points between the stainless steel articles to be brazed;

- placing the articles in a furnace;

- heating the furnace to a temperature lower than the temperature at which the metal powder of the brazing paste is completely melted; and

- allowing the stainless steel articles to set by lowering the temperature.

2. The method according to claim 1, wherein the particle size is less than 45 μιη.

3. The method of claim 1 or 2, wherein the particle size is less than 20 μιη.

4. The method according to claims 1-3, wherein the temperature at which the metal powder of the brazing paste is completely melted is above 1250 degrees C.

5. The method according to claims 1-4, wherein the heating includes the steps of: - steadily raising the temperature from room temperature to a temperature at which the solvents and/or binder evaporates; - keeping the temperature at which the solvents and/or binder evaporates until most or all of the binder an/or solvent is evaporated;

- elevating the temperature to the brazing temperature; and

- keeping the brazing temperature until the joints have been formed.

6. A brazed plate heat exchanger comprising a number of heat exchanger plates provided with a pressed pattern of ridges and grooves adapted to keep the plates on a distance from one another under formation of interplate flow channels, the heat exchanger further comprising port openings, wherein selective communication between the interplate flow channels and the port openings is achieved by providing areas around the port openings of each plate on different heights, such that the areas around the port openings of neighbouring plates either contact one another or do not contact one another, hence blocking or allowing for communication between the port opening and the interplate channel, respectively, characterized in that the heat exchanger is at least partly brazed by a method according to claims 1 to 5.

7. The heat exchanger according to claim 6, wherein the areas around the port openings that are contacting one another are provided with a brazing material having a melting temperature lower than the brazing temperature.

8. The heat exchanger according to claim 7, wherein the brazing material provided at the areas around the port openings that are contacting one another is a brazing material identical to the brazing material as defined in claim 1, however comprising more than 8% Si and/or up to 1,5% B.

9. A brazing alloy comprised in a brazing paste composed of a powder of said alloy and an organic binder and/or a solvent, said solvent being adapted to vaporize during a brazing cycle, wherein said brazing alloy comprises:

trace amounts of C and S;

2-2.5% Mo;

12-13% i;

17.5-18.5% Cr;

6-8.3% Si;

4.5- 5.5% Mn;

all percentages being given by weight,

balance being iron (Fe).

10. The brazing alloy of claim 9, wherein the brazing alloy is a mechanical blend of a stainless steel powder and a stainless alloy having a high percentage of Si and Mn.

1 1. The brazing alloy of claims 9 or 10, wherein the brazing alloy powder has a particle size of less than 106 μηι.

12. The brazing alloy of claims 9 - 11, wherein the brazing alloy powder has a particle size of less than 45 μιη.

13. The brazing alloy of claims 9-12, comprising:

trace amounts of C and S;

2.1- 2.4% Mo;

12.2-12.8% Ni;

17.6-18.3% Cr;

6.2- 6.8% Si;

4.6- 5.3% Mn;

all percentages being given by weight,

balance being iron (Fe).

14. The brazing alloy of claims 9-13, comprising:

trace amounts of C and S;

2.1-2.4% Mo;

12.2-12.4% Ni;

17.8-18.1% Cr;

6.3- 6.7% Si;

4.7-4.9% Mn

all percentages being given by weight,

balance being iron (Fe).

15. The brazing alloy of claim 10, wherein the particle size of the stainless steel powder is less than 22 μιη.

16. A brazing material comprising a brazing alloy according to anyone of claims 9-15, characterized in that said brazing alloy comprises a first alloy powder having a first grit size and a second alloy powder having a second grit size, wherein the grit size of the first alloy powder, differs from the grit size of the second alloy powder.

17. The brazing material of claim 16, wherein the first grit size is smaller than 100 μιη.

18. The brazing material according to any of the claims 16 and 17, wherein the second grit size is smaller than 40 μιη.

19. The brazing material according to any of the claims 16 - 18, wherein the first grit size is smaller than 40 μιη and the second grit size is smaller than 20 μιη.