SE539565C2 - Brazing method for brazing articles, a brazed heat exchangerand a brazing alloy - Google Patents

Abstract

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 um,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 articlesto be brazed, whereafter the articles are placed in a furnace. The furnace is heated toa temperature lower than the temperature at which the metal powder of the brazingpaste is completely melted, whereafter the stainless steel articles are allowed to setby lowering the temperature.

Description

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

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

Summary of the invention J oining tests have been conducted with 316 base materials and different blends of pure3l6L PM and boron free F3l2 fillers. The Boron free F3l2 fillers have, according toDTA-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 toor lower than 1250 °C. Thus, the joining process can be seen as activated diffusionbonding 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, morespecifically plate heat exchangers with a boron free, iron based brazing materialcomprising Si and Mn as melting point depressants. Moreover, it has been found thatthe 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 themelting 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, thecircumferential skirt and the areas surrounding the port openings may be brazed with abrazing material having a melting point lower than the brazing temperature (NB - thisbrazing material may comprise more silicone or even boron as melting pointdepressant), while the heat exchanging areas may be brazed with a brazing material according to the invention.

Moreover, it has been found that it the particle size of the brazing material is crucial forachieving the best results. Smaller particle sizes give stronger joints. Moreover, testshave shown that it is possible to join articles of stainless steel by applying small-grainparticles 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 ofstainless steel when brazed at a temperature of 1250 degrees C comprises a metalcontent comprising: Trace amounts of C and S; 2.3% Mo; l2,3% Ni; 17,9% Cr; 6.4% Si; 48% Mn; Balance being iron (Fe), the metal content of the brazing material being in powder formand having a particle size less than 106 um. 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% Ni;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 ofgrain sizes of the metal powders comprised in the brazing material are characterised bythe smallest particle size in terms of larger contact area per weight unit, significant poresize of the resulting joint, and the resulting creep Characteristics. This means that it ispossible 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.

F igures 5 - 8: Diagrams showing furnace temperature and pressure of various brazingcycles used in experiments according to the present disclosure.

Figure 9: A diagram showing tensile strength of samples produced at various brazingtemperatures.

Figure 10: A diagram showing Time-Temperature effect on mechanical strength onG577.

Figures 11 - 13: Diagrams showing a comparison of tensile strength for different fillerof 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 presentdisclosure.Figure 33: A diagram showing burst pressures of various samples of the presentdisclosure.

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 l - 34.

Fil/ers The tested alloy, below called G577, comprises trace amounts of C and S, 2.3% Mo,l2.9% Ni, l8,4% Cr, 83% Si, 5,6 % Mn, balance being Iron (Fe). To evaluate fillerswith different Silicon contents, different blends of alloy G577 powder were mixed withpure 3l6L sinter powders (PM) into a paste. Alloy G577 was diluted with 3l6L PMpowders in ratios 75/25 and 50/50. Also, fillers comprising 3l6L PM powders and binder only were tested, showing surprisingly good results.

Moreover, the bonding properties with respect to particle size distribution were alsoevaluated. Two different sievings of Alloy G577, i.e. >45 um and Braze Paste Alloy_ Particle Metal C S M0 Ni Cr Si Mn T_s0l T_liqfiller batch L0t_No Size_ ContNo um wt% F313-B»D» 204 1649135 5106 93 2.3 13.1 19.1 7.3 5.0 1280 13409302 F313P- 15_009 G577 545 91 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 131591XX F313- 15_010 G577 545 91 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 131591XX 316LP- 15_003B G14637 525 92 0.025 0.007 2.1 10.4 16.5 0.6 1.4 92XX /645 F313- 15_011 G577 5106 90 0.028 0.006 2.3 12.9 18.4 8.3 5.9 1250 131590XX F313/316Lf 15_015A 50/50 545/25 91 0.027 0.007 2.2 11.7 17.5 4.4 3.6 91XX F313/316Lf 15_016B 72/25 5106/22 90 0.027 0.007 2.3 12.3 17.9 6.4 4.8 90XX F313/316Lf 15_015B 75/25 545/22 91 0.027 0.007 2.3 12.3 17.9 6.4 4.8 91XX F313/316Lf 15_016A 50/50 5106/22 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 beseen, there is a melting onset of the alloy at about 1280 degrees C, and a second peak atabout 1340 degrees C. The onset temperature is the temperature where the alloy starts tomelt, 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 (redline 1) and a brazing material being equal to G577 but with an additional content ofboron (green line 2) are shown. As can be seen, the melting temperature of the Boroncontaining brazing material is signiflcantly lower than for the G577 brazing f1ller -1180 degrees C for the brazing material containing boron and 1310 degrees C for G577,respectively. Vertical axis shows DSC/(mW/mg).

T ests of mechanical strengthThe 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 toprovide contact points between neighbouring heat exchanger plates while holding theplates on a distance from one another. The samples were coated with brazing materialusing a stencil, see figure 3. The stencil is identical to a stencil used for actual coating ofheat exchanger plates with brazing material. The chevron angle, i.e. crossing angle, ofthe neighbouring IDS:es is 45°. Although the same pattern is used for the pasteapplication and measures are taken to select samples with similar amounts of paste, theamount of filler varies from sample to sample. The Variation in paste amount results indifferent 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 5106 um PSD and 6% Si (left). Sample 3 with 5106 umPSD and 4% Si (right).

T ests concerning actual heat exchangers 4 heat exchangers were initially manufactured, where the heat transfer area, includingport regions, wherein a paste comprising G577 alloy was applied to areas surroundingport openings and the heat exchange area, i.e. the area provided with herringbonepattem. Flanks were robot dispensed with P3 l2D paste, i.e. a paste having low meltingpoint by addition of boron with the standard dispensing program. All units leaked in theport area. It was therefore concluded that G577 brazing material did not provide “fluidtight” joints. This is probably due to inherent cavities in the brazing joint, which will bedescribed later. The robot dispensing program was therefore modified so as to applypaste 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.

Brazíng cycles “Boron free” fillers should theoretically not gain anything from a diffusion step duringbrazing. Contrary, by minimising time at elevated temperatures, grain growth could bereduced and thereby increasing the base material strength. Different brazing cycles have therefore been tested. See Figures 5 to 6 below, wherein temperature and pressure of the brazing furnace is shown as a function of time in HH:MM. It should be noted that thefollowing results, if not otherwise stated, were obtained with a brazing cycle according to Figure. 6.

Figure 5 shows furnace temperature of 1160 °C; 120 min. Pink line (1) representsfurnace 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 l250°C; 120 min. Pink line (1) representspressure.

Figure 8 shows furnace temperature of 1250°C; 90 min + 1l50°C; 60 min. Pink line (1)shows furnace pressure.

ResultsFigure 9 shows the tensile strength of G577 applied on SS316 IDS having a platethickness 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 agrain size of less than 106 um. Please note that there is a trend indicating that shortertime at the highest temperature yields a stronger joint. This might be due to the fact thatshorter time periods at the elevated temperatures reduces grain growth of the base material.

Figure ll shows comparison of tensile strength for 0,3 mm coil thickness with differentfillers, heat treated at 1250 C according to the brazing cycle of Fig. 6. “GenII” is abrazing material identical to G577, except for the addition of l,1% B. 316L PMcontains 0,4% Si. Cu-foil is heat treated at 1130 C. The brazing materials containing 4%and 6% Si were obtained by mixing (3577 with stainless steel powder in the relations50/50 and 75/25, respectively. Please note that the small-grain (less than 45 um) 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 treatedat 1250 °C according to the brazing cycle of Fig. 6. Cu-foil is heat treated at 1130 °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 treatedat 1250 °C. Cu-foil is heat treated at 1130 °C. Most consistent results achieved with small-grain braze filler containing 6% Si.

T ensíle strength testingIn 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 um. Heat treated at 1250 ”C according to Fig.6.

Figure 15 shows brazing filler comprising 8% Si, <45 um grain size (top) . <106 um grain size (bottom).

Figure 16 shows brazing f1l1er comprising pure 316L powder having a grain size of 22um. Please note that the structure of the base material seems totally unaffected by thebrazing 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 um. Please note thatthe base material seems unaffected by the brazing material and that the pores aresignificantly larger than for the joint obtained by the 22 um pure stainless steel powderbrazing filler.

Figure 18 shows brazing joint With a brazing material comprising 6% Si (i.e. a mixtureof 75% G577 with a grain size of 106 um and 25% pure Stainless steel powder having agrain size of 22 um). Please note that the brazing material has entrained well into thebase material, however without eroding the same. The pore size of the brazed joint is, however, rather large.

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

Rapture 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 isof no use providing a brazing material that gives a stronger joint than the material it issupposed 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 #l5_0l6B (see table l), i.e. a filler comprising 6% Si Figure 25 shows filler #l5_0l6A (see table l), 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 1160 degrees C). lO Figure 28 shows brazing material comprising 4% Si, grain size of 106 um. 0,5 mm coilthickness. As can be seen, in this test, the brazing material was the weak link for threeout of four joints. It should be bome 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 um PSD. All ruptures in brazing material.

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

Ductilíly ofjoínts.

It is a well known problem of brazed heat exchanger brazed with the prior art brazefillers comprising boron that they tend to be less ductile than copper brazed heatexchangers. This problem is, however, overcome according to the present invention. InFig. 31, the force vs. elongation for different brazing joints are shown. One brazingmaterial 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 issignificantly larger than for all the other brazing material, including copper, i.e. it is lessductile 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 coilthickness, BT 1250 °C.

Figure 32 shows Force vs Elongation for “best off” tensile samples and 0,3 mm coilthickness, BT 1250 °C.

Burstpressures of prototype unitsBurst 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 ll brazed with 106 pm braze filler comprising 4%, 6% and 8% Si, respectively, a 45 pmbraze filler comprising 4% Si and a prior art braze filler comprising boron (“GenI1”) areshown. 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 materialsaccording to the invention having 4, 6 and 8% Si, respectively, for plate thicknesses of0.3, 0.4 and 0.5 mm, respectively. A copper braze is shown for comparison. Figure 34ashows the 4% Si, Figure 34b shows the 6% Si, figure 34c shows the 7-8 % Si and figure34d shows the copper brazed joint (copper foil). As can be seen, the copper brazingjoints 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-l8 units Figure 34 shows joint strength plots for iron brazed brazing materials containing 4, 6and 8% Si, respectively, for different base material thicknesses. A copper brazed joint isshown 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 Activated Diffusion Healing (ADH) Effect of T ime-T emperature on sample strength The IDS samples heat treated at 1160 °C and 1210 °C, have lower mechanical strengthcompared to IDS heat treated at 1250 °C, figure 9. This is discussed in more detail inRD101184. Low strength is also obtained for the pure 3l6L PM, heat treated at 1250°C, figure 9. Although diffusion bonding is obtained between the different particles ofthe 3 l6L PM as well as between the 3l6L PM and the base material, figure 14, the high porosity of the joint results in low mechanical strength. 12 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 thelowest 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 1150 °C give higher values. This may be due to stronger base material.

Joint strength for 0,3 mm coil thíckness For the IDS-316 in 0,3 mm coil thickness, heat treated at 1250 °C, the rupture alwaysoccur in the base material in the vicinity of the joint, figures 13-15. Higher mechanicalstrength is obtained for these samples compared to samples heat treated at lowertemperatures, figure 9. Moreover, no significant difference in strength can be seenbetween fillers containing 4% Si, 6% Si or 8% Si, when subjected to the same “heatload”. No difference can be seen due to the particle size distribution of the filler either.

See figure 11.

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 alsooccurs in the base material in the vicinity of the joint, figure 19-21. Higher mechanicalstrength is obtained for these samples compared to 0,3 mm coil thickness, Nosignificant difference can be seen between 4% and 6% Si. No significant difference can be seen for <45 um particle size compared to 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 jointstrength between the different fillers, as well as between the samples with the samefillers, is probably due to variations in the amount of applied paste. More paste results inlarger joints and bigger cross section areas. Figure 3 shows two samples after capillarydispensing, where the left sample gave 4085N and the right sample 4896N. As can beseen, 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 13 tensile testing, since the rupture runs through the base material, as can be seen from figure 22 and 23.

The GenIII samples With 5 6% Si have longer elongation prior to rupture compared tothe GenII samples and “boron free” samples With 7-8% Si. This indicates a higher ductility for i 6% Si samples, figures 25-26.

Erosíon versus Sí-content The “Boron free” filler With 7,3% Si, have an approximate solidus temperature (Tsol) of1280 °C, figure 1. The Tsol for the filler With 8,3% Si is approx 1250 °C, figure 2.However, metallographic investigations of CP-150114-06 and CP-l50223-0l stillshowed a substantial dissolution of the base metal at 1250 °C, figures 13 and14, despiteheat treating temperatures being lower than measured Tsol of the fillers. As can be seenfrom figures 13 to 19, the erosion rate decreases With decreasiiig Si-content. Slightdissolution of the base metal can be seen for 6% Si at 1250 °C. Hardly any erosion for 4%Si.

Claims (15)

14 Amended 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-l3% Ni; 17.5-l8.5% 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 um,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 tobe brazed; - placing the articles in a fumace; - heating the furnace to a temperature lower than the temperature at which the metalpowder 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 um.

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

4. The method according to claims l-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 orall 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 platesprovided with a pressed pattern of ridges and grooves adapted to keep the plates on adistance from one another under formation of interplate flow Channels, the heatexchanger further comprising port openings, wherein selective communication betweenthe interplate flow channels and the port openings is achieved by providing areas aroundthe port openings of each plate on different heights, such that the areas around the portopenings of neighbouring plates either contact one another or do not contact oneanother, hence blocking or allowing for communication between the port opening andthe 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 openingsthat 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 atthe areas around the port openings that are contacting one another is a brazing materialidentical to the brazing material as defined in claim 1, however comprising more than 8% Si and/or up to l,5% B.

9. A brazing alloy comprised in a brazing paste composed of a powder of said alloy andan organic binder and/ or a solvent, said solvent being adapted to vaporize during abrazing cycle, wherein said brazing alloy comprises: trace amounts of C and S; 2-2.5% Mo; 12-13% Ni; 16 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.

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

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

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; 17 4.7-4.9% Mnall percentages being given by weight, balance being iron (Fe).

15. l5. The brazing alloy of claim 10, Wherein the particle size of the Stainless steel powder is less than 22 pm.