How to braze with brazing material

Abstract

A method for brazing is provided. [Solution] A brazing material for brazing a brazed plate heat exchanger including a plurality of heat exchanger plates having a pressed ridge and groove pattern, a method for brazing with a brazing material including a brazing alloy containing at least one melting point depressant and a metal similar in composition to the heat exchanger plates, wherein the brazing material includes a mixture between particles of molten brazing material having a solidus / liquidus temperature below the brazing temperature and particles of unmelted brazing material having a solidus / liquidus temperature above the brazing temperature, and wherein 50% by weight or more of the brazing alloy is unmelted brazing material. The method includes: an application step of applying the unmelted brazing material to the contact surfaces of the heat exchanger plates; and a heating step of heating the brazing material to a brazing temperature that is a temperature above the liquidus temperature of the molten brazing material particles and below the solidus temperature of the unmelted brazing material particles.

Description

[Technical Field]

[0001] The present invention relates to a brazing material for brazing a brazed plate heat exchanger including a plurality of heat exchanger plates having a pattern of pressed ridges and grooves that provide contact points between adjacent heat exchanger plates, thereby holding the heat exchanger plates at a distance from one another and forming interplate flow paths for a medium to exchange heat between the heat exchanger plates. The brazing material includes a brazing alloy that includes at least one melting point depressant and a metal similar in composition to the heat exchanger plates. [Background technology]

[0002] In the art of brazing, one or more materials are joined together by applying a brazing material to the contact surfaces of the materials to be joined, after which the temperatures of the materials to be joined and the brazing material are increased to a temperature at which the brazing material melts but the materials to be joined do not melt.

[0003] In the field of brazed plate heat exchangers, the most common base material and brazing material are stainless steel and copper, respectively. While this material combination is highly advantageous in many ways, it cannot be used in some cases. For example, copper is prohibited in some countries for drinking water applications, and copper is excluded when ammonia is used as a refrigerant because copper forms water-soluble complexes in the presence of ammonia.

[0004] For applications where copper cannot be used, nickel can be used as a brazing material. However, nickel as a brazing material has the drawback of producing brittle brazed joints due to the melting point depressants used to lower the melting point of nickel. The melting point depressants are usually silicon, boron, phosphorus, or a combination thereof.

[0005] Yet another option is to use stainless steel based brazing materials, i.e. stainless steel mixed with melting point depressants such as silicon, boron and / or phosphorus. However, the use of stainless steel with melting point depressants has the disadvantage that the melting point depressants tend to migrate into the substrate, which lowers the melting point of the substrate and thus leads to erosion and "burn-through", a phenomenon caused by the melting point depressants in the brazing material lowering the melting point of the plate material to such an extent that the plate material melts throughout the thickness of the plate and thus causes holes through the plate in the heat exchanger plate.

[0006] A solution to the erosion problem is presented in Patent Document 1, filed by the applicant of the present application. Here, the amount of melting point depressant is so small that the brazing material does not melt during brazing. During the brazing process, the substrate and brazing material are not joined by dissolving into each other, but the unmelted brazing material and the substrate are joined by diffusion. The joining method disclosed in Patent Document 1 is advantageous in that it causes virtually no erosion, but a disadvantage is that the joint containing particles of unmelted brazing material becomes porous. Porosity can sometimes lead to a weakened joint.

[0007] It is well known that brazing alloys containing melting point depressants such as silicon, boron, and / or phosphorus do not have a single temperature melting point. Rather, such brazing alloys have a melting point range with a lower temperature ("solidus temperature") at which the brazing alloy begins to melt, and a higher temperature ("liquidus temperature") at which the brazing alloy is completely melted.

[0008] It is an object of the present invention to provide a brazing material that provides a brazed joint with low or no porosity while reducing erosion of the substrate. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Swedish Patent Application No. 1550718-9 Summary of the Invention [Means for solving the problem]

[0010] The present invention solves or at least alleviates the above and other problems by providing a brazing material comprising a blend between particles of a molten brazing material having a solidus temperature and a liquidus temperature below the brazing temperature and particles of a non-melting brazing material having a solidus temperature and a liquidus temperature above the brazing temperature, wherein the ratio between the molten brazing material and the non-melting brazing material is such that an alloy formed by the molten brazing material and the non-melting brazing material has a solidus temperature below the brazing temperature and a liquidus temperature above the brazing temperature.

[0011] The particles of the molten brazing material may contain boron as a melting point depressant due to its fast diffusion rate and high melting point depressant effect.

[0012] To resemble the composition of heat exchanger plates, the molten brazing material particles may contain 1.5-3% Mo, 10-15% Ni, 16-20% Cr, 7-10% Si, 5-7% Mn, 1-2% B, and the remainder Fe.

[0013] For the same reason, the non-melting brazing material particles may contain 1.5-3% Mo, 10-15% Ni, 16-20% Cr, 5-7% Si, 5-7% Mn, the remainder being Fe.

[0014] To take advantage of the rapid diffusion of boron, the composition of the molten brazing material and the non-molten brazing material may be the same except that the molten brazing material contains 1-2% boron.

[0015] To meet the desired brazing temperature, the non-melting brazing material may have a solidus temperature above 1250°C, and the molten brazing material has a liquidus temperature below 1250°C.

[0016] To obtain a non-porous brazed joint while reducing erosion of the substrate, the alloy formed from the molten brazing material and the unmolten brazing material in proportions of brazing material may have a solidus temperature below 1250°C and a liquidus temperature above 1250°C.

[0017] In the following, the invention will be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a typical DTA-TGA curve of four typical brazing alloys containing one or more melting point depressants. [Figure 2] 1 is a photomicrograph of a polished prior art brazed joint in which a brazing material having solidus and liquidus temperatures below the brazing temperature was used to join heat exchanger plates. [Figure 3] 1 is a photomicrograph showing a polished prior art brazed joint in which a brazing material having a solidus temperature and a liquidus temperature higher than the brazing temperature was used. [Figure 4] 1 is a photomicrograph showing a polished brazed joint according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Two DSC (differential scanning calorimetry) curves for a typical brazing alloy containing one or more melting point depressants are shown in Figure 1. Briefly, a DSC curve represents the difference in the amount of heat required to increase the temperature of a sample and a reference measured as a function of temperature.

[0020] Referring to FIG. 1, the upper DSC curve includes a first section from 700°C to about 1120°C. In this section, the heat required to heat the sample (in this case, the brazing alloy) is rather constant. From about 1120°C to 1164°C, i.e., section 2, much more energy is required to increase the temperature. This is due to the melting of the brazing material. From about 1164°C to about 1180°C, i.e., section 3, the energy required to increase the temperature returns to approximately the same value as during section 1, and in section 4, the brazing material is completely melted and the energy required to increase the temperature becomes constant again.

[0021] To deduce from these curves the so-called solidus and liquidus temperatures, i.e. the temperatures at which all brazing materials are in the solid and liquid states, respectively, these temperatures are usually referred to as start and end temperatures, where the start temperature is the temperature of the intersection between the mean tangent of the first interval and the mean tangent of the second interval, and the end temperature is the temperature of the intersection between the mean tangent of the third interval and the mean tangent of the fourth interval. In the following, the start and end temperatures of the DSC curves are considered to be the solidus and liquidus temperatures of the brazing materials.

[0022] In the following figures, cross-sectional photographs of the brazed joints are shown. All of these photographs show the brazed joints between the intersecting ridges and grooves of adjacent heat exchanger plates. The brazing material was applied in the form of a paste containing a solvent, a binder, and a powder of the brazing alloy.

[0023] FIG. 2 shows a prior art brazed joint between the ridges and grooves of adjacent heat exchanger plates contained within a plate heat exchanger. The brazing material is an alloy containing stainless steel and a melting point depressant (boron and silicon), and the brazing temperature is higher than both the liquidus and solidus temperatures of the brazing material—in other words, the brazing material is fully melted during brazing. It may be noted that the brazing material was applied near the intersection between the ridges and grooves of adjacent plates, rather than at the intersection, as disclosed by the applicant in WO 2015 / 062992 A1. As can be seen in FIG. 1, the brazed joint is uniform (i.e., the joint contains no or very few pores). However, some erosion of the plate material can be seen around the joint. This erosion is due to melting point depressant elements in the brazing alloy migrating into the plate material, thereby lowering the melting point of the plate material. Therefore, the plate material to which the melting point depressant migrates may melt and become part of the brazed joint, thus reducing the thickness of the plate material in the vicinity of the brazed joint. In severe cases, the erosion of the plate material may lead to so-called "burn-through", which means that there are holes through the heat exchanger plates. This is of course catastrophic for a plate heat exchanger, as holes through the plates lead to internal leaks in the heat exchanger.

[0024] To reduce the erosion and burn-through problems caused by brazing materials such as those shown in FIG. 2, applicants have experimented with brazing materials having solidus and liquidus temperatures higher than the brazing temperature—in other words, the brazing material does not melt during brazing. Instead of joining heat exchanger plates by melting and subsequent solidification, as occurs with the brazing material of FIG. 2, brazing materials having solidus and liquidus temperatures higher than the brazing temperature join the plates by diffusion. FIG. 3 shows a typical brazed joint achieved with a brazing material having liquidus and solidus temperatures higher than the brazing temperature. As can be seen in this figure, the brazed joint is completely free of erosion—this is not surprising, since the amount of melting point depressant in the brazing material is insufficient to allow the brazing material itself to melt. Even if some of the melting point depressant migrates into the plate material, the percentage of the melting point depressant in the plate material would not be sufficient to melt the plate material. It may be mentioned that in contrast to FIG. 2, the brazing material was applied to the contact points between the ridges and grooves of adjacent plates.

[0025] It will also be seen that the central portion of the brazed joint is porosity-free. The pressure between the ridges and grooves of the intersecting adjacent plates is sufficient to fuse the particles of brazing material together and form a porosity-free joint. However, there are porosity on the periphery of the brazed joint.

[0026] The test showed that the strength of the brazed joints in Figures 2 and 3 was measured in N / mm 2 Although the results are approximately equal in terms of pore size, the effective, i.e., pore-free joints achieved with the brazing material of FIG. 3 are smaller than those achieved with the joints according to FIG. 2, indicating a lower joint strength. On the other hand, there is no erosion.

[0027] As stated in the "Summary of the Invention" section, it is an object to provide a brazing material that produces a pore-free joint and that produces little or no erosion of the substrate or plate material.

[0028] This is achieved by mixing a non-melting brazing alloy, i.e., an alloy with solidus and liquidus temperatures above the brazing temperature, with a powder of a melting brazing alloy, i.e., a brazing material with liquidus and solidus temperatures below the brazing temperature.

[0029] All embodiments have in common that more than 50% (by weight) of the brazing alloy is a non-melting brazing alloy. All embodiments also have in common that the brazing material is in the form of a paste, and that the paste comprises particles of brazing material, with a certain percentage of the particles being made from molten brazing material and the remaining particles being made from non-melting brazing material. Except for the particle origin of the brazing material, the paste comprises a solvent and a binder used to form a paste of particles.

[0030] Typical particle sizes for the present brazing materials are less than 100 microns, meaning that the total surface area of ​​the particles is many times greater than the surface area of ​​the substrate that the brazing material covers.

[0031] As mentioned above, a major problem with molten brazing materials containing melting point depressants is that the melting point depressants tend to migrate into the substrate and depress its melting point as the substrate melts, which causes erosion of the substrate. The erosion process is very time-dependent—the longer the brazing time, the more erosion occurs. However, according to embodiments, the time available for the melting point depressants to migrate into the substrate is regulated in an efficient manner. When the molten brazing material melts, the melting point depressants begin to migrate, but because the surface area of ​​the non-melting particles is much larger than the area of ​​the substrate in contact with the brazing material, most of the melting point depressants migrate into the non-melting brazing material particles and not into the substrate. Of course, this leads to "erosion" of the non-melting particles, but it also leads to the molten brazing material becoming starved, which means that once all or most of the non-melting brazing material has been "eroded" or melted, there is not enough melting point depressant left in the molten brazing material to migrate into the substrate to the extent that its melting point is below the brazing temperature. Thus, erosion of the substrate is largely avoided.

[0032] As an example, the non-melting brazing material particles could be made from pure stainless steel of the same type as the base material, but best results have been achieved by providing the non-melting brazing material in the form of particles made from stainless steel mixed with a small amount of a melting point depressant, likely due to faster depletion / dilution of the molten brazing material.

[0033] Tests have shown that a 90-75 / 10-25 mixture of unmelted brazing material particles containing (by weight) 2% Mo, 12.5% ​​Ni, 18.2% Cr, 6.3% Si, 5% Mn, balance Fe, and molten brazing material particles containing (by weight) 2.1% Mo, 13.4% Ni, 18.4% Cr, 8% Si, 5.3% Mn, 1.1% B, balance Fe, produces strong, pore-free joints that exhibit no or very little erosion of substrates made from 316 stainless steel when brazed at 1250°C in an inert gas atmosphere of 1-1.5 mbar.

[0034] The non-melting brazing material according to the above has a solidus temperature of about 1260°C and a liquidus temperature of about 1370°C, i.e. both the solidus temperature and the liquidus temperature are higher than the brazing temperature of 1250°C.

[0035] This molten brazing material has a solidus temperature of about 1110° C. and a liquidus temperature of about 1195° C., i.e., both the solidus and liquidus temperatures are below the brazing temperature.

[0036] Using a simple interpolation procedure, the solidus and liquidus temperatures of an alloy made from a brazing material containing 10% molten brazing material and 90% unmelted brazing material according to the above are estimated to have a solidus temperature of 1245° C. and a liquidus temperature of 1352° C., and an alloy containing 25% molten brazing material and 75% unmelted brazing material is estimated to have a solidus temperature of 1222° C. and a liquidus temperature of 1326° C. In other words, the solidus and liquidus temperatures of both alloys are such that the solidus temperature is lower than the brazing temperature and the liquidus temperature is higher than the brazing temperature.

[0037] By performing the same interpolation between the solidus and liquidus temperatures of the unmelted brazing material containing particles of pure stainless steel and the molten brazing material according to the above, alloys obtained for molten brazing material contents of 27% to 53% have solidus and liquidus temperatures similar to those disclosed above that are lower and higher than the brazing time, respectively (i.e., solidus and liquidus temperatures lower and higher than the brazing time, respectively).

[0038] The time required to form a homogeneous alloy from particles of molten and unmolten brazing material depends primarily on the particle size of the unmolten brazing material particles, with smaller particles exhibiting shorter times to form a homogeneous alloy.

[0039] The use of non-melting brazing material made from stainless steel particles is economically advantageous because such particles in a variety of particle sizes (sixes) can be purchased "off the shelf," whereas brazing material particles made from molten brazing material cannot.

[0040] The present invention is particularly directed to brazing with brazing materials containing melting-point depressant elements capable of migrating into substrates, such as stainless steel substrates brazed with stainless steel brazing materials containing melting-point depressant elements in the form of silicon, boron, phosphorus, and / or manganese. Among these melting-point depressant elements, boron is the most efficient in terms of melting-point depressant effect as a percentage function and also has the fastest diffusion rate into the substrate or nearby unmelted particles with lower boron percentages. Boron also tends to form brittle borides, primarily with chromium in stainless steel. Aside from the extreme brittleness of borides, the formation of chromium borides "consumes" a significant amount of chromium, thereby not providing the erosion resistance properties expected after boride formation. Because chromium borides are primarily formed in the liquid phase, and the present invention shortens the time the brazing material is in the liquid phase, the present invention significantly reduces problems due to boride formation, thereby improving both the braze strength and corrosion resistance of the joint.

[0041] Silicon and phosphorus are less efficient melting point depressants in terms of melting point depressant effect as a function of percentage, therefore larger amounts of these melting point depressants are required and therefore are equally problematic in terms of brittle phase formation.

[0042] Manganese is a melting point depressant that, in addition to its melting point depressant effect, also provides good wetting between the brazing material and the substrate. However, manganese tends to vaporize during brazing (especially if the brazing is performed under vacuum), which means that the melting point of the finished brazed joint may be higher than the initial melt of the brazing material. This is not usually a problem, but if unnecessarily large amounts of manganese are used, there may be problems with fouling of the brazing furnace.

[0043] Preferably, the brazing material is provided in the form of a paste comprising fused and unfused brazing material particles, a solvent to provide the paste with flow properties for its application, and a binder adapted to hold the brazing material particles together when the solvent evaporates. However, other forms are possible. Optionally, the brazing material may be provided as a green body, i.e., a body comprising fused and unfused brazing material in powder form, pressed so that low-temperature sintering occurs between the particles. It should be noted that, according to the present invention, the green body should not be heat-treated before it is used as a brazing material in a brazing process, in order to equalize the compositional differences of the brazing material particles.

Claims

1. 1. A method of brazing a brazed plate heat exchanger, comprising: a brazing material for brazing a brazed plate heat exchanger, the brazing material comprising a plurality of heat exchanger plates having a pattern of pressed ridges and grooves that provide contact points between adjacent heat exchanger plates, thereby holding the heat exchanger plates at a distance from one another and forming inter-plate flow paths for a medium to exchange heat between the heat exchanger plates; the brazing material comprising a brazing alloy including at least one melting point depressant and a metal similar in composition to the heat exchanger plates; the brazing material comprises a mixture between particles of molten brazing material having a solidus temperature and a liquidus temperature below the brazing temperature and particles of unmelted brazing material having a solidus temperature and a liquidus temperature above the brazing temperature, the ratio between the molten brazing material and the unmelted brazing material being such that an alloy formed by the molten brazing material and the unmelted brazing material has a solidus temperature below the brazing temperature and a liquidus temperature above the brazing temperature, and wherein an applying step of applying the unmelted brazing material to the contact surface of the heat exchanger plate is such that 50% by weight or more of the brazing alloy is the brazing alloy; a heating step of heating the brazing material to the brazing temperature, the brazing temperature being at or above the liquidus temperature of the particles of the molten brazing material and at or below the solidus temperature of the particles of the unmolten brazing material; and further comprising an adjusting step of adjusting a brazing time so as to prevent the melting point depressant element from migrating from the brazing material to the contact surface of the heat exchanger plate and the contact surface of the heat exchanger plate from becoming lower than the brazing temperature. wherein the particles of the non-melting brazing material contain 1.5-3% Mo, 10-15% Ni, 16-20% Cr, 5-7% Si, 5-7% Mn, and the remainder is Fe; A method of brazing with brazing material.

2. The method for brazing with the brazing material includes: the heating step heating the brazing material to at least 1222°C and up to 1260°C. A method of brazing with the brazing material of claim 1.

3. The method for brazing with the brazing material includes: the heating step heating the brazing material to at least 1245°C and up to 1260°C. A method for brazing with the brazing material according to claim 1 or 2.

4. The method for brazing with the brazing material includes: the heating step comprising heating the brazing material to 1250°C. A method for brazing with the brazing material according to any one of claims 1 to 3.

5. The method for brazing with the brazing material includes: the particles of the molten brazing material include boron; A method for brazing with the brazing material according to any one of claims 1 to 4.

6. The method for brazing with the brazing material includes: At least 75% by weight of the brazing alloy are particles of non-melting brazing material; A method for brazing with the brazing material according to any one of claims 1 to 5.

7. The method for brazing with the brazing material includes: the particles of the non-melting brazing material have a solidus temperature of 1260°C; A method for brazing with the brazing material according to any one of claims 1 to 6.

8. The method for brazing with the brazing material includes: The particle size of the unmelted brazing material and / or the molten brazing material is less than 100 μm; A method for brazing with the brazing material according to any one of claims 1 to 7.

9. The method for brazing with the brazing material includes: the composition of the molten brazing material and the unmolten brazing material is the same except that the molten brazing material contains 1-2% boron; A method of brazing with a brazing material according to any one of claims 1 to 8.

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