RU2106942C1 - Process of soldering of telescopic structures - Google Patents

Abstract

FIELD: manufacture of telescopic structures having external and internal shells of heterogeneous materials. SUBSTANCE: multilayer structure having several internal sealed spaces is assembled for soldering and is soldered in furnace. Inert gas under excessive pressure in injected into furnace for compression of internal shells 2, 3. Space 5 bordering on extreme internal shell 3 is filled with gas under pressure. This pressure is counterpressure relative to external pressure in furnace and corresponds in value to difference between external pressure and maximum permissible pressure. Permissible pressure is determined by yield point of material of this shell under temperature of soldering. EFFECT: increased output of good production thanks to provision of good contact between soldered surfaces. 1 dwg

Description

 The invention relates to power engineering, in particular to soldering telescopic structures that contain an outer and inner shell of dissimilar materials.

 Such structures are made in the form of interconnected shells: an inner shell made of heat-conducting metal, such as bronze, and an outer shell made of high strength steel. The shells form a two-layer package and are interconnected by ribs of the inner shell, bonded to the outer by high-temperature soldering. To ensure a dense and durable soldered seam during soldering, it is necessary to press the joined shells to each other.

 A known method of soldering two-layer telescopic structures containing the outer and inner shells intended for use as heat exchangers in power engineering. The method includes assembling, sealing the shells with the formation of a cavity between them, evacuating it, heating, soldering and cooling in an inert gas atmosphere. The shells are pressed against each other by creating excessive external pressure in the furnace [1].

 Known soldering three-layer structures of heat exchangers [2].

 The use of known methods for soldering structures containing several inner shells is difficult, since the increased pressure required to compress the increased number of shells leads to unacceptable local deformations of the extreme shells of the package in places located above the cavities, and does not provide a universal fit of the connected surfaces.

 The objective of the invention is the creation of a technology for soldering telescopic multilayer structures, eliminating the possibility of deformations of the extreme inner shells and the formation of joints in soldered joints.

 The problem is solved due to the fact that the cavity adjacent to the extreme inner shell is filled with gas under pressure corresponding to the difference between the external pressure and the maximum allowable pressure determined by the yield strength of the material of this shell at the soldering temperature. This circumstance ensures that the pressure of the inner shells is pressed against each other and to the outer shell without causing internal stresses in their material exceeding the yield strength.

 EFFECT: increased yield of products by ensuring tight contact between soldered surfaces and the absence of deformation of the walls of the inner shells.

 The method in accordance with the invention is as follows.

 A high-temperature brazing of a multilayer telescopic structure is carried out, containing several inner shells (two more often, three less often). The outer power shell is made of 10X18H10T stainless steel, the inner ones are made of BrX0.8 bronze.

 The multilayer structure assembled for soldering is welded at the ends of the shells to form internal cavities.

The cavity adjacent to the outermost shell is connected to an inert gas — argon or a mixture of argon and nitrogen. The structure is placed in a container or compression oven. The space of the container or furnace is filled with inert gas and the specified gas pressure is created, which is necessary to compress the shells to each other. This gas pressure in the furnace is external to the cavities of the multilayer structure, the shells of which must be connected during brazing. The cavity adjacent to the outermost shell is filled with gas under pressure corresponding to the difference between the external pressure and the maximum allowable pressure, determined by the yield strength of the material of this shell at the soldering temperature. This gas pressure in the cavity is counter-pressure with respect to the external gas pressure and is preliminary calculated so that the pressure drops between the shells are sufficient to compress the shells along the soldered surfaces to each other, but do not lead to local deformations thereof. As a result of creating back pressure in the cavity, the load on the extreme inner shell is reduced, which allows it to maintain its stability during the soldering process. Other internal cavities are evacuated to a residual pressure of less than 1 • 10 ^-3 mm RT. Art. A silver-based alloy is used as solder. Soldering temperature 970 ± 10 ^o C with a shutter speed of 30 minutes The preloading of the shells is carried out throughout the soldering process.

 Metallurgical analyzes of soldered joints and pneumohydro tests of structures were carried out. No leakage was detected. The yield of products is 95%.

 Example. The heat exchanger was brazed in the form of a three-layer telescopic structure.

 The drawing shows a diagram of this design.

 The structure comprises an outer power shell 1 with ribs inside, a first inner shell 2 with ribs outside and a second outermost shell 3 with ribs outside. The inner shell 2 forms a cavity 4 with the outer shell 1, and the inner shell 2 forms a cavity 5 with a shell 3.

 The external gas pressure and back pressure in the cavity 5, necessary for preloading the shell, are assigned according to the calculation carried out before soldering, taking into account the given dimensions of a particular design.

где σ_т - предел текучести бронзы БрХ0,8 при температуре пайки, равный 0,7 кгс/мм².The external pressure P _nN necessary for preloading the shells 2 and 3 and selecting a gap at the soldering temperature was determined from the ratio

where σ _t is the yield strength of BrX0.8 bronze at a brazing temperature of 0.7 kgf / mm ² .

h ₀ - the total thickness of the two inner shells, calculated as the difference between the radii of the shell 2 at the junction and shell 3 at the point of contact with the furnace gas (R ₂ -R ₁ ) and equal to 11 mm
R ₂ is the radius of the inner shell 2 at the junction with the outer shell 1, equal to 73.5 mm

где l₃ - расстояние между ребрами оболочки 3, равное 4 мм,
h₃ - толщина оболочки 3, равная 0,9 мм.The external pressure in the furnace as a result of the calculation is 0.105 kgf / mm ² . This pressure creates in the material of the shell 3 between the ribs at the temperature of the soldering voltage, determined from the ratio

where l ₃ - the distance between the ribs of the shell 3, equal to 4 mm,
h ₃ - shell thickness 3, equal to 0.9 mm

The voltage between the ribs of the shell 3 is 1.04 kgf / mm ² , which exceeds the yield strength of the bronze from which the shell 3 is made σ _t = 0.7 kgf / mm ² ), and can cause deformation of the intercostal sections at the soldering temperature.

.To reduce the pressure on the shell 3, it is necessary to introduce argon into the cavity 5, creating a pressure in it equal to the difference between the external pressure and the maximum allowable yield strength of the material of this shell at the soldering temperature. This maximum allowable pressure was determined previously from the ratio

.

,
где
P_обол - давление, воздействующее на оболочку 2;
σ₂ - напряжение в оболочке 2;
l_r - расстояние между ребрами оболочки 2, равное 2 мм;
h_r - толщина оболочки 2. равная 2,5 мм.It amounted to 0.072 kgf / mm ² . The total argon pressure on the inner shells 2 and 3 cannot be more than this value. To ensure this condition, argon was introduced into cavity 5, creating counterpressure in it (P _int - P $\genfrac{}{}{0ex}{}{\text{max}}{\text{ext.}}$ ) equal to 0.035 kgf / mm ² . The inner shells 2 and 3 were affected by a total pressure of 0.07 kgf / mm ² . In turn, this pressure caused a tension in the shell 2 between its ribs, determined by the ratio

,
Where
_Obol P - pressure acting on the shell 2;
σ ₂ is the stress in the shell 2;
l _r is the distance between the edges of the shell 2, equal to 2 mm;
h _r - shell thickness 2. equal to 2.5 mm

This stress was 0.02 kgf / mm ² , which did not exceed the yield strength of bronze at the soldering temperature.

 Thus, with the above selection according to the invention, the external pressure and back pressure in the casing 3 exerted a total pressure on each of the inner shells, providing tight contact between the soldered surfaces and the absence of deformation in the shells between the ribs.

 Metallographic analysis and pneumohydro tests confirmed the absence of defects in the welded structure and the depressurization of the soldered joint.

Claims (1)

 The method of brazing telescopic structures, mainly heat exchangers, containing outer and at least two inner shells made of dissimilar materials, including assembling, sealing the shells with the formation of cavities between them, evacuating the cavity and pressing the shells against each other by external pressure, heating, soldering and cooling, characterized in that the cavity adjacent to the outermost shell is filled with gas under pressure corresponding to the difference between the external pressure and the maximum allowable the pressure value, which is selected depending on the yield strength of the material of the outer inner shell at the soldering temperature.