Design of brazed joints
1. Introduction
An optimum brazed joint can only be achieved ifthere is close co-operation between design andproduction departments.
A series of constructional considerations must betaken into account before carrying out any brazingwork:
First of all the operating conditions (type of load,magnitude of the load and its direction; media,temperatures) must be established. In addition, thebase material and any heat treatment it is subjectedto must be defined. The brazing alloy and brazingtechnique are then selected. The fit of theworkpieces must be selected depending on thebrazing technique.2. Fits (gap widths)
0 – 0.1 mm Vacuum-brazing, gap-brazing0 – 0.2 mmInert gas brazing, gap-brazing0.05 – 0.2 mmFlux-brazing, gap-brazing by
machine
0.05 – 0.5 mmFlux-brazing, manual gap-brazing> 0.5Flux-brazing, manual joint-brazing
As a rule of thumb, an overlap length of between 3
and 6 times the sheet thickness can be used (seeFigure 1) or alternatively a value in accordance withthe nomogram (see Table 5).
3. Sizes of brazed joints
Brazed joints must be of such a size that they canbe subjected to as high loads as the base material.The brazed joints are calculated using the laws ofthe theory of the strength of materials. If oneassumes there to be predominantly static loads atroom temperature, no major brazing defects andsuitable combinations of materials, the following canbe used for calculating the brazed joints:σB Brazed joint ≈ 200 MPa (N/mm)
2
τB Brazed joint ≈ 100 MPa (N/mm)
2
Figure 1: Examples of overlapped gaps
Unnecessarily large overlap lengths lead to moredefects and make „through-brazing“ more difficult.
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Design of brazed joints
4. Determining the amount of brazing alloyThe required amount of brazing alloy can be
calculated from the maximum gap volume, plus ca.15 % for each fillet to be formed. For cylindricalbrazed joints, the wire diameter of the brazing alloyring can be determined using the nomogram (seeFigure 2).
b = width of brazed gapt = insertion depthd = wire diameter• • • •
Read off the gap width from the y-axis (verticalaxis)
Draw a horizontal line from this point to thecurve having insertion depth t.
From the intersection point, draw a vertical lineto the x-axis (horizontal axis) and read off thesought after wire diameter d.
If the answer is an intermediate value, the nextlarger wire diameter must be chosen.
Insertion depth t [mm]
5. Design regulations•
Make parallel hemmings (see Figure 4)
Butted joints
Overlap joints
Wire diameter d [mm]
Incorrect
Figure 2: Relationship between the brazed gapwidth, insertion depth and brazing alloy wirediameter for brazing cylindrical joints
Correct
Figure 4: Incorrect and correct gap cross-sections
Figure 3: Schematic representation of thearrangement of a brazing alloy wire ring for filling thegap between cylindrical components
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Design of brazed joints
•
On melting, the brazing alloy must maintaincontact with the gap. Keep the brazing alloy flowdistance as short as possible (see Figure 5).
Surface press fit; poorKnurled press fit; good
Figure 7: Brazing pressed components
A: Surface press fit; poor
B: Knurled press fit, very good•
Do not sink nipples, bolts, etc. during the brazingprocedure on workpieces having diameters > 30mm due to possible internal stress (see Figure8).
Take into account size changes caused by heatexpansion when using a combination of differentmaterials so that the required gap width ispresent at the brazing temperature.
Make movement of the brazing alloy / fluxpossible (see Figure 9).
Figure 5: Surface brazing with inserted brazing alloyrings•
If possible, arrange workpieces so that they areself-positioning. When brazing in normal air, asurface press fit may not be used as too littleflux is present in the brazing gap to keep the gapwalls bare metal. Self-positioning of thecomponents can be achieved using a three-linepress fit (see Figure 6) or a knurled press fit (seeFigure 7).
Concentric press fit; poorThree-line press fit; good
•
•
Brazing alloy ring 4003
BeforeAfter
Figure 8: Brazing threaded connection pieces
Figure 6: Brazing pressed components
A: Surface press fit; poorB: Line press fit; good
BrazeTec GmbH, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang
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Design of brazed joints
Keep de-burring–phases small and flat so that theycan be filled up with brazing alloy (Figure 11)
Included flux
Outflow opening
for flux
PoorGood
Figure 9: Avoiding flux inclusions
Figure 11: Design of brazed joints
•
Avoid erratic changes in cross-section on brazedjoints subjected to high loads (see Figure 10).
Abrupt change in cross section Gradual change in cross-section
Figure 10: Cross-sectional changes
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Design of brazed joints
• Containers – connection pieces – brazed joints
Problem: Due to local heating, the connection piecesbecome larger in diameter but the bore not.
On cooling down the connection piece becomessmaller but the bore not.
Consequence: High stresses in the brazed jointseam with a danger of cracking on cooling downSolution: Knock out the container walls or weld onbrazed connection pieces.
Heated state
Solution 1
Cracked brazed joint as a resultof stress on cooling down
Solution 2
Figure 12 and 13: Brazing pipes into thick-walled containers
BrazeTec GmbH, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang
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Design of brazed joints
6. Examples of constructions(Tables 1 – 4)
BrazeTec brazing alloys in brazed joints
A selection of technically important examples now follows.
(Favourable gap width range: 0.05 ... 0.2 mm; the gap widths are not shown to scale in the tables but areenlarged).
Good Poor Good Good
PermissibleGood Good Good
PermissibleTable 1: Bolts
Wall thickness
>= 1 mm α >= 50°
Good Good Good
Good >Poor Good
Sockets and fittings after installation (usually)
Brazingalloy
Good
Table 2: Pipes
Poor Good
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Design of brazed joints
GoodGood Exceptionally Good
Good Poor Good Possible Good
Table 3: Nipples and flanges
Good Good Exceptionally Possible
GoodPoor Good Good
Good Good GoodWith flame field burner
Table 4: Containers made of sheet metal
BrazeTec GmbH, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang
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Design of brazed joints
Bolts-diameter
Tensile strength ofthe base material2
[N/mm]
Table 5: Nomogram for determining the overlap length for overlapped brazed joints; e.g. sheets,
pipes inserted into each other and inserted bolts.
BrazeTec GmbH, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang
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Design of brazed joints
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BrazeTec GmbH, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang
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