The presentation is about the Vacuum and Hydrogen brazing, describes its procedure along with the advantages and disadvantages and applications.

1. Prepared by Guides
Raghavendra Darji(385) Dr. S.N.SomanRaghavendra Darji(385) Dr. S.N.Soman
M.E.-Part-Il Dr. J.Krishnan
(Welding Tech.)
Metallurgy and Material Science Department,
Faculty of Technology and Engineering,
The M.S.University, Baroda.
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2. Introduction
Brazing procedure
Brazing Furnaces and atmospheres
Different types of brazingDifferent types of brazing
Furnace brazing
Vacuum Brazing
Hydrogen Brazing
References
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3. Brazing is broadly defined as a group of joining processes that
produces bonding by the melting and re-solidification of a filler
material in the space between the surfaces to be joined.
The filler material used is required to wet the faying surfaces and be
drawn to fill the space between them by capillary action.drawn to fill the space between them by capillary action.
The filler material should have the liquidus above 4500 C and below
the solidus of the base material.
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4. In order to achieve optimum strength in a brazed joint the following
steps are normally followed.
1. Design of the joint,
2. Clean the faying surfaces,
3. Selection of flux, filler material and its placement,
4. Selection of brazing process,
5. Post cleaning and inspection
6. Heat treatment.
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6. The assembly or the region of the part to be joined is heated to a
temperature of at least 450 °C
The assembled parts and brazing filler metal reach a temperature
high enough to melt the filler metal but not the parts.
The molten filler metal, held in the joint by surface tension, spreadsThe molten filler metal, held in the joint by surface tension, spreads
into the joint and wets the base metal surfaces.
The parts are cooled to solidity or freeze. The filler metal , which is
held in the joint by capillary attraction and anchors the parts
together by metallurgical reaction and atomic bonding.
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7. Batch type, with either air or controlled atmospheres, in which work
pieces are loaded and unloaded manually,
Continuous type with either air or controlled atmospheres, which
feature an automatic conveying system,feature an automatic conveying system,
Retort type , with controlled atmospheres ,
Vacuum type.
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8. Torch Brazing,
Dip Brazing,
Induction Brazing,
Resistance Brazing,
Infra Red Brazing,
Furnace Brazing
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9. In furnace brazing, temperatures of 2050° to 2100° F (1120° to
1150° C) and above are not uncommon, especially when brazing
stainless steels with nickel-based filler metals or carbon steel with
copper filler metal.
Multiple joints on the same assembly can be brazed simultaneously.
Undesirable atmosphere constituents can be controlled orUndesirable atmosphere constituents can be controlled or
eliminated.
Multiple atmospheres or chambers make various types of processing
operations possible.
The process is highly repeatable, ideally lending itself to automated
production and data acquisition.
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10. Vacuum brazing is a term for various metal joining or brazing
processes that take place in a chamber or retort below atmospheric
pressure, otherwise known as a vacuum furnace. Vacuum brazing is
brazing in a furnace using a vacuum atmosphere.
Furnaces are categorized as hot wall or cold wall, depending on the
location of the heating and insulating components.location of the heating and insulating components.
Assemblies are bright and clean (shiny) after vacuum brazing
because the extremely low amount oxygen in a vacuum atmosphere
prevents oxidation of parts.
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11. Vacuum brazing is particularly useful where base metals are
processed that adversely react with other atmospheres, or where
entrapped fluxes or gases are intolerable. Vacuum brazing is widely
used to braze base metals of stainless steel, super alloys and carbon
low alloy steels.
Vacuum brazing offers the combination of high cleanliness and
uniform heating and cooling or rapid cooling. Vacuum brazing is
ideal for oxidation sensitive materials such as those used in the
aerospace industry.
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12. The reaction that takes
place in a vacuum is a
physical one in which
the low pressures (high
vacuum),combined
with sufficiently highwith sufficiently high
temperatures
,dissociates the
metallic oxides and
produces atomically
clean surfaces.
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13. Clean, oxide free surfaces are imperative to ensure sound brazed
joints of uniform quality.
Uniform capillary attraction may be obtained only when all grease,
oil, dirt, and oxides have been removed from both the filler metal
and the base metal before brazing.and the base metal before brazing.
Chemical and Mechanical methods can be employed for cleaning.
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14. The choice of cleaning process depends on the following:
Nature of contamination
Specific base metal to be cleaned
Degree of cleanliness required for brazing
Part configuration
Need to remove or provide barrier for coating for
Undesirable element such as Al, Ti, N2 ,etc.
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15. Emulsion cleaning in insoluble hydrocarbons and water (good for
removal of oils and cutting fluids)
Cleaning in water-base alkaline cleaners (good for removal of oils
and cutting fluids)
Solvent cleaning in mineral spirits, alcohol, acetone , chlorinatedSolvent cleaning in mineral spirits, alcohol, acetone , chlorinated
hydrocarbons (good for removal of mineral oils and cutting fluids).
Poor effect when used for removal of water-soluble oils.
Vapour degreasing in trichloroethylene, trichloroethane,(excellent
for removal of mineral oils and cutting fluids). Poor effect when used
for removal of water soluble oils.
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16. All parts shall be chemically cleaned to remove oils, dirt and greases
prior to attempting one of these mechanical cleaning methods:
Grit blasting using chilled cast iron, or stainless steel grits or
powders. (Blasting with Aluminium oxide or silicon carbide should
be avoided)be avoided)
Machining or grinding is acceptable methods provided that joint
clearances are not disturbed.
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17. A properly designed vacuum brazing cycle is a very critical step in
the vacuum brazing process.
Initial Pump Down: Vacuum furnace pump down to 8 * 10-4 torr to
5 * 10-4 torr, depending upon the material to be brazed.
Initial Heating: The initial ramp should be 20º to 30º F/minute.
Cement Burn off: For critical materials (e.g. nickel-base super alloys)
particularly, with heavily applicated parts (e.g. surface braze build-
up) a burn off soak is strongly recommended to avoid too high a
pressure rise.
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18. Stabilizing soak: A soak temperature shall be about 50ºF below
solidus temperature of the brazing slurry for duration of 15 to 30
minutes, or until the pressure drops below the desired level,
whichever is longer. It serves two purposes:
It allows the temperature throughout the load to equalize so all
parts in the load will reach brazing temperature at approximatelyparts in the load will reach brazing temperature at approximately
the same time during the next heating cycle.
It ensures that vacuum pressure levels are low enough before
proceeding (ramping) to brazing temperature.
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19. Heating to brazing temperature: The final heating rate to brazing
temperature is very critical. The rate must be fast enough to avoid
excessive liquidation of the brazing alloy and subsequent alloying
with and erosion of the base metal.
For thin materials heating rates of 50º to 75º F/minute are essential.
Rates of 30º to 50º F/minute are the most frequently used in the
industry.
Brazing Temperature: It is desirable to use the lowest brazing
temperature within the recommended brazing range consistent with
producing a satisfactory joint. In some application minimum brazing
temperatures are essential:
when using pure copper brazing filler,
to fill large gaps with wide gap nickel alloys
when brazing very thin materials
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20. Brazing Soak: In general, time at brazing temperature should be
long enough to ensure that all sections of a work piece and all parts
within the load reach the desired brazing temperature.
Cooling from brazing temperature: For materials that do not require
solution heat treatment or hardening, it is recommended that the
load be vacuum cooled from brazing temperature to a temperature
at least 50ºF below the solidus temperature of the brazing slurry,at least 50ºF below the solidus temperature of the brazing slurry,
before initiating the gas quenching system.
If the part requires heat treatment from brazing temperature, then
the gas quench must be initiated at the end of the brazing soak
period.
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21. Unloading the furnace: In
theory parts can be unloaded
when they are below 400ºF
without discoloration.
However, to ensure that thereHowever, to ensure that there
is no possibility of
discoloration on critical,
heavier parts, it is
recommended that all load
thermocouples t be below
200ºF before opening the
furnace.
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22. Prior to brazing one has to take care of the following:
Clearance For Vacuum Furnace Brazing,
Differential Metal Expansion (DME)
Overlap And Surface Flatness Brazing
Surface Conditioning Prior To Vacuum Brazing
Brazing Filler Metal Selection
Assembly & Fixturing For Brazing
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23. Extremely clean
Flux-free braze joints with high integrity and superior strength
Improved temperature uniformity
Lower residual stresses owing to slow heating and cooling cycle,Lower residual stresses owing to slow heating and cooling cycle,
resulting in dramatically improved mechanical and thermal
properties of the material
Age hardening or heat treating of the work piece is part of the
metal-joining process, but all in a single furnace cycle
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24. Nickel and Iron based alloys containing aluminium and/or titanium
Refractory metals
Reactive metals
Ceramics & metal to ceramics
Exotic base metals such as Aluminium, Titanium, Zirconium,
Niobium, Molybdenum, and Tantalum
Beryllium brazing
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25. Hydrogen brazing is a braze process that uses the cleaning
(reducing) properties of high purity hydrogen to improve the flow
characteristics of the braze alloy.
The hydrogen atmosphere reduces surface oxides on the parent
material, enabling the braze alloy to flow (wet) more effectively tomaterial, enabling the braze alloy to flow (wet) more effectively to
create a high integrity braze joint.
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26. Hydrogen brazing can be performed within a sealed retort furnace
or a hump back belt furnace. Both furnace types surround the part
being processed within a high purity hydrogen atmosphere).
As the furnace load is heated above the liquidus temperature of the
braze alloy, the hydrogen atmosphere reduces surface oxides
present of the parent material and improves the wettingpresent of the parent material and improves the wetting
characteristics of the braze alloy.
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27. Cleanliness – the reduction of surface oxides on the parent material
improves the cleanliness and integrity of the braze joint.
Increased braze alloy and parent material options – enables the use
of high vapour pressure braze alloys and parent materials thatof high vapour pressure braze alloys and parent materials that
cannot be brazed within a vacuum atmosphere.
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28. Medical devices
Electronic devices
Aerospace parts (precision machined stainless steel components)
High vapour pressure braze alloys (Cu, Ag, etc.)
High cleanliness assemblies
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29. Hydrogen brazing can be applied to a multitude of materials such as
stainless steel, copper and some nickel based alloys.
Titanium alloys cannot be brazed in a hydrogen atmosphere.
Hydrogen brazing reduces surface oxides at the faying surfaces,
resulting in a clean, high integrity braze joint that improves the
service characteristics of the finished part.
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30. 1. R.S.Parmar, Welding processes and Technology Page no:499-505
2. ASM Handbook Volume-6
3. Introduction to furnace brazing by Dr. Diran Apelian, Bruce
Boardman, Roger Fabian, Dan Herring, Harb Nayar, Robert
Peaslee.(from internet).
4. Paper published on vacuum brazing titled “ Issues in vacuum
brazing” by Janusz Kowalewski & Janusz Szczurek from internet.brazing” by Janusz Kowalewski & Janusz Szczurek from internet.
5. http://www.wallcolmonoy.com
6. AWS Brazing handbook.
7. http://www.azom.com/article.aspx?ArticleID=8305
8. http://www.altairusa.com/brazing-vacuum-active-metal.php
9. http://www.bodycote.com
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