Download - Chemical Energy in Welding-Related Processes
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Chemical Energy in Welding-
Related Processes
A small number of slides adapted from Prof. F. Lawrences Class Notes.
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Oxyacetylene Process
Acetylene
- Density = 0.61
- M.P = -81.8 C
- Colorless, odorless.
-Produces 6300F or
3482 C flame when
combust in oxygen.
- Percussion sensitivewhen stored underpressure.
Acetylene at first generated asneeded because it couldnt be
stored under pressure. Later
storage by adsorbing in
acetone and sawdust allowed
safe storage. Now stored in
acetone and porous ceramiccore.
2 2 2 2CaC H O C H
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Complete combustion
2C2H2 + 5 O2 -> 4CO2 + 2H2O DH 2CO + H2 + DH
Secondary combustion:
4CO + 2H2 + 3 O2(from air) ->
4CO2 + 2H2O+ DH
Acetylene flame lit first with no oxygen. Oxygen added - reducing (carburizing)
flame.
More oxygen - neutral flame.
More (excess) oxygen - oxidizing flame.
Extinguish flame in reverse order..
Combustion of C2H2
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Hottest part of the oxy-acetylene
flame just in front of inner cone
(blue-white part of flame).
Absolute hottest flame
produced using a slightly
oxidizing flame
Combustion of C2H2
Flame Adiabatic Temperature?Calculation for other combustiblegases!
reaction
o
T
p
prod reag prod reag T
H C dT
D D
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Adiabatic Heat Calculation
D
D
D
D D
122
1298, 2
298,
298,
500
500, 298,298
52, 400 0 0
52,400
2
o o o o
Reaction 298, PbO 298, Ni 298, O
o
Reaction
o
Reaction
o o
Reaction Reaction p,products p,reactants
Illustration Problem
Pb O PbO
H H H H
H Cal
H Cal
H H C C
D
D
D
5001
500, 2298
5003 3
500,298
3 5 212
500,
52,400
52,400 10.6 4.0 10 5.63 2.33 10
7.16 1.0 10 0.4 10
51,998
2
o
Reaction p, PbO p, Ni p, O
o
Reaction
o
Reaction
o
298, PbO
dT
H C C C dT
H x T x T
x T x T dT
H Cal
H Given in th
1
2; ;
2p, PbO p, Ni p, O
ermodynamic tables
C C C Given in thermodynamic tables
T - Given in Kelvin
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Acetylene does not have the highest
heat of combustion (DH) nor the highest
burning velocity (V) BUT it has thehighest combustion intensity = V x H!
Combustion Intensity
- Hydrogen has thehighest flame velocity- Propane and butanehave the highestheat of combustionper volume
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Acetylene is explosive that its use is frequently prohibited in factories
(e.g. Caterpillar.)
Acetylene in contact with Cu, Hg, and Ag with impurities creates
Acetylides which are violently explosive and shock sensitive.
THEREFORE, DO NOT use acetylene flames on alloys with more than
67% Cu, that is, dont use acetylene to weld BRASS!
Safety when Using C2H2
e.g.H-C C-H H-C C-Na
H-C C-Cu
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Make sure needle valves are closed.
Regulators are backed off.
Open main valves
Adjust pressure
Crack open acetylene needle valve.
Ignite,
Adjust flame.
Crack open oxygen needle valve.
Adjust flame.
Shut down in reverse order; finally, open
needle valves to bleed off gases.
Oxy-Gas Welding
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Oxy-acetylene welding is a
two-handed process if fillermetal is added. Many metals
can be welded but the
adjustment of the flame and
the use of fluxes varies with
the metal.
Oxy-Acetylene Welding
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Diffused Heat Source
Diffused Heat Sourcee.g. Oxy-acetylene & Gas Metal Arc
Focused Heat Sourcee.g. Laser Beam & Electron Beam
Temperature
Distance
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Oxy-fuel (flame) cutting uses flames to bring the metal to the
temperature at which it will react with an oxygen jet to burn the metal.
No melting occurs!
Not all metals can be flame cut! Carbon steel can but stainless steel
and aluminum cannot. The necessary conditions for successful flame
cutting are enumerated above.
Oxy-Gas Cutting
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Uncuttable Alloy Systems?
Why not Stainless Steels?
High melting temperature oxide layers, Cr2O3
Why not Aluminum Alloys?
High melting temperature oxide layers, Al2O3 High thermal conductivity
Why not Titanium Alloys?
Oxygen and carbon pickup
Why not Copper Alloys? High thermal conductivity
Possibility of Acetylide formation
Why not Cast Alloys?
Molten SiO2 layer covering kerf
Oxy-Gas Cutting
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The torch tip, oxygen pressure
and travel speed must be properly
adjusted.
Too slow - shuts down.
Too fast - too much drag, wont
cut completely through particualrly
at edges.
Drag
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Stack cuttingCutting an edge
preparation for
welding
Flame Cutting Operations
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Rail Failures and Welding
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Thermit or Thermite Welding
Casting?Welding?
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Aluminothermic Welding
For maximum efficiency, the magnetite thermite mixture should contain
23.7% aluminium and 76.3% iron oxide (mass percent).
Using hematite, iron (III) oxide, the themite mixture should contain 25.3%
aluminum and 74.7% iron oxide (mass percent).
The reaction using Fe3O4 produces a substantially larger amount of
energy/mole reaction. The reaction using Fe2O3 produces more energy/gram
of thermite mixture.
Temperature is raised to 2000-2200oC.
3 4(s) (s) 2 3(s) (s)
2 3(s) (s) 2 3(s) (s)
3Fe O + 8Al 4Al O + 9Fe H = -3347.6 kJ/mol
Fe O + 2Al Al O + 2Fe H = -851.5 kJ/mol
D
D
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p,Fe- p,Fe- p,Fe- L p,L
Problem No. 1
Assume combustion of one mole of C H produces 140kCal at 298K
the maximum temperature of one mole of Fe exposed to theC H
combustion can be calculated as:
140,000= C L C L C L C
2 2
2 2
,
T
p
p
dT
Solve for T
Note that the C terms represent heat capacities of the several states
of Fe and L the L terms represent the latent heats of
transformation and fusion. After substitution of the several C ter
max
298
max .
, ( , , , ),
D
oReaction
ms
and L values, the problem becomes one of integration.
Problem No. 2
Now try to calculate H
FeO Al Al O Fe
Problem No. 3
Identify other alloy systems that can be joined using thermite
1700,
2 33 2 3
reactions.
Other Thermite Systems
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Identify other alloy systems that can bejoined using thermite reactions.
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Safety of Thermit Welding
Thermite should not be used near flammable materials; smallstreams of molten iron released in the reaction can travelconsiderable distances and may melt through metal containers,ignite their contents, etc.
Flammable metals with relatively low boiling points such as Zincshould be kept away from thermite, as contact with such metalscould potentially boil superheated metal violently into the air, where itcould then burst into flame as it is exposed to oxygen. The boilingpoint of Zinc at 1665 F (907 C) is about 2500 F (1371 C) below
the combustion temperature of thermite. Thermite must be used with care in welding pipes or other items with
air cavities, as thermal expansion of trapped gases may causebursting.
Generally, the ignition of thermite should be timed so that individuals
handling it have ample time to get away.
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Major Thermite Applications
Rail Joining
Competes against Flash Butt Welding
Rebar Joining
Competes against SMAW and FCAW
Steel Hull Plate Joining
Build aluminum superstructures so not top heavy
Used explosive bonding to bond the plates together
Get aluminum-steel transition
Potential Hazards
Get aluminum in the presence of rust, start of a thermit reaction
Aluminum will burn
Jet fuel ignited thermit reaction
Galvanic reactions?
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Other Thermite Applications
Military Applications
Thermate -TH3 is a mixture of thermite and pyrotechnicadditives for incendiary purposes. Its composition by weight
is generally thermite 68.7%, Ba(NO3)2 29.0%, S 2.0% andbinder 0.3%.
Ba(NO3)2 increases its thermal effect, creates flame inburning and significantly reduces the ignition temperature.
Ames Process an adaptation of the thermite reaction for
obtaining pure Uranium (as part of the Manhattan Procjectat Ames Laboratory).