Chemical Energy in WeldingRelated Processes
A small number of slides adapted from Prof. F. Lawrence’s Class Notes.
Oxyacetylene Process
CaC2  H2O  C2 H2
• Acetylene…
-
Density = 0.61
M.P = -81.8 ˚C
Colorless, odorless.
Produces 6300˚F or
3482 ˚C flame when
combust in oxygen.
- Percussion sensitive
when stored under
pressure.
Acetylene at first generated as
needed because it couldn’t be
stored under pressure. Later
storage by adsorbing in
acetone and sawdust allowed
safe storage. Now stored in
acetone and porous ceramic
core.
Combustion of C2H2
Complete combustion
2C2H2 + 5 O2 -> 4CO2 + 2H2O
DH<0
Primary combustion:
C2H2 + O2 -> 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
Hottest part of the oxy-acetylene
flame just in front of inner cone
(blue-white part of flame).
Flame Adiabatic Temperature?
Calculation for other combustible
gases!

prod reag
DH 
Treaction

To

prod reag
DCpdT
Absolute hottest flame
produced using a slightly
oxidizing flame
Adiabatic Heat Calculation
Illustration Problem
Pb 
1
2
O2  
PbO
o
o
o
o
1
DH298,
Reaction  H298, PbO  H298, Ni  2 H298,O2 
o
DH298,
Reaction   52,400  0  0  Cal
o
DH298,
Reaction  52,400 Cal
o
o
DH500,
Reaction  DH298,Reaction  
500
298
o
DH500,
Reaction  52,400  
500
o
DH500,
Reaction  52,400  
500
298

  Cp,products   Cp,reactants dT
Cp, PbO  Cp, Ni  21 Cp, O dT
 2


 
 10.6  4.0 x103T  5.63  2.33 x10 3T
298 

 21 7.16  1.0 x103T  0.4 x105T 2  dT
o
DH500,
Reaction  51,998 Cal
o
H298,
 Given in thermodynamic tables
PbO
Cp, PbO ;Cp, Ni ; 21 Cp,O2   Given in thermodynamic tables
T - Given in Kelvin

Combustion Intensity
- Hydrogen has the
highest flame velocity
- Propane and butane
have the highest
heat of combustion
per volume
Acetylene does not have the highest
heat of combustion (DH) nor the highest
burning velocity (V) BUT it has the
highest combustion intensity = V x H!
Safety when Using C2H2
• Acetylene is explosive that its use is frequently prohibited in factories
(e.g. Caterpillar….)
e.g. H-C  C-H  H-C  C-Na
• 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, don’t use acetylene to weld BRASS!
H-C  C-Cu
Oxy-Gas Welding
• 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-Acetylene Welding
Oxy-acetylene welding is a
two-handed process if filler
metal is added. Many metals
can be welded but the
adjustment of the flame and
the use of fluxes varies with
the metal.
Diffused Heat Source
Temperature
Focused Heat Source
e.g. Laser Beam & Electron Beam
Diffused Heat Source
e.g. Oxy-acetylene & Gas Metal Arc
Distance
Oxy-Gas Cutting
• 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.
Uncuttable Alloy Systems?
Oxy-Gas Cutting
• 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
Drag
• The torch tip, oxygen pressure
and travel speed must be properly
adjusted.
• Too slow - shuts down.
• Too fast - too much drag, won’t
cut completely through particualrly
at edges.
Flame Cutting Operations
Cutting an edge
preparation for
welding
Stack cutting
Rail Failures and Welding
Thermit or Thermite Welding
Casting?
Welding?
Aluminothermic Welding
3Fe3O4(s) + 8Al(s)  4Al2O3(s) + 9Fe(s)
Fe2O3(s) + 2Al(s)  Al2O3(s) + 2Fe(s)
•
DH = -3347.6 kJ/mol
DH = -851.5 kJ/mol
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.
Other Thermite Systems
Problem No. 1
Assume combustion of one mole of C2H 2 produces 140kCal at 298K ,
the maximum temperature of one mole of Fe exposed to the C2H 2
combustion can be calculated as:
140,000= 
Tmax
Cp,Fe-  L   Cp,Fe-  L   Cp,Fe-  L L  Cp,L dT
Solve for Tmax .
298
Note that the Cp 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 p terms
and L values, the problem becomes one of integration.
Problem No. 2
o
Now try to calculate DH1700,
Reaction
3 FeO  2 Al  Al 2O3  3 Fe
Problem No. 3
Identify other alloy systems that can be joined using thermite reactions.
• Identify other alloy systems that can be
joined using thermite reactions.
Safety of Thermit Welding
• Thermite should not be used near flammable materials; small
streams of molten iron released in the reaction can travel
considerable distances and may melt through metal containers,
ignite their contents, etc.
• Flammable metals with relatively low boiling points such as Zinc
should be kept away from thermite, as contact with such metals
could potentially boil superheated metal violently into the air, where it
could then burst into flame as it is exposed to oxygen. The boiling
point 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 cause
bursting.
• Generally, the ignition of thermite should be timed so that individuals
handling it have ample time to get away.
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?
Other Thermite Applications
• Military Applications
– Thermate -TH3 is a mixture of thermite and pyrotechnic
additives for incendiary purposes. Its composition by weight
is generally thermite 68.7%, Ba(NO3)2 29.0%, S 2.0% and
binder 0.3%.
– Ba(NO3)2 increases its thermal effect, creates flame in
burning and significantly reduces the ignition temperature.
– Ames Process – an adaptation of the thermite reaction for
obtaining pure Uranium (as part of the Manhattan Procject
at Ames Laboratory).