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. Lawrence’s Class Notes.
                   Oxyacetylene Process


• Acetylene…
  - Density = 0.61
  - M.P = -81.8 ˚C
  - Colorless, odorless.
  - Produces 6300˚F or      Acetylene at first generated as
    3482 ˚C flame when      needed because it couldn’t be
    combust in oxygen.      stored under pressure. Later
                            storage by adsorbing in
  - Percussion sensitive    acetone and sawdust allowed
    when stored under       safe storage. Now stored in
                            acetone and porous ceramic
    pressure.
                            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!




         Absolute hottest flame
         produced using a slightly
         oxidizing flame
Adiabatic Heat Calculation
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….)
• 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!
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


                            Focused Heat Source
                            e.g. Laser Beam & Electron Beam
Temperature




                                       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           Stack cutting
welding
Rail Failures and Welding
Thermit or Thermite Welding




                   Casting?
                   Welding?
                           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.
Other Thermite Systems
• 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).

				
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posted:7/20/2011
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