Chemical Energy in WeldingRelated Processes A small number of slides adapted from Prof. F. Lawrences Class Notes. Oxyacetylene Process CaC2 H 2O C2 H 2 Acetylene - Density = 0.61 M.P = -81.8 C Colorless, odorless. Produces 6300F or 3482 C flame when combust in oxygen. - Percussion sensitive when stored under
pressure. Acetylene at first generated as needed because it couldnt 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 H<0 Primary combustion:
C2H2 + O2 -> 2CO + H2 + H Secondary combustion: 4CO + 2H2 + 3 O2(from air) -> 4CO2 + 2H2O + H 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! Treaction prod reag H To CpdT prod reag Absolute hottest flame
produced using a slightly oxidizing flame Adiabatic Heat Calculation Illustration Problem Pb 21 O2 PbO o o o H298, Reaction H298, PbO H298, Ni 1 2 o H298, O2
o H298, Reaction 52,400 0 0 Cal o H298, Reaction 52,400 Cal 500 o o Cp,products H500, Reaction H 298,Reaction 298 C
p,reactants dT 500 o C H500, Cp, Ni 21 Cp, O2 dT Reaction 52,400 298 p, PbO 500
3 T 0.4 x105T 2 dT 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 (H) 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, dont 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, wont 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) H = -3347.6 kJ/mol H = -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: Tmax 140,000= C p,Fe- L C p,Fe- L C p,Fe- L L C p,L dT 298 Solve for Tmax . Note that the C p 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 H1700, 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).
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