chemical energy in welding-related processes

8/2/2019 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
http://upload.wikimedia.org/wikipedia/commons/2/25/Geschweisster_schienenstoss.jpeg


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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?
http://upload.wikimedia.org/wikipedia/commons/f/f9/Rebar_and_shingles.jpg


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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).

chemical energy in welding-related processes

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