PowerPoint to accompany
Welding
Principles and Practices
Third Edition
Sacks and Bohnart
Gas
Welding
Chapter 5
1
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
Oxyacetylene Welding
• Definition:
– Join metal by heating surfaces to be joined to
melting point with gas flame
– Fusing metal into homogeneous mass
– Letting it solidify into a single unit
• Flame in cone reaches temperatures as high as
5800º to 6300ºF
• Filler rod may or may not be used to intermix
with molten pool
5-2
Oxyacetylene Process
• During first part of this century used as major
welding process both for fabrication and
construction, maintenance and repair
• Today use limited for industrial production
purposes
• Still used for brazing, soldering, welding
metals with low melting points; general
maintenance and repair work
5-3
History of Oxyacetylene Welding
• Early Egyptians, Greeks, and Romans used
alcohol or oil flame to fuse metals
• In nineteenth century various gases tested
– In 1847, Robert Hare fused platinum with
oxyhydrogen flame
– In 1880, production of oxygen and hydrogen
through electrolysis of water made distribution of
gases in cylinders under pressure
– Late 1800s experiments done with oxygen-coal gas
and air-hydrogen flames
5-4
Discoveries Leading to the
Oxyacetylene Process
• In 1836, Edmund Davey discovered acetylene gas
• In 1862, acetylene gas produced from calcium
carbide
• In 1895, Thomas L. Willson produced calcium
carbide commercially
• Used for residential lighting
• In 1895, LeChatelier discovered that combustion
of acetylene with oxygen produced flame hotter
than any other gas flame
5-5
Discoveries Leading to the
Oxyacetylene Process
• In 1900, Edmond Fouche invented high
pressure acetylene torch
• Later designed low pressure torch (worked on
injector principle)
• In 1906 Eugene Bourbonville brought first
welding torch to this country
• Process first used for maintenance and repair
• During World War I oxyacetylene welding
became production tool
5-6
Gases
• Oxyacetylene welding process uses two
principal gases
– Oxygen and acetylene
• Other gases can be used for cutting and heating
– Propane
– Natural gas
– Mapp® gas\
5-7
Various Fuel Gas Efficiencies
Fuel Gas
Btus Usable heat
(ft3)
(s/ft2)
Normal
Flame Oxygen
velocity
temp. per ft3 of fuel (ft/s)
Acetylene
Mapp®2
Propane
Natural gas3
Hydrogen
1433
2381
2309
918
275
5,420ºF
5,301ºF
5,190ºF
5,000ºF
4,600ºF
12,700
5,540
5,500
5,600
7,500
1.04
2.4
4.00
1.50
0.25
17.7
7.9
11.9
15.2
36
5-8
Various Gas Cylinders
Thermadyne Industries, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5-9
Oxygen
• Gaseous Chemical element in air
– Necessary for life
• Most abundant chemical element in crust of
Earth
• No color, odor, or taste
• Does not burn, but supports combustion
– Substances that do not burn in air will in oxygen
5 - 10
Oxygen Production
• Two commercial processes used in production
of oxygen
– Separation of water into oxygen and hydrogen by
electrolysis of water
– Separation of air into oxygen and nitrogen by
liquefying air
5 - 11
Oxygen Distribution
Aluminum
cylinders
• Distributed
in steel or aluminum cylinders
Used for medical gases (smaller size)
Made of high strength 6061-T6 alloy
Steel cylinders
Made from single plate of high-grade steel
Heat treated to develop maximum strength and
hardness
Seamless, drawn-steel vessels with iron neck ring
shrunk on at top and cylinder value screwed into neck
High pressure – undergo rigid testing
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5 - 12
Valve Mechanisms
• Must be opened to release
oxygen
• Double-seated valve perfectly
tight when completely open or
closed
– Valve protected from damage by
iron cap that screws on neck ring
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 - 13
Gas Cylinders
• Charged with oxygen at pressure of about
2,200 p.s.I. at 70ºF
• Gas temperature
– Increase: gas expands and increases pressure
– Decrease: gas contracts and reduces pressure
• Safety device on cylinder valve to blow off
oxygen when excess pressure
– Occurs, oxygen lost
– DO NOT store where cylinder can be overheated
5 - 14
Capacity of Cylinder
• Three cylinder sizes used for welding and
cutting
• Gas suppliers fill tanks with varying amounts
– Large size: 220-244 cubic feet of oxygen
• Full: 148 to 152 pounds; Empty: 130-133 pounds
– Middle size: 110-122 cubic feet of oxygen
• Full: 89 to 101 pounds; Empty: 79-93 pounds
– Small size: 55-80 cubic feet of gas
• Full: 67 pounds; Empty: 60 pounds
5 - 15
Safety Precautions
• Take special care to keep oil and grease away
from oxygen
• When using, do not place cylinders where oil
might drop of them from overhead bearings
• Never use oxygen in pneumatic tools or to start
internal combustion engines
• Never use oxygen to blow out pipe or hose
lines, dust clothes or create head pressure in
tank of any kind
5 - 16
Safety Precautions
• Do not store oxygen cylinders near acetylene
generator, carbide, acetylene, or other fuel-gas
cylinders
• Do not use cylinder as roller or lift it by cap
• Keep cylinders away from welding operation
and close cylinder valve when work completed
• Keep cylinders away from any electrical
contact
5 - 17
Acetylene
• Most widely used of all fuel gages
– Both welding and cutting
• Generated as result of chemical reaction that
takes place when calcium carbide comes in
contact with water
• Tests show oxyacetylene flame temperatures
up to approximately 6,300ºF
– Very rapid rate of preheating
– Burns with smoky flame, gives off carbon, has
peculiar odor
5 - 18
Characteristics of an Effective
Welding Fuel Gas
•
•
•
•
High flame temperature
High rate of flame propagation
Adequate heat content
Minimum chemical reaction of flame with base
and filler metal
Acetylene most closely matches all these
requirements and used for welding purposes.
5 - 19
Acetylene Production and
Distribution
• Commercial acetylene made from calcium
carbide – referred to as carbide
– Gray, stonelike substance
– Product of smelting coke and lime in electric
furnace
– Several sizes available
• Distributed in standard steel drums
– 100 pounds for use in acetylene generators
5 - 20
Acetylene Cylinders
• Constructed differently from oxygen cylinders
– Free acetylene should not be stored at pressure
above 15 p.s.i.
• Safety solved by packing cylinders with porous
material saturated with acetone
– Acetone – liquid chemical having property of
dissolving or absorbing many times the volume of
acetylene
• Strong steel container (packed completely full)
• Handle with care
5 - 21
Three Basic Acetylene
Cylinders
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5 - 22
Valve Mechanisms
• Acetylene drawn off through valve
– Some cylinders valve located in recessed top
– Others valve located on convex top
• Simpler in construction than oxygen valve
– Does not have to stand high pressure
• Opened only about 1 ½ turns
– Can be turned off quickly in case of fire
• Safety fuse plugs also provided
5 - 23
Capacity of Cylinder
• Full cylinder of acetylene has pressure of about
225 p.s.i.
• Two sizes used for welding and cutting
– Large size contains 300 cubic feet of acetylene and
weighs about 232 pounds when full
– Small size contains 100 cubic feet and weighs 91
pounds when full
• Two special sizes (10 cubic ft. and 40 cubic ft.)
• Not all acetylene in cylinder can be used
5 - 24
Safety Precautions
Remember acetylene will burn.
It will form explosive mixture with air.
• Do not leave acetylene cylinders on their sides
– Store valve end up
• Store cylinders in well-protected, ventilated,
dry location
– Away from highly combustible material or sources
of heat
– Keep valve cap on when not in use
5 - 25
Safety Precautions
• Use warm (not boiling) water on the valve if it
becomes clogged with ice (Never a flame!)
– Fusible safety plugs melt at boiling point of water
• Handle acetylene cylinders carefully
– Damages could cause leakage
• Advise supplier immediately if acetylene leaks
around valve spindle when valve opened
– Close spindle and move cylinder to outside area
• Never tamper with fuse plugs
5 - 26
Propane Gas
• Hydrocarbon present in petroleum and natural gas
• Used primarily for oxyfuel heating, cutting,
soldering and brazing
• Sold and transported in steel cylinders containing
from 20 to 100 pounds of liquefied gas
– Also supplied by tank car and bulk delivery
• Oxypropane flame temp less than oxyacetylene
– Takes longer to bring steel to melting point
– Used for soldering and alloy brazing
5 - 27
Mapp® Gas
• Liquefied acetylene compound
– Fuel gas for oxyfuel heating and cutting
• Strong smell (aid in discovering leaks)
• Mixed with oxygen, flame is 5,301ºF
• Heating and cutting somewhat slower
– Lower temperature
• Overall expenses lower due to reduced
handling costs and lower gas costs
5 - 28
Mapp® Gas
• Use as fuel gas for heating and cutting is
growing
• Distributed in bulk or steel-cylinders
– Shutoff valve similar to ones on acetylene cylinder
• Liquefied and stabilized so can be used at
pressures as high as 375 p.s.I. at 170ºF
• Explosive limits lower than acetylene
• Can be stored in free state and at high pressures
5 - 29
Safety in Handling Mapp® Gas
• Forms an explosive mixture with air
• Same general precautions used with handling
acetylene cylinders should be observed
• Safest of industrial fuels
– Explosive limits of Mapp® gas vapor in air and
oxygen much narrower than acetylene, same as
propane and natural gas
– Can smell it at concentrations as low as 0.01
percent
5 - 30
Manifold Distribution
• Supplies both oxygen and fuel gas to number
of work stations
• Acetylene manifolds must be equipped with
flash arrester to prevent flashback through
manifold into cylinders
• Each cylinder connected to manifold by means
of an individual pigtail flash arrester and
backcheck valves
5 - 31
A typical station
outlet for both
oxygen and
acetylene
pipelines from
the source.
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5 - 32
Manifold Regulator
• Reduces pressure to 50 or 75 pounds in line
that goes to station outlets in shop
– Full pressure of 2,000 p.s.i. in
oxygen manifold pipes
• Work station also equipped
with acetylene and oxygen
regulator for further pressure
control
Thermadyne Industries, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 - 33
Acetylene Generators
• Two general types
– Calcium carbide dropped into water
– Water allowed to drip on carbide
• Carbide-to-water generators used to produce
acetylene gas for welding and cutting
– Water absorbs heat given off by chemical reaction
– Gas purified by bubbling through water
5 - 34
Acetylene Generator
Rexarc
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5 - 35
Acetylene Generators
• Two classes of generators
– Low pressure (less than 1 p.s.i.)
– Medium pressure (1 to 15 p.s.i.)
• Generating capacities range from
– 30 cu feet of acetylene per hour for small portable
– 6,000 cu feet acetylene per hour for mediumpressure generator
5 - 36
Acetylene Generator
• Further classified as
stationary or portable
Example of portable
acetylene generator
Rexarc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 - 37
Protective Equipment
• Functions may be combined in one device or
provided by separate devices
– Located in main supply line, head of each branch
line in manifold system or at each location where
fuel gas withdrawn
• Backflow protection should be provided to
prevent fuel gases from flowing into oxygen
system
• Flashback protection should be provided to
prevent flame from passing into fuel-gas
system
5 - 38
Oxyhydrogen Welding (OHW)
• Form of gas welding once used extensively
– Today have limited use
• Flame produced by burning two volumes of
hydrogen with one volume of oxygen
– Temperature of ~ 4,100ºF
– Almost invisible
• Similar equipment to that used for
oxyacetylene welding
– Need regulator specifically designed for hydrogen
5 - 39
Oxyhydrogen Welding (OHW)
• Relatively low flame temperature
– Used principally in welding metals with low
melting points
– Used extensively in welding of lead
• Thicknesses of lead up to 1/4 or 3/8 inch
• No deposit of carbon
5 - 40
Flash Arrestors
• Generally make of sintered metal alloy
• Prevents flame from moving upstream of
arrestor
• Does not protect torch or tip
– Neither does check valve
Best way to assure that accidents don’t happen
is to follow all safety operating procedures.
5 - 41
Internal View of a Check Valve
Prevents the Reverse
Flow of Gases
Thermadyne Industries, Inc./Victor Equipment Company
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 - 42
Internal View of a Flashback
Arrestor
Thermadyne Industries, Inc./Victor Equipment Company
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 - 43