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© 2012 Delmar, Cengage Learning
Chapter 31
Oxyfuel Gases and Filler Metals
© 2012 Delmar, Cengage Learning
Objectives
• Explain the chemical reaction that takes place in any oxyfuel flame
• List the major advantages and disadvantages of the different fuel gases
• Demonstrate an ability to choose correct filler metals
• Explain what conditions affect the selection of filler metal
© 2012 Delmar, Cengage Learning
Introduction
• Oxyfuel processes– Consist of a number of separate processes
• Burn fuel gas with oxygen
– Oxyfuel flame was used for fusion welding as early as the first half of the 1800s
– Early use of oxygen with hydrogen or acetylene gas often resulted in flashbacks
– Welding was dangerous until the development of the torch mixing chamber
• Gave a more uniform flame
© 2012 Delmar, Cengage Learning
Introduction (cont'd.)
– Early 1900s: oxyacetylene flame became more popular
– Today: oxyacetylene flame is seldom used on metal thicker than 1/16 inch
• Other process are faster, cleaner, and cause less distortion
© 2012 Delmar, Cengage Learning
Uses of the Oxyfuel Flame
• Increased use of oxyfuel flame – Cutting ferrous metals
• Cutting torch– Used by hand or machine
– Rapidly cuts out steel parts
• Large number of manufactured items are touched in some way by the oxyfuel flame– Expanding role
© 2012 Delmar, Cengage Learning
Characteristics of the Fuel-Gas Flame
• Flame condition and purity of gas – Affect flame temperature
• Optical pyrometer– Gives an accurate temperature reading
– Where the temperature is measured makes a difference
• Differences in heat values may also be misleading– Depending on how they are obtained
© 2012 Delmar, Cengage Learning
Fuel Gases
• Most fuel gases used for welding are hydrocarbons– Atoms are bound together tightly to form molecules
– Each molecule of a specific gas has the same type, number, and arrangement of atoms
• Acetylene: two hydrogen and two carbon atoms• Propane: eight hydrogen and three carbon atoms
© 2012 Delmar, Cengage Learning
FIGURE 31-2 Chemical formulas for two hydrocarbons used as fuel gases. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Fuel Gases (cont'd.)
• Number of oxygen atoms to completely combust the fuel gas varies– Combustion of acetylene is divided into two
separate chemical reactions• Primary combustion• Secondary combustion
– Final products of all clean-burning hydrocarbon flames are the same
© 2012 Delmar, Cengage Learning
FIGURE 31-4 Primary flame reaction (with acetylene as the fuel gas). © Cengage Learning 2012
© 2012 Delmar, Cengage Learning
FIGURE 31-5 Secondary flame reaction.© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Flame Rate of Burning
• Combustion rate – Speed at which a flame burns
– Determined by heat energy required to break the bonds
– Higher combustion rate: more prone the mixture is to backfire or flashback
© 2012 Delmar, Cengage Learning
Acetylene (C2H2)
• Characteristics– Produced by mixing calcium carbide with water
– Colorless, lighter than air, and has a strong garlic smell
– Used inside acetylene cylinders to absorb and stabilize the gas
– Withdrawal rate of gas should not exceed one-seventh of the total cylinder capacity per hour
© 2012 Delmar, Cengage Learning
Heat and Temperature
• Neutral oxyacetylene flame – Burns at about 5589 degrees Fahrenheit
• Maximum temperature of a strongly oxidizing flame is about 5615 degrees Fahrenheit
• Flame burns in two parts– Inner cone and outer envelope
• High temperature of oxyacetylene flame: Concentrated around the inner cone
– More heat is produced in the secondary flame but the temperature is much lower
© 2012 Delmar, Cengage Learning
Liquefied Fuel Gases
• Obtained in individual cylinders or bulk tanks– Pressure in cylinder is not an indication of the level
of gas in the tank
• Results of high withdrawal rates – Drop in pressure
– Lowering of cylinder temperature
– Possible freezing of the regulator
© 2012 Delmar, Cengage Learning
FIGURE 31-14 The heat absorbed by the liquid propane causes it to change to a gas. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Methylacetylene-Propadiene (MPS)
• Characteristics– Many in use today as fuel gases
• Cutting• Heating• Brazing• Metallizing• Welding
– Mixture of two or more gases
– All manufacturers provide MPS gases as liquefied gases in pressurized cylinders
© 2012 Delmar, Cengage Learning
Production
• Approximately twenty-six MPS gases are sold– Mixed by a local supplier as cylinders are filled
– Premixed to the supplier
• Cylinders should be moved enough to remix before use– Piccolo tube improves the mixing of the gas
© 2012 Delmar, Cengage Learning
Temperature and Heat
• MPS gases– Neutral oxyfuel flame
• About 5301 degrees Fahrenheit
– Heat of primary flame • About 570 Btu/ft3
– Much better than acetylene for heating, brazing, and some types of cutting
• Slower burn rate makes welding difficult
© 2012 Delmar, Cengage Learning
MAPP
• Characteristics– Trade name for stabilized liquefied mixture of
methylacetylene and propadiene gases
– Oxy MAPP combusts with a high-heat, high-temperature flame
– Gases mixed to produce MAPP have the same atomic composition
– Oxy MAPP has a neutral flame of 5301 degrees Fahrenheit
– MAPP safety advantage: odor
© 2012 Delmar, Cengage Learning
FIGURE 31-17 MAPP gas molecules.
© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
FIGURE 31-19 Explosive limits of MAPP gas in air. MAPP Products
© 2012 Delmar, Cengage Learning
Propane and Natural Gas
• Characteristics– Limited use in welding industry
– Often used for heating the shop
– Both obtained from the petroleum industry
– Chemically, propane is C3H8; natural gas is mostly methane (CH4) and ethane (C2H6)
© 2012 Delmar, Cengage Learning
Hydrogen
• Characteristics– Oxyhydrogen produces only a primary combustion
flame• Flame is almost colorless
– Not widely used in welding because of cost
– Fastest burning velocity of the fuel gases
– Much lighter than air
– Low flame temperature restricts oxyhydrogen flame to cutting
© 2012 Delmar, Cengage Learning
Filler Metals
• Divided into groups– Welding: prefix letter R
– Brazing: prefix letter B
– Buildup, wear resistance surfacing, or both
• Tubular welding rods – Designated with an RWC prefix
• Some filler metals are classified both as a braze welding rod and a brazing rod
© 2012 Delmar, Cengage Learning
Ferrous Metals
• Characteristics– Mainly iron
– Other elements are added to change:• Strength• Corrosion resistance • Weldability• Other physical properties
– Specifications and classes have minimum and maximum limits for alloys that are added
© 2012 Delmar, Cengage Learning
Ferrous Metals (cont'd.)
• Physical changes are most often affected by changes in the percentage of alloys of:– Carbon
– Silicon
– Manganese
– Chromium
– Vanadium
– Nickel
– Molybdenum
© 2012 Delmar, Cengage Learning
Mild Steel
• Ferrous metal filler rods – Generally classified by the AWS as:
• Mild steel• Low alloy steel• Cast iron
– Mild steel and low alloy steel • Most frequently gas welded• Easily welded without flux
– Cast iron and stainless steels • Require fluxes and special techniques
© 2012 Delmar, Cengage Learning
Cast Iron
• Characteristics– Cast iron filler rods for gas welding are small,
round, or square iron castings
– CI stands for cast iron
– High-temperature, borax-based flux must be used
– Class RCI is lower-strength filler metal
– RCI-A welding rods have a higher tensile strength
© 2012 Delmar, Cengage Learning
Summary
• Oxyacetylene welding process – Most desirable in many cases
– Wide variety of fuel gases offers the welder unique challenges
– Acetylene process is costly• Other cheaper fuels do not have all of the
characteristics of acetylene
– Proper selection of an oxyfuel filler metal is critical