© 2012 delmar, cengage learning chapter 31 oxyfuel gases and filler metals

© 2012 Delmar, Cengage Learning

Chapter 31

Oxyfuel Gases and Filler Metals


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


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


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


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


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


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


FIGURE 31-2 Chemical formulas for two hydrocarbons used as fuel gases. © Cengage Learning 2012


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


FIGURE 31-4 Primary flame reaction (with acetylene as the fuel gas). © Cengage Learning 2012


FIGURE 31-5 Secondary flame reaction.© Cengage Learning 2012


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


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


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


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


FIGURE 31-14 The heat absorbed by the liquid propane causes it to change to a gas. © Cengage Learning 2012


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


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


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


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


FIGURE 31-19 Explosive limits of MAPP gas in air. MAPP Products


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)


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


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


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


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


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


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


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

© 2012 delmar, cengage learning chapter 31 oxyfuel gases and filler metals

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