Unit 24

Cutting with Oxyfuels and Other Gases

Objectives

• Students will:– Write the names and characteristics of common fuels used

for cutting. – State and apply recommended safety practices for using

oxyfuels. – Select appropriate pressures for using oxygen and

common fuel gases for cutting. – Cut steel with oxyfuels– Pierce steel with oxyfuels

Important Terms

• Oxyfuel: combination of pure oxygen and combustible fuel gas to produce a flame

• Oxyfuel cutting: process in which steel is heated until it burns and is then removed to leave a kerf

• Kerf: a thin cut• Slag: a by-product that is formed during

the combination of oxygen with hot steel• Brass: mixture of copper and zinc

Important Terms (continued)

• Base metal: main piece of metal• Backfire: a loud snap that blows out the

flame• Flashback: gases burning back inside the

torch that causes a hissing noise• Slag box: metal container of water or sand

placed to catch hot slag and metal• Plasma: a group of charged particles that

conduct electrons across a gap

Oxyfuel Processes

• No one gas is the safest or the best for all jobs

• Oxyfuels are used for welding, brazing, cutting, and heating metals

• Oxygen and fuel gases are stored under pressure in tanks or cylinders, carried through valves and hoses, then mixed inside of torch assemblies before being burned as they are released through the tip

Heat

• Oxyfuels burn between 5,000°F and 6,000°F

• Temperature is raised enough to soften metal for bending or shaping– NO melting takes place– Can be used to make a sharp bend in a

specific piece– Also used to reduce stress from welding

Brazing

• Process of bonding with metals and alloys that melt at or above 840°F

• The base metal is heated until the brazing alloy melts, flows, and bonds to it

• The two metals do not mix

Fusion Welding

• Joining of metal by melting it together• The torch is used to heat two pieces until

the metal from each runs together to form a joint

• Joints are as strong as the base metal itself

Oxygen

• Not a fuel—will not burn on its own• Combines with other substances to cause

them to burn at a higher temperature• Must be 99.5 percent pure to support the

combustion of iron

Acetylene

• Fuel gas most suitable for welding• Produces a cleaner weld than most other

fuel gases• Produces a more controllable flame• Unstable and very hazardous• Most stable at pressures below 15 psi

Other Gases

• Propane and natural gas– Used for general heating and cutting, but not

for welding• MAPP® gas

– Mixture of methylacetylene and propadiene gases

– Qualities similar to acetylene, but more stable– Suitable for brazing, cutting, and heating

Oxyfuel Cutting

• Obtain a neutral flame• Mark the line of cut• Hold the flame over the corner/edge of the

metal at a slight angle• Once the metal turns red, press the

oxygen lever and move the torch across the metal at a steady rate

• Keep the flame about 1/8 inch from the metal

Improving Cuts

• Excessive preheat: if the flame is too hot, or the torch moves too slowly, the surface melts before the metal is heated through

• Clearance: the distance from the torch to the metal is correct when the tips of the primary flames are almost touching the metal surface

Improving Cuts (continued)

• Cutting speed– Too fast will result in an incomplete cut with

rough edges– Too slow results in a melted top edge and

gouging• Gas pressure

– Too high: dish shape to the kerf– Too low: cut is not complete at the bottom

Piercing

• Pierce: to make a hole• Hold the torch at a normal preheat

distance above the mark• When heated to red color, raise the torch

one-half inch or more and slowly press oxygen lever

• Move the tip sideways in a circular motion until a hole breaks through

Cutting Pipe

• Small pipe– Pierce a hole in the top– Cut a kerf to the left and one to the right– Rotate the pipe and repeat on the underside

• Large pipe – Hold the torch at a right angle to the pipe– Move the torch around the pipe to make the

cut

Plasma Arc Cutting

• Used for cutting nonferrous metals• Uses an electric arc and argon (an inert

gas)• Cuts metal faster than conventional oxyfuel

torches• Not as efficient in cutting thicker metals