oxyfuel & gasess

8/9/2019 oxyfuel & Gasess

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Oxyfuel Cutting

Definition: To sever or remove metal through a high-

temperature chemical reaction between oxygen and thebase metal.

Abbreviation: OFC - OxyFuel Cutting

With special equipment, OFC process can be used to cutplates up to 7 foot thick. OFC can be used to cutstraight lines or curves because the oxygen jet provides

a 360 degree cutting edge. Study torch terminology and function shown by figures

14.1, 14.2 and 14.7.


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Kerf and Drag

Kerf - The width of the oxyfuel cut. Factors influencing

the kerf include the type of tip used, the size of theoxygen port, fuel and oxygen flow rates and travelspeed. (Metal thickness is an indirect factor.) The width

of the cut is very important to maintain dimensionaltolerance.

Drag - When the travel speed is increased to the point

that the oxygen stream lags at the bottom of the cut,the amount of lag is referred to as “Drag”. See figure14.3. Some drag is generally acceptable, but excessive

drag will result in a poor quality cut.


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Kerf and Drag (cont.)


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How it Works The oxyfuel cutting process relies upon 3 different, high

temperature, chemical reactions between the oxygenand iron. Each reaction produces heat.

The preheat flame is used to warm a localized spot tothe ignition temperature (also called “kindling point”).This temperature is 1600F for iron.

Upon reaching the ignition temperature, a stream ofhigh-purity oxygen initiates the chemical reactions and

the cutting action. At this point the process can be self-sustaining (the preheat flame is not required).

The oxygen should be a minimum of 99.5% pure for

optimum cutting.


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Fuel Gases Functions of the fuel gas:

Raise steel temperature to the ignition point. Add heat to the steel to aid cutting reaction.

Shield the oxygen cutting stream from the air.

Help remove rust, mill-scale and other contaminationfrom the steel surface.

Several fuel gases are available. Each provides unique

properties, advantages and disadvantages. Study figure 14.4 - Types of oxyfuel flames. A neutral

flame is generally used for cutting.


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Factors to Consider when

Selecting a Fuel Gas

Time required for preheat.

Will the gas impair cutting speed?

Cost and availability in the desired package.

Cost of preheat oxygen needed to burn the gaseffectively.

Ability to use the fuel for other operations (welding,heating, brazing).

Inherent safety factors (stability, etc.).

Available equipment: Torches and Tips used must

be appropriate for the fuel selected.


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Fuel Gas Choices

Acetylene (C2H2) - Widely used, good availability and

user familiarity, high flame temperature and heat-transfer characteristics. Acetylene is unstable atpressures above 15 psi gage pressure and has a wide

explosive limits range (2.5 to 80%). Methylacetylene-Propadiene Stabilized (MPS, C3H4) - A

mixture of several fuel gases with operating

characteristics similar to Acetylene. However it requiresabout double the oxygen volume to achieve a neutralflame (oxygen costs will be higher). Higher regulator

pressures may be used.


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Fuel Gas Choices (cont.) Natural Gas (mostly methane, CH4) - Produces a lower

temperature, less intense flame than acetylene. Thusmore gas must be used to achieve similar results. Atwo-piece tip must be used. Natural gas is delivered vialow-pressure pipelines.

Propane (C8H8) - Highest total heat value, but alsohighest oxygen usage. Available in liquid form andeasily transported to remote work-sites. Narrow

explosive limits range (2.3 to 9.5%) and can be used athigh (150 psi) line pressure.

Propylene (C3H6) - Sold under a variety of brand names

(MAPP gas, etc.). Similar to MPS.


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Advantages of OFC

Faster than mechanical methods.

Curved shapes and thick sections can be cut.

Low cost equipment when compared to

mechanical tools.Portability of process.

Can make abrupt changes in cut direction.Large pieces can be cut in place.

Economical method for beveling weld joints.


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Disadvantages of OFC

Poor dimensional tolerances when compared to

machine tools.

Limited to readily oxidized metals (steel and castiron).

Safety hazards associated with flames and hot slag.

Ventilation and fume control required.

Hardenable steels may be damaged by OFC.Process must be modified for alloy steels and cast

irons.


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Equipment

Manual Equipment - Typically used for operations

that do not require a high degree of accuracy or cutquality. Often used for maintenance chores, cuttingscrap metal, at remote locations and on large

cumbersome pieces.

Mechanized Equipment - Used for accurate, highquality work and large volume jobs. Mostfabrication shops have one or more mechanizedOFC cutting tools (line burner, pipe beveler, multi-axis cutting table).


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Manual Equipment

Torches - Premixing or Tip mixing (fig. 14.1 & 14.2)

Cutting Tips - Use the right type and size for the fuelgas used and steel thickness. (fig. 14.5 & 14.6)

Hoses for oxygen and fuel gas. (see p. 363)

Regulators for oxygen and fuel gas. (see p. 364)

Striker, tip cleaner, wrench, clamp.

Personal protection equipment:tinted goggles shade 3 to 5

gloves and leather, wool or cotton clothing).

Steel toed boots.


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Mechanized Equipment

In addition to the equipment used for manualcutting, the following are often found onmechanized cutting apparatus:

A machine to move the torch: usually motorized,sometimes hand-cranked. (see figures 14.8 - 14.14)

Special torch (fig. 14.7) and torch adjustments

A cutting table to support work and catch slag. Automatic torch igniter.

Cutting machines can be portable or stationary.


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Operation -

Follow Manufacturer’s Instructions!

Regulators - NEVER grease or oil a regulator. Keep themclean. Do not hang anything (ex.: oil bottle) from aregulator. Operate at the pressure recommended for the tip.

Hoses - Use the proper size and type of hose for the cuttingtorch and application. Keep hoses in good shape. Do not

“fix” with tape or other unapproved method. Flashback - Burning of the flame in or behind the torch

mixing chamber. Often caused by overheating the tip or

failure to purge the hoses prior to lighting torch. Seriouscondition!

Backfire - The momentary recession of the flame into thetorch tip. Clean the tip and torch. If condition continues

have torch serviced. Fairly common condition.


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Operation (cont. 1) Starting the Torch - Turn on the fuel gas slightly, ignite with

a spark lighter (never use matches or a cigarette lighter),adjust the fuel and oxygen to achieve a neutral flame.

Know the difference between a neutral, carburizing andoxidizing flame. Read about flame adjustment, study fig.14.4.

Start at the edge when possible. Open oxygen stream whenthe steel under the preheat flame is reddish yellow.

Flame cones should be held 1/16 to 1/8 inch from thesurface. When piercing (not at an edge) remember to lift thetorch about 3/8 inch away from surface and slowly open the

cutting oxygen valve. This helps prevent tip clogging.


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Operation (cont. 2)Maintain tip-to-work distance. Move at a speed that

produces a steady spark stream. Sheet metal should be cut with the torch turned to a

sharp leading angle.

Thin plate (3/16 to 7/16”) should be cut with the torchat a slight leading angle.

Plate 1/2” and over should be cut with the torch

perpendicular to the surface. Use angle iron or other straight-edge for straight cuts

and bevels. Use upward progression for vertical cuts.

Turning off the Torch - Remember O-A-O.


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Plate Beveling and Gouging

The preheat flame is a more critical factor because the

the bevel angle increases the preheat efficiencydecreases. Special tips are available for steep angle,large thickness applications.

Multiple torches may be used to achieve the desired joint bevel. Study figures 14.14 to 14.17.

Oxy-Fuel Gouging is often used to “back-gouge” a weld

joint in preparation for a back weld. Gouging is alsoused to remove defective sections of weld. Special tipsare used.


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Quality Indicators:

Proper Cut Angle

Flatness of the cut

Sharpness of the cut at the preheat edge

Dimensional Tolerances of the cut shape

Tolerances of 1/32 to 1/16 are achievable

Adherence of slag

Cut surface uniformity


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Factors that Affect Cut Quality

Type of Steel

Thickness of SteelQuality of Steel

Condition of Surface

Intensity of preheat flame

Cutting tip size

Purity of Oxygen and flow rate

Cleanliness and condition of tip

Cutting speed and movement uniformity

Study figures in book and on bulletin board.


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Materials

Low-carbon steels are readily cut.

Low-Alloy steels are generally easy to cut but mayharden and crack adjacent to the cut.

Preheating (400 - 600F) is often used when cutting

low-alloy steels.

Cast Iron - High carbon cast irons do not cut well usingtechniques used for low-carbon steel.

Use an oscillating motion (fig. 14.24), larger cuttingtip, a carburizing flame and higher flow rates.

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