06b oxyacetylene cutting

Cutting & Other Edge Preparation Processes
1. Mechanical Cutting
i. Sawing
ii. Milling, Planing and Shaping
iii. Shearing
iv. Water Jet Cutting
2. Thermal Cutting
i. Oxyfuel Gas
ii. Plasma
iii. Laser
iv. Thermal Lancing
v. Air / Arc Gouging
OxyFuel Cutting
1. Introduction
THE OXYFUEL GAS cutting process employs a torch with a tip (nozzle). The
functions of the torch are to produce preheat flames by mixing the gas and
the oxygen in the correct proportions and to supply a concentrated stream
of high purity oxygen to the reaction zone. The oxygen oxidizes the hot metal
and also blows the molten reaction products from the joint.
3. Modes of Operation
The torch may be moved manually or by a mechanized carriage.
The accuracy of a manual operation depends largely on the skill of the
operator.
Mechanized operation generally improves the accuracy and speed of the
cut and the finish of the cut surfaces.
OxyFuel Cutting
KERF and DRAG in OxyFuel Cutting
OxyFuel Cutting
KERF and DRAG in OxyFuel Cutting
KERF: When a piece is cut by an OC process, a narrow width of metal is
progressively removed. The width of the cut is called a kerf

DRAG: When the speed of the cutting torch is adjusted so that the oxygen
stream enters the top of the kerf and exits from the bottom of the kerf along
the axis of the tip, the cut will have zero drag.
If the speed of cutting is increased, or if the oxygen flow is decreased, the
oxygen available in the lower regions of the cut decreases. With less oxygen
available, the oxidation reaction rate decreases, and also the oxygen jet has
less energy to carry the reaction products out of the kerf. As a result, the
most distant part of the cutting stream lags behind the portion nearest to
the torch tip. The length of this lag, measured along the line of cut, is
referred to as the drag.
OxyFuel Cutting
The Chemistry of OxyFuel Cutting
THE PROCESS OF oxygen cutting is based on the ability of high- purity oxygen
to combine rapidly with iron when it is heated to its ignition temperature,
above 1600°F (870°C). The iron is rapidly oxidized by the high-purity oxygen
and heat is liberated by several reactions.
The balanced chemical equations for these reactions are the following:
(1) Fe + 0 Fe0 + heat (267 kJ), first reaction
(2) 3Fe + 202 Fe304 + heat (1120 kJ) second reaction
(3) 2Fe + 1.502 Fe203 + heat (825 kJ), third reaction
The tremendous heat release of the second reaction predominates over that of the first
reaction, which is supplementary in most cutting applications. The third reaction occurs
to some extent in heavier cutting applications.
OxyFuel Cutting
Preheating Fuels
Same fuel gases as used in OxyFuel Gas Welding
•
Acetylene
•
Propylene

•
Propane
•
Methyl Acetylene Propadiene (MPS)
•
Natural Gas
OXYGEN
OXYGEN USED FOR cutting operations should have a purity of 99.5 percent or higher.
Lower purity reduces the efficiency of the cutting operation.
A one percent decrease in oxygen purity to 98.5 percent will result in a decrease in
cutting speed of approximately 15 percent, and an increase of about 25 percent in
consumption of cutting oxygen. The quality of the cut will be impaired, and the
amount and tenacity of the adhering slag will increase. With oxygen purities below
95 percent, the familiar cutting action disappears, and it becomes a melt-and-wash
action that is usually unacceptable.
OxyFuel Cutting
Advantages
(1) Steels can generally be cut faster by OFC than by mechanical chip removal processes.
(2) Section shapes and thicknesses that are difficult to produce by mechanical means can be severed
economically by OFC.
(3) Basic manual OFC equipment costs are low compared to machine tools.
(4) Manual OFC equipment is very portable and can be used in the field.
(5) Cutting direction can be changed rapidly on a small radius during operation.
(6) Large plates can be cut rapidly in place by moving the OFC torch rather than the plate.
(7) OFC is an economical method of plate edge preparation for bevel and groove weld joint designs.
Disadvantages
(1) Dimensional tolerances are significantly poorer than machine tool capabilities.
(2) The process is essentially limited commercially to cutting steels and cast iron, although other readily
oxidized metals, such as titanium, can be cut.
(3) The preheat flames and expelled red hot slag present fire and burn hazards to plant and personnel.

(4) Fuel combustion and oxidation of the metal require proper fume control and adequate ventilation.
(5) Hardenable steels may require preheat, post heat, or both to control their metallurgical structures
and mechanical properties adjacent to the cut edges.
(6) Special process modifications are needed for OFC of high alloy steels and cast irons.
OxyFuel Cutting
Equipment
1. Manual
2. Mechanized

Manual:
(1) One or more cutting torches suitable for the preheat fuel gas to be used and the range
of material thicknesses to be cut
(2) Required torch cutting tips to cut a range of material thicknesses
(3) Standard Oxy-Acetylene Welding Equipment
Torches:
The functions of an OFC torch are as follows:
(1) To control the flow and mixture of fuel gas and preheat oxygen
(2) To control the flow of cutting oxygen
(3) To discharge the gases through the cutting tip at the proper velocities and volumetric
flow rates for preheating and cutting
These functions are partially controlled by the operator, by the pressures of incoming
gases, and by the design of the torch and cutting tips.
OxyFuel Cutting
Equipment
Manual Cutting Tips:
Cutting tips are precision machined copper-alloy parts of various designs and
sizes. They are held in the cutting torch by a tip nut.
All oxygen cutting tips have preheat flame ports, usually arranged in a circle
around a central cutting oxygen orifice. The preheat flame ports and the
cutting oxygen orifice are sized for the thickness range of metal that the tip

is designed to cut. Cutting tips are designated as standard or high speed.
OxyFuel Cutting
Cutting Procedure
Manual Cutting:
SEVERAL METHODS CAN be used to start a cut on an edge.
1. The most common method is to place the preheat flames halfway over
the edge, holding the end of the flame cones 1/16 to 1/8 in. (1.5 to 3
mm) above the surface of the material to be cut.
2. The tip axis should be aligned with the plate edge.
3. When the top corner reaches a reddish yellow color, the cutting oxygen
valve is opened and the cutting process starts.
4. Torch movement is started after the cutting action reaches the far side
of the edge
It is often necessary to start a cut at some point other than on the edg"e of a viece of
metal. This techniaue is known as piercing.