1.2 Oxy Gas Welding_PV IST

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IIW Specialist Syllabus

Welding Processes and Equipment

1.2 Oxyfuel Gas Welding

Trainer: Eng. Pedro Vilaa

June 2011

The basis of this presentation was developed

by


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1.2 Oxy Gas Welding 1

OXYFUEL GAS WELDING (OFW) is a

manual process in which the metal

surfaces to be joined are melted

progressively by heat from a gas flame

and are caused to flow together and

solidify without the application of pressure

to the parts

OFW can be applied with or without filler

metal

The most important source of heat for

OFW is the oxyacetylene welding (OAW)


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Oxygen and fuel are stored in

separate cylinders

The gas regulator attached to each

cylinder, whether fuel gas or oxygen,

controls the pressure at which the gas

flows to the welding torch

The mixed gases then pass through

the welding tip and produce the flame

at the exit end of the torch tip


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The equipment is versatile, low-cost, self-sufficient, and usually

portable

It can be used for preheating, postheating, welding, braze welding, and

torch brazing, and it is readily converted into oxygen cutting

The process can be adapted to short production runs, field work,

repairs, and alterations

The oxy-acetylene welding process (OAW) is by far the most important,

and probably the most versatile, of the oxy-fuel gas welding (OFW)

processes but is tending to be superceded by other welding processes,

e.g., the Tungsten Inert Gas (TIG) welding process


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Table: Approximate Maximum Flame Temperatures

deg C

Oxy-acetylene 3,100 to 3,300

Oxy-propane 2,500

Oxy-hydrogen 2,370

Oxy-coal-gas 2,200

Air-acetylene 2,460

Air-coal-gas 1,871

Air-propane 1,750

The gas acetylene (C2H2) is the most important fuel gas employed,

because it has the highest calorific (heat) value. Other hydrocarbon

gases are also used, e.g. Liquid Petroleum Gas (LPG), Propane, etc.


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In practical applications the oxy-acetylene welding process (OAW) is the

only one applied in welding

Higher heat power density

Hotter for all the distances from the center of the flame

More reducing action

Easy to adjust with Oxygen

The oxi-propane flame is more applied in brazing when temperature and

heat power should be lower

Propane and butane are more safe to use fuel gases

The propane, butane and natural gas allow greater autonomy as they canbe easily stored in large volumes.


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Acetylene (C2H2) storing:

When under pressure of 203 kPa and above,

acetylene is unstable, and a slight shock can cause it

to explode, even in the absence of oxygen or air

By dissolving purified and dried acetylene in liquid

acetone, a cylinder such as that shown in Figure can

be used to store about 7.79 m3 of acetylene under a

pressure of 1.7 MPa

The cylinders must be stored in an upright position to

keep the acetone from escaping during use


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Manifolded supplies may be used for both acetylene and oxygen

cylinders, although where the oxygen consumption is high it is more

practical to use an oxygen generator/evaporator plant, the oxygen being

delivered and stored in the more compact liquid form.

In each instance the gas is stored away from the workshop in a special

building designed to relevant safety requirements, the gas is piped from

the store to the supply points in the workshop.

The acetylene is conveyed in steel tubes (copper is not used because

of the danger of producing the explosive compound: copper acetylide).

The line is protected by non-return valves and flash-back arresters at

each outlet. The cylinder is also protected by large-capacity flashback

arresters.


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Oxygen cylinder(Capacity, 220ft3 at 2 000 lb / in2, Weight 145 lb)

Acetylene cylinder(Capacity, 250ft3 at 250 lb / in2, Weight 215 lb)


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OXYFUEL GAS WELDING (OFW) STATION - Equipment


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1. Welding torch: efficient and light, producing a flame of the right shape and

temperature with controls that are easy to adjust allowing for the quick and easy

changing of nozzle tips, thus affording a wide range of blowpipe power

2. Oxygen and acetylene gas regulators: to reduce the high pressure in a gas

storage cylinder to a lower working pressure, and at the same time maintaining

a steady supply, free from pressure fluctuations3. Supplies of oxygen and acetylene gases, which must be safely stored in

cylinders and suitably piped to the welding areas


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4. Protection equipment, such as suitable colour-tinted goggles,

manufactured to the appropriate standard, e.g. BS 679, and

other protective clothing, such as chrome-Ieather gloves,

aprons, etc

Filter glasses for goggles are invariably green, their main

function being to reduce the glare from the flame cone and

molten metal to a level comfortably acceptable to the eyes ofthe welder, while also protecting the eyes from hot sparks

They are produced in different densities or shades, appropriate

to light or heavy glare. Modern filters are photo chromatic, i.e.

they change shade automatically


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Fire lighters

Protection goggles


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5. Supplies of suitable filler rods in convenient diameters and lengths. These are

usually from 1,2mm to 4,8mm in diameter and 1mm in length

6. Certain other equipment, such as flexible, high-pressure rubber hoses,

coloured red for fuel gas and black for oxygen, with properly designed

connections, threaded left-hand for fuel gas and right-hand for oxygen, to

prevent accidental exchange

Also, safety devices, such as flash-back arresters, or hydraulic backpressure

valves. Equipment such as refractory-surfaced work tables, gas economizers,

etc are desirable but cannot be regarded as absolutely essential


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Types of torches/burners architecture

Features of high-pressure with no injector blowpipe


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Procedure for adding a filler rod into the weld pool


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Fluxes:

A flux prevents the oxidation of molten metal. The flux (material) is

fusible and non metallic During welding, flux chemically reacts with the oxides and a slag is

formed that floats to and covers the top of the molten puddle of metal

and thus helps keep out atmospheric oxygen and other gases

Except for lead, zinc, and some precious metals, OFW of nonferrous

metals, cast irons, and stainless steels generally requires a flux

In welding carbon steel, the gas flame shields the weld adequately,

and no flux is required

Adjustment for correct flame atmosphere is important, but the absence

of flux results in one less variable to control


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The maximum temperature of the oxy-acetylene flame is 3,100 to

3,300 C and the centre of this heat concentration is just off the

extreme tip of the white cone. Combustion is recognised as taking

place in two main stages of combustion

T f Fl i OAW


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Types of Flames in OAW

In oxy-acetylene welding the character of the flame is most important. Certain

technical terms must be learned in this connection

When the acetylene and oxygen are in equal proportions the resultant flame is

said to be neutral; when there is an excess of oxygen the flame is said to

be oxidising; and if more acetylene is present than oxygen the flame is

said to be carburising, orreducing

A reducing flame is on that, because of its need for oxygen will reduce surface

oxides, such as iron oxide. A strictly neutral setting is correct, but the slightest

excess of acetylene may keep scaling (oxidation) to a minimum, particularly

when welding stainless steels

For example, non-ferrous alloys and carbon steels may require a reducing

flame, while zinc-bearing materials may need an oxidising flame


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Three types of flame setting: mixture ratio


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FERROUS AND NONFERROUS METALS THAT CAN BE WELDED BY OAW:

(a) - Match base metal ; (b) - No Flux required


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Acetylene: 2 C2H2+5 O2=4 CO2+2 H2O

Propane: C3

H8

+5 O2

=3 CO2

+4 H2

O

Natural Gas: CH4+2 O2=CO2+2 H2O

Combustion with other fuel gases

Lighting a blowpipe


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Lighting a blowpipe

The correct pressures, as recommended for the appropriate nozzle, should

first be set, initially the fuel gas by opening slightly the blowpipe acetylene

valve and regulating it to the correct pressure by the pressure-regulator screw

This procedure is repeated for the oxygen supply, the oxygen valve then

being closed. The fuel gas is turned on, ignited and adjusted so that the flame

just ceases to form soot but is not blown away from the nozzle tip

The oxygen is now turned on at the blowpipe valve and adjusted until the

acetylene feather just disappears, to obtain a neutral flame setting


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Lighting a blowpipe (cont.)

To extinguish the flame, the fuel gas should be turned off first, followed by the

oxygen. In the event of backfires with either design of torch, the fuel gas

should be turned off first to prevent the internal temperatures from being

destructively high and damaging the blow-pipe body


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Use of grooves to facilitate oxyfuel gas

welding of t> 4.8 mm thick plates

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