06a oxyacetylene welding

oxyacetylene welding

1 Introduction

Oxy-fuel gas welding includes any welding operation that uses combustion with oxygen as a heating medium The process involves melting the base metal and usually a filler metal, using a flame produced at the tip of the welding torch

2. Advantages and Limitations

• Control over heat input and temperature

• Ideally suited for repair welding , welding thin sheets, tubes and small dia pipes.

• Thick sections weld, but are not economical

• Low cost equipment, portable and versatile enough to be used for a variety of related operations such as bending, straightening, preheating,

post-heating, surfacing, brazing and braze welding

3 Fuel Gases

Fuel gases when burned with oxygen, must have the following properties,

• High flame temperature

• High rate of flame propagation

• Adequate Heat Content

• Minimum chemical reaction of flame with base and filler metals

Acetylene most closely meets the above mentioned requirements

Some of the commercially available fuel gases are,

• Propylene

• Natural Gas

• Proprietary gases based on above mentioned gases

3 Fuel Gases

4 Equipment for Oxy-Fuel Welding

Basic equipment consists of

• Cylinders for fuel gas and oxygen cylinders

• Cylinders are fitted with Regulators and Gauges for Pressure and flow

• Hoses for conveying gases to the torch

• A torch (with mixer) and a tip combination

4 Equipment for Oxy-Fuel Welding

The Acetylene Cylinders

BECAUSE FREE ACETYLENE, under certain pressure and temperature may dissociate explosively into its hydrogen and carbon constituents, cylinders

to be filled with acetylene are initially packed with a porous filler Acetone, a solvent capable of absorbing 25 times its own volume of acetylene per atmosphere of pressure, is added to the filler

By so dissolving the acetylene and dividing the interior of the cylinder into small, partly separated cells within the porous filler, a safe acetylene-filled container is produced Acetylene cylinders are available in sizes containing from 10 to 420 ft3 (0.28 to 12 m3) of the gas The cylinders are equipped with fusible safety plugs made of a metal that melts at about 212°F (100°C) This allows the gas to escape if the cylinder should be subjected to excessive heat, resulting in a relatively controlled burn rather than rupturing the cylinder.

5 Oxy-Fuel Welding Torch

A typical torch consists of

• Torch Handle

• Flash Back Arrestor

6 The Regulators

A REGULATOR CAN be described as a mechanical device for maintaining the delivery of a gas at some substantially constant reduced pressure even though the pressure at the source may change

Regulators are adjustable pressure reducers, designed to operate automatically after an initial setting There are basically two categories

Single Stage Regulator

The output pressure of the single-stage type exhibits a characteristic known as rise or drift This is a slight rise or drop in the

delivery pressure that occurs as the cylinder pressure is depleted This characteristic is usually detrimental only when a large quantity of the gas is withdrawn from a high-pressure cylinder at a single usage Periodic readjustment of regulator pressure will correct any detrimental effects

Two Stage Regulators

Two-stage regulators are essentially two single-stage regulators operating in series within one housing They provide constant delivery pressure as cylinder pressure is depleted.basic principle

7 The Oxy-Acetylene Flame

Theoritically the complete combustion of acetylene is represented by the chemical equation

The combustion takes place in two stages

When the gases issuing from the torch tip are in the one-to-one ratio indicated in equation, the reaction produces the typical brilliant blue inner cone This relatively small flame creates the combustion intensity needed for welding steel The flame is termed neutral because there is no excess carbon or oxygen to carburize or oxidize the metal The end products are actually in a reducing status, a benefit when welding steel

7. The Oxy-Acetylene Flame - Temperatures

8 The Oxy-Acetylene Flame – Three Basic Types

i. Neutral (or balanced): A 1:1 ratio of oxygen and Acetylene It obtains additional oxygen from the air and provides complete combustion It

is generally preferred for welding The neutral flame has a clear well defined luminous cone, indicating that the combustion is complete Most welding procedures use a neutral flame.

ii. Carburizing (Excess Acetylene): The inner cone has a feathery edge This white feather is called the acetylene feather If the feather is

twice as long as the cone the flame is called 2X flame The carburizing flame may add carbon to the weld

iii. Oxidizing (Excess Oxygen): It has a shorter envelope and a smaller pointed white cone The reduction in the length of the inne white cone

is a measure of excess oxygen The flame tends to oxidise the weld metal and is used to weld only specific metals

8. Applications