GasWeldingAndCutting-Rama Sir.pdf

7/28/2019 GasWeldingAndCutting-Rama Sir.pdf

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Module-II of Manufacturing Science-I

Lecture Notes of Chinmay Das 7

2.1 GAS WELDING AND CUTTING1. Principle

In oxy-fuel gas welding (OFW) the heat is obtained from the combustion of a fuel gas such as

acetylene in combination with oxygen. The process is a fusion welding process wherein the joint is

completely melted to obtain the fusion. The heat produced by the combustion of gas is sufficient to melt

any metal and as such is universally applicable. The fuel gas generally used is acetylene because of the high

temperature generated in the flame. This process is called oxy-acetylene welding.

C2H2 + O2 2 CO + H2 + 448 KJ/ mol (18.75 MJ/ m3of acetylene)

The carbon monoxide (CO) and hydrogen produced in the first stage further combine with atmospheric

oxygen and give rise to the outer bluish flame, with the following reaction.

4 CO + 2H2 + 3 O2 4 CO2 + 2H2O + 812 KJ /mol (35.77 MJ/ m3

of acetylene)Though higher amount of heat is produced in the second stage, since it is distributed over a larger area, the

temperature achieved is of the order of 1200 to 20000

C in the flame which may be used for preheating the

metal. The inner white cone temperature is of the order of 31000

C, which is used for melting the joint.

Types of gas flame: There are three types of flame which are discussed here.

Neutral Flame: A certain amount of oxygen is required for complete combustion of fuel gases. When the

oxygen supply varies, the flame appearance obtained would also vary. In neutral flame all the acetylene

present is completely burned and thus allthe available heat in the fuel gas is

released. So this is the most desirable

flame to be used in oxy-acetylene

welding. Due to neutral flame no chemical

change and no oxidation in molten metal

occur.

Figure: 2.1.1: Neutral flame

Carburising or Reducing Flame: When less than theoretically correct amount of oxygen is provided, part

of the combustible matter is left as it is

and it results in a reducing or

carburising flame. This flame has threedistinct layers namely inner white cone,

intermediate reddish zone and outer

blue zone. The length of intermediate

zone which is also called flame feather

is an indication of the excess acetylene

present. Since unburned carbon present

goes into the weld metal pool, the metal

appears to boil. This excess carbon

causes the steel to become extremely hard Figure: 2.1.2: Carburising flame

and brittle. The temperature of this flame is lowest among all the three flames. Since this flame provides a

strong reducing atmosphere in the welding zone, it is useful for those materials which are readily oxidize

like oxygen free copper alloys. It is also used for high carbon steels, cast iron and hard surfacing with high

speed steel and cement carbides.Oxidising Flame: When oxygen is in excess, it is called the oxidising flame which is similar to the neutral

flame with the exception that the inner white

cone is smaller in size having high

temperature of around 33000

C. There is an

excess amount of oxygen present in the flame

which badly oxidizes the weld metal. Because

of burning of metal, the weld pool foams,

sparks and produces loud noise. Figure 2.2.3: Neutral flame


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The presence of excess oxygen in this flame causes an oxide film to form quickly which provides a

protective cover over the base metal pool. It is useful for welding copper base alloys, zinc base alloys, cast

iron, manganese steel etc.

2. Oxy-Acetylene Welding EquipmentAn oxy-acetylene welding outfit consists of supply units of oxygen, acetylene with associated

regulators, welding torch and safety devices.

Gas Cylinders: The oxygen is normally stored in strong cylinders made up of steel (black colour) at apressure ranging from 13.8 MPa to 18.2 MPa. The regulators have right hand screws and a safety cap is

provided on the top of the cylinder to allow any leakage of oxygen. The oxygen cylinder should not be

stored near to any combustible gas cylinder.

Figure 2.2.4: Oxy-acetylene welding outfit

Acetylene is normally available in two forms: acetylene storage cylinder and acetylene generator.

Free acetylene if stored at a pressure more than 200 KPa becomes

unstable and may explode. This instability places special

requirements on the storage of acetylene. A storage cylinder (maroon

or red colour) is filled with a mixture of calcium silicate, a material

that is 92 percent porous.

The cylinder is then filled with acetone, which is the

solvent agent of acetylene gas and which has an absorptive capacity

of up to 35 volumes of acetylene per volume of acetone per

atmosphere of pressure. This enables about 420 volumes of acetylene

to be compressed at 18 bar. Under these conditions, the gas is present

in the form in which it is to be used. Acetylene comes out of the

acetone solution at a slower rate as the pressure in the cylinder is

released. The rate depends on the temperature of the gas.In acetylene generator it is produced by a reaction between

calcium carbide and water which is instantaneous.

CaC2 + 2 H2O C2H2 + Ca (OH2)

Figure 2.2.5: Acetylene cylinder


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This uses small granulated carbide and produces 250 litres of acetylene per kilogram of carbide. The

generator is self contained (i.e. there is no

separate gas holder) and consists of a

water tank surmounted by a carbide

hopper, in the top which is a diaphragm.

The carbide feed valve is controlled by the

diaphragm, which is actuated by thepressure of the gas generated in the tank.

When the pressure falls, carbide flows in

to the tank; as the pressure builds up the

flow ceases. The gas pressure at which the

generator will work is adjusted by means

of a spring fitted to the opposite side of

the diaphragm, ensuring close control of

pressure with generation strictly in

accordance with the demand. The carbide

Figure 2.2.6: Acetylene generator hopper is either made of glass or is fitted

with windows so that the quantity of carbide remaining in the hopper can be ascertained at a glance. Owing

to relatively large volume of water into which the small sized carbides fall, there is no possibility of over

heating and the carbide is completely slaked. The sludge, which collects at the bottom of the tank, and isemptied each time the generator is charged, consists of a thin milky fluid. The impurities in crude acetylene

consist chiefly of ammonia, hydrides of phosphorous, sulphur and nitrogen, and there are also water vapour

and particles of lime. These impurities must be removed before the gas is suitable for welding. So it is

filtered and washed and chemically purified by passing it through salts of ferric iron. The generator

provides acetylene up to safe pressure of 100 KPa. The colour of acetylene cylinders is maroon or red and

regulators have left hand screws. These cylinders are fitted with several fusible plugs at their bottom which

may melt and give way to acetylene to escape to atmosphere in case of burst of cylinder at high temperature

and pressure.

Gas Pressure Regulators: In order to reduce the pressure of either oxygen or dissolved acetylene from the

high pressure of the storage cylinder to that required at the blow pipe (desired pressure at the welding torch

may be somewhere up to 2.5 bar gage and 1 bar gage for oxygen and acetylene), a regulator or reducing

valve is necessary.

Good regulators are essential to

ensure the even flow of gas to the

blowpipe. The gas enters the

regulator at the base via a fine

sintered metal filter and the

cylinder pressure is indicated on

the first gauge. The gas then

enters the body of the regulatorR

through the aperture A, which is

controlled by the valve V. The

pressure inside the regulator rises

until it is sufficient to overcome

the pressure of the spring S,

which loads the diaphragm D.The diaphragm is therefore

pushed back and the valve V, to

which it is attached, closes the

aperture A and prevents any

more gas from entering the

regulator.

Figure 2.1.7: Single stage pressure regulator


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The outlet side is also fitted with a pressure gauge which indicates the working pressure on the blowpipe.

Upon gas being drawn off from the outlet side the pressure inside the regulator body falls, the diaphragm is

pushed back by the spring, and the valve opens, letting more gas in from the cylinder. The pressure in the

body R therefore depends on the pressure of the springs and this can be adjusted by means of a regulator.

Regulator bodies are made from brass forgings and single stage regulators are fitted with one safety valveset to relieve pressure of 16 -20 bar, should it be rendered inoperative by misuse, it ruptures at pressures of

70-80 bar and vents to the atmosphere through a vent in the bonnet. Single stage regulators are suitable for

general welding with maximum outlet pressure of 2.1 bar and for scrap cutting and heavy duty cutting,

thermic lancing and boring with outlet pressures 8.3-14 bar. A two stage regulator consists of two single

stages in series within one body forging. This reduces pressure in two stages and gives a much more stable

output pressure than the single stage regulator.

Hoses: The currently available industrial gases may be piped through

the tubes of steel, stainless steel, brass, bronze and cloth-reinforced

rubber flexible hose. The latter one is the most common method of

piping oxygen and acetylene which comes in black, green and red in

colour.

The green hose is usually used to transport oxygen; the red hose is

usually for fuel gas hose and the black hose is used for transport ofother industrially available welding gases. Available lengths are from

5 to 20 m, with bore diameter 4.5 mm for maximum working

pressure of 7 bar, 8 mm for a maximum of 12 bar and 10 mm for a

maximum working pressure of 15 bar.

Figure 2.1.8: Cross section of

cloth-reinforced rubber flexible

hose

Welding Blowpipe or Torch: A welding torch mixes oxygen and acetylene in the desired proportions,

burns the mixture at the end of the tip, and provides a means for moving and directing the flame. Welding

torches are classified as either injector type or positive pressure (equal pressure) type.

The principal difference between these two types of torches is in the mixers which should do following

things.

Mixes the gases usually oxygen and acetylene for proper combustion at the tip. Arrest flash backs, which may occur through improper operation. Any flame which happens to

travel back through the tip should be stopped at the mixer.

Equal pressure or high pressure blow pipe is simply a mixing device to supply approximately equal

volumes of oxygen and acetylene to the nozzle, and is fitted with regulating valves to vary the pressure of

the gases as required. The low pressure or injector type blowpipe has an injector nozzle inside its body

through which the high pressure oxygen streams. The oxygen draws the low pressure acetylene into the

mixing chamber and gives it the necessary velocity to preserve a steady flame, and injector also helps to

prevent backfiring. The torch may be of brass, aluminium, stainless steel etc.

Figure 2.1.9: Equal pressure welding torch


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Figure 2.1.9: Exploded view of welding torch

Figure 2.1.10: Injector type welding torch

Welding torch tips are made of a high thermal conductivity material likecopper. This helps to keep the tip sufficiently cool in order to prevent melting of

tip and any flash back due to over heating of the tip. The tip size is measured as

the diameter of the central hole through which the gaseous mixture passes.

For welding thicker material large sized hole is used which will supply more

combustible gases and more heat. The central hole of the tip should be circular and

smooth to provide streamlined flow. The tip may be solid one piece gooseneck

type fitted directly onto the mixer portion of the blowpipe or multiple pieces

assembled together and threaded to screwed on the head of the blowpipe.

Figure 2.1.11: Torch tips


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Spark Lighter: It is used for initiating burning of

combustible gases instantaneously. It consists of a

pointed stone and a rough surface to produce spark

when rubbed together.

Goggles: They are used to protect the eyes of the

operator from harmful heat and radiation of

infrared and ultraviolet rays produced during thewelding.

Apron and Gloves: they are used for the

Figure 2.1.12: Goggles protection of clothes and hand of the operator.

3. Filler Metals: Filler metals are used to supply additional materialto the weld zone during the welding. They are available as rod or wire

made of metals compatible with those to be welded. These consumable

filler rods may be bare, or they may be coated with flux. The purpose

of the flux is to retard oxidation of the surfaces of the parts being

welded, by generating gaseous shield around the weld zone. The flux

also helps to dissolve and remove oxides and other substances from the

workpiece and so contributes to the formation of a stronger joint. The

slag developed protects the molten metal puddles of metal againstoxidation as it cools.

Figure 2.1.13: Filler metal rod

4. Welding Technique: To light the flame, the acetylene valve on the torch is opened slightly andlighted with the help of a friction spark lighter. The flame draws the oxygen from the atmosphere and thus

results in a reducing flame. Then the acetylene valve is opened to get the required flow of acetylene. The

oxygen valve is then slowly opened till the intermediate flame feather of the reducing flame recedes into

the inner white cone. The actual adjustment of the flame depends on the type of material to be joined.

Metal Flame Metal Flame

Mild steel Neutral Aluminium Slightly carburisingHigh carbon steel Reducing Brass Slightly oxidising

Grey cast ironNeutral, Slightly

oxidisingCopper, bronze

Neutral, Slightly

oxidising

Alloy steel Neutral Nickel alloy Slightly carburising

Lead Neutral

Table-I: Flame adjustment for oxy-acetylene welding

The choice of the torch size depends on the thickness of the metal to be joined. Larger torch tip

sizes cause higher amount of oxygen and fuel to flow out causing the release of more heat. All joints except

outside corner joint require a filler metal to be added to fill the joint. This is done with the help of a welding

rod whose composition depends on the parent metal of the joint.

The torch tip should be positioned above the metal plate so that the white cone is at a distance of

1.5 to 3.0 mm from the plate. The torch should be held at an angle of 30 to 45

0

from the horizontal plane.The torch movement along the joint should be either oscillating or circular. In forehand welding, the torch

is moved in the direction of the tip. This tends to preheat before the white cone of the tip melts it. In

backhand welding the torch moves backwards. The outer blue flames are directed on the already welded

joint. This allows the joint to be continuously annealed relieving the welding stresses. This welding allows

a better penetration as well as form bigger weld. Backhand welding is generally used for thicker materials.

When the welding rod is used to provide filler material, it is necessary to hold it at a distance of 10

mm from the flame and 1.5 to 3.0 mm from the surface of the weld metal pool or puddle. This way the rod

gets preheated and when dipped into the puddle would readily get melted.


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Figure 2.1.14: Forehand and backhand welding techniques

Oxy-fuel welding can be used for all the types

of joints in all positions. Overhead usage

requires additional skill to safeguard the welder.

The various butt joint edge preparations are

shown in the

adjacent figure.

Thicker plates

require more than

one pass of the gas

torch along the

length to complete

the joint. In multi

pass welding, the

first pass (root

pass) is very

critical in anywelding operation.

Figure 2.1.15: Butt edge preparations Figure 2.1.16: Weld appearance

5. Gas Cutting: It is possible to rapidly oxidise (burn) iron and steel when it is heated to a temperaturebetween 800 to 10000 C. When a high pressure oxygen jet with a pressure of the order of 300 KPa is

directed against a heated steel plate, the oxygen jet burns the metal and blows it away causing the cut. This

process is used for cutting steel plates of various thicknesses (can go up to 2 m) mainly because the

equipment required is simple and can be carried anywhere without handling the heavy steel plates.

Oxy-acetylene gas cutting outfit is similar to that of the oxy-acetylene welding except for the torch

tip. Here the torch tip has a provision for preheating the plate as well as providing the oxygen jet. Thus thetip has a central hole for oxygen jet with surrounding holes for preheating flames. The cutting tip should be

chosen for the intended application. The size is normally dependent on the thickness of the plate which

determines the amount of preheating as well as the oxygen jet flow required for cutting.

After the steel is heated to the kindling temperature which is about 8700C, it gets readily combined with

oxygen giving iron oxide with the following reactions:

3 Fe + 2 O2 Fe3O4 + 6.67 MJ/Kg of iron

2Fe + O2 2FeO + 3.18 MJ/Kg of iron

4 Fe + 3 O2 2Fe2O3 + 4.90 MJ/Kg of iron


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All the above reactions are exothermic in nature and as such would provide a good amount of heat to

preheat the steel. But this energy may not be sufficient to bring the steel to its kindling temperature, and

hence preheating flames may have to be continued as somewhat lower rate. The heat generated causes the

metal to melt and get blown away by the oxygen pressure. About 30 to 40 % of metal is simply blown

away, while the rest is oxidised.

The cutting can start at the edge or in

the middle of the plate. After the platehas reached the kindling temperature,

the operator should release the oxygen

jet to start the cutting, moving the

torch in the forehand direction to

achieve the desired cut. Drag is the

amount by which the lower edge of

the drag line trails from the top edge.

Figure 2.1.17: Difference in torch tips for welding and cutting

A good cut is characterised by very small or

negligible drag. When the torch is moved too rapidly, the

metal at the bottom does not get sufficient heat to get oxidised

and cut and hence there is a large drag. When the torch ismoved slowly, all the preheated metal is burnt away by the

oxygen jet and a large amount of slag is generated.

Figure 2.1.18: Drag Figure 2.1.19: Gas cutting

Though the gas cutting is more useful with thick plates, thin sheets (less than 3 mm) can also be

cut by this process taking special precautions. Tip size chosen should be as small as possible. If small tips

are not available, then the tip is inclined at an angle of 15 to 20 degrees.

Gas cutting can be done manually or by a

machine. The manual cutting is used for

general purpose work and for straight linecutting. In machine cutting the torch is

mounted on a rail and both rail and the

torch can move simultaneously along the

two mutually perpendicular axes in the

horizontal plane with the help of servo

Figure 2.1.20: torch position for cutting thin sheets motors. There is provision in the machine

to hold more than one torch so that large number of identical pieces can be cut at the same time.


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Oxygen cutting would be useful only for those materials which readily get oxidised and the oxides have

lower melting points than the metals. So it is most widely used for ferrous materials. But it cannot be used

for materials like aluminium, bronze, stainless steel which resist oxidation. Cutting of high carbon steels

and cast irons require special attention due to formation of heat affected zone (HAZ) where structural

transformation occurs.

6. Advantages of gas welding: It is one of the versatile methods of welding. The same equipment with a range of torches would

be used for welding, cutting, brazing and braze welding.

Rate of heat generation is less so thin sheets can be welded. As the source of heat and filler metal are separated, the metal deposition can be easily controlled

and heat properly adjusted giving rise to a satisfactory weld.

Welding equipment is portable and can be operated at remote places. The cost of equipment is not so high. Heat affected zone (HAZ) is very narrow.

7. Limitations of gas welding: Heavy sections cannot be joined efficiently. For heavy sections proper penetration may not be achieved. Slower speed of welding compared electric arc welding. Flux used in the filler metal provides fumes which are irritating to the eyes, nose, throat and lungs. More safety is recommended in gas welding. Acetylene and oxygen are expensive gases. Prolonged heating of the joint may results in large HAZ.

8. Applications: For joining of thin materials. For joining materials in whose case excessively high temperature or rapid heating and cooling of

the job would produce unwanted changes in the metal.

For welding both ferrous and non-ferrous metals. In automotive &aircraft industries, project site works, workshops etc.

9. Reference: Welding and Welding Technology by R.L.Little, MacGraw-Hill, NewYork, page 1-27 The Science and Practice of Welding by A.C.Davis, Cambridge University Press, page 255-269 Principles of Manufacturing Materials and Processes by J.S.Campbell, , MacGraw-Hill, NewYork,

page 380-388.

Manufacturing Engineering and Technology by Kalpakjian and Schmid, Pearson Education, page775-779

Manufacturing Technology by P.N.Rao, TMH, page368-377. Advance Welding Technology by S.A.Rizvi, Katson Books, page9-20.


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10. Review Questions:1. Describe the principle of oxy-fuel gas welding process.

2. What is the gas pressure at the weld metal zone in case of gas welding?

3. What other fuel gases that can be used in oxy-gas welding?

4. Explain how you can produce neutral, oxidising and carburising flames in gas welding.

5. Where you will suggest the use of carburising and oxidising flames?

6. Why acetylene is stored in acetone at high pressure?7. Why maximum pressure of acetylene is limited to 100KPa in acetylene generator?

8. Why two pressure gauges are mounted in each gas cylinder?

9. Why oxygen and acetylene gas regulators have different thread type?

10. Explain how gas regulators function.

11. Why gas regulators are provided with gas cylinder in case of gas welding?

12. What are different materials for hose pipe to carry gases from the cylinder to weld metal zone?

13. Is it necessary to change the welding torch if one replaces acetylene cylinder with acetylene

generator?

14. Differentiate between equal pressure and injector type blowpipes or torches.

15. What are the common materials used for blowpipes in gas welding?

16. What is the material of torch tip? What will happen if it becomes very hot?

17. How the gas is ignited in oxy-fuel welding?

18. How back fire is arrested in oxy-fuel welding?19. Write the advantages of forehand welding techniques.

20. Why welding torch is inclined to the horizontal surface during the welding?

21. How you will identify that acetone is coming with acetylene from the cylinder?

22. Describe the welding edge preparations with neat sketch for gas welding.

23. Why filler material is required in gas welding? How it is supplied to the weld metal zone?

24. Why filler rod is placed ahead of welding torch during the welding operation?

25. What is heat affected zone (HAZ) in welding? What is its importance in gas welding?

26. How overhead welding is done by gas welding?

27. Describe Linde welding and block welding? (Beyond syllabus)

28. Which are the best materials for gas pressure regulator body?

29. Explain the advantages and application of oxy-acetylene welding.

30. Write the principles of oxy-fuel gas cutting.

31. Write a short note on oxy-hydrogen welding. (Beyond syllabus)

32. What do you understand by the term kerf in gas cutting? Explain its relevance.

33. What are the parameters that decide the drag in oxy-fuel gas cutting? Explain how a good cut

is achieved.

34. Explain the effect of torch speed on the cut in gas cutting.

35. How a gas cutting torch is different from welding torch?

36. Show the cross section of a gas cutting operation labeling important details.

37. Why it is difficult to cut materials like aluminium, stainless steel, and bronze in gas cutting?

38. How cast iron can be cut in gas cutting?

39. Why pre-heating and post -heating are essential in case of high carbon steel gas cutting?

40. How thin materials can be cut by gas welding?

41. Describe the flame used in gas cutting with proper reasoning.

42. What do you mean by kindling temperature?

43. Describe the automated gas cutting process.

44. What is powder cutting? (Beyond syllabus)45. The pressure of oxygen is more in gas welding or gas cutting?

46. Why should copper lines not used for acetylene gas?

47. Write the factors on which the diameter of tip opening or orifice of gas welding torch depends.

48. Write the advantages of multiple piece tip over solid tip of welding torch.

49. What is the correct procedure to rebuild a dirty welding tip?

50. Explain various weld movements possible in gas welding with neat sketches.

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GasWeldingAndCutting-Rama Sir.pdf