WELDINGWELDING

Welding is a process of joining similar metals by the Welding is a process of joining similar metals by the application of heat with or without application of application of heat with or without application of pressure and addition of filler material. The result is pressure and addition of filler material. The result is a continuity of homogenous material, of the a continuity of homogenous material, of the composition and characteristics of two parts which composition and characteristics of two parts which are being joined together.are being joined together.

Welding may be classified under two broad Welding may be classified under two broad headings:headings:

(1)Plastic welding, (2) Fusion welding(1)Plastic welding, (2) Fusion welding

WELDINGWELDING

In the plastic or In the plastic or pressure weldingpressure welding, the pieces to be joined , the pieces to be joined are heated to a plastic state and then forced together by are heated to a plastic state and then forced together by external pressure.external pressure.

Ex. Forge welding, resistance welding, thermit welding, gas Ex. Forge welding, resistance welding, thermit welding, gas weldingwelding

In the In the fusion weldingfusion welding, the material at the joint is heated to , the material at the joint is heated to molten state and allowed to solidify. Ex. Gas welding, arc molten state and allowed to solidify. Ex. Gas welding, arc welding, thermit weldingwelding, thermit welding

In the weld, metal solidifies from liquid state and is In the weld, metal solidifies from liquid state and is therefore essentially a casting. In a single – run weld , long therefore essentially a casting. In a single – run weld , long columnar grains will grow from the sides of the weld and columnar grains will grow from the sides of the weld and will meet at the centre of the weld deposit forming a plane will meet at the centre of the weld deposit forming a plane of weakness.of weakness.

In a multi-run weld, the first run, is as before. The second In a multi-run weld, the first run, is as before. The second run normalizes the first layer causing considerable grain run normalizes the first layer causing considerable grain refinement (giving stronger joint). But the possibility of slag refinement (giving stronger joint). But the possibility of slag and gas inclusions will increase. and gas inclusions will increase.

The contour of weld by forming notches can affect both fatigue and low temperature properties of a structure, (welding is a skilled work).

Heat Affected Zone The effect of welding heat upon the metal immediately

adjacent to the weld is likewise important and dependent on chemical composition and thermal conductivity of parent metal.

HAZ ranges from over heated area near the weld metal to an under annealed structure farther away. Pre heating is often employed when welding C.I and high carbon steel, since it slows down the cooling rate of parent metal.

Fusion WeldingFusion Welding

1.Oxy-fuel gas welding 2.Arc Welding – (a) Consumable electrode (b) Non-consumable electrode 3.Solid state welding – Ultrasonic, Friction,

Resistance

Gas WeldingGas Welding

Done by burning a combustible gas with air or oxygen in a concentrated flame of high temperature. The purpose of the flame is to heat and melt the parent metal and filler rod of a joint.

Advantages: Equipment is inexpensive Versatile Serves adequately in many job and general repair shops Types: Oxy-acetylene welding Air-acetylene welding Oxy-hydrogen welding

GAS WELDING TECHNIQUEGAS WELDING TECHNIQUE According to the position of welding, all welds are According to the position of welding, all welds are

classified into:classified into: Down hand or flat weld deposited on a horizontal Down hand or flat weld deposited on a horizontal

surface so that the flame is above the weld. surface so that the flame is above the weld. Vertical welds deposited on a vertical surface Vertical welds deposited on a vertical surface

(upwards or downwards) (upwards or downwards) Inclined welds deposited up or down an inclined Inclined welds deposited up or down an inclined

surfacesurface Horizontal welds deposited on a vertical surface in a Horizontal welds deposited on a vertical surface in a

horizontal directionhorizontal direction Overhead welds deposited on a horizontal surface in Overhead welds deposited on a horizontal surface in

any direction so that the weld is above the flame.any direction so that the weld is above the flame.

AIR-ACETYLENE WELDING This process uses a torch similar to a Bunsen burner

and operates on the Bunsen burner principle. The air is drawn into the torch required and mixed with the fuel flame. The gas is then ejected and ignited, producing an air-fuel flame.

The common fuels used are acetylene, natural gas, propane and butane. This type of welding has limited use since the temperature is lower than that attained by other gas processes.

The air-fuel welding processes are used in lead welding and many low-melting-temperature metals and alloys like in brazing and soldering processes.

OXY-HYDROGEN WELDING

The oxygen-hydrogen process were once used extensively to weld low temperature metals like aluminium, lead and magnesium, but it is not popular today because more versatile and faster welding process such as TIG (tungsten inert gas) and MIG (metal inert gas) have replaced the oxygen-hydrogen flame. The process is similar to oxygen-acetylene system, with the only difference being a special regulator used in metering the hydrogen gas.

OXY-ACETYLENE WELDING

Accomplished by melting the edges or surface to be joined by gas flame and allowing the molten metal to flow together, thus forming a solid continuous joint upon cooling.

This process is suitable for joining metal sheets and plates having thickness of 2 to 50 mm.

With material thicker than 15 mm, additional metal called filler metal is added to the weld in the form of welding rod.

The composition of the filler rod is usually the same as that of the part being welded. To remove the impurities and oxides present on the surfaces of the metal to be joined and to obtain a satisfactory bond a flux is always employed during the welding except mild steel which has more manganese and silicon that act as deoxidizing agents.

GAS FLAMEGAS FLAME The correct adjustment of the flame is important for The correct adjustment of the flame is important for

reliable works. A neutral flame is produced when O2 and reliable works. A neutral flame is produced when O2 and acetylene are supplied to torch in equal volumes. The heat acetylene are supplied to torch in equal volumes. The heat is generated with a pair of chemical reactions which occur is generated with a pair of chemical reactions which occur at the at the inner coneinner cone, where the temperature reaches between , where the temperature reaches between 3050 to 3450 deg C.3050 to 3450 deg C.

CC22HH22 + O + O22 → 2CO + H → 2CO + H22 + heat + heat

The secondary combustion process in the The secondary combustion process in the outer envelopeouter envelope, , flame temperature is around 2100 deg C near the inner flame temperature is around 2100 deg C near the inner cone and around 1250 deg C at the end point of the flame.cone and around 1250 deg C at the end point of the flame.

4CO + 2H4CO + 2H22 + 3O + 3O2 2 → 4CO→ 4CO2 + 2 + 2H2H22O + heatO + heat

A A neutral flame has 2 definite zonesneutral flame has 2 definite zones 1. Sharp1. Sharp brilliant cone extending short distance from the tip of brilliant cone extending short distance from the tip of

torch – develops heat, temp 2100 deg Ctorch – develops heat, temp 2100 deg C 2. Outer cone or envelope, faintly luminous bluish colour – 2. Outer cone or envelope, faintly luminous bluish colour –

protects the metal from oxidation.protects the metal from oxidation. Uses:Uses: Welding steel, stainless steel, CI, Cu, Al Welding steel, stainless steel, CI, Cu, Al Carburizing flame, Carburizing flame, temp 2700 deg C, there is excess of temp 2700 deg C, there is excess of

acetylene. The flame has acetylene. The flame has 3 zones, 3 zones, 1. Sharp inner cone1. Sharp inner cone 2. Intermediate cone of whitish colour2. Intermediate cone of whitish colour 3. Bluish outer cone3. Bluish outer cone Oxidizing flameOxidizing flame, temp 3400 deg C, there is excess of oxygen, , temp 3400 deg C, there is excess of oxygen,

the flame has the flame has 2 zones2 zones 1. Small inner cone with Purple tinge1. Small inner cone with Purple tinge 2. Envelope2. Envelope This flame is necessary for welding brass. In steel this results in This flame is necessary for welding brass. In steel this results in

large grain size, high brittleness large grain size, high brittleness

WELDING EQUIPMENTWELDING EQUIPMENT Welding torchWelding torch – tool for mixing oxygen & acetylene – tool for mixing oxygen & acetylene Welding tipWelding tip – diameter of tip opening depends on the type of metal to – diameter of tip opening depends on the type of metal to

be welded and its thickness.be welded and its thickness. Pressure regulatorPressure regulator – to reduce the cylinder pressure to the required – to reduce the cylinder pressure to the required

working pressure. For working pressure. For << 25 mm25 mm thickness, reg. pre. is 0.15 to 0.7 thickness, reg. pre. is 0.15 to 0.7 kgf / kgf / cmcm22 and for and for >> 25 mm25 mm thickness, reg. pre. is 0.98 thickness, reg. pre. is 0.98 kgf / cmkgf / cm2.2.

Hose & hose fittingsHose & hose fittings Goggles, Gloves & spark lighterGoggles, Gloves & spark lighter Gas cylinderGas cylinder – – Oxygen gas cylinder, charged with 40 L at 154 kgf / cmOxygen gas cylinder, charged with 40 L at 154 kgf / cm22 Acetylene gasAcetylene gas – – lowlow (0.07 (0.07 kgf / cmkgf / cm22) & ) & highhigh pressure (1.0 pressure (1.0 kgf / cmkgf / cm22) ) Low pressure acetylene is generated and used in site. To prevent air Low pressure acetylene is generated and used in site. To prevent air

blowing back, a back pressure valve must be introduced between the blowing back, a back pressure valve must be introduced between the blow pipe & gas holder.blow pipe & gas holder.

Every valve has a safety device to provide against dangerous pressure.Every valve has a safety device to provide against dangerous pressure. In addition to the above welding rods & fluxes are employed.In addition to the above welding rods & fluxes are employed.

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