WELDING

Introduction
Welding is the metal joining method wherein localized coalescence is produced either by heating the
metal to suitable temperatures, with or without use of filler metal or by application of pressure.
The filler material has similar composition and melting point temperatures as the base metal. It is
used to fill gap between the joint surfaces. Welding can be applied to both ferrous (iron based)
and non-ferrous (non-iron based) metals and their alloys e.g. wrought iron, cast iron, carbon steels,
cast steels, alloy steels, stainless steels, aluminium, copper, magnesium, nickel and zinc. Over the years
welding has emerged as a successful fabrication process owing to its several merits, though it has its
demerits also.
Advantages of Welding
1) A good weld is as strong as the base metal.
2) General welding equipment is inexpensive and now also available in portable form.
3) It facilitates considerable freedom in design.
4) Many metals/ alloys both similar and dissimilar can be joined by welding.
Disadvantages of Welding
1) Welding gives out harmful radiations and fumes.
2) It results in residual stresses and distortion in work pieces.
3) Edge preparation of work-pieces is required before welding.
4) Structure of welded joint isn’t the same as that of parent metal

Positions of Welding:
Basic welding positions are as follows
1) Flat- face of the weld is horizontal and the electrode (almost vertical) is above the base metal.
2) Horizontal- axis of the weld is horizontal.
3) Vertical- axis of the weld is vertical and welding is done horizontally.
4) Overhead- face of the weld is nearly horizontal, electrode almost vertical and welding is
done from below the base metal.

Welded Joints
1. Butt Joint – When material thickness does not exceed 5mm, often welded by simply placing
the sheared straightened edges close to each other with a gap of 1 to 2 mm. This is “square butt joint”.
Other types of butt joint are – single V butt joint, double V butt joint
2. Lap Joint – plates overlap each other. The width of the lap is usually between 3 to 5 times the
thicknesses of the plate. Welds are made on both sides of the lap.
3. Fillet Joint – is a T-joint. The purpose of edge preparation is to achieve the required
penetration and strength. Different types of fillet joints are – square, single bevel, double bevel,
single J and double J
Arc Welding
Arc welding comprises of those welding processes in which the source of heat is electricity i.e.
coalescence is brought about by heating the workpiece with an electric arc struck between an electrode
and the workpiece, with or without the use of filler metal depending upon the base plate thickness.
The electric supply maybe direct current (dc) or alternating current (ac), depending on the characteristics
desired of the weld.

Principle of Operation
To understand the generation of arc in arc welding, let us consider dc power supply in which the
positive pole (anode) and the negative pole (cathode) remain the same, unlike ac supply in which they
keep on interchanging periodically. The arc column is generated between the anode and cathode (of
a dc power supply), one of which shall lie on the electrode and the other at the base metal. The
cathode and anode are brought in mutual contact for a short while and then drawn apart to establish an
air gap. Negatively charged electrons (which have a small mass) are easily disassociated from the
metal at the negative pole or cathode and accelerated towards the anode, striking it with high kinetic
energy. Kinetic energy, which is mathematically equal to mv22/2. which m being mass, v being velocity
possessed by these electrons is enormous in spite of its small mass by virtue of its high velocity.
Electric charge disrupts electron flow in its orbit, making it to fly off at a tangent from its orbital path
to seek another positive nucleus to which it will reassociate itself.
Heat is produced by the three following ways
1. On hitting the anode, kinetic energy of electrons is liberated as heat energy.
2. When charge on the electron is received by the anode, an electromotive force (emf) is induced in it,
which gets directly transferred into heat energy.
3. When positively charged ions going from anode to cathode, intermingle with electrons heat is
produced.
Electrode

Functions of Welding electrodes

1. Initiation and maintenance of welding arc.
2. Protect the molten metal from deleterious effects of oxygen and nitrogen in the air.
3. Provide a slag blanket on the weld
4. Means of introducing alloying elements not contained in the core wire.

Constitution of the arc: The arc stream comprises of three areas of heat namely
1) anode area
2) plasma area
3) cathode area
Anode area: area of high heat attributed to electrons striking the anode and current capacity of
electrons.
Plasma area: area of low heat resulting from the atomic collision of some electrons with positive ions.
Cathode area: area of medium heat, attributed to positive ions striking the cathode.
Selections of heat zone and power supply are crucial because in a dc arc column system, two-thirds of
the energy released is at the anode. Whereas the same in an ac system is equalized between anode
and cathode areas and plasma area is the area of medium heat. When dc power supply is used, the
positive and negative terminals can be located in two ways viz.
i) Positive Ground/ Straight Polarity In this condition, the base metal is the positive pole
or anode and the electrode, the cathode. Electron flow occurs from electrode to work.
Penetration of the bead formed in welding into the base metal is high.
ii) ii) Negative Ground/ Reverse Polarity In this condition, the base metal becomes the cathode
and electrode becomes the anode. Electron flow occurs from work to electrode. The weld bead
has shallow penetration into the base metal
When ac power supply is used, the terminals continuously interchange between the base metal and the
electrode and the weld bead has penetration intermediate than the same in cases of straight and reverse
polarity.

Gas Welding
Definition: Gas welding process is a fusion welding process. It joins metals using the heat of
combustion of oxygen/air and a fuel gas mixture. The intense heat produced melts and fuses together
the edges of the parts to be welded.

Oxy-Acetylene Welding
Principle of Operation: When acetylene is mixed with oxygen in correct proportions in the welding
torch and ignited, the flame resulting at the tip of the torch is sufficiently hot to melt and join the parent
metal.

Different types of flames

1. Neutral flame
2. Neutral flame
3. Oxidizing flame
Neutral Flame - Equal volumes of oxygen and acetylene are mixed. - Temperature of the flame is
about 3260 °C. - The flame has nicely defined inner cone which is light blue in color, surrounded by
outer envelope of darker blue than the inner cone. - Suitable for: Mild Steel, Stainless Steel, Cast Iron,
Copper, Aluminium.
Oxidizing Flame - After neutral flame is established, supply of oxygen is further increased. - Burns
with a loud roar. - Hotter than neutral flame and temperature rise as high as 3500°C. - Suitable for:
Copper Base Metals, Zinc Base metals, Ferrous metals such as manganese steel and cast iron.
Reducing Flame - If the volume of oxygen supplied to the neutral flame is reduced, the resulting flame
will be carburizing or reducing flame, i.e. rich in acetylene. - The flame has an approximate temperature
of 3038 °C. - Suitable for welding high carbon steel.

Advantages of gas welding

 Probably the most versatile welding process.
 Welder has considerable control over the temperature of the metal in the weld zone.
 Rate of heating and cooling is relatively slow.
 Since the source of heat and filler metal are separate, the welder has control over filler metal
deposition. Equipment is low cost, self sufficient and usually portable.

Disadvantages of gas welding

 Heavy sections cannot be joined economically.

 Flame temperature is less than the temperature of the arc.
 Refractory metals and reactive metals cannot be welded by this process.
 More safety problems are associated with it.
 Flux shielding gases are not so effective.