WELDING AND ITS TYPE

WELDING

Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material. 

Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

TYPES

·        Plastic Welding or Pressure Welding

The piece of metal to be joined are heated to a plastic state and forced together by external pressure

(Ex) Resistance welding

·        Fusion Welding or Non-Pressure Welding

The material at the joint is heated to a molten state and allowed to solidify

(Ex) Gas welding, Arc welding

Classification of welding processes:

     (i). Arc welding

1.      Carbon arc

2.      Metal arc

3.      Metal inert gas

4.      Tungsten inert gas

5.      Plasma arc

6.      Submerged arc

7.      Electro-slag

    (ii). Gas Welding

1.      Oxy-acetylene

2.      Air-acetylene

3.      Oxy-hydrogen

    (iii). Resistance Welding

1.      Butt

2.      Spot

3.      Seam

4.      Projection

5.      Percussion

(iv)             Thermit Welding

(v)               Solid State Welding

1.      Friction

2.      Ultrasonic

3.      Diffusion

4.      Explosive

(vi)             Newer Welding

1.      Electron-beam

2.      Laser

(vii)           Related Process

1.      Oxy-acetylene cutting

2.      Arc cutting

3.      Hard facing

4.      Brazing

5.      Soldering

Arc welding methods

1. Metal arc welding

            It is a process of joining two metal pieces by melting the edges by an electric arc. The electric arc is produced between two conductors. The electrode is one conductor and the work piece is another conductor. The electrode and the work piece are brought nearer with small air gap. (3mm app.)

            When current is passed an electric arc is produced between the electrode and the work piece. The work piece and the electrode are melted by the arc. Both molten piece of metal become one. Temperature of arc is about 4000°c Electrodes used in arc welding are coated with a flux. This flux produces a gaseous shield around the molten metal. It prevents the reaction of the molten metal with oxygen and nitrogen in the atmosphere. The flux removes the impurities from the molten metal and form a slag. This slag gets deposited over the weld metal. This protects the weld seam from rapid cooling. Fig.1 shows arc welding process.

Equipments:(Refer Fig 2)

·        A welding generator (D.C.) or Transformer (A.C.)

·        Two cables- one for work and one for electrode

·        Electrode holder

·        Electrode

·        Protective shield

·        Gloves

·        Wire brush

·        Chipping hammer

·        Goggles

Fig. 1 Arc Welding

Fig 2 Electric Arc Welding Equipments

Advantages:

Ø      Most efficient way to join metals

Ø      Lowest-cost joining method

Ø      Affords lighter weight through better utilization  of materials

Ø      Joins all commercial metals

Ø      Provides design flexibility

Limitations:

1.      Manually applied, therefore high labor cost.

2.      Need high energy causing danger

3.      Not convenient for disassembly.

4.      Defects are hard to detect at joints.

2. Carbon arc welding:

          In carbon arc welding, the intense of heat of an electric arc between a carbon electrode and work piece metal is used for welding. DC power supply is used. The carbon electrode is connected to negative terminal and work piece is connected to positive terminal, because positive terminal is hotter (4000°c) than the negative terminal (3000°c) when an arc is produced. So carbon from the electrode will not fuse and mix up with the metal weld. If carbon mixes with the weld, the weld will become weak and brittle. To protect the molten metal from the atmosphere the welding is done with a long arc. In this case, a carbon monoxide gas is produced, which surrounds the molten metal and protects it.

            Carbon arc welding is used to weld both ferrous and non ferrous metals. Sheets of steel, copper alloys, brass and aluminium can be welded in this method.( Refer Fig 3)

Fig 3 Carbon Arc Welding

Comparison of A.C. and D.C. arc welding

	Alternating Current (from Transformer)	Direct Current (from Generator)
1	More efficiency	Less efficiency
2	Power consumption less	Power consumption more
3	Cost of equipment is less	Cost of equipment is more
4	Higher voltage – hence not safe	Low voltage – safer operation
5	Not suitable for welding non ferrous metals	suitable for both ferrous non ferrous metals
6	Not preferred for welding thin sections	preferred for welding thin sections
7	Any terminal can be connected to the work or electrode	Positive terminal connected to the work Negative terminal connected to the electrode

GAS WELDING

Oxy-Acetylene welding

      In gas welding, a gas flame is used to melt the edges of metals to be joined. The flame is produced at the tip of welding torch. Oxygen and Acetylene are the gases used to produce the welding flame. The flame will only melt the metal. A flux is used during welting to prevent oxidations and to remove impurities. Metals 2mm to 50mm thick are welded by gas welding. The temperature of oxyacetylene flame is about 3200°c. Fig 3 shows Gas welding equipments.

Gas Welding Equipment

1. Gas Cylinders

Pressure

Oxygen – 125 kg/cm²

Acetylene – 16 kg/cm²       

       2. Regulators

            Working pressure of oxygen 1 kg/cm²

            Working pressure of acetylene 0.15 kg/cm²          

Working pressure varies depends upon the thickness of the work pieces welded.

3. Pressure Gauges

4. Hoses

5. Welding torch

6. Check valve

7. Non return valve

Fig- 4 Gas Welding Equipment

TYPES OF FLAMES

•         When acetylene is burned in air, it produces a yellow sooty flame, which is not enough for welding applications

•         Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent blue envelope is called Carburizing flame (30000c)

•         This flames are used for hardening the surfaces

•         Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral flame (It has a balance of fuel gas and oxygen)

•         Most commonly used flame because it has temperature about 32000c

•         Used for welding steels, aluminium, copper and cast iron

•         If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more fierce is called Oxidizing flame

•         Has the highest temperature about 34000c

•         Used for welding brass and brazing operation

Fig 4 shows the types of flames.

Fig 5 Types of Gas Flames

 Advantages

1.      Equipment has versatile

2.      Same equipment can be used for oxy acetylene cutting and brazing by varying the torch size

3.      Heat can controlled easily

Disadvantages

1.      Slower process

2.      Risk is involved in handling gas cylinders

GAS CUTTING

•         Ferrous metal is heated in to red hot condition and a jet of pure oxygen is projected onto the surface, which rapidly oxidizes

•         Oxides having lower melting point than the metal, melt and are blown away by the force of the jet, to make a cut

•         Fast and efficient method of cutting steel to a high degree of accuracy

•         Torch is different from welding

•         Cutting torch has preheat orifice and one central orifice for oxygen jet

•         PIERCING and GOUGING are two important operations

•         Piercing, used to cut a hole at the centre of the plate or away from the edge of the plate

•         Gouging, to cut a groove into the steel surface

Fig 6 Automatic Gas Cutting

Fig 7 Manual Gas Cutting

Weld joint:

There are 5 basic joint types in welding

•         Butt joint: Two materials are in the same plane, joined from the edges.

•         Corner joint:The corners of two materials form a right angle and joined.

•         Lap joint: Two parts overlaps.

•         Tee joint: One part is perpendicular to the other, making a T shape.

•         Edge joint: Edges of the two materials joined.

Types of weld

1.      Fillet weld: Used in T joints,corner joints, lap joints.

2.      Groove weld:Used in butt joints.

3.      Plug weld: Used in lap joints.

4.      Slot weld: Used in lap joints.

5.      Spot weld: Used in lap joints.

6.      Seam weld: Used in lap joints.

7.      Flange weld:Used in edge joints.

8.      Surfacing weld:Not a joining process, it is used to increase the thickness of the plate, or provide a protective coating on the surface.

Fig 8 Types of Weld Joints

Weldability is the ease of a material or a combination of materials to be welded under fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service

Brazing and Soldering

Brazing

It is a low temperature joining process. It is performed at temperatures above 840º F and it generally affords strengths comparable to those of the metal which it joins.  It is low temperature in that it is done below the melting point of the base metal.  It is achieved by diffusion without fusion (melting) of the base

          Depending upon the method of heating, brazing can be classified as

1.      Torch brazing

2.      Dip brazing

3.      Furnace brazing

4.      Induction brazing

Fig 9 Brazing

Advantages:

Ø      Dissimilar metals which canot be welded can be joined by brazing

Ø      Very thin metals can be joined

Ø      Metals with different thickness can be joined easily

Ø      In brazing thermal stresses are not produced in the work piece. Hence there is no distortion

Ø      Using this process, carbides tips are brazed on the steel tool holders

Disadvantages:

Ø      Brazed joints have lesser strength compared to welding

Ø      Joint preparation cost is more

Ø      Can be used for thin sheet metal sections

Soldering

                 It   is a low temperature joining process. It is performed at temperatures below 840ºF for joining. 

Soldering is used for,

•         Sealing, as in automotive radiators or tin cans

•         Electrical Connections

•         Joining thermally sensitive components

•         Joining dissimilar metals

Fig 9 Soldering

Ropes

Rope:-

Wire rope is a type of rope which consists of several strands of metal wire laid (or 'twisted') into a helix. Initially wrought iron wires were used, but today steel is the main material used for wire ropes.

Rope construction:-

Wire:-

Steel wires for wire ropes are normally made of non-alloy carbon steel with a carbon content of 0.4 to 0.95%. The wires are patented [6](heat process) 70 and 2450 N/mm² _. The very high strength of the rope wires enables wire ropes to support large tensile forces and to run over sheaves with relatively small diameters.

Strands :-

n the so-called cross lay strands, the wires of the different layers cross each other. In the mostly used parallel lay strands, the lay length of all the wire layers is equal and the wires of any two superimposed layers are parallel, resulting in linear contact. The wire of the outer layer is supported by two wires of the inner layer. These wires are neighbours along the whole length of the strand. Parallel lay strands are made in one operation. The endurance of wire ropes with this kind of strand is always much greater than of those (seldom used) with cross lay strands. Parallel lay strands with two wire layers have the construction Filler, Seale or Warrington.

Spiral ropes:-

In principle, spiral ropes are round strands as they have an assembly of layers of wires laid helically over a centre with at least one layer of wires being laid in the opposite direction to that of the outer layer. Spiral ropes can be dimensioned in such a way that they are non-rotating which means that under tension the rope torque is nearly zero. The open spiral rope consists only of round wires. The half-locked coil rope and the full-locked coil rope always have a centre made of round wires. The locked coil ropes have one or more outer layers of profile wires. They have the advantage that their construction prevents the penetration of dirt and water to a greater extent and it also protects them from loss of lubricant. In addition, they have one further very important advantage as the ends of a broken outer wire cannot leave the rope if it has the proper dimensions.

Stranded ropes :-

Stranded ropes are an assembly of several strands laid helically in one or more layers around a core. Most types of stranded ropes only have one strand layer over the core (fibre core or steel core). The lay direction of the strands in the rope can be right (symbol Z) or left (symbol S) and the lay direction of the wires can be right (symbol z) or left (symbol s). This kind of rope is called ordinary lay rope if the lay direction of the wires in the outer strands is in the opposite direction to the lay of the outer strands themselves. If both the wires in the outer strands and the outer strands themselves have the same lay direction, the rope is called a lang lay rope (formerly Albert’s lay or Lang’s lay). Multi-strand ropes are all more or less resistant to rotation and have at least two layers of strands laid helically around a centre. The direction of the outer strands is opposite to that of the underlying strand layers. Ropes with three strand layers can be nearly non-rotating.

Left-hand ordinary lay (LHOL) wire rope (close-up). Right-hand lay strands are laid into a left-hand lay rope.

Classification of ropes according to usage:-

Depending on where they are used, wire ropes have to fulfil different requirements.

• Running ropes (stranded ropes) are bent over sheaves and drums. They are therefore stressed mainly by bending and secondly by tension.
• Stationary ropes, stay ropes (spiral ropes, mostly full-locked) have to carry tensile forces and are therefore mainly loaded by static and fluctuating tensile stresses.
• Track ropes (full locked ropes) have to act as rails for the rollers of cabins or other loads in aerial ropeways and cable cranes. In contrast to running ropes, track ropes do not take on the curvature of the rollers. Under the roller force, a so called free bending radius of the rope occurs. This radius increases (and the bending stresses decrease) with the tensile force and decreases with the roller force.
• Wire rope slings (stranded ropes) are used to harness various kinds of goods. These slings are stressed by the tensile forces but first of all by bending stresses when bent over the more or less sharp edges of the goods.

Depending on where they are used, wire ropes have to fulfil different requirements. The main uses are:

Safty:-

The wire ropes are stressed by fluctuating forces, by wear, by corrosion and in seldom cases by extreme forces. The rope life is finite and the safety is only given by inspection for the detection of wire breaks on a reference rope length, of cross-section loss as well as other failures so that the wire rope can be replaced before a dangerous situation occurs. Installations should be designed to facilitate the inspection of the wire ropes.

Lifting installations for passenger transportation require that a combination of several methods should be used to prevent a car from plunging downwards. Elevators must have redundant bearing ropes and a safety gear. Ropeways and mine hoistings must be permanently supervised by a responsible manager and the rope has to be inspected by magnetic method with that inner wire breaks can be detected too.

The lay of a wire rope describes the manner in which either the wires in a strand, or the strands in the rope, are laid in a helix

Terminations:-

The end of a wire rope tends to fray readily, and cannot be easily connected to plant and equipment. There are different ways of securing the ends of wire ropes to prevent fraying. The most common and useful type of end fitting for a wire rope is to turn the end back to form a loop. The loose end is then fixed back on the wire rope. Termination efficiencies vary from about 70% for a Flemish eye alone; to nearly 90% for a Flemish eye and splice; to 100% for potted ends and swagings.

Right-hand ordinary lay (RHOL) wire rope terminated in a loop with a thimble and ferrule.

Thimbles:-

When the wire rope is terminated with a loop, there is a risk that it will bend too tightly, especially when the loop is connected to a device that spreads the load over a relatively small area. A thimble can be installed inside the loop to preserve the natural shape of the loop, and protect the cable from pinching and abrading on the inside of the loop. The use of thimbles in loops is industry best practice. The thimble prevents the load from coming into direct contact with the wires.

Wire clips:-

A wire rope clamp, also called a clip, is used to fix the loose end of the loop back to the wire rope. It usually consists of a u-shaped bolt, a forged saddle and two nuts. The two layers of wire rope are placed in the u-bolt. The saddle is then fitted over the ropes on to the bolt (the saddle includes two holes to fit to the u-bolt). The nuts secure the arrangement in place. Three or more clamps are usually used to terminate a wire rope

Swaged terminations:-

Swaging is a method of wire rope termination that refers to the installation technique. The purpose of swaging wire rope fittings is to connect two wire rope ends together, or to otherwise terminate one end of wire rope to something else. A mechanical or hydraulic swager is used to compress and deform the fitting, creating a permanent connection. There are many types of swaged fittings. Threaded Studs, Ferrules, Sockets, and Sleeves a few examples. Swaging ropes with fibre cores is not recommended.

Wedge sockets:-

wedge socket termination is useful when the fitting needs to be replaced frequently. For example, if the end of a wire rope is in a high-wear region, the rope may be periodically trimmed, requiring the termination hardware to be removed and reapplied. An example of this is on the ends of the drag ropes on a dragline. The end loop of the wire rope enters a tapered opening in the socket, wrapped around a separate component called the wedge. The arrangement is knocked in place, and load gradually eased onto the rope. As the load increases on the wire rope, the wedge become more secure, gripping the rope tighter.

Potted ends or poured sockets:-

Poured sockets are used to make a high strength, permanent termination; they are created by inserting the wire rope into the narrow end of a conical cavity which is oriented in-line with the intended direction of strain. The individual wires are splayed out inside the cone, and the cone is then filled with molten zinc, or now more commonly, an epoxy resin compound.

Eye splice or Flemish eye:-

An eye splice may be used to terminate the loose end of a wire rope when forming a loop. The strands of the end of a wire rope are unwound a certain distance, and plaited back into the wire rope, forming the loop, or an eye, called an eye splice. When this type of rope splice is used specifically on wire rope, it is called a "Molly Hogan"

The ends of individual strands of this eye splice used aboard a cargo ship are served with natural fiber cord after the splicing is complete. This helps protect seaman's hands when handling.