Satish Final Project

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1. CLASSIFICATION OF WELDING

1.0(a) INTRODUCTION

Def: - Welding is a process of joining two or more pieces of the same or dissimilar

materials to achieve complete coalescence. This is the only method of developing monolithic

structures and it is often accomplished by the use of heat or pressure. At times it may be used

as an alternative to casting.

Presently welding is used extensively for fabrication of vastly different components

including critical structures like boilers and pressure vessels, ship, off-shore structures,

bridges, storage tanks and spheres, pipe lines, railway coaches, anchor chains, missile and

rocket parts, nuclear reactors, fertilizers and chemical plants, earth moving equipment, pateand box girders, automobile bodies, press frames and water turbines. Welding is also used in

heavy plate fabrication industries, pipe and tube fabrication, joining drill bits to their shanks,

automobile axles to brake drums, lead wire connection to transistors and diodes, sealing of

containers of explosives like nitro-glycerine and welding of cluster gears.

1.0(b) CLASSIFICATION OF WELDING:-

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1.1 SUBMERGED ARC WELDING: Submerged arc welding is a process in which continuous copper coated spooled wire is

used in conjunction with loose granulated flux poured ahead of the arc so as to provide a

protective a protective media to ward off the atmospheric gases from reacting with the molten

metal pool. The electrode wire diameter may range between 2 and 10 mm. both ac and dc

with electrode positive is the preferred choice.

SAW is mainly used in the download welding position in both automatic and semi-

automatic modes. The former is a more popular mode and asset-up for the same is shown in

figure.

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The weld joint produced by submerged arc welding is of very high quality and

consequently this process finds extensive use in joining thick plates in long, linear seams as

are encountered in ships, pressure vessels, bridges, structural work, welded pipes and nuclear

reactors.

1.2 GAS TUNGSTEN ARC WELDING:

Gas tungsten arc welding(GTAW) or tungsten inert gas(TIG) welding employs a non-

condensable tungsten electrode with an envelope of inert shielding gas(argon, helium etc) to

protect both the electrode employed varies and the weld pool from the detrimental effects of

surrounding atmospheric gases.

Both ac and dc power sources are used for GTAW. The tungsten electrode employed

varies in diameter from 0.5 to 6.5 mm and the current carrying capacity varies accordingly

between 5A and 650A. The welding torch used for carrying current higher than 100A is

normally water cooled. The process is mainly in its manual mode.

GTAW is an all-position welding process and gives the highest quality welds amongst

the commonly employed arc welding processes and is, therefore, extensively used for

welding most of the industrially useful metals and alloys usually in thin grades. Aircraft

industry, rocket and missile fabrications, chemical and nuclear plant fabrications are the

typical user industries of this process.

1.3 GAS METAL ARC WELDING:

In gas metal arc welding (GMAW) process a consumable wire, of 0.8 to 2.4 mmdiameter and wound in spool form, is fed at a preset speed through a welding torch wherein it

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Three types of flames are used in oxy-acetylene welding. The nature of the flame

depends upon the ratio of the two gases. The neutral flame is most often used for welding of

most of the materials like low carbon steels, cast steels, cast iron etc. The oxidizing flame has

higher proportion of oxygen than acetylene and is used for welding of Mn-steel, brass and

bronze whereas the carburizing flame has higher proportion of acetylene in it and is used for

welding aluminium, nickel etc.

The heat transfer to the work in this process is very poor (about 30%) and may lead to

wide HAZ around the weld. The welding speed is also accordingly low. Typical applications

of oxy-acetylene welding include welding of root run in pipe and other multi-run welds, lightfabrications like ventilation air-conditioning ducts, and motor vehicle repairs. A large percent

of general repair work is also done by this process.

1.5 GMAW ARC SPOT WELDING:

Normal GMAW equipment can be used for making spot welds between the lapped

sheets by providing a special torch with a nozzle attached to it. A vented metal nozzle of a

shape to suit the application is fitted to GMAW gun and is pressed against the work piece atthe desired spot. The operation is carried out for a period of 1 to 5 seconds and a slug is

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melted between the parts to be joined. Timing is usually controlled automatically with the

help of a timer.

No joint preparation is required except proper cleaning of the overlapped areas. Argon

and co2 are the shielding gases commonly used for GMAW arc spot welding.

GMAW arc spot welding process can be used most efficiently for down hand welding

position. It can be successfully employed for horizontal position but fails for overhead

welding position.

This process does not require a hole to be made in either member, thus it differs from

plug welding in that respect. As the upper member is required to be melted through and

through its thickness is normally restricted to 3 mm. The thickness of the second member is

not important.

GMAW arc spot welding can be successfully used on aluminium, mild-, low alloy-and stainless steels.

1.6 GTAW ARC SPOT WELDING:

In this process the equipment used is basically the same as for conventional GTAW

except that the control system includes timing device and the torch nozzle is modified to

develop a spot weld at the intended place. GTAW arc spot welding may be done with ac or

DCEN (direct current with electrode negative). DCEN is used for all materials except

aluminium for which ac with continuous superimposition of high frequency current is

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employed. The torch nozzle is made of copper or stainless steel and is often water cooled as

the arc is enclosed completely within the nozzle. The torch nozzle, usually about 12 mm

inside diameter, is provided with venting ports to affect gas flow and escape. The shielding

gas used is either helium or argon with a flow rate of 2.5 to 4.5 lit/min.

To accomplish a spot weld, the arc is initiated by the high frequency discharge for

which the outline circuitry.

Normally no filer metal is used but when required it is fed with the help of special

wire feeder. Filler wire addition improves nugget configuration and helps in overcoming

crater cracking.

This process is mainly used in its semi-automatic mode but it can be mechanized and

even controlled by numerically controlled system to achieve high rate production.

GTAW arc spot welding is widely used in the manufacture of automatic parts and

parts for electronic components and appliances. It is particularly useful for applications where

access to a lap joint can be gained only from one side.

1.7 ELECTRON BEAM WELDING:

In electron beam welding (EBW) a beam of electrons is used to melt for welding. The

electron beam, emitted from a heated filament, is focused on to the desired spot on the work

piece surface with the help of focusing coil. The work piece which is placed in the vacuum

chamber can be moved to create the necessary welding speed.

The penetrating power of the electron beam depends upon the speed electrons which

are controlled by the magnitude of the accelerating voltage. Depending upon the accelerating

voltage the EBW are rated as low voltage and high voltage types with the range of 15-30KV

and 70-150 KV respectively. Below figure shows the schematic representation of the triode

type EBW unit.

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The EBW welds are very narrow and can be of the full penetration type with width to

penetration ratio 1:20 compared with 5:1 of shielded metal arc welding and 2:1 of gas metal

arc welding. The energy density of electron beam being nearly 5*108W/mm2 it is, therefore,

possible to melt and weld any known metal. Due to high energy density of the electron beam

the heat affected zone (HAZ) is extremely narrow and high welding speeds can be reached.

EBW is widely used in the electronics, nuclear, missile and aircraft industries.

Typical applications include cluster gears, intricate valve arrangements made of corrosion

resistant alloys for automobile industry as well as pressure capsules, and missile hull frames.

A portable EBW unit has also been developed for in-flight repair welding of satellites.

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1.8 FLASH BUTT WELDING:

Flash welding is similar to that of resistance butt welding except that it is

accompanied by arching and flashing. Flash welding consists of one fixed and one movable

clamp to hold clamp the work pieces firmly as well as force them together, a heavy dutysingle phase transformer with a single turn secondary, along with equipment to control

welding current, movement of the clamp, force and time. With a voltage of about 10 volts

across the clamps, heavy current flows along the asperities across the contacting faces of

work pieces. As the points of contact are melted and the metal is squeezed out in a shower of

fine molten metal droplets, the contact is broken arching takes place across the gap. Due to

flashing the contaminants from the contacting surfaces are removed and the surfaces are

heated to a uniform temperature. The flash can be removed by subsequent machining. Basic

arrangement for flash butt welding process is shown in figure.

Transformer used for flash butt welding are single phase which can, thus, place an

unbalancing load on normal 3-phase supply from the mains. This necessitates the use of

special transformer which can distribute the load uniformly. In flash butt welding the pieces

to be welded must be held with enough force to avoid slipping and that of the upsetting force

of up ot twice that of the upsetting force. The upset force is around 70MPafor mild steel and

nearly 4 times that for high strength materials.

Flash butt welding is extensively used for welding mid steels, and alloy as well as

non-ferrous metals like aluminium alloys, nimonic alloys (80% Ni+20% Cr) and titanium.

Dissimilar metals may be flash welded if their flashing and upsetting characteristics aresimilar.

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Typical uses of flash butt welding include welding of wheel rims, cylindrical

transformer cases, cylindrical flanges, and seals for power transformer cases. The aircraft

industry utilizes flash butt welding to manufacture landing gears, control assemblies and

hollow propeller gears while the petroleum industry uses oil drilling with fittings attached by

flash welding. Other uses of the process include welding of rails, steel strips, window framesand heavy duty chain links.

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2. HEAT FLOW IN WELDING

All fusion welding processes involve heat flow during welding to accomplish the

desired joint. Depending upon the heating and cooling cycles involved different types of

micro structures are obtained in weld bead and the heat affected zone(HAZ). This leads to

varying mechanical properties of different zones of a weldment, necessitating PWHT (post

weld heat treatment) to obtain uniform structure and the required service behaviour. Apart

from the metallurgical affects of heat flow in there are other phenomena involved including

distortion, residual stress, physical changes and chemical modifications. Thus, to achieve a

weldment of desired specifications to perform satisfactorily in service it is essential to know

the effects of heat during welding. This can well be achieved by knowing the temperature

distribution during welding so as to determine the cooling rates in different directions with

respect to the weld axis.

2.1 Temperature Distribution in Welding

Temperature distribution in welding depends upon the nature of the welding process

used, type of the heat source employed, energy input per unit time, configuration of the joint,

type of joint(butt, fillet, etc.), physical properties of the metal being welded, and the nature of

the surrounding medium i.e. ordinary atmospheric conditions or underwater. Although it

beyond the scope of to analyze all these aspects of the heat flow in detail brief descriptions of

the following cases are included.

(A)Arc Welding

(i) Linear butt welds,

(ii) Circular butt welds,

(iii) Fillet welds.

(B) Resistance Welding

(i) Upset butt welding,

(ii) Spot welding.

(C) Electroslag Welding

(D)Underwater Welding

2.2 Temperature Distribution in Arc Welding

Nearly 90% of welding in world is carried out by one or the other arc welding

process; therefore it is imperative to discuss the problem of temperature distribution in arcwelding in the maximum possible detail to arrive at the best possible understanding of the

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problem. Because linear butt welds are perhaps the most used type of welds in welded

fabrication therefore this type of joint will be detailed the most.

2.3 Temperature Distribution in Linear Butt Welds

Heat flow in welding is mainly due to the heat input by the welding source in a

limited zone, and its subsequent flow into the body of the workpiece by conduction. A

limited amount of heat loss is by way of convection and radiation as well but that can be

accounted for by allotting heat transfer efficiency factor at the accounting of heat input. So,

the problem of temperature distribution can be seen as a case of heat flow by conduction

when the heat input is by a moving heat source.

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2.4 Temperature Distribution in a Semi-infinite Plate (3-Dimensional case)

Considering a case of laying a single weld bead, using a point heat source, on thesurface of a very large and thick plate (work piece), as shown in Fig. 2.5.

Let us assume that the Z-axis is placed in the direction of thickness of the plate

downwards. For determining the temperature distribution the solution of equation must

satisfy the following conditions.

Since welding is done by a point heat source, the heat flux through the surface of the

hemisphere drawn around the source must tend to the value of the total heat, Qp, delivered to

the plate, as the radius of the sphere tends to zero. If R is the radius of the sphere, then the

total heat flowing through the hemispherical surface of the heat source as given by Fouriers

Equation will be,

q = -2(pi) R. k. (dT/dR)

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2.5 Efficiency of Heat Sources

It is evident that to solve the temperature distribution problems we require knowing

the efficiency (n) of the heat source used for welding ; where n is defined as,

n = Energy transferred to the workpiece/Energy generated by the heat source

Thus, if the efficiency (n) of the heat source is known, the energy (Q) transferred form

it to the workpiece, can be determined. In arc, electroslag and electron beam welding.

Q = n V.I

where V and I are the arc voltage and welding current respectively.

In consumable electrode welding processes like SMAW, SAW, GMAW and FCAW,

using deep or a.c. the heat going to both the electrode and the workpiece finally lands on the

workpiece through transfer of molten metal. Thus, the heat transfer efficiencies of these

processes are high. In SAW process, the heat transfer n is further increased because the arc

remains under a blanket of flux, the heat loss to the surroundings is, thus, minimized.

The efficiency of heat transfer in ESW is lower than that in SAW, mainly owing to

heat loss to the water-cooled copper shoes and, to lesser extent, by radiation and convection

from the surface of the molten slag.

In EBW (electron beam welding) process the welds are produced by the phenomenon

of key holing. These keyholes act like black bodies to the heat source and trap most of its

energy; leading to very high efficiency of heat transfer in EBW process.

Heat Source Efficiencies of Arc, Beam and Flame Welding Processes

S.no Welding Processes Efficiency, n (%)

1. Gas tungsten arc welding (GTAW)

(a) dcen

(b) a.c.

2. Shielded metal arc welding (SMAW)

3. Gas metal arc welding (GMAW)

4. Submerged arc welding (SAW)

5. Electroslag welding (ESW)

6. Electron beam welding (EBW)

7. Laser beam welding (LBW)

8. Oxy-acetylene gas welding (OAW)

50-80

20-50

65-85

65-85

80-99

55-82

80-95

0.5-70

25-80

2.6 Weld Characteristic

With the processes in which heat is used, the pattern of energy conversion to heat and

its subsequent dissipation after welding is a major factor influencing the utilization of theprocess and the properties of the joint. These factors influence,

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(i) The welding speed, and

(ii) The size of the heat affected zone.

With high heat input conditions, allowing high welding speeds, heat dissipated in the

workpiece is minimized but never be reduced below half the total heat available.Inefficient processes or processes use at relatively low speeds result in losses three or

four times greater than this. A measure of the efficiency of utilization of heat is given

for the 2-dimensional heat flow case by weld characteristic devised by wells. For

fusion welds this is a non-dimensional term Vd/4a encountered in equations. Thus,

Weld characteristic (W.C.) = Vd/4a

Where, V = weld speed, mm/sec

d = melted width, mm

a = thermal diffusivity, mm2/sec.

2.7 Weld Bead Dimensions

Based on Rosenthals equation for 3-d heat flow Christensen et al have derived

theoretical relationships between the weld dimensions and the welding conditions, using

dimensionless parameters D and n which are related to each other as shown in Fig. 2.7, and

can be expressed by the following relationships.

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Dimensionless depth, D = p.v/2a

and dimensions operating parameter, n = Qp.v/ (4.pi.a2pc(Tm-To))

where, p = weld penetration,

Tm = welding point of work material.

2.8 Heat Flow in Fillet Welds

A simple approach to analyze heat flow in T-type fillet welds is to assume that the

total heat supplied from the arc is distributed in the three plates in the ration of their

thickness. Temperature distribution in the three plates forming the fillet joint can then be

determined individually with the help of forming the filet joint can then be determined

individually with the help of formulas used for determining temperature distribution for

laying bead-on-plate on moderately thick plates. This approach implies that if three plateshave equal thicknesses, temperatures at points equi-distant from the centre of the weld should

be same in the three plates. This, however, does not hold good at early stages of heat flow

from the weld centre though all three heat distributions approach similarity as the time

passes. This leads to a conclusion that the bead-on-plate analysis can be applied successfully

to fillet welds for determining temperature distribution except (i) at the early stages of

welding, and (ii) o the points close to the arc. The deviation is large when the arc is passing

just over the point under consideration. However, it decreases and ultimately the temperature

distributions show no differences as the time passes, as shown in fig.

Such a deviation at earlier stages of welding fillet joint can be accommodated byintroducing a factor which approaches unity with time. If equation is multiplied by this factor,

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the temperature deviations at earlier stages can be duly accounted for finding the true

temperature distribution in fillet welds. For this purpose an exponentially varying factor of

the form (1-Ae-Bt) is considered most appropriate where A and B are constants, the

magnitudes of which depend upon the thickness ratios of three plates, etc. If equation is

expressed in a simple form as,

(T-To) b = Q f (E,y,z)

2.9 Heat Flow in Upset Butt Welding

In general the welding processes require that a certain temperature be reached and

maintained long enough for the weld to be completed. For example, in upset or resistance

butt welding the aim is for an interface temperature of the order of the solidus temperature,

Ts, of the work material. This temperature when reached needs to be held only until the oxide

layer at the interface has been dislodged by fragmentation or diffusion, or until sufficient

lengths of the work pieces have been heated to permit upsetting.

Let us consider upset butt welding of two round bars of steel, each of length l and

cross-sectional area A. Assume that the interface is raised instantaneously to the solidus

temperature, Ts, and that the opposite ends of bar are held at room temperature. To determine

the temperature variation, along the length of the bar, with time let us consider an element of

thickness dx of the bar centered x cm from the interface as shown in fig. Assume that the

interface has been at temperature Ts long enough so that the element has a temperature above

the room temperature, To, but has not admitted as yet its steady state temperature.

Applying Fouriers equation, the heat flow at section x is given by,

q = -kA [d0/dx]

where, 0 = T-To,

k = thermal conductivity of bar material which is assumed to be independent of

temperature, T.

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2.10 The Effect of Position of Head Surface

The heat transfer coefficient, h, and the heat flux [q/a] depend, to a great extent, upon

the conditions in which the generated vapours separate from the heated surface. These

conditions are most favorable in the case of horizontal heated surfaces, the heated side of thesurface facing upwards. The aforementioned equations hold good for such conditions. If the

heated side of the work faces downwards, the conditions in which vapours separate from the

surface deteriorate sharply and the peak heat flux diminishes by as much as 40%. This is

because the motion of the fluid is only in a thin layer underneath the work; the rest of the

fluid below that layer remains stationary.

Fig. represents the commonly accepted nature of convection currents above and below

a horizontally placed heated flat plate.

2.11 Metallurgical Effects of Heat Flow in Welding

Using equations given in the earlier sections it is possible to determine temperature at

any given point during quasi-stationary state of welding and from such a data it is possible to

draw thermal histories for any point of interest. If sufficient number of such thermal histories

are known for different points along a transverse section with respect to the weld centerline

then such thermal histories can be utilized to draw isotherms for different temperatures

keeping the weld pool as the innermost isotherm representing the solidus temperature of the

material being welded as shown for slow and fast welding in Fig. 2.23.

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2.12 Experimental Determination of Cooling Rates in Welding

Take a 16mm thick steel plate of sufficient width and length (say 300mm x 300mm)so that quasi-stationary state will be established after welding has proceeded through a length

of 50mm. Mark it on bottom side. Drill 3-4mm deep holes with 1mm diameter drill bit at

points 1,2,,6. Imbed the hot junctions of Alumel-chromel thermocouple in these holes

which are filled up with high temperature brazing material-using oxy-acetylene brazing torch.

The other ends of these thermocouples are connected to temperature recorders for recording

thermal histories of these points during welding on the top side.

2.13 Critical Cooling Rate

It is the fastest rate at which steel can be cooled without the appearance of martensite,or stated conversely, it is the slowest rate at which the steel can be cooled that will still

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temperature. Thus, for example, the austenite in a medium carbon steel break down to ferrite

and pearlite in the temperature range of 700 to 500o C, whereas if the cooling rate is

somewhat higher, pearlite formation can be partly or wholly suppressed, and bainite will

form at a lower temperature. At extreme cooling rates, even bainite formation can be

suppressed, and martensite will form at a still lower temperature.

2.16 Continuous Cooling Transformation Diagrams (CCT

Curves)

A CCT diagram is a record of the transformation behavior of that steel under continuous

cooling conditions which can be correlated fairly closely with the kind of continuous cooling

occurring in the vicinity of a weld. From such diagram it is possible to determine whether or

not martensite or brittle structure is likely to form under given welding conditions. The

farther to the right and lower the curves on the diagram the more hardenable the steel and

more difficult the welding.

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Successful welding of materials of high thermal conductivity and thick sections in any

martial or the welding of hardenable steels requires a controlled rat of cooling because when

heated to a high temperature during welding and cooled rapidly thereafter, they harden.

Welding under such conditions without du control of cooling rate may produce embrittlement

in HAZ (heat affected zone) parallel to the weld joint. With proper preheating the rate of

cooling is reduced and consequently the metal in and around the weld bead do not harden.

The preheat temperature for arc welding is the temperature at which the work piece must be

maintained and below which it must not fall until the welding is complete.

If steel is welded without preheat than the total drop in temperature will be fromabout 1540 degrees to about 30degrees (room temperature) i.e. about 1500degrees. In case it

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is preheated, to say 300degrees, the drop will be reduced to about 1200degrees. This results

in reduced cooling rates particularly at the important intermediate temperatures 800 to

500degrees.In multi-run welds the succeeding bead may be deposited on metal that has been

preheated (by the preceding beads). The more rapidly the beads are deposited on each other,

the higher, obviously, is the preheat or interposes temperature.

PWHT (post weld heat treatment) is intended primarily as a stress-relief

treatment. For welding most of the high carbon as well as the high alloy steels post heating is

as important as, if not more, preheating. Although preheating does control the residual

stresses to develop and approach the dangerous level affecting the service life of a

component. If due attention is not paid to these aspects of preheating and PWHT, it may lead

to a catastrophic failure of a welded component or structure.

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3. PREHEAT AND POSTWELD HEAT TREATMENT

3.1 AIMS OF PREHEATING: -

Apart from what is stated above, the main aims of preheating are:

1. To reduce the heat losses from the weld area, that in turn reduces the cooling

rate of the weld,

2. To reduce cracking by preventing the formation of hard surfaces(due to the

formation of martensite in the case of steels),

3. To reduce the expansion and contraction rates thus reducing distortion and

residual stresses,

4. To burn grease, oil, and scale from the joint area leading to faster welding

speeds,

5. Preheating also keeps the weld beads more fluid with flatter surfaces thus

avoids stress concentration due to notch effect,

6. Preheating also brings some steels above the temperature where brittle fracture

might occur during welding,

7. To help allow sufficient time for hydrogen to diffuse out of the weld and HAZ.

This may also modify the microstructure of the weld, making it less susceptible

to H2-embrittlement.

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3.2 Electrical Resistance Heating

It is generally recognised that the electrical resistance heating methods offer greater

flexibility in meeting the requirements of temperature distribution and uniformity over a wide

range of preheating applications. In addition, electrical resistance systems are portable, and

attractive in terms of capital cost. These units when used in their automatic mode reduce the

labour costs considerably.

3.3 Power Sources

Specialized power sources with solid-state switching are now the norm in resistance heating

units. When these power sources are used in conjunction with variable voltage control they

prove to be ideal for unusual shapes and sizes. Certain industries such as aero-engineering

and nuclear steam generation have greatly benefitted from the development of variable

voltage facility with their considerably reduced operating and maintenance costs. These

power sources range between 8 KVA transformation for pipe butt welds and 132 KVA heat

treatment units for on site simultaneous multi preheat operations over a small site area withboth fixed and variable voltage.

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3.4 Temperature Control

Open flame techniques are still widely practiced although it is known that these are the least

satisfactory from the temperature control point of view. For improved control of preheat

temperature set-point temperature controllers are used. There are different types of these

controllers viz., one called on-off type monitors the temperature from the thermocouple and

compares, it to the set temperatureturning the heater on if it is below and off if it is

above that temperature. Most reliable system, however, employs solid-state switching with

phase control which supplies only that amount of power which is required to keep it at a

constant temperature. Still another is called burst fire control which involves switching on the

thyristor when the supply voltage is at zero and switching off after a number of supply cycles.

With proper choice of cycle time this unit can be used for accurate control of preheat

temperature even for thin components like thin wall boiler tubes.

3.5 Applications

The resistance heating process is particularly advantageous for the welding of pipes,

axles and similar other shapes. Figure shows a wraparound resistance heater used for

preheating pipes. It consists of a nichrome wire element embedded in 25 mm thick ceramic

fibre blanket which is housed in a stainless steel shell.

Preheating for Rotating weld seams : Electrical resistance heating elements

cannot readily be fitted directly onto the work piece where fixed head, automatic welding

system requires component rotation, as for circumferential welds in pressure vessels. Under

these circumstances, it is more convenient to provide non-flexible heating units at the lower

part of the circumference away from the component, with transfer primarily by radiation, as

shown fig. the heating unit may be either electrical resistance panels, manufactured as a strip

or a coil element design, or surface combustion gas radiant panels operating on LPG

(liquefied petroleum gas) or natural gas.

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3.6 ADVANTAGES AND LIMITATIONS OF PREHEATINGS METHODES

1. Flame heating

2. Electric Resistance Heating

3. Induction Heating

4. Gas-Flame Generated Infrared Ray

Heating

5. Quartz Lamp Heating

1. Flame Heating

The advantages are low cost and portability while the drawbacks are: minimal

precision and repeatability, non-uniform temperature distribution, and the need for operator

skill.

2. Electric Resistance Heating

Advantages of this process are:

(i) Continuous and even heat can be maintained throughout the welding operation and

during long breaks;

(ii) Temperature can be adjusted quickly and accurately;

(iii) Welders can work in relative comfort and they need not stop to raise the preheat

temperature;

Uneven heat can be obtained easily. That includes the heat needed for the top

and bottom halves of a pipe or where pipes are attached to heavier sections as

in valves.

Limitations of the process include,

1. element may burnout during preheating process,

2. a resistance element may short itself out to the pipe producing arc

spot which can be the cause for crack initiation in service.

3. Induction Heating

With induction heating, high heating rates are possible;

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Temperatures can be controlled within narrow ranges; Local hot spots can be avoided more

easily; and the coils last for a long time.

Amongst the limitations of induction preheating are:

(iv) High initial cost,

(v) Equipment is bulky and thus not easily portable,

(vi) Provides limited means to compensate for non-uniform wall thickness and

geometries as for different energy requirements of the top and bottom of a

pipe in the horizontal position,

(vii) The power has to be turned off during welding,

(viii) Extra set up required if extra coils are needed.

4. Gas-Flame generated Infrared Ray Heating

This process uses economical fuel and suitable control equipment is also available

for it. Also, no insulation is used in the area to be heated.

The only disadvantage is that separate furnaces have to be used.

5. Quartz-Lamp Heating

The advantages of quartz lamp heating are fast response time, efficiency, cleanliness,

fast cooling down and quick turn around.

The limitation of this process is a high initial equipment cost. Also, quartz lamps are

fragile and sensitive to contamination. When employed for preheating pipes separate furnaces

must either be fabricated or made available for each pipe diameter.

3.7 Preheating Of Carbon Steels

Low carbon steels (_


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a material with a notch impact value below 8-10 J. it is, therefore, recommended to refer to

the impact value curve for the material when it is to be fabricated into a structure under

severe stress condition. Even a temperature increase of 10degrees, say from -20to-10degrees

can, at times, make a considerable improvement in the service behaviour of the component.

3.8 POSTWELD HEAT TREATMENT (PWHT)

Post-weld heat treatment is intended primarily as a stress-relief treatment. Forwelding some of the higher range carbon steels, PWHT is as important as preheating.

Although preheating control the cooling rate but the development of the residual stresses

always remains a possibility. Unless these stresses are removed cracks may develop when the

work piece is cooled to the room temperature, otherwise the part may be distorted, especially

after the machining operation, if employed. Apart from stress relieving PWHT can also

accomplish stabilization as well as reformation of structure of weldment.

3.8.1 POSTWELD STRESS RELIEVING:

The residual stresses developed due to welding can be quite severe and may lead to

failure when the component is put into service. The simple solution, if practicable, is to

anneal the whole weldment but this is often not practical due to size and shape of the

fabricated component. Full annealing may also result in grain growth with consequential

reduction in mechanical strength of the component. To overcome this difficulty the following

alternatives are usually practiced.

1. Sub-critical or low temperature annealing, and

2. Local low temperature stress-relieving heat treatment in and around the

weld.

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However, local heating has to be done very carefully, to avoid uneven heating as

well as overheating.

3.8.2 STABIISATION OF STRUCTURE:

The rapid cooling associated with welding often leads to metastable structure whichmay change with time, due to natural ageing, with possibility of embrittlement of the

structure. Full annealing is a simple method to homogenize the structures but that may cause

grain growth, coarsening of the metallurgical structure and undesirable loss of mechanical

properties in other parts of the component. The alternative PWHT method for materials like

low carbon and low alloy steels is to normalize the component to give a relatively stress-free

and comparatively stable structure.

However, if a proper annealing or normalizing operation is not possible, it may be

necessary to use a sub-critical PWHT to give as uniform structure as possible; avoiding at thesame time grain growth or precipitation of a brittle phase.

3.8.3 REFORMATION OF STRUCTURE:

Post weld reformation of the structure of materials such as mild steel and low alloy

steels is often desirable and is relatively easy to affect by normalizing. An undesirable

elevated temperature precipitate, such as chromium carbide in austenitic stainless steel, can

sometimes be redissolved by heating the material into appropriate temperature range, higher

than that which caused the original trouble, and then cooling it rapidly.

With certain other examples, a totally different approach may be needed. For

example, to eliminate martensite in the vicinity of a weld in a high manganese austenitic

steel, it is desirable to heat the weldment to about 800 oC, into the fully austenitic condition,

and then quench in water to prevent martensite transformation.

It is evident from the above description that to achieve the desired quality weld

different PWHTs are required depending upon the material of the structure involved. Some of

the typical PWHT processes used extensively in the fabrication industry include the

following.

1. Annealing

2. Sub-critical annealing

3. Normalizing

4. Stress relieving

5. Quench annealing

6. Hardening and tempering

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PREHEAT TEMPERATURE FOR WELDING WITH

BASIC COATED ELECTRODES

For Basic Electrode Operating (preheating)temperature in oC for different joints and

plate thickness in mm

Jominy

distance

Electrode

dia

1st run in U or V butts Fillets

Mm mm 13 25 38 50 13 25 38 50

3 1.6

2.0

2.4

3.15

4

5

5.8

0

0

0

0

0

0

0

0

0

0

0

0

0

0

100

0

0

0

0

0

0

150

100

0

0

0

0

0

0

0

0

0

0

0

0

100

100

0

0

0

0

0

150

150

100

0

0

0

0

150

150

100

100

0

0

0

4 1.6

2.0

2.4

3.15

4

5

5.8

0

0

0

0

0

0

0

100

100

0

0

0

0

0

150

100

100

0

0

0

0

200

150

100

100

0

0

0

100

0

0

0

0

0

0

150

100

0

0

0

0

0

150

150

100

100

0

0

0

200

200

150

100

100

0

0

5 1.6

2.0

2.4

3.15

4

5

5.8

0

0

0

0

0

0

0

150

150

100

0

0

0

0

200

150

150

100

0

0

0

250

200

150

150

0

0

0

150

100

0

0

0

0

0

200

200

150

100

0

0

0

250

250

200

150

150

100

0

300

250

200

200

150

100

100

6 1.6

2.0

2.4

3.15

150

0

0

0

250

200

150

100

300

250

200

200

400

300

250

200

200

200

100

0

300

300

250

200

350

350

300

250

350

350

300

300

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PRE HEAT TEMPERATURE FOR WELDING WITHORGANIC NEUTRAL AND AC COATED ELECTRODES

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For organic

neutral and

acidic electrode

Operating (preheating)temperature in oC for

different joints and plate thickness in mm

Jominy

distance

Electrod

e dia

1st run in U or V butts Fillets

Mm mm 13 25 38 50 13 25 38 50

3 1.6

2.0

2.4

3.15

4.0

5.0

5.8

6.3

8.0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

10

0

0

0

0

0

0

0

0

0

150

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

100

0

0

0

0

0

0

0

0

150

100

0

0

0

0

0

0

0

150

150

100

0

0

0

0

0

0

4 1.6

2.0

2.4

3.15

4.0

5.0

5.8

6.3

8.0

0

0

0

0

0

0

0

0

0

100

0

0

0

0

0

0

0

0

150

10

0

0

0

0

0

0

0

0

100

100

100

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

150

100

0

0

0

0

0

0

0

200

150

100

100

0

0

0

0

0

200

150

150

100

100

0

0

0

0

5 1.6

2.0

2.4

0

0

0

15

0

10

0

20

0

15

0

200

200

150

150

100

0

200

200

150

250

200

200

250

250

200

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3.15

4.0

5.0

5.8

6.3

8.0

0

0

0

0

0

0

0

0

0

0

0

0

0

10

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

100

0

0

0

0

0

150

100

0

0

0

0

150

150

100

0

0

0

6 1.6

2.0

2.4

3.15

4.0

5.0

5.8

6.3

8.0

100

0

0

0

0

0

0

0

0

25

0

20

0

15

0

0

0

0

0

0

0

30

0

25

0

20

0

15

0

10

0

0

0

0

0

300

250

200

150

100

0

0

0

0

200

150

100

0

0

0

0

0

0

300

250

200

150

100

0

0

0

0

350

300

250

200

150

100

0

0

0

350

350

300

250

200

150

100

0

0

7 1.6

2.0

2.4

3.15

4.0

5.0

5.8

6.3

150

0

0

0

0

0

0

0

30

0

25

0

20

0

10

0

0

0

35

0

30

0

25

0

20

0

15

0

350

300

300

250

200

100

0

0

250

200

100

0

0

0

0

0

350

300

300

200

150

100

0

0

400

350

350

250

200

150

100

0

450

400

350

300

250

200

150

100

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5. FATIGUE STRENGTH

Fatigue of metals is very misunderstood if only because the very word fatigue is so

inappropriate. It implies an exhaustion of some type, the loss properties, when it is in fact

nothing of the sort. The name arose in the nineteen century from very inadequateobservations and has been used ever since. What we call fatigue in metal is the starting and

progression of cracks under the fluctuating loads. We often read hairline cracks under. This

can mean any sort of crack, whether it be in a tea cup or power station. The reader is here

advised to cast aside all previous nations and build up a picture for himself on the following

fundamentals.

1. Neither metals nor the weld joining them are as smooth as they look, they have pits,

grooves and cracks.

2. Metal tear or fracture when they are stretched more than a certain amount.

3. The pits, grooves and cracks in a metal under load cause high strains over very small

areas. Fluctuating loads will create small tears or fractures, which increase in length with

each application of the load- a sort ratchet effect develops because the metal around the

crack is stretched and compressed plastically at each load application. The surrounding bulk

of metal will control the amounts by which the crack can open.

At this stage the crack must be thought of as bending so small that it does not affect the

overall strength of component in the same way that an oil hole in the shaft or drainage hole ingirder will not influence strength of those items. It is only when the fatigue crack become

large enough to affect the component strength that we can start talking about failure.

Conventionally a fatigue failure occurs

When the crack has reduced The section so much that the tensile strength of material is

reached on remaining section or the crack is large enough to start brittle fracture. In certain

products a fatigue crack can undesirable when its affects the operation of a unit without

causing fracture. For example a fatigue crack which goes through the wall of the tank, pipe

line. or pressure vessel cause leakage even though there is no structural collapse. A crack

can change the stiffness of structure and make it to resonate at loading frequencies it was

designed to avoid. The last two effects are, in fact turned to advantage in some areas. The

presences of cracks in helicopter rotor blades and the tubular jibs of very large dragilines is

detected by pressurizing the hollow members and measuring pressure drops which occur if

fatigue cracks are present. It has been proposed that cracks in offshore platforms could be

detected by monitoring the natural frequencies of the platform which may change if cracks

occur.

The rate at which a crack spreads propagates is the commonly used word, depends on

the size of the stress fluctuations in the cracked member and size of crack itself. These

combine to produce the amount of plastic deformation, or stretch, at crack tip.

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If, for the sake of argument, we assume that in welded components the cracks and groove are

all much the same size start with, the only important effect is the stress fluctuaction. The

stress that the metal sees is the uniform stress magnified by any local shapes changes, and

the degree of magnification is called the stress concentration, caused by holes, grooves, steps

and, course, welds.

The number of stress fluctuation which member will stand before failure is called the

fatigue failure. This can be described by more clearly in a diagram showing the variation of

stress with time, such might occur in a hydraulic press or conveyor pulley.

The stress in the press starts at zero and increasing as the pressing, or forming operation

proceeds and then dies away as the pressure is reduced. At any particular point on pulley

shaft the stress changes from tension to compression as it rotates.

In the press the stress range is equal to the maximum stress where is in the pulley the stress

range is twice the maximum stress. In welded components it is the stress range which decide

the fatigue life and is , therefore, very important. The relation between stress range and

fatigue life is most simply shown as graph produced from experimental results.

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Such graph is called an SN curve or wohlerline. At very low stress ranges we might find that

the pulleys did not fail at all so there is cut-off, shown dotted on the SN curve. At the high

stress we run into region where the performance of pulley is limited by the yielding of the

steel so we have an upper limit beyond which this type of component cannot go. In practice,

of course, the maximum allowable design stress is lower than yield stress. In structure like

the hydraulic press there can be local stress concentrations, and although the main body of

structure will be working below the allowable design stress concentration working well above

that level, so we need to know the hole of the SN curve.

Since longest life will be obtained with the lowest stress the best design will have the

minimum of stress concentration and the hole structure will be evenly stressed. It is

impossible to avoid all stress concentrations and the most difficult ones to avoid arise from

welded joints themselves. Indeed different types of weld joints have differing degrees of

stress concentration and their relative fatigue lives are not always in the most obvious order.

For example a butt weld in plate might be expected to have a shorter fatigue life than an

unjointed plate with a welder bracket carrying no load. Unfortunately, if butt welded plate

with a welded bracket weld. The crack will start at the toe of the weld and spread into plate

until failure occurs.

A whole range of welded joints have been analysed to produce design SN curves. Joints

having similar fatigue performance are grouped under categories to make life easier for

designer. The answer is not however clear cut.

Every specimen is slightly different as is item from a production run and the fatigue test

results shows a great deal 0f scatter. A mean line can be drawn through test results but for

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practical purpose the designer needs to be sure that only an acceptable proportion of his

products will fail before the design life.

It is sometimes the practice to divide the mean life by a factor to make sure that early fatigue

failures do not occur. This is rather crude device and a more refined method is used in which

the test results, and service experience, are analysed to give SN lines with certain probability

of failure. The mean line gives a situation where 50% of the components designed for that

life will have failed by the time that life has been reached. Any other line can be drawn with

other probabilities of survival.

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6. BRITTLE FRACTURE

In any manufactured components there may be a chance of section, holes or welded

joints containing cracks or other defects. These features cause local magnification of stress.

This can be measured by various techniques.

Although we may say that the design stress is below the yield stress, there will be then be

areas where the stress is higher and there will even be areas which have yielded under the

first load. For example, the stress alongside a round hole in a plate is theoretically three times

that in that in the plain part of the plate, so with design stress of 2/3 yield stress, the stress

alongside the hole exceeds yield, but does not reach three times the yield stress because

plastic strain occurs at the edge of the hole and the stress remains at the yield stress.

In the same way that the tensile test piece necks or thins so lateral concentration

occurs at the edge of the hole to allow plastic flow to, happen.

NOTE: Very high strength steels and other materials do not have a clear yield point so

special care has to be taken in design. The plastic behaviour of mild steels is great and

unappreciated benefit to the engineer.

Cracks and other very sharp notches produce much greater magnification of stress than that

produced by a round hole and so plastic strain at the tip of the crack sets in a correspondingly

lower working stress. The plastic strain occurs over a very small area and becomes high. At

this level of strain in simple tensile test specimen there would be extensive necking or

thinning but at the tip of a crack the small plastic zone is surrounded by material which is still

under elastic stress and maintains its original shape. This restrains the plastic material and

prevents lateral contraction so that high stresses are setup in all dimensions tri-axial stressing.

There comes a point when steel cannot support these high stress and fractures very suddenly,

this fracture travels at high speed through the metal and is called brittle fracture. The amount

of stretch, which can be accepted by a metal at the tip of notch before fracture measure of its

notch ductility or fracture toughness and can be measured in laboratory. There are some

metals which have such high fracture toughness that they will not fracture suddenly but

merely tear from notch; others may be so lacking in fracture toughness that, like glass they

will break at the notch in a brittle way at low stress. Between these two limits of behaviour,structural steel can exhibit a variety of levels of fracture toughness. What is particularly

important is that the same piece of steel can have high fracture at room temperature. The

stress strains at the notch tip are functions of the notch shape and size. This means that the

level of notch ductility required to prevent brittle fracture is controlled by the size and type of

notch. It is convenient to summarise at this point the features which can influence the risk of

brittle fracture.

(a) Fracture toughness

(b) Notch size

(c) Tensile stress

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In the weld structure we have the possibility of notches in the form of weld defects

and we have high stress in and around the weld occurring as the sum of the residual stress

due to welding and the applied load. The welding operations may have reduced the

fracture toughness of the parent material in the heat affected zone and fracture toughness

of the weld metal depends on the composition of the consumable and welding procedure.

It will be apparent that this subject can be very complicated, and for many

applications the risk of brittle fracture are controlled by selecting materials on the basis of

a simple test, which measure the toughness of the material. This test is the charpy test;

samples of the steel are taken and cut into 16mm square bars which have a notch cut into

one side. These are placed in a machine in which a pendulum swings into specimen is

measured as the loss of height by the pendulum. This is simply read on on a calibrated

scale in foot pounds or joules and is measure of the fracture toughness of the steel from

which the sample charpy energy at various temperature immediately prior to testing.

The values of charpy energy and temperature required for a particular application depends

on the stressing, the thickness and strength of the steel, and minimum service

temperature.

For components which are to have purely compressive or low tensile service stresses.

i.e. less than 30% of yield stress, it may be unnecessary to call for charpy tests as the

requirements are so low as to be satisfied by almost any steel. Similarly the weldable

carbon and carbon manganese steel have sufficient fracture toughness to permit their use

in many products without charpy tests up to about 16mm thickness at temperatures down

to -200

C.

In some types of product it is necessary to carry out charpy tests on samples of the welded

joints as well as the parent material to ensure that they have sufficient fracture toughness

in the heat affected zone and weld metal.

Standard specifications for various products give the minimum charpy energy and the test

temperature which must appear in the steel specification and the designer must comply

with such specification, or, if his product is not covered by a specification , he should

seek specialist advice.

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7.0 WELDING CHARACTERSTICS

Fusion Weld ZoneFusion Weld Zone

Fig : Characteristics of a

typical fusion weld

zone in oxyfuel gas

and arc welding.

Solidification of Weld metalSolidification of Weld metal

Solidification begins with formation of columnar

grains which is similar to casting

Grains relatively long and form parallel to the heat

flow

Grain structure and size depend on the specific

alloy

Weld metal has a cast structure because it has

cooled slowly, it has grain structure

Results depends on alloys ,composition and

thermal cycling to which the joint is subjected.

Pre-heating is important for metals having high

thermal conductivit

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Heat affected ZoneHeat affected Zone

Heat effected zone is within the metal itself

Properties depend on

Rate of heat input and cooling

Temperature to which the zone was raised

Original grain size ,Grain orientation , Degree of prior cold

work

The strength and hardness depend partly on how originalstrength and hardness of the base metal was developed

prior to the welding

Heat applied during welding Recrystallises elongated

grains of cold worked base metal

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Destructive Techniques

Tension Test :

Longitudinal and transverse tension tests are performed

Stress strain curves are obtained

Bend test :

Determines ductility and strength of welded joints.

The welded specimen is bend around a fixture The specimens are tested in three-point transverse bending

These tests help to determine the relative ductility andstrength of the welded joints

Other destructive testing Fracture Toughness Test:

Corrosion and creep tests

Testing of spot welds

Tension-Hear

Cross-tension

Twist

Peel

Non-Destructive testing : Often weld structures need to be tested Non-Destructively

Non-Destructive testing are :

Visual

Radiographic

Magnetic-particle

Liquid-penetrant

Ultrasonic

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TENSILE STRENGTH TEST

a. This test is used to measure the strength of a welded joint. The width thickness of the test

specimen are measured before testing, and the area in square inches is calculated by

multiplying these before testing , and the area in square inches is calculated by multiplying

these two figures. The tensile test specimen is then mounted in a machine that will exertenough pull on the piece to break the specimen. The testing machining may be either a

stationary or a portable type. A machine of the portable type, operating on the hydraulic

principle and capable of pulling as well as bending test specimens, is shown in figure. As the

specimen is being tested in this machine, the load in pounds is registered on the gauge. In the

stationary types, the load applied may be registered on a balancing beam. In either case, the

load at the point of breaking is recorded. Test specimens broken by the tensile strength test

are shown in figure.

b. The tensile strength, which is defined as stress in pounds per square inch, is calculated by

dividing the breaking load of the test piece by the original cross section area of the specimen.

The usual requirements for the tensile strength of welds are that the specimen shall pull not

less than 90 percent of the base metal tensile strength.

c. The shearing strength of transverse and longitudinal fillet welds is determined by tensile

stress on the test specimens. The width of the specimen is measured in inches. The specimen

is ruptured under tensile load, and the maximum load in pounds is determined. The

shearing strength of the weld in pounds per linear inch is determined by dividing

the maximum load by the length of fillet weld that ruptured. The shearing strength inpounds per square inch is obtained by dividing the shearing strength in pounds per linear inch

by the average throat dimension of the weld in inches. The test specimens are made wider

than required and machined down to size.

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Weld Tension test Welding results in metallurgical (and often compositional) differences in

the weld joint, and it is important to know the effects of these changes on, mechanical

properties. The tension testing of welds is somewhat more involved than for base metal

because the weld test section is heterogeneous in nature, composed of the deposited weld

metal, the HAZ and the unaffected base metal. Tensile test specimen can be either transverse

or longitudinal depends on the loading on the welded joint.In tension test, strength,

elongation and reduction area are of primary importance.

If the weld metal strength exceeds that of the base metal, most of the plastic strain occurs in

the base metal, with resultant necking (local reduction in area of the cross-section by

stretching) and failure outside of the area. In such a case, the test does not give an indication

of the weld ductility. When the weld strength is considerably lower than that of the base

metal, most of the plastic strain occurs in the weld. Transverse weld specimens may provide a

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measure of joint efficiency in terms of strength, but do not provide a good ductility

measurement of the weld. But, however, transverse specimen is generally used.

Tensile testsare used to compare the weldment to the base metal mechanical valuesand specification requirements. The weldment is sliced into coupons, and then each end of

the coupon is pulled in opposite directions until the coupon fails (breaks). A tensile test

machine is shown in Figure. Tensile tests are made to determine the following: Ultimate strength of the weld. This is the point at which the weld fails under tension.

Yield strength of the weld. This is the point at which the weld yields or stretches under

tension and will not return to its original dimensions.

Elongation. This is the amount of stretch that occurs during the tensile test. It is measured

by

placing gauge marks on the sample or coupon before testing and comparing the after-break

distance with the original gauge marks.

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Residual Stresses:

Caused because of localized heating and coolingduring welding, expansion and contraction of theweld area causes residual stresses in the work

piece.

Distortion,Warping and buckling of welded parts

Stress corrosion cracking

Further distortion if a portion of the weldedstructure is subsequently removed

Reduced fatigue life

Stress relieving of welds :

Preheating reduces reduces problems caused by

preheating the base metal or the parts to be welded

Heating can be done electrically,in furnace,for thinsurfaces radiant lamp or hot air blast

Some other methods of stress relieving : Peening,

hammering or surface rolling

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Root and Face bend tests

Root and face bend tests are another simple low cost method of testing. It gives very simple

to understand results and will show any signs of poor fusion or weaknesses such as porosity

within the weld. There are numerous variations on this method, we will look at one of the

most simplest methods.

Whether the sample piece is bent root up or root down decides whether it is a root or face

bend test, with the root on the outside of the bend, in tension that would be a root bend test.

Once a suitable section of the weld is selected, it is prepared for testing. The test piece is then

put into a bending jig and force applied to it directly over the welded area. The piece shouldbend around without cracking. A crack would show a weakness of the weld. A neatly bent

strip would show the weld is as strong as the parent metal. This is another testing method that

is suited to students learning welding due to its ease and low cost. When used in recorded

circumstances, a test procedure would be issued, specifying the details such as radius of the

punch used and degrees it needs to be bent to. Another version of these tests is a side bend, as

pictured below.

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Preparing

The first step is to select a suitable section to test. These are best taken from a start/stop

position, central 50mm of a butt joint, and not directly from either end of a larger run, the

sides of the strip to be tested need to be fully welded and square. Remove any high spots of

the weld before cutting you test strip. It is also a good time to mark the root and face side,

either marker pen, or by hard stamping.

Cutting the strip can be done by almost any process, sawing, flame cutting, or abrasive disc.

The strip needs to be narrow enough for you to be able to exert the force, around 25mm wide

seems to work well for most simple tests.

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Cut the strip, and de-burr the edges, taking extra care on long edges, this is an important step,

or it may create false results caused by normal bending cracks that occur at sharp edges. The

test piece should be allowed to cool naturally between each step, and not quenched, as this

can make the heat affected zone (HAZ) brittle.

Bending

Once the test strip is prepared, you can set up your bending jig. A small hydraulic press or fly

press is adequate for this purpose. The top punch should have a radius of approximately twice

that of the material thickness being tested. The die needs to have sufficient clearance to allow

the top punch to pass through it, plus twice the material thickness. For example, for testing a

5mm thick sample, you would use a punch with a radius of 10mm (diameter of 20mm). Then

the bottom needs an opening of 30mm.

Align the test strip in the tool, with the weld centralised between the bottom die, then apply

pressure to bend it to 90degrees, further if you so wish.

The results

This is fairly self explanatory really, if the weld doesnt break, or show signs of cracking, its

good. The strip should show a nice even radius, if it has a sharp angle change mid way, this

would be down to the weld beginning to fail. If the weld does break during testing, you will

need to see where it has broken.

If the strip broke into two, with virtually the entire weld still on one side, that would point to

undercut, or poor fusion.

If the weld had broken into two, look carefully, you may see signs of a slag inclusion, or

porosity, which has weakened the weld in-line with the defect.

The images below show an example of a passed bend test, the root is at the top of the picture,

and in tension. The weld has shown no signs of defects in this case. In the event of a failure

you would expect to seethe edge of the root peeling away from the parent metal. You will

notice the face of the weld has been ground flush, this is to allow the sample to be bent

evenly.

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8. NON DESTRUCTIVE TESTS

Visual Inspection (VT)

Visual inspection is often the most cost-effective method, but it must take place prior

to, during and after welding. Many standards require its use before other methods, because

there is no point in submitting an obviously bad weld to sophisticated inspection techniques.

The ANSI/AWS D1.1, Structural Welding Code-Steel, states, "Welds subject to

nondestructive examination shall have been found acceptable by visual inspection." Visual

inspection requires little equipment. Aside from good eyesight and sufficient light, all it takes

is a pocket rule, a weld size gauge, a magnifying glass, and possibly a straight edge and

square for checking straightness, alignment and perpendicularity.

Before the first welding arc is struck, materials should be examined to see if they

meet specifications for quality, type, size, cleanliness and freedom from defects. Grease,paint, oil, oxide film or heavy scale should be removed. The pieces to be joined should be

checked for flatness, straightness and dimensional accuracy. Likewise, alignment, fit-up and

joint preparation should be examined. Finally, process and procedure variables should be

verified, including electrode size and type, equipment settings and provisions for preheat or

postheat. All of these precautions apply regardless of the inspection method being used.

During fabrication, visual examination of a weld bead and the end crater may reveal

problems such as cracks, inadequate penetration, and gas or slag inclusions. Among the weld

detects that can be recognized visually are cracking, surface slag in inclusions, surface

porosity and undercut.

On simple welds, inspecting at the beginning of each operation and periodically as work

progresses may be adequate. Where more than one layer of filler metal is being deposited,

however, it may be desirable to inspect each layer before depositing the next. The root pass of

a multipass weld is the most critical to weld soundness. It is especially susceptible to

cracking, and because it solidifies quickly, it may trap gas and slag. On subsequent passes,

conditions caused by the shape of the weld bead or changes in the joint configuration can

cause further cracking, as well as undercut and slag trapping. Repair costs can be minimized

if visual inspection detects these flaws before welding progresses.

Visual inspection at an early stage of production can also prevent underwelding and

overwelding. Welds that are smaller than called for in the specifications cannot be tolerated.

Beads that are too large increase costs unnecessarily and can cause distortion through added

shrinkage stress.Visual inspection can only locate defects in the weld surface. Specifications or applicable

codes may require that the internal portion of the weld and adjoining metal zones also be

examined. Nondestructive examinations may be used to determine the presence of a flaw, but

they cannot measure its influence on the serviceability of the product unless they are based on

a correlation between the flaw and some characteristic that affects service. Otherwise,

destructive tests are the only sure way to determine weld serviceability.

]

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Radiographic Inspection (RT)

X-RAY RADIOGRAPHY

X-rays are produced when high speed electrons, in the form of a beam called

cathode ray, strike a metal target placed in an x-ray tube. The velocity at which the electrons

strike a metal target is determined by the tube voltage where the tube voltage is the potential

difference between the source of electrons, called the cathode and target. Since electrons in

motion comprise an electric current and moving electrons determine the magnitude of the

electric current which is referred to as the current for an x-ray tube.

The energy of penetrating radiation is often expressed in terms of electron volts,

kilo electron volts or million electron volts. The energy of the x-rays produced at the target is

proportional to the square of the mean velocity of the electrons; and the shorter the

wavelength of the x-rays, the greater their penetrating power. The velocity v acquired by an

electron because of a difference of potential V can be calculated by setting the K.E equal to

the work done in accelerating the electron and is given by the following expression.

mv2 = 1.610-12V

Where, m = mass of each electron, (9.110 -28g)

v = velocity of electron, cm/sec,

V = tube voltage, volts

The intensity of the x-rays is directly proportional to the tube current and depends

on the tube voltage raised to the power greater than 2.5. the efficiency of the x-ray production

is given by the following expression :

E =1.410-7ZV

Where, E = Efficiency, %

Z = atomic number of target material,

V = tube voltage, volts

The above expression indicates that efficiency of x-ray production is low at low

voltages. At 300KV, only about 3% of the energy of the electrons is converted to x-rays. The

rest of the energy of the electrons ay the anode appears in the form of heat. The amount of

heat generated at the target of a tube is proportional to the product of the tube voltage and

tube current. Consequently, it is necessary to cool the target.

There are two types of x-ray tubes which can be used for radiography, a gas tube

and a Coolidge tube. The Coolidge type x-ray tube is a heated tungsten wire filament. Mostx-ray filaments operate in the range of 6 to 15 volts and use a current of 3 to 5 amperes to

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heat the filament. The electron current from a heated filament is a function of the temperature

of the filament.

In radiography it is desirable to have a source of x-rays which approaches an ideal

point source. This can be achieved by proper design of the filament and focusing cup, but it

does complicate the problem of heat removal. In the usual case, a compromise has to be made

on the size of the focal spot which can be used safely. The finer the focal spot, the better will

be the radiographic image. The power requirements for x-ray machines range from 50 to

24000KV. A 24000KV x-ray machine is capable of photographing approximately 500mm

thick steel.

Test procedure: The x-ray tube, welded test component and the photographic film are set up

as shown. The x-rays are allowed to fall upon the test specimen. Some of the x-rays are

absorbed. The extent of this absorption, as already stated earlier, depends on the presence of

voids, foreign inclusions or cracks in weld metal. As a result, the radiation passing through

the weld and falling upon photographic plate or film behind it will produce areas and light

spots to some defects in the interior of the weld. The quality of the resulting picture depends

on the intensity of the radiation source, the angle of inclination of the x-rays, the type and

thickness of the metal. X-raying is especially effective in locating cracks, lack of fusion,

under cutting, slag inclusion, porosity, pin holes and blow holes. X-rays are used for

weldments of all types of materials viz. steel, aluminium, magnesium etc. Radiography is

extensively used in the pipe line industry to ensure proper weld quality. However, x-ray

inspection is slow and expensive NDT method.

Penetrameters are used to determine the sensitivity of the radiograph by placing iton the test piece. These are made of the same material that is being inspected. Thus if

penetrameter can be seen clearly on the radiograph, any change in thickness of test piece will

be seen clearly. Although film radiography is slow and expensive, it is one of the most

popular NDT methods for locating subsurface defects. However, with the introduction of

automated welding system the production rate is increasing very fast thus a wrong set of

welding parameters can lead to a large number of defective pieces before the defect is

detected leading to increased number of overall scraped components. This is leading to the

introduction of radioscopy for quick and online detection of welds.

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X-rays are produced by high-voltage generators. As the high voltage applied to an

X-ray tube is increased, the wavelength of the emitted X-ray becomes shorter, providing

more penetrating power. Gamma rays are produced by the atomic disintegration of

radioisotopes. The radioactive isotopes most widely used in industrial radiography are Cobalt

60 and Iridium 192. Gamma rays emitted from these isotopes are similar to X-rays, except

their wavelengths are usually shorter. This allows them to penetrate to greater depths than X-rays of the same power, however, exposure times are considerably longer due to the lower

intensity.

When X-rays or gamma rays are directed at a section of weldment, not all of the radiation

passes through the metal. Different materials, depending on their density, thickness and

atomic number, will absorb different wavelengths of radiant energy.

The degree to which the different materials absorb these rays determines the

intensity of the rays penetrating through the material. When variations of these rays are

recorded, a means of seeing inside the material is available. The image on a developed photo-

sensitized film is known as a radiograph. The opaque material absorbs a certain amount of

radiation, but where there is a thin section or a void (slag inclusion or porosity), less

absorption takes place. These areas will appear darker on the radiograph. Thicket areas of thespecimen or higher density material (tungsten inclusion), will absorb more radiation and their

corresponding areas on the radiograph will be lighter.

The reliability and interpretive value of radiographic images are a function of their

sharpness and contrast. The ability of an observer to detect a flaw depends on the sharpness

of its image and its contrast with the background. To be sure that the radiographic exposure

produces acceptable results, a gauge known as an Image Quality Indicator (IQI) is placed on

the part so that its image will be produced on the radiograph . IQls used to determine

radiographic quality are also called penetrameters. A standard hole-type penetrameter is a

rectangular piece of metal with three drilled holes of set diameters. The thickness of the piece

of metal is a percentage of the thickness of the specimen being radiographed.

A penetrameter is not an indicator or gauge to measure the size of a discontinuity or the

minimum detectable flaw size. It is an indicator of the quality of the radiographic technique.

Radiographic images are not always easy to interpret. Filmhandling marks and streaks, fog

and spots caused by developing errors may make it difficult to identify defects. Such film

artifacts may mask weld discontinuities.

Surface defects will show up on the film and must be recognized. Because the angle of

exposure will also influence the radiograph, it is difficult or impossible to evaluate fillet

welds by this method. Because a radiograph compresses all the defects that occur throughout

the thickness of the weld into one plane, it tends to give an exaggerated impression of

scattered-type defects such as porosity or inclusions.

An X-ray image of the interior of a weld may be viewed on a fluorescent screen, as well as on

developed film. This makes it possible to inspect parts faster and at lower cost, but image

definition is possible to overcome many of the shortcomings of radiographic imaging by

linking the fluorescent screen with a video camera. Instead of waiting for film to be

developed, the images can be viewed in real time. This can improve quality and reduce costs

on production applications such as pipe welding, where a problem can be identified and

corrected quickly.

By digitizing the image and loading it into a computer, the image can be enhanced and

analyzed to a degree never before possible. Multiple images can be superimposed. Pixelvalues can be adjusted to change shading and contrast, bringing out small flaws and

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discontinuities that would not show up on film. Colors can be assigned to the various shades

of gray to further enhance the image and make flaws stand out better. The process of

digitizing an image taken from the fluorescent screen - having that image computer enhanced

and transferred to a viewing monitor - takes only a few seconds. However, because there is a

time delay, we can no longer consider this "real time." It is called "radioscopy imagery."

Existing films can be digitized to achieve the same results and improve the analysis process.

Another advantage is the ability to archive images on laser optical disks, which take up far

less space than vaults of old films and are much easier to recall when needed. Industrial

radiography, then, is an inspection method using X-rays and gamma rays as a penetrating

medium, and densitized film as a recording medium, to obtain a photographic record of

internal quality. Generally, defects in welds consist either of a void in the weld metal itself or

an inclusion that differs in density from the surrounding weld metal.

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Porosity

Caused by gases released during melting of the weld area buttrapped during solidification, chemical reactions, Contaminants

They are in form of spheres or elongated pockets

Porosity can be reduced by

Proper selection of electrodes

Improved welding techniques

Proper cleaning and prevention of contaminants

Reduced welding speeds

Slag Inclusions Compounds such as oxides ,fluxes, and electrode-coating materials

that are trapped in the weld Zone

Prevention can be done by following practices : Cleaning the weld bed surface before the next layer is deposited

Providing enough shielding gas

Redesigning the joint

CracksCracks

Cracks occur in various directions and various locations

Factors causing cracks:

Temperature gradients that cause thermal stresses in theweld zone

Variations in the composition of the weld zone.

Embrittlement of grain boundaries

Inability if the weld metal to contract during cooling

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Cracks in Weld Beads

Fig : Crack in a weld bead, due to the factthat the two components were notallowed to contract after the weld

was completed.

Ultrasonic Inspection (UT)

Ultrasonic Inspection is a method of detecting discontinuities by directing a high-frequency

sound beam through the base plate and weld on a predictable path. When the sound beam's

path strikes an interruption in the material continuity, some of the sound is reflected back.

The sound is collected by the instrument, amplified and displayed as a vertical trace on avideo screen - Fig. 5.

Both surface and

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