Resistance welding

Introduction :

All welding process are fusion welding process. The resistance welding process is also a fusion welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence, the name is resistance welding.

How it works?

In this process, a low voltage and very high current are passed through the joint for a very short time. This high heats the joint, due to the contact resistance at the joint and melts it. The pressure on the joint continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as 

H = k I² R t

Where 

H = The total heat generated in the work ( J )

I = electric current ( A )

t = time for which the electric current is passing through the joint ( s )

R = Resistance of the joint ( ohms ) 

k = A constant to account for the heat losses from the welded joint 

In this process, the amount of heat released is directly proportional to the resistance. It is likely to be released at all of the mentioned points below :

• The resistance of the electrodes
• Contact resistance between the electrode and workpiece
• Contact resistance between the two workpiece plates
• The resistance of the workpiece plates

All of above a large amount of heat is to be generated to have an effective fusion is at the interface between the two workpiece plates. 

Because of the squaring in the above equation, the current I needs to be precisely controlled for any proper joint.

At first, apply force and current through the electrodes contacted with metal parts to be welded and resistance heat is generated at the interface of metal parts and hence the metal parts melt and form the joint. Through a large current flows, there is no danger of an electric shock because the only low voltage is impressed.

Electrodes for resistance welding :

The electrodes in resistance welding carry the very high current required for fusion, as also transmit the mechanical force to keep the plates under pressure and in alignment during fusion. They also help to remove the heat from the weld zone thus preventing overheating and surface fusion of the work. For both of the purpose in this process, the electrode should have higher electrical conductivity as well as high hardness. 
Copper is alloyed form is generally used for making electrodes. Though pure copper has high electrical and thermal conductivity, it is poor in mechanical properties.

Copper-cadmium alloys have the highest electrical conductivity with moderate strengths and therefore are used for welding non-ferrous materials such as aluminium and magnesium alloys.

Copper-chromium alloys have slightly lower electrical conductivities than above but better mechanical strength. These are used for resistance welding of low-strength steels such as mild steels and low alloy steels. 
When cobalt and beryllium are added to copper, its conductivity is decreased to a great extent but the strength is increased. Hence, these are used for welding higher heat-resisting alloys such as stainless steels and steels with tungsten and other such alloying elements.  

Advantages of resistance welding :

• High-speed welding.
• Economical welding process.
• Easily automated.
• Easy operation so that suitable for high rate of production.
• No flux require such as solder is necessary.
• Electric facility is required in some cases due to use of large current.
• Possible to weld dissimilar metals as well as metal plates of different thickness.
• Very little skill is required to operate resistance welding machine.

Disadvantages of resistance welding :

• Visual inspection is difficult because welded portion can't be checked from the outside.
• Initial equipment cost is high.
• Lower tensile and fatigue strength of welded joint that formed by using this process.
• This welding process are limited only to lap joints.

The various resistance welding processes of interest are mentioned below :

• Resistance spot welding 
• Resistance seam welding 
• Projection welding 
• Upset welding 
• Flash welding

Thermit welding

Introduction :

Thermit welding is a process, which was traditionally used for the welding of very thick plates. Though this was used for welding large sections such as locomotive rails, ship hulls and broken large castings, its use has decreased nowadays because of the availability of other simpler methods such as submerged arc welding. 

A thermit mixture that uses for welding steels is aluminium and iron oxide. One question arises in your mind that what is thermit mixture?
The heat source utilized for fusion in this welding process is the exothermic reaction of the thermit mixtures. The thermit reaction starts when the mixed thermit powder is brought to its ignition temperature of 1200 ⁰C. 

How it works?

The thermit welding in which the molten metal obtained by the thermit reaction is poured into the refractory cavity made around the joint. It is a similar process as casting. The two pieces to be joined are properly cleaned and the edge is prepared. Then wax is poured into the joint so that a pattern is formed where the weld is to be obtained. Around the joint moulding, a flask is kept and sand is rammed carefully around the wax patterns as shown in below figure providing necessary pouring basin, risers and sprue. For the run off the molten wax, the bottom opening is also provided. The wax is melted through this opening which is also used to preheat the joint and make it ready for welding.

The thermit mixture which is mixed with fluxes is filled into a ladle through the bottom opening. The opening is initially closed. The igniting mixture which is normally barium peroxide or magnesium is placed at the top of the thermit mixture. The igniting mixture is lighted by means of a heated metal rod, whereby the complete reaction takes place and molten metal is produced. The bottom plug of the ladle is opened and the metal is allowed to flow into the mould prepared. The weld joint is allowed to cool slowly. Thus weld is formed. If we making a fast weld, thermit welding also provides a reasonably strong weld. The strength if thermit welded joint reaches that of a forged metal without any defects.

Applications :

The main application of this welding technique is in the repair works of rails in railways.

Advantages of thermit welding :

• Very large and heavy parts are also joined.
• No external power source is required it just use the heat of the chemical reaction.
• It is also used for building up large wobblers.

Disadvantages of thermit welding :

• Weld may contain gas mainly hydrogen so that the slag inclusion problem occurs.
• The metal which has low melting point can't weld by this process.
• Low deposition rate with operating factor.
• Only ferrous parts may be weld by this process.
• It is the high-temperature process so that cause distortion and changes in Grain structure in the weld region.

Electron beam welding

Introduction :

Electron beam welding is a powerful beam welding process. The heat source in electron beam welding for melting joints is a focused beam of high-velocity electrons. The electron beam upon impinging on the workpiece releases the necessary heat by converting its kinetic energy. 
Electron beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. It is often performed under vacuum conditions to prevent dissipation of the electron beam.

How it works?

The cathode within the electron gun is the source of a stream of electrons. The electrons are accelerated towards the anode because of the large potential difference that exists between them. The potential difference between that are used are of the order of 30 kV to 175 kV. The higher the potential difference, the higher would be the acceleration. The current levels are low, ranging between 50 mA to 1000 mA. Depending on the accelerating voltage, the electrons would travel at the speed of 50000 to 200000 km/s. The depth of penetration of the weld depends on this electron speed which in turn is dependent upon the accelerating voltage.

The electron beam is focused by means of an electron magnetic lens so that the energy is released in a small area. When the high-velocity electron beam strikes the workpiece all the kinetic energy is converted to heat. As these electrons penetrate the metal, the material that is directly in the path is melted and a keyhole is formed melting the metal around the beam. As the beam traverses, the keyhole would also travel along, with the molten metal being pushed back which when solidified and thus forms the joint.

Advantages of electron beam welding :

• The penetration of the beam is high. 
• The depth-to-width ratios between 10:1 to 30:1 can be easily realized with electron beam welding.
• It is also possible to closely control this penetration by controlling the accelerating voltage, beam current and beam focus. 
• The process can be used at higher welding speeds, typically between 125 to 200 mm/s.
• Filler metal or flux are not needed to be used in this process of welding.
• The heat liberated is low and also is in a narrow zone. Thus, the heat-affected zone is minimal as well as weld distortion is eliminated.
• It is also possible to carry out electron beam welding with workpieces in an open atmosphere.
• The other advantage of using a vacuum is that the weld metal is not contaminated.

Limitations of electron beam welding :

• EBW is the most costly welding process due to vacuum enclosure.
• This process requires a vacuum chamber containing a hard vacuum.
• Only small to medium size items can be welded.
• Though the welding itself can be done very fast, overall EBW is time-consuming.
• The equipment is complex and there are quite a few process variables involved.

You can also check it out :

• Applications of electron beam welding
• Adcvanatages and disadvantages of electron beam welding

Laser beam welding

Introduction :

Laser beam welding is a technique of welding which used to join multiple pieces of metal through the use of laser. It is frequently used in high volume applications using automation, such as in the automotive industries.
Now we all know that laser is a concentrated beam of coherent monochromatic radiation. Normal white light consists of a number of colours and a number of waves. As a result, it is not possible to use a monochromatic source as laser, provided all waves are of a single phase. This is achieved by means of stimulation. Because of coherency, it is possible to concentrate the laser beam by means of an optical lens to a spot of any desired size without appreciably losing any of its intensity. Thus, the laser beam is a high-energy source of heat to melt a joint for fusion welding in laser beam welding.

Laser beam welding operates in two fundamentally different modes :

• Conduction limited welding 
• Keyhole welding 

The mode in which the laser beam will interact with the material it is welding will depend on the power density of the focus laser spot on the workpiece.

How it works?

There are generally two types of laser which are used in welding operation such as solid-state lasers and gas lasers.

In solid-state lasers, light is emitted from a glass on a single crystal that is doped with transition elements such as chromium for ruby. When a beam of normal white light impinges on the crystal, the outer shell electrons of the dope elements go to a higher energy metastable state. They return to the normal state after emitting the extra energy in the form of a photon. All the photons that are stimulated to emit at a given instant will form coherent radiation, which can be concentrated by optical lenses. Thus, the output would be normally in pulses. The power ratings of such units may be up to 2 kW.

In gas lasers, the gas such as carbon dioxide molecules is excited to the higher vibrational energy level by means of an electric discharge. The transition from this high-energy level to the normal level generates the radiation which is coherent and gets focused by means of the usual optical lenses. Continuous-wave gas lasers using carbon dioxide gas with powers up to 20 kW are used for laser beam welding. 

With low power lasers typically less than 1 kW, the penetration would not be much and the weld is obtained by means of complete welding of the joint near the surface. But as the power increased because of that the large heat density obtained. That large density causes the metal at the centre of the jet to be vaporized with a keyhole being formed similar to that of electron beam welding. The temperature within this keyhole can reach as high as 25000 ⁰C. This technique permits large welding speeds depending on laser size.  

There are three types of lasers that are commonly used in welding operation such as CO₂, Nd: YAG and fibre laser. The Nd: YAG laser light is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 % for most metals. Using standard optics, it is possible to achieve focused spot sizes diameter as small as 0.025 mm. On the other side, the CO₂ laser has an initial reflectance of about 80 to 90 % for most metals and requires special optics to focus the beam to a minimum spot size of about 0.075 to 0.100 mm diameter. However, whereas the Nd: YAG lasers might produce power outputs up to 500 watts, the CO₂ system can easily supply 10 kW of power and more.


Advantages of laser beam welding :

• Heat input is close to the in the minimum required to fuse the weld metal, thus heat-affected zones are reduced and workpiece distortions are minimized.
• Time for welding thick sections is reduced and the need for filler wires and elaborate joint preparations is eliminated by employing the single pass laser welding procedures.
• No electrodes are required.  
• LBM being a non-contact process so distortions are minimized and tool wears are eliminated.
• Welding in areas that are not easily accessible with other means of welding can be done by LBM.
• The joining of small spaced components with tiny welds very easily because of laser beam can focused on a small area.
• Wide variety of materials including various combinations can be welded very easily.
• Thin welds on small diameter wires are less susceptible to burn back than is the case with arc welding. 
• Metals with dissimilar physical properties, such as electric resistance can also be welded by LBW.
• No vacuum or X-Ray shielding is required. 
• Welds magnetic materials also. 
• Aspect ratios means depth-to-width ratio of the order of 10:1 are attainable in LBM.
• Faster welding rate.
• No flux or filler metal required.
• Single-pass two-side welding. 
• Shorter cycle and higher up times. 

Limitations of laser beam welding :

• Joints must be accurately positioned laterally under the beam.
• Final position of the joint is accurately aligned with the beam impingement point.
• The maximum joint thickness that can be welded by laser beam is somewhat limited. 
• The materials have high thermal conductivity and reflectivity like Al and Cu alloys can affect the weldability with lasers. 
• An appropriate plasma control device must be employed to ensure the weld reproducibility while performing moderate to high power laser welding. 
• Lasers tend to have low energy conversion efficiency less than 10 percent. 
• Some weld-porosity and brittleness can be expected, as a consequence of the rapid solidification characteristics of the LBM. 

Laser beam welded parts are often required additional steps before and after the actual weld. we can see this additional steps below :

Process before welding :

• CAD/CAM product design and weld design.
• Tooling design and fabrication.
• Parts cleaning.
• Strategic Sourcing and Subcontractor Contract Management

Process after welding :

• Leak test.
• Metallurgic evaluations.
• Post weld thermal treatment.
• Not destructive testing.

You also check it out the :

• laser beam welding application

Plasma arc welding

Introduction :

In plasma arc welding plasma is the state of the matter when part of the gas is ionized making it a conductor of electric current. It is the state of matter present in between the electrode in an arc. The plasma arc welding closely resembles that of the TIG welding in that it also uses a non-consumable tungsten electrode and a shielding gas such as argon.
The plasma welding process was introduced to the welding industry as a method of bringing better control to the arc welding process in lower current ranges. 

How does it work?

In plasma arc welding, the plasma arc is tightly constrained and a small amount of pure argon gas flow is allowed through the inner orifice surrounding the tungsten electrode to form the plasma gas.

To initiate the arc in PAW, a low current pilot arc is obtained between the electrode and the constricting nozzle which is heated to an extremely high temperature and ionized so that it becomes electrically conductive. The arc in this type of welding is concentrated and straight. and similar to tungsten inert gas welding plasma to transfer an electric arc to a workpiece. The metal to be welded is melted by the intense heat of the arc and metal fuses together thus welding is done.

In plasma welding torch a tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is produced between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded.

The plasma gas itself is not sufficient to protect the weld metal therefore, a large volume of inert-shielding gas is allowed to flow through an outer gas nozzle surrounding the inner nozzle. Plasma gases are normally argon. The torch also uses a secondary gas surcharge, argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing the oxidation of the weld.

The power source used is DC with electrode negative for better electrode life. 
The actual welding by PAW is done by means of a technique called a keyhole.

Equipment Required List :

• Power Supply
• Plasma Console (sometimes external, sometimes built-in)
• Water re-circulator (sometimes external, sometimes built-in)
• Plasma Welding Torch
• Torch Accessory Kit (Tips, ceramics, collets, electrodes set-up gages)

Advantages of plasma arc welding :

• The main advantages of this are lies in the control and quality produced in the part being welded.
• The torch design allows for better control of the arc, as well as a higher tolerance for in torch standoff distance.
• Welds are typically cleaner and smoother in PAW process.
• Smaller heat affected zone results welds are very strong.
• Metal deposit rates are high.

Limitations of plasma arc welding :

• Welding equipment is expensive.
• Nozzle surrounding the electrode needs a frequent replacement.
• Relatively high startup costs.
• High skilled workers required.

Shielded metal arc welding

Introduction :

Shielded metal arc welding is also called as manual metal arc welding. It is the most extensively used manual welding process, which is done with stick electrodes. Though in the USA, its use is decreasing in comparison to the other arc welding process. In India, it still is the most used arc welding process. This process is highly versatile and can be used extensively for both simple as well as complicated joints. 

How it works?

The typical shielded metal arc welding set up with an AC power source. The electrodes for the welding operation should be selected properly, depending on the requirements of the welding.
It can be done with either an AC or DC power source. The typical range of the current usage may vary from 50 to 500 A with voltages from 20 to 40 V.

Shielded metal arc welding uses a metallic consumable electrode of a proper composition for generating arc between itself and the workpiece. The molten electrode metal fills the weld gap and joins the workpiece.

The electrode is coated with a shielded flux of suitable composition. The flux melts together with the electrode metallic core, forming a gas and slag, shielded the arc and weld pool. The flux cleans the metal surface, supplies some alloying elements to the weld and then slag is removed after the solidification.

The main points to be considered in this welding process are the following :

• Composition of the base metal, which determines the electrode composition.
• Tensile strength of the required joint.
• The thickness of the base metal. ( for thinner metal the current setting should be lower )
• Required metal deposition rate.
• Type of arc welding equipment used.
• Weld position - Flat, horizontal, vertical or overhead.

Advantages of shielded metal arc welding :

• A job of any thickness can be welded by shielded metal arc welding accept very small thickness below 3 mm may give rise to difficulties in welding because of their lack of rigidity.
• Simple, portable and inexpensive equipment.
• Suitable for outdoor applications.
• Wide variety of metals welding done by this process.
• Also, a wide variety of welding positions and electrodes are applicable.

Limitations of shielded metal arc welding :

• The slow speed of the welding process.
• The typical weld metal deposition rates may be in the range of 1 to 8 kg/h in the flat position.
• A lot of electrode material is wasted in the form of unused end, slag and gas.
• There are some chances of slag inclusion in the bead. 
• Some precautions are needed to reduce moisture pick-up so that it does not interfere with the welding.
• Fumes make difficult the process control.

Carbon arc welding

Introduction :

In this welding process produces joining of metals by heating them with an arc between a carbon electrode and the work-piece.
This is the earliest of the arc-welding process but is not used nowadays in many applications because it replaced by twin carbon arc welding.

Electrodes Selection :

In this process, the electrode is made of either carbon or graphite. In contrast to graphite electrodes, carbon electrodes are soft so that can't take up very high current densities. The arc with the carbon electrodes is more controllable. Lower current also adds to the higher electrode life. For use in carbon arc welding, the electrode should be of uniform structure and as far as possible and free from impurities. The life of a graphite electrode is higher than that of the carbon electrode.

How does it work?

In the carbon arc welding, the required filler metal is supplied through a separate filler rod. The arc can be obtained between the carbon electrode and the workpiece. The arc heats and melts the edges of the workpiece thus forming a joint.

In this process generally, a DC power supply with electrode negative is used for single electrode carbon arc welding to minimize the heat generation near to the electrode side so that wear of the electrode is maintained at the minimum rate.

Advantages of carbon arc welding :

• Because of the separation of the heat source from the filler metal, better control of the heat input is possible.
• Low distortion of the workpiece.
• This process is easily automated.

Limitations of carbon arc welding :

• The major problem is the blowholes that are caused because of the turbulence associated with the DC power source.
• This process is not suitable for the overhead or vertical welding position but very high mechanized welding speeds could be obtained by the process in the flat position.
• A carbon of electrode contaminates weld material with carbides.
• Unstable quality of the weld caused porosity.

Electric arc welding

Introduction :

In electric arc welding, generation of heat by an electric arc is one of the most efficient methods. The electric arc welding process makes use of the heat produced by the electric arc to fusion-weld metallic pieces. This is one of the most widely used welding processes because of the ease of use and high production rates that can be achieved economically.

Principle of arc :

An arc is generated between two conductors of electricity, cathode and anode when they are touched to establish the flow of current and then separated by a small distance.
An arc is sustained electric discharge through the ionized gas column is called plasma between two electrodes.
The electrons liberated from the cathode move towards the anode and are accelerated in their movement. When they strike the anode at high velocity, a large amount of heat is generated.
In order to produce the arc, the potential difference between two electrodes should be sufficient to allow them to move across the air gap. 
For the convenience of explanation, we have chosen a direct current arc for the above description. But even with an arc of the alternating current ( AC ), it would be similar, with the main difference that the cathode and anode would change continuously and as a result, the temperature across the arc would be more uniform compared to a DC arc. 

How does it work?

The arc welding is a fusion welding process in which the heat required to fuse the metal is obtained from an electric arc between the base metal and an electrode.

First of all, metal pieces to be weld are thoroughly cleaned to remove the dirt, dust, grease, oil etc. After that, the workpiece should be firmly held in suitable fixtures. Insert a suitable electrode in the electrode in the electrode holder at the angle of 60 to 80 degree with the workpiece.

Select the proper current and polarity. The spot is marked by the arc at the places where welding is to be done. The welding is done by making contact of the electrode with the workpiece and then separating the electrode to a distance to produce an arc.

When the arc is obtained, heat is produced and melts the work below the arc, and forming a molten metal pool. A small amount of depression is formed in the work and the molten metal is deposited around the edge of this depression. After the completion of welding, the electrode holder should be taken out quickly to break the arc and the supply of current is switched off.

Arc welding equipment :

The main requirement in an electric arc welding is the source of electric power. They are essential for two types :

Alternating current ( AC ) machines 

• Transformer 
• Motor or engine-driven alternator

Direct current ( DC ) machines 

• Transformed with DC rectifier
• Motor or engine-driven alternator

The arc welding machines can also be divided into two types :
The first one is the constant current welding machines and the second one is droop curve machines.

Advantages of electric arc welding :

• low cost.
• Simplicity and portability of the tool.
• Versatile process.
• Wide choice of the electrode.
• Higher welding speed.
• No flux required.

Limitations of electric arc welding :

• Wastage of material.
• Less productive due to continuous wire process.
• Proper alignment and care of electrode required.
• Radiation impacts more extreme.

Difference between annealing and tempering

In this article you can check it out the difference between two manufacturing process annealing and tempering. First of all you should know about both of this process. 

What is annealing ?
Annealing is a hardening process, in which material is heated above the re-crystallization temperature and then is cooled in furnace / oil /water. 

What is tempering ?
Tempering is a stress removal process, in which material is heated below the re-crystallization temperature and then is cooled in air.

Annealing is used for :

• Soften a metal for cold working
• Improve machinability
• Enhance electrical conductivity

Tempering is used for:

• Hardness
• Ductility
• Toughness
• Strength
• Structural stability

Now we can check the difference between both the two process :

• Annealing is softening the metal after work hardening while tempering is reducing brittleness after quench hardening.
• Annealing increase ductility and toughness, stress relieving while tempering is the process of introducing toughness.

Difference between quenching and tempering

In this article you can check it out the difference between two manufacturing process quenching and tempering. First of all you should know about both of this process. 

What is quenching ?
Quench means rapid cooling so in this process rapid way of bringing metal to cool and back to room temperature after the heat treatment process.

What is tempering ?
Tempering is the process used to increase the toughness and is usually performed after hardening to reduce some of excess hardness.

Now we can check the difference between both the two process :

• Quenching is the process of heating the material above the re-crystallization temperature and cooling it suddenly while in tempering the material is heated to a temperature below the re-crystallization value and hold for few hours. 
• Quenched steels are brittle and tempering toughens them.
• The material becomes brittle, hard, ability to withstand wear and vibrations in quenching while tempering removes internal stress and improve a bit of ductility to the hard material.

Defects in welding

In view of the severe thermal regime through which the welding process proceeds the weldments are likely to be affected and if proper care is not taken they are likely to end up with certain defects.

Distortions have been discussed in greater detail earlier, and we will see the other defects here. The likely defects are :

• Undercut
• Incomplete fusion
• Porosity
• Slag inclusion 
• Hot cracking 
• Cold cracking 
• Lamellar tearing 
• Lack of penetration or Excess penetration

Now we can see the above defects in details :

• Undercut : 

The undercut is an extremely common welding defect it appears like a small notch in the weld surface.
It is generally attributed to the improper welding technique or excessive welding current.

• Incomplete fusion :

It occurs when individual weld beads don't fuse together or don't fuse properly to the base metal that you are welding. This will be seen as a discontinuity in the weld zone. 

The main cause for this defect is improper penetration of the joint and wrong design of the joint or incorrect welding technique including the wrong choice of the welding parameters.

• Porosity :

It is caused by the presence of gases which get entrapped during the solidification process. The main gases that cause porosity are :

• Hydrogen
• Oxygen 
• Nitrogen 

There may be also some other gas that causes porosity like helium, argon and carbon dioxide that are also present in weld pool but in view of their insolubility, they do not cause porosity.

Porosity if present in large would reduce the strength of the joint.

Make sure all material are clean before start welding and try to using low hydrogen electrodes that may reduce the chances of porosity.

• Slag inclusion :

Slag is formed by the reaction with the fluxes. It is generally lighter. In view of its low density, it will float atop of the weld pool and would be chipped off after solidification.

Some of the factors that cause slag inclusion are :

• The high viscosity of weld metal 
• Rapid solidification 
• Insufficient welding heat
• Improper manipulation of the electrode
• Undercut on the previous pass

Also, in multi-pass welding the slag solidified in the previous pass is not cleaned before depositing the next bead, which may cause slag inclusion.

Slag inclusion is like porosity, weakens the metal by providing discontinuities.

• Hot cracking :

It generally occurs at high temperature and the size can be very small to visible.

The crack in most parts is intergranular and its magnitude depends upon the strains involved in solidification. 

It generally happens only in steels and its caused by deformities in the structure of the steel. They are more likely to form during the root pass when the mass of the base metal is very large compared to the weld metal deposited.

It can be prevented by preheating the base metal, increasing the cross-sectional area of the root bead, or by changing the contour or composition of the weld bead.

• Cold cracking :

Cold cracking occurs at room temperature after the weld is completely cooled. It can be seen in the heat-affected zone. 
Mainly cause for cold cracking is :

• Excessive restraint of the joint which induces very high residual stresses.
• Martensitic transformation making the metal very hard as a result of rapid cooling.

Pre and post-heating of the weldment help in reducing the cooling rates and the consequent locking of the stresses.

• Lamellar Tearing :

It is generally seen at the edge of the heat-affected zone. It appears as a long and continuous visual separation line between the base metal and heat-affected zone.

This is caused by the presence of the elongated inclusions such as Mn Fe and S in the base metal. 

Lamellar tearing can also be caused by the weld configuration which gives rise to high residual tensile stresses in the transverse direction.

• Lack of penetration :

In complete penetration happens when your filler metal and base metal are not joined properly and the result is a gap or crack appears.

Welds that suffer from incomplete penetration are weak at best, and they will likely fail if you apply much force on them.

Welding terms and definitions

In this article, you can learn the definitions of some of the welding terms that are generally in used.

• Baking :

It is the material support provided at the root side of a weld to aid in the control of penetration.

• Base metal :

The metal to be joined or cut is called the base metals.

• Bead or Weld bead :

Bead is the metal added during a single pass of welding. 

The weld bead appears as a separate material from the base metal.

• Crater :

In arc welding, a crater is a depression in the weld-metal pool at the point where the arc strikes the base metal plate.

• Deposition rate :

The rate at which weld metal is deposited per unit time is called deposition rate.

It is normally expressed as kg/h.

• Fillet weld :

The metal fused into the corner of a joint made of two pieces placed at approximately 90 degrees to each other is called fillet weld.

• Penetration :

It is a depth up to which the weld metal combines with the base metal as measured from the top surface of the joint.

• Puddle :

The portion of the weld joint that is melted by the heat of welding is called puddle.

• Root :

It is the point at which the two pieces to be joined by welding are nearest.

• Tack weld :

A small weld, generally used to temporarily hold the two pieces together during actual welding, is the tack weld.

• The toe of weld :

Toe of the world is the junction between the weld face and the base metal.

• Torch :

In gas welding, the torch mixes the fuel and oxygen and controls its delivery to get the desired flame.

• Weld's face :

It is the exposed surface of the weld.

• Weld metal :

Weld metal is the metal that is solidified in the joint. 

It may be only a base metal or a mixture of base metal and filler metal.

• Weld pass :

A single movement of the welding torch or electrode along the length of the joint which results in a bend is a weld pass.