Repairs

Repair Methods

Failure of welded joints

Repairs on cast iron valves

Cracks in shafts

Repairs on Crankshafts
Heat exchanger, Pump, and Pipe repairs.

Repairs on Pipe joints and fittings
Non-Destructive test
Metal locking
Welding, Brazing, and Soldering
Propeller repair
Rudder repair
Selection of electrode

Defects in welding

Repair Methods

The most commonly used methods of joining two materials are:
a. Brazing - where a filler material having a low melting point is used with a gas Welding torch to join non-ferrous materials.
b. Welding - which varies from gas welding to electric arc welding to special underwater welding. In repair work, arc welding is most common for larger items, whereas gas welding is used for smaller pipework or thin sheet metal.
c. Inert gas welding - in special cases where it is necessary to shroud the arc with inert gases such as argon.
d. Cement for temporarily stopping leaks in piping.
e. Chemicals used for bonding, such as plastic Putty.

Failure of welded joints

Welded joints can fail in different ways. Insufficient current in electric arc welding can cause a failure at the side of the weld, due to lack of fugal. Internal stresses can be set up if proper heat treatment is not carried out to stress relieve the joint.
Failure of the base material at the boundary of the heat-affected zone can also take place. Undercut occurs at the toe of a fillet.
It is caused by the parent metal melting at the fillet edge and running down into the fillet. This creates a stress riser.
Joint preparation is very important to the success of the welding operation. The included angle or should not be less than 60°. Factors controlling the distortion caused by welding are preparation of joint, correct root gap, and the way the welding is carried out. If the metal in the joint is prevented from contracting when cooling down, it sets up a strain, which leads to tensile stress. These are called residual stresses and are minimized by proper heat treatment. 

Repairs on Cast Iron valves

Cast iron valves are vulnerable to corrosion by graphitization. The iron corrodes away leaving behind a thin shell of graphite, which fractures as soon it is subjected to stress. It is very easy to overlook such an occurrence, due to the appearance of graphite.
Ship's seawater valves are very prone to such graphitization. The presence of graphite further has a galvanic effect on the valve seat, if made of copper-base alloys, resulting in further damage. If the defective area can be machined out, a new seat can be fitted.
Welding of Cast Iron components.
Cracked or broken castings are often required to be welded. The area around the break must be cleaned thoroughly with kerosene, after which the area is wiped dry. By rubbing white chalk over the area, fine cracks will be easily visible. Crack arresting holes can now be drilled, to prevent the cracks from propagating further. A narrow strip along each end of the joint must be ground, to remove the surface layer from the casting. This surface layer is known as requires to be removed before welding can be carried out. If the thickness is less, it may be butt welded, else it may require to be beveled. Special electrodes called NIFE are used, which contain nickel.
A normal weld would not work because of the excess carbon in cast iron, which makes the weld brittle. These special electrodes are expensive and hence another option is to have a thin layer called a  'buttering layer', which prevents the absorption of carbon.
for thicker plates, a carbon block is used as a heat sink, to prevent the casting from overheating. Which could change the structure of the weld.
When gas welding cast iron, the job must be pre-heated, in order to prevent cracking, and also to get a uniform expansion of metal to be welded. Annealing the weld is also easier if pre-heating has been done.
A flux is needed to break down the oxides within the parent metal and the filler material. It also removes any gas pockets. If the flux is not used, the infusible slag mixes with the iron oxide causing inclusions and blow holes in the weld. The rod is dipped in the flux before allowing it to be inserted in the weld. The procedure for welding cast iron is to direct the flame to the bottom of the Vee or bevel joint until the base metal starts to melt. When a puddle is formed at the bottom of the weld joint, the flame is moved up, to melt the sides. The welding rod is introduced after the entire welding area has become molten. If any impurities adhere to the welding, the rod should be tapped to remove them. After completion of the weld, the casting is re-heated to relieve the stresses and then allowed to cool slowly.

Cracks in shafts

Cracks can be either visually seen or detected by dye penetrant or magnetic particle detection tests. The depth of the crack should be ascertained. If it is shallow, it may be ground out, so that its effect as a stress raiser is reduced.  The straightness of the shaft should be checked.
Repairing of cracks in cylinder blocks
Cracks result from excessive temperature fluctuations. Excessive heat around the areas such as exhaust valves could also result in thermal crack formation. There are several methods to effect the repair.
One is to drill and tap a series of over-lap holes along the length of the crack. Threaded rods are then screwed into these holes. When the entire length of the crack has been covered, the exposed end of the threaded rod is peened off.
Another variation is to bridge the crack with specially formed bridges or locks. These prevent the crack from widening. Special jigs are used to drill the holes which form the recesses for the locks. The locks are evenly spaced to cover the entire crack. With the locks hammered in place, overlapping holes are drilled between the bridges. These holes are now tapped and threaded pins of a special alloy are screwed in, the ends of which are peened off. This is allowed by grinding with a wheel, and another light peening.  

Repairs on Crankshafts

Crankshafts and crankpins are usually repaired for uneven wear, which makes them 'out of round'. One solution is to re-condition by hard chrome plating, which is then ground down to the correct size. If however, this is not possible, then 'metal spraying' is also applied to build up the surface. Metal for spraying is usually carbon steel. The journal is first prepared, and then the spraying is carried out as soon as possible, to avoid oxidation of the cleaned surfaces. After spraying, the journals are ground down to the correct size, as this steel coating is too hard for machining. Wet grinding is preferred so as to prevent the expansion of the metal layer from the heat of the grinding. 

Heat exchanger, Pump, and Pipe repairs.

Leaks in a multi-tubular heat exchanger.
If there is leakage from the tubes, the tubes are usually plugged. If the leakage is from the junction between the tube end and the tube plate this can be rectified by expanding the tube end with a roller expander.
Plugs are made of wood - either soft in the form of a conically tapered shape, which can be hammered in place. The angle of the taper must be correct, if too large, it may not be possible to hammer in the plug. If the angle is too less, the plug may not be able to stop the leak. Some manufacturers provide metal pugs with the heat exchanger, and the plug is usually of a non-ferrous material.
Repairs on Pumps
Pump impellers normally erode and if the water velocity is high, they corrode rapidly near the tip, Copper base alloys are normally used for the material of the impeller. Wear rings are used to maintain clearances within limits. These are normally replaced when the wear approaches 2 mm. However, the erosion of the casing may change this clearance and affect pump operation. The damage to the casing may change this clearance and affect pump operation. The damage to the casing is built up by welding. Shaft sleeve wear can take place due to gland packing being excessively tightened. The shaft material has high corrosion resistance, e.g. Monel Alloy K.

Repairs on Pipe joints and fittings

Pipe joints crack in use and need to be welded, using special processes like TIG or MIG. Manual welding using approved electrodes is often acceptable. The use of the backing gas is essential where the weld root is inaccessible. Filler material for welding contains deoxidants. Where the seawater is in contact with the weld, the weld metal should be 70:30 cupro-nickel alloy having iron addition. For pipe sizes below 108 mm (4") nominal diameter, brazing is acceptable.
Avoid any modifications which increase the water speed or turbulence. If there are any crevices, the metal should be resistant to corrosion or sealed and protected from seawater contact, e.g. rubber-lined. Avoid accumulation of debris in tanks and pipe-work. When making repairs, use materials that match. Stagnant water often leads to pitting corrosion. If the tank is not in use, preferably drain it.

Non-Destructive test

Non-Destructive Test (NDT) or Non- Destructive Evolution (NDE) is used to detect defects such as crack, corrosion, etc.,

NDTs are conducted directly on the Objects. Also when the equipment is in service it can be tested, It is possible to check the component completely, it is faster and with very little preparation required. Various non-destructive tests are used to check whether material or component is fit for further use, or should be discarded. The choice of the test to be used depends on various factors.

Following are NDTs described in the article:-

Visual inspection

Liquid penetration test

Radiographic test

Ultrasonic test

Eddy current test

Magnetic particle test

1. Visual inspection:- A skilled inspector can find a flow in components by visual inspection, it is very cost-effective. Surface cracks can be seen using a magnifying lens. The sound produced by a component when it is hanged and stroked by a hammer is an indication of a flaw inside the component.

2. Liquid penetration test:- It is a low-cost method of inspection for the detection of surface cracks in non-porous materials. It can detect casting & forging defects, cracks and leakages in new products, and fatigue cracks in components that are in service. This method does not require special skills. 

The red Dye penetration method is normally used on board. 

a. The surface is cleaned by using cleaning fluid in an aerosol can. 

b. The dye penetrant from the can is sprayed on the top of the surface. 

c. The penetrant seeps into the surface crack. 

d. After adequate penetration time, the excess penetrant is removed using the cleaning fluid. 

e. The developer is applied, to draw penetrant out of the flaw. 

f. This gives the red visible indication of flow, which can be seen clearly in the normal white light.

3. Radiographic test:- A short-wavelength electromagnetic radiation (High energy photons) is used to detect hidden flaws in the material. Welding, forging, and castings are inspected by this method. This is used to check welds in pressure vessels, either on the whole weld or in certain critical areas. The rays are directed onto special radiographic film, which is then developed to expose any possible defects in the work.

a. An X-Ray machine or a radioactive source like Iridium-192 or Cobalt-60 is used as a source of photons. 

b. The radiation penetrates through the test object and falls on the photographic film.

c. The Intensity of radiation emerging from the test object varies depending on the flow.

d. This can be seen in the negative, the dark areas are signs of defects.

4. Ultrasonic test:-Ultrasonic short pulse waves of frequency 0.1-15MHz are used. This uses high-frequency waves, which are transmitted through the material to be tested, and the reflections are analyzed to find any flaws or to ascertain the thickness of the material.

a. Test can detect the flaws in the components, also used to determine the thickness of the plate.

b. A transducer is connected to a diagnostic machine is passed over the object being inspected.

c. The transducer sends pulse waves through the surface of the object and receives the sound reflected back to the device.

d. Reflected ultrasound comes from an interface such as the back wall of the object or from an imperfection.

e. The CRT screen on the calibrated diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance taken for the reflection to the transducer.

f. In the CRT it can be seen that the sound wave is reflected from the bottom and from the fault, by which the location of the fault can be detected.

5. Eddy's current test:- The principle of electromagnetism is the basis for this test. The test is used for crack detection & thickness measurement.

a. An A.C is applied to a coil placed near the test object which generates a changing magnetic field around the coil. 

b. This caused a current induction in the test object. 

c. Eddy currents flow in a circular path, a flow inside the test object disturbs the eddy current and creates a measurable response.

6. Magnetic particle test:- They are conducted on ferromagnetic materials and can detect surface and subsurface defects. These are used on ferrous components to detect the presence of any subsurface cracks. The material to be checked is magnetized and sprayed with a special powder. This collects on the location of the crack and highlights the area.  

a. Finely divided ferromagnetic particles, applied over the surface. 

b. The magnetic field is applied to the material under test.

c. The discontinuities in the surface or sub-surface will cause a leakage field, the particles will be gathered and held by the leakage field.

d. The collection of particles forms an outline of the discontinuity and indicates its location, size, shape, and extent.

Metal locking

A. Metal Locking or metal stitching is a precise mechanical method of making permanent repairs to broken casting and mechanical parts.
There is no heat involved in the method, thus post-repair there is no need to correct any misalignment and warpage, also the repair can be carried out in the restricted area where it is hazardous to carry out hot work. This method of repair does not require any major disassembly.
a. A Jig to drill the holes across the fracture equidistantly,
b. Dumbbell-shaped Nickel alloy metal-lock keys. Which is a ductile alloy. They are available in different sizes.
c. Studs whose heads can be sheared off.
d. A Mechanized chisel to join the drill holes, size is such that to match the contour of the key.
e. Drill machine and drill bits of required size to match the keys and studs.
f. Hammer to tap the keys.
g. A chisel for the removal of remnant rough metal.
h. Grinder to ground the burrs.

B. Cast Iron components when cracked are not advised to repair by welding whereas metal locking is a suitable method to carry out such repair. Also, steel, bronze, or aluminum structures or components are also repaired by metal locking.

C. Procedure:- A skilled technician or engineer is required to carry out this repair. Shore-based workshops approved by classification societies are generally available for such repairs.

Following are the steps to carry out metal locking:-
a. With the help of a Jig & Drill machine, series of holes are drilled across the line of fracture, to the depth of the workpiece.

b. Drill holes are then joined in common by using a mechanized chisel.

c. Keys are then driven into the aperture and peened into a metal-to-metal condition, causing the key to virtually become integral with the parent metal.

d. Holes are then drilled along the line of the fracture, then screwed in with the studs.

e. Stud as screwed in should bite into the predecessor, forming a pressure-tight joint. This makes the casting a more rigid body. The heads of the studs are sheared off.

f. The remnant rough metal is removed by chisel. Protruding burrs are grounded by a grinder and a coat of paint is applied as required.
 

Welding, Brazing, and Soldering

TIG welding:-  Tungsten Inert gas welding Also know as Gas tungsten arc welding is a manual weld process that uses a non-combustible electrode made up of tungsten, an inert or semi inert gas mixture, and separate filler material. especially useful for welding thin materials, this method is characterized by a stable arc and high-quality welds. however, it is required significant operator skills and can only be accomplished at relatively low speeds. it can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. it is often used when quality welds are extremely important.

MIG welding:- Metal Inert Gas welding, Also known as Gas metal arc welding is a semi-automatic or automatic welding process that uses a continuous wire feed as an electrode and an inert or semi inert shielding gas to protect the weld from contamination using an inert gas as a shield. A constant voltage DC power source is most commonly used, but a constant, as well as an alternating current, can also be used. Welding speed is relatively high due to automatically fed continuous electrodes but is less versatile because it requires more equipment than the simple shield metal arc welding (SMAW) process.  

Brazing:- Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.

Brazing differs from welding in that it does not involve melting the workpieces. The filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal and is then cooled to join the workpieces together. A major advantage of brazing is the ability to join the same or different metals with considerable strength.
High-quality brazed joints require that parts be closely fitted, and the base metals exceptionally clean and free of oxides. Joint clearances of 0.03 to 0.08 mm are recommended for the best capillary action and joint strength. Cleanliness of the brazing surfaces is also important, as any contamination can cause poor wetting (flow). Cleaning prior to brazing may be done by chemical cleaning and abrasive or by mechanical cleaning. In the case of mechanical cleaning, it is important to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry.
Unless brazing operations are contained within an inert or reducing atmosphere environment (i.e. a vacuum furnace), a flux such as borax is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, liquid, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core.
A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.
Some of the more common types of filler metals used are Aluminium-silicon, Copper, Copper-silver, Copper-zinc (brass), Copper-tin (bronze), Gold-silver, Nickel alloy,  Silver, etc.

Soldering:- soldering is a technique where different components are placed in a PCB design. Soldering pencil, solder, flux, and soldering rod stand are the devices used for the process. The joints are made at very low temperatures.
De-soldering is a process where it may be necessary to disconnect or remove components or wires from the soldered board or wired circuit.
Before soldering a component on the board, the component is first tinned. To solder, a copper wire on the board, remove the sleeve from the wire, and clean the copper wire. Dip the copper wire in the flux and heat it with a solder machine along with the solder. This process is called tinning. Heat the tinned wire and solder till the solder flows freely over the tinned wire.
Soldering pencil (iron), has a polished tip that is used for heating and is designed with an insulated handle. Solder is a soft metal alloy that melts when contacted with the soldering iron, melting point can be as low as 90 deg cel and up to 450 deg cel. The available types of solders are lead, lead-free, hard, glass, and flux-core. The type of flux can be acid, rosin, or water-soluble.

Propeller repair

Repairs to Propellers can be done by argon arc welding or sigma argon arc welding. The normal procedure followed by the workshop is :
a. Removal of dirt and oxides by grinding. Cracks are to be drilled out and ground or machined to a Vee or double Vee joint.
b. The metal to be welded is pre-heated to the required temperature (above 200°C).
c. Appropriate filler rods are utilized, compatible with the parent metal. After welding is complete, the metal should be stress-relieved by heating to 500°C, and cooling at a controlled rate. Care to be taken to avoid wrapping of the blades during the heating and repair work. Serious cracks in the blade will require careful consideration, with respect to repair methods, cost of repair vs cost of replacement, time available, and availability of spares and dry-dock time.
There are various options that can be examined :
a. Drill out the crack arresting holes at the roots of the crack and monitor for further spread, if is insufficient for repair, and the extent of the crack is limited.
b. Possibility of carrying out metal-locking.
c. Cropping of the blades.
If the extent of the crack is large, then drilling holes could lead to pieces of bladed breaking away. This may cause further damage, vibration, and reduced reliability.
Cropping may be a solution to vibration problems, as removal of the diagonally opposite blade tip will create no static imbalance. Effect on the performance of the vessel will not be excessive (less than 10% reduction in speed).
Cavitation is to be expected, due to the cropping of the tips. Also a change in the 'barred speed' range, with possible rising of exhaust temperatures and specific fuel consumption of the engine.

Rudder repair

Repairs on Rudder Twisted / Bent
Rudder stock Repairs must be considered on a case-by-case basis, so the following information is just a guideline. If 'L' be the length of the rudder stock over which the twist appears uniform, 'd' be the diameter of the twisted portion and A the angle of twist in degrees:
a. For angle A < L / d, i.e. for small angles of twist, the stock should be acceptable for further service without any heat treatment. This is subject to the rudder stock being free of defects such as cracks.
b. For L / d < angle A < 5 L / d, the stock is to be stress relieved by heat treatment to 625° C, the soaking time being one hour for every 25 mm thickness.
c. For angle A >5 L/ d, the rudder stock is to be fully annealed or normalized, by heat treatment at 900° C, the soaking time is 1/2 hour for every  25 mm thickness. This should be cooled slowly in the furnace for full annealing or cooled in the air in case of normalizing.
d. The furnace should be checked to be wide enough to accommodate the whole rudder stock, and have properly calibrated instruments. Welding repairs should be carried out prior to this heat treatment so that it will also confirm the post-weld heat treatment requirements.
Repair of Rudder stock by welding
Before caning out any welding, it is necessary to check the material of the rudder stock by chemical analysis, to ascertain its suitability for welding. This is usually mentioned in the original certificate for the rudder stock. If this is not available, an analysis of the representative drilling of the stock will show: When carbon content <0.25% or carbon equivalent <0.4%. It is weldable, where carbon equivalent = (C + Mn/6 ).
If the stock was originally made by welding two parts together, it is weldable, -unless the problem is associated with a weld failure. Repairs are normally done by removal of the rudder stock in the dry dock. Excavation of 'U' type is made to allow good access for welding, removing the minimum of material, which must ensure that all defective material is removed. The complete removal of defective material is verified before welding, by a Magnetic particle test or at least by Dye penetrant test.
Welding consumables are to be of an approved low hydrogen type depositing weld material with mechanical properties similar to that of the parent stock. Only certified Welders are used, and the welding is to be witnessed by Class / other competent authority.

Selection of electrode

American welding society (AWS) and American standards for testing and materials (ASTM) have set up certain requirements for electrodes to ensure uniformity in electrode manufacturing.

Thus different manufacturer's electrodes that are within the classification of AWS & ASTM should have the same characteristics.

A specific symbol is given in each type of electrodes, such as E-6010, E-7010, and E-8010.

The prefix E indicates the electrodes for the electric arc welding.

In the symbol, the first two digits designate the minimum allowable tensile stress in thousands of psi (pound square inch) of the deposited weld metal.

The third digit indicates the welding position for which the electrode is designed.

1- any position

2- horizontal and flat only.

The fourth digit indicates the type of coating and current used for the electrode by using the number from 0-8.

The third and fourth digits are often identified together to know the welding material, quality of the weld, and amount of penetration.

Welding position and type of joint are important factors in determining the size of the electrode. For example, if it is welding of a thick metal section with a narrow V-cut, a smaller diameter electrode is always used to ensure the full penetration at the root of the weld.

3/16 Inch is the largest diameter electrode which can be used for vertical and overhead welding positions regardless of the plate thickness. This electrode takes about one-half of the time to deposit the equal quantity of the weld metal, as the 1/4 Inch electrode does.

The deposition rate and joint preparation are also important in the selection of electrodes.

Fast freeze, fill freeze and fast fill are the classifications of electrodes for welding mild steels.

Fast freeze electrodes produce a strong, deep penetrating arc and fast freeze deposits. These types of electrodes have little slag and produce flat beads. They are widely used in all position welding but mostly preferred for vertical and overhead welding. 

Fill freeze electrodes produce a moderate force arc deposition rate and they have complete slag coverage. these types of electrodes are widely used in all position welding for repair works.

Fast fill electrodes are the heavy coated, iron powder electrodes producing a soft arc and fast deposition rate. These types of electrodes have heavy slag and produce smooth weld deposits. They are suitable for flat position welding. 

Rules for selecting the right electrodes:

Never use an electrode of diameter greater than the thickness of the metal to be welded.

Use small diameter electrodes for welding narrow V-grooves.

For welding alloy materials, pick the electrode which is similar in composition to the base metal.

For economy, use the largest electrode which is feasible for the work. 

refer to the electrode amperage chart below:

Defects in welding

1. Cracks:
1. It is a fracture type of discontinuous development characterized by a sharp tip and a high ratio of length and width to opening displacement.
2. They are caused due to poor ductility of the base metal, high velocity of arc, electrodes with high hydrogen content, and concave-shaped weld bead. 
3. due to the effects of welding crack, the strength of the weld is reduced, the effective thickness of the base metal is reduced and joints get broken.
2. Porosity and blowholes:
1. porosity is a group of small holes, whereas, blowhole or gas pocket is a comparatively bigger isolated hole or cavity. they occur mainly due to entrapped gases. The parent metal melt by means of the arc tends to absorb gases like hydrogen, carbon monoxide, nitrogen, and oxygen if they are present around the molten weld metal pool. 
2. Porosity and blowhole may also occur if tungsten is present in the weld metal and improper use of the electrode.
3. Due to the effects of porosity and blowholes, the workpiece experiences entrapment of hydrogen, oxygen, and carbon monoxide and a reduction in the ductile property of the base metal.
3. Slag inclusion: 
1. It is the non-metallic solid materials entrapped in the weld metal or between the weld and the base metal. 
2. The causes are a high rate of solidification, improper arc length, impurities in the base metal, and filler wire.
3. Due to the effects of slag inclusion, the strength of the joint is reduced and the impurities get adhered to the base metal.
4. Undercutting: 
1. In undercutting, a groove gets formed in the parent metal along the sides of the weld bead. 
2. The causes are high electrode diameter, the higher velocity of the arc, impurities present in the job surface, and more length of the arc.
3. Due to the effects of the undercutting, the metallurgical properties of the base of the solid part of the weld get reduced and the strength of the weld reduces.
4. Grooves reduce the thickness of the plate and thus the area along with the bead, which in turn weaken the weld.
5. Incomplete fusion and penetration: 
1. Incomplete fusion is a weld discontinuity in which fusion doesn't occur between weld metal and fusion faces. this absence of fusion may occur at any location within the weld joint and may present in fillet weld and groove welds. 
2. Penetration is the distance from the base plate top surface to the maximum extent of the solid piece of the weld. Incomplete joint penetration is described as a joint root condition in a groove weld in which weld metal does not extend through the joint thickness.
3. It is the failure of filler metal or base metal to completely fill the root of the weld.
4. The causes of Incomplete fusion are the improper position of the electrode entrapment of oxides and slag, improper size of the electrode, and low supply of hear input. Due to the effects of the incomplete fusion, the weld is non-uniform and the strength of the weld is reduced.
5. The causes for the incomplete penetration are the high velocity of the arc, improper alignment of the tack-weld setting of the workpieces, long length of the arc, and wrongly held electrode. Due to the effect of incomplete penetration, the strength of the weld is not uniform and breakage occurs in the joints.
6. Distortion: 
1. While welding a job, base metal under the arc metal, base metal ahead gets preheated the base metal portion already welded starts cooling. There is a good amount of temperature difference at various points along the joint and thus at ant distance certain area of base metal expand and other including weld bead contact.
2. The causes are improper welding sequence, improper alignment and tack weld setting of the pieces, slow velocity of the arc, and high residual stresses in the base metal. Due to the effects of the distortion, the strength of the weld and ductile property of the base metal gets reduced. 

Q. Slot welding process? Construction of rudder?

Q. What will be the size of the electrode for a 12mm steel plate?

Q. Overhead welding procedure & Precautions?

Q. Vertical welding procedure & Precautions?

Q. TIG & MIG?

Q. Metal arc welding precautions?

Q. Two copper pipes joining without welding?

Q. Checks in welding?

Q. S/W pipe leakage? Welding method to be used? electrode size and current? formula?

Q. C. I welding precautions? 

Q. ND tests for welding defect detection.

Q. Ultrasonic test and its limitations.

Q. Precautions while doing DPT.

Q. Bilge keel welding?