Welding reviews 

1.Inverters

Inverters are fast taking over from other types of welding machines. These machines are able to provide AC and smooth stable DC output at high-efficiency levels, and feature lightweight construction. The machines operate on either 240 V or 415 V AC input current and immediately rectify this to DC using a series of high temperature diodes. This DC current is stored in filter capacitors and then converted to an oscillated AC output in an oscillator stage at a much higher frequency than the input supply. This high-voltage/high-frequency signal is then fed into a high-efficiency transformer primary coil and high-frequency AC current is produced in the secondary coil. The frequency can be anything from 5 kHz upward, depending on the design and type of output required. Due to the high-frequency AC generated by the oscillator, the weight of transformers can be reduced dramatically because there is no magnetic loss or heat loss through the windings, and much greater transformer efficiencies can be achieved. Now that a high-frequency, low voltage, but high current power supply has been created it can be used as high-frequency AC welding power for MMAW or GTAW of aluminium. Alternatively the AC can then be rectified into DC current again and passed through a second filtering system to produce a very smooth current flow. Welding machines with an output frequency of around 5 kHz demonstrate a characteristic whistling sound during welding. Inverter welding machines have very good electronic controls that can regulate the voltage and current. They are also very efficient and highly portable due to their reduced weight.

The inverter cycle 1. Mains current rectified to DC and stored. 2. DC is oscillated to a high frequency. 3. High-frequency/high voltage AC is transformed to low voltage AC. 4. AC rectified to produce DC. 5. DC filtered to a smooth current. Inverters and advantages 1 Weight reduction, hence portability. 2 Multi process application SMAW,GTAW,GMAW 3 DC output for variety of applications 4 Efficiency 80-90 percent 5 Best performance 6 Ease of controls 

2.Basic SMAW

  Control of arc blow: 1. Use AC instead of DC 2. Weld away from work lead 3. Use shorter arc length 4. Change welding direction Size of electrode to be used 1.Thickness of plates 2.Welding position 3.Type of joint 4.Welder skill

Filter plate shade  number for SMAW : 10-14 as per electrode dia. Electrode selection for SMAW 1.Base metals  2.Type of joint. 3.Fitup 4. Amount of weld metal required 5.Welding position 6.Type of power supply7.Welder skill Problems due to moisture in electrodes: Electrodes flux coating absorbs moisture from atmosphere and cause hydrogen in weld.This results in cracking and hydrogen embrittlement. The drying and re baking  procedure recommended for each type of electrode by suppliers  to be strictly followed for getting best weld quality. Restarting of an arc in existing weld: strike arc 25 mm away and come back to where weld was stopped and continue to weld to get good quality free of defectshttp://www.welding.com  

3.Welding metallurgy :

  The reactions that take place are typical to welding. 1 Melting 2.Solidification 3. Gas metal reactions 3.Slag metal reactions 4.Surface phenomena 5.Solid state reactions. Preheating temperature: This is a function of carbon equivalent and base metal thickness and should be maintained over a specific area on either side of weld during welding. Post heating:Immediately after completion of welding , the joint area is heated to a higher temperature to reduce cooling rate and avoiding cracking in hardenable steels. Interpass temperature: Every weld pass should have temperature not less than preheat temperature before depositing subsequent weld pass. Cold cracking:This is a serious problem and strictly to be controlled.Stresses,hydrogen level to be minimised to avoid cold cracking which can occur any time after welding is completed.Defects which cause high level of stress Concentration lead to cold cracking.Low hydrogen electrodes, pre heating, inter-pass temperature control, post heating, stress relieving require to be attended during welding procedure development. Hot cracking:This occurs during weld solidification due to low melting constituents present at grain boundary.In steels Manganese  to Sulphor Ratio to be maintained as 30 to avoid hot cracking. Residual stresses: These are locked up stresses due to differential heating and cooling associated during welding.The magnitude of transverse and longitudinal residual stresses is of great concern during service as failures occur if not minimised. Distortion: Permanent deformation during welding fabrication is of great worry and involves  lot of expenditure for correction.Weld sequencing and control of stresses to be followed. Factors for distortion and residual stresses: 1.Heat input 2 Melting point 3. Coefficient of thermal expansion between metals 4. Phase changes 5. Heat treatment 6. Welding techniques 7. Applied stresses 8, Preheat Welding discontuities: These are to be understood as acceptable or not acceptable.They are due to solidification,processing by welding,heat treatment and service conditions such as stresses,corrosion,fatigue loading etc. Planar discontinuities : Cracks,Lack of fusion,Lack of penetration, under cut. Volumetric discontinuities are slag inclusions,pores,cavities etc.Geometrical type are excess reinforcement, root cavity etc. Estimation of heat input: Heat input in joules/m is given by the ratio of (voltage*current)/(speed of welding).This can be used to compare various processes and their relative metallurgical importance Weldability: It comprises process, material and service weldability. Welding procedure specification development should take care of all these three aspects.Risk of under bead cracking can be judged by carbon equivalent: %c + (Mn+Si )/6 + (Cr +Mo + V)/5 + ( Ni + Cu)/15 Low carbon steels with %C less than 0.15 have excellent weldability and %C between 0.15-0.30  good, and medium carbon steels 0.3-0.45 fair, high carbon steels 0.45-1.0 poor. Worldwide low carbon steels find extensive use in welded fabrication.Many commonly used welding processes are 1.Shielded metal arc welding2.Gas metal arc welding 3.Submerged arc welding4.Gas tungsten arc welding5 Electro slag welding6.Electro gas welding 7.Resistance welding 8.Friction welding. There are very reputed consumable suppliers in the world to give high quality jointshttp://www.esab.com High strength low alloy steels are extensively used in structural fabrication : Bridges, ship building,Construction etc.Structural welding code AWS D1.1 may be followed to avoid problems due to hydrogen cracking.http://www.twi-global.com Hydrogen embrittlement (case study): Cracks were observed in high tensile QT steel.The welds were completed by using shielded metal arc welding using matching electrode( E-11018M).The cracks were observed after heat treatment.Initially it was attributed to reheat cracking mistakenly till Welding-technologist analysed   the reason for such type of cracking.All factors 1.Preheat 2.Baking of electrodes 3.Post heating were  ensured during investigation trials.The cracks persisted after prolonged periods after completion of welding.This was definitely a case of hydrogen embrittlement.But all precautions were taken. The solution was found in reducing time interval between electrode drawl and use which kept the hydrogen in the weld to a minimum level to avoid cracks.It should be remembered always that the acceptable  diffusable hydrogen level in welds comes down drastically as the tensile strength increases.(Courtesy:IIW.journal).

4.Welding codes and standards

  Product manufacture requires meticulous use of relevant codes and standards as applicable to the specific product. ASME is referred all over the world for pressure vessel manufacture. http://www.asme.org ASME section IX is important for Welding procedure establishment and welder qualification. WPS(welding procedure specification),PQR(procedure qualification record) and welder qualification are all required during production of product and or assembly.Every detail mentioned in the code should be applied .Inspection authorities demand these documents for verification any time. Welding standards of importance AWS 1. A5.1-91 specifications for carbon and low alloy steel for SMAW 2.A 5.4-81 specifications for covered corrosion resisting chromium and Cr-Ni steel welding electrodes 3.A5.5-81 specification for low alloy steel covered arc welding electrodes 4.A5.11-90 specification for Ni Ni alloy welding electrodes for shielded metal arc welding 5 A 5.12-80 specification for tungsten arc welding electrodes Apart from AWS many standards prepared by UK,Europe,CANADA,Japan,India,France are also very popular. 5.K-TIG process This is a patented process for welding Stainless steels,low alloy steels,Titanium alloys using key hole TIG process.Single pass full penetration welds are possible with high welding speeds.Special purpose equipment with automatic controls gives reliable welds of good quality. 6.welding defects Defects are discontinuities and to be understood very clearly .While testing by NDT the inspector analyses and classifies as acceptable or not acceptable. Geometrical defects and process associated defects are essential part of fabrication. www.weldingandndt.com Repairs are more difficult and if wrong procedures are adopted,may lead to  premature failures. 7.Welding defects and remedies Causes and remedies for some important weld defects are discussed as below. 1.Abrupt changes in weld profiles for SMAW,SAW Causes Too low or too high current,voltage, Improper electrode manupulation(SMAW),Improper flux depth,High moisture in coatings or flux Remedies Use recommended current and voltage,Use correct electrode manipulation techniques,Proper moisture control,Correct flux depth 2.Excessive reinforcement on face and root Causes More current,Wrong manipulation,Large root opening Remedies Current current levels,Electrode manipulation at correct speeds, Specified root opening,Control the electrode displacement 3.Excessive concavity,covexity in fillet welds Causes High current in concave fillet welds,Low current in convex fillet welds,Wrong electrode angle,Improper electrode manipulation Remedies Correct current levels,Proper electrode manipulation,Correct electrode angle. 4.Under cut Causes High current,Wrong travel angle,electrode manipulation,Long arc and high arc voltage Remedies Use correct settings for all the above mentioned. 5.Porosity Causes Moisture in coatings,orfluxes or shielding gases,In adequate shielding,High current,High arc voltage,Contaminants. Remedies Removal of contaminants,Correct levels of parameters,Baking of electrodes and fluxes,Adequate shielding. Slag inclusions Causes Slag entrapment inSMAW,Submerged arc welding; Remedies Cleaning between passes,Selection of type of coating,flux,Proper electrode manipulation,Correct groove preparation Tungsten inclusions Causes Tungsten particles entrapped during welding when  tungsten electrodes touch molten puddle. Remedies Proper usage and handling of tungsten electrode during welding 6.Hot cracks Causes Solidification cracking caused by excess carbon and sulphur,Restraint on the joints Remedies Proper cooling rate,Selection of matching electrode 7.Cold cracks Causes Delayed cracking due to hydrogen levels beyond acceptable limits Remedies Preheating,Postheating,delayed cooling,Use of low hydrogen electrodes 8.Lack of fusion Causes Partial melting of weld metal due to wrong welding techniques,low welding current,high welding speed,improper cleaning between passes,improper joint fitup, Remedies Use of correct parameters,and procedures. https://youtu.be/HrcPZ3DsBJs 8.Characteristics of electrode coatings Electrode coatings are important for arc characteristics.There are specified functions based on type of coating.As per AWS electrode designation the coating constituents differ.Selection of electrodes is dictated by constituents and their function. https://youtu.be/3lIHbeYvAZs Welding placements