Before Welding 1. Safety: Safety: To check all operations are carried out in complete compliance with local company or National safety law i.e (permits to work are in place) 2. Documentation: Documentation : To check, a) Specif Specifica icatio tion n (year (year & revi revisio sion) n) b) Corr Correc ectt revi revised sed draw drawin ings gs.. c) Welding procedure specifications (WPS) & welder approvals (WQT) d) Calibration certificates of welding equipment / ancillaries ancillaries & all inspection instruments. e) Material & consumable certificates. 3. Welding process & ancillaries: ancillaries : To check welding equipment & all related ancillaries (cables, regulators, ovens, quivers etc.) 4. Incoming Consumables: Consumables: To check all pipe / plate & welding consumables for size, t ype & condition. 5. Marking out preparation & set up: up : To check, a) Correct Correct method of of cutting cutting weld preparati preparations. ons. (Pre-heat (Pre-heat for thermal thermal cutting cutting if if applicable) applicable) b) Correct Correct preparation preparation (relev (relevant ant bevel angles, angles, root root face, root root gap, root root radius, radius, land etc.) etc.) c) Correct Correct pre-weldin pre-welding g distortion distortion control. control. (Tacking (Tacking,, bridging, bridging, jigs, jigs, line up clamps clamps etc.) etc.) d) Correc Correctt pre-heat pre-heat appli applied ed prior prior to tack tack weldin welding. g. e) All tack tack weld welding ing to use use monit monitore ored d & inspec inspectio tion. n.
During Welding a) b) c) d) e) f) g)
To check, Pre-he Pre-heat at values values (heatin (heating g method, method, locati location on & control) control) In process process distortion distortion control control (sequence (sequence or balanced balanced weldin welding) g) Consumable Consumable control control (speci (specificati fication, on, size, size, condition condition & any specia speciall treatments) treatments) Process Process type type & all related related variabl variable e parameters parameters (voltage (voltage,, amperage, amperage, travel travel speed) speed) Purging Purging gases gases (type, (type, pressure pressure / flow & control control method) method) Welding Welding conditions conditions for root root run / hot pass pass & subseque subsequent nt run & inter-run inter-run cleaning. cleaning. Minimum Minimum or maximum maximum inter-pass inter-pass temperature temperature (temper (temperature ature & control controlling ling method) method)
After Welding a) Visual inspection of the welded joint (including (including dimensional check) b) NDT requirements (method & qualification of operator) c) To identify repairs from assessment of visual of NDT reports d) Post weld heat treatment (PWHT) (Heating (Heating method & temperature recording system) e) To re-inspect with with visual/NDT after PWHT (if applicable) f) Hydro test procedures procedures (for pipelines pipelines or pressure vessels) vessels) Repairs: Repairs: To check, a) Excavat Excavation ion proc procedu edure re (appr (approva ovall & executi execution) on)
Approval of the NDT Procedures. c Repair procedure. c) Execut Execution ion of of approv approved ed re-we re-weldi lding ng proce procedure dure.. d) To re-inspect re-inspect the the repair repair area with with visual visual inspection inspection & approved approved NDT method) method) e) To submit submit inspectio inspection n reports reports & all related related document documents s to the QC QC department. department. After all , responsibilities of welding inspector are, To observe all actions related to weld quality th roughout production. This will include a fina l visual inspection of the weld area. To record all production inspection points record showing all identified weld imperfections. To compare all reported information with the acceptance levels / criteria & clauses with the applied application standard. : Used to assess root penetration & fusion in double sided but welds & the internal faces of single
sided butt welds. welds. Test is carried out for a welder approval test. The specimen is normally cut by hacksaw hacksaw through the weld faces to a depth stated in the standard. It is then weld in a vice & fractured with a hammer blow from the rear. Once fracture has been made then both fractures are inspected for imperfections. As we are checking weld quality, test is qualitative mechanical test.
Generally performed magnifications greater than 10X Determine micro structural constituents, presence of inclusions, presence of microscopic defects, nature of cracking etc. Require fine grinding & polishing to produce a mirror finish. Pictures of micro specimens are called photomicrographs .
Generally performed using magnifications 10X or lower. Determine depth of fusion, depth of penetration, effective throat, weld soundness, degree of fusion, presence of weld discontinuities, weld configuration, number of weld passes etc. Some macro specimens need only be rough ground but mostly fine grinding & even polishing pictures of macro specimens are called photo macrographs
1) Mode of Operation : Manual 2) Principle of Operation:Arc is struck between short flux bonded metal electrode & the work piece. Both the electrode & work piece surface melt to form a weld pool. Simultaneously melting of flux coating on the rod will form a gas & slag which protects the weld pool from the surrounding atmosphere. One of the weld run is completed the slag must be chipped off. 3) Basic Equipment requirement:
Transformer rectifier (constant current (dropping) characteristic) b) Power / Power return cables c) Electrode holders.
g) Holding oven (temp. up to 200 C) d) Visor with lens e) Electrode f) Electrode oven (bakes up to 350 C) 4) Arc striking: The arc is struck by striking the electrode on the surface of the plate & withdrawing it a small distance. Maintain short, constant arc length. 5) Weld refining & cleaning : Refining & cleaning compounds within the bonded flux. 6) Process variable parameters: a) OCV (Open Circuit Voltage) Requirement to initiate or re-ignite the arc & change with type of electrode being used. Arc voltage changes with change in arc length. b) Current : Type & value of current will be determined by the choice of electrode classification, diameter, material type, thickness & welding position. c) Polarity : AC/DC+/- (Electrode + or - & polarity reversible or straight) or electrode coating being used. d) Full electrode specification & diamter : Should be correctly written on electrode & electrode box. °
Electrode pre-use: Basic coated electrodes (i) should be baked at 350 C for 1 Hr. (ii) Held in holding ovens at 150 C (iii) Finally in a heated quiver (around 70 C0 with welder for welding. f) Speed of travel: High dependant on the skill of a welder. 7) Consumables: Core solid wire between 350 & 450mm & 2.5-6mm diameter, covered with bonded flux coating core wire generally low quality steel. Electrodes are grouped depending on the main constituent in their flux coating. The common groups arc Basic – Calcium carbonate & calcium fluoride (Electrode no. ending with5,6&8) Cellulosic – Cellulose (Electrode no. ending with 0 &1) Rutile – Titania (Electrode no. ending with 2,3, &4) 8) Typical imperfections: i) Slag inclusions : Poor welding technique & insufficient inter run cleaning. ii) Hydrogen cracks : Incorrect electrode type or baking procedure. 9) Advantages: i) Range of consumables. ii) A ll positional. 10) Disadvantages i) High level of generated fumes. ii) Hydrogen control 11) Positional capabilities : All positional but depend on consumable type. °
1) Mode of Operation: Manual but can be semi-automatic 2) Principle of operation: Small diameter solid wire and shielding gas (inert gas) is used. The arc is produced between a non-consumable electrode (tungsten) & the work piece. Operator must control the arc length & also add filler metal into the weld pool 3) Basic equipment requirements: a) Transformer / Rectifier (constant current (drooping) characteristic) b) Head / Hose assembly. c) Power return cable. d) Torch head assembly e) Gas cylinder, hoses, regulators, flow meter. f) Visor with lens. g) Fume extraction. 4) Arc Striking : The arc striking (scratch start) the core wire onto the plate and withdrawing cause contamination of the tungsten and weld metal to work on this high frequency arc is used cause interference. To work on this, lift arc is used where the electrode is touched on to the plate & is withdrawn slightly. 5) Arc and Weld Shielding: Inert gas (pure argon & helium) is used to shield arc & weld. Gas cut-off delay is used to shield weld metal at the end of a run.
Weld refining & Cleaning: Very clean high quality drawn wire is used. Process variable parameters: a) Voltage : Changes with change in arc length & type of gas being used. b) Current : Changes with change in tungsten diameter. Slope in & slope out controls the current at the start & end of the weld.
Polarity : DC –ve for steels , AC for Aluminum. d) Inert Gas type: Pure gases argon & helium are used. Nitrogen added for copper welding. Mixture (Ar+ He) gives good gas cover & penetration. e) Gas Flow rate : Should be correct for given joint design & position as given in approved welding procedures. f) Purging : Purging gas pure argon used to reduce atmospheric root oxidation. g) Tungsten type : Thoriated tungsten for DC and zirconated tungsten for AC. Too fine vertex angle will melt the tungsten tip. With AC, the tungsten end is chamfered & forms a ball end during aluminum welding Consumable : High quality drawn wire & inert gas (pure argon or helium or mixture of both) a) Typical imperfection: Tungsten inclusions: Caused by a lack of welder skill, too high current & incorrect vertex angle. Crater pipes : Caused by poor weld finish technique or incorrect use of current decay. a. Weld/root oxidation : If using insufficient gas cut-off delay or purge pressure.
High quality weld b) Disadvantages:
b) All positional
c) Low inner run cleaning
Small range of consumables. Positional capabilities: All positional.
b) High ozone levels.
c) Low productivity
Metal Inert Gas (MIG) Welding Process Metal Active Gas (MAG) Welding Process 1. Mode of Operation : Semi-automatic 2. Principle of Operation : Copper coated or uncoated small diameter solid continuous wire from a spool & shielding gas (Argon + CO2) is used. Arc is produced between a metal electrode wire & the work piece to form a weld pool. 3. Basic equipment requirements:
Transformer / Rectifier (constant voltage (flat) characteristic) b) Head / hose assembly c) Wire liner d)Power return cable e) Wire feed unit, wire spool f) Gas cylinder, hoses, regulators, flow meter g) Visor with lens i) Fume extraction. 4. Arc Striking: The arc is struck in three different metal transfer modes. b) Dip transfer : The wire short circuits the arc & the molten metal forming on the wire is transferred by the wire dipping into the weld pool. ii) Spray transfer : The wire does not make contact with the weld pool. The molten metal at the tip of the wire transfers to the weld pool in the form of spray of small droplets. iii) Pulsed transfer : Uses pulses of current to fire a single global of metal across the arc gap.
Arc & Weld shielding : Cylinder fed inert / active gas shield for arc & weld.
Weld refining & cleaning : Very clean, high quality drawn wire. 7. Process variable parameters: OCV (Open Circuit Voltage): Require to initiate or re-ignite the arc. Depend on type of gas being used & type of transfer achievable. Current /Wire feed speed (WFS): Increasing the wire feed speed automatically increases the current in the wire. Polarity : DC –ve (Electrode positive – Reversible) Gas type : Mixture of argon & Co2 (5-20%) to get good penetration, stable arc, very little spatter. Gas flow rate : Adequate to give good coverage over solidifying & molten metal to avoid oxidation & porosity. Inductance: Causes a backpressure of voltage to occur in the wire & operates only when there is a change in current. Reduce level of spatter. Electrode diameter: (Generally produced in 0.6/0.8/0.1/1.2/1.4&1.6mm diameter. Contact tip/drive roller & liner sizes : Both should be of correct size for the wire being used. Loss in contact between the wire & contact tip will reduce current pick. Contact tip should be replaced regularly. The drive roller pressure is only hand tight just to drive the wire. Liner should be of correct type & size for the wire. Wire Feed Speed (WFS) : Incrasing the wire feed speed automatically increases the current in the wire. Consumables : High quality drawn wire & inert active gas. Typical imperfections: i) Burn through : Incorrect metal transfer mode. ii) Solica inclusions : Caused by poor inter run cleaning. Advantages: i) Material thickness range b) High productivity (o/f) c) Continuous electrode Disadvantages i) Small range of consumables b) High ozone levels c) Protection for site working. Positional Capabilities: Dip – All positional Spray – Flat only Pulse – All positional
Mode of Operation: Usually automatic but it can be semi-automatic. Principle of Operation: Granular flux & bare solid wire is used. Arc is submerged hence no visible sign of arc. Arc melts the electrodewire, flux & some base metal to form weldpuddle. Basic equipment requirements: i) Transformer / rectifier (constant voltage( flat )characteristic) ii) Head/Hose assembly iii )Power return cablei iv) Wire feed unit v) Flux hopper / delivery / recovery system vi ) Run on/off tabs vii) Tractor carriage viii) Fume extraction. Arc Striking: Wire contact is made by the advancement of the wire by mechanical drive. Arc & Weld shielding: Granular flux uses to generate protective gases & slag, & to add alloying elements to the weld pool. Weld refining & cleaning: Refining & cleaning compounds within the flux Process variable parameters: a) OCV (Open Circuit Voltage): Required to initiate or re-ignite the electric arc. b) Arc Voltage: Changes with arc length. Arc is submerged any changes in arc length will change in weld metal composition . c) Current / WFS (Wire Feed Speed): Increasing the wire feed speed automatically increases the current in the wire. d) Polarity: AC/DC +/- . e) Flux type & size:i) Fused fluxes: acidic type. Agglomerated fluxes (Bonded fluxes): basic type. The shape of the flux is granular f) Electrode wire type & diameter: High quality wire in 2.4 – 6 mm diameter in coils. Large diameter reduces penetration. g) Electrode wire / flux specification: Composition & grading is selected to suit the electrode & parent metal. h) Wire Feed Speed( WFS): Increasing the wire feed speed automatically increases the current in the wire. Consumables: High quality drawn wire & granular flux. Typical welding imperfections: (i) Centerline cracks : Deep & narrow welds. (ii) Shrinkage cavities: caused by a weld depth / width ration of > 3/2
Advantages (i) High productivity (ii) No visible arc light Disadvantages (i) Restricted in positional welding II) Variable compositions (Arc length) Positional Capabilities: Flat only, but may be H/V butt welds.
Location : Parent Material Steel Types : Any steel type Susceptible Micro structure : Low through thickness ductility Lamellar tears are terrace like separations in the base metal. They are caused by shrinkage stresses in the through thickness direction of the plate just below HAZ.Lamellar tears can cause a serious failure. Micro impurities such as sulphides & silicates which occur during steel manufacture, causes this poor through thickness ductility this may lead to lamellar tearing. Prevention of Lamellar Tearing a) Checking the chemical analysis & for laminations with UT & PT on plate edges. b) Change of weld design c) Use weld metal buttering layers d) Minimize restraint f) Use pre formed ‘T’ piece for critical joints.
HAZ Longitudinal Weld metal Transverse or Longitudinal Steel type : All hardenable steels HSLA steels & QT steels Susceptible microstructure : Martensite Also referred to as cold cracking, delayed cracking or HAZ cracking. Hydrogen cracking may occur in the HAZ or weld metal, depending on the type of steel being welded. Hydrogen may be absorbed into the arc from water on plates, moisture in the air, paint or oil on the plates or the breakdown of gas shielding etc. If the HAZ or weld has some harden ability then the chances of hydrogen cracking is more.
The four minimum critical factors & their values, where hydrogen cracking is likely to occur, arc considered to be: a) b) c)
Hydrogen content: Hardness : Stresses :
> 15ml/100 gm of deposited weld metal >350 VPN > 0.5 of the yield stress
: 2:1. Also called hot plastic tear with sharp edges & is treated as a crack. 3. Solid inclusions: include metallic & non-metallic inclusions that may be trapped in the weld during the process of welding. May be caused by a) Lack of welder skill (incorrect welding technique) b) Poor manipulation of the welding process, or electrode. c) Incorrect parameter settings, i.e) voltage, current, travel speed. d) Magnetic arc blow. e) Incorrect positional use of the process or consumable f) Incorrect inter-run cleaning. 4. Lack of Fusion: is a lack of union between two adjacent areas of material. A serious imperfection as produce areas of high stress concentration. This may caused by a) Lack of welder skill (incorrect welding technique) a) Poor manipulation of the welding process, or electrode. b) Incorrect parameter settings i.e) voltage, amperage, travel speed. c) Magnetic arc blow. d) Incorrect positional use of the process, or consumable. e) Incorrect inter-run cleaning. 5. Surface & Profile : Generally caused by poor welding techniques. Surface or profile imperfections are as follows. a) Incompletely filled grove: May bring the weld below its design throat thickness. Spatter: Not a major factor but should be cleaned off before inspection as it mask other imperfections. Can cause micro cracking. Arc Strikes (Stray arc or Stray Flash) can cause several types of cracks to occur. Normally be NDT inspected & then required. Incomplete root penetration: Can cause by too small a root gap, insufficient current or poor welding technique. Bulbous or irregular contour : Causes sharp stress concentrations at the toes & may also contribute to overall poor toe blend. Irregular bead width: Is a surface imperfection. Should be regular along its linear length. Undercut: Depression at the toe of a weld. Caused by incorrect welding technique, too high current & the welding position. Severity can be measured by its length depth & sharpness. Root concavity (Suck back): Caused when using too high a gas backing pressure in purging. Also produced when welding with too large a root gap & depositing too thin a root bead. Excess penetration / Burn through: Caused by using too high a welding current & or slow travel speed, large root gap &/or small root face for the current or process being used. Accompanied by burn through, which is a local collapse of the weld puddle causing a hole or depression in the final weld root bead. Root Oxidation: May take place when welding reactive metals such as stainless . steels with contaminated or inadequate purging gas flow. 6. Mechanical Damage: Surface material damage caused during the manufacturing process. Damage can be caused by Grinding, Chipping , Hammering , Breaking of welded attachments by hammering using needle guns to compress weld capping runs can cause local stress concentrations & should be repaired prior to completing the job. 7. Misalignment: Two forms of misalignment a) Linear misalignment. b) Angular misalignment.Linear misalignment can be controlled during weld set up by tacking, bridging, clamping etc. Excess weld metal height is always measured from the lowest plate to the highest point of the weld cap. Angular misalignment can be controlled by balance welding, offsetting or use of jigs, clamp etc.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
E 51 33
To assist arc ignition To improve arc stabilization. To produce a shielding gas to protect the arc column. To refine & clean the solidifying weld metal. To form a slag, which protects the solidifying, weld metal. To add alloying elements. To control hydrogen content of the weld metal To form a core at the end of the electrode, which directs the arc. To reduce the cooling rate. It acts as a deoxidizing agent.
B 160 2 O H
- Electrode - Tensile & Yield Strength (TS-510-650N/mm2,, YS-380N/mm 2,) - Toughness 28 & 47 Joules First digit 28J, Second digit 47J Testing temperature : - 20 C - Electrode Coating – B for Basic - Electrode efficiency - Welding position – 2 for all positions except vertical down - Electrical parameters – O for DC polarity as recommended & AC Min OCV ( Open circuit voltage) not recommended. - Low hydrogen potential (After baking)
E 46 3 1Ni
Electrode Tensile & Yield strength (TS 530-680N/mm2 , YS 460N/mm2) Toughness 47 Joules (3 for -30 C) Any light alloying composition. (Mn – 1.4, Ni – 0.6-1.2) B - Flux coating type (B-Basic) Electrical parameters & efficiency (AC+DC, Recovery% >125 < 160) 4 - We lding posit ion (4 for flat butt & f ille ts) H5 - Low hydrogen potential (after baking)
E 80 1 18 G
Electrode Tensile Strength x 1000 (TS 80000), YS 68 – 80000) Welding position (1 for all positions) Electrode coating * electrical characteristic (18 for basic +25% Fe powder, AC or DC+) - Low alloy steels ( Ni, Cr,Mo& V)
: Fused fluxes are mixed together & baked at a very high temperature where all the components
become fused together. When cooled the resultant mass resembles a sheet of black glass, which is then pulverized into small particles. These particles again resemble small silvers of black glass. They are hard, reflective, irregular shaped, & cannot be crushed in the hand. Fused fluxes tend to be of the acidic type produce comparatively low quality weld metal in terms of the mechanical properties of tensile strength & toughness.
fluxes are a mixture of compounds that are baked at a much lower temperature & are essentially bonded together by bonding agents into s mall particles. The recognition points of these types of fluxes is easier, as they are full, generally sound granules, that are easi ly crushed & can also be very brightly colored as coloring agents may be added in manufacture as a method of i dentification unlike fused fluxes. Agglomerated fluxes tend to be of the basic t ype & will produce weld metal that is of much higher quality in terms of strength & toughness
ULTRASONIC TESTING Advantages 1. Ferrous & non-ferrous alloys can be tested. 2. Can easily detect lack of sidewall fusion. 3. No major safety requirements. 4. Portable with instant results. 5. Able to detect sub-surface defects. Measures depth & through wall extent. 6. Can test heavy wall thickness job.
1. 2. 3. 4. 5.
Disadvantages High operator skill level. Difficult to interpret Requires calibration. No permanent record (unless automated) Not easily applied to complex geometry.
RADIOGRAPHIC TESTING 1. 2. 3. 4. 5.
Advantages Permanent record. Most materials can be tested. Detects internal flaws. Gives a direct image of flaws. Fluoroscopy can give real time imaging.
1. 2. 3. 4. 5.
Advantages Low operator skill level. Applicable to non-ferromagnetic materials. Low cost. Simple, cheap & easy to interpret. Portability.
Disadvantages 1. Skilled interpretation required. 2. Access to both sides required. 3. Sensitive to defect orientation (possible to miss planner flaws) 4. Health hazard. 5. High capital cost.
1. 2. 3. 4. 5.
1. 2. 3. 4. 5.
Disadvantages Careful surface preparation required. Surface breaking flaws only. Not applicable to porous materials. No permanent record. Potentially hazardous chemicals.
MAGNETIC PARTICLE TESTING Advantages Disadvantages Pre-cleaning not as critical as with DPI 1. Ferromagnetic materials only. Will detect some sub-surface defects 2. Demagnetization may be required. Relatively low cost. 3. Direct current flow may produce arc Simple equipment. strikes. Possible to inspect through thin coatings. 4. No permanent record. 5. Required to test in two directions.
Residual stresses are defined as those stresses remaining inside a material after a process has been carried out. The process used is welding & the stresses are caused by the heat of welding producing load expansion & contraction to take place. These stresses causes stress corrosion. Cracking to occur also affect dimensional stabilit y. The amount of contraction is controlled by : The volume of weld metal in the joint; t he thickness, heat input, joint design. Offsetting may be used to finalize the position of the joint. In plates or pipes arc prevented from moving by tacking, clamping or jigging etc. The movement caused by welding related stresses is called distortion,
Three basic directions of distortion are i) Longitudinal ii) Transverse iii) Short transverse A high percentage of residual stresses can be removed by heat treatments. The peening of weld faces (with the use of a pneumatic needle gun) will only redistribute the residual stress.
: Welding inspector have to ensure that the safe working practices are strictly followed. Several area where safety in welding requires arc as follows: . 1) Welding / cutting process safety. Electrical safety. Welding fumes & gases (use & storage of gases) Safe use of lifting equipments. Safe use of hand tools & grinding machines. General welding safety awareness. 1)Welding / cutting process safety: a)Removing any combustible materials from the area. Checking all containers to cut or welded arc fume free (permits to work etc) a) Providing ventilation & extraction where required. b) Keeping oil & grease away from oxygen. c) Appropriate PPE is worn at all times. d) 2) Electrical Safety: a)
Ensure that insulation is used where required & that cables & connections are in good condition. All electrical equipment must be regularly tested & identified. 3)Gases & Fume Safety: Gases & fumes may come from electrodes, plating, base metals & gases used in & produce during the welding process. Dangerous gases include ozone, nitrous oxides & phosgene, which are extremely poisonous & will result in death when over exposure occur. Cadmium chromium & other metallic fumes are extremely toxic & will result in death if over exposure results. 4)Lifting Equipments: It is essential the correct lifting practices are used for slinging. Should be regularly inspected. Care should be taken for cutting corners, as it is more dangerous. Don’t stand beneath a load when lifting is going on. 5)Hand tools & grinding machines: Hand tools should always be in a safe & serviceable condition & should always be used in a safe & correct manner. Use cutting discs for cutting grinding discs for grinding only. 6)General welding safety awareness: Be aware of the hazards in any welding job & always minimize the risk. Always refer safety advisor if any doubt exists.
All thermal cutting processes must satisfy two functions to used a cutting / gouging process. 1. A high temperature (capable of melting the materials being cut) 2. A high velocity (capable of removing the molten materials in the cut) Plasma Cutting: Utilizes the temperatures reached from the production of the plasmas from certain types of gases. Nitrogen gas plasma can reach a temperature of over 20,000 C but temperature of air plasma is much lower. There are two different types of plasma cutting process whic h are: Transferred arc (Used cutting conductive materials) Non-transferred to arc (Used for cutting non conductive materials) Arc cutting & gouging. Temperature attained by an electric arc can be used in cutting processes. There are three types of processes, the main differences being in the consumables & the gas used in producing the velocity required. Conventional cutting / gouging electrodes. Oxy-arc cutting / gouging. Arc – air cutting / gouging. Conventional cutting / gouging electrodes : The consumables consist of a light alloy central core wire, which is mainly to give rigidity & a heavy flux coating, which provides elements that produce arc energy. The arc is truck in a conventional way to MMA welding, however the melts the base material, which is then pushed away by using a pushing action with the electrode. °
Oxy-Arc cutting / gouging: Require a special type of electrode holder. The consumables arc tubular in section & arc coated with a very light flux coating. The arc is struck & compressed oxygen may be activated at the torch head. The heat of the electric arc melt the base metal or alloy & the velocity to remove it is provided by the compressed oxygen. This process is generally used for decommissioning / scrapping plant as the cut s urface is generally not consistent. Arc – Air cutting / gouging: Used for gouging old welds removing materials. The consumable is a copper coated carbon electrode. The gas used is compressed air. The process is basically a “melt & blow process”. The main disadvantage is high level of noise produced & the volume of fumes generated. The coat face will require dressing. A safety precaution is to use correct ear protection & breathing supply system.
Fluxes for Submerged arc welding 1) Fused fluxes : Fused fluxes are mixed together & baked at a very high temperature where all the components become fused together. When cooled the resultant mass resembles a sheet of black glass, which is then pulverized into small particles. These particles again resemble small silvers of black glass. They are hard, reflective, irregular shaped, & cannot be crushed in the hand. Fused fluxes tend to be of the acidic type produce comparatively low quality weld metal in terms of the mechanical properties of tensile strength & toughness. 2) Agglomerated Fluxes: Agglomerated fluxes are a mixture of compounds that are baked at a much lower temperature & are essentially bonded together by bonding agents into small particles. The recognition points of these types of fluxes is easier, as they are full, generally sound granules, that are easily crushed & can also be very brightly colored as coloring agents may be added in manufacture as a method of identification unlike fused fluxes. Agglomerated fluxes tend to be of the basic type & will produce weld metal that is of much higher quality in terms of strength & toughness
Residual Stresses & Distortion Residual stresses are defined as those stresses remaining inside a material after a process has been carried out. The process used is welding & the stresses are caused by the heat of welding producing load expansion & contraction to take place. These stresses causes stress corrosion. Cracking to occur also affect dimensional stability. The amount of contraction is controlled by :
The volume of weld metal in the joint; the thickness, heat input, joint design. Offsetting may be used to finalize the position of the joint. In plates or pipes arc prevented from moving by tacking, clamping or jigging etc. The movement caused by welding related stresses is called distortion, Three basic directions of distortion are i) Longitudinal ii) Transverse iii) Short transverse A high percentage of residual stresses can be removed by heat treatments. The peening of weld faces (with the use of a pneumatic needle gun) will only redistribute the residual stress.