Before Welding
Before Welding
1
1. Safety. Safety: To check all operations are carried out in complete compliance with local company or National safety law i.e: 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)
(permits to work are in place) 2
2. Documentation. Documentation: : To To check,check, a)
a) SpecifSpecificaicatiotion (n (year year & r& revievisiosion)n) b)
b) CoCorrrrecect ret revivised sed drdrawawiningsgs.. c)
c) Welding Welding procedure procedure specifications specifications (WPS) (WPS) & & welder welder approvals approvals (WQT)(WQT) d)
d) Calibration certificates Calibration certificates of welding of welding equipment / anciequipment / ancillaries & llaries & all inspection all inspection instruments.instruments. e)
e) Material Material & & consumable consumable certificates.certificates. 3.
3. Welding process & ancillariesWelding process & ancillaries: To check welding equipment & : To check welding equipment & all related ancillaries (cables, regulators, ovens, all related ancillaries (cables, regulators, ovens, quiversquivers etc.)
etc.) 4.
4. Incoming ConsumablesIncoming Consumables: To check all pipe / : To check all pipe / plate & welding consumables for size, tplate & welding consumables for size, t ype & condition.ype & condition. 5.
5. Marking out preparation & set upMarking out preparation & set up: To check,: To check, a)
a) Correct method oCorrect method of cuttif cutting weld prng weld preparatieparations. (Prons. (Pre-heat for te-heat for thermal chermal cutting iutting if applicf applicable)able) b)
b) CorreCorrect prepact preparation (ration (relevrelevant bevel angant bevel angles, roles, root face, root face, root gap, rooot gap, root radiust radius, land etc., land etc.)) c)
c) CorreCorrect pre-wct pre-weldinelding distorg distortion contion control. (Ttrol. (Tackingacking, bridg, bridging, jiging, jigs, line up cls, line up clamps etcamps etc.).) d)
d) CorCorrecrect pre-t pre-heat apheat appliplied pried prior to tacor to tack welk weldinding.g. e)
e) All tAll tack wack weldelding to uing to use mose monitnitoreored & insd & inspecpectiotion.n.
During Welding
During Welding
To check, To check, a)
a) PrePre-he-heat valat values (heaues (heatinting methog method, locd, locatiation & contron & control)ol) b)
b) In proIn process cess distodistortion rtion contrcontrol (sol (sequence equence or balor balanced wanced weldinelding)g) c)
c) ConsumaConsumable conble control (trol (specispecificatification, sizeon, size, condi, condition & any stion & any speciapecial treatl treatments)ments) d)
d) ProceProcess typss type & all ree & all related vlated variablariable paramete parameters (vers (voltageoltage, ampera, amperage, travge, travel speel speed)ed) e)
e) PurgiPurging gasng gases (es (type, type, presspressure / ure / flow flow & con& control trol methodmethod)) f)
f) WeldiWelding conding conditions tions for rofor root run / ot run / hot pashot pass & subs & subsequesequent run & nt run & interinter-run cl-run cleaning.eaning. g)
g) MinimMinimum or maxium or maximum intemum inter-pass r-pass tempertemperature (ature (tempertemperature & cature & controlontrolling meling method)thod)
After Welding
After Welding
a)
a) Visual inspection of Visual inspection of the welded joint (the welded joint (including dimensional check)including dimensional check) b)
b) NDT requirements (method NDT requirements (method & qualification of & qualification of operator)operator) c)
c) To identify repairs from To identify repairs from assessment of visual assessment of visual of NDT reportsof NDT reports d)
d) Post weld heat treatment (PWHT) (HPost weld heat treatment (PWHT) (Heating method & temperature recording system)eating method & temperature recording system) e)
e) To re-inspect wTo re-inspect with visual/NDT after PWHT ith visual/NDT after PWHT (if applicable)(if applicable) f)
f) Hydro test procHydro test procedures (for pipeliedures (for pipelines or pressure vessnes or pressure vessels)els) Repairs
Repairs:: To check,To check, a)
a) ExcExcavatavation pion procroceduedure (are (apprpprovaoval & execl & executiution)on)
b)
b)
Approval Approval of of the the NDT NDT Procedures. Procedures. c c Repair Repair procedure.procedure. c)c) ExeExecutcution oion of appf approvroved reed re-we-weldilding prng proceoceduredure.. d)
d) To re-iTo re-inspect tnspect the repaihe repair area wir area with visuath visual inspel inspection & ction & approvapproved NDT meted NDT method)hod) e)
e) To submiTo submit inspt inspectioection reportn reports & all rs & all related delated documentocuments to the Qs to the QC deparC department.tment. After all , responsibilities of welding inspector are,
After all , responsibilities of welding inspector are, To observe
To observe all actions related to weld quality thall actions related to weld quality th roughout production. This will include a finaroughout production. This will include a fina l visual inspection of the l visual inspection of the weldweld area.
area. To record
To record all production inspection points record showing all identified weld all production inspection points record showing all identified weld imperfections.imperfections. To compare
To compare all reported information with the all reported information with the acceptance levels / criteria & clauses with acceptance levels / criteria & clauses with the applied applicationthe applied application standard.
standard.
:: Used to assess root penetration & fusion in double sided but welds & the internal faces of singleUsed to assess root penetration & fusion in double sided but welds & the internal faces of single sided butt welds
sided butt welds. . Test is carried out for Test is carried out for a welder approval test. The specimen is normally cut by a welder approval test. The specimen is normally cut by hacksahacksaw through the w through the weldweld 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 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 has been made then both fractures are inspected for imperfections. As we are checking weld quality, test is qualitative mechanical test.
mechanical test.
Generally performed magnifications greater than 10X Generally performed magnifications greater than 10X
Determine micro structural constituents, presence of inclusions, presence of Determine micro structural constituents, presence of inclusions, presence of microscopic defects, nature of cracking etc.
microscopic defects, nature of cracking etc.
Require fine grinding & polishing to produce a mirror finish. Require fine grinding & polishing to produce a mirror finish. Pictures of micro specimens are called photomicrographs Pictures of micro specimens are called photomicrographs..
Generally performed using magnifications 10X or Generally performed using magnifications 10X or lower.lower.
Determine depth of fusion, depth of penetration, effective throat, weld soundness, Determine depth of fusion, depth of penetration, effective throat, weld soundness, degree of fusion, presence of weld discontinuities, weld configuration, number of degree of fusion, presence of weld discontinuities, weld configuration, number of weld passes etc.
weld passes etc.
Some macro specimens need only be rough ground but mostly fine grinding & 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:
a)
Transformer rectifier (constant current (dropping) characteristic) b) Power / Power return cables c) Electrode holders.d)
Visor with lens e) Electrode f) Electrode oven (bakes up to 350°C) g) Holding oven (temp. up to 200°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.
e)
Electrode pre-use: Basic coated electrodes (i) should be baked at 350°C for 1 Hr. (ii) Held in holding ovens at150°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.
6)
Weld refining & Cleaning: Very clean high quality drawn wire is used. 7) 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.
c)
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.
b)
Advantages:a)
High quality weld b) All positional c) Low inner run cleaning b) Disadvantages:a.
Small range of consumables. b) High ozone levels. c) Low productivity Positional capabilities: All positional.Metal Inert Gas (MIG) Welding Process Metal Active Gas (MAG) Welding Process
1. Mode of Operation : Semi-automatic2. 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:
a)
Transformer / Rectifier (constant voltage (flat) characteristic) b) Head / hose assembly c) Wire liner d)Power return cablee) 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.
5.
Arc & Weld shielding : Cylinder fed inert / active gas shield for arc & weld.6.
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.
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.
Location : 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) Hydrogen content: > 15ml/100 gm of deposited weld metal b) Hardness : >350 VPN
c) Stresses : > 0.5 of the yield stress
d)
Temperature : <300°CPrevention of hydrogen Cracking
a) Use a low hydrogen process & electrode b) Maximise arc energy.
c) Minimise restraint
d) Ensure plate is dry & free from rust, oil, paint or other coatings. e) Control interpass temperature
f)
Ensure welding is carried out under controlled environmental conditions. g) Ensure pre-heat is applied. h)Use a constant & correct arc length.Location : Weld center ( longitudinal )
Steel types : All
Susceptible microstructure : Columnar grains
( In the direction of solidification )
Also called hot cracking occurs during solidification of welds in steel, having high sulphur content or contaminated with sulphur. Another cause is the depth / width ratio of the weld which refers to as deep narrow welds. Therefore if we have a combination of deep narrow welds with a high incidence of sulphur then we are increasing the chances of hot cracking. During welding, sulphur in or on the plate may be re-melted will join with the iron to form iron-sulphides. Iron sulphides are low melting point impurities, which will seek the last point of solidification of the weld, which is the weld centerline.
Prevention methods for solidification cracking
a)
Use low dilution processes b)Use high manganese consumables c)Maintain a low carbon content d) Minimize restraint / stress e) Specify low sulphur content of plate f)Remove laminationsg)
Through cleaning of preparation h)Minimize dilutionLocation : Weld HAZ ( longitudinal ) Steel types : Autenitic stainless steels
Susceptible microstructure : Sensitised grain boundaries
During welding of austenitic stainless steels when heated to the welding temperature a portion of base metal reaches temperatures in 500°- 900°C range & with this temperature range the chromium & carbon present in the metal combine to
form chromium carbides. These chromium carbides typically found along the grain boundaries of the structure. This result of their formation is the reduction of the chromium content within the grain itself adjacent to the grain boundary called “ chromium depletion”
Resulting reducing the chromium content below that desired. After such an effect we can say that the stainless steel has been sensitized that is to say it has become sensitive to corrosion. If no further treatment is given, corrosion will appear parallel to
the weld toes, within the HAZ. This corrosion will occur only when the weld is subsequently put in service. This s commonly known as weld decay.
Prevention of weld decay in stainless steel:
a) Use parent material with a carbon content below 0.03%
a) To add stabilizing elements such as niobium & titanium to the plate & electrodes to stabilize the steel. b) Maximum interpass temperature controls applied
c)
After welding heating to 1100°C & quenching.All heat treatment are basically cycles of three elements, which are a) Heating b) Holding & Soaking 3)Cooling.
Heat treatments given to metals are as follows.
1) Annealing 2) Normalizing 3) Hardening 4) Tempering 5) Stress Relieving. 6) Pre-heating.
The methods/sources that may be used to apply heat to a fabrication may include: i) Flame burners / heaters - pre-heating.
ii) Electric resistance heating blankets – Preheating & PWHT.
iii) Furnaces – Annealing, Normalizing, Hardening, Tempering. The tools use to measure the temperatures of furnaces & heated materials may include:
a) Temperature indicating crayons (tempil sticks) – preheating, PWHT. b) Thermo-couples – All heat treatments.
c) Pyrometers (Optical, resistance, radiation) – Furnace heat treatments. d) Segar cones – Furnace heat treatments.
1) Annealing: The term annealing generally means to bring a metal or alloy to its softest or most ductile natural condition. Annealing is a heat treatment process that may be carried out on steels, & most metals to regain the softness ductility. In steels we can carry out two kinds of annealing:
a) Full annealing (including solution annealing) b) Sub critical annealing
In full annealing of steels the steel is heated above its UCT (Upper Critical Temperature) & allowed to cool very slowly in a furnace for 1 hour every for every 25mm of thickness. The furnace is then turned off & the steel remains in the furnace to cool. This produces a large or course grain structure that is soft & ductile but has very low toughness. The UCT range of plain carbon steels is between 723-910°C. In sub critical annealing the steel is heated to temperatures well below the lower critical
temperature (723°)
2) Normalizing: Generally used for steels. The steel is heated above its UCT & soaked for 1 hour for every 25mm of thickness. Once the soaking time finished the steel is removed from the furnace to cool in still air. This produces a much finer grain structure than annealing & although the softness & ductility is reduced, the strength & hardness is increased.
3) Hardening: The steel is heated above its UCT & soaked for 1 hour for every 25mm of thickness. Once the soaking time has finished the steel is removed from the furnace to quench in a cooling medium. Brine is considered to be the fasted
cooling media followed by water & then oil. This produces a fine grain martensitic structure that has very high hardness & strength, though ductility is almost zero, with very low toughness.
4) Tempering: Temper means to balance. When tempering steel we re-balance the properties of excessive hardness & brittleness by decreasing the hardness & increasing the level of toughness. The steel is re-heated after hardening & the balance of hardness to toughness is adjusted as the temperature is increased from 220°-723°C. At 723°C all martensite has
been tempered removing brittleness & returning the ductility. The fine structure is retained giving high strength & further improving the toughness.
5) Stress Relieving or PWHT: The purpose of stress relieving is to relieve internal elastic stress that has become trapped inside the weld during welding. In stress relieving the steel may be heated between 200-950°depending on the steel type.
During stress relieving yield point is very important yield point is the point where steel can no longer support elastic strain & becomes plastically deformed (i.e) Plastic strain occurs. This means that the steel will no longer return to its original dimension. When steel is heated the yield point is suppressed, which means that the elastic strain will now start to become plastic strain. The higher the temperature then generally the more elastic strain will be converted to plastic strain.
6) Pre-heating: Pre heat is used
a) To control the structure of the weld metal & HAZ on cooling.
b) To improve the diffusion of gas molecules through on atomic structure. c) To control the effects of expansion & contraction.
In this the steel is heated to a temperature dependant on the type of steel being heated treated, but normally less than 350°
C. Pre-heating used to control the formation of H2 cracks.
All materials arriving on site should be inspected for Size
Condition
Type / Specification
Some imperfections associated with plate are as follows:
a) Laminations: Contain impurities & major inclusions in the ingot. When rolled out these major inclusions may exist throughout the plate thickness cause laminations.
b) Segregation bands: Occur in the center of the plate & are low melting points impurities such as sulphur or phosphorous. Cannot be detected by NDT. Can only be found on etched surfaces.
c) Laps: Caused during rolling when overlapped metal does not fuse to the base material due to insufficient temperature & or pressure.
Plate Inspection
should be checked for condition of the plate. Corrosion, mechanical damage, laps & lamination should be checked. Type & specification should be stamped on plate should be checked for dimensions(length, width) & Thickness of the plate. Other checks need to be made such as heat treatment condition, distortion, tolerance, quality storage & identification.
Pipe Inspection
Corrosion, mechanical damage, wall thickness, quality, laps, and laminations. Type & specification should be stamped on pile. Should be checked for dimensions (inside, outside diameter of pipe, length & thickness of pipe, whether seam or seamless other checks also need to be made such as heat treatment condition, distortion, tolerance, quantity, identification & storage.1) Harness Tests : Hardness is the ability of a material to resist indentation. Test is used to check the level of hardness across the weld.
Types of hardness test are
a) Rockwell scale – Impressing a diamond or steel ball b) Vickess pyramid VPN – Impressing a diamond
c) Brinell BHN – Impressing 5 or 10mm diameter steel ball d) Shore Scwerescope – Measures resilience
Most of the hardness tests are carried out by
i) Impressing a ball or a diamond into the surface of a material under a fixed load.
ii) Then measuring the resultant indentation & comparing it to a scale of units (BHN/VPN etc.) Hardness surveys are generally carried out across the weld also at the weld junction / fusion zone.
2) Toughness Tests : Toughness is the ability of a material to absorb impact energy & resist fracture. Test is used to check the resistance to impact loading.
Types of toughness test are,
a) Charpy V (Joules) – Specimen weld horizontally in test machine, notch to the rear b) Izod (Ftlbs) – Specimen held vertically in test machine, notch to the front.
c) CTOD or Crack Tip Opening Displacement testing (mm)
In charpy V & Izod test, the fracture toughness is assessed by the amount of impact energy absorbed by a small specimen of 10mm2during fracture by a swinging hammer. The notch is 2mm deep, 0.25 root radius & notch 45
°.
3) Tensile test: Used to measure tensile strength (N / mm2) (Ductility as E%). Strength is the ability of a material to resist a force (normally tension). Types of tensile tests are: a) Transverse tensile test & b) All weld metal tensile test a) Transverse tensile test: i) Reduced section: Used to test the strength of the weldment ii) Radius reduced section: Can be used to assess the strength of the weld metal. . b)All weld metal tensile test : Used to test weld metal for UTS, yield point & elongation or E%
Transverse tensile test are taken across the weld . In reduced tensile test the specimen is first cut & then reduced. In radius reduced tensile test the weld metal is turned down .All weld metal tensile test are carried out by electrode manufacturers to determine weld metal strength & also ductility as elongation (E%).After fracture, the pieces are placed back together & the elongation is calculated as E%.
4) Macro Examination tests: used to check the internal level f quality in the weld. A macro specimen is normally cut from a stop/start position in the root or hot pass of a welder approval test. The start/stop position is marked out during a welder approval test by the welding inspector. Once cut the specimen is polished using finer grit papers & polishing at 90°to previous
polishing direction for smooth surface. It is then etched in the acid solution, which is normally 5-10% nitric acid in alcohol (Carbon steels). After etching for the correct time, the specimen is then washed & dried. A visual inspection should be carried out at all stages. Finally a report is then produced on the visual findings, then compared & assessed to the levels of acceptance in the application standard. Macro samples may be sprayed with clear lacquer after inspection, for storage purposes.
5) ) Bend Test: . Ductility is the ability of a material of plastically deform under tension. Bend test is used to check weld ductility & fusion in the area under stress.
In bend test the former is moved through a guide (guided bend test) or rollers, & the specimen is bend to the desired angle Types of bend test are: a)Face bends b)Root bends c)Side bends d)Longitudinal bends
Face bend – The face of the specimen is in tension & root is in compression. Root bend – The root of the specimen is in tension & face is in compression. Side bend – Any side of the specimen is in tension & other side is in compression.
Generally bend tests are carried out for welder approval tests, though they may also be used during procedure approval. Bend test is qualitative method of mechanical testing.
6) Fillet weld fracture tests: Used to check root fusion in fillet welds. This test is carried out for welder approval test. The specimen is normally cut by hacksaw through the weld face to a depth of 1-2mm. It is then fractured with a hammer blow from the rear. Once the fracture has been made both fractured surfaces are inspected for imperfections. Finally the line of root fusion is observed for continuity. Any straight line would indicate a lack of root fusion & as per most of the standards this is, sufficient to fail the welder. As we are checking weld quality, test is qualitative mechanical test.
7)
Nick break tests: Used to assess root penetration & fusion in double sided but welds & the internal faces of single sided butt welds. Test is carried out for a welder approval test. The specimen is normally cut by 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.:
Welding imperfections are material discontinuities caused by, or during the process of welding. Can be classified into the following groups.
1Cracks,2 Gas pores & porosity 3 Solid inclusions 4 Lack of fusion 5 Surface & profile 6 Mechanical damage 7Misalignment.
1. Cracks: Occur in welded materials. To occur crack there are three criteria that must be present a) Force b) Restraint c) A weakened structure.
Typical types of cracks are: Hydrogen cracks
Solidification cracks Lamellar tears
Materials causing crack during welding can be evaluated under the term weldability. Materials, which welded by common welding processes, is called weldability. Cracks are classed as planer imperfections as they generally have length & depth. 2. Gas Pores & Porosity: Gas filled cavities smaller than 1.6mm diameter, which are created during solidification. Porosity are gas pores <1.6mm diameter, which are generally grouped together. A singular gas filled cavity = or > 1.6mm diameter is termed as “blow hole”. Porosity is mainly produced when welding improperly cleaned plate, or when using damp welding consumables.
Shrinkage cavities are created during solidification of welds of high depth: width rate. This may occur when the d: ration is >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. To assist arc ignition
2. To improve arc stabilization.
3. To produce a shielding gas to protect the arc column. 4. To refine & clean the solidifying weld metal.
5. To form a slag, which protects the solidifying, weld metal. 6. To add alloying elements.
7. To control hydrogen content of the weld metal
8. To form a core at the end of the electrode, which directs the arc. 9. To reduce the cooling rate.
10. It acts as a deoxidizing agent.
E - Electrode
51 - Tensile & Yield Strength (TS-510-650N/mm2,, YS-380N/mm2,)
33 - Toughness 28 & 47 Joules First digit 28J, Second digit 47J Testing temperature : - 20°C
B - Electrode Coating – B for Basic 160 - Electrode efficiency
2 - Welding position – 2 for all positions except vertical down
O - Electrical parameters – O for DC polarity as recommended & AC Min OCV ( Open circuit voltage) not recommended. H - Low hydrogen potential (After baking)
E - Electrode
46 - Tensile & Yield strength (TS 530-680N/mm2 , YS 460N/mm2)
3 - Toughness 47 Joules (3 for -30°C)
1Ni - Any light alloying composition. (Mn – 1.4, Ni – 0.6-1.2)
B - Flux coating type (B-Basic) 5
-Electrical parameters & efficiency
(AC+DC, Recovery% >125 < 160) 4 - Welding position (4 for flat butt & fillets) H5 - Low hydrogen potential (after baking)
E - Electrode
80 - Tensile Strength x 1000 (TS 80000), YS 68 – 80000) 1 - Welding position (1 for all positions)
18 - Electrode coating * electrical characteristic (18 for basic +25% Fe powder, AC or DC+) G - Low alloy steels ( Ni, Cr,Mo& V)
1)
: Fused fluxes are mixed together & baked at a very high temperature where all the componentsbecome 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 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 Disadvantages
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. High operator skill level. 2. Difficult to interpret 3. Requires calibration.
4. No permanent record (unless automated) 5. Not easily applied to complex geometry.
RADIOGRAPHIC TESTING
Advantages Disadvantages
1. Permanent record.
2. Most materials can be tested. 3. Detects internal flaws.
4. Gives a direct image of flaws.
5. Fluoroscopy can give real time imaging.
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.
PENETRANT TESTING
Advantages Disadvantages
1. Low operator skill level.
2. Applicable to non-ferromagnetic materials. 3. Low cost.
4. Simple, cheap & easy to interpret. 5. Portability.
1. Careful surface preparation required. 2. Surface breaking flaws only.
3. Not applicable to porous materials. 4. No permanent record.
5. Potentially hazardous chemicals.
MAGNETIC PARTICLE TESTING
Advantages Disadvantages
1. Pre-cleaning not as critical as with DPI 2. Will detect some sub-surface defects 3. Relatively low cost.
4. Simple equipment.
5. Possible to inspect through thin coatings.
1. Ferromagnetic materials only. 2. Demagnetization may be required. 3. Direct current flow may produce arc
strikes.
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; 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.
:
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.
a) Checking all containers to cut or welded arc fume free (permits to work etc)
b) Providing ventilation & extraction where required.
c) Keeping oil & grease away from oxygen.
d) Appropriate PPE is worn at all times. 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 :
