Triangle of Fire:-
Heat
Fuel
Oxygen: 21% in Air
Spread of Fire:
Three modes:
1) Conduction: Travel from one side to another side
2) Convection: It is applicable in Chemical and Gases. One thing collapse another
3) Radiation: Laser is the form of Radiation
Principles of Extinguishing:-
1) Heat ------- Oxygen
2) Smothering: mean to cover with anything.
3) Cooling: Water etc.
Classification of Fire:
A) Class fire: wood , paper, cloth,rubber, etc. ( Extinguisher Type: Water)
B) Liquids: ( Oils, Paints, etc) (Foam: AFFF (Aqus film forming foam protiene)
C) Gases: ( Sui gas, LPG, CNG, Hydrogen) ( DCP: Dry Chemical Powder( Ammonium Phosphate +Silicon)
D) Light metals: Like magnesium use in color form ( Dry Chemical Powder)
E) Electric, Electrical equipment ( Co2, Halotron) Halotron has more density than air.
Remember: Smoke will increase less of oxygen.
Carbon will be increase.
Back Draft:
A fire that has burned out all the oxygen in a room leaving only superheated gases.
Indications:
1) Yellow or brown smoke exits in puff form.
2) Incomplete Combustion caused dark colours.
3) Un burned carbon
Signs & Symptoms:
Yellow or brown smoke, Smoke which exits small holes in Puff
2) Extremely hot surrounding environment
Always occurred in Basement
Suit: Black Smoke
Cracked Windows
Slight Vibration of glass pan
Air Suction
Preventive Measures:
Immediate Evacution
Ventilation
Careful Entry
Boundry cooling
Fire Suit
Flash Over:-
Temperature: 500c (930 F)
Stages of Flashover:-
1) Initial Fire procedure layer of Hot Smoke
2) Hot Smoke spreads across the ceiling
3) Hot smoke bounded by the walls
4) Depth of Buoyant smoke layers grows
Fires always travel from ceiling to Wall.
Radiated heat, Heat the combustible materials
Flammable gases are given off
Surface Temperature attained from.
Brick : 250c
Concrete: 800c – 900c
Preventive Measures:-
1) Careful entry ( Sitting beside the door) Always use Nomex Material Turn Out gear
2) Use of Appropriate PPE ( Personal Protective Equipment)
Fire Prevention System:-
1) Hydrant system
2) Wet Hydrant system anytime available
3) Dry: No water in lines, Stored in Tank Fixed Installation
4) Sprinkler System: Installed in Walls inside
5) Operate Automatically.
6) Heat & Smoke Detectors: Is a fixed fired detection system.
7) To detect heat
8) Smoke Detector: To detect smoke
9) Fire Alarm system: Fixed fire appliances:-
10) Nozzels, Hose, Hose reels, (Nozel: 2.50 inches) ( Reels: 75 Inches)
Emergency Exits:-
1) Fire stairs
2) Fire doors
3) First Aid Fire Appliances
4) Fire Extinguisher
5) SCBA: Self contained breathing apparatus
Gas / Smoke Masks
Respirators: Effective
EEBD: Emergency Escape Breathing Devices: ( 5-10 minutes)
EMBOW Bags:
Fire plans:-
1) Sketch of a building
2) Fire Fighting Appliances
3) Installations
4) Emergency evacuation plan
Classification of Fire Volume:
Small Fire: 1-2 Jets
Medium Fire: 3-8 Jets
Large Fire: 9-18
Major Fire: 18 + plus
Extinguisher Parts:
1) Neck Ring
2) N2 20%
3) Water (80%)
4) Plastic Belt
5) Syphone Tube
6) Barrel
7) Bottom Ring
What is P A S S?
P: Pull the Pin
A: Aim the nozzle at fire
S: Squeeze the lever
S: Sweep
Monday, July 11, 2011
Saturday, March 26, 2011
Friday, March 25, 2011
Thursday, March 24, 2011
Saturday, March 19, 2011
Thursday, February 24, 2011
Thursday, February 10, 2011
Wednesday, February 9, 2011
Tuesday, February 8, 2011
What is the material applied on the flanges or valves to obtain smooth surface for application of cold tape?
Ans: Moulding compound shall be hand applied to obtain smooth surface for application of cold tape on flanges / valves
What is the minimum overlap of field wrapping(cold tape) on shop wrapping?
Ans: Wrapping shall start and finish to give a minimum 75mm overlap onto the adjoining shop coating.
What is cold type of wrapping?
Ans: PVC backed bituminous compound tape used for field wrapping is called cold wrapping.
What should be the minimum staggering of inner & outer wraps?
Ans:- The overlaps of the inner and outer wraps shall be staggered from each other by minimum distance of 100mm
What should be the overlap between inner & outer wrap?
Ans:- The inner and outer wraps shall be overlapped by 25mm
How much should be the depth of pulling of Inner Fibre glass tissue into the hot enamel?
Ans: The inner wrap of Fibre-glass tissue pulled in such a manner that the same is imbedded half way into the enamel without touching the steel surface.
What should be the minimum thickness of enamal on any point on pipe?
Ans: The enamel shall have minimum thickness of 2.4 mm when measured on top of the weld with an overall thickness of 4mm.
Which material is used as inner and outer wrapping?
Ans: Fibre glass tissue consisting of a uniformity porous mast of chemically resistant boro-silicate glass containing not less then 5%
What is upper yield point with reference to above stess-strain graph (UTS)?
Ans: The highest stress that the metal can withstand under sustained load without continuing to elongate under same load is called the upper yield point
What is stress?
Ans: It is defined as the applied load per unit cross section of the specimen. The common unit are PSI(Pound Per Square Inch), KPA, MPA, KG/CM
What are the factors upon which the mechanical properties of material are dependant?
Ans: Mechanical properties of any material of construction are dependent on:-
A) Chemical composition of the material
B) Method by which the material is manufactured
A) Chemical composition of the material
B) Method by which the material is manufactured
What are the Failures with reference to the structural design?
Ans: Failure of a structural part can occur by:-
A) Excessive elastic deformation
B) Excessive non-elastic deformation
C) Fracture
Any design has to guard against these perceived failures.
A) Excessive elastic deformation
B) Excessive non-elastic deformation
C) Fracture
Any design has to guard against these perceived failures.
Friday, January 28, 2011
What is film contrast?
Ans: The factor of the film, which affects the contrast, is known as Film contrast
What is subject contrast?
Ans: The factor of the specimen, which affects the contrast, is known as subject contrast.
What do you mean by Radiographic Contrast?
Ans: Density difference between the two adjacent areas of the Radiograph is known as contrast Radiographic contrast is the combined effect of the following.
(A) Subject contrast
(B) Film Contrast
(A) Subject contrast
(B) Film Contrast
What are the Criteria for Selection of IQI or Penetrametre?
Ans: Penetrametre should be made of same material as that of the specimen.
The selection of IQI should be made as:-
(A) For carbon steel & low Alloy steel:- Carbon Steel IQI
(B) For High Alloy Steel & Stainless steel: Stainless Steel IQI
(C) For Aluminum & Aluminium Alloy: Aluminum IQI
(D) Copper & Copper Alloy: Copper IQI
The selection of IQI should be made as:-
(A) For carbon steel & low Alloy steel:- Carbon Steel IQI
(B) For High Alloy Steel & Stainless steel: Stainless Steel IQI
(C) For Aluminum & Aluminium Alloy: Aluminum IQI
(D) Copper & Copper Alloy: Copper IQI
What are the commonly used IQI?
Ans: Commonly used IQI are:-
(A) Wire type Penetrametre
(B) Plate type Penetrametre
(C) Step type Penerametre
(D) Step-Hole type Penetrametre
(A) Wire type Penetrametre
(B) Plate type Penetrametre
(C) Step type Penerametre
(D) Step-Hole type Penetrametre
What is the general requirement of Radiographic Sensitivity?
Ans: General requirement of Radiographic sensitivity are:-
>2%(Less then two percent) - Good
<2%(More ten two percent) - Not acceptable
>2%(Less then two percent) - Good
<2%(More ten two percent) - Not acceptable
What is the function of fixer?
Ans: It removes all unexposed silver grains and clouded film starts to become clear.
What is subject contrast?
Ans: The factor of the specimen, which affects the contrast, is known as subject contrast.
What are the ingredients of Fixer?
Ans: Fixing Agent:
(A) Sodium thiosulphate commonly known as Hype. Commonly used fixing agent
(B) Ammonium thiosulphate. Used as rapid fixing agent.
(A) Sodium thiosulphate commonly known as Hype. Commonly used fixing agent
(B) Ammonium thiosulphate. Used as rapid fixing agent.
What is the function of Stop bath?
Ans: Stops the developing action by neutralising the alkaline developer.
What is the ideal developing temperature?
Ans: Below 18c and above 24c, developing is not recommended
What is the affect of temperature on Developer?
Ans: The Developer supplies more electrons at high temperature and reduces the developing time. Opposite is the case when the temperature is lower.
What is the function of Restrainer present in Developer?
Ans: It opposes to reduce the unexpected silver bromide. It acts as antifogging agent.
What is the function of Accelerator present in Developer?
Ans: It encourages the developer to supply more electrons.
What are the Ingredients of Developer?
Ans: Developing Agent: Metol, Hydroquinone and Pencilone
(B) Accelerator :- Sodium carbonate
(C) Restrainer :- Potassium Bromate
(D) Preservative:- Sodium Sulphate
(B) Accelerator :- Sodium carbonate
(C) Restrainer :- Potassium Bromate
(D) Preservative:- Sodium Sulphate
What are the main steps in film processing?
Ans: Main steps in film processing are:-
(A) Developing
(B) Stop bath
(C) Fixture
(D) Washing
(E) Drying
(A) Developing
(B) Stop bath
(C) Fixture
(D) Washing
(E) Drying
How the Radiographic films are classified?
Ans: The Radiographic films are classified as:-
(A) Class- I :- Highest contrast, Lowest speed
(B) Class-II :- High contrast, Low speed
(C) Class-III:- Medium contrast, Medium Speed
(D) Class-IV :- Low contrast , High Speed
(A) Class- I :- Highest contrast, Lowest speed
(B) Class-II :- High contrast, Low speed
(C) Class-III:- Medium contrast, Medium Speed
(D) Class-IV :- Low contrast , High Speed
What is the function of Subbing material, Emulsion & protective layer in Radiographic film?
Ans: Subbing Material:- It provides the sticky action to the emulsion, as the emulsion does not adhere directly on the base material.
Emulsion: It contains silver bromide gelatin (Generally animal bone marrow)
Protective Layer: It is coated on emulsion in order to protect the same from physical damage, abrasion and stress mark.
Emulsion: It contains silver bromide gelatin (Generally animal bone marrow)
Protective Layer: It is coated on emulsion in order to protect the same from physical damage, abrasion and stress mark.
What is the different base material tried so far for Radiographic film?
Ans: The materials so far tried for base is:-
(A) Glass
(B) Cellulose Nitrate
(C) Cellulose Acetate
(D) Cellulose Triacetate
(E) Polyester (Most suitable material to be used as base material)
(A) Glass
(B) Cellulose Nitrate
(C) Cellulose Acetate
(D) Cellulose Triacetate
(E) Polyester (Most suitable material to be used as base material)
What are the main constituents of a Radiographic film?
Ans: The Radiographic films consist of the following:-
(A) Base Material
(B) Subbing layer
(C) Emulsion and
(D) Protective layer / Super coat
(A) Base Material
(B) Subbing layer
(C) Emulsion and
(D) Protective layer / Super coat
Where the fluorescent screen finds its use?
Ans: The fluorescent screens are widely used for medical purpose to reduce the exposure time.
How the intesification factor depends on energy of radiation?
Ans: More is the energy of radiation, more is the intensifying action
How the Intesification factor depends on thickness of foil?
Ans: The intensification factor increases with the increase in the thickness of the foil. Intensification increases maximum corresponding to the range of photoelectron in that meetal. After further increase it remains practically constant. if the thickness further increases, greater number of Gamma photos will be attenuated and this will reduce the produce of photoelectrons.
How the intensification factor depends on metal of foil?
Ans: For a given radiation source, the number of electrons produced depends on the nature of the metal foil. Intensification factor increases with atomic number of the metal. For Gamma Ray radiography generally Lead screen are used.
What are the factors upon which the intensification factor depends?
Ans: Intensification factor due to metallic screens depends on the following:-
(A) Metal of foil
(B) Thickness of foil
(C) Energy of Radiation
(D) Specimen thickness
(A) Metal of foil
(B) Thickness of foil
(C) Energy of Radiation
(D) Specimen thickness
What are the types of Lead screens?
Ans: Types of Lead screens are:-
(A) Lead folt screen
(B) Lead oxide screen
(A) Lead folt screen
(B) Lead oxide screen
What are the types of Radiographic screens generally used?
Ans: Types of radiographic screens generally used are:-
(A) Lead Screen
(B) Fluorescent screen or salt screens
(A) Lead Screen
(B) Fluorescent screen or salt screens
What is the function of Radiographic screens?
Ans: It intensifies the radiographic images on the film
What do you mean by Roentgen?
Ans: Roentgen is the old unit for exposure. It is defined as the amount of X or Gamma radiation which liberates 1e s u of charge of either sign in 1C. C of air at S T P.
1R = 1 e s u / C C of air at STP
= 2.58 X 10 / C/ Kg air.
1R = 1 e s u / C C of air at STP
= 2.58 X 10 / C/ Kg air.
What do you mean by exposure?
Ans: It is defined as the quantity of X or Gamma radiation that produces in air, ions carrying 1 coulomb (C) of charge (of either sign) per Kg of air. The unit of exposure is C/Kg.
How the source strength of Radiographic Isotopes expressed?
Ans: The source strength of Radiographic Isotopes expressed in terms of Curie.
What are the governing factors for exposure from particular Radioisotopes?
Ans: There are three factors for governing the exposure with a given kilovoltage for X- Ray or with the Gamma Ray from particular radioisotopes.
(A) Milliamperage (X-Ray) or source strength (for Gamma Ray)
(B) Focal spot to film distance or source to film distance
(C) Time of exposure
(A) Milliamperage (X-Ray) or source strength (for Gamma Ray)
(B) Focal spot to film distance or source to film distance
(C) Time of exposure
Thursday, January 27, 2011
What are the factors on which the density of Radiographic film depends?
Ans: The density of Radiogrphic films depends upon the following:-
(A) Total amount of radiation emitted by X-ray or Gamma ray
(B) Amount of radiation reaching the specimen.
(C) The amount of radiation passing through the specimen.
(D) Intensifying action of the screen if used.
(A) Total amount of radiation emitted by X-ray or Gamma ray
(B) Amount of radiation reaching the specimen.
(C) The amount of radiation passing through the specimen.
(D) Intensifying action of the screen if used.
Name the instrument used for measuring density of photographic or Radiographic film?
Ans: Densitometre is an instrument for measuring the density of photographic and Radiographic film.
What do you mean by photographic Density?
Ans:
It is the quantiative measurement of film blackness. It is expressed as:-
Where , D is density.
Io - Light intensity incident on the film.
I1- Light intensity transmitted through the film.
It is the quantiative measurement of film blackness. It is expressed as:-
Where , D is density.
Io - Light intensity incident on the film.
I1- Light intensity transmitted through the film.
What is the depth of Penetration is steel by cobalt-60, Cesium-137, Iridum-192 & Thulium-170?
Ans:
Penetration is steel by:-
(A) Cobalt - 60 - 9(inch)
(B) Cesium - 137 - 3 1/2 (inch)
(C) Iridum - 192 - 3 (inch)
(D) Thulim - 170 - 1/2 (inch)
Penetration is steel by:-
(A) Cobalt - 60 - 9(inch)
(B) Cesium - 137 - 3 1/2 (inch)
(C) Iridum - 192 - 3 (inch)
(D) Thulim - 170 - 1/2 (inch)
Name the Isotopes, which emits Gamma Ray?
Ans:
(A) Iridium- 192
(B) Cobalt- 60
(C) Cesium- 137
(D) Thulium- 170
(A) Iridium- 192
(B) Cobalt- 60
(C) Cesium- 137
(D) Thulium- 170
What is the charges on A particle , B Particle, Gamma Particle & Compare their relative penetration?
Ans: Charges on radiation are:-
(A) Alpha particle: Positive charge & less penetrating in comparison to Beta Particle & Gamma Ray. They can be stopped by a thin sheet of paper.
(B) Beta Particle: Negative charge & have definite range of penetration. Easily absorbed in the matter.
(C) Gamma Ray: No charge & highly penetrating.
(A) Alpha particle: Positive charge & less penetrating in comparison to Beta Particle & Gamma Ray. They can be stopped by a thin sheet of paper.
(B) Beta Particle: Negative charge & have definite range of penetration. Easily absorbed in the matter.
(C) Gamma Ray: No charge & highly penetrating.
What are the Types of Radiation emitted by Isotopes?
Ans: There are three types of Radiation as:-
(A) Alpha particle
(B) Beta particle
(C) Gamma Ray
(A) Alpha particle
(B) Beta particle
(C) Gamma Ray
What should be the content of chlorine in water while conducting hydrotest for Carbon Steel & Stainless Steel pipes?
Ans:
Carbon steel : 250PPM
Stainless Steel : 30PPM
Carbon steel : 250PPM
Stainless Steel : 30PPM
Why the Electrode E7018 is called Low Hydrogen Electrode?
Ans: The low hydrogen electrodes have in their coating ingredient, which produces carbon Di-Oxide during melting. This Co2 gives a gaseous shielding for the metal and prevents atmospheric hydrogen from entering in Arc atmosphere. By this way the weld metal has low level of hydrogen.
Where the use of Electrode E 7018 is recommended?
Ans: The use of electrode E7018 is recommended for welding the following:-
(A) For high strength steel
(B) For high thickness carbon steel plates
(C) Higher carbon equivalent material
(A) For high strength steel
(B) For high thickness carbon steel plates
(C) Higher carbon equivalent material
What is the Post Heat Temperature for Alloy Steel Material?
Ans: Post Heat Temperature for Alloy Steel Material is 300c.
What is the rate of heating & cooling during stress reliving for Alloy Steel material?
Ans: The rate of heating & cooling for Alloy Steel material is 100c/hr
What is the soaking period during stress reliving for Alloy steel material?
Ans: Soaking period for Alloy Steel material is 2hrs.
What is the soaking temperature during stress reliving for Alloy Steel Materials?
Ans: Soaking temperature for Alloy steel material is 720C.
What is the pre-heat temperature during stress reliving for Alloy steel materials?
Ans: Pre-Heat temperature for AS materials is 180c.
What is the rate of heating & Cooling during stress reliving for carbon steel material?
Ans: The rate of heating & cooling for carbon steel material during stress reliving is 150c/hr.
What is the rate of heating & cooling during stress reliving for carbon steel material?
Ans: Soaking period for carbon steel material during stress reliving is 1Hour.
What is the Soaking temperature during stress reliving for carbon steel material?
Ans: Soaking period for carbon steel material during stress reliving is 620c.
Is post heating required for carbon steel material above 19.05MM thk
Ans: No. Post heating is not required for carbon steel material of any thickness.
What is the Pre-Heat Temperature for carbon steel above 19.05mm thk
Ans: Pre-Heat temperature for carbon steel above 19.05mm is 80c.
Why Post-Heating is done on some pipe after the welding is over?
Ans: To maintain uniform homogenous structure.
What is mean by "PWHT"? Why it is require?
POST WELD HEAT TREATMENT" This is done to remove residual stress left in the joint which may cause brittle fracture.
What are the Common Welding Defects?
Lack of Penetration:
This defect occurs at the root of the joint when the weld metal fails to reach it or weld metal fails to fuse completely the root faces of the joint. As a result, a void remains at the root zone which may contain slag inclusions.
Cause:
(A) Use of incorrect size of electrode in relation to the form of joint.
(B) Low welding current
(C) Faulty fit up and inaccurate joint preparation
Lack of Fusion:
Lack of fusion is defined as a condition where boundaries of unfused metal exist between the Weld metal & base or between the adjacent layers of weld metals.
Cause:
(A) Presence of scale, dirt, oxide, slag and other non-metallic substance which prevents the weld metal to reach melting temperature.
(B) Improper deslagging between the weld pass.
Precaution:
(A) Keep the weld joint free from scale, dirt, oxide, slag and other non-metallic substance.
(B) Use adequate welding current
(C) Deslag each weld pass thoroughly
(D) Place weld passes correctly next to each other.
Under Cut:
This defect appears as a continuous or discontinuous groove at the toes of a weld pass and located on the base metal or in the fusion face of a multipass weld. It occurs prominently on the edge of afillet weld deposited in the horizontal position.
Cause:
(A) Excessive welding current
(B) Too high speed of Arc travel
(C) Wrong electrode angle
Rectification:
The defect is rectified by filling the undercut groove with a weld pass. If undercut is deep & contains slag, it should be chipped away before rewelding.
Slag Inclusion:
Non metallic particles of comparatively large size entrapped in the weld metal are termed as slag inclusion.
Cause:
(A) Improper cleaning of slag between the deposition of successive passes.
(B) Presence of heavy mill scale, loose, rust, dirt, grit & other substances present on the surface of base metal.
Precaution:
(A) Clean the slag thoroughly between the weld pass.
(B) Keep the Joint surface (especially gas cut surface)and bare filler wire perfectly clean.
(C) Avoid undercut & gaps between weld pass.
(D) Use proper welding consumables.
Porosity:
The presence of gas pores in a weld caused by entrapment of gas during solidfication is termed as porosity. The pores are in the form of small spherical cavities either clustered locally or scattered throughout the weld deposit. Sometimes entrapped gas give rise to a single large cavity called blowholes.
Cause:
(A) Chemically imperfect welding consumables, for example, deficient in deoxidiser.
(B) Faulty composition of base material or electrode, for example, high sulphur content.
(C) Presence of oil, greese, moisture and mill scale on the weld surface.
(D) Excessive moisture in the electrode coating or submerged -arc flux.
(E) Inadequate gas shielding or impure gas in a gas-shielded process.
(F) Low welding current or too long an Arc.
(G) Quick Freezing of weld deposit.
Crack:
Fracture of the metal is called crack. Two types of cracks:-
Cold Crack:
Cold crack usually occur in HAZ of the base metal when this zone becomes hard and brittle due to rapid cooling after the weld metal has been deposited & sufficient hydrogen has been absorbed by the weld metal from the Arc atmosphere.
Precaution:
(A) Use of low carbon equivalent materials.
(B) Higher heat input during welding.
(C) Preheating.
(D) Use of low hydrogen electrode
This defect occurs at the root of the joint when the weld metal fails to reach it or weld metal fails to fuse completely the root faces of the joint. As a result, a void remains at the root zone which may contain slag inclusions.
Cause:
(A) Use of incorrect size of electrode in relation to the form of joint.
(B) Low welding current
(C) Faulty fit up and inaccurate joint preparation
Lack of Fusion:
Lack of fusion is defined as a condition where boundaries of unfused metal exist between the Weld metal & base or between the adjacent layers of weld metals.
Cause:
(A) Presence of scale, dirt, oxide, slag and other non-metallic substance which prevents the weld metal to reach melting temperature.
(B) Improper deslagging between the weld pass.
Precaution:
(A) Keep the weld joint free from scale, dirt, oxide, slag and other non-metallic substance.
(B) Use adequate welding current
(C) Deslag each weld pass thoroughly
(D) Place weld passes correctly next to each other.
Under Cut:
This defect appears as a continuous or discontinuous groove at the toes of a weld pass and located on the base metal or in the fusion face of a multipass weld. It occurs prominently on the edge of afillet weld deposited in the horizontal position.
Cause:
(A) Excessive welding current
(B) Too high speed of Arc travel
(C) Wrong electrode angle
Rectification:
The defect is rectified by filling the undercut groove with a weld pass. If undercut is deep & contains slag, it should be chipped away before rewelding.
Slag Inclusion:
Non metallic particles of comparatively large size entrapped in the weld metal are termed as slag inclusion.
Cause:
(A) Improper cleaning of slag between the deposition of successive passes.
(B) Presence of heavy mill scale, loose, rust, dirt, grit & other substances present on the surface of base metal.
Precaution:
(A) Clean the slag thoroughly between the weld pass.
(B) Keep the Joint surface (especially gas cut surface)and bare filler wire perfectly clean.
(C) Avoid undercut & gaps between weld pass.
(D) Use proper welding consumables.
Porosity:
The presence of gas pores in a weld caused by entrapment of gas during solidfication is termed as porosity. The pores are in the form of small spherical cavities either clustered locally or scattered throughout the weld deposit. Sometimes entrapped gas give rise to a single large cavity called blowholes.
Cause:
(A) Chemically imperfect welding consumables, for example, deficient in deoxidiser.
(B) Faulty composition of base material or electrode, for example, high sulphur content.
(C) Presence of oil, greese, moisture and mill scale on the weld surface.
(D) Excessive moisture in the electrode coating or submerged -arc flux.
(E) Inadequate gas shielding or impure gas in a gas-shielded process.
(F) Low welding current or too long an Arc.
(G) Quick Freezing of weld deposit.
Crack:
Fracture of the metal is called crack. Two types of cracks:-
Cold Crack:
Cold crack usually occur in HAZ of the base metal when this zone becomes hard and brittle due to rapid cooling after the weld metal has been deposited & sufficient hydrogen has been absorbed by the weld metal from the Arc atmosphere.
Precaution:
(A) Use of low carbon equivalent materials.
(B) Higher heat input during welding.
(C) Preheating.
(D) Use of low hydrogen electrode
Sunday, January 23, 2011
GTAW
After the discovery of the electric arc in 1800 by Humphry Davy, arc welding developed slowly. C. L. Coffin had the idea of welding in an inert gas atmosphere in 1890, but even in the early 1900s, welding non-ferrous materials like aluminum and magnesium remained difficult, because these metals reacted rapidly with the air, resulting in porous and dross-filled welds.[2] Processes using flux-covered electrodes did not satisfactorily protect the weld area from contamination. To solve the problem, bottled inert gases were used in the beginning of the 1930s. A few years later, a direct current, gas-shielded welding process emerged in the aircraft industry for welding magnesium.
This process was perfected in 1941, and became known as heliarc or tungsten inert gas welding, because it utilized a tungsten electrode and helium as a shielding gas. Initially, the electrode overheated quickly, and in spite of tungsten's high melting temperature, particles of tungsten were transferred to the weld. To address this problem, the polarity of the electrode was changed from positive to negative, but this made it unsuitable for welding many non-ferrous materials. Finally, the development of alternating current units made it possible to stabilize the arc and produce high quality aluminum and magnesium welds.[3]
Developments continued during the following decades. Linde Air Products developed water-cooled torches that helped to prevent overheating when welding with high currents.[4] Additionally, during the 1950s, as the process continued to gain popularity, some users turned to carbon dioxide as an alternative to the more expensive welding atmospheres consisting of argon and helium. However, this proved unacceptable for welding aluminum and magnesium because it reduced weld quality, and as a result, it is rarely used with GTAW today.
In 1953, a new process based on GTAW was developed, called plasma arc welding. It affords greater control and improves weld quality by using a nozzle to focus the electric arc, but is largely limited to automated systems, whereas GTAW remains primarily a manual, hand-held method.[5] Development within the GTAW process has continued as well, and today a number of variations exist. Among the most popular are the pulsed-current, manual programmed, hot-wire, dabber, and increased penetration GTAW methods.
Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. However, some welds combining thin materials (known as autogenous or fusion welds) can be accomplished without filler metal; most notably edge, corner, and butt joints.
To strike the welding arc, a high frequency generator (similar to a Tesla coil) provides a spark; this spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart. This high voltage, high frequency burst can be damaging to some vehicle electrical systems and electronics, because induced voltages on vehicle wiring can also cause small conductive sparks in the vehicle wiring or within semiconductor packaging. Vehicle 12V power may conduct across these ionized paths, driven by the high-current 12V vehicle battery. These currents can be sufficiently destructive as to disable the vehicle; thus the warning to disconnect the vehicle battery power from both +12 and ground before using welding equipment on vehicles.
An alternate way to initiate the arc is the "scratch start". Scratching the electrode against the work with the power on also serve to strike an arc, in the same way as SMAW ("stick") arc welding. However, scratch starting can cause contamination of the weld and electrode. Some GTAW equipment is capable of a mode called "touch start" or "lift arc"; here the equipment reduces the voltage on the electrode to only a few volts, with a current limit of one or two amps (well below the limit that causes metal to transfer and contamination of the weld or electrode). When the GTAW equipment detects that the electrode has left the surface and a spark is present, it immediately (within microseconds) increases power, converting the spark to a full arc.
Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.[7]
Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.[8][9]
[edit] Operation modesGTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, positively charged ions flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.
[edit] SafetyLike other arc welding processes, GTAW can be dangerous if proper precautions are not taken. The process produces intense ultraviolet radiation, which can cause a form of sunburn and, in a few cases, trigger the development of skin cancer. Flying sparks and droplets of molten metal can cause severe burns and start a fire if flammable material is nearby, though GTAW generally produces very few sparks or metal droplets when performed properly.
It is essential that the welder wear suitable protective clothing, including leather gloves, a closed shirt collar to protect the neck (especially the throat), a protective long sleeve jacket and a suitable welding helmet to prevent retinal damage or ultraviolet burns to the cornea, often called arc eye. The shade of welding lens will depend upon the amperage of the welding current. Due to the absence of smoke in GTAW, the arc appears brighter than shielded metal arc welding and more ultraviolet radiation is produced. Exposure of bare skin near a GTAW arc for even a few seconds may cause a painful sunburn. Additionally, the tungsten electrode is heated to a white hot state like the filament of a lightbulb, adding greatly to the total radiated light and heat energy. Transparent welding curtains, made of a polyvinyl chloride plastic film, dyed in order to block UV radiation, are often used to shield nearby personnel from exposure.
Welders are also often exposed to dangerous gases and particulate matter. Shielding gases can displace oxygen and lead to asphyxiation, and while smoke is not produced, the arc in GTAW produces very short wavelength ultraviolet light, which causes surrounding air to break down and form ozone. Metals will volatilize and heavy metals can be taken into the lungs. Similarly, the heat can cause poisonous fumes to form from cleaning and degreasing materials. For example chlorinated products will break down producing poisonous phosgene. Cleaning operations using these agents should not be performed near the site of welding, and proper ventilation is necessary to protect the welder.[10]
Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.
[edit] ApplicationsWhile the aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry. In addition, GTAW is often used to make root or first pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium.[11] Because the weld metal is not transferred directly across the electric arc like most open arc welding processes, a vast assortment of welding filler metal is available to the welding engineer. In fact, no other welding process permits the welding of so many alloys in so many product configurations. Filler metal alloys, such as elemental aluminum and chromium, can be lost through the electric arc from volatilization. This loss does not occur with the GTAW process. Because the resulting welds have the same chemical integrity as the original base metal or match the base metals more closely, GTAW welds are highly resistant to corrosion and cracking over long time periods, GTAW is the welding procedure of choice for critical welding operations like sealing spent nuclear fuel canisters before burial.[12]
Engineers prefer GTAW welds because of its low-hydrogen properties and the match of mechanical and chemical properties with the base material. Maximum weld quality is assured by maintaining the cleanliness of the operation—all equipment and materials used must be free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil and grease, alcohol or similar commercial solvents may be used, while a stainless steel wire brush or chemical process can remove oxides from the surfaces of metals like aluminum. Rust on steels can be removed by first grit blasting the surface and then using a wire brush to remove any embedded grit. These steps are especially important when negative polarity direct current is used, because such a power supply provides no cleaning during the welding process, unlike positive polarity direct current or alternating current.[13] To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTA welding in windy or drafty environments increases the amount of shielding gas necessary to protect the weld, increasing the cost and making the process unpopular outdoors.
Because of GTAW's relative difficulty and the importance of proper technique, skilled operators are employed for important applications. Welders in the U.S. should be qualified following the requirements of the American Welding Society or American Society of Mechanical Engineers. Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. If there is too much heat input, however, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. Additionally, if the welder holds the welding torch too far from the workpiece, shielding gas is wasted and the appearance of the weld worsens.
If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, it can be identified with radiography and prevented by changing the type of electrode or increasing the electrode diameter. In addition, if the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that electrode be ground with a diamond abrasive to remove the impurity.[14] GTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, positively charged ions flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.
The equipment required for the gas tungsten arc welding operation includes a welding torch utilizing a nonconsumable tungsten electrode, a constant-current welding power supply, and a shielding gas source.
[edit] Welding torchGTAW welding torches are designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling systems are most often used for low-current operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.
The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively. The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. Collets are sized according to the diameter of the tungsten electrode they hold. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder.
The size of the welding torch nozzle depends on the amount of shielded area desired. The size of the gas nozzle will depend upon the diameter of the electrode, the joint configuration, and the availability of access to the joint by the welder. The inside diameter of the nozzle is preferably at least three times the diameter of the electrode, but there are no hard rules. The welder will judge the effectiveness of the shielding and increase the nozzle size to increase the area protected by the external gas shield as needed. The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance, offers greater visibility. Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to improve the control shielding gas flow to reduce turbulence and introduction of contaminated atmosphere into the shielded area. Hand switches to control welding current can be added to the manual GTAW torches.[15]
Power supplyGas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.[16]
The preferred polarity of the GTAW system depends largely on the type of metal being welded. Direct current with a negatively charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material. Direct current with a positively charged electrode (DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance.
Alternating current, commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. Surface oxides are still removed during the electrode-positive portion of the cycle and the base metal is heated more deeply during the electrode-negative portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high-frequency voltage to encourage ignition.[17]
SteelsFor GTA welding of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface. Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases.[24]
[edit] Copper alloysTIG welding of copper and some of its alloys is possible, but in order to get a seam free of oxidation and porosities, shielding gas needs to be provided on the root side of the weld. Alternatively, a special "backing tape", consisting of a fiberglass weave on heat-resistant aluminum tape can be used, to prevent air reaching the molten metal.
[edit] Dissimilar metalsWelding dissimilar metals often introduces new difficulties to GTAW welding, because most materials do not easily fuse to form a strong bond. However, welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation. In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (for example, using a stainless steel filler metal with stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials.
When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials.[25]
[edit] Process variations[edit] Pulsed-currentIn the pulsed-current mode, the welding current rapidly alternates between two levels. The higher current state is known as the pulse current, while the lower current level is called the background current. During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations, making it useful for specialized applications.[26]
[edit] DabberThe dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades.[27]
[edit] Hot wireWelding filler metal can be resistance heated to a temperature near its melting point before being introduced into the weld pool. This increases the deposition rate of machine and automatic GTAW welding processes. More pounds per hour of filler metal is introduced into the weld joint than when filler metal is added cold and the heat of the electric arc introduces all of the heat. This process is used extensively in base material build up before machining, clad metal overlays, and hardfacing operations.
Posted by: Adnan
This process was perfected in 1941, and became known as heliarc or tungsten inert gas welding, because it utilized a tungsten electrode and helium as a shielding gas. Initially, the electrode overheated quickly, and in spite of tungsten's high melting temperature, particles of tungsten were transferred to the weld. To address this problem, the polarity of the electrode was changed from positive to negative, but this made it unsuitable for welding many non-ferrous materials. Finally, the development of alternating current units made it possible to stabilize the arc and produce high quality aluminum and magnesium welds.[3]
Developments continued during the following decades. Linde Air Products developed water-cooled torches that helped to prevent overheating when welding with high currents.[4] Additionally, during the 1950s, as the process continued to gain popularity, some users turned to carbon dioxide as an alternative to the more expensive welding atmospheres consisting of argon and helium. However, this proved unacceptable for welding aluminum and magnesium because it reduced weld quality, and as a result, it is rarely used with GTAW today.
In 1953, a new process based on GTAW was developed, called plasma arc welding. It affords greater control and improves weld quality by using a nozzle to focus the electric arc, but is largely limited to automated systems, whereas GTAW remains primarily a manual, hand-held method.[5] Development within the GTAW process has continued as well, and today a number of variations exist. Among the most popular are the pulsed-current, manual programmed, hot-wire, dabber, and increased penetration GTAW methods.
Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. However, some welds combining thin materials (known as autogenous or fusion welds) can be accomplished without filler metal; most notably edge, corner, and butt joints.
To strike the welding arc, a high frequency generator (similar to a Tesla coil) provides a spark; this spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart. This high voltage, high frequency burst can be damaging to some vehicle electrical systems and electronics, because induced voltages on vehicle wiring can also cause small conductive sparks in the vehicle wiring or within semiconductor packaging. Vehicle 12V power may conduct across these ionized paths, driven by the high-current 12V vehicle battery. These currents can be sufficiently destructive as to disable the vehicle; thus the warning to disconnect the vehicle battery power from both +12 and ground before using welding equipment on vehicles.
An alternate way to initiate the arc is the "scratch start". Scratching the electrode against the work with the power on also serve to strike an arc, in the same way as SMAW ("stick") arc welding. However, scratch starting can cause contamination of the weld and electrode. Some GTAW equipment is capable of a mode called "touch start" or "lift arc"; here the equipment reduces the voltage on the electrode to only a few volts, with a current limit of one or two amps (well below the limit that causes metal to transfer and contamination of the weld or electrode). When the GTAW equipment detects that the electrode has left the surface and a spark is present, it immediately (within microseconds) increases power, converting the spark to a full arc.
Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.[7]
Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.[8][9]
[edit] Operation modesGTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, positively charged ions flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.
[edit] SafetyLike other arc welding processes, GTAW can be dangerous if proper precautions are not taken. The process produces intense ultraviolet radiation, which can cause a form of sunburn and, in a few cases, trigger the development of skin cancer. Flying sparks and droplets of molten metal can cause severe burns and start a fire if flammable material is nearby, though GTAW generally produces very few sparks or metal droplets when performed properly.
It is essential that the welder wear suitable protective clothing, including leather gloves, a closed shirt collar to protect the neck (especially the throat), a protective long sleeve jacket and a suitable welding helmet to prevent retinal damage or ultraviolet burns to the cornea, often called arc eye. The shade of welding lens will depend upon the amperage of the welding current. Due to the absence of smoke in GTAW, the arc appears brighter than shielded metal arc welding and more ultraviolet radiation is produced. Exposure of bare skin near a GTAW arc for even a few seconds may cause a painful sunburn. Additionally, the tungsten electrode is heated to a white hot state like the filament of a lightbulb, adding greatly to the total radiated light and heat energy. Transparent welding curtains, made of a polyvinyl chloride plastic film, dyed in order to block UV radiation, are often used to shield nearby personnel from exposure.
Welders are also often exposed to dangerous gases and particulate matter. Shielding gases can displace oxygen and lead to asphyxiation, and while smoke is not produced, the arc in GTAW produces very short wavelength ultraviolet light, which causes surrounding air to break down and form ozone. Metals will volatilize and heavy metals can be taken into the lungs. Similarly, the heat can cause poisonous fumes to form from cleaning and degreasing materials. For example chlorinated products will break down producing poisonous phosgene. Cleaning operations using these agents should not be performed near the site of welding, and proper ventilation is necessary to protect the welder.[10]
Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.
[edit] ApplicationsWhile the aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing such as those used in the bicycle industry. In addition, GTAW is often used to make root or first pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium.[11] Because the weld metal is not transferred directly across the electric arc like most open arc welding processes, a vast assortment of welding filler metal is available to the welding engineer. In fact, no other welding process permits the welding of so many alloys in so many product configurations. Filler metal alloys, such as elemental aluminum and chromium, can be lost through the electric arc from volatilization. This loss does not occur with the GTAW process. Because the resulting welds have the same chemical integrity as the original base metal or match the base metals more closely, GTAW welds are highly resistant to corrosion and cracking over long time periods, GTAW is the welding procedure of choice for critical welding operations like sealing spent nuclear fuel canisters before burial.[12]
Engineers prefer GTAW welds because of its low-hydrogen properties and the match of mechanical and chemical properties with the base material. Maximum weld quality is assured by maintaining the cleanliness of the operation—all equipment and materials used must be free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil and grease, alcohol or similar commercial solvents may be used, while a stainless steel wire brush or chemical process can remove oxides from the surfaces of metals like aluminum. Rust on steels can be removed by first grit blasting the surface and then using a wire brush to remove any embedded grit. These steps are especially important when negative polarity direct current is used, because such a power supply provides no cleaning during the welding process, unlike positive polarity direct current or alternating current.[13] To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTA welding in windy or drafty environments increases the amount of shielding gas necessary to protect the weld, increasing the cost and making the process unpopular outdoors.
Because of GTAW's relative difficulty and the importance of proper technique, skilled operators are employed for important applications. Welders in the U.S. should be qualified following the requirements of the American Welding Society or American Society of Mechanical Engineers. Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. If there is too much heat input, however, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. Additionally, if the welder holds the welding torch too far from the workpiece, shielding gas is wasted and the appearance of the weld worsens.
If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, it can be identified with radiography and prevented by changing the type of electrode or increasing the electrode diameter. In addition, if the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that electrode be ground with a diamond abrasive to remove the impurity.[14] GTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, positively charged ions flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.
The equipment required for the gas tungsten arc welding operation includes a welding torch utilizing a nonconsumable tungsten electrode, a constant-current welding power supply, and a shielding gas source.
[edit] Welding torchGTAW welding torches are designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling systems are most often used for low-current operations (up to about 200 A), while water cooling is required for high-current welding (up to about 600 A). The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.
The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively. The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. Collets are sized according to the diameter of the tungsten electrode they hold. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder.
The size of the welding torch nozzle depends on the amount of shielded area desired. The size of the gas nozzle will depend upon the diameter of the electrode, the joint configuration, and the availability of access to the joint by the welder. The inside diameter of the nozzle is preferably at least three times the diameter of the electrode, but there are no hard rules. The welder will judge the effectiveness of the shielding and increase the nozzle size to increase the area protected by the external gas shield as needed. The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance, offers greater visibility. Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to improve the control shielding gas flow to reduce turbulence and introduction of contaminated atmosphere into the shielded area. Hand switches to control welding current can be added to the manual GTAW torches.[15]
Power supplyGas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.[16]
The preferred polarity of the GTAW system depends largely on the type of metal being welded. Direct current with a negatively charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material. Direct current with a positively charged electrode (DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance.
Alternating current, commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. Surface oxides are still removed during the electrode-positive portion of the cycle and the base metal is heated more deeply during the electrode-negative portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high-frequency voltage to encourage ignition.[17]
SteelsFor GTA welding of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface. Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases.[24]
[edit] Copper alloysTIG welding of copper and some of its alloys is possible, but in order to get a seam free of oxidation and porosities, shielding gas needs to be provided on the root side of the weld. Alternatively, a special "backing tape", consisting of a fiberglass weave on heat-resistant aluminum tape can be used, to prevent air reaching the molten metal.
[edit] Dissimilar metalsWelding dissimilar metals often introduces new difficulties to GTAW welding, because most materials do not easily fuse to form a strong bond. However, welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation. In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (for example, using a stainless steel filler metal with stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials.
When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials.[25]
[edit] Process variations[edit] Pulsed-currentIn the pulsed-current mode, the welding current rapidly alternates between two levels. The higher current state is known as the pulse current, while the lower current level is called the background current. During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations, making it useful for specialized applications.[26]
[edit] DabberThe dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades.[27]
[edit] Hot wireWelding filler metal can be resistance heated to a temperature near its melting point before being introduced into the weld pool. This increases the deposition rate of machine and automatic GTAW welding processes. More pounds per hour of filler metal is introduced into the weld joint than when filler metal is added cold and the heat of the electric arc introduces all of the heat. This process is used extensively in base material build up before machining, clad metal overlays, and hardfacing operations.
Posted by: Adnan
GTAW
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.[1]
GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.[1]
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