Welding Procedures And Inspection

Front 1

What is Welding

Back 1

• Welding involves joining two or more pieces of metal using heat and sometimes pressure.
• Arc welding uses the heat of an electric arc.
• The heat of the arc is used to melt and fuse the parts with or without the filler metal.
• The weld joint is usually specially prepared.

Front 2

What is Metal Arc Welding

Back 2

• Arc welding is a fusion welding process.
• The electric arc provides the necessary heat.
• The heat is produced between the base metal and the electrode.
• Temperatures range from 3000-8300oC.
• A pool of molten metal forms between the work and the arc location.
• The welder manipulates the molten pool of metal as required.

Front 3

What is Shielded Metal Arc Welding

Back 3

• Uses a coated consumable electrode.
• Small droplets of molten metal are deposited at the weld joint.
• Metal deposition is depended on current flow.

• The weld is shielded by the gases produced from the coating.
• Most popular, wide application, high speed process for thin sections.
• Adaptable for flat, vertical, and overhead positions.

• Extensively used for pressure vessel and piping system manufacture and repair.

Front 4

Describe Arc Welding Equipment

Back 4

• Uses either AC or DC sources depending on electrode type.
• Portable motor generators are the most satisfactory type of power source.
• Generator is variable voltage and adjusts to maintain current flow.

• AC is supplied by single phase transformers.
• Uses variable voltage taps or variable inductors to obtain current values.

• Another source of DC is a rectifier.

Front 5

Describe Electrodes used in welding

Back 5

• Carbon, bare wire, and electrodes with
combustible coatings generally require DC.
• Electrodes with coverings of principally minerals are, in general, used for AC and DC.
• Combustible coatings provide a gaseous cloud to shield the weld.
• Mineral coatings produce a slag around the
metal globules as they form.
- the slag protects the weld as it cools.

Front 6

Which group has developed an Electrode Classification

Back 6

• The American Welding Society (AWS) has developed an identification system for SMAW electrodes.

Front 7

Describe Electrode Classification

Back 7

• The AWS system consists of a four figure number proceeded by the letter “E”.

• The letter “E”:
– Welding electrode
• The first two digits:
– The tensile strength in thousands of psi.
– 1 psi = 6.9 kPa
• The third digit:
– Denotes the weld positions.
• The fourth digit:
– Denotes the electrodes characteristics

Front 8

What are the different Electrode Weld Positions

Back 8

1. All positions
2. Flat horizontal fillet
3. Flat only
4. Down hand only

Front 9

What are some examples of Electrode Characteristics

Back 9

3 - rutile, potassium, AP AC or DC, LP
7 – iron powder, iron oxide SP AC or DC, MP

Front 10

Describe Electric Arc Machines

Back 10

• Most machines for pressure vessel work
are DC.
• DC creates a more manageable welding
arc.
• DC produces better control and is the
preferred current source.

Front 11

Describe Straight Polarity

Back 11

• Straight polarity more heat at the
electrode.

• Faster welding speeds.
• Shallow wide angled weld bead.

Front 12

Describe Reverse Polarity

Back 12

• Assuming conventional current if the
electrode holder is + then the polarity is
reversed.
• More heat at the work piece.

• Narrow weld bead.
• Better penetration
• Good for overhead welding.

Front 13

Describe Submerged Arc Welding

Back 13

• Welding beneath a layer of granular flux.
• Produces neat uniform seams.
• Machine electric welding process.
• Produces welds that would be difficult to reproduce using hand methods.
• The granular material protects the weld as it progresses.

• The arc is not visible during the welding process.
• The electrode does not come in contact with the base metal.
• Very little gas or fumes are created.
• Can be manual or automatic.
• Using steel wool between the electrode and work piece is one way to start the arc.

• Arc is struck beneath the flux.
• Very little spatter or sparks.

• The molten flux is lighter and forms a slag
on top of the weld bead.
• This slag protects the weld from oxidation.
• Allows the weld to cool slowly.
• Produces a smooth uniform bead.
• Excessive unused flux can be recycled.

Front 14

Describe Gas Metal Arc Welding (GMAW/MIG)

Back 14

• Semi automatic or automatic process.
• Continuous consumable wire electrode.
• Uses a shielding gas fed through gun.
• Usually uses constant voltage DC but AC can be used.
• Originally developed to weld aluminum.

• Four primary methods of metal transfer:
– Globular
– Short circuiting
– Spray
– Pulsed spray

• Cost of inert gases initially prohibited its use.
• When semi-inert carbon dioxide gas became common it gained in use in welding steels.
• Popular in automotive production.
• Outdoor use is limited due to shielding gas.

Front 15

Describe Gas Tungsten Arc Welding (GTAW/TIG)

Back 15

• The gas tungsten arc welding process (GTAW) joins metal by the heat of an arc between a non-consumable tungsten electrode and the work piece, with or without the addition of a filler metal

• Gas tungsten arc welding can join most metals, but is particularly suited to the welding of difficult to weld metals where high quality defect free welds, are required

Front 16

Describe the GTAW Process

Back 16

• Gas tungsten arc welding is a highly skilled, low production process.
• Most GTAW applications involve manual feeding of the filler metal to the arc.
– A semi-automatic variant involves the feeding of a preheated continuous filler wire to the arc.
• More recently with the advent of new power sources which can produce pulsing arcs, combined with advances in computer controlled servomotors, GTAW torches have been mounted on carriages and travel on orbital tracks to produce precision tube to sheet and thin walled pipe welds.
– However, from a production standpoint, GTAW cannot match the deposition rates of semi-automatic welding processes such as gas metal arc welding (GMAW), flux-cored arc welding (FCAW) and submerged arc welding (SAW).

• The arc melts the base metal and forms a molten puddle
• An inert coverage gas usually shields the arc at the tip of the non-consumable tungsten electrode and the molten weld puddle.
• Some applications utilize active/inert gas mixtures for shielding.

Front 17

What are the Advantages of GTAW

Back 17

• It is well suited to the welding of difficult to weld nonferrous and highly alloyed steels.
– Stainless steels, coppers, aluminums and magnesiums and some refractory type metals such as titanium, are readily joined using GTAW.
• GTAW is particularly suited to the welding of dissimilar metals.
• When correctly applied, it is the preferred process for welding thin materials in all positions.
• The GTAW process is extremely clean.
– There is no spatter and flux is not required.
– Fume rate is low.
• GTAW is a relatively low cost process, requiring conventional power sources and gas coverage equipment.

Front 18

What are the Disadvantages of GTAW

Back 18

• The process demands high operator skills.
• The utmost cleanliness, in the preparation and handling of the base material and the filler metal, is required.
– Contaminants, on the base metal or dirty filler metal, can introduce porosity into the weld pool.
• Loss of coverage gas can result in overheating of the tungsten, causing particles of tungsten to enter the weld pool.
• Touching of the tungsten electrode into the deposited weld metals, in addition to gas coverage of the tungsten tip and weld pool, makes it necessary to devise back purging of the exposed root face, which adds cost by increasing set up time.

Front 19

Describe Brazing

Back 19

• Brazing does not produce fusion.

• Brazing involves heating usually with an oxyacetylane torch

• Temperatures are above 450oC.
• A bronze filler rod is used.
• The heat bronze and flux create a grain structure bond between the surfaces.
• Brazing is extensively used to repair cast-iron and malleable parts.

Front 20

Describe Weld Faults

Back 20

• Weld defect tolerance depends on type of service.
• Service condition must be known before suitable inspections can be done.
• Power Engineers regularly meet with welding inspectors.

Front 21

Describe Cracks

Back 21

• Cracks are the first and most serious category of structural defects.
• There are several ways to categorize cracks.
• Surface cracks
• Underbead cracks etc.

Front 22

Describe Porosity

Back 22

Porosity is spherical or tube-like defects, cavities, or voids caused by gases
trapped inside the weld or gases that have evolved to the surface.

Front 23

Describe Slag Inclusions

Back 23

Slag inclusions are solid non-metallic inclusions entrapped below the surface in the weld metal or more often, between deposited weld metal and the base metal. Internal solid inclusions can occur randomly, continuously, or intermittently.

Front 24

Describe Undercut

Back 24

• Undercut is a melting away of the parent metal which can reduce the thickness of the welded joint. (Fig. 10)
• Produces points of stress.
• Caused by excessive currents, incorrect manipulation of electrode, incorrect electrode angle or type.

Front 25

Describe Overlap

Back 25

• Overlap is an excess of deposit metal, which is not fused to the base metal. It is both wasteful an a site for acute entrance angles and potential fatigue failure.

Front 26

Describe Underfill

Back 26

• Underfill is insufficient weld deposit, resulting in a reduction in the thickness of the groove.

Front 27

Describe the types of Non-Destructive Testing

Back 27

• Visual
• Dye-penetrant
• Magnetic particle
• Radiographic
• Ultrasonic

Front 28

Describe Visual Inspection

Back 28

• Visual inspection is quick and inexpensive.
• It is extensively used by examiners.
• Most surface conditions can be readily detected.
• Improper edge and joint preparation, root gaps, etc. can be caught early.
• Inspections usually take place before and during welding.
• Good visual inspection can save many hours of time as well as money by correcting faults before applying more expensive testing.

Front 29

Describe Liquid Penetrant Testing (PT)

Back 29

Liquid penetrant testing (PT) is an NDE technique for detecting flaws that are open to the surface.
– The surface, being inspected, cannot be rough or porous as these conditions will produce excessive background and interfere with the PT inspection.
PT is based upon the principle of capillary action (wetting action).
– E.g. Liquid in a straw.
– E.g. Reading the level of water in a cylinder or beaker.

The liquid penetrant must have the ability to wet the surface of the component, being inspected.
This provides the surface of the component with a uniform liquid coating. The liquid must be capable of migrating.
For a liquid to have these desired properties, it all depends upon surface tension and contact angle.
– The height to which the liquid rises up the straw is proportional to contact angle and surface tension.
– It is inversely proportional to liquid density.
Liquid penetrant testing can be used to examine pressure components that are welded, cast, rolled or forged.

Front 30

What are the different types of Liquid Penetrant Testing (PT)

Back 30

There are two different types:
• Fluorescent
• Dye
There are also four PT methods:
• water-washable,
• postemulsifiable - oil based,
• postemulsifiable – water based,
• solvent-removable.

Front 31

Describe Fluorescent Dye Testing

Back 31

• Detects defects that extend to the surface.
• Good for magnetic or non-magnetic materials.
• Uses a penetrating fluid that fluoresces under UV light.
• Fluid is applied and allowed to migrate into any flaws.
• The excess is then wiped off.
• The flaws will fluoresce under the UV light.

Front 32

Describe Dye Penetrant Testing

Back 32

• Simplified version of fluorescent testing.
• Indicates surface discontinuities with bright red lines or dots on a white background.
• Requires a minimum of equipment.
– Three brushes.
– Three cans, one each of:
• Cleaner
• Dye penetrant
• Developer

Front 33

What is the procedure for performing a PT Test

Back 33

When completing a PT, there are six
essential steps required:
1.Pre-cleaning.
2.Penetrant application.
3.Excess penetrant removal.
4.Developer application.
5.Inspection/interpretation.
6.Post-cleaning.

Front 34

What should be considered when selecting the PT Technique

Back 34

When selecting a PT technique, the following
points should be considered:
• Size of component being tested.
• Surface condition of the component being
tested.
• Location.
• Sensitivity of test required.
• Typical characteristics of the flaws to be detected.

Front 35

Can PT Testing be used for Leak Detection

Back 35

• Penetrant testing can be used for leak detection by applying the dye to an area suspected of leaking and then applying the developer to the other side.
• The dye will be drawn through the weld and become visible.
• This method of leak detection can be done with either type of penetrant testing.

Front 36

What is Magnetic Particle Testing

Back 36

Magnetic particle testing (MT) is an NDE technique for detecting flaws that are either surface, or subsurface. The MT technique can only be applied on ferromagnetic materials.

The principle behind MT techniques is that in the presence of discontinuities, the magnetic flux in a material is distorted.
– The distortion is a function of the orientation of the discontinuity to the magnetic field (flux lines).
• The distortion is greatest when the discontinuity is perpendicular to the magnetic field.

• When distortion of the magnetic field is great enough, a pair of magnetic poles, which act as small magnets, are established at the discontinuity.
• Applied magnetic particles are attracted to the poles and will gather at the discontinuity.
• Magnetic particles can be applied as wet or dry particles; available in various colors (silver-grey, black, red, yellow and green) and as fluorescent particles.
• The availability of various colors is necessary to obtain the maximum contrast between the surface of the component and the discontinuity.
• Fluorescent particles are extremely visible when viewed under ultraviolet light and have a high contrast with the surface being examined.

Front 37

What are the Uses of Magnetic Particle Testing

Back 37

Magnetic particle testing can be used to examine ferromagnetic pressure components that are welded, cast rolled or forged.
• Some of the typical discontinuities that can be detected are:
– Cracks from quenching.
– Cracks from grinding.
– Cracks from fatigue.
– Cracks from stress corrosion.
– Lack of penetration.
– Cracks from welding.

Front 38

What is the Procedure for Magnetic Particle Testing?

Back 38

When completing MT, there are essentially seven steps that are followed:
1. Pre-cleaning the component.
2. Pre-demagnetization of the component.
3. Magnetizing the component.
4. Application of magnetic particles.
5. Interpretation.
6. Post-demagnetization of the component.
7. Post-cleaning.
Pre-demagnetization is important as the component being tested may have been magnetized previously for testing.
Post-demagnetization is required to prevent damage to moving parts, arc deflection during welding, interference with machining and interference with coating or painting.

Front 39

What are the considerations for Selecting the Magnetic Particle Testing

Back 39

When selecting an MT technique, the following points must be considered:
• The type of material being tested (must be ferromagnetic).
• The size and geometry of the component.
• The most suitable type of MT method, (dry or wet).
• The most suitable type of particles (visible or fluorescent).
• Will it be a shop or site test?
• Typical defects expected (i.e. size, location and orientation).

Front 40

Describe Radiographic Testing

Back 40

Radiographic testing (RT) is an NDE technique used for detecting flaws that are internal or on the inside surface. It is one of the oldest NDE techniques used in the pressure equipment industry.

The principle behind RT techniques is that in the presence of flaws, there will be a differential absorption of penetrating radiation. The unabsorbed radiation passes through the test component and exposes a film. The exposed film indicates the varying amounts of radiation passing through the component and gives a permanent record of the test.

Penetrating radiation can be x-rays or gamma rays.

• X-rays are produced by high-speed electrons striking a metal target, causing a transfer of energy. An x-ray tub, in an x-ray machine, produces these high-speed electrons.

• Gamma rays are emitted from radioisotopes, such as Cobalt 60 and Iridium 192, as they decay (disintegrate).

• The ability for x-rays and gamma-rays to penetrate materials varies.

Front 41

What are the uses of Radiographic Testing

Back 41

Typical discontinuities that can be detected are:
– Porosity.
– Incomplete Penetration.
– Incomplete Fusion.
– Thickness Variations.
– Corrosion.
– Pitting.
– Slag Inclusions.
Radiography can only detect cracking when cracking is oriented, parallel to the radiation beam.

Front 42

What is the procedure for Radiographic Testing

Back 42

When completing an RT, there are four essential steps:
– Source selection.
– Set up.
– Exposure of test component to the radiation source.
– Film development.
It is interesting that approximately sixty percent of the time is spent, on set-up.

Front 43

What are considerations when selecting Radiographic Testing

Back 43

When selecting an RT technique, the following points are considered:
• The size and geometry of the component being tested.
• Typical defect type, size location and orientation.
• Shop of field testing.
• Personnel safety.
• Generally flaws must be at least as large as 2 % of the penetration thickness, to be detected.
• Access to both sides of the component is required to place the film.

Front 44

Describe Ultrasonic Testing

Back 44

• High frequency vibrations are very sensitive to cracks and defects than other types of testing.
• Ultrasonic waves are directed into the material, reflected, and measured.
• Ultrasonic inspection can be carried out on in service and out of service equipment.
• Rules for UT are listed in ASME Section 1.

Front 45

Describe the Ultrasonic Testing Procedure

Back 45

• The sound waves penetrate materials at some known velocity, depending on the composition and density of the material.
• Providing there is a good coupling medium between the vibrating wafer (transducer) and the material to be inspected, the sound waves penetrate the material and reflect back through the couplant (usually water, glycerine, or oil), to the transducer. The couplant is the material that is used to make the connection between the probe and the material being tested.
• The transducer, in turn, converts the signal to an electrical impulse, which is then revealed as an indication on an oscilloscope.
• The height and width of the indication can be related to the thickness of the material, being examined.
• The exact depth, location and profile of the flaws can be determined by UT.