RADIOGRAPHY EXAMINATION - IDENTIFICATION OF DISCONTINUITIES IN WELD ~ FABRICATION & RELATED TERMS

IDENTIFICATION OF DISCONTINUITIES IN WELD

Discontinuities is basically interruption in the structure of material. This discontinuities may occur in base metal, on weld of material or on HAZ (Heat Affected Zone). The number of discontinuities that is acceptable for given material is depend on type of discontinuities and different parameters so it is important to identify the given discontinuity and either acceptable or not as per given applicable codes. So the different types of general discontinuities as shown below,

1) Lack of Fusion : The Lack of Fusion is the type of discontinuity which may appears on between rungs of weld on at the edges of weld. Basically Lack of Fusion is where the weld filler metal does not properly fuse with the base metal or previous weld pass material. The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.

Lack of fusion is most serious weld discontinuity occur on weld joint. If lack-of-fusion defects and cracks are compared, it is evident that much more attention is paid to cracks than to lack of fusion although lack of fusion is as serious a defect as a crack. On the basis of position the lack of fusion classified as,

1) lack of side-wall fusion,

2) lack of inter-run fusion,

3) lack of fusion at the root of the weld.

The Diagrammatic view and the view on RT Film of Lack of Fusion as shown below,

Pic.001

2) Cracks : Crack is tight linear separations of metal that can be very short to very long indications. Cracks are grouped as hot or cold cracks. Hot cracks usually occur as the metal solidifies at elevated temperatures. Cold cracks occur after the metal has cooled to ambient temperatures. Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Different types of cracks occurs in weld as,

1) Longitudinal Cracks.

2) Transverse Cracks.

3) Centre Cracks.

4) Throat Cracks.

5) Root Cracks.

6) Under bead Cracks.

7) Root Cracks.

8) Hot Cracks.

9) Toe Cracks.

10) Cold Cracks.

11) Crater Cracks.

The Diagrammatic view and the view on RT Film of Cracks as shown below,

Pic.002

3) Porosity : Porosity is the result of gas entrapment in the solidifying metal. Porosity is actually black round or irregular spot in clusters or in rows. All porosity is a void in the material it will have a radiographic density more than the surrounding area. Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.

The Diagrammatic view and the view on RT Film of Porosity as shown below,

Pic.003

4) Cluster porosity: Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process.

The Diagrammatic view and the view on RT Film of Cluster Porosity as shown below,

Pic.004

5) Root Concavity: Root or Internal concavity or suck back is condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. Root concavity is caused by shrinkage of the weld pool in the through-thickness direction of the weld. Melting of the root pass by the second pass can also produce root concavity. On a radiograph it looks similar to lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

The Diagrammatic view and the view on RT Film of Root Concavity as shown below,

Pic.005

6) Incomplete penetration (IP): Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment. Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides root region of the joint are unfused due to,

A) Excessive Root Face in Butt Weld

B) Misplaced Weld

C) Too small Root Gap

D) Too small Bevel Angle

E) Use of Too large diameter Electrode.

The Diagrammatic view and the view on RT Film of Incomplete Penetration as shown below,

Pic.006

7)Slag inclusions: Slag inclusions are non metallic particles trapped in the weld metal or at the weld surface. Slag inclusions result from faulty welding technique, improper access to the joint or both. Tungsten inclusions are tungsten particles trapped in weld metal deposited with the gas tungsten arc welding process. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

The Diagrammatic view and the view on RT Film of Slag Inclusion as shown below,

Pic.007

8) Undercut: Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

The Diagrammatic view and the view on RT Film of Undercut as shown below,

Pic.008

9) Mismatch or Offset : Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image is a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.

The Diagrammatic view and the view on RT Film of Mismatch or Offset as shown below,

Pic.009

10) Inadequate weld reinforcement: Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be more (darker) than the image density of the surrounding base material.

The Diagrammatic view and the view on RT Film of Inadequate Weld Reinforcement as shown below,

Pic.010

11) Excess weld reinforcement : Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

The Diagrammatic view and the view on RT Film of Excess Weld Reinforcement as shown below,

Pic.011

12) Tungsten Inclusions : Tungsten inclusions are tungsten particles trapped in weld metal deposited with the gas tungsten arc welding process. Dipping the tungsten electrode in the molten weld metal or using too high current that melts the tungsten can cause inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas ( TIG ) welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is denser than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on the radiograph.