Slide 4 of 81

      

      

      

      

      

     

      

                                                                          

                

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

COMMONLY OBSERVED DISCONTINUITIES IN FUSION WELDING

TYPE EXISTS REMARKS

Lamination Pm Base metal, genarally near mid thickness of section.

Delamination Pm ,,

Seams & laps

Pm Base metal surface, often at all times longitudinal.

Lamellar tears

Pm Invariably near the HAZ in flange plate of T-butt joint.

Cracks Pm, Wm Wm/Pm

Restraint, Hot, Brittle & Under bead cold cracks; which may be either in longitudinal or transvers direction.

Crater cracks Wm Usually with multi axial cracks at the point of termination.

Fissures Wm Micro cracks generally in fully austenitic stainless steel & less ductile metal.

Stray flash Pm Appears away from the weld seam as a trail of arc spots with micro fissures, excesively brittle & hard character.

Spatters Pm Globular weld particles ejected out of an arc zone & scattered shabbily around over the base metal.

Pm = Parent metal; Wm = Weld metal; Pm/Wm = Junction of weld & Pm = Parent metal; Wm = Weld metal; Pm/Wm = Junction of weld & base metalbase metal Continued...

Weld decay & stress corros- ion cracks

Pm Precipitation of chromium carbide in austenitic stainless steels & severely degrading the corrosion resistance property in HAZ; which may also be associated with the stress corrosion cracks.

Oxidation Wm Inadequacy in gas shield or gas purge from the root side causes a heavy black scale or an extremely rough crinkled appearance.

Craters Wm An unfilled concave crater causes a point of stress raiser.

Underfill Wm Inadequate weld metal filling and causing weakness.

Undercut Wm/Pm Groove made by the arc force & left unfilled, causes severe stress concentration.

Overlap Wm/Pm Accumulation of weld, without fusion, causes an extremely voilent point of sstress raiser.

Lack of fusion

Wm, Wm/Pm

Lack of union between the two weld beads or weld & base metal causes stress concentration.

Lack of penetration

Pm Inadequacy of through thickness fusion depth.

Solid particle inclusion

Wm Trapped slag particle, tungsten or oxide (Al2O3) in weld.

Gas inclusion Wm Gas voids contained within the weld causes: Blow hole, Gas pore, Piping, Worm holes, Linear, Clustered or Scattered porasity.

Pm = Parent metal; Wm = Weld metal; Pm/Wm = Junction of weld & Pm = Parent metal; Wm = Weld metal; Pm/Wm = Junction of weld & base metalbase metal

      

                    

                    

                                                                                          

- due to segregation- due to segregation

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

Lamellar Tearing:

      

                    

                    

                                                                                          

• Dilution

• Heat input

The dilution:

Fm

Pm

Wm

Wm = Fm + Pm

Dilution% = Pm / Wm X 100

Pm = Melted Parent metal

Fm = Melted Filler metal

The heat input:

Kj / mm = I.V./ S.1000

I = Current across the arc.

V = Voltage across the arc.

S = The rate of arc motion mm/Sec.

‘‘Too low or high heat input both may equally be proved detrimental.’’

      

                    

                    

                                                                                          

Solidification cracks due to the bead factor:

W = Bead Width

P = Bead Depth

W

P P

W

W/P>1 W/P<1

That is W/P

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                        

     

Cracks is detected in a radiograph, only when it produces a change in thickness that is parallel to the x-ray beam. It appears often zig-jagged with faint irregular line. Cracks can also appear sometime as "tail" to an inclusion or porosity.

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                         

    

Undercut is an erosion of the base metal next to the toe of the weld face. It appears in radiograph as a dark irregular line on outer edge of the weld.

                                                                                         

    

Root undercut is an erosion of the base metal next to the root of the weld. It appears in radiographic images 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 the straight edge

                                                                                         

    

Root concavity or suck back is a condition where the weld metal has contracted as it cools down & has been drawn up into the root of the weld. On a film it appears similar to the lack of root penetration but the line has irregular wide edges and placed in the middle.

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                         

    

Cold lap is a condition where the weld metal does not fuse with the base metal or the previous weld bead (interpass cold lap). The arc does not melt the base metal and causes the molten puddle to flow into the base metl without the proper bonding.

                                                                                         

    

Incomplete fusion is a condition where the weld metal does not fuse with the base metal. Appearance on radiograph is usually a darker line or lines oriented in the direction of the weld seam along the weld joining area.

      

                    

                    

                                                                                          

                                                                                        

    

Burn through (icicles) results when too much heat causes weld to pierce through. Lumps of weld metal sag through the seam creating a thick globular condition on the root face. On a radiograph, burn through appears as dark spots surrounded by light globular areas.

Whiskers are the short lengths of electrode wire, visible on the top or bottom surfaces of the weld or contained within theweld. On radiograph they appear as light, "wire like" indications.

      

                    

                    

                                                                                          

                                                                                         

    

Lack of penetration occurs when the weld metal fails to penetrate through the joint. Allows a linear stress riser like discontinuity from which a crack may initiate. The appearance on a radiograph is a dark well-defined straight edges that follows the land or root face down the center of a joint.

      

                    

                    

                                                                                          

•Gas pore _ singular.

•Blowhole _ singular.

•Scattered Porasity.

•Cluster Porasity.

•Linear Porasity.

•Piping.

• Worm holes.

Gas inclusion

Fine Severe

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                  

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                        

    

Porosity appears often as dark round irregular spots in clusters or rows. Sometimes it is elongated and may have an appearance of a tail. This is the result of gas attempting to escape while the metal is still in a liquid state & is called wormhole porosity. All porosity is indeed a void will have a darker density than the surrounding.

                                                                                         

    

Cluster porosity is caused when electrodes are contaminated with moisture or hydrocarbon. It appears like regular porosity in a film but the indications will be grouped close together.

      

                    

                    

                                                                                          

• Oxide inclusion/ Puckering

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                       

     

Slag inclusions are the nonmetallic solid materials trapped in weld or between the weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

      

                    

                    

                                                                                          

                                                                                        

    

Tungsten inclusions. Tungsten is a brittle and dense material used as an electrode in tungsten inert gas welding. If an incorrect welding procedures & skill is performed, then only the tungsten gets trapped. Radiographically, tungsten is more dense than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on the radiograph

                                                                                        

    

Oxide inclusions are usually visible on the surface of a weld mtal (especially aluminum). Oxide inclusions are less dense than the surr -ounding metals and, thus it appears as dark irregular shaped discon -tinuity in radiograph. This is also referred as puckering in ISO.

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                         

    

The radiographic image is a noticeable difference in density between the two mismatched 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.

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                        

    

Excessive reinforcement is an area of a weld added in excess of that specified by the drawings and codes. The appearance on a rad-iograph is a localized & less darker area. A visual examination will easily determine if the weld reinforcement is in excess.

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

      

                    

                    

                                                                                          

                                                                                         

    

Underfilling is an area where the deposited weld metal is less than the required thickness. It is easy to determine by RT films, because the image density in the area of inadequacy will be darker than the surrounding image density.

Distortion:Distortion is an unavoidable phenom -enon of fusion welding.

Type:

• Longitudinal distortion; &

• Transvers distortion.

• Angular distortion;

Distortion:

Remedy: only to minimise.

• Reduce the cause of shrinkage forces;

• Make use of the shrinkage forces; &

• Balance the shrinkage forces.

Reduce the cause of shrinkage forces:

1. Do not weld __ if possible.

2. Reduce the number of joints.

3. Improve the joint design & fit-up.

4. Avoid excessive root gap & mismatch.

5. Avoid over welding.

6. Reduce the number of runs.

7. Use larger size electrodes.

8. Use iron powder type electrode.

9. Use semi or fully mechanized welding.

RO

RF

GA

GA=Groove / Included angleRF = Root faceRO = Root opening

Fusion faces

© Shoulder

©©

Obviously the poor fit-up demands more metal to be filled in and thus the more shrinkage / distortion.

t

W

W

Effective throat area gets reduced in proportion to the root gap and an over welding by 1.6mm to a 6 mm given fillet size, the cross section area of weld increases by a margin of 56%.

Make use of shrinkage forces:

Balance the shrinkage forces:

Use an appropriate welding scequence.

i.e. Back step & intermittent welding techniques.

Use external force:

i.e. Tack weld, Jigs, Fixtures & Clamps.