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18-06-2014, 03:49 PM
Post: #1
Defects and Discontinuities

Defects and Discontinuities



.ppt  1378527162-000043.ppt (Size: 1.25 MB / Downloads: 241)

Defect

A flaw or flaws that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability.

Discontinuity

An interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect

Misalignment

Definition: Amount a joint is out of alignment at the root

Cause: Carelessness. Also due to joining different thicknesses (transition thickness
Prevention: Workmanship. Transition angles not to exceed 2.5 to 1.

Repair: Grinding. Careful on surface finish and direction of grind marks. Inside of Pipe /Tube difficult

Undercut

Definition: A groove cut at the toe of the weld and left unfilled

Cause: High amperage, electrode angle, long arc length, rust

Prevention: Set machine on scrap metal. Clean metal before welding.

Repair: Weld with smaller electrode, sometimes must be low hydrogen with preheat. Sometimes must gouge first

Undercut typically has an allowable limit. Different codes and standards vary greatly in the allowable amount.
Plate - the lesser of 1/32” or 5% (typ.)

Insufficient Fill

Definition: The weld surface is below the adjacent surfaces of the base metal

Cause: Improper welding techniques

Prevention: Apply proper welding techniques for the weld type and position. Use stripper beads before the cover pass

Repair: Simply weld to fill. May require preparation by grinding

Insufficient Fill on the Root Side (suckback)

Definition: The weld surface is below the adjacent surfaces of the base metal at the weld root.

Cause: Typically improper joint preparation or excessive weld pool heat.

Prevention: Correct cause. (see next slide)

Repair: Backweld to fill. May requireremoval of weld section by grinding for access to the joint root

Cause for Insufficient Fill at the Root

Some liquids, like water or molten steel, try to cover as much surface area of whatever they are in contact with as possible

Welding a root pass too wide can also cause the bead to sag (overhead position).

Removing a root pass by grinding

1. Recreate the groove geometry as closely as possible.
2. Use a saw or die grinder and 1/16 - 1/8” cut off wheel to recreate root opening. Remember repairs are sometimes required to be made with a smaller electrode.
3. Open the groove angle. Be careful to leave the proper root face dimension.
4. Feather the start and stop to blend smoothly into and out of the existing weld.

Insufficient Reinforcement

Definition: Specifically defined by the standard. Typically, Underfill may be up to 5% of metal thickness not to exceed 1/32” as long as the thickness is made up in the opposite reinforcement. Not applied to fillet welds.

Cause: On root reinforcement - Too little filler metal will cause thinning of the filler metal. In OH position, too hot or too wide will cause drooping of the open root puddle

Prevention: Use proper welding technique. Use backing or consumable inserts. Use back weld or backing

Repair: Possibly simply increase the face reinforcement. If backwelding is not possible, must remove and reweld

Improper Weld Contour

Definition: When the weld exhibits less than a 1350 transition angle at the weld toe

Cause: Poor welding technique

Prevention: Use proper techniques. A weave or whip motion can often eliminate the problem

Repair: The weld face must be feathered into the base plate

Overlap

Definition: When the face of the weld extends beyond the toe of the weld

Cause: Improper welding technique. Typically, electrode angles and travel speed.

Prevention: Overlap is a contour problem. Proper welding technique will prevent this problem

Repair: Overlap must be removed to blend smoothly into the base metal. Be careful of deep grind marks that run transverse to the load. Also be careful of fusion discontinuities hidden by grinding. Use NDT to be sure

Base Metal Discontinuities

Lamellar tearing
Laminations and Delaminations
Laps and Seams

Laps and Seams

A mill-induced discontinuity in which results from a lump of metal being squeezed over into the surface of the material.
If beyond acceptable limits, must be removed and repaired or discarded.

Porosity

Single Pore
Uniformly Scattered
Cluster
Linear
Piping

Single Pore

Separated by at least their own diameter along the axis of the weld

Uniformly Scattered Porosity

Typically judged by diameter and proximity to a start or stop
often caused by low amperage or short arc gap or an unshielded weld start

Porosity

preheat will help eliminate
may need an electrode with more deoxidizers
Use run-on/run-off taps
restart on top of previous weld and grind off lump

Heat-affected zone microstructure alteration

add drawing of HAZ of groove weld with leaders to:
grain refinement
grain growth
hardened areas
softened areas
precipitate suseptable areas

Size or dimension

If it renders the part unusable, it is a defect.
If it is outside the allowable limit, it renders the part unusable.
Things don’t have to be perfect, just within the acceptable tolerance. Working to perfection is too time consuming and costly

Hammer marks

Stress risers
Unsightly
Unnecessary

REPAIR TECHNIQUES

May involve:
different process
different procedure
different preheat/PWHT
different electrode
smaller electrode
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