A discontinuity or discontinuities that by nature or accumulated effect (for example, total crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability. See DISCONTINUITY and FLAW.

Defects in welds are points, areas or volumes of a weld that are unsound, indicating that there is either a geometric or metallurgical discontinuity in the structure. Such defects may involve regions where metal is absent and there is no solid present (e.g., pores, voids, cracks), regions where there are low-density (compared to the weld metal) non-metallic inclusions (e.g., entrapped slag), regions where there are high-density

(compared to the weld metal) inclusions (e.g., tungsten inclusions), or various geometric discontinuities (eg, lack of penetration, missed seam, mismatch, or undercut).

Defects in welds can arise from one or more of the following sources:

(1) Improper joint design, preparation, alignment, or fit-up

(2) Inherent base or filler metal characteristics

(3) Process characteristics

(4) Environmental factors

Regardless of origin, defects almost always act as points of stress concentration, often reduce the cross sectional load-bearing area, and sometimes degrade the properties of the metal, especially ductility and toughness.

Joint-Induced Defects

Improper or inappropriate joint design, preparation, alignment, or fit-up can lead to the following types of defects:

(1) Lack of complete penetration of the joint groove or seam because of improper design or inappropriate process or parameter selection

(2) Mismatch or surface offset due to misalignment of joint elements

(3) Severe distortion caused by unbalanced masses or excessive heat input

(4) Porosity resulting from entrapped air or volatile contaminants

(5) Shrinkage, voids or cracks resulting from poor fit-up or excessive restraint

(6) Underfill caused by poor fit

(7) Excessive dilution resulting from improper design or process selection.

Fusion-Zone Defects

Potential defects that can occur in the fusion zone of a weld include:

(1) Porosity caused by dissolved gases being released on solidification

(2) Entrapped slag within or between passes from the coatings of electrodes, the cores of flux-cored wires, or other sources, in processes employing slag

(3) Solidification hot cracks resulting from low-melting constituents at grain boundaries being pulled open by shrinkage stresses

(4) Severe macro-segregation resulting from gross unmixed dissimilar base metals or unmatched fillers and base metals

(5) Cold cracks caused by hydrogen embrittlement

(6) High-density inclusions resulting from contamination by non-consumable tungsten electrodes used in gas tungsten arc welding.

Partially-Melted Zone Defects

The three major defects in the partially-melted zone in fusion welds are solidification hot cracks, back filled hot cracks, and hydrogen cold cracks.

Heat-Affected Zone Defects

Defects in the heat-affected zone of fusion welds include:

(1) Hydrogen cold cracks

(2) Liquation, reheat, or strain-age cracks

(3) Stress-corrosion cracks, weld decay cracks, or knife-line attack cracks (e.g., in sensitized stainless steels).

(4) Lamellar tears in base metals containing extensive non-metallic inclusions in the form of stringers

 

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