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05 B - Surface and subsurface alterations


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Absorption
The impurities that are absorbed and retained in surface layers of the base material, possibly leading to embrittlement or other property changes
Alloy depletion
Loss of critical alloying elements are from the surface layers, with possible loss of properties in the metal.
Cracks
Narrow ruptures or separations either at or below the surface that alter the continuity of the material. Cracks are characterized by sharp edges and length-to-width ratios of 4:1 or more. They are classified as macroscopic (can be observed with magnification of 10X or less) and microscopic (requires magnification of more than 10X)
Hardness changes
Hardness differences at or near the surface.
Heat affected zone
Regions of the metal that are affected by the application of thermal energy; the regions are not melted but are sufficiently heated that they undergo metallurgical changes that affect properties. Abbreviated HAZ, the effect is most prominent in fusion welding operations
Inclusions
Small particles of material incorporated into the surface layers during processing; they are a discontinuity in the base material. Their composition usually differs from the base material.
Intergranular attack
Various forms of chemical reactions at the surface, including intergranular corrosion and oxidation
Laps, folds, seams
Irregularities and defects in the surface caused by plastic working of overlapping surfaces.
Pits
Shallow depressions with rounded edges formed by any of several mechanisms, including selective etching or corrosion; removal of surface inclusions; mechanically formed dents; or electrochemical action.
Plastic deformation
Refers to microstructural changes from deforming the metal at the surface; it results in strain hardening.
Recrystallization
Formation of new grains in strain hardened metals; associated with heating of metal parts that have been deformed.
Redeposited metal
Metal that is removed from the surface in the molten state and then reattached prior to solidification.
Resolidified metal
A portion of the surface that is melted during processing and then solidified without detaching from the surface. The name remelted metal is also used for resolidified metal.
Recast metal
Both redeposited and resolidified metal.
Residual stresses
Stresses remaining in the material after processing.
Selective etch
A form of chemical attack that concentrates on certain components in the base material.