Defects Materials in Radiography NDT
(toc)
1. Introduction
A discontinuity can occur any time in the history of a piece
of metal. If it is introduced during the initial production from the molten
state, it is termed as inherent discontinuity. If caused during further
processing, fabrication or finishing, it is called processing discontinuity.
Finally, if it arises during the use of the end either due to environment, load
or both, it is called service discontinuity.
2: Casting Defects
Casting the process of causing liquid metal to fill a cavity
and solidify into a useful shape.
The discontinuity that can occur during casting process are
given below:
a, Non-metallic inclusions: Non-metallic inclusions within the molten metal. are caused by the Impurities in the starting material and most of the non-metallic matter being lighter, rise to the top of the ingot. but some are trapped within, because the molten metal above them hardens before it could reach the surface. These Inclusions are irregular in shape.
b, Porosity: It is spherical or nearly spherical shaped and is caused by the entrapped gas in the molten material.
c, Pipe:
The molten metal. after being poured Into a mold, starts to cool and It
solidities. The solidification process starts from the surface and travels
towards the center of the ingot. On solidification, the molten metal contracts.
Since the center of the ingot is the last to cool and solidify, most of the
shrinkage as observed in the center. This results in a cavity called “PIPE”. It
may extend from the top towards the interior of the ingot along the axis.
d, Cold-shut: Cold shut as formed when molten is poured over solidified metal. When the metal
is poured, it hits the mold too hard and spatters small drops of metal. When
these drops of metal hit higher up on mold, they stick and solidify. When the
rising molten metal reaches and covers the solidified drops of metal, a crack
like discontinuity is formed. Cold shuts can also be formed by the lack of
fusion between two intercepting surfaces of molten material of different
temperatures.
e, Hot Tear
(Shrink Crack): Hot tear is caused by unequal shrinking of light and heavy
section of a casting as the metal cools. In casting having light and heavy
section, the light sections, being smaller, solidify faster they shrink faster
pulling the heavier section towards the, as they are hotter and do not shrink
as fast.
f, Shrinkage
Cavity: Shrinkage cavity is caused by lack of enough molten metal to fill the
space created by shrinkage of the solidifying metal, just as a “PIPE” is formed
inan ingot. It can be found anywhere in the cast product, unlike the “pipe” in
the ingot, which always occurs only at the top portion of the ingot.
g, Micro
Shrinkage: Shrinkage can also occur in the casting at the mold gate, ie., at
the entrance to the mold through which the molten metal is poured. Shrinkage
occurs, if metal at the gate solidifies or is blocked off while some of the
metal beneath is still molten. Shrinkage which occurs at the gate appears as a
number of small holes called "micro shrinkage". Micro shrinkage can
also occur deeper within the metal, if the mold is improperly designed.
h, Blow
holes: Blow holes are small holes on the surface of the casting and are caused
by external gas emanating from the mold itself.
3: Forging Defects
Forging is the process of working of metal into a useful
shape by hammering or pressing. Most forging operations are carried out hot,
although certain metals can be cold forged. The defects that can occur during
forging are given below:
a, Forging
lap: A forging lap is discontinuity caused by folding of metal into a thin
plate on the surface of the forged material. It is due to the mismatching of
the mating surfaces of the two forging dies in "closed-die forging"
or abrupt changes in grain direction. It is always open to the surface.
b, Forging
bursts or cracks: It is a rupture caused by forging at improper temperatures.
Forging a metal at too low a temperature can cause this defect. They may be
internal or may occur at the surface.
4: Welding Defects
Welding is the process of joining metals. Two basic types of
welding are used, fusion welding and pressure welding.
4.1: Fusion welding
It is essentially a casting process, in which a metal is
melted and cast in the joint and is made to fuse with the parts to be joined.
The metal is supplied by filler rods, which are generally of composition
similar to the metal being welded. There are cases in which no filler rod is
used for example, in thin sheet, where part of the sheets are melted to produce
the filler metal.
4.2: Pressure welding
In this process, pressure and heat are applied to the pieces
to be welded. The pressure produces plastic deformations, the het then produces
recrystallization across the boundary, resulting in the formation of new
crystals which are an integral part of both pieces and thus the welding takes
place.
All critical welds require NDT for assurance of quality or
as a means to enable repairs.
The possible defects (Fig.) that can occur in fusion weld metals are listed below:
1. Overlap 2.
Lack of Fusion
3. Undercut 4.
Porosity
5. Crack 6.
Slag Inclusion
Fig.: Typical Weld Defects
a, Overlaps: A fin of surplus metal on the end of a billet may be caught and folded into the surface during subsequent rolling. The bent-over metal, while forced tightly against the main stock, will not bond to it. The resulting lap is a possible initiation point for a fatigue crack.
b, Crater cracks: Crater cracks are caused in the weld bead by improper use of heat source, either when a weld is started or stopped. A crater crack can also occur at the temporary stop of the weld. Crater cracks are primarily of three types - transverse, longitudinal and multiple star-shaped.
c, Stress cracks : These are cracking of weld metal and base metal in or near the weld zone and they are usually caused by high stresses set up by localized dimensional changes. Stress cracks are most likely to occur when weldments are of heavy sections. These cracks usually occur transverse to the weld in a single pass weld and longitudinal in a multiple pass weld.
d, Porosity: Porosity is the term for the gas pockets or voids free of any solid material that are frequently found in weld metals. Porosity can come from gases released by the cooling weld metal and from gases formed by chemical reactions in the weld metal. Porosity may be scattered uniformly throughout the entire weld, isolated in small areas or concentrated at the root. Gas pores are usually spherical in shape, although they may also occur as non-spherical pockets along grain boundaries. Most welds contain some amount of porosity which may be micro or macro in size
e, Slag inclusions: This term is used to describe the oxides and other non-metallic solid material that are entrapped in the weld metal or between weld metal and base metal. Slag inclusions may be caused by contamination of the weld metal by the atmosphere, but in most cases, they are generally derived from electrode covering materials or fluxes, employed in arc welding operations. In multilayered welding operations, failure to remove the slag between passes will result in slag inclusions in these zones. Slag inclusions, are generally linear and may occur either as short particles or long bands
f, Tungsten inclusions: In the gas tungsten arc-welding process, the occasional touching of the electrode to the work or to the molten weld metal, particularly in manual operation, or excessive currents during arc welding may transfer particles of the tungsten into the weld metal. These are called 'tungsten inclusions'.
g, Lack of fusion: Lack of fusion or incomplete fusion as it is frequently termed, describes the failure of adjacent weld metal and base metal or inter weld passes to fuse together completely. This failure to obtain fusion may occur at side wall or in the inter pass region. Lack of fusion is usually elongated in the direction of welding and may have either rounded or sharp edges depending on how it is formed.
h, Lack of Penetration: Lack of penetration is due to failure of weld metal to extend into the root of the joints. The most frequents cause for this type defect is the unsuitable groove design for the selection welding process.
i, Undercut:
During welding of the final or cover pass, the exposed upper edges of the weld
preparation tend to melt or run down into the deposited metal in the weld
groove. Undercutting occurs when insufficient filler metal is deposited to fill
the resultant, at the edge of the weld bead. The result is a groove that may be
intermittent or continuous and parallel to the weld bead. Undercutting may be
caused by excessive welding current, incorrect arc length, high speed,
incorrect electrode manipulation, etc..