Discontinuity
-Any break or interruption in the normal physical structure of an article is called discontinuity or defect.
1. Casting Defects :
a. Porosity - It is round or nearly round, and is caused by entrapped gas in the molten material.
b. Non-metallic inclusions - These are of irregular shape and consist of slate like impurities accidentally included in the molten metal.
c. Pipe - This is caused by shrinkage as the molten metal solidifies. It may extend deeply into the centre of the ingot.
d. Cold-Shut - Cold shut is formed when molten metal meet with already solidified or relatively cold metal. Cold shut can also be formed by the lack of fusion between two intercepting surfaces of molten material of different temperatures.
e. Hot-Tears - A hot-tear is caused by unequal shrinking of light and heavy sections of a casting as the metal cools.
f. Micro-shrinkage - This defect results from contraction during solidification where there is not an adequate opportunity to supply filler material to compensate for shrinkage.
g. Shrinkage cavity - It is caused by a void left in cast metals as a result of solidification shrinkage. It can be found any where in the cast product.
h. Blow holes - Blow holes are small holes on the surface of the casting caused by gas which is not within the molten metal external gas. This external gas comes from the mold itself.
2. Rolling defects :
a. Laminations - Defects with separation or weakness generally aligned parallel to the worked surface of the metal. It may be the result of pipe, blisters, seams, inclusions or segregations elongated and made directional by working.
Lamination defects may also occur in metal powder compacts.
b. Stringers - It is the longer and thinner configuration of non-metallic
inclusions aligned in the direction of working commonly, the term is associated with elongated oxide or sulfide inclusions in metals.
c. Seams - It is unwelded fold or lap which appears as a crack, usually resulting from a defect obtained in casting or in working. Seams are always open to the surface.
3. Forging Defects :
a. Forging Lap - A forging lap is a discontinuity caused by folding of metal in a thin plate on the surface of the forging. It is due to the mis-matching of the mating surfaces of the two forging dies 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 a too low temperature may cause this defect.
They are either internal or may occur at the surface.
4. Welded pipe and Tubing discontinuities :
a. Seams - It results from lack of fusion at the weld. This seam may appear either on the inside or outside of the pipe. This comes from faulty welding.
b. Laminations - This may be in the plate stock or sloop from which the pipe is made.
5. Seamless Pipe and Tube discontinuities
-a. Slug - This discontinuity forms on the inside of the pipe caused by the piercing mandrel. Slug has been deposited in the pipe wall as the result of severe metal build up on the mandrel.
b. Gonging - Caused by friction between the sizing mandrel and the inside surface of the pipe or tube.
c. Seams or stringers - Found in the bar - stock from which the pipe is made.
6. Extrusion discontinuities :
This can result from the extruding process itself. If the metal does not flow through the die properly, there can be cracks or galling in the finished part. This can be the case in hot extrusions as well as in cold extrusions also. Again, if the original bar stock has contained a crack or porosity, the same discontinuities would show up in the formed part also.
a. Grinding cracks - Grinding cracks can be caused by stresses which are built up from excess heat created between the grinding wheel and the metal. It occur at a right angle (crosswise) to the rotation of the grinding wheel.
Cracks have no relation to the grain direction of metal. Grinding cracks may or may not occur across the grain in hard metals.
b. Heat-treating cracks - Unequal cooling between light and heavy sections of a part which is being heat-treated results in cracking.
c. Explosive forming cracks - It is caused by the explosive force overstressed the material while shaping it to the contour of the dia. A discontinuity is most likely to develop at a point in the dia where the most extreme deforming of the stock occurs or where the die changes contour abruptly.
d. Fatigue cracks - These cracks usually start from stress-concentration points which themselves are open to the surface in most of the cases. Nicks, grinding cracks, forging laps, even poorly finished surfaces, are all examples of discontinuities which might result in fatigue cracks. Some fatigue cracks are sub-surfaces which are originating from porosity or non-metallic inclusions.
8. Welding defects :
a. Crater cracks - Crater cracks are caused at the weld bead by improper use of the heat source either when a weld is started or stopped. A crater crack can also occur at the temporary stop of the weld also. There are 3 types of crater cracks ;
(i) Star crater crack - A roughly star - shaped crack,
(ii) Longitudinal crater crack - Crater crack that parallels the direction of the weld-bead.
(ii) Transverse crater crack - Crater crack that runs across the weld and is limited to the area of the crater.
b. Stress cracks - Stress cracks in welds are the result of stresses created during the cooling of a restrained (rigid) structure. Stress crack usually occurs transverse (across) the weld in a single pass weld and longitudinal in a multiple pass weld.
c. Porosity - It is caused by the gas which remain entrapped in the weld.
d. Slag inclusion - It can occur during arc welding. As the electrode melts so does its oxide coating and it mixes with the molten metal. This result in the weld-bead is slag inclusions.
e. Tungsten inclusions - Excessive current during tungsten arc welding can cause the tungsten electrode to melt and deposit in the weld. This called tungsten inclusions.
f. Lack of penetration - It results from incomplete penetration into the parent metal or may be caused by backing, or by the molten puddle. It occurs at the root of the weld.
g. Lack of fusion - It is a failure of the weld to fuse with the parent metal or may be caused by a failure of the weld passes themselves to fuse. This particular discontinuity occurs farther up in the weld than the root.
h. Undercut - This discontinuity occurs where the welder has melted and flushed out some of the parent metal in the line of fusion. It has to be open to the surface and can be inspected visually. If there is undercut, some of the parent metal has been melted away.