Defects in Materials
NATURE OF DEFECTS
Microscopic Defects
Macroscopic Defects
ORIGIN OF DEFECTS
Inherent Defects
Processing Defects
Service Defects
DETERIORATES PHYSICAL and MECHANICAL PROPERTIES of MATERIALS
DETECTION of DEFECTS
Destructive Testing
Non-Destructive Testing
2. DISCONTINUTIES & DEFECTS
• NATURE OF DEFECTS
– Microscopic Defects
– Macroscopic Defects
• ORIGIN OF DEFECTS
– Inherent Defects
– Processing Defects
– Service Defects
• DETERIORATES PHYSICAL and MECHANICAL
PROPERTIES of MATERIALS
• DETECTION of DEFECTS
– Destructive Testing
– Non-Destructive Testing
3. CASTING DEFECTS : Inclusions
• Inclusions are nonmetallic
materials (oxides or sulphides)
• a lower melting point than the
metal
• relative brittleness
• these defects occur near the
surface as a “skin” effect.
• occur at the centre of the
casting
• they tend to be irregular in
shape, not spherical or oval.
4. CASTING DEFECTS : Porosity
• gas trapped in the molten
metal
• formed by
– release from the molten
metal itself
– release from the green sand
mold, water vapour, or
turbulence caused when
pouring the metal
• form of small pockets or voids
appears as round, irregular or
elongated shapes
5. CASTING DEFECTS : Shrinkage
• Metal contracts or shrinks when changing from the liquid
to the solid state and defects will occur in a casting unless
sufficient molten metal is available to “feed” it.
• Defects may take the form of cavities, branch-like tears
• Shrinkage cavities occur usually at hot spots in the
casting
6. CASTING DEFECTS
• Piping : Central Cavity
• As the casting
solidifies, the metal
contracts if there is
not an adequate
supply of molten
metal to the centre of
the casting
• usually occurs in pure
metals and alloys
having narrow ranges
of solidification
temperature
• causes lamination
Hot Tears : If a section
begins to shrink while
still hot and there is not a
sufficient supply of liquid
metal, the resulting
internal stress will tear
the metal. This is
because while hot, the
metal has relatively low
strength.
10. Open Die Forging Defects
Fracture -
– exhausted ductility
– Intergranular fracture
in hot working
• Barreling - Friction
• Solution -
– limited deformation per
step
– Process anneal
between steps
13. Rolling Defects : Seams
• Surface irregularities, such as
cracks, on the slab or billet are
stretched out and lengthened during
rolling
• Seams may also be caused by
folding of the metal due to improper
rolling
• Seams are surface discontinuities
and on finished bars will appear as
either continuous or broken straight
lines
14. Rolling Defects : Stringers
• Nonmetallic inclusions in slabs or billets, that are thinned
and lengthened in the direction of rolling, by the rolling
process, are called stringers.
15. Rolling Defects
• If the cold mill rolls or hot mill coils
happen to have too much crown, the
cold mill will roll out the center and
create center buckle.
• If cold mill rolls or hot mill have too
little crown, the cold mill will roll out
the edges and create edge wave.
17. Extrusion Defects
Defects in Extrusion
• Surface Crack – Lack of proper
lubrication
• Axial Hole or Funneling
18. Fatigue Failure
Fractography of Fractured Surface
direction of
rotation
final
rupture
crack origin
dull, fibrous
brittle failure
smooth circular
“beachmark”
practical example of fatigue failure