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MME 345, Lecture 44
Casting Defects2. Effect of defects on propertiesRef: J. Campbell, Castings, Butterworth-Heniemanne, 1992
Summary of casting defects
Effect of defects on Yield strength
Fracture toughness
Fatigue
Ductility
Ultimate tensile strength
Leak tightness
Residual strength
2/23
Compact defects Inclusions
Gas porosity
Core blow
Planar defects Layer porosity
Tears and cracks
Films
3/23
In the past, some castings containing large undetected defects
have gone on to serve useful lives in critical applications
Others, however, have failed (e.g., the Tay Bridge disaster in Scotland, UK)
The mixed performance arises because of the elementary point that
the size of the defect is often of much less importance than its form and position.
a large pore in a low-stressed area of the casting may be far less detrimental
than a small region of layer porosity in a sharp corner subject to a high tensile stress
To have blanket specifications requiring the elimination of all types of defect from
every area of the casting is therefore not appropriate
• may result in the scrapping of many serviceable castings
The most logical and effective control over casting performance is achieved by
• specifying separate designated regions of the casting, and then
• each separate region being required to contain no defects above the critical size
appropriate for that location4/23
The list of properties which a casting may be required to possess can be long.
It might include, for instance, high-temperature properties such as creep resistance,
high-temperature fatigue resistance, or oxidation resistance.
Alternately, room temperature properties such as resistance to corrosion
in specific environments may be required.
In this lecture, we shall confine ourselves to the more usually specified properties
such as strength, toughness, ductility, and leak-tightness.
Different defects have different effects on each of these properties
5/23
Yield Strength or Proof Stress
Because no substantial deformation has
taken place, it is logical to assume that the
YS / 0.2PS will be unaffected by most defects
The only effect will be that due to the
reduction in area
but since most defects occupy at most only a few
per cent of the area of the casting, this effect is
usually hardly detectable
UTS and E are, however, noticeably affected
The 1 % or so of reduction in 0.2PS because of the
1 % or so loss of area of layer porosity cannot be detected in the scatter of the results.
6/23
7/23
A notable exception to the constancy of the
yield stress is found for the embrittlement of
steels by grain boundary precipitates of
sulphides
• The large effect on yield strength is
consistent with the large area which the
sulphides cover on the grain boundaries,
and the weakness of the sulphide phase
• intergrannular brittle failure by a fast-running
crack through the grain boundaries becomes
favoured over extensive plastic deformation
of the matrix
Steel embrittlement by grain boundary precipitation of sulphides
8/23
Fracture Toughness
Fracture toughness is a material property, independent of the size of gross defects.
useful in the prediction of the shapes and sizes of defects which might lead to failure
For an Al alloy
KIC = 32 MPa m1/2
sy = 240 MPad = 11 mm (inside crack)d = 9 mm (surface initiated crack)
For inside crack of diameter d, the critical defect size
According to the fracture mechanics, fracture may begin when the stress-intensity
factor K exceeds a critical value, the fracture toughness K1C.
edge cracks
are somewhat more serious than
centre cracks
• These are comparatively large defects (11 mm / 9 mm)
can be detected relatively easily by non-destructive tests
prior to the casting going into service
• Most current radiography standards which state “no linear
defects of any size” or which reject aluminium alloy
castings for flaws more than approximately 1 mm in size,
may not be logical
9/23
Steels have high fracture toughness,
with correspondingly good tolerance to large
defects at low applied stress
However, when they are used in highly
stressed applications, the permissible defect
size is reduced again
When the applied stress equals the yield stress, the greatest resistance to
crack extension offered by the materials is controlled by the value of (K1C/sYS)
this parameter is a valuable measure of the defect tolerance of a material
d = 2K1C2/ps2
Permissible defect size diminishes with increasing strength
as fracture toughness falls with increasing strength
For stronger irons, the permissible defect size is below 1 mm
this is particularly difficult to detect, and sets a limit to the stress at
which a strong ductile iron casting may be used with safety
10/23
• Poor properties of flake irons and the excellent
properties of some steels and titanium alloys;
ductile irons occupy an interesting middle ground
• In general, it seems that most groups of alloys can
exhibit either high strength and low toughness, or
high toughness and low strength.
11/23
For brittle materials, the use of fracture toughness is
more appropriate measure of the reliability of the casting
than ductility.
• Any crack would have to exceed certain critical value before
failure, even if such failure is eventually of a brittle failure.
properties of Al-7Si-0.4Mg alloy as a function of iron
intermetallic particles
little affected by ageing
and retains a respectable
value regardless of low ductility
12/23
Fatigue
The fatigue performance of cast alloys is generally poor.
• The initiation of fatigue crack during stage 1 of the fatigue process is short.
This is due to the presence of defects (pores, oxide films).
The growth and propagation of crack in stage 2 is, however, remarkably slow.
• This is due to the result of irregular branched nature of the crack, which appeared to have
follow a zigzag path through the as-cast structure.
• This contrasts with the wrought alloys where the fine-grained structure allows the crack to
spread unchecked along a straight path.
Thus, if crack initiation can be slowed, then the fatigue lives of cast metals
might be extended considerably, perhaps in excess of those of wrought alloys.
13/23
The following factors appeared to
have beneficial effect on fatigue
performance of cast materials:
• Shot blasting due to the introduction
of compressive stress at the surface
• Filtering the liquid prior to casting
and/or improving casting methods
(e.g., low pressure, and uphill casting)
due to reduced defects
• Metal mould casting due to reduced
size in defects
14/23
Ductility
Why should an assembly of holes in a
matrix affect the ductility so remarkably ?
All second phase additions (including pore) has deleterious effect on ductility.
This is due to the lack of cohesion of the second phase with the matrix when deformation starts.
Thus all particles are soon acting as holes.for only 1 vol.% pore,
ductility falls from a
theoretical maximum of
100 % to an approx. 10 %
1%
15/23
Since failure occurs at 45°, the theoretical
elongation is equal to the linear dimension of the cross-section, l.
For test piece with single pore of size d,
the elongation is equal to the half of the remaining sound length, i.e., (l -d)/2.
In general, for a spacing S in an array of micropores, we have
Elongation = s – d
= 1/n1/2 – (f/n)1/2
= (1 – f1/2) / n1/2
n = no. of pores per unit area = 1/s2
f = area fraction of pore on fracture surface = nd2
16/23
Film defects were less important than an equal area fraction of compact inclusions in lowering the ductility of casting.
Micro-inclusions are more effective than
maco-inclusions in lowering ductility.
When f = 1, ductility falls to zero.
This situation can easily happen when turbulence
during filling can generate oxide films, which
occupy the whole of the cross-section of the test
piece.
there were between 100 and 1000 times the
number of microinclusions compared to film-type
defects in a given area of fracture surface
17/23
Ultimate Tensile Strength
Ultimate tensile strength comprises
1. yield strength plus
2. additional work strengthening during plastic deformation before failure.
The behaviour becomes more complicated to understand
than the behaviour of yield stress or ductility alone.
UTS = proof stress, when
1. ductility = 0, or
2. work strengthening effect = 0
(mainly occurs at high temperature when rate of recovery exceeds rate of hardening;
ductility can be high for this case)
At room temperature, UTS increases with
1. increasing ductility (since YS/0.2PS is relatively insensitive to many variables), and
2. work hardening.
18/23
For aluminium alloys, UTS increases
due to a reduction in defects.
mainly due to the increase in ductility
When cracks or films occupy the
majority of the cross-section,
the casting will be highly injurious.
the UTS falls to zero as the crack
occupies progressively more of
the area under test
19/23
Layer porosity has similar effect on UTS as it has on ductility.
Have larger effect when porosity is layered perpendicular to the applied stress and “stitched”
together to form a single crack.
even this marked reduction in
UTS is not as serious as would
be expected if the layers had
been cracks
Leaks are seldom caused by gas porosity.
because the distribution of gas porosity is not random; they are distributed at specific distances,
and are kept apart by the presence of dendrite arms.
Leakage due to gas porosity occurs
only when the sample contains an impossibly amount of high gas content (~20 – 30 vol.%.)
Leak Tightness
Most leaks in light alloys and al-bronze castings are the result of oxide inclusions.
They fall into 2 categories:
1. Old, thick oxide films that are in suspension are jamming/bridging together as the metal rises
2. Folding of new oxide films into the metal due to turbulence. These poorly wetted, folded over
dry side to dry side bi-films constitute the major leak paths through the walls of casting.
Sometimes results due to shrinkage porosity.
especially in long-freezing-range alloys where porosity adopts sponge/layer morphology;
porous metals resulted due to poorly fed shrinkage will produce leak, especially after machining
21/23
Residual Stress
Unseen and unsuspected,
residual stresses can be the most damaging defects of all.
the stress can be so large that it can outweighing the effect of all other defects
Never specified to be low in any standards.
Practically impossible to measure in any NDT test.
Residual stress is added up to the applied stress, and put the casting
near to its point of failure even at a relatively small applied loads.
The remedy is, of course, the application of stress relief by thermal treatment.
22/23
Next ClassMME 345, Lecture 45
Casting Design Considerations