4. History
• Rolling to final thickness (14 cm) consisted
of three sessions each of which was
performed at 1230°C.
• three slabs were stacked on top of each and
air-cooled to room temperature
5. Problem
After a stack of three slabs were air-cooled to
room temperature, a transverse crack as long as
76 cm and extending through the entire thickness
was consistently observed in the middle slab. In
contrast, no cracks were detected in either the
top or bottom slabs.
8. Analysis
Visual Inspection
Macrostructure
No evidence for measurable macroscopic plastic
deformation of the failed slab.
Texture
The original crack surface had a texture different
from that produced by the final separation.
11. Analysis
Material Verification
Chemical
composition of
the alloy
Ni- (26–30)%Mo-2%Fe-1%Cr-1%Co-1%Si-1%Mn-
0.02%C
Chemical analysis
of the failed slab
Ni-26.94%Mo-0.7%Fe- 0.31%Cr-0.1%Mn-0.02%Si-
0.003C
Similar results were obtained for the top and bottom slabs ruling out any deviation from the
specified composition as a contributory factor to the failure.
13. The only applied stress is a bearing stress* resulting from the slab weight;
Only the middle and bottom slabs are acted upon by a bearing stress
(compressive stress) and that the bearing stress acting on the bottom slab is
twice that acting on the middle slab.
If the alloy density is ρ and the width, length, and thickness of the slab are
w, l, and t, respectively, the bearing stress σв acting on the middle slab is given
by:
12/15/2016Material Science
13
σв = (wltρ)/wl= tρ
Analysis
Stress Analysis
15. *where wl is the bearing area.
*Given that
ρ of the alloy is 9.22 g/cm³
t= 13.97 cm
∴the bearing stress acting on the middle slab was
∴And the bearing stress acting on the bottom slab was:
12/15/2016Material Science
15
σв = (wltρ)/wl= tρ
σв = (9.22)* (13.97) = 124 MPa
2*σв =248 MPa
Analysis
Stress Analysis
16. Analysis
Stress Analysis
Both the middle and bottom slabs were cooled under the
influence of bearing stresses.
Middle slab was sandwiched between the top and bottom
slabs, it would be expected to cool at a slower rate.
17. Analysis
Stress Analysis
During cooling, both the middle and bottom slabs tend to plastically deform
at those temperatures where the applied bearing stress exceeds the yield
strength
For example @ 1095°C, the tensile yield strength (Sy) of the material is 55
MPa, while @ 980°C, the yield strength is increased to 110 MPa
Note: When the yield strength exceeds the bearing stress with continued cooling, the
deformation becomes only elastic.
18. Analysis
Stress Analysis
Like in compression test the frictional forces will restrict deformation in the
surface layers of middle and bottom slabs forming tensile stresses
perpendicular to their cross sectional area which is maximum at the mid-
length. From the above analysis the crack was initiated with the help of the
tensile stress reaching it is maximum value at the mid-length.
19. Analysis
Stress Analysis
As the bearing stress on the bottom slab is greater than that of the bottom so
the tensile stress will be also greater but the bottom slab did not fail so the
magnitude of the tensile stress would not be the only cause of the failure of the
middle slab and that another factor must have been involved.
23. 870°C
29.1 wt% Mo
Disordered solid solution above
this temperature
the alloy undergoes a generalized long range ordering reaction resulting in the
transformation of the fcc phase into an ordered tetragonal structure (Ni4Mo).
below this temperature
less than
29.1wt% Mo
Ni₄Mo precipitates from the parent fcc phase
during cooling.
Analysis
Microstructural Characterization Examination by transmission
electron microscopy
binary
Ni-Mo
system
24. Analysis
Microstructural Characterization
Experiments show that an exposure of the slab material for
only
10 min at 760°C reduces the tensile ductility from 55 to 3%
corresponding to an increase in hardness from Rb 98 to Rc 34
and a corresponding increase in yield strength from 400 MPa
to 772 MPa.
27. Analysis
Mechanism of Crack Propagation
Source of weakening
Creep
deformation
Precipitation
of embrittling
phases
28. Analysis
Mechanism of Crack Propagation
Fractography revealed that the cracks developed in the middle slab had propagated
by an intergranular mechanism.
Intergranular fracture:
Crack propagation is
along grain boundaries
(grain boundaries are
weakened or
embrittledby impurities
segregation etc.)
30. Consistent with the results of microstructural characterization, this indicated that the
middle slab
became defective by slow cooling, resulting in the precipitation of Ni4Mo and
corresponding loss of ductility
Analysis
Stress Required for Crack Propagation
Why the bottom slab did not fail!
34. Analysis
Cause of Failure
Based upon experimental observations, the cause of
failure was the relatively slow cooling rate of the middle
slab as a result of being sandwiched between the top and
bottom slabs.This made the middle slab defective by
extensive precipitation of the embrittling Ni4Mo phase
36. Never stack slabs of the alloy on top of
each other to avoid slow cooling of the
middle slabs.
37. Produced by (sec1)
Ibrahim Ahmed Mohammed (1)
Ibrahim Abd elFatah Ali (2)
Ibrahim Magdy Ibrahim (3)
Abo Baker Eid Hamed (4)
Ahmed Ashraf Mohammed (5)
Ahmed Akram Mohammed (6)
Ahmed Elsayed Hamed (7)
Ahmed Elsayed Mohammed (8)
Ahmed Ehab Moustafa Ahmed (9)
Ahmed Gamal Sayed Hessen (10)
