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EuroWire – May 2011
60
technical article
The three-pass drawing of a mother wire
with a surface crack was repeated in the
experiment and FEM analysis. The obtained
crack deformation behaviour for each
drawing pass is also shown in
Figure 10
.
4.2 FEM analysis of U-shaped crack
deformation behaviour during
repeated drawing
Next, a wire with a U-shaped crack was
modelled and the drawing process was
similarly analysed by finite element
analysis.
Figure 11
shows examples of
crack deformation behaviour for 10mm
diameter wires with a crack of breadth
(a)=0.73mm and depth (h)=0.10mm
(1%) and with a crack of a=0.73mm and
h=0.60mm (6%) during repeated drawing
at α=6° and R/P=20%.
As shown in
Figure 11
, under condition
I, the bottom of the crack rises during
repeated drawing, and thus the surface
crack is removed after the first pass.
Under condition II with the greater depth,
however, the right side of the crack is tilted
so that it overhangs the left side and forms
an overlapping crack (defect), indicating
that the crack cannot be removed by
drawing. Moreover, a deep crack develops
in the wire although it appears to be small.
Namely, the behaviour of the U-shaped
crack during drawing depends on the
depth (h).
The results for the concave and U-shaped
cracks were compared. An overlapping
defect develops from the concave crack,
regardless of the depth (h), whereas for the
U-shaped crack the depth (h) serves as a
parameter; that is, the crack with a shallow
(h) is removed, but the crack remains on
the wire when (h) is greater.
It is considered that the shape of both
sides of the crack significantly affects its
removal under condition I in
Figure 11
.
5 Conclusion
Wire breakage was investigated using
experimentation and finite element
method analysis. A focus was on wire
breakages caused by the presence of a
foreign substance, in or on the wire, or
by transversal cracks that develop on the
surface of wires due to the effect of fatigue,
inclusions or surface flaws.
The obtained results are summarised
below.
1) Surface flaws, foreign materials and
excessive drawing stress generated
by seizing are the causes of wire
breaking. However, many of the causes
of breakage in wires with a diameter of
50μm or less are speculated to be due
to the presence of inclusions that are
formed during casting
2) It is thought that there is a danger of
wire breakage when Di/Do is 0.3 or
higher, and that the highest frequency
of wire breakage is where Di/Do is
approximately 0.7. This was found
to be because drawing stress moves
rapidly upward when an inclusion
passes through the die
3) The result of FEM analysis agrees with
the experimental result; therefore, it is
possible to estimate crack deformation
behaviour by FEM analysis to predict
the state after drawing
4) The
mechanism
underlying
the
removal of a crack in a wire rod is the
rise of the bottom of the crack during
drawing
5) The behaviour of a U-shaped crack
during drawing depends on the depth
(h)
6) Where there is a greater depth,
however, the right side of the crack
is tilted so that it overhangs the left
side and forms an overlapping crack
(defect), indicating that the crack
cannot be removed by drawing.
Moreover, a deep crack develops in the
wire, although it appears to be small
Appearance
Name
Morphology and features
Scab
Foliate mark; rod surface is rubbed bare
Transversal crack Crack perpendicular to rolling direction
Bump
Scale-like cracks on surface
Scratch
Concave crack due to scratching in rolling
direction
Rolled in material Dent resulting from pressing against foreign
substances such as metal chips
Over-filled Defect resulting from continuous indenting in
rolling direction
▲
▲
Figure 7
:
Mesh deformation after repeated drawing of wire with foreign material on the surface examined by FEM
▲
▲
Table 2
:
Classification of surface cracks on wire rod
7
▲
▲
Figure 8
:
Drawing model of wire
▼
▼
Figure 9
:
Work hardening diagram of tested
stainless steel wire
Drawing direction
A) Mother wire
B) 1 pass
C) 2 passes
Transversal
crack
A
0
A
1
is a cross-sectional
area before and after the
processing
Die
Crack
Drawing
direction
Strain
Stress (MPa)