EoW May 2011

technical article

It is considered that the shape of both sides of the crack significantly affects its removal under condition I in Figure 11 .

Drawing direction

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 ▼ ▼ Figure 9 : Work hardening diagram of tested stainless steel wire

C) 2 passes

A) Mother wire

B) 1 pass

▲ ▲ Figure 7 : Mesh deformation after repeated drawing of wire with foreign material on the surface examined by FEM

Crack

Die

A 1 is a cross-sectional area before and after the processing 0 A

Transversal crack

Drawing direction

▲ ▲ Figure 8 : Drawing model of wire

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

Concave crack due to scratching in rolling direction

Scratch

Rolled in material Dent resulting from pressing against foreign substances such as metal chips

Over-filled Defect resulting from continuous indenting in rolling direction

▲ ▲ Table 2 : Classification of surface cracks on wire rod 7

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.

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

Stress (MPa)

Strain

60

EuroWire – May 2011