WCA July 2011

The surface cracks that develop on wire rods during upstroke rolling are classified as shown 7 in Table 2 ; however, there is no clear solution to this problem. In particular, only a small number of studies have reported on surface cracks formed during drawing 8–11 . In this study, wire rods that developed circumferential cracks during casting and rolling are used as mother wires and drawn repeatedly. The growth and removal of these cracks are examined in the experiments and by FEM analysis. Stainless steel (SUS304) rod wires were mechanically scratched in the axial direction using a lathe and analysed by experiments and FEM. Rod wires that were mechanically marked to form V-shaped, concave, and U-shaped cracks in the cir- cumferential direction were used as specimens. FEM software, MSC/Marc Mentat 2008 R1, was used in this study. Figures 8 and 9 and Table 3 show the model used in the FEM analysis, its material constant and the parameters of the V-shaped crack, respectively. Coefficient of friction(μ) was set at 0.05. Moreover, the model was assumed to be axis-symmetric in the FEM analysis to save calculation time. experiment and FEM analysis A crack with a depth of h=0.8mm (8%) was cut on an 8mm diameter wire and the change in its shape was experimentally and analytically examined after each drawing pass. The initial crack on the mother wire was asymmetrically V-shaped. 4.1 Comparison of results of

Drawing direction

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 Dent resulting from pressing against foreign substances such as metal chips

Scratch

Rolled in material

Over-filled Defect resulting from continuous indenting in rolling direction

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

Three dimensions FEM code MSC/Marc Mentat 2008r1 was used in this study. The results of FEM analysis are consistent with the experimental results. Regardless of whether the foreign material is located in or on the wire, it does not undergo deformation because of its hardness, even if drawing is repeated. This leads to a high value of Di/Do, increasing the drawing stress and increasing the chance of wire breakage.

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

4 Analysis of a wire drawing with surface cracks

Stress (MPa)

Surface cracks develop on rods or wires because of mishandling during casting, hot rolling, drawing or transport, or because of the improper winding of wires 6 .

Strain

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Wire & Cable ASIA – September/October 2007 July/August 2011