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Wire Drawing

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Basic principals

The art or process of wire drawing like the name implies is to draw a wire of a bigger diameter through a hole with smaller diameter hereby reducing the diameter through plastic deformation while the volume remains the same.

To do this a tool called a tungsten carbide drawing die is used. See picture below.

If we assume that the inlet Diameter = Do and the length of the wire prior to drawing = Lo then the cross section Area prior to drawing Ao is Pi/4 x Do^2 and the volume V = Lo x Ao

If the diameter after it passed the die is D1 then the cross section Area after the wire has passed through the die is A1 and A1= Pi/4 x D1^2 and since the volume remains the same i.e. V = Lo x Ao = L1 x A1 where L1 is the length of the wire after it has passed through the die.

The ratio of length after drawing to the length prior to drawing L1/Lo is called elongation.

1 - A1/Ao is called the area reduction

and 100 x (1 - A1/Ao) is called percentage of area reduction.

Wire defined as a round string of metal dates back to early civilization 3000 B.C or earlier. The process of producing a wire by drawing a round piece of metal through a drawing die hereby reducing its diameter dates back to around 1300 A.D.

Here we will not dwell more on the history of steel wire drawing other than establish the fact that the industry continues to improve the manufacturing process.

We will instead present the various machine types in use and point out the features of the various designs.

The most common steel wire rod diameter is 5.5 mm or 7/32"

Steel wire work hardens during plastic deformation and the ductility (the degree of elasticity) is reduced while the tensile strength increases.

In general it is possible through subsequent or sequential passes through ever smaller dies to reduce the cross section area of a wire between 85-95%.

Further area reduction will require an intermediate anneal to restore ductility.

The degree of total area reduction possible without intermediate annealing depends on the composition of the steel.

A high carbon steel wire without alloys like rope wire can be drawn from size 5.5mm to size 1-0.8mm, while a carbon steel wire with 1.8% Manganese (Mn) work hardens with less amount of area reduction.

In order to establish production practices it is therefore necessary to know the work hardening characteristic of the steel quality (=grade) you want to process.

We will not elaborate further in this chapter but the importance becomes apparent by the following example.

A MIG welding wire of grade DIN 8559 SG2 (typical alloy composition C 0.1% Si 0.85% Mn 1.5%) can be drawn from size 5.5mm - 1.2mm without an intermediate anneal, while a MIG welding wire of grade DIN 8559 SG3 (typical alloy composition C 0.1% Si 1.0% Mn 1.7%) requires an intermediate anneal.

However a MIG welding wire of grade DIN 8559 SG2 (typical alloy composition C 0.1% Si 0.85% Mn 1.5) can be classified as of grade AWS A 5.18:ER 70S-6 as long as the Mn content is over 1.4 as the AWS classification (American Welding Association) for grade 70S-6 calls for 1.4-1.8% Mn

The maximum amount of area reduction theoretically in each die pass, if there was no friction and the inlet tensile before passing through the die and after it had passed through the die was the same is 50%. In reality there is friction and there is an increase in tensile as the wire passes through the die.

In a single pass it is often possible for low carbon steels like grade C 1006 wire for rebar to take as much as 40% area reduction.

However in products where subsequent draws are needed to reach the desired finish diameter much lower area reductions are used.

For a draw from 5.5mm inlet to 2 mm the standard is to choose an average area reduction per die of about 30%.

A lower area reduction per die gives you less of a reduction in ductility.

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