Intermediate Wire Processing

Heat treatment
As previously briefly mentioned the wire drawing process is a plastic deformation process. For a material to be possible to draw it must be ductile - and to be ductile a steel wire must have a suitable large grain crystal structure. During drawing the steel work hardens and it is therefore necessary to anneal or stress relief the wire. Through heat treatment it is also possible to alter the steel characteristics to fit the demand of a specific application.
Annealing
The purpose of annealing is:

To soften the steel and improve ductility,
To relieve internal stresses induced by some previous treatment (drawing, cold rolling, or uneven cooling after hot rolling the wire rod)
To remove coarseness of grain.
The operation consists of: heating the steel to a certain temperature, "soaking" at this temperature for a time sufficient to allow the necessary








Annealing
The purpose of annealing steel wire is:

1. To soften the steel and to improve ductility,

1. To relieve internal stresses induced by some previous treatment (drawing, cold rolling, or uneven cooling after hot rolling the wire rod)

1. To remove coarseness of grain.

The operation consists of: heating the steel to a certain temperature, "soaking" at this temperature for a time sufficient to allow the necessary changes to occur and then bycooling at a predetermined rate.
Sub-critical anneal
It is not always necessary to heat the steel into the critical range. Mild steel products which have to be repeatedly cold worked in the processes of manufacture are softened by annealing at 500?to 650癈 for several hours. This is known as "process" or "close" annealing, and is commonly employed for wire and sheets. The recrystallisation temperature of pure iron is in the region of 500癈 consequently the higher temperature of 650癈 brings about rapid recrystallisation of the distorted ferrite Since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. As shown, Fig. 1b illustrates the structure formed consisting of the polyhedral ferrite with elongated pearlite (see also Fig. 2).
Prolonged annealing induces greater ductility at the expense of strength, owing to the tendency of the cementite in the strained pearlite to "ball-up" or spheroidise, as illustrated in Fig. 1c. This is known as "divorced pearlite". The ferrite grains also become larger, particularly if the metal has been cold worked a critical amount. A serious embrittlement sometimes arises after prolonged treatment owing to the formation of cementitic films at the ferrite boundaries. With severe forming operations, cracks are liable to start at these cementite membranes.

Figure 1. Effect of annealing cold-worked mild steel

Figure 2. Effect of annealing at 650癈 on worked steel. Ferrite recrystallised. Pearlite remains elongated (x600)
2 principal methods are used batch or continuous annealing.The modern batch furnace is either of double vacuum type or an inert gas furnace. The continuous annealing furnace is commonly a so called tube furnace using an inert gas shield. Batch annealing usually consists of 24-30 hrs 670癈, soak 12 hrs, slow cool 4-5 days. Continuous annealing is used commonly used for stainless austenitic grades such as 400 and 300 series. Processing speed is determined mainly by the length of the furnace although the length are restricted particularly in fine wire processing by the back tension. The cycle is approximately up to 660癈 20 sec, soak and cool 30-40 sec. There is little chance for grain growth but it produces a wire that can be further drawn and reduced in size.
Changes on annealing

Figure.3 The iron carbon diagram
Consider the heating of a 0,3% carbon steel. At the lower critical point (Ac1) each "grain" of pearlite changes to several minute austenite crystals and as the temperature is raised the excess ferrite is dissolved, finally disappearing at the upper critical point (Ac3), still with the production of fine austenite crystals. Time is necessary for the carbon to become uniformly distributed in this austenite. The properties obtained subsequently depend on the coarseness of the pearlite and ferrite and their relative distribution. These depend on:
a) the size of the austenite grains; the smaller their size the better the distribution of the ferrite and pearlite. b) the rate of cooling through the critical range, which affects both the ferrite and the pearlite.
As the temperature is raised above Ac3 the crystals increase in size. On a certain temperature the growth, which is rapid at first, diminishes. Treatment just above the upper critical point should be aimed at, since the austenite crystals are then small.
By cooling slowly through the critical range, ferrite commences to deposit on a few nuclei at the austenite boundaries. Large rounded ferrite crystals are formed, evenly distributed among the relatively coarse pearlite. With a higher rate of cooling, many ferrite crystals are formed at the austenite boundaries and a network structure of small ferrite crystals is produced with fine pearlite in the centre.

Carbon Steel Wire - Patenting
The purpose of patenting is to produce a wire with a uniform austenite grain structure, suitable for production of wire for products like rope construction.
In the patenting operation the wire is passed through tubes in a furnace at about 970 Deg. C. This high temperature treatment produces uniform austenite of rather large grain size. The subsequent cooling – in air or molten lead (lead patenting) – is rapid (e.g. coarse process wire or wire rod), and the patented wire structure will consists of very fine pearlite with little or no separation of primary ferrite.









Heat-treatment of High Carbon Steel Wire - Patenting




Patenting consists of passing the wire through tubes in a furnace at about 970oC. This high temperature treatment produces uniform austenite of rather large grain size. The subsequent cooling ?in air or molten lead ?is rapid since the sections treated are generally small (e.g. wire rods), so that the resulting structure consists of very fine pearlite preferably with no separation of primary ferrite

Wire ropes are usually made from carbon steel wires ranging from 0,35 to 0,5% carbon, and before drawing the material is subject to a heat-treatment known as patenting.
The large crystals would give rise to brittleness if the material was left in the heat-treated condition, but this effect is not noticed after a few drawing passes. Variation in hardness - either softer or harder - can be produced by tempering martensite, but such material does not draw so well as patented wire, which is able to withstand reductions of area up to 90%. The strength is explained on the basis of the reduced ferrite cells and the alignment of cementite in fibres.

Hardenability, mass effect, ruling section
Hardenability is the measure of the depth to which a steel will harden on quenching.
The maximum hardness is mainly a function of the carbon content. The hardenability of steel depends on:

The quenching medium and method of quenching.
Composition of the steel and method of manufacture.
Section of the steel. The so-called "Mass effect" arises from the fact that even with the most severe quench the cooling of the wire rod is progressively slower, from the outside to the centre due to the low thermal conductivity of the steel. It must be appreciated, therefore, that it is the rate of cooling of a piece of steel which determines the properties resulting from a quenching process, and not mass or weight.