Wire drawing by using the Pressure Coating Device EM 20

 

1      Introduction

Today the wire industry needs to turn to production methods that provide better quality at lower costs. Changes are necessary in production methods to provide better quality and accordingly:

 

 According to the importance of a constant product quality it is necessary to achieve high reliability in quality of the drawing process.

In wire drawing, as well as other metal forming processes,  the quality of the lubrication is very important for the product quality, the efficiency of the process and the process limits. For a stable lubrication with a lubricant film between the drawing die and wire, the necessary pressure of the lubricant goes up to the value of the strain strength of the drawing material. The limits for the supposed average die pressure  are at the wire drawing between 0,5...2,5 ( - die pressure, kfm Ė average strain strength). These limits describe the tribological conditions in the lubricant film between the drawing die, the wire and the complexity of the material flow at the drawing process.

For the drawing of wire, lubricant-carriers are used. They are based on zinc-phosphate, borax and lime. The basic elements of the lubricants are in general sodium, calcium and aluminium soaps. This procedure based on the combined utilisation of lubricant carriers and powderized metallic soaps  had a firm place in drawing shops for several decades. In any case it is not possible to produce sufficient lubricant pressure directly in front of the transforming zone, wich is required for a stable drawing process, because the carrier of the lubricant parts into the die follows the wire and the quantity of the lubricant particles in motion are influenced by a number of properties  such as the consistence or the wire surface, the granulation and the purling ability of the lubricants or the drawing speed.

Insufficient lubricant pressure within the transforming area of the drawing die leads to insufficient technological efficiency and security and poor quality of the produced wire.

  

2      Pressure Coating System EM 20

 This process combined pressure lubrication and pressure coating the wire will be coated with a solid lubricant with high viscosity by using pressure and temperature (Figure 1).

The development of the contineously working lubrication-technology wich offers the possibility to produce solid lubricant films under avoidance of the liquid phase (in form of water dispersing lubricants) with the connected disadvantages. The thickness of the lubricant film depends on the selected pressure in the pressure chamber, the drawing velocitiy, the temperature and the die design. An automatic control keeps the pressure in a small tolerance field. The same applies to the temperature wich is thermostated and fixed on a selected level.

 
Fig. 1: Pressure block of the EM 20 (in diagram form)

The newly developed procedure for the combined drawing and coating of wire bases on the property of the main components of the dry lubricants, the alkali-salts and the ground-alkalisalts of higher molecular fatty acids. These materials are nearly incompressible and can in the first approximation be regarded as liquids unter the simultaneous effect of pressure and temperature. The lubrication coating can be produced as thick, adhesive and temperature- and pressure stable, that the wire for a multi-staged drawing (2,4 or more stages in succession) has to be coated only once with the suitable developed pressure-coating-device.

 This newly system enables to receive much better lubricant conditions. The lubrication between the die and drawing material is hydrodynamic on the whole surface, nearby independent from the cross-section profil of the material. The embrace pressure in the pressure chamber leads simultaneous to a more homogeneous transforming of the material.

 

3 Advantageous effects of the pressure-coating process

The process of the pressure coating leads to a whole range of advantages. Some of them are described below in detail:

a)     Elimination of wet carrier coatings for many applications

b)     Lower coefficient of friction between wire and die

c)      Greatly reduced lubricant consumption and consequently reduced waste management costs

d)     Reduction of die wear

e)     Increase of drawing speeds

f)        Improved internal stress distribution resulting in better mechanical properties

g)     Realisation of larger reduction degrees

h)      Smaller diameter tolerances achievable

i)        Higher quality of the finished product

 
a)     Elimination of wet carrier coatings for many applications

For the drawing of long metallic materials, lubricant carrier coatings on the basis of zinc phosphates, borax and lime will be applied. The lubricants themselves consist mostly of calcium-, sodium- and aluminium soaps as the basic ingredient. These processes as a combination between lubricant carrier coatings and powdered metallic soaps in the grains are common for the drawing of large- and medium sized diameters since decades.

This technology does not lead in any case to sufficient lubricant pressure conditions for a stable drawing process directly before and in the transformation zone of the drawing tool, because the loose lubricant grains will be transported with the wire in direction of the transforming area of the die and the quantity of the lubricant particles depends on many factors influenced by the constitution of the wire surface, the grain size, the purling abilities of the wire and the drawing speed.

In the pressure-coating unit EM 20 the lubricants will be transported directly into the pressure chamber. By pressure- and temperature-effects, the lubricants become viscous and will be pressed on the wire. The quantity of lubricants applied on the wire is independent of the constitution of the wire surface. Without problems, it is possible to apply the lubricant coat on stainless steel wires with bare and shiny surfaces, mechanical descaled wires and fresh galvanised wires without any further pre-treatment. The quantity of the lubricants and the thickness of the lubricant film on the wire surface can be altered to suit requirements.

A mathematical model was the fundamental to acquire the essential coefficients, counter-effects and parameters, which could be confirmed by a manifold of practical drawing cases. On the basis of the Navier-Stokeís equation for a power-free, incompressible and convection-free liquidity, the conditions in the hydrodynamic active ring-shaped channels of the drawing tool and the pressure chamber could be described sufficiently exact. In the hydrodynamic active ring-channel with the length l, the lubricants will be moved with the viscosity η and the density ρ as well by the pressure difference Δp along the length l as also the wire speed vz. Consequently the lubricant- mass-currency will be calculated as follows:

RS  - Radius bore of the die        RD Ė Radius wire

 The increase of the pressure and/or the drawing speed generates positive effects to the thickness of the lubricant film on the wire surface.

 b)     Lower coefficient of friction between wire and die

Figure 2 shows the different frictions of the contact areas. Drawing with liquid lubricants is shown in zone III, while the drawing with the combination of lubricant carriers and powdered metallic soaps takes place in zone I and II. In the ideal case the lubrication is hydrodynamic with a perfect liquidity friction. In the production process under real conditions, the conventional lubricant application only rarely achieves this perfect lubrication quality over the whole drawing process. The normal friction is a mixed friction as shown in fig. 2.


Fig. 2:
Stribeck-Curve
The pressure-coating and lubrication system EM 20 grants the constant lubricant application of the wire and ensures optimal frictional conditions. The hydrodynamic lubrication conditions and with that, the liquid friction (as shown in zone III) occur over the whole drawing process. Direct contacts between wire and drawing tool will be completely prevented. 
 
c)      Greatly reduced lubricant consumption and consequently reduced waste management costs

In addition to the elimination of the lubricant carriers the lubricant consumption can be greatly reduced. This reduction will be realised by the consistent lubricant application over the whole length of the wire by transporting only as much lubricants as really required for the transforming process. The lubricant consumption can be reduced to less than the half and consequently reduced waste management cost of the lubricant waste.  

d)     Reduction of die wear

The reduction of die wear results of the conditions described under c). The hydrodynamic lubrication leads to the greatly increase of die longevity by generating ideal frictional conditions. Local cold weldings, the reason for premature die wear can be completely eliminated.

 e)     Increase of drawing speeds

Although from the machine-technical view, very high drawing speeds are feasable, these speeds cannot be utilised for a large range of applications. The maximum possible drawing speed will be determined by different factors, e.g. the application of powdered lubricants depends essentially on the quantity of lubricant particles, transported with the wire. Higher drawing speeds will result in an insufficient application of the lubricants as well as an insufficient lubricant pressure in the transforming area of the die. The problems (metallic contacts between wire and drawing tool, essential increase of the temperature on the wire surface with the peril of martensite-generation, but also the loss of lubrication-effects of the lubricants produced by the increase of the temperature) caused by insufficient lubrication are limiting the drawing speed required for a stable drawing process. At least the drawing speed for some applications is restricted by the pre-treatment of the wire in continuous pass coatings and the subsequent drying periods. 

The engagement of the pressure-coating unit EM 20 enables a stable quality of the lubrication and a predominantly hydrodynamic lubrication over the whole transforming area of the wire can be achieved. As a result of that, higher drawing speeds are possible. Carbon-steel wires can be drawn two-times faster as conventionally. Steel profiles with the intensified peril of cold weldings, especially in the contour-edges of the profile can be drawn up to ten-times faster as conventionally. The same can be said for special materials (e.g. Ti, Ta, Nb). Even though, these materials have a strong tendency to cold weldings, by generating a perfect liquidity friction, the drawing speed can be increased up to fifty (50)-times. It has been considered that together with these increases of the drawing speed, lubricant carriers can be eliminated and at the same time the longevity of the drawing tools will be significantly extended.

f)        Improved internal stress distribution resulting in better mechanical properties

After exceeding the floating limits, ductile materials have deformation capabilities up to the break, each depending on the internal stress distribution, whereby under multiple axial pressure stress conditions a larger deformation capability will be achieved than under tensile stress conditions.

As shown in fig. 3, the wiredrawing as transformation process is in comparison to other transformation processes relatively unfavourable concerning itís internal stress distribution and itís deformation capability up to the break. The figure 3. shows schematically  the influence of the kind of stress respectively the transforming process on the deformation capability of each material in dependency to the deformation stability kf  in connection with the stress medium value δm. With a decrease of friction in the reduction area (see section b) during the wire drawing, the working power diminishes. The lower required drawing power results in lower tensile stress conditions in the transformation area and therewith in an improved stress condition with increased deformation capability.

  
Fig. 3: Magnitude of the transformation capability at different transformation processes in comparison to the break deformation during tension- pressure- and torsion trials as per Stenger (schematic)

 

A further influence on the deformation capability, has the transforming with increased lubricant pressure. The lubricant pressure will be applied from outside and increases in the entrance and drawing angle to coefficients in the magnitude of the deformation stability. This embracing pressure results in deformations of the wire before it gets into contact with the drawing tool (fig. 4 and 5). The occurring embracing pressure improves essentially the relation of the stress conditions at the transformation (fig. 3). The deformation capability up to the break is higher.


Fig. 4: Deformation of the wire in the beginning of the reduction area with different chamber pressures pk.

 

     


Fig. 5: Illustrations of the deformation area of a stainless steel wire (X5CrAl15.5) showing the drawing without pressure (upper illustration) and with a pressure of 2500 bar (lower illustration). The lower illustration shows clearly the strangling of the wire by the embracing pressure. The improvement of the stress conditions results in a more homogeneous transforming.

The improvement of the stress conditions of the transforming is a result of the increased hydrostatic pressure stress distribution (embracing pressure). Fig. 6 and 7 are showing how the engagement of the pressure coating applies a more homogeneous transformation over the cross section. As well the different curvature profiles in fig. 6 as the different formation of the filaments particularly in the core area of a composite material (fig. 7) proof that definitely.

 
Fig. 6: Curvature profile over the cross section of a wire during the drawing without pressure (left illustration) and with pressure (right illustration). The relative displacement within the texture over the cross section is definitely lower with the pressure drawing. (Schiermeyer)

    
Fig. 7: Cross-section of a composite material after the drawing without pressure (left illustration) and with pressure (right illustration). Proceeding from the different formation of the filaments, particularly in the core area, the influence of the higher pressure proportion on the homogeneity of the transformation over the whole cross-section can be identified.

 

Under these transforming conditions it is suggested that subsequently to the drawing, a higher plasticity reserve exists, which possibly allows to eliminate annealing steps after the drawing in order to reduce the stress conditions.  

g)     Realisation of larger reduction degrees

The explanation under f) shows that the transforming conditions generated by the pressure-coating system EM 20 result in a higher deformation capability of the wire. In combination with the quality of the applied lubricant film on the wire the system enables the realisation of larger reduction degrees without the generation of texture- and surface defects.

h)      Smaller diameter tolerances achievable

The engagement of the EM 20 guarantees a constant lubricant application. Following to this, the thickness of the lubricant film on the wire can be defined. The stability of the lubrication conditions during the whole drawing process with simultaneously longer die-life and dimensional accuracy of the drawing tools permits the realisation of smaller dimensional tolerances in the drawing tool.

i)        Higher quality of the finished product

The higher finished product quality amounts to the combination of the optimal lubrication conditions, the consequently higher surface quality and the improved stress conditions during the transformation process. Decisive are the transformation conditions and following to that, a secured finished product quality during the whole process.

 

4      Installation of the pressure coating unit EM 20

Pressure coating units EM 20 are working nearby trouble-free in wire drawing mills since more than 2 years in multiple shift operation. Figure 8 and 9 shows the common alignment of the pressure coating unit EM 20 to a wire-drawing machine. The compact design and itís especially designed adapter permits the installation to almost every wire- drawing machine. The pressure coating unit EM 20 replaces the transformation of the first drawing tool. It is easy to install within one hour and requires only 4 bores at the wire-drawing machine for the adapter. By means of the adapter, the pressure block will be adjusted to the requirements of the wire-drawing machine (Height of incoming wire, inclination of the capstan, direction to the pay-off etc.).

                
Fig. 8 and 9:
Alignment of the pressure coating unit EM 20 to the wire-drawing machine


Fig. 10: Solid lubricant film on the wire after leaving the pressure coating system EM 20

The controls of all in industrial-operation working pressure coating units EM 20 are designed to be coupled with the wire-drawing machine. The process control works fully automatic. Furthermore they are equipped with a range of alarms to grant a trouble-free production and to secure the finished product quality. By thermostating  all areas, necessary for the coating-process, the permanent readiness will be guaranteed. The different, optional selectable lubricant pressures, which are adjusted to the respective drawing case, can be kept in narrow tolerances nearby constantly by means of a pressure-meter and the frequency-converter of the extruder. The pressure coating unit EM 20 will be operated with a separate touch pad (fig. 11) and requires only a quick instruction to the operator.            


Fig. 11: Working scheme of the touch pad (also in English language available) to control the pressure coating unit EM 20.



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