Сomparative analysis of high carbon steel behavior on contact surface with a tool in different methods of deformational nanostructuring

Every method of deformational nanostructuring is characterized by typical scheme of the processing. One of the main aspects of such methods is the behavior of the processed metal on the contact surface with a tool. The aim of this study is to analyse the distribution of deformation from the contact surface of the workpiece to the bulk in different methods of deformational nanostructuring. High carbon steel was chosen for the analysis. Simulation was performed in ABAQUS software. Methods based on different kinds of deformation were chosen for the comparison: method based on simple shear in one deformation zone, method based on simple shear with torsion, method based on simple shear in two deformation zones with torsion, and continuous method of combined deformational processing by drawing with torsion. Data of the distribution of main strains, tangential strains, equivalent strains, and the Mizes stress from the contact surface to the depth of 0.05 mm to the bulk was obtained. The results of simulation can be used for prediction of steel behavior under different kinds of deformational processing.


Introduction
Manufacturing processes of metal ware production are based on different methods of deformational processing. It is evident that during such kind of processing metal microstructure changes causing the change of its mechanical properties. One of the important tasks of state-of the-art material science is to nd effective ways to increase strength of metals. It is well known that in order to improve metals' strength it is necessary to choose the complex effect on microstructure combining methods of different physical nature: to use alloying elements, to apply any kind of thermal treatment or deformational processing.
When analyzing the existing approaches to increase of mechanical properties in metals [1] one can make the following conclusions. Firstly, at present time it is not enough to use any method which is based on the only one strengthening mechanism for steels. Secondly, choice of the grade of steel or alloying elements for it should be based on the prediction of the effects which can be probably occur on every stage of steel treatment. In order to implement different strengthening mechanisms during processing high carbon steel are preferable. Eutectoid steel in cooling has the only one critical point and at ambient temperature microstructure consists only from pearlite. It proves that properties of this steel will be uniform all over the whole volume of the processed workpiece. It makes it possible to deform this kind of steel with high values of deformation degree and to store the de nite value of its ductility. Thirdly, the most effective way to increase strength properties in steel is to use the deformational processing. It leads to the decrease of grain size of the processed metals. In this case Hall-Petch equation makes it possible to predict the level of mechanical properties after deformational processing.
Existing methods of metal processing are based on different kinds of plastic deformation: tensile, compression, bending, and torsion. Every kind of plastic deformation causes peculiar changes in microstructure to its deformation scheme and stress-strain state in the processed metal.
Drawing is the main operation for wire production. At drawing the wire is processed by tensile and compression deformation which result in speci c texture formation. In [2][3][4][5] the results of pearlitic steels microstructure evolution during drawing are presented. In uence of microstructure formation on mechanical properties of carbon steel wire with different carbon content was studied as well.
Bending as the kind of plastic deformation which is the basic one at wire descaling operation. During bending different parts of wire are deformed by tensile and compression deformation. Alternate bending is widely used in different technological processes both as auxiliary (coiling and recoiling) and separate (sheet forging, strengthening) operation. In contrast to traditional methods of metal processing in which total deformation is limited by dimensions of workpiece or the part the total deformation in bending actually in not limited at all. It increases the e ciency of the process. Peculiarities of wire bending are presented in [6,7].
Twisting makes it possible to arrange several wires into one metal part -cable or rope. Torsion deformation is characterized by very complicated scheme. There is no consistent opinion about stressstrain state of the processed metal under torsion deformation. One of the point of view is based on the hypothesis that torsion deformation is maximum on the surface. Tangential stresses are considered to change in different ways from maximum values on the surface to the zero in the center of the processed workpiece [8]. In uence of torsion deformation on microstructure of pearlitic steel wire is presented in [9,10]. One of the important aspects of any method of deformational processing is the contact interaction between the workpiece and a tool. Stress-strain state of the workpiece in the contact area with a tool depends on hard friction conditions, increase of temperature in the contact zone and as a result can cause damage and fracture of the workpiece [20-22 and others]. It can limit the possibility of implementation of SPD methods into industrial manufacturing technologies.
The aim of this study is to compare the level of deformation on the contact surface between a workpiece and a tool in different SPD processes in order to estimate their possibility to be applied at large scale production. For the comparison following methods based on different kinds of deformation were chosen: method based on simple shear in one deformation zone, method based on simple shear with torsion, method based on simple shear in two deformation zones with torsion, and continuous method of combined deformational processing by drawing with torsion.

Materials And Methods
High carbon steel was chosen for the analysis (Table 1). Method based on simple share in one deformation zone. Equal channel angular pressing (extrusion) (ECAP/ECAE) was chosen for the investigation as the method based on simple share in one deformation zone [11]. During ECAP a billet is multiple pressed through a special die in which the angle of intersection of two channels is usually 90 0 (Fig. 1) [12].
ECAP is considered to be one of the classical SPD methods. It is used to study the peculiarities of microstructure and properties formation for different metals and alloys [12,15,17,18 and others].
Method based on simple share with torsion. This method includes operation of reduction treatment, combined with shear [23,24]. Implementation of the method includes deformation of the metal according to the drawing scheme due to the applied attractive force through two consecutively located conical wiredrawing dies 1 and 2 with simultaneous rotation of one of the dies (2) (Fig. 2). semi-nished products by drawing with shift [23] The speci c aspect of this deformational processing is that additional shear deformation appear in the deformation zone due to both the eccentricity and applied transverse moment which cause the rotation of the second die.
Method based on simple shear in two deformation zones with torsion. This method is known as twist extrusion (TE) [25,26]. TE is based on pressing out a prism specimen through a die with a pro le consisting of two prismatic regions separated by a twist part (see Fig. 3) [26]. As the specimen is processed, it undergoes severe deformation while maintaining its original cross-section.
The mode of deformation in these zones is simple shear in the transversal layers, as in HPT. In terms of strain, at the rst approximation, the billet during TE like passes through two "transparent" Bridgman anvils (see arrows in Fig. 3). High hydrostatic pressure in the deformation zone is ensured by application of backpressure.
Continuous method of combined deformational processing by drawing with torsion. The laboratory setup was used for experimental part which makes it possible to combine drawing with torsion (Fig. 4).in drawing with torsion of carbon steel wire.
Two consequently arranged along the longitudinal symmetrical axis dies (2, 4) are installed into the frame 1. The four-roll system 3 has the autonomous engine which makes it possible to apply torsion deformation to the moving wire 5 (moving direction is marked by an arrow). The novel step of this setup is to adjust these tools simultaneously on the moving wire.

Results And Discussion
Method based on simple share in one deformation zone. Distribution of deformation from the contact surface to the bulk of the processed workpiece for the method based on simple shear is presented in Fig. 5. The results show that strains distribution from the contact surface with the die to the bulk is rather uniform. Hence, this scheme of plastic deformation is contributory to the processed metal. Values of the Mizes stress are uniform all over the cross section of the billet (Fig. 6). It will be not cause any cracks or damage of the processed workpiece. From this point of view ECAP can be applied for several times to the same workpiece in order to get the uniform microstructure in the whole volume of the billet.
Method based on simple share with torsion. This method is characterized by combination of deformation by drawing with torsion. As it was mentioned before, the construction of setup makes it possible to achieve the scheme of simple share (see Fig. 2). As it is seen from Fig. 7, this scheme is characterized by high values of tangential strains. Values of equivalent strains change periodically. It may cause damage of the processed workpiece.
Distribution of the Mizes stresses is presented in Fig. 8. Due to the eccentricity of the tool and rotation of a die (see Fig. 2) values of the Mizes stresses start to increase in the entrance of the deformation zone.
High values of this parameter can cause damage of the processed workpiece especially in its center. From this point of view this methods is recommended for processing metals with high ductility.
Method based on simple shear in two deformation zones with torsion. Results of simulation show that the workpiece undergoes intensive deformational processing (Fig. 9). Distribution of deformation is uniform from the surface to the bulk of the processed workpiece. But change of high values of the deformational parameters proves about the intensive interaction of the workpiece with the tool. It can lead to cracks formation on the surface of the billet.
Complicated scheme of deformations appear in the processed workpiece in drawing combined with torsion. Maximum values of deformation parameters are observed in 0.2 s after the beginning of the process. It testifyes the high intensity of such kind of deformational nanostructuring. From the obtained values of the Mizes stresses (see Fig. 12) it is evident that deformational processing by drawing with torsion does not lead to damage of the workpiece saving its' ductility. These results match well with known aspects that using torsion deformation during drawing, for example in roller dies, results in decrease of friction between a wire and a die. It simpli es intrusion of deformation in bulk of the processed workpiece.

Conclusions
Simulation of different methods of deformational nanostructuring showed following peculiarities of high carbon steel behavior on the contact surface between the processed workpiece and a tool: 1. Equal channel angular pressing is the method which is characterized by simple shear in one deformation zone. During this method the distribution of deformations from the surface area of the billet to the bulk is uniform. The level of deformations is not very high that is why the workpiece can be processed many times in ECAP.
2. High level of deformation is observed in method based on simple shear with torsion. It may cause damage of the processed workpiece. It is preferable to apply this method of deformational nanostructuring to ductile metals and alloys.
3. Twist extrusion is the method based on simple shear in two deformation zones with torsion. Rather complicated scheme of deformation makes it possible to get the uniform structure of the processed workpiece from the surface to its axis.  Principle scheme of the laboratory setup for combined deformational processing in drawing with torsion of carbon steel wire Distribution of deformation in the workpiece from high carbon steel for the scheme of simple share: amain strains, b -tangential strains, c -equivalent strains, d -Mizes stress Values of the Mizes stress on the contact surface of the workpiece with the die for the method of deformational nanostructuring based on the scheme of simple share: a -scheme of simulation; b -the entrance to the deformation zone; c -the exit of the deformation zone Values of the Mizes stress on the contact surface of the workpiece with the tool for the method of deformational nanostructuring based on the scheme of simple share with torsion: a -scheme of simulation; b -the entrance to the deformation zone; c -the exit of the deformation zone Figure 9 Distribution of deformation in the workpiece from high carbon steel for the scheme of simple share in two deformation zones with torsion: a -main strains, b -tangential strains, c -equivalent strains, d -Mizes stress Distribution of deformation in the workpiece from high carbon steel for the method of deformational nanostructuring based on drawing with torsion: a -main strains, b -tangential strains, c -equivalent strains, d -Mizes stress