Автор: V.V. Kukhar, O.V. Vasylevskyi
Категории:
dieforging
The characteristics of the stressstrain state in workpieces are investigated by the experimental method of coordinate grid during new intensive modes of a stretchforging in combined dies. Lows of influences of modes with the fixed reduction at increase of angle of rotation at common aggravation of compressing of metal layers in a crosssection have been detected. The modes of stretchforging with the fixed angles of rotation and increasing of reduction leads to the growth of intensity of a strain on a crosssection with the best compressing of metal layers of a medial zone.
Keywords: forging of shafts, stretchforging, combined dies, coordinate grid, stressstrain state, compacting work of metal layers
UDC 621.73
V.V. Kukhar /D.Sc.(Eng.)/
SHEI “Priazovskyi State Technical University”, Mariupol, Ukraine
O.V. Vasylevskyi
LLC “Metinvest – Mariupol MechanicalRepair Plant”, Mariupol, Ukraine
Experimental research of distribution of strains and stresses in workpiece at different modes of stretchforging with rotation in combined dies
Introduction
Increasing of the longitudinal size of the workpiece at stretchforging is produced by compression of it’s crosssection by different working tools: flat die, convex die, cutout die, combined dies. The calibers of the traditional cutout and combined dies for forging of shafts with round crosssectional have a rhombic or radius(round) profile. Currently there is an active development of innovative ways of forging, which can intensify the compacting work of internal layers of the ingot at low coefficients of total reduction of crosssection [1]. The bulk of these methods are sent to creation of macroshift of material of workpiece in the deformation zone by complication of form of dies for the intermediate forging and combining of tools. From this point of view an actual scientific and practice task is development of the energysaving modes of deformation of workpieces at stretchforging with the use of the traditional combined dies due to realization of methods of intensive strains and achievement of high degree of compacting work of metal layers for providing of production of metal forgings shafts with the required operating properties.
Analysis of the last researches and literature
In the work [2] the process of forging of ingots by profiled dies with receiving of threebeam or fourbeam billet is offered. The further stretchforging with macroshifts from profiled workpiece is demands of tool change and is conducted of a cooling of metal of a semiproduct. Technological realization of the method [3] assumes of forging of billet in the beginning by the flat dies and then it’s rotation in round of longitudinal axis in dies with round cutout without increase in length of a shaft. The article [4] in which conditions of a change of the sizes of workpiece and an emergence of macroshifts are analysed is devoted to researching of a stretchforging of ingots by profiled dies. It is obvious that achievement of positive influence of macroshifts effect on indicators of quality of forgings due to complication or increase in quantity of sets of the working tool is economically justified only during forging of ingots from the highalloyed expensive brands of steel.
In the work [5] probes of modes of a stretchforging of billets in the combined dies with a rotation of workpiece in round of longitudinal axis in relation to conditions of forge and press shop of the LLC “Metinvest – the Mariupol Mechanicalrepair Plant” enterprise that organized on the basis of maintenance shops of PJSC "Iyich Iron and Steel Works of Mariupol" are begun. A stressstrain condition of shaft forgings during working was investigated by finiteelement modeling. The development of scientifically reasonable recommendations about a choice of rational modes of forging by such tool demands of experimental research of influence of sizes of upset reduction and angles of rotation of billet round of longitudinal axis on compacting work of material of a forgingpart and geometrical characteristics of a crosssection.
Object of research and statement of tasks
The aim of this research is experimental studying a stressstrain state at various modes of a stretchforging of cylindrical billets with rotation in round a longitudinal axis in the combined dies.
For achievement of the specified aim the tasks are set: to develop the methodological approaches for definition of the strain condition of workpieces in relation to processes of forging of shaft by a stretchforging in the combined dies; to establish of influence of stretchforging modes on distribution of strains and stresses in a crosssection of the zone of deformation of workpieces; to determine the best values of upset reduction and angles of a rotation of workpiece round a longitudinal axis for achievement of highquality compacting work of metal layers of billet by a crosssection in the deformation zone at stretchforging.
Materials of research
The six samples with a diameter = 50 mm and length = 100 mm were made from antimonial lead (ССу brand) for performance of experimental research. The samples were made by pressing in the form of two halves of semicircular crosssection, and coordinate grid with a step = 3 mm was put on the inner part of one of halves. The soldering by Wood's alloy was carried out (fig. 1, a) for receiving of continuous samples which marked on one of ends by signs "0", "1", "2", "3", "5", "8" (fig. 1, b), and the marking for the performance of rotation of workpiece a round of longitudinal axis on the fixed corners = 30°, 60° и 90° was put at other end (fig. 1, c).
Fig. 1 Preparation and forging of experimental samples:
a – soldering by Wood's alloy of samples with coordinate grid;
b – marking on the ends of soldered samples;
c – forging of sample with marks in combined dies;
d – sample after forging mode;
e – mechanical scheme of strains
The model of cutout anvils were made for laboratory experiments in scale 1:10 to the size of a productive nature: width = 30 mm, the radius of the notch in the lower anvil = 30 mm. The material of anvils is steel 45 (0.45% Carbon). These anvils were fixed in a stamp block (Fig. 1, c), mounted on a universal testing machine (0.2 MN), and carried out the deformation of lead samples (Fig. 1, d) with the entire width of the anvil in the middle of the length of the workpiece (which corresponds to the amount of feed = 30 mm, the relative feed rate = 1.0). So, the influence of hard and not deformed ends at modes of stretchforgings was also taken into account (Fig. 1, d and e), which receives the stretch in the longitudinal direction due to making compressions (Fig. 1, e). In real conditions it is necessary to remove the workpiece along the front of feed with relative feed £ 1.0 and hold running along the diameter, having the given values of upsetting and angles of rotation for the implementation of the next step of stretch forging.
The samples were divided into two groups to study the effect of influence of stretch forging modes on the controlled indicators, each of which was assigned rotation angle, value of reduction (upsetting) and quantity of reduction till to full rotation of the workpiece to 360°. In the first group of samples ("0", "1" , "2") varying of the rotation angle was performed at a fixed value of reduction = 5 mm: a sample "0" – = 30°, n = 12; a sample "1" – = 60°, n = 6; a sample "2" – = 90°, n = 4. In the second group of samples ("3", "5", "8") varying with the value of reduction was performed at a fixed tilting of rotation angle = 60°: a sample "3" – = 5 mm, n = 6; a sample "5" – = 6.6 mm, n = 6; a sample "8" – = 9 mm, n = 6. Thus, the study was carried out at relative reduction: = 0.1; 0.132 and 0.18.
To investigate the stressstrain state as a basis, the experimental method of grids was chosen [6]. The vertical line was determined before the soldering of the samples = 16 (Fig. 2, a), which is at a distance (then ) from the end of the sample. Along this line initial height () and the width () of each cell of the grid was measured. A feature of measurements was that the base rate of the cells of the grid took the width and height with the presence of an orthogonal basis in the form of two crossing lines in the middle of the cells (Fig. 2, b). This facilitates the measurement of the central angle of cell shift of the grid relatively to the initial angle = 90°, as well as finale size and (see Fig. 2, b) after forging and desoldering of the samples. Accordingly, the deformation refers to the center of the cell with the cross of lines in the middle, the material was considered isotropic. Measurements were performed using microscope (model: BMI10) and according to scanned images of the grid.


а

b

Fig. 2 Measurement scheme of the grid:
a – the numbering of cells at length and height;
b – the cell is before and after forging
The forging with rotation involves the conversion of the original round crosssection of the workpiece into a round section of a forgingpart, so the stressstrain state of the material in the area of deformation can be taken as axisymmetric. The components of strains in elementary cells were calculated as
; ; . (1)
Components responsible for the shift:
; . (2)
Accordingly, the intensity of strains for the accepted conditions:
. (3)
According to the hypothesis of a unique curve, we have unique functional relationship between the intensities of stresses and intensity of strains e^{i} for given conditions of thermomechanical deformation of material: . The similarity of kinematics of deformation of workpieces from different materials was allowed. Then the quantity of the deformation along the height of semifinished item depends on the type of curve of hardening. The dependence for antimonide lead CCy was determined after the tests (coefficient of determination = 0.9997):
. (4)
The approximation of hardening curve of the steel 12XHMФA (C 0.090.16% , Cr 0.60.9% , Ni 1,01,4%, Mo 0.150.3% , V 0.10.2% ) at the temperature = 1100° C and velocity of deformation = 10 с1 [7] gives the following relationship (R^{2} = 0.9838):
. (5)
The conditions of deformation were taken monotone, so the coefficients of hardness of the scheme of stress and strain state (according to G.A. SmirnovAliaev [8]) were considered:
; ; . (6)
where и – the components of stress and strain for cell.
For axisymmetric deformation the coefficient of hardness of the scheme of stressstrain state was calculated as
. (7)
The results of processing of experimental data are shown in Fig. 3 and Fig. 4.
The increasing of the angle of rotation of the workpiece at a fixed relative reduction ( = 0.1) is accompanied by a decrease of the average values of the intensities of strains in the deformation zone, deterioration in compacting work of metal layers (see Fig. 3). The moving of values of the coefficients of hardness of stressstrain state in the middle of the height of the crosssection in a hard area also confirms the ability of appearance here of stretching stresses at angles of rotation = 60° and 90°, despite the fact that the maximum values of parameter are not in the central area of the workpiece.
Fig. 4 Graphs of the distribution of intensity of strains ,
intensity of stress for lead CCy () and steel 12XHMФA () billets, and coefficients of scheme of stressstrain state at height:
а – sample "3" ( = 60°; = 0.1);
b – sample "5" ( = 60°; = 0.132);
c – sample "8" ( = 60°; = 0.18);
1 – experimental points, 2 – data approximation
At different values of reduction and angle of rotation = 60° the maximum of intensities of strains is observed at middle of height of the samples (see Fig. 4), increases with a rise of values of upsetting . Growth of reduction values also leads to fewer of layers of metal of billet which are located in a hard area, and the average value of coefficients of harness of stressstrain state belongs to a soft area. The qualitative difference between the stress intensity values for the same deformation modes of workpieces from different materials connected with different mechanism of their hardening under given thermomechanical conditions of deformation.
Conclusions
The technique of experimental determination of the influence of the modes of the stretch forging on the distribution of the strains and stresses in a crosssection of the workpiece, which takes into account the mechanical and kinematics conditions of its deformation with rotation in combined anvils, is developed. The fact that in fixed reductions (upsetting) the increasing of the angle of rotation of the workpiece around the longitudinal axis allow to reducing an average value of intensity of deformation along the crosssection with increasing of a share of stretching deformation on the midpoint of the height of the workpiece was found. At fixed values of the angle of rotation the increasing of reduction (upsetting) leads to rising of allaverage intensity of strains with a maximum in the middle of the height of the deformation zone and reduction of dispersion of values of the coefficients of hardness of scheme of stressstrain state. The best results from the point of view of achieving a qualitative compacting works of the metal on the crosssection of deformation zone showed the modes with the angle of rotation = 60° and relative reduction = 0.18.
References
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