RU2400320C1 - Hard alloy draw die for drawing items out of hardly deformed alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention is designed for substantial rise of operational resistance of hard-alloyed draw dies designed for drawing items out of alloys with high ultimate stress limit of nichrome type. The draw die has a drawing channel consisting of deformation, calibrating and output zones.

EFFECT: favourable distribution of stresses in body of draw die sharply reducing probability of its destruction facilitated by ratio of length of output zone of drawing channel to total length of channel 0,24-0,32.

11 dwg, 1 ex

Description

The invention relates to the manufacture of cold-drawn products from hard-to-deform alloys, that is, from alloys having a high tensile strength such as nichrome and the like, and more particularly, to the shape of a drawing channel of carbide dies used in said production.

It is known from international and domestic practice that in the manufacture of nichrome prefabricated products, in particular with a non-circular cross section, considerable technological difficulties have to be overcome, since the shape of the strip section obtained by the traditional method of flattening a round workpiece with rolls of a rolling mill has a barrel-shaped shape due to uneven broadening metal [1]. The convexity of the small face of the strip is obtained naturally, that is, it is not controlled by the tool and, as a result, can vary quite arbitrarily for many reasons: depending on the size of the compression, friction coefficient, wear of the surface of the rolls, temperature fluctuations, etc. Thus, it is technologically difficult to perform a strip with compliance with tolerances and with small deviations in the cross-sectional area along the length of the rolled blanks. At the same time, stability of the area is given great importance during the operation of heating elements, since when passing an electric current in places of reduced cross section, the metal overheats, oxidizes and gas saturates, while losing proper service properties.

A blank of nichrome in the form of a sheet or tape, obtained by the traditional method of flat rolling [2-5], is intermediate, then the sheet or tape is divided by longitudinal cutting into strips of finished sizes. For the first time in domestic practice, under the conditions of the applicant, an alternative technological direction was selected, investigated and successfully applied in the production of nichrome strips - using the drawing process, implemented on subsequent grounds.

A sharp decrease over the past 17-18 years in the volumes of batches of nichrome strips ordered by ferrous metallurgy enterprises from Russian consumers has led to the fact that the above-described production scheme for flat rolling using ingots weighing up to 500 kg turned out to be unprofitable. In view of the situation prevailing in the market of this type of rolled metal, in the applicant’s production, on the basis of research conducted, a highly cost-effective technological regulation for the small-scale production of nichrome strips and wire is developed and implemented according to the following technological scheme:

- smelting of the alloy in an ILK-type furnace using raw materials, deoxidizers, modifiers and fluxes established by a charge card;

- filling casting of conical ingots weighing 55-60 kg;

- rolling of ingots heated in a chamber furnace on a small-grade wire mill 300 for riot strip blanks or for wire rod of circular cross section;

- etching of strip blanks or wire rod with subsequent application of a lubricating coating;

- sharpening the front ends of the billets and wire rod and dragging them to strips or wires of finished finished size in several passes on a single drum drawing machine such as VSG through carbide dies with intermediate annealing; lubricant - sodium soap powder with additives.

It is known from the prior art [6-14] that practically all bibliographic sources devoted to drawing contain information about the shape and size of the drawing channel. Moreover, in the overwhelming majority of them, for obvious reasons, the authors focus on the parameters of the working and calibrating zones of the channel, briefly consider the lubrication and inlet zones, and the amount of information on the channel outlet zone is extremely poor. In particular, it was mentioned that its purpose is reduced to the function of protecting the exit edge of the gage zone from chipping in order to increase the stability of the gage belt, as well as to prevent injury to the surface of the drawn metal on the said edge when the drawing process is unstable, namely: vibration of the front end, axis mismatch channel and stretched strip or round billet, dynamic jerks at the time of starting the drawing machine and when the front end of the product is broken, etc.

An exception is the source [7], in which on p.183-184 the shape of the exit zone is considered more extensively. Firstly, it was noted that the “inverse cone" (as the exit zone was called in [7]) is important for dies intended for wire production on multiple-draw machines with sliding. The applicant does not consider these details, since the nature of the sliding process is beyond the scope of this application. Secondly, it is indicated that the shape of the outlet zone can be conical, hemispherical or combined. Thirdly, information is provided that the exit zone shifts the deformation zone closer to the middle of the die height. Passing to generally accepted terminology, the last phrase should be stated as follows: the exit zone shifts the deformation zone closer to the middle of the length of the drawing channel of the die.

In connection with the foregoing, this object [7] is adopted as the closest analogue (hereinafter referred to as NBA).

It is believed [6, 8, 11-14, etc.] that the angle of inclination of the generatrix of the outlet zone to the channel axis should be 30-45 °, that is, its total angle is 60-90 °. The known sources say practically nothing about the length of the exit zone. At the same time, the length of the outlet zone becomes a very important component of measures to increase the hardness of carbide wire when drawing strips and wires of alloys with high strength properties, which, when drawing, are further increased as a result of the effect of cold hardening in the deformation zone. It is to such alloys that nichromes belong. In addition, an objective and economically justified desire to reduce drawing routes by increasing private hoods additionally significantly increases the load on the tool.

During the commercial operation of the known hard-alloy die under the conditions of the applicant, for example, in the process of drawing a strip of nichrome, it shows extremely unstable and generally unsatisfactory resistance to complete destruction, namely: in the best case, it was possible to stretch the die into it, for example, for a strip with cross-sectional dimensions of 4 × 40 mm 100-200 kg of metal, that is, 2-4 riots of the workpiece with a mass of one riot ~ 50 kg. In the worst case (for example, with the dimensions of the hot-rolled billet made with a plus tolerance of +0.5 mm), the die was forced to decommission (replace) after drawing the first riot due to its destruction; a steel clip of a die was also broken.

The objective of the proposed technical solution is to repeatedly increase the operational stability of carbide dies designed for drawing strips and wires of hard-to-deform alloys, that is, alloys having a high tensile strength, nichrome and the like, by more favorable distribution of spacer stresses in the fiber body, sharply reducing the likelihood of its destruction.

The problem is solved in that the ratio of the length of the output zone of the drawing channel to the total length of the drawing channel is 0.24-0.32.

In Fig. 1, as an example of obtaining strips, when drawing which the drawing tool experiences the greatest (i.e., critical) loads, the spacing forces vectors Q and F are shown, acting on the carbide insert 2 and steel cage 3 during drawing; lines 1-1 schematically show typical fracture sites of the carbide insert 2 of the known die-like die. In Fig.2 shows a section aa (Fig.1) carbide insert 2 and a steel cage 3 of the known die; figure 3 is a similar section aa (figure 1) insert 2 and clip 3 of the proposed die. The interpretation of the positions indicated in FIGS. 2 and 3 is given in the description text.

The following is a calculation of the spacer stress arising from the force Q and acting on the die along a large facet of the channel cross-section when drawing a strip of nichrome grade X20H80-N.

In accordance with the well-known upper bound formula [15], the average normal stress σ _nsr acting on the wall of a large face of the die channel in the case of a planar deformed state (that is, for strips) is:

σ _nsr / 1.15σ _s = 1 + 0.25l _od / h _cp ,

where σ _s is the average cold deformation resistance of nichrome X20N80-N; σ _s = 1200 MPa; l _od - the length of the deformation zone; h _cp is the average thickness of the deformation zone.

For the passage of drawing from the thickness of the hot-rolled billet 6 mm to the thickness of the intermediate billet 5 mm, that is, for the practice of drawing the strip of maximum absolute compression along the larger side of the section Δh _max = 1 mm (therefore, Δh _max / 2 = 0.5 mm) and with a minimum half-angle of the working area of the channel α = 3 °, these parameters will be equal to:

l _od = (Δh _max / 2): tg3 ° = 0.5 / 0.0524 = 9.54 mm;

h _cp = (6 + 5) / 2 = 5.5 mm.

Then σ _nsr = 1.15 · 1200 (1 + 0.25 · 9.54 / 5.5) = 1978 MPa.

The obtained value significantly exceeds the permissible bending stress of the VK8 hard alloy, equal to 1666 MPa [16], and this circumstance is undeniable evidence of the objective reason for the destruction of the die, especially taking into account the following conditions of real production: wear of the drawing channel due to the high adhesive ability of nichrome, unstable or insufficient supply of lubricant to the deformation zone, dynamic loads on the die at the time of the start of drawing or when the machine stops due to the breakage of the front about the end, misalignment of the drawing channel and the direction of the drawing force, etc.

In addition, it should be noted that in the known drawing, the action of the vectors of the spacer forces Q (taken as the main ones when considering the situation, since they are many times higher than the forces F) has an extremely unfavorable direction (Fig. 2) compared with the direction of action of these vectors in the proposed drawing (figure 3). From the consideration of figure 2 it follows that in the known die force Q is directed to the rear surface 4 of the carbide insert 2 resting on the clip 3, and since in this place the total cross-section bb of the insert 2 and the clip 3 has a reduced value, then this in principle, the cross section cannot adequately counteract the force Q and, as a result, the fracture of the carbide insert in this situation becomes inevitable, taking into account the above production conditions, which is confirmed in practice.

In the proposed drawing, the points of application of the action of the forces Q and F are substantially displaced in the opposite direction to the drawing direction (Fig. 3), and the said forces Q and F are counteracted by the total cross-section in-insert 2 and clips 3, and this cross-section has a significant an increased value compared to the cross-section bb of the known die both due to the increased thickness of the insert 2 at this place, and due to the thicker section of the cage 3 at this place (Fig. 3). In addition, in the case of the proposed die (Fig. 3), the supporting surface 5 of the drawing insert, interacting with a much larger section of the steel sleeve 3, does not rest directly on the holder, but through a layer of brass solder 6, which is designed to securely fix the insert in the holder by its fixation with molten brass. This circumstance additionally compensates for the negative impact of the spacer forces Q and F.

In the text of the NBA description [7, p.183-184] it is stated: “The length of the outlet zone is, in relation to the diameter of the calibrating zone, from 8 for small dies to 0.05 for large dies, and the absolute height (obviously, the absolute length is a note of the applicant) - respectively, 5.6-1 mm. " When used according to the NBA [7] recommendations regarding the length of the output zone l _{o. zone} = 8 ... 0.05d _channel , for usually obtained by drawing bars of the largest diameters of 50-80 mm calculated according to [7] l _{output. zones} = 50 ... 80 · 0.05 = 2.5 ... 4 mm. Therefore, the ratio of l _{o zone} / l of the _channel is 2.5 ... 4/25 = 0.1 ... 0.16. It is these values of this relationship that occur in the drag - NBA.

If we take into account the recommendation [7] for large dies about the ratio of the length of the outlet zone to the thickness of the stretched strip of nichrome reduced to a circle, then, for example, even the quadruple lower limit of this ratio will be 0.05 · 4 = 0.2. Since the recommendation contained in the NBA relates to the drawing of a round bar, in order to use it, it is necessary to bring the section of a rectangular strip to a round section. As noted above, for this particular case, the largest strip of 4 × 40 mm rectangular nichrome profiles manufactured in the applicant’s production was selected for which, in the first drawing pass, a hot-rolled billet with a cross-section of 6 × 43 mm was stretched to an intermediate billet with a cross-section of 5 × 42.5 mm; its cross-sectional area F = 5 · 42.5 = 212.5 mm ² .

, то есть сечению промежуточной заготовки 5×42,5 мм соответствует равновеликий круг диаметром 16,45 мм. Тогда согласно рассчитанному по [7] отношению, равному 0,2, получим максимальное значение

. Таким образом при полной длине канала твердосплавной волоки для волочения изделий из нихрома, равной 25 мм, отношение максимального значения

к l_канала составит 3,29/25=0,13, то есть это значение находится по НБА в интервале 0,10-0,16, то есть согласно волоке - НБА.After reduction of the considered strip using the formula area of a circle F = πd ^2/4 to obtain a round cross-section corresponding circle diameter

, that is, the cross section of the intermediate workpiece 5 × 42.5 mm corresponds to an isometric circle with a diameter of 16.45 mm. Then, according to the ratio of 0.2 calculated according to [7], we obtain the maximum value

. Thus, with the full length of the channel carbide for drawing products from nichrome, equal to 25 mm, the ratio of the maximum value

to l of the _channel will be 3.29 / 25 = 0.13, that is, this value is in the NBA in the range of 0.10-0.16, that is, according to the drag - NBA.

, а длина выходной зоны с учетом принятого выше отношения 0,2 составит

. Тогда отношение

составит 2,45/25≈0,1. Полученное отношение является нижним его пределом в интервале согласно волоке - НБА.Upon receipt in the applicant’s production of a nichrome strip of smallest dimensions 2 × 20 mm in the first drawing pass, a workpiece with a cross section of 6 × 25 mm is pulled onto an intermediate workpiece with a cross section of 5 × 23.5 mm, the cross-sectional area of which is 5 · 23.5 = 117.5 mm ² . The diameter of the corresponding circle for it will be

, and the length of the exit zone, taking into account the ratio of 0.2 adopted above, will be

. Then the ratio

will be 2.45 / 25≈0.1. The resulting ratio is its lower limit in the range according to the drag - NBA.

Below is the rationale for the claimed range of the ratio of the length of the output zone to the full length of the drawing channel equal to 0.24-0.32.

, следовательно, соответственный круглый профиль (после корректировки отношения 0,05 до значения 0,2) будет иметь диаметр 6/0,2=30 мм. При верхнем пределе интервала, равном 0,32,

следовательно, соответственный круг имеет диаметр 8/0,2=40 мм. (Воспользоваться рекомендацией из [7] об этом соотношении, равном 0,05, не представляется возможным, поскольку в этом случае диаметры соответственных прутков будут равны 6/0,05=120 мм и 8/0,05=160 мм. Получение прутков таких диаметров волочением практически исключено.) Полученные волочением прутки таких диаметров (30 и 40 мм) действительно являются одними из самых крупных. Однако, если воспользоваться рекомендацией из [7] и рассчитать l_{вых. зоны}, опираясь на величину указанного в [7] отношения для крупных прутков 0,05, то фактическая длина l_{вых. зоны} составит:With the lower limit of this interval and the total length of the drawing channel 25 mm, the length of the outlet zone

therefore, the corresponding round profile (after adjusting the ratio of 0.05 to 0.2) will have a diameter of 6 / 0.2 = 30 mm. With the upper limit of the interval equal to 0.32,

therefore, the corresponding circle has a diameter of 8 / 0.2 = 40 mm. (It is not possible to use the recommendation from [7] about this ratio of 0.05, since in this case the diameters of the respective rods will be 6 / 0.05 = 120 mm and 8 / 0.05 = 160 mm. diameters by drawing are practically excluded.) Rods of such diameters (30 and 40 mm) obtained by drawing are indeed one of the largest. However, if we use the recommendation from [7] and calculate l _{out. zone} , based on the value specified in [7] the ratio for large rods of 0.05, then the actual length l _{out. zones} will be:

- for a bar with a diameter of 30 mm l _{out. zones} = 30 · 0.05 = 1.5 mm;

- for a bar with a diameter of 40 mm l _{out. zones} = 40 · 0.05 = 2 mm.

Obviously, such values of l _{o. Zones} cannot be considered acceptable for practical use.

In order to assess the level of stresses developing in the body of the carbide die, the calculation of the stress fields was carried out according to the method described in [18]. Thus, an assessment was made of the qualitative picture of the fields of the sought stresses and their distribution over the volume of the insert body. Qualitative pictures of the stress fields acting in the known and proposed dies are shown in FIGS. 4, 5, 6, 7; moreover, the critical zones of action of these stresses differ in a lighter tone. The figures show:

- known dragging from the side of the output zone, that is, the rear view of the insert (figure 4);

- a known dragging from the entrance area, that is, the front view of the insert (Fig. 5);

- the proposed drawing from the output zone, that is, the rear view of the insert (Fig.6);

- the proposed draw from the input zone, that is, the front view of the insert (Fig.7).

A comparison of the patterns of the stress fields arising from the spacer forces Q acting on the die shows the following obvious advantages of the proposed technical solution, namely: the areas of hazardous stress zones in the insert body are significantly reduced (Figs. 6 and 7) compared with similar zones in the insert body known die (figure 4 and 5).

When performing the drawing channel with the declared elongated output zone, the distribution of the stress fields developing in the insert body under the action of the spacer forces applied to the channel walls changes significantly in a favorable direction (Fig.6 and 7):

- the cumulative effect of these stresses in the weakened places of the insert is smoothed out;

- the intensity of these stresses decreases;

- in the zone of maximum action of these stresses they are opposed by a thicker section of the drawing insert, as well as a more massive part of the steel cage.

As a result of using the proposed shape of the drawing channel, made with a significantly elongated outlet zone, based on the applicant's data on the drawing of industrial lots of 15 sizes of strips made of nichrome grade X20H80-N, a significant increase in the resistance of carbide dies against destruction was achieved, namely: the amount of metal drawn to replace the destroyed the draw is up to 4000-5000 kg instead of the previously existing maximum mass of the stretched metal until 200 kg is destroyed.

In addition to the qualitative picture of the stress fields from the same source [18], the level of elastic deformations of the insert (from VK8 hard alloy) in the working and calibrating zones of the channel arising in the known die during drawing and shown from the input zone was calculated (Fig. 8) and from the outlet zone with its specific length of 3 mm, i.e. with respect to l _{out. zone} / l _channel = 0.12 (Fig.9); similarly - for the proposed die with a specific length of the output zone of 6 mm, that is, with the stated ratio l _{o. zone} / l of the _channel = 0.24 (figures 10 and 11). From a comparison of the patterns of elastic deformations (they differ in Figs. 8–11 in a lighter tone) acting in the known and proposed dies, the following conclusions are drawn:

1) the zone of elastic deformations in the known fiber has a much more developed character, that is, the area of their impact in the known fiber is much higher than in the proposed fiber;

2) a comparison of the zones in one and the other case indicates that in the proposed die (FIGS. 10 and 11) this zone is shifted (shifted) against the direction of drawing in comparison with the same zone in the known die (FIGS. 8 and 9), then there is in the proposed die the load on the tool is distributed more evenly, and thickened sections of both the insert and holder resist the dangerous stresses (Fig. 3);

3) the picture of elastic deformations having an arcuate distribution over the width of the insert (i.e., along the long side of the strip section) is in full accordance with the arcuate view of the fracture of the insert during its destruction (Fig. 1).

The following is an example of a specific implementation of the claimed technical solution when it is used in the technological process for producing a strip of nichrome X20H80-N with cross-sectional dimensions of 4 × 40 mm that meets the requirements of the standard [17].

A conical ingot with a diameter of 85/120 and a height of 650 mm, obtained by filling castings in a cast iron mold, after heating to a rolling temperature in a gas chamber furnace, is rolled on a small-section wire mill 300 onto a billet with a cross section of 6 × 43 mm with winding into a riot. After alkaline-acid etching, washing in water and rolling of the end of the billet, it is pulled on a single drum drawing mill VSG1 / 650 for 4 passes to a finished strip of 4 × 40 mm with intermediate annealing in a shaft furnace at a temperature of 950 ° C and etching after each pass. In the first and second passes, carbide die with a channel, in which the outlet zone is elongated in accordance with the formula of the claimed technical solution, is used as a prerequisite. Specifically: l _{out. the} draw _zone for the first pass as the most loaded is 7.5-8 mm, therefore, the ratio of its length to the total length of the drawing channel is l _{o. zone} / l of the _channel = (7.5 ... 8) / 25 = 0.3-0.32. For the second pass of the route l _{out. zone} = 6.5-7 mm, therefore, the ratio of its length to the total length of the channel (6.5 ... 7) / 25 = 0.26-0.28. In the third and fourth passes, the effect of the increased thickness of the hot-rolled billet (due to the plus tolerance on its dimensions) is practically imperceptible, therefore, the claimed ratio can be maintained in the region of the lower limit of the interval, that is, take it equal to 0.24-0.26. With this value of this ratio, the length of the output zone is l _{o. zones} = (0.24 ... 0.26) · 25 = 6-6.5 mm. Exactly the same values of this ratio are maintained for other sizes of nichrome strips, the number of which in the production of the applicant, as noted above, is 15 positions.

The use of the proposed technical solution for dies used in the manufacture of the applicant upon receipt of a nichrome wire with a diameter of 0.8 to 10 mm (which exceeds the upper limit established by GOST 12.7666.1-90) also showed its practical significance and undoubted effectiveness. Of course, the load on the tool in the case of drawing a wire rod of circular cross section to a wire of finished diameters is significantly reduced compared to the drawing of strip profiles, however, in this case, a significant increase in the operational resistance of carbide drawing inserts against fracture was achieved.

In the production of the applicant, upon receipt of strips of nichrome, intentionally provides for a deviation in the lower side from the axis of the drawing channel of the direction of action of the drawing force applied to the front end of the strip, in order to reduce the diameter of the riot of the stretched strip to the required value determined by the dimensions of the working space of the furnace for annealing the cured metal . Due to the high strength and elastic characteristics of the alloy, a riot after the end of the drawing operation, as a rule, exhibits spring properties, that is, it can unfold arbitrarily, increasing its diameter, if special measures are not provided for to compensate for this phenomenon. One of them is the use of a drum drawing mill with a reduced drum diameter; in particular, in the applicant’s production, instead of the drawing required for this purpose, at the mill VSG1 / 1000, it is carried out at the mill VSG1 / 650. Instead of a drum with a diameter of 1000 mm laid for the VSG1 / 1000 drawing machine, a drum with a diameter of 650 mm is installed on this machine, grinding the original drum or installing it from the VSG1 / 650 machine, which simultaneously increases the maximum drawing force required when drawing strong alloys such as nichrome. This makes it possible to have a diameter of an extended unfolded riot of about 1000 mm, and such a riot is annealed without problems in a shaft electric furnace. However, this measure (deviation of the drag force) leads to additional load on the lower part of the drawing insert and, ultimately, exacerbates its tendency to fracture in the form of a fracture, shown in figure 1. Nevertheless, this tendency was fully eliminated by using the claimed technical solution.

As for the operational stability of the carbide insert, when using the claimed technical solution for the most mass profile with a section of 3 × 30 mm, it is 5000-5500 kg of stretched metal. The destruction of dies for strips of all sizes without exception in the form of a fracture of carbide inserts when using a tool with the declared parameters has stopped completely; dies fail, as a rule, either due to wear of the drawing channel in the form of nichrome sticking (which unacceptably worsens the surface quality of the strips) or due to a decrease in the required dimensional accuracy of the finishing profiles (for the same reason).

The carbide drawing channel in the applicant’s production is obtained by EDM cutting using a precision machine AGIECUT CLASSIK 2S (Switzerland), using an L63 brass wire with a diameter of 0.25 mm as a consumable electrode. The grade of carbide is usually VK8. The machine is equipped with the following devices and systems:

- highly adapted software system;

- feedback of the processing center with the control computer, carried out using optical devices;

- a system for maintaining a stable temperature regime of the working medium — distilled deionized water — in order to minimize thermal distortions and stabilize the parameters of the electric arc;

- air conditioning system;

- high-precision devices providing step discreteness in the process of EDM cutting, equal to 0.1 microns.

In this regard, obtaining the parameters required for the drawing channel according to the claimed technical solution, including the output zone, is quite reliable, that is, with the necessary accuracy.

Information sources

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18. http: /www.solidworks.ru/com/index.html.

Claims (1)

Carbide drawing for drawing products from hard to deform nichrome alloys having a drawing channel, consisting of deformation, calibrating and output zones, characterized in that the drawing channel is made with a ratio of the length of the output zone to the total length of the drawing channel equal to 0.24-0.32 .

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