Metal Forming ProcessesDr. Pulak M. Pandey http://paniit.iitd.ac.in/~pmpandey

Introduction Practically all metals, which are not used in cast form are reduced to some standard shapes for subsequent processing. Manufacturing companies producing metals supply metals in form of ingots which are obtained by casting liquid metal into a square cross section. Slab (500-1800 mm wide and 50-300 mm thick) Billets (40 to 150 sq mm) Blooms (150 to 400 sq mm)

Sometimes continuous casting methods are also used to cast the liquid metal into slabs, billets or blooms. These shapes are further processed through hot rolling, forging or extrusion, to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.

Sequence of operations for obtaining different shapes

Primary Metal Forming Processes

Rolling Forging Extrusion Tube and wire drawing and Deep drawing Although Punching and Blanking operations are not metal forming processes however these will be covered due to similarity with deep drawing process.

Rolling

Change in grains structure in rolling

Salient points about rolling

Rolling is the most extensively used metal forming process and its share is roughly 90% The material to be rolled is drawn by means of friction into the two revolving roll gap The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area The geometry of the product depend on the contour of the roll gap Roll materials are cast iron, cast steel and forged steel because of high strength and wear resistance requirements Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish

In rolling the crystals get elongated in the rolling direction. In cold rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone

The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough. In the deformation zone the thickness of the strip gets reduced and it elongates. This increases the linear speed of the at the exit. Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverses. When the angle of contact exceeds the friction angle the rolls cannot draw fresh strip Roll torque, power etc. increase with increase in roll work contact length or roll radius

Pressure during rollingTypical pressure variation along the contact length in flat rolling. The peak pressure is located at the neutral point. The area beneath the curve, represents roll force.Friction in rolling: It depends on lubrication, work material and also on the temperature. In cold rolling the value of coefficient of friction is around 0.1 and in warm working it is around 0.2. In hot rolling it is around 0.4. In hot rolling sticking friction condition is also seen and then friction coefficient is observed up to 0.7. In sticking the hot wok surface adheres to roll and thus the central part of the strip undergoes with a severe deformation.

Roll passes to get a 12 mm rod from 100 x 100 mm billet

Roll configurations in rolling millsTwo-high and three-high mills are generally used for initial and intermediate passes during hot rolling, while four-high and cluster mills are used for final passes. Last two arrangements are preferred for cold rolling because roll in these configurations are supported by back-up rolls which minimize the deflections and produce better tolerances.

Various Roll Configurations (a) Two-high (b) Three-high (c) Four-high (d) Cluster mill (e) Tandem mill

back

Other deformation processes related to rolling

Forging

Forging is perhaps oldest metal working process and was known even during prehistoric days when metallic tools were made by heating and hammering. Forging is basically involves plastic deformation of material between two dies to achieve desired configuration. Depending upon complexity of the part forging is carried out as open die forging and closed die forging. In open die forging, the metal is compressed by repeated blows by a mechanical hammer and shape is manipulated manually. In closed die forging, the desired configuration is obtained by squeezing the workpiece between two shaped and closed dies.

On squeezing the die cavity gets completely filled and excess material comes out around the periphery of the die as flash which is later trimmed. Press forging and drop forging are two popular methods in closed die forging. In press forging the metal is squeezed slowly by a hydraulic or mechanical press and component is produced in a single closing of die, hence the dimensional accuracy is much better than drop forging. Both open and closed die forging processes are carried out in hot as well as in cold state. In forging favorable grain orientation of metal is obtained

Open and closed die forging

back

Grain orientation in forging

Forging

Machining

back

Barreling in forging

Flash less forging or precision forging

Self reading in forgingFullering Edging Cogging Upsetting Heading Swaging Radial forging etc.

Go through any book on Manufacturing processes by Kalpakjian, Groover or Degarmo

Extrusion

It is a relatively new process and its commercial exploitation started early in the nineteenth century with the extrusion of lead pipes. Extrusion of steels became possible only after 1930 when extrusion chambers could be designed to withstand high temperature and pressure. In extrusion, the material is compressed in a chamber and the deformed material is forced to flow through the die. The die opening corresponds to the cross section of the required product. It is basically a hot working process, however, for softer materials cold extrusion is also performed.

Direct and Indirect Extrusion In direct extrusion metal flows in thesame direction as that of the ram. Because of the relative motion between the heated billet and the chamber walls, friction is severe and is reduced by using molten glass as a lubricant in case of steels at higher temperatures. At lower temperatures, oils with graphite powder is used for lubrication. In indirect extrusion process metal flows in the opposite direction of the ram. It is more efficient since it reduces friction losses considerably. The process, however, is not used extensively because it restricts the length of the extruded component.

Impact ExtrusionIt is similar to indirect extrusion. Here the punch descends rapidly on to the blank which gets indirectly extruded on to the punch and to give a tubular section. The length of the tube formed is controlled by the amount of metal in the slug or by the blank thickness. Collapsible tubes for pastes are extruded by this method.

Hydrostatic ExtrusionIn this process the friction between container wall and billet is eliminated, however, this process has got limited applications in industry due to specialized equipment & tooling and low production rate due to high set up time.

Drawing

Large quantities of wires, rods, tubes and other sections are produced by drawing process which is basically a cold working process. In this process the material is pulled through a die in order to reduce it to the desired shape and size. In a typical wire drawing operation, once end of the wire is reduced and passed through the opening of the die, gripped and pulled to reduce its diameter.

By successive drawing operation through dies of reducing diameter the wire can be reduced to a very small diameter. Annealing before each drawing operation permits large area reduction. Tungsten Carbide dies are used to for drawing hard wires, and diamond dies is the choice for fine wires.

Tube drawing

Tube drawing is also similar to wire drawing, except that a mandrel of appropriate diameter is required to form the internal hole. Here two arrangements are shown in figure (a) with a floating plug and (b) with a moving mandrel The process reduces the diameter and thickness of the tube.

Deep Drawing

This operation is extensively used to for making cylindrical shaped parts such as cups, shells, etc from sheet metal. As the blank is drawn into the die cavity compressive stress is set up around the flange and it tends to wrinkle or buckle the flange.

Deformation of workpiece during punch travel

Back

Defects in drawing

(a)Wrinkling in the flange or (b) in the wall (c) tearing, (d) earing, (e) surface scratches

The effect of wrinkling and buckling can be seen from the way a trapezoid on the outer surface of the blank is stretched in one direction and compressed in another direction to become a rectangle on the cup drawn.

Wrinkling and buckling is avoided by applying a blank holder force through a blank holder. Blank holder force increases friction and hence the required punch load. Therefore, blank holder force should be just enough to prevent wrinkling of the flange. The edges of the punch and die are rounded for the easy and smooth flow of metal. Sufficient clearance is also provided so that sheet metal could be easily accommodated. In sufficient or large clearance may result into shearing and tearing of sheet. A drawn cup can be redrawn into a smaller cup but it must be annealed to prevent failure.

Punching and Blanking Punching and blankingoperations are not metal forming operations but are discussed together with metal forming because of their similarity with deep drawing operation. Objective of punching and blanking is to remove material from the sheet metal by causing rupture, the punch and die corners are not provided with the any radius. Tool steel is the most common material for tool and die. Carbides are also used when high production is needed.

Comparison of metal forming processes

Self reading for your interestDefects in metal forming processes and their remedies. (use Kalpakjians book)

Defects in Rolling

Defects in forging

Defects in extrusion

Surface cracking piping Internal cracking

Ya. I. et Mamuzi, V. N. Danchenko al.: The heat conditions of the cold pilger rolling Ya. V. V.Frolov, Frolov

ISSN 0543-5846 METABK 45 (3) 179-184 (2006) UDC - UDK 539.377:669122.2 =111

The heat conditions of the cold pilger rollingReceived - Primljeno: 2005-10-21 Accepted - Prihvaeno: 2006-03-28 Review Paper - Pregledni rad

The influence of technological factors and constructive parameters of the cold pilger rolling mill on the warm conditions of deformation is observed. The dependences of the heat conditions of pilger rolling on deformation mode and preheating temperature are shown. There was a conclusion drawn about the expediency of combination of warming and cooling of the metal for keeping the deformation zone temperature within limit, which corresponds to the minimal intensity of the metal strengthening. Key words: metal foming, tube, cold pilger rolling, warm deformation Toplinski uvjeti hladnog pilger valjanja. Promatra se utjecaj tehnolokih imbenika i konstruktivnih parametara pilger stana na toplinske uvjete deformacije. Prikazana je ovisnost toplinskih uvjeta pilger stana na nain deformiranja i temperaturu predgrijavanja. Donesen je zakljuak da se isplati kombinirati zagrijavanje i hlaenje metala da bi se temperatura zone deformiranja drala unutar granica koje odgovaraju minimalnom intenzitetu uvrivanja metala. Kljune rijei: oblikovanje metala, cijev, hladno pilgerovanje INTRODUCTION One of the most important finish metallurgical products are seamless tubes. After the hot rolling a part of these tubes goes to the end-user, the other part serves as a billet to the cold pilger rolling. The tubes, made by the cold pilger rolling, are used by traditional and perspective consumers - machine-building enterprises. At this, a consumer makes the higher demand to the assortment and the product quality. The following factors can be related with such demands: the usage of hardly-deformed materials, which have the high users quality and the manufacturing of the tubes with the complicated shape of the cross-section - so called hollow profiles. The preciseness of the geometrical dimensions, the surface quality, the structure and mechanical characteristics of the ready tubes require under-recrystallisation temperatures for finish size deformation. One of the wide-using methods of making such tubes is cold pilger rolling (Figure 1.) [1 - 4]. By this method the part of billet (hot-, cold rolled or extruded etc. tube) 1 that equals to feeding volume, gets into moving zone of working cage 2, where rolls 3 with variable dies radius are installed. The variation of dies radius is provided by special calibrationYa. V. Frolov, V. N. Danchenko, National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukraine, I. Mamuzi, Faculty of Metallurgy University of Zagreb, Sisak, Croatia

of tool. In the rolling process summary deformation zone (working cone) 4 is formed there. Its internal surface is formed by the mandrel 6. The consequent deformation of the parts of billet with the low level of partial strains in working cone is the tube with finish size 5. At the middle of XX century a considerable progress in the field of technology and pilger rolling equipment for manufacturing of precision tubes was indicated. At that time the concepts such as cold and warm tubes rolling were formed, they had a new sense for technology and equipment improveing. PARAMETRES OF HEAT CONDITIONS The metal temperature determines by the equation of thermal balance [5] all conditions of heating and cooling of metal and tool at the pilger tubes rolling Nb + Nf + N = N + N +N + N (1)

where: Nb, Nf, and N, respectively, thermal capacity of incoming components, entering with a billet, from deformation and friction; N, N, N, and N - respectively, thermal capacities of outgoing components, deflected by the rolled tube through contact surface with the rolls, through the mandrel and out to the environment. Thermal balance (1) allows determining the correlation of the income and outcome of 179

METALURGIJA 45 (2006) 3, 179-184

Ya. V. Frolov et al.: The heat conditions of the cold pilger rolling heat as for the deformation zone in total (working cone) and for its separate parts. Apart from the technological factors (initial temperature of metal, type of lubrication-cooling liquid (LCL), value of feeding and rolling route) some mills constructive parameters affect the volumes and ratio of balance components, where it needs to underline the fast motion of the mill (number of double strokes of the cage per minute ) and the way of lubrication-cooling liquid is applied to the working cone. Apart from the appointed factors, the metal temperature is affected by the heat, released from the friction, quantity of which depends on difference of natural and forced rolled radius, tool profile and lubricants effectiveness. In the balance component, reflecting the quantity of heat, released because of the deformation, the character of hardening characteristics of metal right plays the particular role during the deformation process. The length of the working cone determines the duration of the impact of as incoming so and outgoing balances components and cannot be the independent factor, which determines the heating conditions of rolling. - working cage with the rolls has relatively low movement , not much than 100 double strokes per minute; - the length of the part of the tube, rolled per one double stroke of rolls, equals to the product of the drawing out factor and the volume of the feeding , does not exceed 15 20 mm for the steel tubes; - the water emulsion, as LCL, is applied on the summary stroke of cage of its length of focus of deformation (working cone) and on the rolls.

Types of heat conditions At the present time some types of the warm rolling modes for cold pilger rolling mills found their application: - cold rolling - without warming of a billet and with the cooling by lubrication-cooling liquid (LCL); - warm rolling - warming of a billet, absence of cooling by LCL; - rolling without any warming and cooling LCL (emulsionless rolling), at that the metal temperature raises at the expanse of warming, appeared as a result of deformation and friction. It historically developed that under the way of cold rolling of the tubes understood the rolling in the moving rolls cage, when the process of deformation goes under the following circumstances: - the billet-tube comes to the rolls at the room temperature; 180

In such conditions the temperature of the surface of the working cone does not exceed 100 - temperature of the water component of emulsion boiling. The emulsion boiling, because of, for instance, the excessive feeding, appears to be a gross violation of the rolling technology. At such temperature the overwhelming majority of the steels and alloys, used for the tubes manufacturing, do not have the activation of the processes, which counteract the strengthening. Thus, we have a classic cold deformation. The lowering of intensity of the metal strengthening, noted during the billets warming before its feeding to the rolls up to the temperature 150300 and named the effect of the warm deformation, initiated the way of the warm tubes rolling. This method today is wide used on mills, what rolls tubes from austenite steels. The following conditions define the tubes rolling by that way: - the billet is heated before the feeding to the rolls up to the temperature 150 300 ; - mills high speed does not exceed 100 double strokes per minute; - the volume of the product of the factor of drawing out and the volume of the feeding sums up to 60 mm; METALURGIJA 45 (2006) 3, 179-184

Ya. V. Frolov et al.: The heat conditions of the cold pilger rolling - some salty lubricant is applied on the billet, cooling by the liquid of the focus of deformation and rolls does not exist. The temperature of the working cone under that condition comes up to 500 . Despite to that lowering of the strengthening intensity [6] were discovered in the major steels as the temperature rises, most expansion the way of the warm rolling has received for austenitic steels. The main reason of that is the continuous nature of lowering of the level of strengthening character with the temperature raising within the interval from 100 up to 500 right for that kind of steels. For other materials the nature of changing of the level of strengthening character is not of one single meaning within that temperature interval (Figure 2.). For example, ball bearing steel ShH 15 (by GOST) in that temperature interval shows change of reducing and increasing of hardening intensity. Steel 20 (by GOST), deformed with 40 % strain shows slow increasing of flow stress for all researched temperatures. From the Figure 3., it is possible to see the differences in behavior of Steel 20, during the deformation with 10, 20, 40 and 60 % strain. The flow stress is increasing with the level of stain at all temperatures. The reason for is an common using of temperature increasing and temperature reducing factors. In order to lower the maximal temperature of the working cone during the steels rolling, which have the range of the lowering of level of the strengthening characteristics lays in the field of lower temperatures, than austenitic steels do, there is a variety of the ways of warm rolling without prior heating of the billet and fluid cooling of the working cone - emulsionless rolling. Emulsionless rolling is characterized by the following conditions: - tube-billet comes to the rolls at the room temperature; - mills high speed does not exceed 100 double strokes per minute; - the volume of the product of the factor of drawing out and the volume of the feeding sums up to 25 mm; - some salty easily melted lubricant is applied on the billet, cooling by the liquid of the focus of deformation and rolls does not exist. The temperature of the working cone under that conditions comes up to 400 450 . The experience of use of the emulsionless way of rolling revealed that the achieved increase of the degree of the deformation in this case is accompanied by the major lowering of the tools resistance and deterioration of the tubes surface quality. The influence of deformation parameters on HEAT CONDITIONS OF PILGER ROLLING Let us examine in details the appointed factors, which determine the actual temperature of the working cones metal. The results, shown below, are based on the experimental research, carried out in the State Tube Institute (Dnepropetrovsk, Ukraine) for rolling tubes made of chrome-

activity of the dynamic deformation aging process [5, 7]. Increasing of strain moving the starting point of this process to the side of low temperatures. Cold pilger rolling with low partly strains gives a possibility of stopping intensive increasing of flow stress and using the reducing of hardening intensity. Due to low level of partly strains on working cone, in pilger rolling using of the effect of the warm deformation for other hard deformed materials is possible. Necessary condition of that is effective METALURGIJA 45 (2006) 3, 179-184 181

Ya. V. Frolov et al.: The heat conditions of the cold pilger rolling nickel-titanium steel 08-18-10 on the mills HPT32 (charts 1 on the drawing 4 - 7), HPT55 (charts 2 on the drawing 4 - 7) and HPT90 (charts 3 on the drawing 4 - 7). These mills are intended for rolling of billets with external diameter 32, 55 and 90 mm. The dependences of the maximal increase of the temperature of the working cone (t) on the initial temperature of the billet, shown on the Figure 4., have the identical character: at the raising of the initial temperature of metal (t0), increase of the temperature diminishes. As it follows from the Figure 5., rising of the number of the double strokes of the cage and the increase of the metal temperature, at the other conditions equal, tighten between each other by the linear dependence. - HPT32, the dependence has a linear character, but coming to the mills of a grater dimension-type - HPT55 and HPT90 the intensity of the temperature increase diminishes. It is seen from the Figure 7., that the factor of drawing out puts strong influence upon the temperature increase of the metal, at that the intensity of this influence practically does not depend on the mill dimension-type.

The intensity of the volume of feeding dependence on the temperature increase (Figure 6.) depends on the dimension-type of the mill: for the mill of small dimension-type

Also during the process of the experimental research it was determined that during the rolling without fluid cooling the rolls after 60 80 minutes, as the work started, come off to the stable level of the temperature, which corresponds to 220 250 . The influence of the heat exchange between the metal and the mandrel, in case last does not cool forcedly, is assumed to be considered as inessential and not to take into account when calculating. The improvement of the cold pilger rolling mills by the way of usage of the weight balancing of the inertial powers of the moving mass of the working cage and con-rods gave the opportunity to essentially raise the speed of the mills. This brought to that the metals temperature exceeded 100 , and the water emulsion started to boil on the working cone. The emulsion boiling on the working cone brings to the appearance of the steam cover on its surface, which sharply lowers the effectiveness of the heat removal. This circumstance and also a wish of improvement of the quality of the rolling tubes surface stipulated for usage as LCL of the compositions on the basis of mineral or synthetic oils, so called oil LCL. Such LCL, though they lose to the water emulsion intensity of the heat removal at the temperature up to 100 , but save their characteristics at the temperature rising. The method of measurement of working cone temperature [4] allows calculating change of temperature along the length of a cone in different modes of rolling. One of METALURGIJA 45 (2006) 3, 179-184

182

Ya. V. Frolov et al.: The heat conditions of the cold pilger rolling the features of this method is calculating of the increase of the temperature from deformation heat by experimentally confirmed dependency: tf = t0 en t0 where: t0 - initial (preheating) temperature for partly deformation on working cone /, n - empirical factor, - strain /%. Diagrams of rated distribution of temperature along the length of working cone for rolling with emulsion and low-viscosity oil cooling on route 57 3,75 35 1.5 mm on mill HPT55 with steel 20 (by GOST) and feeding m = 8 mm are shown on Figure 8. The calculations were made with assumption that in this case supply of coolant is made through round nozzles installed on working cage. (2) sented. Experiment were carried out without liquid cooling of working cone and rolls. The measurements of temperature of working cone were performed on the start - charts a, middle - charts b and finish of working cone - c.

However, even with application of progressive methods of supply of coolant to working cone on rolling mill KPW 25 VMR, temperature of metal is exceeding 100 . So, for example, at rolling of stainless austenite steel pipes without preheating with m = 25 mm and n = 200 min1, temperature of metal runs up to 220 . At this temperature, the same as in case of emulsionless rolling, there can be an effect of warm deformation, which decreases power of rolling, or activation of deformation aging process, which impedes deformation. The different types of steel show different reaction on common influence of preheating and strain during pilger rolling. On the Figures 9. and 10. experimental dependences of vertical component of rolling force (VRF) and working cone temperature (WKT) on preheating temperature are preMETALURGIJA 45 (2006) 3, 179-184

As it can be seen from Figures 9. and 10., the temperature of working cone reaches to the level of 300 600 . It renders a negative influence on stability and lifetime of rolls, mandrels and technological grease, though reducing of vertical component of rolling force (VRF) and absence of destruction of metal gives possibility to intensification of process. Most effective way of overcoming this contradiction is precision cooling of working cone and rolls. Taking the above-mentioned examples into account, there is a doubt about appropriateness of term cold rolling to intensive modes of deformation. It concerns, particularly, rolling at long-range high-speed rolling mills on which the process of non-stop rolling is realized (KPW HMRK). The process of rolling without shutdown of the mill and switching-off the feeding at recharging of another billet favours to stabilization of temperature on higher level at the expense of elimination of temperature 183

Ya. V. Frolov et al.: The heat conditions of the cold pilger rolling losses during shutdown of the mill for recharging. Thus, modern modes of squeezing characterized by capacity of mills about 200 - 1600 mph, practically, leave no chance to classical cold rolling. narrow (up to 30 ) temperature interval, where durability characteristics of metal are on the lowest level and rolls and mandrel has satisfactory lifetime. These methods are: preliminary heating of billets and usage of heat from deformation and contact friction, together with fluid cooling of working cone and rolls. CONCLUSIONS 1. The intensive modes of the deformation of the precision pipes rolling with the modern pilger rolling mills do not appear as cold rolling, even if prior heating, before the billet comes to the rolls, does not exist. At such modes of deformation the achievement of the warm deformation temperatures happens at the expense of the heat of forming and friction without prior billet warming. 2. The cold pilger rolling in condition of combination of metal preheating with using of effective lubricationcooling liquid and methods of cooling gives possibility to hold working cone temperature at narrow interval, where intensity of hardening is lowest. REFERENCES [1] I. Mamuzi, V. M. Drujan, Theory, materials and technology for steel tubes, Hrvatsko metalurko drutvo, Zagreb 1996. [2] M. Kostelac, Tehnologije hladnog pilgerovanja elinih cijevi, elaborat, Institut za metalurgiju Sisak, Sisak 1987. [3] Machinery and Equipment to the Manufacture of Tubes Using Cold Pilger Process, Mannesmann Demag Httentechnik, Bericht, Mnchengladbach, 1986. [4]Das Kaltwalzen von rohren, Meer - Bericht, Demag - Meer, 151/4. [5] V. F. Frolov, V. N. Danchenko, Ya. V. Frolov, Holodnaya pilgernaya prokatka trub - Dniepropetrovsk: Porogi, 2005. [6] G. I. Gulyaev, V. F. Frolov, V. V. Sergeev, V. V. Frolov: The ways of raising draft per pass at cold tube pilger mills, - 43rd MWSP CONF. PROC., ISS 39 (2001), 149 - 162. [7] Yu. V. Zilberg, Ya. V. Frolov: Opredelenie napryazheniya tekuchesti po eksperimentalnim znacheniyam sili prokatki // The modern problems of metallurgy. Naukovi visti. Plastic metal deformation. Dniepropetrovsk: System technologies (2002) 5, 186 - 190. [8] Ya. V. Frolov: Opredelenie temperaturi pri holodnoy pilgernoy prokatke trub, Metallurgical and coalmining manufacturing (2003) 3, 326 - 341.

Moreover, there is growing actuality of usage of warm deformation effect not only for austenite steels, but for other steels and alloys as well. Effectiveness of deformation modes during periodical rolling of pipes can be increased by methods of retention of working cone temperature in

184

METALURGIJA 45 (2006) 3, 179-184

Pipe & Tube ProductionUDC 621.774.36

Effect of Deformation Conditions on Mechanical Properties of Metal During Tube Cold Pilger RollingYa. V. FrolovNational Metallurgical Academy of Ukraine 4 Gagarin Ave., Dnipropetrovsk 49600, UkraineDetermined quantitative index allows estimating the impact of thermal, kinematic and deformation parameters of a process on the change of mechanical properties of material during tube cold Pilger rolling. The equation of connection of deformation conditions and change of mechanical properties of metal during cold Pilger rolling was obtained. Keywords: DEFORMATION, PARAMETERS, EFFECT MODE, MECHANICAL PROPERTIES, METAL, COLD PILGER ROLLING, TUBES

Introduction Both deformation and thermal factors affect the change of mechanical properties of metal through cold Pilger rolling. Final temperature and mechanical properties for each previous control cross-section of working cone become initial for the subsequent cross-section. Moreover, this process is continuous lengthwise working stand. Accordingly, deformation conditions during cold Pilger rolling should change along with the change of characteristic for metal which is in the current cross-section of total deformation zone. Following methods are currently applied to estimate mechanical properties of metal at cold Pilger rolling: 1. Reference data derived from standard testing on tensile-testing machines. The advantage of this method is the ease of use and a great scope of experimental investigations performed. At the same time, yield strength and ultimate strength, which are fundamental characteristics of metal, and functions presenting their change depending on amount of reduction (hardening curves) do not consider the effect of heat released in the process of treatment and effect of relationship between diameter and wall thickness reductions. 2. Yield stress values obtained by average pressure recalculation when rolling quadrangular pieces [1]. This method considers the effect of heating-up but does not cover the impact of relationship between diameter and wall thickness reductions. 3. Values of ultimate strength and yield

strength obtained in the process of testing pieces cut out from the working cones on tensile-testing machines. Application of this estimation method enables deformation conditions to be completely compatible and to consider the impact of relationship between diameter and wall thickness reductions. The disadvantage of the method is an inefficient consideration of heating-up impact. However, this disadvantage can be eliminated by using analytical dependences for determination of current temperature of working cone metal when calculating the deformation conditions [2]. Results and Discussion Determination of index of metal mechanical properties change through cold Pilger rolling. The ratio of ultimate strength US to yield strength YS obtained when testing micropieces cut out from a working cone and accomplished according to technology GTI 241-26-02 [3] makes it possible to determine a limit of mechanical properties of metal k and metal recovery S under conditions of cold Pilger rolling by following dependences: - for stock tube

- for finished tube after rolling

USst k st YSst USft YSftk

(Eq. 1)

ftx of

(Eq.2) total

- for

control

cross-section

36

Metallurgical and Mining Industry, 2010, Vol. 2, No. 1

Pipe & Tube Productiondeformation zone

USx YSxkst k

kx

(Eq.3)

- to estimate the usage of limit of mechanical properties in total deformation zone

ft

S

(Eq.4)

- to estimate the usage of limit of mechanical properties in the area of total deformation zone before control cross-section x

kst kx

Sx

(Eq.5)

Change of mechanical properties under effect of cold and warm Pilger rolling was estimated for following groups of processing routes: Group 1 method of tool calculation providing a tapered plug and curve line of pass at cold rolling 4 routes (1 for a large size mill, 2 for medium and 1 for small); Group 2 method of tool calculation providing a tapered plug and curve line of pass at warm rolling 4 routes (3 for a large size mill and 1 for small); Group 3 method of tool calculation providing a tapered plug and curve line of pass at warm rolling with tube supply increased by 50% 4 routes (2 for a large size mill, 1 for medium and 1 for small); Group 4 method of tool calculation based on reduction proportionality principle at cold rolling 6 routes (3 for a large size mill, 2 for medium and 1 for small). Mechanical tests were carried out within one batch of tubes made of steels AISI 304, 316 and 31803 for each case of rolling. As a result, it was determined that limit of mechanical properties for all rolling methods decreases after rolling. For stock tubes, the range of k values was 1.56-2.4, and for tubes after rolling 1.01-1.5. Value of mechanical properties usage S for all methods of calculation and rolling methods is within the interval 1.44-1.98. Analysis of these results showed that the bigger S value is, the more intensively the limit of mechanical properties of metal is used, which negatively characterizes the process of deformation and increases the probability of microfractures in metal. Equation for connection of deformation parameters and mechanical properties of metal at cold Pilger rolling. At cold Pilger rolling, metal is deformed along a working cone, therefore the most

widespread coordinate axes for the description of change for the process parameters are x/l the ratio of control cross-section place to working cone length and total amount of reduction which is composed of diameter reductions D and wall thickness reductions t. In the investigated rolling routes, the relative engineering strain changed from 28% up to 98%, by diameter from 24% up to 67%, by wall thickness from 21% up to 57%. For above-specified rolling cases, the dependences for the change of mechanical properties usage on total deformation S=f(), ratio of diameter reductions and wall thickness reductions S=f(D/t), amount of diameter reduction S=f(D/), and wall thickness reduction in total deformation S=f(t/) were constructed. An increase in amount of diameter reduction D/ in the rolling route is determined to cause a stable growth of S values with various intensity for all cases of rolling (Figure 1).2.2 2 1.8 1.6 1.4 1.2 1

Group 1 Group 3 Group 2 Group 4

0

0.2

0.4

0.6

0.8

1

Figure 1. Interrelation of usage for mechanical properties of metal on amount of diameter reduction in total deformation

Determination of boundary data for dependence S=f(D/) showed that at D/=0 Pilger rolling process was impossible from the kinematic point of view, and accordingly the limit of tube mechanical properties was equal to the limit of billet mechanical properties. At D/=1, there is no wall thickness deformation which provides the maximum values of usage for mechanical properties. This conclusion corresponds to known literature data about negative impact of free reduction on mechanical properties of tubes. Analysis of observed dependences revealed the fact that change of usage for mechanical properties S on the amount of diameter reduction in total deformation D/ could be presented as follows:

Metallurgical and Mining Industry, 2010, Vol. 2, No. 1

37

Pipe & Tube Production(Eq. 6) where bD factor of mechanical properties usage (for route group 1 it equals to 0.97, for route group 2 0.92, for route group 3 0.7, and for route group 4 0.53). Rearrangement of dependence (6) enables to obtain the equation of connection for deformation parameters and mechanical properties of metal at cold and warm Pilger rollingln(

S ebD ( D / )

2. The quantitative index for estimation of the effect of thermal, kinematic and deformation parameters of the process on change of mechanical properties of material during tube cold Pilger rolling is obtained. 3. The equation of connection of deformation conditions and change of mechanical properties of metal at cold Pilger rolling is deduced. References1. Yu. V. Zilberg and Ya. V. Frolov. Suchasni Problemy Metalurgii. Naukovi Visti, Dnipropetrovsk, 2002, Vol. 5, pp. 186-190.* 2. Ya. V. Frolov. Metallurgicheskaya i Gornorudnaya Promyshlennost, 2003, No. 3, pp. 57-59.* 3. Ya. V. Frolov, A. A. Tereshchenko, S. S. Dudka. Bulletin Naucho-Tekhnicheskoy Informatsii Chernaya Metallurgiya, 2009, No. 9, pp. 61-63.* 4. V. F. Frolov, V. N. Danchenko, Ya. V. Frolov. Tube Cold Pilger Rolling, Porogi, Dnepropetrovsk, 2005, 358 p.* * Published in Russian

USst YSft YSst USftbD

)

D /

(Eq. 7)

Values of factor bD allow comparing the methods of tool calculation in view of impact of thermal, kinematic and deformation parameters of the process on change of mechanical properties of metal during Pilger rolling. Values of factor are in the range from 0, which corresponds to conditions of "ideal" process (mechanical properties do not change), to 1, which is a sign of irrational process (the limit of mechanical properties is used with maximum intensity). Tool shape, stage-by-stage deformation and thermal mode of rolling have the main impact on intensity of usage of mechanical properties. Dependence of factor bD on the specified parameters for investigated rolling routes is described by the following expression:

Received December 02, 2009

. . , , . .

bD

(Eq. 8) where m supply (6-9 mm); cumulative elongation ratio (1.7-7.6); kb factor considering the cross-section form of working cone (0.2-0.27); n factor of stage-by-stage deformation (9.4839.8) which depends on the length of total deformation zone determined by tool shape; k limit of mechanical properties of stock tube which, in this case, allows considering the change of mechanical properties depending on warm deformation effect. When comparing the values of factor bD obtained under dependence (6) for route groups and for each route (8), it is clear that for great values of cumulative elongation ratio it is necessary to use great values of factor kb considering the form of cross profile of working cone. Conclusions 1. Increase of reduction amount in total deformation reduces the difference between ultimate strength and yield strength of metal for all methods and modes of cold Pilger rolling.

m 1 k kb n

38

Metallurgical and Mining Industry, 2010, Vol. 2, No. 1