SE448555B - KIT AND PROCEDURE FOR MANUFACTURING HEAT-SPRING SPRINGS - Google Patents

Description

448 555 In the ovens, the workpieces are heated to a normalization temperature, ie above the temperature AC3, and they are cooled after the forming process. During normalization, heat penetrates through the surface of the workpiece and increases its temperature in accordance with the properties of the material and the heat transfer laws. It is known that the temperature in the inner parts of a workpiece compared to its outer parts decreases after an exponential function. How much the temperature decreases depends on the material's properties and dimensions as well as the annealing time.

From the above it appears that the surface of the workpiece reaches the normalization temperature much earlier than its interior. At the beginning of the annealing, an increase in the austenite grains already begins on the outside, while the temperature inside the workpiece is still below the ACS temperature.

This circumstance is particularly disadvantageous in the manufacture of springs because, in addition to the grain enlargement, a surface oxidation also occurs and at the same time there is a risk of decarbonisation.

Since during the work of the springs the outer fibers are always loaded more heavily than other parts, the said inconveniences in principle impair the mechanical properties and service life of the springs.

Another detrimental effect of the grain enlargement is that this increases the risk of dislocation movements and associated slackening of the spring.

As a result of said phenomenon, the practical life does not even exceed the value 106 for springs with the highest quality.

The present invention has for its object to provide a method and device which makes it possible to eliminate said inconveniences and to avoid the grain enlargement occurring during heating and its harmful effect. Furthermore, the Q should enable the use of such advantageous heat treatment methods as cyclic normalization, bainite curing and impact annealing. According to the invention, grain enlargement, which otherwise occurs in the manufacture of thermoformed springs during normalization, is prevented by heating the workpieces to a maximum temperature of Ac3. + 10 ° C, the heating taking place by direct resistance heating for a maximum of 150 seconds.

An apparatus for carrying out the method according to the invention has a heating unit, a forming unit and a cooling unit, the heating unit being arranged between the forming unit and the cooling unit and having built-in current lines built into a sloping path, abutments arranged next to the current lines, the workpieces fixing clamping units and cutting devices arranged next to the clamping units.

The invention is based on the insight that an improvement of the strength values of the springs can only be achieved if the fineness and uniform distribution of the grains of the material are ensured.

This is achieved according to the invention on the one hand by heating the entire cross section of the material at the same time due to direct resistance heating and the applied temperature does not exceed the temperature Ac3 + 10 ° C, thereby already in itself preventing a grain enlargement. In addition, the electromagnetic field arising during heating increases the crystallization rate, whereby the grain fineness is also affected in a favorable direction. In spring production, direct resistance heating is thus applied in such a way that normalization temperature and time are lower here resp. shorter than in known procedures. All these factors together lead to optimal conditions during normalization.

The method according to the invention also enables the use of cyclic heating already known per se, which also advantageously affects the mechanical properties of the product. The cyclic normalization consists of repeated warming to the temperature range between AC3 and Ac3 + 10 ° C and of cooling cycles between the individual heatings. 448 555 The number of cycles must be determined in accordance with the quality and properties of the material in order for the material to obtain a completely uniform austenite structure. As a result of the cyclic normalization, the grain fineness and thus also the strength are further improved.

The invention also enables the use of such advantageous methods as bainite hardening and abutment tempering in spring production. Through the bainite hardening, the mechanical data of the springs can be further improved and through the impact-wise tempering, tempering brittleness can be avoided.

Due to the bainite hardening in spring production, martensite structures can be avoided. Thus, no particularly unfavorable stress conditions occur with springs, which leads to better parameters for strength, impact work, elongation and contraction of the material thus treated. At the same time, the adherent dimensions of the same material quality are also increased. This is achieved by the fact that at the end of the heating the interior of the heated material has a higher temperature than its outside.

The solution according to the invention can be applied in the manufacture of many kinds of springs, for example helical springs, leaf springs, stabilizers, anti-roll bars, etc. Further details of the invention are explained in more detail in connection with exemplary embodiments and the accompanying drawing. Fig. 1 shows a front view of a heating unit for the device according to the invention, Fig. 2 a section along the line 2-2 through the unit shown in Fig. 1, Fig. 3 a cross-section through a container for the cooling unit and Fig. 4 a section along line 4-4 through the container shown in Fig. 3. n; The heating unit shown in Figs. 1 and 2 is provided with a known feeding device. Rods 2 are laid on the gutter 1 of the feeding device by means of a crane, the diameter of which corresponds to the set dimensions of the springs. The rods 2 slide on the oblique channel 1 towards the charging discs 3 and end up in the grooves arranged on the periphery of these charging discs 3 in step with the periodic rotation of the charging discs. During the rotation of the loading discs 3, the rods 2 roll out of the grooves on the opposite side and end up on a sloping path belonging to the heating unit 5. On the side of the loading discs 3 facing the sloping web 5 there is a cover 4 to prevent early rolling out of the rods 2.

The rods 2 on the inclined path 5 pass through a spring-displaceable flap port 10 into a housing 9 surrounding a heating unit and get stuck on abutment 6. It is suitable to delimit the path of the rods 2 on the inclined path 5 by means of guide plates. In this way, the rods 2, after abutment against the abutments 6, lie with their ends exactly on the current conductor 7 of the heating unit. The current conductors suitably consist of cooled contact surfaces of copper.

The rods 2 are pressed with a clamping unit 8 against the current conductors 7.

The clamping unit 8 is suitably operated hydraulically. Adjacent to the clamping unit's 8 clamping jaws resp. the current conductors 7 there are cutting devices l2. The rods 2 are cut through these cutting devices 12 to the desired set length. Both the current conductors and the clamping jaws of the clamping unit 8, but also the units of the cutting device 12, are on one side adjustably arranged so that the power supply, fixing and cutting can be adjusted for arbitrary detail dimensions.

The heating unit is suitably surrounded by a closed housing 9 so that the temperature of the rods 2 is not affected by temperature differences Qenumnt the surroundings and so that the rapid heating of the material resp. its annealing must be able to be performed with an accuracy of ÉZOC. To maintain this accuracy, the heating unit is provided with a temperature sensing means connected to a control device, which, however, for the sake of clarity, is not shown in the drawing. The housing 9 is provided on both sides with a flap door 10, which is opened through the bars at their in- resp. rollout. The closing of the flap doors 10 takes place automatically by means of spring force. 448 555 The rods 2 rolling out from the heating unit and cut to the desired length now end up on transport rollers ll. Several heating units are suitably connected to the rolling path formed by the transport rollers 10. The heating units can symmetrically output to the roller coaster from two sides. In this way, the relatively long heating cycle time is divided and comes closer to the cycle time of other work steps. If, for example, the heating time is l00 sec. and the feed time 20 sec., the cycle time of a heating unit is 120 sec. However, if within a given system on both sides of the roller shutter, three heating units are each arranged, the output interval time for this system amounts to 20 sec.

From the roller walkway, the heated rods 2 end up in a winding device 3, in the manner of the rods 2 screw springs are manufactured in a manner known per se. The hot-formed and still austenitic workpieces now end up on a sloping path 14 shown in Figs. 3 and 4. From there, they roll down into the compartments of an inclined feeding device projecting into the container 15.

The inclined feeding device is suitably designed as a belt conveyor 16 in which the springs are transported at a given speed to the cooling medium present in the container 15.

Workpieces fall out of the belts conveyor 16 pockets only when their temperature has already returned to about 4000. In the refrigerant, the workpieces sink to the inclined bottom of the container 15 and roll on this inclined surface into the pockets of a dispenser. The discharge device is designed as a vertically arranged belt conveyor 17.

By adapting the height of the container 15 and the speed of the belt conveyor 17 by suitable dimensioning, the workpieces can be cooled for a precisely defined time.

The cooling unit suitably contains several similar containers, from which the finished workpieces end up in separate or common collection systems. In the device according to the invention, a detailed heating element is solved. Q ning by direct resistance heating. After the rods 2 ~ 'É are fixed to the current conductors 7, electric current with a minimum voltage is supplied to a working piece from a three-phase transformer. The temperature of the workpiece increases rapidly due to EA of resistance heat. As the rate of increase decreases exponentially with temperature, the last part of the heating proceeds to normalization temperature relatively slowly. Thus, the control device can with great accuracy break the current when the normalization temperature is reached.

If the heating is performed cyclically, the temperature of the workpieces must be increased to a very small extent above the temperature G3. When this temperature is reached, the current through the temperature sensing means is switched off. At the set lower temperature value, the power switches on automatically and the cycle starts again. The cycles are repeated until the completely austenitic state is reached.

An additional step of the heat treatment can be performed in different ways. The hardened material is carried out after rapid cooling in an oil bath, degreasing with lye, tempering in a salt bath and finally cooling in cold water.

According to the invention, the rapid cooling can also take place according to a technology corresponding to bainite curing. In this case, the rapid cooling takes place in a salt or metal bath.

The temperature of the bath must be higher than that corresponding to the line Ms. The details are then cooled in cold water.

The applicability of bainite curing and the increase of the bainite curable cross-sections at the same material quality have also led to a significant improvement in the mechanical properties. The bainite-curable dimensions are increased by the fact that during the direct resistance heating at the end of the heating, the interior of the material becomes warmer than its exterior. Namely, the current is distributed uniformly in the material to be heated and heats the same uniformly, but the heat losses occur first and foremost on the surface. At normal mains frequency, the "skin" effect can be neglected.

In the case of a sudden tempering, the heating rate of the part is lower than the heating rate during normalization. After heating, the parts must be cooled immediately in cold water. .r _ \ _______ p i PDUR ¿448 555 .i 8 After the heat treatment, the final manufacturing steps can be performed in a known manner.

In the device according to the invention, the refrigerants containing the containers can be easily repositioned, depending on the technique used. At the same time, it is ensured that the cooling media present in the containers can be set to the desired temperature. For this purpose, the containers are equipped with heating and cooling units.

Thanks to the solution preferred according to the invention, an ideal normalization of the spring material can be achieved. As a result, the ultrafine grain structure also occurs in spheroiditis and bainite formed during the heat treatment. The material flow limit of the heat-treated springs thus treated is considerably increased. At the same time, the excellent grain fineness limits the dislocation movements, whereby translational and other after-effects are reduced (elastic after-effect, slackening, elastic synthesis, Bauschinger effect).

The invention enables combined heating with direct resistance heating and various advantageous heat treatment methods, eg bainite curing. During heating, cyclic heating methods can also be applied. Due to these factors, the springs manufactured according to the invention have better mechanical properties than conventional springs. If no oxidation or decarbonization occurs during the heat treatment, the dimensions of the finished parts become more precise, the amount of heat required for the heat treatment amounts to only 15% of the amount of heat used in conventional methods and the tool consumption is reduced.

With the device according to the invention, different springs can be manufactured within a very wide dimension range. With this device, the manufacturing process can be fully automated.

By using the heating unit according to the invention during tempering, tempering brittleness can be avoided. Some low-alloy steels, especially Cr- resp. Cr-Mn steel is H m .__ U 9 448 555 namely in temperate state rather brittle. The embrittlement usually occurs during tempering between normal tempering temperatures of 300 - 550 ° C resp. in such cases where steel with a tendency to temper brittleness is slowly heated.

Namely, there is a possibility, on the border of former martensite crystals, for coarse separation of the brittle phase. However, if the annealing takes place quickly and intermittently by direct resistance heating, this phenomenon can be avoided, since the temperature ranges with emerging annealing embrittlement are very quickly passed and left. Thus, there is no time for the formation of the brittle phase at the crystal boundaries. The transformation of the martensite into a tempering structural element takes place over the entire cross-section but also at the same time inside the individual crystals, so to speak explosive, which leads to an extremely uniform structure. This structure may well withstand dynamic effects.

The production line according to the invention can be built up with relatively low investment costs, has little space requirement, and the manufacturing costs are 30 - 40 f lower than with conventional methods.

The metallographic structure, the mechanical properties and the service life of springs, manufactured according to the method according to the invention, have been significantly improved compared to conventional solutions.

Concrete examples for using the method according to the invention are presented below.

Example l Steel, quality 50 CrV4, coil springs were manufactured in two embodiments. One has 10, 1 mm diameter and an extended length of 1960 mm and the other 11, 25 m and an extended length of 2430 mm.

During manufacture, the workpieces were heated within 92 - 95 sec. to a temperature of 81 ° C. The heating of the 10.1 mm diameter rods took place with 223.6 kcal and the second embodiment with 369.8 kcal. The electrical voltage was 16.6 resp. 20.4 V.

After molding, a bainite hardening of the workpieces took place in a salt bath at 30 ° C. The bainite curing time was 20 minutes.

The cooling was performed in water at room temperature.

The springs thus manufactured have a bainitic structure.

The Bainite's share in the structure was 99% and was uniformly distributed. The grain fineness amounted to l3. . The hardness of the spring material was between 48 and 50 HRc. För- nåilanaetø ”F / xfß amounted to 0.93.

II I).

The static deposition losses were 1.06 resp. 1.62%.

In lifetime tests, the springs were subjected to 2 x 106 loads repeatedly without breaking. During the service life tests, the frequency was 0.2 Hz and the springs were also subjected to dynamic loading.

Example 2 Some of the helical springs described in Example 1 were manufactured so that they were toughened after heating. During the toughening, hardening oil with a temperature of 500 ° C was used as the cooling medium.

After curing, degreasing was carried out in a lye bath at 60 ° C.

The material was then rinsed with water at room temperature.

Thereafter, tempering of the material was performed in a salt bath at 320 - 330 ° C. The tempering time was 20 min. After tempering, cool with water of room temperature.

The details thus manufactured had a spheroidite structure.

The distribution of the spheroidite was completely uniform, the grain fineness amounted to l2.

The hardness of the material was 49 - 51 HRC and the ratio 5 F / ¿B was 0.9. The static settlement losses amounted to between 4.5 and 6%. The lifetime tests were performed up to 1.6 x 106 loads. The test frequency was 0.2 to 0.3 Herz. n 448 555 §xempe1 3 From material 50 CrV4, leaf springs were made down 900 mm long, 80 mm wide and 6 mm thick. The material was heated to 802 ° C cyclically. The number of cycles was 8, with the first warm-up lasting 90 sec. and the cooling 8.4 sec. The second warm-up lasted 17 sec. and the cooling 8.4 sec. The additional cycles were identical to the other cycle. The lowest temperature at the cycles was eoo ° c.

The lower temperature value of the cyclic heating was 60 ° C. For heating inherits 1757 kcal.

After forming, the material was bainite cured in a salt bath at 308 ° C for 20 minutes. It was then cooled in water at room temperature.

The workpieces had then been given a bainite structure with 99% bainite, whereby the distribution of the bainite was completely uniform.

The grain fineness amounted to 14.

The hardness of the material was 48 - 49.5 HRC and the ratio d'r / 6'B was 0.95.

Example 4 A stabilizer with a diameter of 20 mm and a length of 2400 mm was made of steel 55 Cr3.

During production, the normalization temperature amounted to 8340 C, whereby the heating took place continuously for about 100 sec. For heating, 1037.3 kcal was used and the voltage used was 13.8 V. The cooling took place in hardening oil at 50 ° C.

The details were treated by bumping. The tempering temperature was 41 ° C, the tempering time 100 sec. The tempering took place with 10.6 V voltage and the cooling took place in water at room temperature. 448 555 U The metallographic structure of the workpieces was also bainitic here with 98 ~ 99% bainite and a completely even bainite distribution. The grain fineness amounted to 12.

The hardness of the material was between 50.5 and 52.5 HRc and the ratio 5 F / ø'B was 0.91. u; We The static settlement losses were between 1.4 and 2.5%.

Lifespan tests were performed with up to 2 x 10 6 loads and 0.2 - 0.3 Hz. ~ = ¿Example 5 Coil springs according to examples 1 and 2 were manufactured according to a conventional method. During manufacture, the rods were heated in an oven to 830 - 860 ° C and then formed.

After molding, the workpieces were toughened in an oil bath at 50 ° C and then run alone at 330 to 40 ° C.

The springs thus manufactured showed a spheroidal structure, but the grain distribution was rather uneven and the carbides appeared in spots in coagulated form. The grain fineness was usually S-9.

The hardness of the springs had a spread of É 4 to 5 HRc, in contrast to the springs in examples 1 - 4, where the scattering weather amounted to É 1 - 1.2 HRC. In the material of the conventionally manufactured springs, the F / most amounted to 0.9. During the lifetime experiments, all springs were broken already below the value 0.5 X l06.

From the above it appears that by means of the method according to the invention a product can be manufactured whose structure is significantly more advantageous than with products manufactured according to conventional methods. Thus, this new product has in principle better mechanical properties and a longer service life. At the same time, the spring production according to the invention is significantly simpler, cheaper and faster than E according to conventional methods. e

Claims (11)

10 15 25 30 448 555 13 PATENT REQUIREMENTS A method for producing hot-wound springs, wherein elongate spring wire along a heating distance of a predetermined length is heated by direct resistance heating and the heated spring wire is hot-wound into a spring and then cooled, known for austenitization to a temperature of not more than 10oC of that the spring wire is heated in and over Ac3 and for a maximum of 150 seconds, the spring wire in the form of a wire section of the same length as the heating distance being held in it. 2. A method according to claim 1, characterized in that the heating is carried out cyclically. Process according to Claim 1 or 2, characterized by curing. 4. A method according to claims 1 or 2, in that the cooling takes place by means of a bainite feature, in that after cooling, a continuous annealing is carried out, also that using direct resistance heating. Device for the manufacture of hot-wound springs from rods, which by direct resistance heating for a maximum of 150 seconds are heated to a temperature between Ac3 and Ac3 + 10 ° C, wound and then cooled, characterized in that between a rod supply device (1, 3) and a forming device (13), which opens into a cooling container (15), is arranged a heating device (6 - 12) designed for direct resistance heating of the rods (2) on an inclined path (5) and that the heating device at the sides of the web has in it built-in current supply contact pieces (7), stops (6) for holding a rod (2) entering the web (5) against the contact pieces (7), for pressing the rod (2) ) clamping units (8) formed towards the contact pieces (7) and closing devices (12) arranged next to the clamping units (8). 448 555 10 15 20 25 14. 6. Device according to claim 5, characterized in that the heating unit is provided with a puriodically operating input device. Device according to claim 5 or 6, characterized in a closed housing (9) provided with resilient folding doors (10). Device according to Claim 5, 6 or 7, in that the heating unit is different in that the heating unit is provided with a temperature sensing means connected to a control device. Device according to one of Claims 5 to 8, characterized in that the cooling unit connected to the forming unit has at least one container (15) filled with cooling medium and provided with a sloping bottom part, a lower part of the container (15) connected dispensing device (17) and an inclined feeding device (16) arranged opposite the dispensing device in the container (15). Device according to claim 9, characterized in that they are designed as bucket belt conveyors (16, 17). Device according to claim 9 or 10, characterized in that the container or container, in that the input and output device n e t e c k a a dls (15) are provided or provided with a cooling and heating unit. hrs