GB2024871A

GB2024871A - Method and apparatus for the heat treatment of coiled wire or strip

Info

Publication number: GB2024871A
Application number: GB7922509A
Authority: GB
Original assignee: Stahlwerke Roechling Burbach GmbH
Current assignee: Stahlwerke Roechling Burbach GmbH
Priority date: 1978-07-08
Filing date: 1979-06-28
Publication date: 1980-01-16
Also published as: AT388391B; CH640272A5; FR2430454B1; IT1202951B; IT7924120A0; DE2830153A1; FR2430454A1; BE877252A; GB2024871B; DE2830153C2; ATA438779A

Abstract

A method for the heat treatment of coiled wire or strip. Coils are austenitized and hardened while being excited to produce natural vibrations, and subsequently annealed. The excitation can for example be provided by a vibrating hook 1 conveying the coil 2 from an austenitizing furnace to a quenchant bath 3. The quenchant bath has means such as pumps 5 for passing a quenchant axially through the coil. <IMAGE>

Description

SPECIFICATION Method and apparatus for the heat treatment of coiled wire or strip The invention relates to a method and apparatus for the heat treatment of coiled wire or strip, wherein the coils are austenitized, hardened while being excited to produce natural vibrations or resonance and subsequently annealed.

Such apparatus in which the material to be treated is excited to produce natural vibrations are known from German Patent Specification 21 58459 and German Offenlegungsschrift 23 27 603. Good results are obtained with this apparatus, but it has been found that appreciable improvements are possible particularly if greater throughputs per unit of time are to be achieved.

The problem underlying the invention is to achieve a particularly high degree of uniformity in the mechanical properties of the heat-treated wire or strip.

This problem is solved in accordance with the invention owing to the fact that excitation to produce natural oscillations commences during or immediately after austenitizing and continues, while the quenchantflows through the coil during the quenching operation, until the entire volume of the coil has undergone martensitic transformation.

In accordance with an embodiment of the invention the height of the coil of wire or strip is increased, from the time when it is received, by 1.2 to three times during or immediately after austenitizing until heat treatment takes place.

The particular advantages achieved with the invention are that the mechanical properties of the heat-treated and subsequently annealed wire or strip are highly consistent throughout the entire coil. Moreover, the measures proposed in accordance with the invention and the apparatus for carrying out same are suitable for installations designed for achieving a high output.

The new method and apparatus to be used for putting the method into practice are described in more detail below: The object of the method, namely, the high degree of uniformity in the mechanical properties of the hardened and annealed coil in all elements, is achieved by exciting the coil, which is about to be received by a vibration-resistant or nonoscillating carrier and has been previously brought to austenitizing temperature, to produce resonant vibrations from the time when the coil begins to be received, and this excitation is continued and maintained, particularly during the stage when the coil is immersed in the quenchant as well as when the coil is fully immersed, until the entire volume or all the elements of the wire have undergone martensitic transformation.

This measure results in, amongst other things, a temporary contact for only a short time between the cold carrier and hot coil and therefore in better compensation for loss of temperature in the areas of temporary contact. In addition, new points of contact keep reappearing owing to the tangential movement of the coil, i.e. no points on the coil are adversely affected.

The excitation of the coil to create natural vibrations during its immersion in the quenchant is equally imperative because this makes it possible for all elements of the coil to be quench-hardened under almost the same conditions. If vibration did not commence until after immersion was completed, the preceding cooling action without vibration of the first immersed elements of the coil would be different from that of the last immersed elements. Moreover, the first cooling stage - and this is crucial for the entire cooling cycle -- would be as uneven or inconsistent as any previous conventional method of cooling coils.

The beating surface of the vibration-free carrier has, in accordance with the invention, a length 1.2 to 3 times, and preferably twice to 3 times greater than the height of the coil. The resonantly vibrating coil received in the austenitizing furnace extends uniformly over this bearing surface during its conveyance to the quenching bath. This measure, which comprises the carrier having a length greater than the height of the coil, counteracts any inadmissible overheating of the quenchant in the area of the wire windings. If several coils are hardened or tempered at the same time, they can in this connection be regarded as one coil.

Moreover, this greater length of coil on a hook, for example, also has the advantage that the hydrodynamic conditions for each individual winding of the coil are even closer. to optimum quenching conditions.

The vibration of parts of the coil in the quenchant results in not only - as in the case of etching -- the advantage of eliminating permanent points of contact between the coil windings and therefore eliminating more or less closed cavities formed by three windings lying in close contact with one another, but also in an increase in the heat transfer coefficient as a result of the constantly changing direction of relative movement between the wire element and quenchant. The temperature dependence of the modulus of elasticity, expressed in limits, and therefore the change in the resonant frequencies with failing temperature should be taken into account when selecting the exciting frequency.

The radial flow of quenchant through the coil is proposed in accordance with the invention as the measure for exciting the coil to produce selfoscillation or natural vibrations from the time when the coil is received until martensitic transformation has been completed. In this connection a flow rate of 0.1 to 5 m/s, and preferably 0.5 to 1.5 m/s, should be maintained.

This rate is the absolute speed to be selected for entry of the quenchant into or its egress from the coil. The relative speed between the quenchant and wire or strip is higher, that is, firstly on account of the vibrations of the wire or strip elements and, secondly, because of the volume ratio or space filling of the wire windings.

It is known practice for the quenchant to flow over one face of the coil (German Patent Specification 21 58 459). In this case the second face is sealed with a plate so that it is necessary for the quenchant to pass radially through the coil.

However, this known measure has disadvantages. The coil must be positioned relatively accurately, and the noise level is increased unnecessarily by the vibrating coil striking against the sealing plate. In contrast, the quenchant flows in accordance with the invention towards both sides of the coil so that the axial rate of flow of the medium in the centre of one face of the coil or even halfway up the coil is nil.

Figure 1 shows a practical embodiment of the quenching bath according to the invention. The vibrating C-shaped hook 1 which carries the coil 2 is immersed in the quenchant 3. The quenchant is caused to flow towards both ends 4 of the coil 2.

The quenchant passes substantially axially from both sides into the interior of the coil 2 and flows through and out of the coil 2 in a radial direction.

As a result the pumps 5 are set for the desired radial rate of flow.

The position of the plane, with the axial rate of flow being zero, can move between a face and the centre of the coil or between the first and second faces during the hardening operation by varying the outputs of the pumps 5 accordingly.

Prior to heat treatment the wire or strip must be brought to the prescribed temperature, usually (in the case of steel) the austenitizing temperature. In many cases a roller hearth furnace is used for this purpose. Figure 2 shows the exit to such a roller hearth furnace closed off by a door 7. One arm 9 of a tilting device or upender, which is connected to the bearing 10 outside the furnace 6, engages in the roller bed 8. As soon as the hook 1 is in the starting position and the coil 2 is arranged on the roller bed above the arm 9, the door 7 is opened and the tilting device and hook are moved towards one another. While the tilting device is pivoted the hook is moved horizontally and vertically in synchronization with this rotary motion so that the coil 2 comes to rest on the hook 1.The pallet 11 is mechanically secured to the tilting device and only removed therefrom and returned to the entrance of the furnace after the coil 2 has been transferred.

It may be expedient to provide the load bearing surface of the hook 1 with a heat-insulating coating. It is also conceivable for the hook 1 to be heated intermittently or continuously, particularly if hooks 1 are kept in a waiting position in front of the furnace 6.

A further measure according to the invention is the extremely rapid transfer of the coil to the carrier and the subsequent rapid immersion. This sequence of movements is performed in less than 30 seconds, and preferably in less than 10 seconds. This time span lasts from the moment when the furnace 6 is opened until the moment when the coil 2 is fully immersed. An additional measure, which is particularly helpful with wire of small diameter, e.g. 5.5 mm, is a protective radiation screen of low heat capacity preferably made of several layers of thin metal foil for encircling the coil during its conveyance from the furnace 6 to the quenching bath.

This rapid transfer from the austenitizing furnace to the quenching bath is important for achieving a high degree of uniformity in the mechanical properties of the hardened and annealed coil because, if transfer of the coil is slower, the outer elements of the wire or strip cool more rapidly than the inner elements resulting in the coil having a variable temperature at the time of hardening. Varying hardening structure and therefore properties varying, for example, from one wire element to another would result.

The method of heat treatment according to the invention consists of three stages, austenitizing, hardening (quench hardening) and annealing. All measures in these three stages of the process must be coordinated with one another to ensure optimum operation. The same applies to intermediate movements or operations between the individual stages which often have considerable influence on the previously and/or subsequently taken measures.

Thus, it is expedient, after the coil is hardened in the same way, to effect weaker excitation of the coil for producing natural vibrations in the case of materials which are sensitive to cracking during hardening so that the quenchant adhering to the coil may be shaken out above a collecting vessel.

The remaining quenchant is removed e.g. in an evaporation chamber during continuous excitation of the coil to produce natural vibrations, a hot gas or hot air flowing radially through the coil. In some cases it is sufficient e.g. for hot air to be forced through the coil without natural vibrations being produced.

It is also the object of this measure not to jeopardize the high degree of uniformity in the mechanical properties of the hardened and annealed coil of wire. If traces of quenchant were caught e.g. between three windings in close contact with one another in the coil, this residue would evaporate only gradually during annealing and the winding elements concerned, in contrast to dry elements of the coil of wire or strip, would remain relatively cold for a much longer time and consequently be annealed for a shorter time.

Figure 3 shows a cross-section through an evaporation chamber 12 by which the coils 2 are rinsed between the hardening and annealing stages of the process. In this chamber 12 the remaining quenchant is removed from the coil 2 by placing a suction pipe 1 3 on one side of the coil while the other end is sealed with a plate 14. Hot gas is conducted into the chamber 12 via the nozzle 15. It flows radially inwards through the coil 2 and is extracted through the pipe 13. The coil 2 is supported by the hook 1 which in turn is supported by the support bar 16 engaging in a chain-type conveyor 1 7. The said chain is moved step by step by wheeis 18. The seal 1 9 prevents hot air from penetrating the second chamber 20 located above the evaporation chamber 1 2. It is advantageous to maintain a slight over-pressure in this chamber 20.

For the hot gas fed through the nozzle 1 5 the rate of flow should be between 1 and 10 m/s and the temperature a maximum of approximately 3000 C.

The hot gas or hot air enriched with the oil vapours can be led outside the chamber 12, for example, through a combustion chamber, in which the oil vapours are burned. The heat liberated can be used for preheating the coil in the evaporation chamber 12. The operations during the final annealing are also of vital importance for achieving a high degree of uniformity in the mechanical properties throughout the entire coil of wire or strip. In accordance with the mode of operation during hardening (Fig. 1) it is therefore proposed according to the invention that a heat carrier should flow radially through the vertically mounted coils from both sides, as shown in Figs. 4 and 5.

This method of annealing coils has the advantage over the conventional annealing of coils, where the coils standing on pallets are guided through a roller hearth furnace, in that the high permeability of the coil lengthened by between 1.2 and 3 times is maintained, the specific radial flow rate allows the mean heat transfer coefficient to be increased by three times and the uniformity of the annealing process is decisively better than that achieved with the conventional method. The efforts made during hardening in the second stage of the process to achieve a high degree of uniformity through all the elements of the wire coil are completed by the described method of annealing.

Figs. 4 and 5 show respectively a cross-section and a longitudinal section through the new annealing furnace 24. The described (Fig. 3) method of conveying the coils 2 by means of hooks 1 and chain-type conveyor 17 is provided.

Hot gas is caused to flow towards both ends 4 of the coils 2. The directions of flow are discernible from Figure 4. The gas is extracted from the furnace 24 via the ventilator 21 and directed on to both front sides 4 of the coil 2 through the flowcarrying pipes 22. In this example the gas is heated by vertically suspended radiant heating tubes 23.

In the annealing furnace 24 the coils 2 preferably occupy the same position as in the evaporation chamber 12, as shown in Fig. 3; they are suspended from a hook 1. This has the advantage that the heat carrier can easily flow radially through the coils 2 as a result of their height being increased by 1.2 to 3 times. The described special version of the annealing furnace 24 ensures a high uniformity during annealing and therefore uniform mechanical properties in all elements of the coil.

The described method and apparatus for applying same have been specifically developed for the heat treatment or hardening of steels which, depending on the choice of quenchant and its temperature, preferably undergo martensitic transformation and, if necessary, also transformation into bainite and which are annealed to a relatively high degree after hardening, depending on the strength required. The method and apparatus, particularly as claimed in Claims 1 to 7 are, however, also suitable for non-transferable steels, e.g. austenitic, stainless steels which must be cooled from a high temperature by quenching in order to prevent the formation of chromium carbide and therefore a loss of corrosion resistance. Non-ferrous metals, which are cooled by quenching from the solution temperature in order to make them easily deformable, can also be treated in accordance with the described method.

Claims

1. A method for the heat treatment of coiled wire or strip, wherein the coils are austenitized and hardened while being excited to produce natural vibrations and subsequently annealed, the excitation commencing during or immediately after austenitizing and continuing during a subsequent quenching operation, with a quenchant flowing through the coil, until the entire volume of the coil has undergone martensitic transformation.

2. A method according to claim 1, wherein from the time when the coil is received, in the process, its height is increased by 1.2 to three times during or immediately after austenitizing until hardening takes place.

3. A method according to claim 1 or claim 2 wherein the exciting frequency lies within the natural frequency range of the individual windings of the coil or parts thereof and is selected to remain within the resonant frequency range of the coil during hardening.

4. A method according to any preceding claim wherein the quenchant is caused to flow towards the ends of the coils from both sides and the two rates of flow are synchronized with one another so that the axial velocity of flow is nil in the centre of one end of the coil or the quenchant flows at the same rate towards both ends of the coil and the selected rate of flow is from 0.1 to 5 m/s.

5. A method according to claim 4 wherein the said rate of flow is between 0.5 and 1.5 m/s.

6. A method according to any preceding claim wherein conveyance of the coil from the austenitizing furnace until it is fully immersed in the quenchant takes less than 30 seconds and, in the event of the product to be heat treated having rapidly changing qualities and small dimensions, the coil is encircled by a protective radiation screen during its conveyance.

7. A method according to claim 6 wherein the said conveyance takes less than 10 seconds.

8. A method according to claim 1 substantially as herein described with reference to the accompanying drawings.

9. Apparatus for putting into practice a method according to any one of the preceding claims, said apparatus comprising an austenitizing furnace, a quenching bath and an annealing furnace, a vibrating carrier being provided to convey a heated coil from the austenitizing furnace to the quenching bath and the annealing furnace and the quenching bath being provided with means for passing a quenchant through the coil.

10. Apparatus according to claim 9 wherein the surface of the carrier which has to receive the hot wire coil is provided with a heat-insulating coating and/or the carrier surface is heated intermittently or continuously.

11. Apparatus according to claim 9 or claim 10 wherein a tilting device or upender is arranged adjacent an exit from the austenitizing furnace so that the hot coil and pallet can be lifted off the furnace hearth, tilted and, after the austenitizing furnace is opened, tilted through a further 900 onto a hook of the carrier provided for taking up the coil.

12. Apparatus according to any one of claims 9 to 11 wherein between the quenching bath and annealing furnace there are provided a chamber and means associated therewith for causing hot gas or hot air to flow radially through the coils vertically mounted on hooks of the carrier, with or without excitation to produce natural vibrations.

13. Apparatus according to any one of claims 9 to 12 wherein the annealing furnace comprises means of causing hot gas to flow towards both ends of the coils, vertically mounted on hooks of the carrier, and to flow radially through the coils.

14. Apparatus according to claim 9 substantially as herein described with reference to the accompanying drawings.

1 5. A wire or strip material heat treated by a method according to any one of claims 1 to 9.