EP0345205B1 - Method for tempering a hollow cylindrical steel product - Google Patents

Description

The invention relates to a method for hardening a cylindrical hollow body made of steel according to the preamble of patent claim 1.

Such a method is known from DE-PS 3721665, which goes back to the same applicant. This method provides for the speed of the hollow body to be cooled to be significantly increased when the martensite start temperature is reached in order to avoid hard cracks during rotary diving. This method has given good results, in particular for containers, that is to say for hollow bodies with closed end faces.

When hardening such objects, however, it is also important that the cooling takes place as far as possible without excessive local differences in the cooling effect, so that overall a uniform structure is ensured. locally different cooling effects can lead to considerable deformations of the hollow body due to the different specific volumes of the structure types. This not only requires the hollow body to be reworked in many ways, but also makes it more difficult, e.g. in the case of steel pipes, often further processing (e.g. transport disruptions due to curved pipes).

While the immersion depth of a container closed on both sides depends on the extent to which its ends have to be cooled, the practical immersion depth for pipes, ie for hollow bodies that are open at the ends, depends on how uniform the material properties should be in the longitudinal direction of the pipe. The uniformity of the cooling on the outside of the tube does not depend on the immersion depth. This is different, however, when cooling the inside of the tube, since the coolant flowing in at the ends is exposed its way to the center is heated so that the cooling effect weakens towards the center. However, with increasing immersion depth, this effect becomes less pronounced. At immersion depths over 80% of the diameter, the cooling effect on the inside is sufficiently uniform over the entire length of the tube, even for tubes with a length of 60 times the diameter, in order to achieve continuous martensitic hardening, provided the wall thickness is not too great (e.g. for one Tempering steel 34 CrMo 4 smaller than approx. 28 mm). For this reason, correspondingly large immersion depths can be regarded as advantageous for achieving uniform properties. A complete immersion of a pipe to be cooled should be avoided, however, since vapor bubbles form inside the pipe, which can escape only with difficulty when immersed and can lead to different cooling windings.

In the case of pipes with a relatively large diameter (above approximately 240 mm diameter), a relatively uniform internal cooling and therefore also a uniform overall cooling usually takes place despite the low immersion depth. However, the situation is problematic for pipes with smaller diameters or for containers that are open on one side, since the thermal and fluidic conditions are considerably less favorable. This applies in particular to the case of intermittent cooling to achieve a bainitic structure which, after quenching, has favorable mechanical properties without further tempering (cf. curve 2a in FIG. 1). With a martensitic hardening (curves 1-3 in Figure 1), ie with quenching to coolant temperature, on the other hand, a uniform structure can usually be ensured by a suitable choice of material for the hollow body, so that even larger differences in cooling effects are still manageable.

It is known that the cooling effect of liquid coolants depends very much on the surface temperature of the object to be cooled. The higher this temperature, the lower the quenching effect. When tempering steel objects, it is often important to ensure that the high temperature range, i.e. to quickly pass through the area between the austenitizing temperature and the martensite starting temperature or the onset of bainite formation in order to prevent the formation of undesirable structural components (e.g. ferrite and pearlite). On the other hand, the temperature range of the martensite formation should be passed through as slowly as possible in order to ensure an even temperature distribution over the wall thickness and thus to avoid the creation of residual stresses in the pipe wall as far as possible.

The procedure according to DE-PS 3721665 leads to good results, particularly with regard to controlling a reduction in the cooling intensity. On the other hand, there is often a desire (in particular for the treatment of thick-walled tubes) to increase the cooling effect when the hollow bodies to be quenched are rotated to be increased beyond what was previously considered feasible. Another problem is that the cooling effect is uneven due to the irregular surface properties of the hollow bodies to be quenched (e.g. by scale).

From EP 0086988 A1 it is known for a cooling device, in which stationary pipes in a moving coolant bath are charged with cooling water inside and outside, to avoid a reduction in the cooling effect due to vapor bubble accumulation on the inside of the completely immersed pipes by the fact that in the through a nozzle is injected with compressed air fed to the pipe face in a helical flow of water. There is no evidence of an increase in the cooling effect due to compressed air in the case of external pipe cooling.

A method for hardening thick-walled, large-format tubular steel tubular bodies is known from US-A-3695598. The hollow body lying horizontally with its longitudinal axis is immersed with a part of its surface in a coolant bath. The part protruding from the coolant bath is heated to approximately 700 ° C. by inductive heating, in which a low-frequency inductor is arranged on the inner and outer surface of the hollow body. Further heating up to about 900 ° C takes place by a second inductor, which is operated at high frequency, only to a certain depth of penetration. The hollow body is rotated slowly and slowly during the treatment, so that the heated parts are immersed in the coolant bath and cooled rapidly. Part of the hollow body is thus constantly cooled while another part is being heated. By repeating heating and cooling several times, i.e. by repeated turning around the longitudinal axis, the treated hollow bodies are to obtain significantly improved mechanical properties.

This method is not comparable to rotary diving according to DE-PS 3721685, in particular because initially not the entire hollow body, i.e. its length, its wall thickness and its circumference, is heated to the austenitizing temperature and only then is it accelerated overall, but rather heating and cooling in each case zone by zone and to a certain depth in the hollow body wall in several heating / cooling cycles. The object of the invention is to provide a method and a device for its implementation, with which the cooling of hollow bodies during rotary diving can be significantly intensified and more evened out.

This object is achieved according to the invention by a method having the features of patent claim 1; Advantageous further developments of this method are characterized in the subclaims 2-7. A device for performing this method is equipped according to the invention with the features of claim 8. Claims 9 and 10 indicate expedient configurations of the device according to the invention.

Fig. 1

ein ZTU-Schaubild für Stahl,

Fig. 2

einen Temperaturverlauf entlang der Rohrlänge beim konventionellen Drehtauchen,

Fig. 3

einen Festigkeitsverlauf entlang der Rohrlänge beim konventionellen Drehtauchen,

Fig. 4

einen Schnitt durch eine erfindungsgemäße Drehtauchanlage,

Fig. 5

einen Temperaturverlauf entlang der Rohrlänge bei erfindungsgemäßem Drehtauchen und

Fig. 6

einen Festigkeitsverlauf entlang der Rohrlänge bei erfindungsgemäßem Drehtauchen."

"In the following, the invention is explained in more detail, inter alia, with reference to FIGS. 1-6.

Fig. 1

a ZTU diagram for steel,

Fig. 2

a temperature curve along the pipe length in conventional rotary diving,

Fig. 3

a course of strength along the length of the tube in conventional rotary diving,

Fig. 4

2 shows a section through a rotary immersion system according to the invention,

Fig. 5

a temperature profile along the length of the tube with rotary diving according to the invention and

Fig. 6

a course of strength along the length of the tube during rotary immersion according to the invention. "

The solution according to the invention provides for a swirling of the coolant below the rotating hollow body to be cooled (eg pipe). The swirling, which could also be achieved, for example, by circulating pumps, is expediently brought about by introducing compressed air, for example with the aid of a nozzle tube arranged below the hollow body. This nozzle tube is located, for example, at the bottom of the coolant tank and runs parallel to the longitudinal axis of the hollow body to be cooled. There are numerous holes on its top through which the compressed air can escape, which causes a strong swirl on the way to the surface of the coolant bath. As a result, the vapor layer (film evaporation) on the surface of the hollow body which occurs in the first cooling phase at a high temperature level is destroyed and in the second cooling phase the resulting vapor bubbles (bubble evaporation) are detached from the surface more quickly. As a result, the cooling effect of the coolant is significantly increased. As the vapor layer collapses and the vapor bubbles detach at low relative speeds between the cooling medium and the surface of the hollow body (e.g. pipe or container) Depends in particular on the nature of this surface (eg roughness), an increase in this relative speed due to the swirling in the case of unevenly formed surfaces has the effect of making the cooling effect on the surface more uniform.

The intensified cooling effect leads to a considerable reduction in cooling times, that is to say, a greater deterrent. This is achieved with extremely simple means (e.g. compressed air introduction). This means that the application area of existing rotary immersion systems can be expanded significantly without great effort. Not only can hollow bodies with thicker walls than previously be quenched, but it is also possible to successfully quench pipes or containers made of steel materials with low contents of alloy elements with the same wall thickness.

The cooling effect can be reduced when the martensite start temperature is reached by simply switching off the compressed air supply. In addition, more gentle cooling can be set if necessary by increasing the speed of the hollow body to be cooled. Finally, the reduction in the immersion depth of the hollow body also results in a reduction in the cooling intensity; however, the risk increases that hollow bodies that are open on one or both sides cool faster on the inside at the open ends. One way of eliminating these different cooling effects on the inside of the hollow body is fundamentally possible by preventing the coolant from entering the interior of the hollow body from the outset. This can be done by temporarily fitting suitable caps on the open end faces of the hollow body. However, such a measure requires considerable handling and apparatus expenditure and therefore appears to be less desirable.

In an advantageous further development of the invention, it is therefore provided, in particular for steel pipes with a diameter of less than 240 mm, that the internal cooling which is reinforced at the open ends by the inflowing coolant is at least approximately compensated for by a corresponding weakening of the external cooling in this area. This is ensured by the swirling of the coolant bath in these end areas e.g. due to local reduction in the pressure of the compressed air supplied for swirling or the coolant supplied with increased pressure (e.g. pressurized water). This enables even microstructure formation (bainite) to be achieved with intercepting cooling even with sensitive materials. The effectiveness of the method according to the invention is explained in more detail with reference to the comparative examples described below.

1st example

Pieces of steel tube of 178 mm in diameter, 14.5 mm in wall thickness and 1500 mm in length were uniformly heated to 980 ° C. in an oven and inserted at a starting temperature of 960 ° C. in a rotary immersion system with a water bath. The rotary immersion device could be lowered so that the pipe sections could be lifted out of the water bath after predetermined quenching times in order to be able to determine the pipe compensation temperature. In a first comparison test, the immersion depth was 90% of the pipe diameter and the speed of the pipe was 80 rpm.

In the conventional procedure, a cooling temperature of 575 ° C. was established in the tube after a cooling time of 18 seconds; with a compressed air supply according to the invention of 0.25 bar overpressure, however, the compensation temperature was already 510 ° C. after only 10 seconds and 450 ° C. after 12 seconds.

2nd example

In a second experiment, two identical tubes were immersed at the same operating temperature as in the first experiment, but with an immersion depth of 50% of the tube diameter, and cooled at 80 rpm with the introduction of compressed air in the rotary immersion system. The pressure of the compressed air was 0.25 bar for the first pipe and 0.5 bar for the second. After a quenching time of 12 seconds, a compensation temperature of 600 ° C. was set in the first tube and a compensation temperature of 453 ° C. in the second.

These results already clearly show that the cooling effect could be increased considerably by introducing compressed air. The cooling time was reduced to about half compared to the prior art.

3rd example

The inadequacy of the previous procedure when cooling hollow bodies with open end faces in a conventional rotary immersion system is evident from the measurement results shown in FIGS. 2 and 3 on a steel tube with 178 mm in diameter, 14 mm in wall thickness and 15 m in length. This tube was quenched from 920 ° C to an average temperature of 450 ° C in order to achieve a bainitic structure. However, the actual temperature distribution was extremely different, as shown in FIG. 2. While at the ends temperatures were already below 400 ° C, the middle range was still around or above 600 ° C. This means that the present temperature differences were up to 250 K. This led to a correspondingly different structure, which is documented, for example, in the yield point values R _t0.5 which differ greatly over the pipe length.

While the values at the ends are around 700 N / mm², because this is where the desired bainitic structure has arisen (corresponding to curve 2 / 2a in Fig. 1), the middle range (approx. 2-3 m from the pipe ends) Determine values of 465-495 N / mm², which indicate a predominantly ferritic / pearlitic structure (corresponding to curve 3 / 3a in Fig. 1).

The effectiveness of the invention in a procedure according to claim 6 can be seen in the measurement results in FIGS. 5 and 6, which were determined on another steel tube which was cooled on a system which is shown schematically in FIG. The material and pipe dimensions corresponded to those of the comparative test according to FIGS. 2 and 3. In FIG. 4, a large number of arrows indicate that the compressed air supply for swirling the coolant bath below the immersed pipe 1 in the area of the pipe start 1a and the pipe end 1b into individual nozzle strips 2a and 2b 2b is divided, which can be subjected to different pressures. In the present example, 8 individual nozzle strips 2a and 2b each with an individual length of 350 mm have been provided on the two pipe ends 1a, 1b. A continuous nozzle strip 2 is arranged in the central region of the tube 1. In principle, this makes it possible to cool a partial length of approximately 3 m each at the two pipe ends to different degrees compared to the central region.

In the case of the exemplary embodiment according to FIGS. 5 and 6, only the two outermost nozzle strips 2a, 2b were each pressurized with compressed air, the pressure of which was reduced to 1.3 bar, while all the other nozzle strips 2a, 2b and the continuous nozzle strip 2 with compressed air from 2, 5 bar were operated. The length of the pipe ends cooled less intensively from the outside was therefore about 700 mm on each side. The depth of immersion of tube 1 was set at 90%.

Under these conditions, the very uniform temperature distribution over the pipe length of about 15 m resulted, as shown in FIG. 5. The scattering range has shrunk to a range of about 30 K, so that the desired temperature for the intercepting cooling was practically reached everywhere. The resulting bainitic structure is correspondingly uniform. This can be seen from the uniform yield strength values of 670-690 N / mm² according to FIG. 6.

To a cooling system according to Figure 4 e.g. To be able to operate steel pipe production as effectively and flexibly as possible (different pipe dimensions and materials), an electronic control system is available that measures the length of the pipe to be cooled when it exits the austenitizing furnace and takes into account the depth, temperature, material, etc. Diameter and the wall thickness of the tube, the position and length of the zones to be cooled to a greater or lesser extent are determined and finally the appropriate nozzle strips are pressurized.

Claims (10)

1. A process for hardening a cylindrical hollow body made of steel in a quenching and tempering treatment, wherein the hollow body which has been heated altogether to austenitisation temperature is cooled in a bath of cooling medium, in particular a water bath, by rotary dipping, in that it is aligned with its longitudinal axis parallel to the surface level of the cooling medium bath, only part of its surface is dipped into the cooling medium bath and it is rotated several times about its longitudinal axis during this cooling, characterised in that the cooling medium bath is at least temporarily swirled by means of compressed gas, in particular compressed air, beneath the hollow body.

2. A process according to Claim 1, characterised in that the swirling is terminated once the martensite starting temperature is reached.

3. A process according to one of Claims 1 to 2, characterised in that the depth of immersion of the hollow body is reduced once the martensite starting temperature is reached.

4. A process according to one of Claims 1 to 3, characterised in that the speed of rotation of the hollow body is increased once the martensite starting temperature has been reached.

5. A process according to one of Claims 1 to 4, characterised in that when cooling the hollow body, which is open at least on one end face, the swirling is performed less strongly in each case in the region of the end or ends with an open end face than in the central region and if applicable in the end region having a closed end face of the hollow body, the reduction of the swirling being metered in such a manner that the weaker external heat dissipation involved therewith corresponds approximately to the greater inner cooling in these end regions.

6. A process according to Claim 5, characterised in that the swirling is performed by supplying compressed air with pressure which differs in zones.

7. A process according to Claim 5 or 6, characterised in that the swirling is performed temporarily only in the central region and if applicable in the region of the closed end face.

8. An apparatus for performing the process according to Claim 1 with a container for holding a cooling medium bath and with a rotation device for rotation of horizontally lying cylindrical hollow bodies in the cooling medium bath, with a system of nozzles being arranged beneath the imaginary axis of the cylindrical hollow body and beneath the intended surface level of the bath of cooling medium, characterised in that the nozzles (2, 2a, 2b), individually or combined in zones, are acted upon by compressed gas at operating pressures which differ from each other.

9. An apparatus according to Claim 8, characterised in that the nozzle system is designed as a horizontally arranged length of pipe, the wall of which has numerous bores in the region of its uppermost generatrix.

10. An apparatus according to Claim 8, characterised in that the rotation device can be set to different heights with respect to the surface level of the bath.

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