EP0632137B1 - Process for the thermal pretreatment of metallic articles - Google Patents

Abstract

Process for the heat treatment of metallic articles such as steel components which distort easily, by means of a roller furnace and presses (so-called quenches) cooperating with the latter. The articles are raised to a predetermined temperature and cooled in accordance with a predetermined characteristic for the cooling rate. After reaching the predetermined temperature, the articles pass out at very high speed through a rapid discharge region of the roller furnace at an even higher temperature, the press parts exerting on the metallic articles pressures which vary as a function of time during the cooling process.

Description

The invention relates to methods for heat treatment of metallic goods (e.g. easily warped Steel parts such as sheet metal, round blanks and the like Like.) by means of a Roller oven and presses interacting with it (so-called Quetten), at which the good to a predetermined temperature brought and between press parts with predetermined Cooling rate curve is cooled.

For the thermal treatment of steels, especially for Hardening, it has been common so far that the blanks on the Austenitizing temperature. To do this Several salt baths are used that are at a fixed temperature be set and that of a blank one after the other be run through.

The blank is brought to the desired temperature quenched, which requires a quenching medium, namely air, water, salt baths and oil.

Quenching certain steels (e.g. higher and lower alloyed tool steels) in air is not possible because with Air does not provide enough heat in a short enough time can be removed from the blank. So there are a number of Steels, which involve the use of salt baths and oil is instructed to deter. These procedures are in the Very questionable with regard to today's environmental ideas, it should also be borne in mind that the quenching in Water due to the components present in the water Is problematic.

It is also known to be subjected to quenching of presses, so-called Quetten. It deals are metal parts, e.g. B. cylinder blocks when it comes to the hardening of saw blade blanks goes. These metal parts press the blank from both sides during the Cooling process. It is in this context too known to cool the press parts or the cylinders, be it with water or oil in order to obtain a cheap, to achieve rapid cooling rate progress.

Härterei Technische Mitteilungen, Vol.45, No. 6, December 1990, pages 325-329 discloses a process for quenching-free heat treatment of steel parts using a roller furnace and an adjoining press cooling device.

In the above-described heat treatment process it's not just about the hardness of a blank, it's supposed to be one too uniform structure should be striven for, if possible evenly distributed over the cross-section, so-called Decarburization zones in the blanks are undesirable because they are large and thin blanks cause the Blank later warped.

It is the pursuit of the present invention, a method to create with which steel blanks are thermally treated can be without the use of oil as Quenching medium would be required.

This is achieved through the in Claim 1 specified features. Preferred embodiments of the method according to claim 1 can be found in the dependent claims.

For one group of methods according to the invention Above all, two characteristics are essential:

The point is to provide a high temperature range that is significantly higher than at those temperatures which previous heat treatment facilities work. In the Technology in general has the prejudice that one is such high temperature range related to heat treatment of steels should be avoided, as such a temperature Changes in carbon content, scaling and Warping the blanks. Through the invention however, it has been recognized that such a high temperature range in the Not only with regard to the problems listed above is harmless, but also significant Advantages, namely, that now Steels can be processed and in particular hardened that So far, when quenching only under oil to the desired ones Properties led. It is essential in this context that the range of particularly high temperatures with a high Speed is passed, this speed the other conditions, in particular the shape and form adapted to the blanks. The amount of temperature depends on the materials chosen, it is also depending on whether the object to be hardened is now large Dimensions in an extension has a smaller thickness or whether it is more or less compact blanks. The same applies to the temperature. The aforementioned Process parameters can be determined by simple tests so that optimal values are achieved.

It has been found that at optimally set Conditions of energy expenditure per workpiece is less than in known methods, albeit with low ones Temperatures is worked.

It is also essential for the present invention that when cooling, the blanks arranged between press parts be, the pressure of the two press parts on each other or is controlled in a time-dependent manner on the blank. On a High pressure phase is followed by a decrease in pressure so that only the dead weight of the upper press on the blank acts, and it is precisely this pressure drop that leads to this has resulted in the blanks even with unfavorable Dimensioning relationships are treated without delay can.

It has also been found that optimal set conditions (temperature and speed in the rapid withdrawal zone and pressure conditions when cooling) individual blanks are decarburization-free, so that a better one Microstructure arises and, as a result, superior Properties could be determined.

It is also essential to the present invention that a cooling unit and a squeeze or press device have been firmly connected to a roller oven. In this manner and way can be turned to the optimal cooling. Here, too, is an essential finding of the present Invention in that the properties of the machined Blank depend on the temperature at which Blank the rapid withdrawal zone at what speed has gone through

In the case of a device which is operated according to the method according to the Invention works, is at the exit of the roller furnace at least one quartet unit, preferably two, the alternately with those at the appropriate temperature blanks can be loaded. This way and Way it is possible to have sufficiently long cooling processes perform and still on the throughput of the roller kiln adapted to work.

The method according to the invention can also be based on blanks such steels can be used, which have also been used to date Air can be hardened. In this context it shows a significantly lower energy expenditure for the couples according to the Invention.

The process can be used on blanks with a thickness of up to 20 mm be applied and shows in this context its Superiority to known methods, especially in required energy expenditure.

It is thought of in the context of handling the Use suitable machines and robots so that time and temperature conditions also defined here be respected.

It turns out that in the method according to the invention many steels only a very small scale layer is formed and that it flakes off easily. Therefore, following that Method according to the invention provided a working stage in which, with the help of compressed air, the igniter from the Blank is removed.

The roller furnaces which can be used according to the invention can be applied to the desired conditions, they are roller ovens known that the temperature range from 850 to 1350 ° C. deal with.

It turned out that not only the quick Hardening tool steels, but also the distortion-free Hardening all types of steel is the essential point of the process according to the invention. Flat products in particular can be used here be hardened without warping.

a) Das schnelle Härten in sogenannten Schnellhärtepressen

b) Das verzugsfreie Härten von Stahlmaterialien

a) Beim schnellen Härten wird die Austenitisiertemperatur, die bei den normal üblichen Werkzeugstählen vorliegt, deutlich erhöht. Die Erhöhung in etwa liegt hier zwischen 100 - 200 K. Durch diese Erhöhung kann die Austenitisierzeit sehr stark verkürzt werden. Wesentlicher Grund dieser starken Temperaturerhöhung ist es, zu gewährleisten, daß das Material unter der Presse noch nicht die Umwandlungstemperatur HC3 unterschritten hat. Dieses bedeutet, daß nach Beendigung der Austenitisierung das Material schnell den Ofen verläßt und unter die Härtepressen gelangt. Die Transportgeschwindigkeit beträgt in diesen Fällen 250 mm/sec. Um eine ungleichmäßige Abkühlung von Rand und Mitte zu verhindern, ist die Auslaufrollenbahn isoliert und stellt einen Tunnel dar. Dieses ist ein wesentlicher Punkt, schwierige Werkzeugstähle zu härten. In der Regel ist der Rand nämlich oft schon wesentlich kälter, als die Mitte. Wesentlich ist, daß das Material einer Ronde mit nahezu gleicher Temperatur Rand - Mitte in die Härtepresse eingelegt wird und nach Schließen der Härtepresse gleichmäßig abkühlt. Werden diese Maßnahmen nicht ergriffen, so hat sich gezeigt, daß die Randbereiche der Materialien schon so kalt geworden sind, daß sie eine wesentlich geringere Härte und ein nicht ausreichend vorliegendes Martensitgefüge aufweisen.

b) Das verzugsfreie Härten wird insbesondere im Härteofen eingestellt. Durch das Einstellen einer bestimmten Temperaturdifferenz in der Austenitisierzone oberhalb und unterhalb der Rollenbahn kann eine derartige Erwärmung gewährleisten, daß sich das Material nicht verzieht. Einerseits wird durch diese Maßnahme bewirkt, daß sich die Temperaturen auf dem Weg zu den Härtepressen so verändern und daß unterhalb und oberhalb des Materials beim Beginn der Abkühlung in den Härtepressen diese Temperatur gleich ist. Auf der anderen Seite wird durch diese Maßnahme die relative Oberflächenbeschädigung gleichgehalten.

Two different phenomena have to be separated:

a) Fast hardening in so-called fast hardening presses

b) The distortion-free hardening of steel materials

a) In the case of rapid hardening, the austenitizing temperature that exists with normal tool steels is significantly increased. The increase is approximately between 100 - 200 K. This increase can shorten the austenitizing time very much. The main reason for this strong temperature increase is to ensure that the material under the press has not yet fallen below the transition temperature HC3. This means that after the austenitization has ended, the material quickly leaves the furnace and gets under the hardening presses. The transport speed in these cases is 250 mm / sec. In order to prevent uneven cooling of the edge and center, the discharge roller conveyor is insulated and represents a tunnel. This is an essential point for hardening difficult tool steels. As a rule, the edge is often much colder than the middle. It is essential that the material of a round blank with almost the same temperature edge-center is placed in the hardening press and cools evenly after closing the hardening press. If these measures are not taken, it has been shown that the edge areas of the materials have already become so cold that they have a significantly lower hardness and an insufficient martensite structure.

b) The distortion-free hardening is adjusted in particular in the hardening furnace. By setting a certain temperature difference in the austenitizing zone above and below the roller conveyor, such heating can ensure that the material does not warp. On the one hand, this measure has the effect that the temperatures on the way to the hardening presses change so that this temperature is the same below and above the material when cooling begins in the hardening presses. On the other hand, this measure keeps the relative surface damage level.

These two points a and b are essential advantages of the Method according to the invention.

It has been found that a very high cooling effect is achieved when a certain thickness of the metal plate is present, which, however, must be as small as possible (high cooling rate and as thick as necessary). Also the Temperature of the cooling water of the indirect cooling of the Hardness presses are vital. You have to Material can be adjusted accordingly. By temperature and the geometric shape of the thickness is very high To achieve cooling effect, the areas of oil hardening covers (high uniformity). In addition, the pressure the hardness presses with regard to even heat transfer also important.

In addition, there are two special cases of hardening, namely partial hardening and hardening of round materials in Molds have been successfully carried out according to the invention. In the following, both variants of this method are described in the Invention described:

Partial hardening

Basis for the partial hardening of flat products in this System is the requirement that a partial area be complete fully hardened, another area unhardened or only tempered is present. For this purpose, the furnace space in the area of Austenitizing zone with a cover of the roller conveyor above and equipped below. The material to be hardened runs through now partially positioned exactly during austenitization an area in which the material has the austenitizing temperature assumes, and on the other hand, a shielded area, in which the estate at the end of the austenitizing zone is a few hundred Kelvin is warmed up colder. Then the good is back removed very quickly and placed under the hardness press. By the subsequent rapid cooling is achieved that the good in full hardness in the fully hardened uncovered area has and significantly lower in the covered area Shows hardness values. This is especially true with knives required a cutting part and a clamping part exhibit. The cutting part should be very hard, the clamping part on the other hand, stay soft.

Hardening cylindrical drills or cylindrical steel parts and also tapered cylindrical steel parts

With cylindrical steel parts it is important to have a distortion-free one Hardening and to achieve a very high curing. To this For this purpose, special forms are made into which the cylindrical components are inserted. These forms will be then covered with a lid and if necessary also the gap between the housing and cover with a protective paste sealed. After that, the lid and housing become identical Plate, quickly transported out of the oven and under cooled to a certain pressure. This ensures that the inner drill bit hardened and through the pressure also be judged. It is therefore a low-warpage or distortion-free hardening of drill blanks is possible. By the Sealing is achieved in that the drill blanks in particular very thin diameters no damage to the surface exhibit. With this method, very high hardness values and a very even distribution of hardness possible. The structure too corresponds to a tough and good structure. The main idea is that Hardening and simultaneous straightening or distortion-free hardening the blanks, as well as recycling from hardening Round materials by inserting them into a mold to the known one Hardening flat products. Here is a high level of uniformity given. Drills were previously hardened in salt baths. By Environmental protection requirements have to be met by the salt baths in Pass over vacuum ovens. These vacuum furnaces do not guarantee anything distortion-free hardening, and on the other hand, by the lower cooling rate not the very high Hardness value achieved.

The invention relates in particular to the hardening of Steel workpieces and preferably on distortion-free hardening of flat products, whatever steel they are made of.

In a preferred method, the temperature in the Roller furnace guided so that the The temperature of the goods only slightly decreases and the goods still into the hardening presses above the austenitizing temperature reached.

Claims (3)

1. Method for thermal treatment of easily deforming steel parts such as sheet metal, rondes and similar materials by means of a roller furnace and presses cooperating therewith (so-called "Quetten"), in which the material

   is brought to a predetermined temperature T, and

   is cooled between press parts at a predetermined developement of cooling rate,

   wherein each steel part, having reached the predetermined temperature T, passes through a region of rapid discharge of the roller furnace at a temperature T₁ > T, T₁ being about 100°C higher as the curing temperatures usually striven for up to now (austenite forming temperature), at a speed of 20 to 40 mm/s or of 150-300 mm/s,

   wherein the press parts exert time-dependingly alternating pressures on the steel part during the cooling process such that after a phase of high pressure occurs a decrease of the pressure, during which only the dead weight of the top press part acts upon the steel part.
2. Method according to claim 1, characterized in that the press parts operate pulsatingly.
3. Method according to claim 2, characterised in that the press parts depending on the material are under pressure for about 3 sec., whereupon follows a period of 3 sec., during which the presses exert only the dead weight on the metallic material.