ITMI20090293A1 - THERMAL TREATMENT AND PLANT FOR THE EXECUTION OF THE SAME. - Google Patents

Description

Description of the patent application for industrial invention entitled "Heat treatment and plant for its execution"

The present invention relates to a method for carrying out a heat treatment, in particular a hardening heat treatment. Said invention also relates to a plant for carrying out a heat treatment, in particular a hardening heat treatment. Hardening is the heat treatment that has been used since ancient times to obtain a steel with a higher hardness than the starting steel.

As is known, the hardening heat treatment consists of heating above the austenitizing temperature, called Ac3, for a period of time such as to guarantee structural uniformity in all points of a mechanical component, and in a sudden cooling with a higher speed than the higher critical speed, resulting in a martensitic structure.

Abrupt cooling can be achieved through water, oil, gas, or in some cases through air.

A particular type of hardening is surface hardening.

The surface hardening heat treatment is applied to all those metal components that must guarantee the mechanical properties associated with high hardness on the surface, while at heart they must guarantee elasticity properties typical of the material without hardening heat treatment. In particular, hardening causes an increase in fatigue resistance and an increase in wear resistance.

The surface hardening heat treatment can be performed using different technologies:

- induction hardening;

- laser hardening;

- EBM hardening (with electron gun);

- hardening with oxyacetylene flame.

The present invention relates to surface hardening performed with induction technology or, preferably, with laser technology.

Recent developments in laser technology using high-power diodes, associated with laser beam control and guiding the laser beam through optical fibers, have made this technology more easily used in the field of surface heat treatments.

The surface hardening heat treatment with laser technology is used with the aim of increasing the hardness of the material in localized areas, while maintaining the core properties of the piece unaltered.

As known, the heat treatment in question, like traditional hardening, is based on two main phases of a thermal cycle: the first phase is defined by a heating above the austenitization temperature, followed by a phase of rapid cooling of the surface of the metal previously exposed to the laser beam.

Surface hardening is due to the change in the structure of the iron and the change in the solubility of carbon within the iron lattice, caused by the heating and cooling cycle. Furthermore, this process involves, as is well known, transformations to the solid state without reaching the local fusion of the treated part. A typical example of a steel that can be surface hardened through this treatment is a common ferritic steel. At room temperature it will essentially consist of ferrite, that is, an α iron matrix with the typical cubic structure with a centered body, and of perlite, with the typical lamellar structure of ferrite and cementite (Fe3C).

This steel, if heated above the austenitization temperature, changes its structure by finding itself within the stability range of iron γ, having the typical cubic structure with centered faces.

In this condition the solubility of the carbon also increases, as the carbon contained in the cementite is solubilized in the γ iron. This γ-iron structure, which at high temperature is called austenite, if slowly cooled below the austenitization temperature becomes unstable, releasing the carbon atoms returning to the ferritic structure with low carbon content.

If the cooling is drastic, there is not enough time for the reverse transformation to prevent the carbon from spreading into the crystal lattice. This creates a distorted tetragonal-shaped structure, called martensite, which is particularly hard, due to the residual stresses accumulated during the transformation. Furthermore, the particularly hard structure leads to a significant increase in the characteristics of resistance to wear and fatigue resistance.

Conventionally, as mentioned above, this type of treatment is carried out through massive heating or through the use of a flame.

The possibility of heating through laser technology allows to treat an extremely reduced thickness of material. This allows for a reduction in the deformation of the mechanical component compared to the case in which the hardening involves the entire piece. In fact, for this very reason, with normal hardening procedures, in almost all cases, a further grinding operation is required to eliminate the effect of deformation.

An advantage of using laser technology is that it is possible to prevent massive heating of the material, given the possibility of locally supplying a portion of the surface of the component with the energy necessary for the hardening treatment.

In addition, only the outermost layer of the material is involved by heating above the austenitizing temperature Ac3, so that, when the laser action is interrupted, the remaining massive part of the mechanical component causes drastic cooling and therefore surface hardening. .

The main process parameters that can be controlled in laser hardening are: power density, beam speed, shape and size of the beam and distribution of the radiation density within the surface area of the component being heated.

Lasers with high-power diodes reach high power levels, of the order of about 10,000 W, and high energy conversion efficiency. The use of optical fibers allows laser technology to be implemented, for example, on an anthropomorphic robotic arm.

The absorption of the radiation emitted by laser diodes by irregular surfaces of some metal components can represent a problem in the case of particularly complex mechanical components.

In particular, irregularities may be due to: the geometry of the surface, the presence of holes or edges, the surface finish of the same, the presence of oxidation or the presence of a previously treated surface.

Therefore, with the aim of improving the homogeneity of the thickness of the treated material, the homogeneity of the hardness profile on the treated surface and in any case the reproducibility of the treatment on an industrial scale, the use of a pyrometer was implemented, measuring the instantaneous temperature of the treated area.

The pyrometer, associated with a laser head, measures the temperature of the heated surface in real time and, through a control unit, processes the signal in order to adjust the instantaneous power of the laser beam. Then the regulation takes place in a closed loop, processing the instantaneous temperature data of the treated area, translating them into information on the power that is supplied to the electromagnetic radiation.

In particular, if the pyrometer measures an increase in the temperature of the heated surface compared to that set, it will act on the power of the beam by decreasing it. On the contrary, if the pyrometer measures a decrease in the temperature of the heated surface, it will act on the power of the beam by increasing it.

A first drawback of the hardening heat treatment is the presence of a surface layer, directly in contact with the atmosphere, which is decarburized.

In fact, the heating of the mechanical component, even if carried out with the characteristics of laser technology, does not prevent the material in the immediate vicinity from heating up by conduction.

This effect ensures that the material, even before being treated, is heated to a temperature that increases the mobility of the carbon atoms towards the outside. Therefore the outermost surface will tend to decarbonise, compromising its hardness and reserving the highest values of hardness for a layer of material about 0.2 mm from the surface. In this way the maximum of the hardness value does not occur on the outermost surface of the piece where it is most needed. This decarburization is called initial decarburization.

A second drawback is due to the need for a drastic cooling which must follow the heating.

In current techniques, this heating is followed by cooling by conduction, due to the transfer of heat from the surface at a higher temperature to the core of the piece at a lower temperature.

This sudden cooling is difficult to obtain in the case of reduced thicknesses and volumes or prolonged exposure of the mechanical component to electromagnetic radiation, as the component tends to heat up completely in a diffused way.

The aim of the present invention is therefore to obviate the drawbacks of the known art.

A first task of the present invention is to ensure that the treated mechanical piece is subject to a significantly lower initial decarburization, compared to the usual laser hardening treatment.

A second task of the present invention is to ensure that, after the heating step, the cooling has a drastic effect, such as to inhibit decarburization after the hardening heat treatment.

A further task of the present invention is that of providing a plant capable of performing the tasks described above.

These and other objects are achieved by a heat treatment according to claim 1 and by a plant for said treatment according to claim 9.

The further advantages and characteristics of the present invention will appear clear from the detailed description that follows of some examples of embodiment, given by way of non-limiting explanation, in relation to the attached drawings in which:

Fig. 1 shows a schematic view of a head of a heat treatment plant according to the invention;

Fig. 2 shows a schematic view of an embodiment of a heat treatment plant according to the invention;

Fig. 3 shows a schematic view of a second embodiment of a heat treatment plant according to the invention;

Fig. 4 shows a schematic view of a possible implementation to an anthropomorphic robotic arm of the plant according to the invention;

Fig. 5 shows a schematic view of a possible implementation to a mobile portal structure of the plant according to the invention;

Fig. 6 shows a schematic view of a possible implementation to a mobile table structure of the plant according to the invention;

Fig. 7 shows a schematic view of an alternative embodiment of a head of a heat treatment plant according to the invention; And

Fig. 8 is a graph showing a comparison between the experimental trends of the microhardness, as the distance of the measurement from the surface varies, of a specimen that has undergone a heat treatment according to the invention and a specimen that has undergone a conventional heat treatment.

Figures 1 and 2 schematically represent the laser head indicated as a whole with the reference 12 of a system 13 for the heat treatment according to the invention.

The heat treatment according to the invention includes:

- an initial cooling of the surface to be treated;

- a heating to a predefined temperature, carried out by means of electromagnetic radiation, emitted by a source of electromagnetic radiation;

- a permanence at this temperature; And

- a final cooling of the treated surface.

One or more embodiments of the method according to the invention will now be described in detail, referring to a hardening treatment in which heating is carried out with laser technology, where the source of electromagnetic radiation is a high-power diode laser.

With reference to Figure 1, the surface of a mechanical component 20 can be schematically divided into three parts: a portion of the surface to be treated 14, a portion of the surface that undergoes heating 16 and a portion of surface that must be cooled 18.

Compared to the usual surface hardening heat treatment, the treatment according to the invention involves an initial cooling phase of the surface to be treated 14.

The cooling performed on the mechanical component 20 can be total, by means of controlled temperature environments, or local through the use of a cooling jet.

The cooling jet can be a fluid, such as air, or preferably an emulsion of liquid nitrogen in gaseous nitrogen.

The nitrogen emulsion is obtained through the use of two tanks 24, 26 whose contents are mixed in a mixer 28. The tank 24 contains nitrogen in the gaseous state, and the tank 26 contains nitrogen in the liquid state. Liquid nitrogen is found at a temperature of about -200 ° C. To adjust the temperature of the cooling jet, liquid nitrogen must be suitably mixed with nitrogen in the gaseous state which acts as a carrier gas. As is well known, by directly addressing liquid nitrogen on the surface of the mechanical component 20, it would cause the embrittlement of the material of which the mechanical component 20 is composed.

The temperature is adjusted by mixing nitrogen gas with liquid nitrogen according to different dosage ratios.

The adjustment of the mixing depends on the cooling profile to be imposed on the surface of the mechanical component 20, on the type of material to be treated, on the heating speed and on the speed of advancement of the laser head 12 in the operating conditions.

The adjustment can be manual or automatic. In manual adjustment, mechanisms known as valves (not shown) may be used. The automatic adjustment can be implemented using an open loop cycle through parameters predefined by the user, or through a closed loop cycle based on parameters measured in real time during heat treatment and processed by a computer program. The real-time measurement can be carried out for example by means of infrared detectors.

In a possible embodiment of the method according to the invention, the cooling can be of a massive nature, i.e. involving the entire component.

This type of cooling can be performed, for example, through the use of cold rooms, controlled atmosphere cold rooms, baths with cooling substances or by any means that causes the entire mechanical component to cool without interfering with the subsequent heating phase.

In the case of massive cooling through a bath with a cooling substance, the use of substances such as ethylene glycol, having a freezing point of about -50 ° C, may be envisaged. The residence time in the cooling medium must be suitable in order to bring the component to a predetermined uniform temperature in the area of interest.

The initial cooling phase has the purpose of preparing the material for the subsequent heating phase, preventing the decarburization of the outermost layers of the material. In fact, in the known laser hardening treatment, the electromagnetic radiations directly heat the surface of the material, but a portion of the surface of the mechanical component adjacent to the heated surface is also heated by conduction.

The second phase of the hardening heat treatment according to the invention consists in a localized heating of the surface.

Said heating takes place by means of a laser system, indicated as a whole with reference 30, with high-power diodes, for example of the type marketed by "LASERLINE". In this type of laser system you can have a selectable output power between 100W and 10,000W.

The laser head 12 is connected by means of an optical fiber 34 to the electromagnetic radiation generator indicated as a whole with reference number 36.

Said laser head 12, in the operating conditions, moves along the direction indicated with A in figure 1. In particular, it is allowed that the heat treatment is carried out either with the head moving along the direction A or with the component moving along the direction B.

In accordance with a possible embodiment, the regulation of the power takes place in a closed cycle as previously described by means of a pyrometer 32 associated with the head 12 of the laser system 30.

The pyrometer 32 instantly measures the temperature of the surface area 16 hit by electromagnetic radiation, the information is processed by a control unit and compared with a value previously set on the machine.

On the basis of the positive or negative sign and the absolute value of the difference between the detected temperature and the selected hardening temperature, said control unit adjusts the instantaneous power supplied to the electromagnetic radiation, allowing to have a heated surface 16 at an almost constant temperature.

The adjustment of the laser power, associated with the initial cooling, allows to keep the hardening parameters constant.

The parameters which during the heating phase affect the hardening treatment and which can be controlled are the heating speed, the hardening temperature, and the residence time at the hardening temperature.

Given a particular type of steel, it is therefore possible to control the heating speed and the hardening temperature by suitably adjusting the relative advancement speed between the laser head 12 and the mechanical component 20. The hardening temperature, set a priori, is controlled by means of the pyrometer 32 and regulated through the control unit connected to the electromagnetic radiation power regulation system.

The last phase of the hardening heat treatment according to the invention is the drastic cooling to which the heated surface of the mechanical component 20 must be subjected, in order to have the transformation of austenite into martensite.

We then proceed with a second cooling jet similar to the one described above, used for the preparation of the surface to be treated 14.

In particular, as in the previous case, it is preferable to use a nitrogen emulsion or other inert gas, in order to prevent any oxidation phenomena due to the use of water or air.

In fact, unlike the known technique where the drastic cooling is obtained passively through the coldest part of the mechanical component 20, in this case the cooling is obtained actively with the use of the jet. It is therefore also possible to control the cooling phase.

In particular, it will also be possible to treat components of limited thickness, where it is not possible to have a part of the material which, having not been heated, cools the most superficial part by conduction.

In the same way it will be possible in the presence of limited thicknesses to obtain a massive hardening of the entire thickness of the mechanical component to be treated. In a further embodiment, passive cooling of the component is provided, ie by conduction of heat with the component portion at a lower temperature. In particular, said form of cooling can be used in components which have been completely cooled and which have a volume and mass such as to guarantee rapid cooling.

Referring now to the total thermal cycle, according to the thermal treatment described above, it is characterized by an initial cooling phase of the mechanical component, be it local or generalized, to a temperature that is generally lower than the ambient temperature.

This phase is followed by a very fast heating to the hardening temperature, which can reach values comparable to the value of the melting temperature of the material.

The last phase is a drastic cooling down to a temperature that can be higher or lower than the ambient temperature.

A possible embodiment of a plant 13 for the hardening heat treatment according to the invention will now be described in detail. System 13 consists of two basic parts, a first part related to heating and a second part related to cooling.

The first part related to heating uses, as previously explained, laser technology. It is understood that any other type of heating is applicable to the present invention, for example heating by induction, having the same characteristics of heating rate, temperature reached and energy concentration. The laser head 12 is connected to the electromagnetic radiation generator 36 through the use of an optical fiber 34 which allows large relative movements between said head 12 and said generator 36.

The emitter 40 of electromagnetic radiation is positioned on the operating surface 21 of the laser head 12, where the pyrometer 32 previously described is also housed. The pyrometer 32 turns out to be orientable, so that it can accurately measure the temperature of the irradiated surface.

The second part of the system is the cooling system used to control the temperature in the treated area, before and after the heating imposed by the laser head 12 previously described.

The cooling system comprises two nitrogen tanks 24 and 26: a first tank 24 containing nitrogen in a gaseous state at room temperature, and a second tank 26 containing nitrogen in a liquid state at a temperature of about -200 ° C. The tanks are connected by means of pipes 42, 44 to the mixer 28.

According to a possible embodiment, the mixer 28 is in turn connected to a control system which, as previously explained, allows to regulate the mixing of the contents coming from the two tanks 24 and 26.

As previously disclosed, the mixer 28 makes it possible to regulate the temperature and pressure of the two jets.

The mixer 28 is connected through a duct 46 to two nozzles 29 and 31: a first nozzle 29 for cooling the surface area to be treated 14, a second nozzle 31 for cooling the treated surface area 18. Both nozzles 29 and 31 can be obtained in the structure of the laser head 12. Under normal operating conditions, the jets, coming from the two nozzles 29 and 31, strike the component in contiguous areas with respect to the area that undergoes heating 16, so that a first jet invests the area that is to be treated 14 and that the second casting invests the area of the surface just treated 18.

In a possible embodiment, two gates 56 and 58 are provided positioned on the operating surface 21 of the laser head 12 dividing the electromagnetic radiation emitter 40 from the nozzles 29 and 31. The gates 56 and 58 have adequate dimensions to prevent direct interaction between said source and the jets coming from said nozzles 29 and 31.

Referring now to figure 3, a further embodiment of the present invention can provide for the use of two mixers 48 and 50: a first mixer 48 for the first nozzle 29 and a second mixer 50 for the second nozzle 31, so as to regulate regardless of the pressure and temperature of the two jets.

According to this embodiment, the mixer 48 is connected to the tanks 24 and 26 through the pipes 43 and 45 and the mixer 50 is in turn connected to the tanks 24 and 26 through the pipes 60 and 62.

The mixer 48 is also connected to the laser head 12 through the duct 52, while the mixer 50 is connected to the laser head 12 through the duct 54.

The use of the two mixers 48 and 50 can be very useful for the independent regulation of the initial cooling and of the final cooling, in this way it is possible to instantly adjust both jets on the basis of parameters instantaneously measured during the operating conditions, for example example through the use of infrared detectors. In the known art, the heating of the area to be treated 14, due to conduction between said surface and the heated one 16, causes the mobility of the carbon atoms in the outermost surface layers to be increased, thus incurring a decarburization of the surface layer which has yet to be subjected to electromagnetic radiation.

As will become clear to the skilled person, directing a cooling jet of emulsified nitrogen on the area 14 that will be subjected to heating causes the increase in the mobility of the carbon atoms to be inhibited.

Furthermore, a further advantage of this additional phase is that it will be possible to improve the repeatability of the treatment, to the benefit of industrialization and automation. By heating in the usual way, depending on the exposure time of the surface 16 to electromagnetic radiation, the surface 14 that still needs to be treated will have a variable temperature according to the speed of advancement of the laser head 12. By acting with cooling, it will become on the other hand, the temperature of the portion of surface 14 which will undergo heating is homogeneous.

By confining the heating of the material to a restricted surface and controlling the temperature of this and the immediately adjacent areas, a better tempering quality will be obtained from the point of view of hardness distribution.

In addition, the use of high temperature gradients allows you to keep the entire hardening process under control in order to contribute to obtaining a high hardness profile especially on the surface. According to a possible embodiment, the initial cooling step can be carried out through cold rooms, cold rooms with controlled atmosphere and baths, which are not shown in the attached figures.

Figure 8 shows in simplified form a comparison between the microhardness trend as the distance from the external surface of the mechanical component treated according to the invention 94 varies and a curve relating to a conventional heat treatment 96. As it is possible to appreciate, in the curve obtained through the heat treatment according to the invention, the maximum value of the micro-hardness is detectable on the external surface and not at about 0.2 mm from the surface as in the known art. In the following we will describe some of the possible implementations of the machine just described.

Figure 4 schematically represents a possible embodiment using an anthropomorphic robotic arm indicated as a whole with the reference number 64.

According to this embodiment, the movement of the laser head 12 is imparted by the anthropomorphic arm 64. In particular in this solution with a single positioning of the component 20, it will be possible to treat several surfaces, even perpendicular to each other.

Figure 5 schematically represents a possible embodiment using a mobile portal indicated as a whole with the reference 66.

As known, the portal structure comprises a generally rectangular base 74 and two shoulders 68 and 70 fixed in proximity to two parallel sides 76 and 78 of the base 74. The shoulders 68 and 70 are capable of translating along the direction of the two parallel sides 76 and 78 of the base 74.

The shoulders 68 and 70 are connected at the top to a crosspiece 72, parallel to a work surface 80 of the base 74. The crosspiece 72 can move upwards or downwards with respect to the working surface 80. The crosspiece 72 is connected to the laser head 12. The mechanical component to be treated rests on the work surface 80.

Figure 6 schematically shows a further embodiment, in which the laser head 12 is stationary and the mechanical component moves relative to the head 12.

The movable table structure, indicated as a whole with the number 82, comprises a base 84 having an operating surface 86 to which a movable component 88 capable of translating along the surface 86 is connected.

The mechanical component to be treated is positioned on the mobile component 88 and therefore can be moved relative to the laser head 12.

The laser head 12 is fixed to a first horizontal body 90, rigidly fixed to a vertical body 92 generally mounted on one side of the base 84.

The skilled person may, in order to meet specific needs, make changes and / or replacements of the described elements with equivalent elements to the embodiments described above, without thereby departing from the scope of the attached claims.

Claims (13)

CLAIMS 1. Method for a heat treatment, comprising: an initial cooling of the surface to be treated (14); a heating to a predefined temperature, performed by means of electromagnetic radiation emitted by a source of electromagnetic radiation; a stay at said predefined temperature; And a final cooling of the treated surface (18). 2. Method according to the preceding claim characterized in that said heat treatment is a quenching heat treatment. 3. Method according to claim 1 or 2 characterized in that said source of electromagnetic radiation is a laser head (12). 4. Method according to any of the preceding claims characterized in that the initial cooling of the surface to be treated (14) is a massive cooling that involves the entire mechanical component (20) to be treated. 5. Method according to any one of claims 1 to 3 characterized in that said first cooling of the surface to be treated (14) is a localized cooling which involves only the surface of the mechanical component (20) to be treated. Method according to any one of the preceding claims, characterized in that the initial cooling and / or the final cooling are obtained by means of an emulsion of liquid nitrogen in gaseous nitrogen. 7. Plant for a heat treatment according to claim 1, comprising: means for the initial cooling of the surface to be treated (14); means for heating the surface to be treated at a predefined temperature comprising an emitter of electromagnetic radiation; and means for the final cooling of the treated surface (18). 8. Plant according to claim 7 characterized in that: said means for heating the surface to be treated (14) at a predefined temperature comprise a generator of electromagnetic radiation (36) connected by means of an optical fiber (34) to an emitter electromagnetic radiation (40), said electromagnetic radiation emitter (40) being positioned on a laser head (12); 9. Plant according to any one of claims 7 or 8 characterized in that: said means for the initial cooling comprise cold rooms or cold rooms with controlled atmosphere or baths with cooling substances. 10. Plant according to any one of claims 7 to 9, characterized in that: said means for the initial and / or final cooling comprise a tank (24) of nitrogen in the gaseous state and a tank (26) of nitrogen in the liquid state, connected through ducts (42, 44) to a mixer (28), said mixer (28) being in fluid connection with a first nozzle (29) and / or with a second nozzle (31) on the laser head (12) by means of a conduit (46); said mixer (28) creating a nitrogen emulsion at a controlled temperature. 11. Plant according to any one of claims 7 to 10, characterized in that: said means for initial and final cooling comprise a tank (24) of nitrogen in the gaseous state and a tank (26) of nitrogen in the liquid state; a first mixer (48) and a second mixer (50) being connected by means of pipes (42, 44, 60, 62) to said tanks; said first mixer (48) being connected by means of the duct (52) to the first nozzle (29), said second mixer (50) being connected by means of the duct (54) to the second nozzle (31), said mixers (48, 50 ) creating a nitrogen emulsion at a controlled temperature. Plant according to any one of claims 7 to 11, comprising at least one control unit for instantaneous regulation of the temperature of the nitrogen emulsion on the basis of parameters measured on the surface of the mechanical component (20). System according to any one of claims 7 to 12, wherein the laser head (12) is connected to a structure selected from the group comprising: an anthropomorphic robotic arm (64), a portal structure (66), and a structure with movable table (82).