WO2003031661A1

WO2003031661A1 - Heat treatment method

Info

Publication number: WO2003031661A1
Application number: PCT/FI2002/000783
Authority: WO
Inventors: Pekka Kemppainen; Jorma JÄÄSKELÄINEN; Erkki Leinonen; Hannu Vuorikari; Eero Smura
Original assignee: Metso Paper, Inc.; Lahden Lämpökäsittely Oy; Urv Uudenkaupungin Rautavalimo Oy
Priority date: 2001-10-08
Filing date: 2002-10-07
Publication date: 2003-04-17
Also published as: DE60208405T2; ES2250700T3; EP1434891B1; FI20011954L; EP1434891A1; DE60208405D1; FI20011954A0; ATE314492T1

Abstract

The invention relates to a heat treatment method, in which method an object/objects is/are heat-treated, in which method the heat treatment takes places in a heat treatment furnace or the like. In the method, in connection with heat treatment, heat transfer of the object/objects is controlled by adjusting the rate of change of the temperature of the object/objects being heat-treated to the desired level in different heat treatment phases.

Description

Heat treatment method

The invention relates to a heat treatment method according to the preamble of claim 1.

In some heat treatment methods known from prior art the objects being heat- treated are first heated to a hardening temperature, after which they are cooled, for example in salt, oil, lead or water baths or in air one batch to be treated at a time.

It is necessary to handle the hot objects when transporting them into the furnace and into cooling and after this further, for example, into a furnace for stress relief annealing, after which they are cooled, for example, to room temperature.

Traditional batch-type heat treatment also consumes a considerable amount of energy, because the objects have to be heated/cooled several times. In addition, the many treatments and transportations require a lot of work. The transportation of hot objects is also problematic in view of occupational safety.

Furthermore, the oil, salt and lead used in cooling are hazardous waste and as such detrimental to environment. Also, oil and salt baths age in use, which is why these cooling mediums have to be replaced from time to time. Figure 2A shows phases of a heat treatment method known from prior art. In the first phase A the heat treatment furnace is charged and the objects to be heat-treated are placed on the grate of the furnace. After this, in phase B, the furnace is charged with energy and the furnace is heated to anneal the objects. In phase C the red-hot objects are taken out of the furnace, for example by a forklift truck, after which they are transported into a cooling tank with the help of a crane. In phase D the cooled objects are lifted up from the cooling tank, and in phase E the objects are transported into the next heat treatment phase, for example into a tempering treatment furnace. In phase F the furnace is charged with energy, the furnace is heated, the objects are annealed and cooled together with the furnace. Finally, in phase G, after the furnace has cooled down to a temperature of e.g. 200 °C, the objects are discharged from the furnace to cool down to room temperature.

A problem related to prior art arrangements is that the cooling capacity cannot be controlled during the cooling process.

It is an object of the invention to provide a heat treatment method that eliminates, or at least minimizes, the problems of the arrangements known from prior art.

An object of the invention is to provide a programmable heat treatment method, in which heat treatment takes place programmably with the desired heat-transfer capacities in different temperature ranges.

A further object of the invention is to provide a heat treatment method that imparts a good quality to the objects that are heat-treated.

To achieve the afore-mentioned objects and those objects that come out later the heat treatment method according to the invention is mainly characterized in what is presented in the characterizing part of claim 1.

According to the invention by adjusting the heat-transfer capacity of the object in the cooling phase (convection, radiation) it is possible to control/adjust the rate of change of the temperature of the object to be cooled to the desired level in differ- ent (heat treatment) cooling phases. This method enables the cooling process to be controlled so that the desired phase changes and/or microstructures can be achieved.

According to an advantageous feature of the invention the cooling rate of the ob- ject is adjusted by changing the convection-based heat-transfer capacity and/or by changing the radiation heat-transfer capacity. Heat transfer by radiation can be controlled by changing the radiation absoφtion of the furnace walls and by changing (adjusting) the absorption and/or convection properties of the heat radiation of the heat-transfer medium and its endothermic mass. In connection with the invention the cooling rate of the furnace and of the object are regulated, for example with the help of the amount of air blown into the furnace and with the flow rate of air (air/gas and/or air/particle). The cooling capacity can be increased by means of a mixture of liquid and gas or of air and secondary gas. The mixture of blast air and liquid (and gas) may also contain solid particles. The cooling capacity can also be adjusted by means of the temperature of the mixture to be blown in. The gas mixture to be blown in may be e.g. an air-water, air-carbon dioxide, argon- carbonic acid mixture (or the like).

According to an advantageous application of the invention the object to be cooled is brought from the heat treatment furnace into a (hardening) cooling space, most suitably into a cloud chamber, where the object is (hardened) cooled in a controlled manner at the desired cooling rate in different temperature ranges to achieve the desired material properties. By adjusting/controlling the rate of change of the temperature of the object in different temperature ranges it is possible to generate desired phase changes in the material of the object and to bring about desired micro structures as a result of the treatment. The cooling rate of the object is adjusted by changing the cooling capacity produced by the system. The cooling of the object/objects takes place in a cloud chamber, into which the needed volume/mass flow of a mixture of air/gas/liquid/solid particle is fed to achieve the desired cooling capacity and rate: A very advantageous and environ- mentally friendly application is to feed an air-water spray, the water being in the form of drops, into the cooling space through one or more nozzles. The cooling nozzles may also direct the air-water spray (or, more commonly, the gas-liquid- solid particle) (flow) directly at the object. In this application of the invention the cooling rate of the object can be controlled to take place locally so that the cooling rate of the different parts of the object is adjusted by directing intensified cooling gas-liquid-particle spray jets locally at the desired areas of the object, for example by applying, in a desired area, the nozzle jet directly and/or by increasing the flow rate and mixture ratio of the cooling medium (gas-liquid-particle mixture) that impinges on the object locally, and/or the areas in which it is desired to slow down the heat flow may be subjected to a weaker cooling medium flow, whereby a low flow rate and/or a less effective heat-transferring mixture ratio is/are used in the flow.

According to a further advantageous feature of the invention the capacity of radiation into the furnace walls is adjusted by changing the absorption properties of the atmosphere and lining of the furnace (i.e. how efficiently the lining takes in ("sucks") or reflects heat radiation). The basic equation of heat radiation is used as a basis here

Q = δ • ε • A (T⁴ _0bject - T⁴waii), where

ε = emissivity of the surface of the object δ = Stefan-Bolzmann constant

A = area

T = temperature.

The heat radiation transfer capacity of the wall surfaces of the furnace can, for example, be altered by using grates that are coated with different kinds of coatings and whose heat radiation absoφtion properties differ from the absoφtion properties of the furnace.

According to further advantageous features of the invention a method of radiation-based heat transfer capacity is to alter the absoφtion properties of the heat radiation of the heat-transferring (blast) gas mixture that has been fed in. According to a further advantageous feature of the invention heat transfer by convection can be regulated by altering the convection properties of the heat- transferring gas mixture.

According to a further advantageous feature of the invention by altering the heat transfer properties of different areas of the object's surface it is possible, if necessary, to equalize the temperature distributions and cooling rates of the object. It is also possible to adjust the transfer and radiation properties of heat by covering desired areas of the object being heat-treated with a ceramic or other correspond- ing layer. It is further possible to locally create cooling capacities of different sizes in the object by controlling the flow rates of the cooling gas in different ways on different surfaces.

According to a further advantageous feature of the invention it is possible to con- trol the cooling process based on measurement results by measuring the temperature with temperature-measuring sensors fastened, e.g. welded to the object. The sensors can be located on the surface of the object, and, when necessary, the temperature can also be measured from within the object with a temperature measurement sensor placed in a hole drilled in the object.

With the heat treatment method according to the invention it is possible to save a considerable amount of energy (about 20...30 %) when the cooling process is halted at an intermediate temperature (instead of cooling down to room temperature). The heat treatment method according to the invention does away with the need to transport the red-hot object from the furnace into a cooling tank or water- air shower or freely into air. This reduces the amount of work required by the handling of the objects (cf. Figures 2A and 2B).

The invention is suited, for example, for the heat treatment method of heavily loaded paper machine components made of ferrous metal, e.g. bearing housings, suspension parts for articulated rolls, and the like. The invention will now be described in more detail with reference to the figure of the accompanying drawing. However, the invention is not strictly limited to the details thereof.

Figure 1A schematically shows an exemplary heat treatment arrangement according to the invention.

Figure IB schematically shows a possible cooling phase in the exemplary heat treatment arrangement of Figure 1A.

Figures IC and ID are schematic illustrations of principle of the temperature and cooling capacity of the object.

Figure 2 A schematically shows a batch-type heat treatment process according to prior art.

Figure 2B schematically shows a heat treatment process according to the invention.

Figures 2C - 2E schematically show an advantageous exemplifying embodiment of the invention, in which a cloud chamber is used.

Figure 3A schematically shows an exemplifying embodiment for controlling the heat treatment process according to the invention.

Figure 3B schematically shows an exemplifying embodiment for controlling the application according to Figures 2C - 2E.

According to the invention by adjusting the heat -transfer capacity of cooling in the object (convection, radiation) the rate of change of the temperature of the ob- ject being hardened (quenched) is controlled/adjusted to the desired level in different heat treatment phases. This method enables the cooling process to be controlled so that the desired phase and/or microstructure changes can be achieved - an embodiment thereof is schematically shown in Figure 1 A. In the heat treatment curve schematically shown in Figure 1A, time t is located on the horizontal axis and temperature T on the vertical axis. The temperatures and treatment times mentioned in the following description are exemplary only and the heat treatment temperatures and times naturally depend on the heat treatment processes, on the objects to be treated and on their materials and desired properties.

According to Figure 1 A, the temperature of the object/objects being heat-treated is first raised to treatment temperature Ti (e.g. to an austenizing temperature of 920 °C (normalization, pearlitization annealing temperature)) with the desired raising rate (20...80 °C/h, e.g. 40 °C/h).

1) Holding time ti; the object/objects are held at the holding temperature Ti in question depending on the heat treatment process for the desired holding time (e.g. wall thickness 100 mm 4 h). At point (1) the controlled/adjusted cooling process is started.

1-2) Phase; lowering of the temperature of the object at the desired rate from temperature Ti to temperature T₂ above the phase change temperature, during the time period t₂. In this phase the temperature is lowered as needed for achieving a uniform temperature in the object. From point (2) to point (3), time pe- riod t , the temperature of the object is equalized, if necessary, whereby the heat capacity of the object is minimized before cooling/rapid lowering of temperature.

3) The object/objects has/have a uniform temperature i.e. its temperature is = T3.

3) Cooling/(hardening) of the object/objects is started with the desired cooling capacity. The cooling capacity is determined by the desired type of micro- structure, the desired properties, depending on the microstructure in the starting phase, and on the composition of the material.

3)-4) Cooling time is - Between phases 3 - 4 of the cooling process there may also be constant temperature stages and/or several different cooling rate stages, whereof an example is shown in Figure IB.

4) Intensified, rapid cooling i.e. lowering of temperature can be stopped below temperature T₄ of the phase change range in question.

4)-5) The temperature is lowered to achieve intermediate temperature T₅, time period t₅.

5) The cooling process is halted. The temperature in question is above e.g. the temperature at which martensite is formed, thereby preventing, for example, the starting of a martensite reaction.

5)-6) The holding time X of constant temperature T₆, by means of which holding time the temperature of the object is equalized. With the holding time it is possible to affect the desired microstructure properties (process time of reactions). By this means it is also possible to affect the microstructure properties produced in the following heat treatment phases.

6)-7) Raise of temperature of the object/objects from temperature T₆ to temperature T₇ (for example to tempering temperature or quenching and tempering temperature) with the raising rate of temperature (e.g. 30 °C/h) desired for the next heat treatment phase, time period t . 7)-8) Holding time t₈ at temperature T₇, T₈ (e.g. tempering temperature, adjustment of residual stress level to the desired level) required for multi-phase heat treatment.

8)-9)-10b) Lowering of the temperature of the object/objects from temperature T₈ to temperature T₉ -> T_]0b at the desired rate; the cooling rate may vary in different temperature ranges (for example according to the desired level of residual stress of the object), time periods t₉, tι₀.

Figure 1A shows a two-phase heat treatment process. The same arrangement may of course be applied in heat treatment with, for example, three or more phases, where, at first, e.g. cooling is carried out, then quenching and tempering and then stress relief annealing.

8)-9)-10a) Lowering of the temperature of the second heat treatment phase at the desired rate to intermediate temperature Tι_0a in heat treatment with three or more phases. After the intermediate temperature Tιo_a has been reached, the temperature of the object is equalized and the heat treatment is continued with phase 3. Cycle 6) - 10a) is repeated with new set values.

Figure IB shows an example in which cooling phase 3 - 4 is halted at an intermediate temperature between phases 3.1 and 3.2.

Figures IC - ID are schematic illustrations of principle of the temperature and cooling capacity of the object. In Figure IC the temperature T of the object is located on the vertical axis and time t on the horizontal axis. As shown in Figure IC a curve 51 representing the temperature of the object starts to sink from the heating temperature, point 1, and the cooling temperature is altered after point 2 and further at point 3 to achieve the desired material properties. In Figures ID time t is located on the horizontal axis and cooling capacity P and heat flow q are located on the vertical axis. The cooling capacity P and the heat flow q as a function of time t are depicted by curve 52 and the phases of temperatures 1, 2, 3 shown in Figure IC are marked in connection with the curve with corresponding numbers. According to Figures IC - ID intensified cooling, curves 51, 52, of the object/objects can be stopped at a de- sired temperature of the object, when providing the cooling spray in a chamber, illustrated in more detail in connection with Figures 2C and 2E, by decreasing or closing nozzle flows and by decreasing the liquid-particle mixture ratio.

Figure 2 A shows the phases of a heat treatment method known from prior art. In the first phase A a heat treatment furnace 10 is charged. Objects 12 to be heat- treated, the objects having been brought near the furnace 10 by a forklift truck 11 or the like, are placed on a grate 13 of the furnace 10. After this, in phase B, the furnace 10 is charged with energy, as shown by arrow 15, and the furnace 10 is heated to anneal the objects 12. In phase C the red-hot objects 12 are transported to a cooling tank 16, e.g. by a forklift truck 11, from which the objects 12 are moved into the cooling tank 16 with the help of a crane 14 or the like and in phase D the cooled objects 12 are lifted up from the cooling tank 16 and in phase E the objects 12 are transported into the furnace of the next heat treatment phase, for example into a tempering furnace 17, after which, in phase F, the furnace is charged with energy 15, the furnace 17 is heated, the objects 12 are annealed and cooled together with the furnace 17. Finally, in phase G, after the furnace 17 has cooled down to a temperature of e.g. 200 °C, the objects 12 are discharged from the furnace 17. The objects 12 have been shifted, for example onto the forklift truck 11 to cool down to room temperature.

Instead, in an arrangement according to the invention, the arrangement being shown in Figure 2B, objects 12 to be heat-treated are transported in the first phase R with a forklift truck 11 or the like onto a grate 13 of a heat treatment furnace 10, in phase S the furnace 10 is charged with energy 15 and heat treatment is carried out, for exam- pie by means of a heat treatment process according to Figure 1A. hi phase T the heat-treated objects 12 are taken out of the furnace 10 e.g. with the forklift truck 11. In an advantageous application of the invention shown in Figures 2C - 2E cloud chamber cooling is used, in which application the cooling i.e. hardening space is a semi-closed space, into which the object/objects are brought to be hard- ened/cooled. Cooling-gas-particle mist is fed into the cloud chamber 30 through one or more nozzles 31. In Figure 2C the cloud chamber is marked with reference number 30. The cloud chamber 30 comprises a set of distributor pipes 36 and mixers 32. A particle, gas and liquid flow is passed into the mixers 32 from ducts 33, 34, 35. From the nozzles 31 of the set of distributor pipes 36 the cooling me- dium is blown into the cloud chamber 30. Exhaust flow of gas is marked with a reference arrow 37, which exhaust flow of gas is passed into exhaust gas suction 38. Figure 2D schematically shows a nozzle structure used, according to the invention, in connection with a cloud chamber 30, which nozzle has been connected to a mixer 32, where the liquid, gas and particle jets passed through ducts 33, 34 and 35 are mixed so that the desired composition is reached for being fed into the chamber 30 through a nozzle 31. The chamber 30 can have the geometrical form of a cylinder or a chamber and the quenching space can be a semi-closed space or a space otherwise equipped with appropriate degassing. The nozzles 31 used in connection with the cloud chamber 30 may be fixed, and the nozzle 31 generates a gas-liquid-particle spray and the adjustment of mixture and flow takes place at the beginning of the set of distributor pipes connecting the nozzles 31. The nozzles 31 may be adjustable, and fixed or movable nozzles 31 may be used to direct the flow, and the adjustment and control of the shaping of the flow opening and flow jet are carried out by means of a controller 39 and the adjustment and control take place in the mixer 32. The mixer 32 may be a mixer that can be controlled continuously or set at correct values in advance, and it is possible to form an entity out of the intermediation of the mixer 32 and the nozzle 31, which entity is moved with the help of the movements of the mountings to the desired place and in the desired blow direction in the cloud chamber 30. Figure 2E schematically shows nozzles 31 connected to a distributor pipe 36, into which distributor pipe 36 a flow is conducted through a mixer 32, into which the flows are conducted through ducts 33, 34, 35 in order to be blown into a cloud chamber 30 through the nozzles 31.

Figure 3A depicts controlling of the heat treatment method. Charging data 22 relating to the objects to be heat-treated and parameters 23 of the furnace are fed into a control unit 20. The temperature inside 24 and on the surface 25 of the object/objects placed in the furnace 21 is monitored and the data is fed into the control unit 20. The control unit 20 processes the data and, with control 28, controls 26 the volume flow and temperature (Vl/Tl) of the cooling air 31 fed into a mixer 25 and the mass flow 27 (or volume flow) and temperature (m2/T2) of an additive possibly mixed with the cooling medium. A cooling fan (flow rate) 32 of the furnace is controlled with control 29. If necessary, temperature data of the cooling air 31 and the additive 27 mixed are measured and given to the control unit 20. Heat treatment data from the control unit 20 is also transmitted and used for heat treatment reports asso- ciated with a quality system 30.

Figure 3B shows a control scheme for the exemplifying embodiment according to Figures 2C - 2E. In this application the cooling of a hot object means that the transferring heat energy of the object is removed from the object at the desired heat trans- fer capacity. According to the figure a thermal model 41 of the object is formed based on the thermal property 42 of the material, the object 43 itself (form, mass, starting temperature, material) and on the cooling curve 44 of the object, after which the cooling capacity 45 needed is determined, when the data is combined with the data 46 of the cooling medium and the data 47 of the mixer of the nozzle. After this, the volume and mass flow needed are provided as a function 48 of time verified on the basis of measurement data 49 obtained by a temperature measurement 53. After this, control and adjustment 54 of the nozzles in each mixer is carried out. Based on the cooling capacity 45 needed and measurement corrections 49, quality reports 55 are determined, with the help of which quality reports it is possible to make the con- trol and adjustment parameter more precise and receive data concerning the temperatures measured from the object/objects.

Claims

1. A heat treatment method, in which method an object/objects is/are heat-treated, in which method heat treatment takes places in a heat treatment furnace or the like, in which method, in connection with heat treatment, heat transfer from the object/objects is controlled by adjusting the rate of change of the temperature of the object/objects being heat-treated to the desired level in different heat treatment phases, characterized in that in the method the cooling process is controlled by adjusting the heat-transfer capacity in order to achieve the desired phase changes and/or microstructure.

2. A method according to claim 1, characterized in that the method comprises the following phases: a) that the heating/cooling process is halted at an intermediate temperature to equalize the temperature distribution of the object, b) the cooling process is halted at an intermediate temperature above the phase change temperature to equalize the temperature, c) that after the cooling phase the cooling process is halted at an intermediate temperature and the treatment is continued with the next heat treatment phase, d) that the cooling phase may comprise one or more intermediate temperatures and/or cooling rate sequences that vary in duration and in temperature level, e) the cooling process is halted at an intermediate temperature before the start of the next heat treatment phase.

3. A method according to claim 1 or 2, characterized in that in the method the different phases of heat treatment are carried out in the same heat treatment furnace or the like without moving the object/objects from the heat treatment furnace or the like before the treatment is completed.

4. A method according to claim 1 - 3, characterized in that in the method the object/objects are heated in a heat treatment furnace and cooled in a cloud chamber (30).

5. A method according to claim 1 - 4, characterized in that in the method the heat- transfer capacity of the object/objects being heat-treated is adjusted by means of convection.

6. A method according to one of claims 1 - 4, characterized in that the convection properties of the heat-transfer medium are regulated.

7. A method according to one of claims 1 - 4, characterized in that the endother- mic phase change properties of the heat-transfer medium are adjusted.

8. A method according to claim 1 - 4, characterized in that in the method the heat- transfer capacity of the object/objects being heat-treated is adjusted by means of radiation.

9. A method according to claim 1 - 4, characterized in that in the method the cooling rate of the object being heat-treated is adjusted by changing the convection-based heat-transfer capacity and/or the radiation heat-transfer capacity.

10. A method according to claim 8 or 9, characterized in that in the method the radiation heat transfer capacity is controlled by changing the radiation absoφtion of the furnace walls.

11. A method according to claim 8 or 9, characterized in that in the method the radiation heat transfer capacity is controlled by changing the heat radiation ab- soφtion properties of the heat-transfer medium.

12. A method according to claim 8 or 9, characterized in that in the method the radiation heat transfer capacity and/or the convection-based heat transfer capacity are controlled by changing the endothermic mass of the heat-transfer medium.

13. A method according to one of claims 1 - 4, characterized in that in the method the amount and flow rate of the heat-transfer medium blown into the furnace are adjusted.

14. A method according to claim 8 or 9, characterized in that in the method radiation capacity and/or convection are adjusted by means of the composition of the heat-transfer medium.

15. A method according to claim 8 or 9, characterized in that in the method radia- tion capacity is adjusted by changing the absoφtion properties of the furnace lining and/or of the heat-transfer medium.

16. A method according to one of claims 1 - 16, characterized in that in the method the heat transfer and radiation properties of the object are adjusted by covering desired areas of the object being heat-treated with a layer that changes said properties and/or by controlling the flow rates of the heat treatment medium in different ways on different surfaces of the object.

17. A method according to one of claims 1 - 16, characterized in that in the method the heat treatment process is controlled based on measurement results obtained by measuring the temperature of the object/objects being heat-treated.