JP2008266729A - Method for heating steel-made workpiece - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heating method with which a temperature difference between steel-made workpieces or the temperature difference between the surface and the core of the steel-made workpiece can be reduced. <P>SOLUTION: The steel-made workpiece 11 in a heating chamber 20 is shifted to a front chamber 15 (an arrow mark 3) as shown in (c) before the temperature of the steel-made workpiece reaches a target temperature, and when the shift is finished, a sectioning door 18 is closed. Then, the steel-made workpiece 11 is held in the front chamber 15 for 0.5-2 min and after the lapse of this holding time, the steel-made workpiece 11 is shifted to the heating chamber 20 as per the arrow mark (4). The steel-made workpiece is taken out from the heating chamber one time or several times and naturally cooled in gas. Since the surface temperature of the steel-made workpiece itself is lowered and the temperature difference between the surface and the center is reduced. In the case of a plurality of the steel-made workpieces, since the temperature of the steel-made workpiece near a heater is lowered, the temperature difference between the steel-made workpiece near the heater and the one far from the heater is reduced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an improvement in a method for heating a steel workpiece such as a gear.

Since gears and shafts are high-strength parts, such steel workpieces are heat-treated for the purpose of improving the strength. For heat treatment, surface strengthening methods such as quenching, carburizing, and nitriding are known. Especially, the vacuum carburizing method which implements carburizing in a vacuum or non-oxidizing gas is widely known (for example, refer patent document 1).
JP 2002-180235 A (FIG. 1)

Patent document 1 is demonstrated based on the following figure.
FIG. 5 is a diagram for explaining the basic principle of the conventional technique, in which a group of processes is shown on the horizontal axis.
In the heating step, the steel material part is heated in nitrogen gas at atmospheric pressure from room temperature to carburizing temperature (900 to 1050 ° C.).
In the activation step, the surface of the steel part is activated by changing the atmosphere to a vacuum at the carburizing temperature.

In the carburizing step, carburization is performed by blowing hydrocarbon gas as the carburizing gas and maintaining in a reduced pressure atmosphere of 1.3 to 2.6 kPa.
In the diffusion step, the atmosphere is changed to a vacuum of 13 to 26 Pa to promote diffusion.
In the temperature lowering step, nitrogen gas is blown, and the steel parts are lowered to 820 to 890 ° C. in nitrogen gas at atmospheric pressure.

Among the above process groups, the following problems occurred in the heating process.
FIG. 6 is an explanatory view of a conventional heating process. A vacuum heating furnace 100 shown in FIG. 6A is a thermoelectric device that includes a door 102 in a furnace body 101, a heater 103 in the furnace, and measures the atmospheric temperature in the furnace. A heating control unit 105 that controls the outputs of the pair 104 and the heater 103 is provided, and a vacuum pump 106 and a nitrogen gas supply source 107 are provided.

  The door 102 is opened, the steel workpiece 110 is charged into the furnace, and the door 102 is closed. The vacuum pump 106 exhausts air and gas in the furnace. The vacuum pump 106 is stopped, nitrogen gas is supplied from a nitrogen gas supply source 107, and the furnace is filled with nitrogen gas at atmospheric pressure. Next, the heater 103 is energized, and the heating controller 105 controls the furnace temperature so that the furnace atmosphere becomes a carburizing temperature (for example, 930 ° C.).

When the steel workpiece 110 is a gear, as shown in (b), a large number of gears 112A, 112B, 112C, 112D, 112E, 112F, etc. are stored side by side in a heat-resistant rack jig basket 111. The jig basket 111 is charged into the furnace.
The temperature T1 of the gear 112A at the upper corner of the rack jig basket 111 and the temperature T2 of the gear 112B at the center of the rack jig basket 111 were each measured experimentally.

Then, as shown in (c), the temperature T1 rose rapidly. Since the gear 112A in the upper corner is directly heated by the heater 103, the temperature rises earlier than anywhere.
On the other hand, the temperature T2 was extremely slow to rise.

  The central gear 112B shown in (b) can hardly be expected to be directly heated by the heater 103 shown in (a). Moreover, since the gap between the gears 112A to 112F is small and the flow of nitrogen gas is stagnant, the convective heat transfer effect is small. Therefore, the gear 112B mainly receives heat by the thermal radiation of the peripheral gears 112C to 112F. Therefore, the temperature of the central gear 112B is the slowest.

It is not preferable that the temperature difference ΔT between the temperature T1 and the temperature T2 is extremely large. The heat histories of the gear 112A and the gear 112B are different, causing heat treatment failure.
When the steel workpiece 110 is a single block, the surface temperature is T1 and the core temperature is T2, and depending on the material, heat cracking or thermoplastic deformation occurs due to thermal stress.

  In any case, a compression technique with a temperature difference ΔT is required.

  This invention makes it a subject to provide the heating method which can make small the temperature difference between the steel workpieces, or the temperature difference of the surface of a steel workpiece, and a core.

  The invention according to claim 1 is a heating method for heating a steel workpiece to a target temperature for heat treatment, wherein the steel workpiece is heated to a target temperature in a heating chamber one or more times. The steel workpiece is taken out of the heating chamber and naturally cooled in gas. Natural cooling refers to cooling by natural convection.

  The invention according to claim 2 is characterized in that the steel workpieces are arranged in a plurality of stages.

  In the invention which concerns on Claim 3, natural cooling in gas is complete | finished before the center temperature of steel workpieces begins to fall.

  In the invention which concerns on Claim 4, the timing which takes out a steel workpiece from a heating chamber is determined based on the surface temperature of a steel workpiece.

  In the invention which concerns on Claim 5, the timing which takes out a steel workpiece from a heating chamber is determined based on the atmospheric temperature of a heating chamber.

  In the invention which concerns on Claim 6, the timing which complete | finishes natural cooling in gas is determined based on the surface temperature of the steel workpiece | work after taking out from a heating chamber, It is characterized by the above-mentioned.

  In the invention which concerns on Claim 7, the timing which complete | finishes the natural cooling in gas is determined based on the elapsed time after taking out from a heating chamber, It is characterized by the above-mentioned.

  The invention according to claim 8 is characterized in that the steel workpiece is a gear for carburizing and quenching.

  In the invention according to claim 1, the steel workpiece is taken out from the heating chamber once or a plurality of times and is naturally cooled in gas. In the steel workpiece itself, the surface temperature decreases, so the temperature difference between the surface and the center is reduced. When there are a plurality of steel workpieces, the temperature of the steel workpiece closer to the heater is lowered, so that the temperature difference with the steel workpiece farther from the heater is reduced.

  The invention according to claim 2 is characterized in that the steel workpieces are arranged in a plurality of stages. Since the temperature of the steel workpiece closer to the heater is lowered, the temperature difference with the steel workpiece farther from the heater is reduced.

  In the invention which concerns on Claim 3, natural cooling in gas is complete | finished before the center temperature of steel workpieces begins to fall. Since the steel workpiece has a high surface and a low center, the temperature of the center rises for a while even if the surface cools. However, if the center temperature starts to decrease, it is supercooled and consumes a lot of heat energy in the next heating. In order to reduce the consumption of heat energy, the natural cooling in the gas ends before the center temperature of the steel workpiece starts to drop.

  In the invention which concerns on Claim 4, the timing which takes out a steel workpiece from a heating chamber is determined based on the surface temperature of a steel workpiece. If the temperature of the steel workpiece, the reliability increases.

  In the invention which concerns on Claim 5, the timing which takes out a steel workpiece from a heating chamber is determined based on the atmospheric temperature of a heating chamber. A temperature sensor for measuring the ambient temperature is always provided in the heating chamber. Since this temperature sensor can be used, the cost is not increased.

  In the invention which concerns on Claim 6, the timing which complete | finishes the natural cooling in gas is determined based on the surface temperature of the steel workpiece | work after taking out from a heating chamber. If the temperature of the steel workpiece, the reliability increases.

  In the invention which concerns on Claim 7, the timing which complete | finishes the natural cooling in gas is determined based on the elapsed time after taking out from a heating chamber. If it is time, it can be easily measured, so there is no cost increase.

  The invention according to claim 8 is characterized in that the steel workpiece is a gear for carburizing and quenching. For gears that require strength, uniform heating is desired. If the temperature difference is small before carburizing and quenching, preferable carburizing and quenching can be performed.

The best mode for carrying out the present invention will be described below with reference to the accompanying drawings. The drawings are viewed in the direction of the reference numerals.
FIG. 1 is a principle view of a vacuum carburizing treatment facility. A vacuum carburizing treatment facility 10 includes a work platform 12 for waiting a steel work 11, a front chamber 15 having a door 13 and a nitrogen gas supply source 14, It consists of an oil quenching tank 17 connected to the bottom of the front chamber 15 and storing oil 16, and a heating chamber 20 connected to the front chamber 15 via a partition door 18.

  The heating chamber 20 includes a furnace body 21, a heater 22 disposed inside the furnace body 21, a thermocouple 23 that detects the ambient temperature, and a temperature detected by the thermocouple 23 so that the temperature is a predetermined temperature. The temperature control unit 24 controls the output of the heater 22, the vacuum pump 25 that exhausts the interior of the furnace body 21, and the nitrogen supply source 26 that supplies nitrogen gas to the furnace body 21.

  The steel workpiece 11 is the same as that described with reference to FIG. 6B, and includes a large number of gears 28 </ b> A and 28 </ b> B arranged in a heat-resistant rack jig basket 27. For convenience, the gear 28A is one of the gears closest to the heater 22, and the gear 28B is the gear farthest from the heater 22.

Next, the heating process performed in the vacuum carburizing treatment facility 10 having the above-described configuration will be described.
FIG. 2 is an explanatory view of the heating process according to the present invention. In (a), the door 13 is opened and the steel workpiece 11 is inserted into the front chamber 15 (arrow (1)). When the charging is finished, the door 13 is closed. The atmosphere in the front chamber 15 is about 100 ° C. nitrogen gas.

  Next, in (b), the partition door 18 is opened and the steel workpiece 11 is charged into the heating chamber 20. Conventionally, the steel workpiece 11 is heated to the target temperature (900 ° C. to 1000 ° C.) in the heating chamber 20. This conventional temperature rise curve is as shown in FIG.

  In the present invention, before the temperature of the steel workpiece 11 reaches the target temperature (maximum heating temperature or carburizing temperature), the steel workpiece 11 in the heating chamber 20 is moved to the front chamber 15 as shown in (c) ( Arrow (3)). Needless to say, the partition door 18 is closed when the movement is completed. Then, the steel workpiece 11 is held in the front chamber 15 for 0.5 minutes to 2 minutes. When this holding time has elapsed, the steel workpiece 11 is moved to the heating chamber 20 as shown by the arrow (4). In the heating process, the atmosphere in the heating chamber 20 was nitrogen gas corresponding to atmospheric pressure.

  In order to accurately measure the heating rise of the steel workpiece 11 in FIGS. 2B to 2C, a thermocouple for measurement is directly attached to the gear 28A and the gear 28B, and the relationship between time and temperature is examined. It was. The temperature of the gear 28A is referred to as T1, and the temperature of the gear 28B is referred to as T2. The results will be described with reference to the next figure.

FIG. 3 is a diagram showing a temperature rise curve of a workpiece according to the present invention, where the horizontal axis is time and the vertical axis is temperature. The “heating chamber” along the horizontal axis indicates that the workpiece has been charged into the heating chamber, and the “front chamber” indicates that the workpiece has been transferred to the front chamber.
The temperature T1 rises rapidly, and the temperature T2 rises gradually, but at the time of P1 (timing), the work was moved to the front chamber at around 100 ° C., so the temperature T1 was changed from 890 ° C. to 790 ° C. It went down at once. And since the workpiece | work was returned to the heating chamber at the time (timing) of P2, temperature T1 begins to rise.

  On the other hand, the temperature T2 becomes flat at the time point P1. This is presumably because the gear 28B is surrounded by other gears, so that it is less affected by the ambient temperature and receives heat from the other gears.

  T101, T101, and T102 indicated by imaginary lines are conventional curves. At time P3, the temperature difference ΔT101 between T101 and T102 was about 300 ° C. On the other hand, the temperature difference ΔT1 between the curves T1 and T2 according to the present invention was about 220 ° C.

  Moreover, since the workpiece | work was moved to the front chamber of about 100 degreeC at the time (timing) of P4, temperature T1 fell at a stretch from 930 degreeC to 850 degreeC. And since the workpiece | work was returned to the heating chamber at the time (timing) of P5, temperature T1 begins to rise.

On the other hand, the temperature T2 becomes flat at the time point P4.
T101 and T102 indicated by imaginary lines are conventional curves. At P6, the temperature difference ΔT102 between T101 and T102 was about 120 ° C. In contrast, the temperature difference ΔT2 between the curves T1 and T2 according to the present invention was about 60 ° C.

As described above, in the heating process, the workpiece is moved from the heating chamber to the front chamber in the middle of heating, so that the temperature T1 (the temperature of the gear arranged at the periphery) and the temperature T2 (the temperature of the gear arranged at the center) ) Was reduced by 30 to 50% compared to the conventional method.
As a result, it has become possible to prevent the occurrence of heating cracks due to an extreme temperature difference.

  As described in the embodiments, the present invention is useful when a large number of gears and shafts are arranged in a rack jig basket (a plurality of shelves). However, it can be applied to a single large workpiece. In this case, the temperature difference between the surface of the large workpiece and the center (core) is reduced.

  In addition, as is clear from FIG. 3, it is expected that when the work is placed in the front chamber for a long time, the work is overcooled, and the central temperature T2 also changes from being flat to falling. Therefore, it is desirable to set the time point P2 and the time point P5 before turning down. If it starts to descend, the next heating time becomes longer, and the heat energy to be input increases. If it is before turning down, the consumption of heat energy can be suppressed. In addition, the heating time (total time) is extended when turning down, but the heating time is not extended before turning down. Therefore, a decrease in productivity can be suppressed.

Next, what is important is setting the timing for moving the workpiece from the heating chamber to the front chamber.
In the experiment, when the target temperature was 930 ° C, the temperature at P1 was 890 ° C. Therefore, it is recommended to detect the surface temperature of the workpiece and to move the workpiece from the heating chamber to the front chamber when this temperature reaches 890 ° C. The surface temperature of the workpiece can be measured with a radiation thermometer or a radiation thermometer. If the temperature of the workpiece, the reliability of the control is increased.

As an alternative, there is control by the temperature of the atmosphere. Ta indicated by a broken line in FIG. 3 is the atmospheric temperature in the heating chamber. Ta increases toward the target temperature immediately after the workpiece is loaded. Although the atmospheric temperature Ta is larger than T1 in principle and the difference between Ta and T1 decreases with time, the tendency of the curves is very similar. Therefore, when the temperature of the atmosphere reaches Ta1 (for example, 920 ° C.), it is recommended to move the workpiece from the heating chamber to the front chamber.
An existing thermocouple can be used to measure the ambient temperature. Since there is no need to provide a radiation thermometer or a radiation thermometer, the equipment cost can be reduced.

Moreover, the timing which returns a workpiece | work from a front chamber to a heating chamber is also important.
In the example, when 60 seconds (1 minute) has elapsed from P1, the workpiece is returned to the heating chamber. It is recognized that the point in time has elapsed is P2. If it is time, certification is easy and the cost of the control system can be suppressed.

  However, it is ideal to manage by temperature. Therefore, the surface temperature of the workpiece is measured with a radiation thermometer or a radiation thermometer, and the time when the temperature from P1 falls to a predetermined temperature is defined as P2.

Another embodiment of the vacuum carburizing equipment will be described.
FIG. 4 is a principle diagram of a vacuum carburizing treatment facility according to another embodiment. The vacuum carburizing treatment facility 30 is connected to a front chamber 32 having an entrance door 31 and a partition door 33 to the front chamber 32. A first heating chamber 34; an intermediate chamber 36 connected to the first heating chamber 34 via a partition door 35; a second heating chamber 38 connected to the intermediate chamber 36 via a partition door 37; The rear chamber 41 is connected to the second heating chamber 38 via a partition door 39, and the oil quenching tank 42 is connected to the bottom of the rear chamber 41. 43 is an exit door, 44 is a thermocouple, 45 is a nitrogen gas supply source, 46 is a vacuum pump, and 47 is a heater.

  The steel workpiece 11 is transferred from the front chamber 32 to the first heating chamber 34, and heating up to P1 in FIG. Next, the steel workpiece 11 in the first heating chamber 34 is moved to the intermediate chamber 36, and the cooling immediately after P1 in FIG. When this cooling is finished, the work in the intermediate chamber 36 is transferred to the second heating chamber 38 and heating to the target temperature is performed. This vacuum carburizing equipment 30 is suitable for a heating method in which a steel workpiece is cooled only once in a heating process.

  If the steel workpieces 11 flow from the left to the right in the figure and do not return to the left, the steel workpieces 11 can be sequentially inserted into the vacuum carburizing treatment facility 30 from the entrance door 31 to increase productivity. be able to. In addition, by returning the steel workpiece 11 from the second heating chamber 38 to the intermediate chamber 36, cooling can be performed a plurality of times. Furthermore, it is possible to give various heating conditions by separately connecting a heating chamber and a carburizing chamber (not shown) to the intermediate chamber 36 in parallel.

That is, the structure of the vacuum carburizing treatment facility for carrying out the method of the present invention is not limited to that shown in FIG.
In the embodiment, nitrogen gas is used. However, as the gas used in the present invention, a gas generally used for heat treatment such as endothermic gas, exothermic gas, and inert gas can be appropriately used. Needless to say.

  The steel workpiece can be of any type as long as it is a mechanical part other than a gear and a shaft.

  The present invention is suitable for a gear heating process.

It is a principle diagram of a vacuum carburizing treatment facility. It is explanatory drawing about the heating process which concerns on this invention. It is a figure which shows the temperature rising curve of the workpiece | work which concerns on this invention. It is a principle figure of the vacuum carburizing processing equipment concerning another example. It is a figure explaining the basic principle of the prior art. It is explanatory drawing of the conventional heating process.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Vacuum carburizing processing equipment, 11 ... Steel workpiece, 15 ... Front chamber, 20 ... Heating chamber, 27 ... Rack jig basket, 28A, 28B ... Gear.

Claims (8)

A heating method for heating a steel workpiece to a target temperature for heat treatment,
A method for heating a steel workpiece, wherein the steel workpiece is taken out of the heating chamber once or a plurality of times and is naturally cooled in a gas while the steel workpiece is heated to a target temperature in a heating chamber. The method of heating a steel workpiece according to claim 1, wherein the steel workpiece is arranged in a plurality of stages. The method for heating a steel workpiece according to claim 1 or 2, wherein the natural cooling in the gas is finished before the center temperature of the steel workpiece starts to decrease. The method for heating a steel workpiece according to claim 1, wherein the timing of taking out the steel workpiece from the heating chamber is determined based on a surface temperature of the steel workpiece. The method for heating a steel workpiece according to claim 1, wherein the timing for taking out the steel workpiece from the heating chamber is determined based on an atmospheric temperature of the heating chamber. The method for heating a steel workpiece according to claim 1, wherein the timing of ending natural cooling in the gas is determined based on the surface temperature of the steel workpiece after being taken out of the heating chamber. The method for heating a steel workpiece according to claim 1, wherein the timing for ending natural cooling in the gas is determined based on an elapsed time after removal from the heating chamber. 2. The method for heating a steel workpiece according to claim 1, wherein the steel workpiece is a gear for carburizing and quenching.

Images