JP6497446B2 - Gas quenching method - Google Patents

Description

  The present invention relates to a gas quenching method in which a workpiece is heated and then cooled using a cooling gas as steel quenching.

  The quenching of steel is a heat treatment technique for obtaining a martensitic structure by rapidly cooling the steel after it is brought to a high temperature state. Conventionally, in order to quench relatively large parts, many liquid quenching methods have been employed in which cooling after heating is performed using a liquid such as oil, water, or a polymer solution having a relatively high cooling property as a coolant. However, in this liquid quenching, boiling occurs non-uniformly during quenching, resulting in a non-uniform cooling rate and unstable quality. In addition, a cleaning process for removing the coolant after quenching is necessary, and the treatment of waste water generated by the cleaning becomes a big problem.

  In view of this, in recent years, an inert gas such as nitrogen gas is used as a coolant, and for example, a gas that rapidly cools or quenches a workpiece by flowing a cooling gas around the workpieces arranged in a furnace. The quenching method is attracting attention.

  Further, in Non-Patent Document 1, as a kind of gas quenching method, isothermal quenching (also called multi-stage quenching) in which a hot gas having a high temperature of about 300 ° C. is used to keep it isothermal for a certain period of time during cooling. ) Is disclosed. In this apparatus, the cooling gas is preheated to about 300 ° C. in advance using factory exhaust heat, etc., and the hot gas is circulated in a gas furnace containing the workpiece heated to about 1000 ° C. The workpiece is isothermally treated at a temperature around 300 ° C. in equilibrium with the temperature of the hot gas. Then, after the temperature equilibration, the work is cooled and the quenching is completed by switching to the circulation of the cooling gas that has passed through the cooler and becomes a low temperature.

  Non-Patent Document 1 describes that by performing such multi-stage quenching, distortion generated in the workpiece is reduced as compared with normal continuous quenching.

  However, in the conventional method of realizing multi-stage quenching using a plurality of gases having different temperatures as in Non-Patent Document 1, a gas furnace, a heat exchanger for gas heating, a cooler for gas cooling, A damper for switching the flow path is required, which complicates the equipment.

  In addition, since the temperature is equalized by the balance between the hot gas temperature and the workpiece temperature, it takes time until the workpiece temperature converges to the target isothermal processing temperature, and the cycle time of the entire quenching process becomes longer. .

Akihiro Hamabe, “Vacuum quenching and carburizing marquenching using hot gas”, Journal of the Vacuum Society of Japan, The Vacuum Society of Japan, 2010, Vol. 53, No. 1, p. 49-52

The present invention is a gas quenching method in which a workpiece made of steel is heated, and the workpiece is cooled and quenched by flowing a cooling gas around the workpiece in a furnace.
During the quenching process before the workpiece reaches the martensitic transformation start temperature, the cooling gas supply is stopped,
The work in the furnace using a vacuum pump to insulation by applying reduced the furnace to a low vacuum state lower than the atmospheric pressure, while maintaining the workpiece temperature to a high intermediate temperature than the martensitic transformation starting temperature, radiation cooling By soaking each part of the work,
After each part of the work is soaked, the supply of the cooling gas is resumed, and quenching is performed so as to pass the martensite transformation start temperature.

  That is, in the quenching method of the present invention, the cooling rate of the workpiece is suppressed by stopping the supply of the cooling gas and quenching the furnace during quenching using the cooling gas. In particular, due to the reduced pressure in the furnace, the cooling effect due to convection is quickly suppressed, and substantially only radiative cooling is achieved. In other words, the inside of the furnace is insulated by the reduced pressure, and the workpiece is temporarily maintained at an intermediate temperature. At this time, heat moves from a relatively high temperature part to a relatively low temperature part in the work, so that each part of the work is soaked. Therefore, when cooling by supplying the cooling gas thereafter, each part of the work passes through the martensite transformation start temperature almost simultaneously and with the same temperature gradient, so that the quenching is performed more uniformly.

  According to the present invention, multistage quenching can be realized without requiring a plurality of gases having different temperatures, and distortion of the workpiece accompanying quenching is reduced by soaking each part of the workpiece. Moreover, compared with the conventional method using a hot gas, cooling to an intermediate temperature and soaking can be performed in a short time, and the cycle time of the entire quenching process is shortened.

Structure explanatory drawing of the gas quenching furnace used for the gas quenching method of this invention. Explanatory drawing which shows the process of the gas quenching method of one Example. The perspective view which shows an example of a workpiece | work. The perspective view of the whole lower link used as a workpiece | work. The characteristic view which shows the distortion amount accompanying hardening by comparing with an Example and a comparative example.

  Hereinafter, an embodiment of the present invention will be described in detail.

  FIG. 1 shows an example of a gas quenching furnace 1 used in the gas quenching method of the present invention. The gas quenching furnace 1 is a vertical furnace having an oblong shape that is vertically elongated when viewed from the front, and a fan 2 that circulates a cooling gas in the gas quenching furnace 1 and agitates the cooling gas in an upper part thereof. Is provided. In the lower part, one or a plurality of trays 3 on which a plurality of workpieces, which will be described later, to be quenched are arranged are arranged. The tray 3 has a lattice shape having a large number of openings so that a cooling gas flow (indicated by an arrow G in the figure) sent by the fan 2 can pass through the tray 3 and flow upward. It is configured. The tray 3 is taken in and out of the furnace through a door (not shown).

  The gas quenching furnace 1 has a sealed structure that can withstand a predetermined reduced pressure state, and includes a decompression pump 4 for decompressing the inside of the furnace. The decompression pump 4 is connected to a space in the furnace through a decompression passage 5, and the decompression passage 5 includes an on-off valve 6 made of an electromagnetic valve or the like.

  The gas quenching furnace 1 includes, for example, a gas introduction passage 7 for introducing a cooling gas made of nitrogen gas, hydrogen gas, helium gas, argon gas, or the like into the furnace, and a cooling gas from the furnace. A gas discharge passage 9 for discharging. The gas introduction passage 7 includes an on-off valve 8 made of an electromagnetic valve or the like, and the gas discharge passage 9 has an on-off valve 10 made of an electromagnetic valve or the like.

  FIG. 2 shows an embodiment of the gas quenching method of the present invention using the gas quenching furnace 1 described above. The workpiece used in this embodiment is obtained by carburizing the surface by gas carburization in advance after machining into a predetermined shape using, for example, chromium steel of SCr420. The target carbon concentration on the surface in the carburizing process is 0.6%. Therefore, the material on the workpiece surface is equivalent to SCr460. The carburizing process is performed in a separate furnace, gradually cooled from the carburizing temperature, and then carried into the gas quenching furnace 1 together with the tray 3 while being reheated to 1050 ° C. for quenching.

  After sealing a door (not shown) of the gas quenching furnace 1, a cooling gas is introduced into the gas quenching furnace 1 through the gas introduction passage 7, and when the cooling gas is filled, the on-off valve 8 is closed and the gas quenching is performed. The inside of the furnace 1 is sealed. Then, the fan 2 is driven to cool the work by forced circulation of the cooling gas. For example, nitrogen gas whose temperature is adjusted to 40 ° C. is used as the cooling gas.

  2A shows the temperature change of the workpiece, FIG. 2B shows the gas cooling, that is, the ON / OFF state of the fan 2, and FIG. 2C shows the pressure reduction in the furnace, that is, the ON / OFF state of the pressure reducing pump 4. However, the workpiece is rapidly cooled by forced circulation of the cooling gas from time t1. Thereby, the temperature of a workpiece | work falls comparatively rapidly. FIG. 2 (a) also shows a bainite transformation curve (B) in which transformation to bainite occurs before the martensite transformation with cooling, but crosses this nose-like bainite transformation curve. In order to prevent this from happening, the rate of temperature decrease by the cooling gas is set.

  Following such a rapid cooling period, before the workpiece temperature reaches the martensitic transformation start temperature, at time t2, the fan 2 is stopped, and the circulation and agitation of the cooling gas are stopped. At substantially the same time, the decompression pump 4 is operated to decompress the interior of the gas quenching furnace 1. Although the cooling by the cooling gas is suppressed by stopping the fan 2, the inside of the gas quenching furnace 1 is further insulated by further reducing the pressure inside the gas quenching furnace 1. That is, the cooling effect by convection is quickly suppressed, and only a slight amount of radiation cooling by radiation from the workpiece surface is achieved. As a result, the cooling rate of the workpiece becomes very small, and the temperature of the workpiece is temporarily held at an intermediate temperature higher than the martensitic transformation start temperature, as shown in FIG. The target intermediate temperature is, for example, 300 ° C., which is slightly higher than the martensite transformation start temperature (Ms).

  During the rapid cooling period between times t1 and t2, there is a slight difference in the cooling rate at each part of the workpiece, and at a portion where the cooling rate is fast, the temperature decrease progresses early as shown by the solid line F in FIG. On the other hand, as shown by the broken line L, the progress of the temperature decrease is delayed at the portion where the cooling rate is relatively slow. Therefore, at time t2, there is a temperature difference in each part. While the work is substantially insulated by stopping the fan 2 and reducing the pressure, the part from the part where the temperature is relatively high in the work. Heat moves to a relatively low temperature, and each part of the work is soaked in the vicinity of a target intermediate temperature (for example, 300 ° C.) slightly higher than the martensite transformation start temperature. That is, the temperature indicated by the solid line F and the temperature indicated by the broken line L in FIG.

  Here, for controlling the stop of the fan 2 and the ON operation of the decompression pump 4 at time t2, the actual temperature of the workpiece is monitored using, for example, an infrared temperature sensor, and soaking is performed in consideration of a delay in temperature change. The fan 2 may be stopped and the decompression pump 4 may be turned on when the predetermined temperature is slightly higher than the intermediate target temperature. Alternatively, the required time from the time t1 until the temperature decreases to a predetermined temperature is experimentally obtained, and when the elapsed time from the time t1 reaches a predetermined value, the fan 2 is stopped and the decompression pump 4 is started. You may make it do. In one embodiment, the initial quenching period between times t1 and t2 is, for example, about 45 seconds.

  When the temperature equalization of each part of the work is completed by maintaining the intermediate temperature, at time t3, the decompression pump 4 is turned off and the cooling gas is reintroduced into the gas quenching furnace 1 through the gas introduction passage 7. Above, the fan 2 is driven and the rapid cooling of the workpiece by the forced circulation of the cooling gas is resumed. The cooling gas may be the same as that in the initial quenching period, for example, nitrogen gas whose temperature is adjusted to 40 ° C. is used.

  By the above rapid cooling, the temperature of the workpiece is lowered across the martensite transformation start temperature (Ms) (that is, passes through the martensite transformation start temperature (Ms)), and quenching is performed. At this time, since each part of the work is soaked, the timing and temperature gradient (cooling rate) when the part of the work passes the martensite transformation start temperature are constant. Therefore, martensite transformation occurs uniformly in each part, and uniform quenching is obtained.

  The required time between times t2 and t3 is, for example, about 30 seconds in one embodiment. As control for restarting cooling at time t3, the time required for soaking may be experimentally obtained, and cooling may be restarted when the elapsed time from time t2 reaches a predetermined value. Alternatively, the actual temperatures at a plurality of locations on the workpiece may be monitored using an infrared temperature sensor or the like, and cooling may be resumed when these temperatures converge to substantially the same temperature.

  In one embodiment, the cooling after time t3 is performed for about 2 to 5 minutes, for example.

  Thus, in the quenching method of the above embodiment, as the gas quenching using a single cooling gas, the first stage, which is a rapid cooling period between times t1 and t2, and the soaking between times t2 and t3. A multi-stage quenching comprising a second stage that is a conversion period and a third stage that is a rapid cooling period after time t3 is realized. Thus, by providing the second stage in which the soaking period is set at an intermediate temperature slightly higher than the martensitic transformation start temperature, uniform quenching can be performed, and distortion associated with quenching is reduced. In addition, since the cooling rate can be quickly reduced using heat insulation by decompression as the second stage, the time required for the first stage and the second stage is shortened, for example, compared to a method using a conventional hot gas. Cycle time is shortened.

  Here, the temperature of the second stage between the times t2 and t3 is higher than the martensitic transformation start temperature (Ms) and lower than the nose-shaped bainite transformation curve as shown in FIG. Set to That is, the intermediate temperature and the period of the second stage are set so that the temperature change characteristic of the workpiece does not cross the bainite transformation curve. Thereby, the transformation to bainite during quenching is suppressed.

  FIG. 3 shows an example of a workpiece suitable for the quenching method of the present invention. This work is a part constituting a part of the lower link 11 (see FIG. 4) in the multi-link type piston crank mechanism of the internal combustion engine. This type of lower link 11 connects an upper link having one end connected to a piston pin and a crank pin of a crankshaft, as described in, for example, Japanese Patent Application Laid-Open No. 2015-42849. As shown in FIG. 2, the upper pin pin boss part has a cylindrical crank pin bearing part 12 fitted in the crank pin at the center and at positions opposite to each other by about 180 ° across the crank pin bearing part 12. 13 and a pin boss portion 14 for a control pin are provided. The lower link 11 has a parallelogram shape close to a rhombus as a whole, and a lower link upper 11A including an upper pin pin boss portion 13 and a control pin pin boss on a split surface 15 passing through the center of the crankpin bearing portion 12. The lower link lower 11B including the portion 14 is divided into two parts. The work of the above embodiment is the lower link upper 11A.

  The pin boss portion 13 for the upper pin in the lower link upper 11A has a bifurcated configuration so as to sandwich the upper link in the axial central portion, that is, a pair of wall-like surfaces facing each other with the central concave portion 16 interposed therebetween. It has become a thing.

  Such a work, that is, the lower link upper 11A is arranged on the tray 3 with the posture shown in FIG. That is, one side surface 17 (see FIG. 4) orthogonal to the dividing surface 15 becomes a bottom surface in contact with the tray 3, and the dividing surface 15 is held in a vertical posture so as to rise vertically from the surface of the tray 3. In the gas quenching furnace 1, the cooling gas is guided in parallel to the dividing surface 15, and the cooling gas flows along the front and back surfaces of the pair of pin boss portions 13 that form a wall shape.

  In quenching such a workpiece, the wall-shaped pin boss portion 13 is generally thinner than the portion near the dividing surface 15 and is widely exposed to the gas flow. 13 is a part with a fast cooling rate, and a thick part near the dividing surface 15 is a part with a slow cooling rate. Moreover, the cooling rate is different between the outer side surface and the inner side surface (surface on the concave portion 16 side) of the wall-shaped pin boss portion 13. As a result, distortion that the wall-shaped pin boss portion 13 is displaced in the axial direction of the lower link 11 is likely to occur with quenching.

  According to the multi-stage quenching method of the above embodiment, such axial distortion of the wall-shaped pin boss portion 13 can be suppressed.

  FIG. 5 shows the amount of change in the distance between the pair of pin boss portions 13 (in other words, the axial width of the concave portion 16) due to the above-described distortion, according to the case of the multi-stage quenching method of the example and the cooling gas as a comparative example The results of a comparative experiment in the case of simple continuous quenching with continued cooling are shown. Here, in the quenching of the example, as a first stage, nitrogen gas at 40 ° C. is sealed at a pressure of 0.6 MPa, circulated by the fan 2 and rapidly cooled for 1 minute, and then reduced to 1 kPa as a second stage. Held for 30 seconds, and as a third stage, nitrogen gas at 40 ° C. was sealed at a pressure of 0.6 MPa, circulated by the fan 2 and cooled for 1 minute. In the comparative example, nitrogen gas at 40 ° C. was sealed at a pressure of 0.6 MPa, circulated with the fan 2, and cooled for 2 minutes and 30 seconds.

  As shown in the figure, according to the multi-stage quenching of the example, the axial strain of the pin boss portion 13 was reduced by half compared to the continuous quenching.

  As mentioned above, although one Example of this invention was described, this invention is not limited to the said Example, A various change including the temperature of processing, time, etc. is possible. The present invention is also suitable for quenching the lower link lower 11B of the lower link 11 shown in FIG. 4, and can be applied to quenching other various parts.

Claims (4)

In a gas quenching method in which a workpiece made of steel is heated and the workpiece is cooled and quenched by passing a cooling gas around the workpiece in a furnace,
During the quenching process before the workpiece reaches the martensitic transformation start temperature, the cooling gas supply is stopped,
The work in the furnace using a vacuum pump to insulation by applying reduced the furnace to a low vacuum state lower than the atmospheric pressure, while maintaining the workpiece temperature to a high intermediate temperature than the martensitic transformation starting temperature, radiation cooling By soaking each part of the work,
A gas quenching method in which the supply of the cooling gas is resumed after each part of the work is soaked, and the quenching is performed so as to pass the martensite transformation start temperature.   The gas quenching method according to claim 1, wherein the work temperature is maintained at a temperature that is higher than the martensitic transformation start temperature and lower than the bainite transformation curve, so that each part of the work is soaked.   The gas quenching method according to claim 1 or 2, wherein the workpiece is previously carburized. A first step of rapidly cooling a workpiece made of steel from a heated state in a furnace with a cooling gas;
In the middle of the temperature drop of the workpiece, the pressure reducing pump stops the supply of the cooling gas to the workpiece and insulates the workpiece in the furnace so that the workpiece maintains an intermediate temperature higher than the martensite transformation start temperature. A second step of reducing the pressure in the furnace to a reduced pressure lower than atmospheric pressure using
A third step of quenching again with a cooling gas after the work has been soaked;
A gas quenching method comprising:

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