RU2690873C1 - Gas hardening method - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: to obtain uniform hardening and avoiding deformation caused by different cooling speeds, method of gas hardening of steel workpieces includes the first stage of cooling in the temperature range (t1–t2), on which the workpiece is subjected to rapid cooling by forced circulation of cooling gas, second stage (t2–t3), at which circulation of cooling gas is stopped and pressure inside furnace is reduced to provide heat insulation, and the third step (starting from temperature t3), wherein the workpiece is cooled by forced circulation of the cooling gas, wherein the pressure is increased. At the second stage, the workpiece is held at an intermediate temperature which is higher than the temperature of the beginning of the martensite transformation to ensure uniform temperature throughout the workpiece.EFFECT: disclosed is a gas quenching method.4 cl, 5 dwg

Description

Technical field

This invention relates to a gas quenching process in which a workpiece is heated and then cooled with a cooling gas, as quenching of steel.

The level of technology

Hardening of steel is a heat treatment technology in order to obtain a martensitic structure by converting steel into a high-temperature state and then rapid cooling. To date, a liquid quenching method has been repeatedly applied, in which cooling after heating is carried out using, as a cooling agent, a liquid, such as oil, water or a polymer solution, which has a relatively high cooling property to conduct quenching of relatively large parts. In such liquid quenching, however, uneven boiling occurs during quenching. As a result, the cooling rate becomes uneven, thereby making the quality unstable. In addition, it is necessary to provide a washing step to eliminate the cooling agent after quenching; in addition, the treatment of the waste water resulting from the washing also becomes a significant problem.

From this point of view, in recent years, attention has been drawn to gas quenching, in which an inert gas, such as nitrogen gas, is used as a cooling agent, and the cooling gas is given the opportunity to flow, for example, around billets placed in a furnace, thereby by conducting rapid cooling or quenching of the blanks.

In addition, Non-Patent Publication 1 discloses, as a type of gas quenching method, isothermal quenching (also called multistage quenching), in which isothermal aging is carried out for some period of time in the middle of cooling with a hot high temperature gas of about 300 ° C. In this method, the cooling gas is preheated to about 300 ° C using the heat from an enterprise or the like, and this hot gas is circulated through a gas furnace that places billets heated to about 1000 ° C, thereby cooling the billets and conducting isothermal treatment of the blanks to a temperature of about 300 ° C, i.e., in equilibrium with the temperature of the hot gas. Then, after temperature equilibration, a switch is made to the circulation of the cooling gas, which has low temperatures by passing through the cooler, thereby cooling the billet in order to complete the quenching.

Non-Patent Publication 1 describes that the deformation of the workpiece is reduced by carrying out such multi-stage quenching compared to ordinary continuous quenching.

However, in the conventional method, in order to perform multi-stage quenching using a variety of gases having different temperatures, similar to non-patent publication 1, it becomes necessary to provide a gas furnace with a heat exchanger for heating the gas, a cooler for cooling the gas, an air valve for switching the channel, etc. This complicates the device.

In addition, the goal is to obtain an isothermal state through an equilibrium between the temperature of the hot gas and the temperature of the workpiece. Therefore, it takes time, during which the temperature of the workpiece reaches the target temperature of isothermal processing, and the cycle time of the tempering treatment as a whole becomes long.

Prior Art Publications

Non-Patent Publication 1: Akihiro Hamabe, “Using Preheated Inhalation Gas for Vacuum and Isothermal Heat Treatment after Carburizing,” Vacuum Society Journal, Japan, 2010, V. 53, No. 1, pp. 49-52.

Summary of Invention

According to the present invention, a gas quenching method is provided in which a billet made of steel is heated and then cooled for quenching by allowing the cooling gas to flow around the billet in a furnace, the gas quenching method comprises:

stopping the supply of cooling gas in the middle of quenching, before the billet reaches the temperature at which the martensitic transformation begins;

reducing the pressure inside the furnace and creating a uniform temperature throughout the workpiece by means of radiation cooling, while the temperature of the workpiece is maintained at an intermediate temperature that is higher than the temperature at which the martensitic transformation begins; and

resuming the supply of cooling gas after the temperature throughout the billet has been made uniform, thereby conducting quenching to pass the temperature of the onset of the martensitic transformation.

That is, in the quenching method of the present invention, in the middle of quenching with the aid of a cooling gas, the supply of the cooling gas is stopped and the pressure inside the furnace is reduced to restrain the cooling rate of the workpiece. In particular, the action of cooling by convection is quickly suppressed by reducing the pressure inside the furnace, resulting in practically only radiation cooling. In other words, the furnace goes into a thermally insulated state by reducing the pressure, so that the workpiece is temporarily maintained at an intermediate temperature. At this time, heat is transferred in the workpiece from a site with a relatively high temperature to a site with a relatively low temperature, thereby making the temperature throughout the workpiece uniform. Consequently, during subsequent cooling through the supply of a cooling gas, the temperatures throughout the billet undergo the temperature of the beginning of the martensitic transformation at almost the same time and with similar temperature gradients. Thus, quenching is carried out more evenly.

According to the present invention, it is possible to perform multi-stage quenching without the need for a plurality of gases with different temperatures, and the workpiece deformation resulting from quenching is reduced by creating a uniform temperature throughout the workpiece. In addition, compared with the traditional method using hot gas, it is possible to carry out cooling and isothermal treatment to an intermediate temperature in a short period of time, thereby reducing the time of the tempering treatment as a whole.

Brief Description of the Drawings

FIG. 1 is an explanatory view of the structure of a gas hardening furnace used in the gas hardening method of the present invention;

FIG. 2 is an explanatory view showing the steps of the gas quenching method of the example;

FIG. 3 is a perspective view showing one example of a blank;

FIG. 4 is a perspective view of the lower link as a whole, which becomes the workpiece; and

FIG. 5 is a characteristic diagram showing the comparison between an example and a comparative example in the amount of deformation resulting from quenching.

The best way of carrying out the invention

Below, an embodiment of the present invention is explained in detail.

FIG. 1 shows an example of a gas hardening furnace 1 used in the gas hardening method of the present invention. This gas hardening furnace 1 is an elliptical vertical furnace that extends in the vertical direction when viewed from the front. It is formed in its upper part with a fan 2, which circulates the cooling gas in the gas hardening furnace 1 and mixes the cooling gas. In its lower part is a single-stage or multi-stage pallet 3, which houses many of the aforementioned blanks as objects of tempering treatment. This pallet 3 has a lattice structure with many openings, so that the flow of cooling gas (indicated by arrow G in the drawing), injected by the fan 2, is allowed to pass through the pallet 3 and then flow in the upper direction. This pallet 3 is fed into and out of the furnace through the door, not shown on the drawings.

The gas hardening furnace 1 has a hermetic structure that is resistant to a predetermined pressure relief state, and is equipped externally with a pressure relief pump 4 to reduce the pressure in the furnace. This pressure-relief pump 4 is connected to the space inside the furnace through the pressure-relief channel 5, and the pressure-relief channel 5 is equipped with an on / off valve 6 with a solenoid valve, etc.

In addition, the gas hardening furnace 1 is equipped with a channel 7 for introducing a gas for introducing a cooling gas, such as nitrogen gas, hydrogen gas, helium gas or argon gas into the furnace, and a channel 9 for releasing gas to release the cooling gas from the furnace. Channel 7 for the introduction of gas is equipped with a two-position valve 8 with a solenoid valve, etc. Channel 9 for the release of gas is similarly equipped with a two-position valve 10 with a solenoid valve, etc.

FIG. 2 shows an embodiment of a gas quenching method of the present invention using the above gas quenching furnace 1. The blank used in this embodiment is a blank prepared by machining chromium steel SCr420 as a base material into a predetermined shape and then preliminarily performing a carburizing treatment on the surface by gas carburizing. The target surface carbon concentration in the carburizing treatment is 0.6%. Consequently, the material on the surface of the workpiece is one equivalent for SCr460. Carburizing treatment is carried out in another furnace. After calcination at the carburizing treatment temperature, the billet is introduced together with the tray 3 into the gas hardening furnace 1 in the state where it is subjected to reheating to 1050 ° C for hardening.

After closing the door (not shown in the drawings) of the gas hardening furnace 1, the cooling gas is introduced into the gas hardening furnace 1 through the gas injection channel 7. After filling with the cooling gas, the inside of the gas hardening furnace 1 goes into an isolated state by shutting off the on-off valve 8, etc. Fan 2 is then activated to cool the workpiece by forcibly circulating a cooling gas. As a cooling gas, for example, nitrogen gas is used, which has a temperature adjusted to 40 ° C.

FIG. 2 (a) shows the change in temperature of the workpiece; FIG. 2 (b) shows the on-off state of the gas cooling or fan 2, and FIG. 2 (c) shows the on-off state of the pressure relief of the furnace or pump 4 to reduce the pressure. From time t1, the workpiece is rapidly cooled by the forced circulation of the cooling gas. Due to this, the temperature of the workpiece is sharply reduced. FIG. 2 (a) also shows the bainite transformation curve (B), where the conversion to bainite occurs as a result of cooling before the martensitic transformation, but the rate of temperature reduction by means of the cooling gas is set to not pass this bainite transformation curve in the form of a protrusion.

Following this period of rapid cooling, before the temperature of the billet reaches the temperature at which the martensitic transformation begins, fan 2 stops at time t2 in order to stop the circulation and mixing of the cooling gas. At almost the same time, the pressure reduction pump 4 is driven to reduce the pressure inside the gas hardening furnace 1. By stopping the fan 2, the cooling with cooling gas is suppressed. However, the inside of the gas hardening furnace 1 becomes thermally isolated by reducing the pressure inside the gas hardening furnace 1. That is, the cooling action by convection is quickly suppressed, resulting in only minor radiation cooling through radiation from the surface of the workpiece. At the same time, the cooling rate of the billet becomes very low, and the temperature of the billet is temporarily maintained at an intermediate temperature that is higher than the temperature at which the martensitic transformation begins, as shown in FIG. 2 (a). The target intermediate temperature, for example, is 300 ° C, which is slightly higher than the temperature (Ms) of the beginning of the martensitic transformation.

During the period of rapid cooling between the times t1-t2 there are some differences in the cooling rate throughout the workpiece. As shown by the solid line F in FIG. 2 (a), the temperature decreases early in the area with a fast cooling rate. In contrast, as shown by the broken line L, the course of lowering the temperature becomes slow in the area with a relatively slow cooling rate. Consequently, at time t2, temperature drops occur between the sections. While the workpiece is practically in a thermally insulated state by stopping the fan 2 and reducing the pressure, heat is transferred from the relatively high temperature section to the relatively low temperature section, and an isothermal state is obtained throughout the target at the target intermediate temperature (for example, 300 ° C ), which is slightly higher than the temperature of the beginning of the martensitic transformation. That is, the temperature shown by the solid line F and the temperature shown by the broken line L in FIG. 2 (a), converge and maintain at nearly 300 ° C.

In this case, in order to control the stopping of the fan 2 and turning on the pump 4 to reduce the pressure, it is not necessary to monitor the actual temperature of the workpiece using, for example, an infrared-type temperature sensor, etc. and stopping the fan 2 and turning on the pump 4 to reduce the pressure when a predetermined temperature is reached, which is slightly higher than the target intermediate temperature in an isothermal state, due to a delay in temperature change. Alternatively, it is not necessary to experimentally determine the time required for the temperature to fall to a predetermined temperature from time t1, and then stop the fan 2 and turn on the pump 4 to reduce the pressure when the elapsed time from time t1 has reached a predetermined value. In one embodiment, the initial rapid cooling period from time t1 to time t2 is, for example, approximately 45 seconds.

After the isothermal state is completed throughout the workpiece by maintaining the intermediate temperature, at time t3, the pump 4 is turned off to reduce the pressure, the cooling gas is reintroduced into the gas hardening furnace 1 through the gas injection channel 7, and the fan 2 is activated to restart rapid cooling of the workpiece through the forced circulation of the cooling gas. The cooling gas may be the same gas as the gas from the initial rapid cooling period. For example, nitrogen gas is used, whose temperature has been regulated to 40 ° C.

Through the aforementioned rapid cooling, the temperature of the workpiece is reduced in order to cross the temperature (Ms) of the beginning of the martensitic transformation (i.e., pass the temperature (Ms) of the beginning of the martensitic transformation) to carry out quenching. At this time, the isothermal state is reached throughout the workpiece. Thus, throughout the billet, the time point and temperature gradient (cooling rate) with the passage of the temperature of the beginning of the martensitic transformation become constant. Therefore, martensitic transformation occurs uniformly throughout the billet in order to obtain uniform quenching.

The required time from time t2 to time t3 is, for example, about 30 seconds in one embodiment. To control the re-start of cooling at time t3, it is sufficient to experimentally determine the time required for the isothermal state and re-start cooling when the elapsed time from time t2 has reached a predetermined value. Alternatively, it is not necessary to monitor the actual temperatures of many areas of the workpiece using an infrared type temperature sensor, etc. and re-start cooling when they converge, on the whole, at the same temperature.

Cooling, starting from time t3, is carried out, for example, for 2-5 minutes in one embodiment.

Thus, in the hardening method of the aforementioned embodiment, since gas hardening uses one cooling gas, multi-stage hardening is performed, including the first stage of the rapid cooling period between time t1 and time t2, the second stage of the isothermal period between time t2 and time t3 and the third stage of the period of rapid cooling, starting at time t3. Thus, in the presence of the second stage as a period for obtaining an isothermal state at an intermediate temperature, which is slightly higher than the temperature of the beginning of the martensitic transformation, it is possible to carry out uniform quenching with a slight deformation resulting from quenching. In addition, it is possible in the second stage to quickly reduce the rate of cooling using thermal insulation by reducing pressure. Consequently, the time required for the first stage and the second stage becomes short. Thus, for example, compared with the traditional method of using hot gas, the cycle time becomes shorter.

In this document, as shown in FIG. 2 (a), the temperature of the second stage between time t2 and time t3 is set to a temperature that is higher than the temperature (Ms) of the beginning of the martensitic transformation and below the curve of the bainite transformation in the form of a protrusion. That is, the intermediate temperature and the period of the second stage are set so that the characteristic of the change in the temperature of the workpiece does not intersect the bainite transformation curve. In this case, the transformation into bainite during quenching is suppressed.

FIG. 3 shows one example of a blank suitable for the hardening method of the present invention. This preform is a component that forms part of the lower link 11 (see FIG. 4) in the multi-link piston crank mechanism for an internal combustion engine. As described, for example, in Japanese Patent Application Publication No. 2015-42849, this type of lower link 11 is a lower link for connecting an upper link with one end connected to a piston pin and a crank shaft connecting rod. As shown in FIG. 4, the link is formed in its center with a cylindrical bearing fragment 12 of the crankshaft connecting rod crankshaft, which sits on the crankshaft connecting rod journal. In addition, it is provided with a fragment 13 of the finger bosses for the upper finger and a fragment of 14 bosses of the finger for the control finger in positions on opposite sides of almost 180 degrees with an insert between the bearing fragment 12 of the crankshaft connecting rod neck. This lower link 11 as a whole forms a parallelogram close to a rhombus. On the dividing surface 15, passing through the center of the bearing portion 12 of the crankshaft crank shaft, two separate parts are formed, the upper part 11A of the lower link, containing the finger bite fragment 13 for the upper finger, and the lower part of the lower link 11B, containing a finger finger piece 14 for the finger management. The blank of the above embodiment is the aforementioned upper portion 11A of the lower link.

Finger pin 13 for the upper finger in this upper part 11A of the lower link has a split structure to place the upper link in its axial direction in its central fragment. That is, it is formed as a pair of wall-like fragments opposite to each other with the central in-depth fragment 16 placed between them.

This preform, i.e., the upper portion 11A of the lower link, is located on the aforementioned pallet 3 with the orientation shown in FIG. 3. That is, it is held in order to have a vertical orientation in which one side surface 17 (see FIG. 4), perpendicular to the dividing surface 15, becomes the bottom surface, i.e. is brought into contact with the pallet 3, and in which the separation surface 15 is vertical from the pallet 3. Then, the cooling gas is directed parallel to the separation surface 15 in the gas hardening furnace 1, and the cooling gas is allowed to flow along the front and rear surfaces of a pair of wall-like fragments 13 finger bosses.

In quenching with respect to such a preform, the wall-like fragment 13 of the finger's lug has a thinner thickness as compared with the part close to the separation surface 15 and is widely exposed for gas flow. Consequently, in general, the stenobular fragment 13 of the finger's boll becomes a fragment with a rapid cooling rate, and the thick fragment in the vicinity of the separation surface 15 becomes a fragment with a slow cooling rate. In addition, the outer surface and the inner surface (the surface on the side of the in-depth fragment 16) of the steniform fragment 13 of the finger bosses differ in cooling rate. As a result, as far as hardening processes are concerned, they tend to have a deformation in which the steniform fragment 13 of the finger's boss 13 is displaced in the axial direction of the lower link 11.

According to the multi-stage quenching method of the above-described embodiment, it is possible to suppress the deformation of such a steniform fragment 13 of the finger lug in the axial direction.

FIG. 5 shows the results of comparative experiments in the case of a multi-stage quenching method from an example and in the case of simple continuous quenching to continue cooling with a cooling gas, as a comparative example, from the point of view of changing the distance between a pair of fragments 13 finger bosses (in other words, the width of an in-depth fragment 16 in axial direction) due to the above deformation. At the same time, in the quenching of the example, as the first stage, nitrogen gas with a temperature of 40 ° C was introduced under a pressure of 0.6 MPa and circulated through the fan 2, thereby conducting a rapid cooling for 1 minute. Then, as a second stage, the pressure was reduced to 1 kPa, then aging was followed for 30 seconds. In addition, as the third stage, nitrogen gas with a temperature of 40 ° C was introduced under a pressure of 0.6 MPa and circulated by means of a fan 2, thereby conducting cooling for 1 minute. In a comparative example, nitrogen gas with a temperature of 40 ° C was introduced under a pressure of 0.6 MPa and circulated by means of fan 2, thereby cooling for two minutes and thirty seconds.

As shown in the drawing, according to the multi-stage quenching of the example, compared with continuous quenching, the result was obtained that the deformation of the fragment 13 of the thumb boss in the axial direction was reduced by half.

As noted above, one embodiment of the present invention has been explained, but the present invention is not limited to the above embodiment. Various modifications are possible, including processing temperature, time, etc. In addition, the present invention is also suitable for quenching the lower part 11B of the lower link for the lower link 11 shown in FIG. 4, and can be applied to the hardening of other various parts.

Claims (10)

1. A gas hardening method in which a billet made of steel is heated and then cooled for quenching by ensuring the flow of cooling gas around the billet in a furnace, the gas hardening method comprising the steps of: stop the flow of cooling gas in the middle of quenching, before the billet reaches the temperature of the beginning of the martensitic transformation; reduce the pressure inside the furnace and establish a uniform temperature throughout the billet by means of radiation cooling, while the temperature of the billet is maintained at an intermediate temperature that is higher than the temperature at which the martensitic transformation begins; and resume the supply of cooling gas, after the temperature throughout the billet was set to uniform, for hardening and passing the temperature of the beginning of the martensitic transformation. 2. The method of gas hardening according to claim 1, wherein the temperature throughout the billet is set uniform, while the billet is maintained at a temperature that is higher than the temperature at which the martensitic transformation begins and below the bainite transformation curve. 3. The method of gas quenching under item 1 or 2, in which the workpiece has a surface that has previously been subjected to carburizing treatment. 4. The method of gas quenching, including: the first stage in which the billet, made of steel, is subjected to rapid cooling in a furnace by means of a cooling gas from a heated state; the second stage in which the supply of cooling gas to the workpiece is stopped and the pressure inside the furnace is reduced in such a way that in the middle of the temperature reduction of the workpiece it is maintained at an intermediate temperature that is higher than the temperature at which the martensitic transformation begins; and the third stage, in which the rapid cooling is again carried out by means of a cooling gas, after the temperature has been set throughout the workpiece to be uniform.

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