JP6350866B2 - Sinter hardening method - Google Patents

Description

  The present invention relates to a sintering hardening method.

  Sinter hardening is known as a technique for manufacturing a high-strength sintered part. Sinter hardening is a method in which a workpiece formed by compression molding a powder material is heated and sintered (sinter), and then the workpiece is hardened by quenching the workpiece in the subsequent cooling process. .

  As an example of this sinter hardening, a method described in Patent Document 1 is conventionally known. In the sintering hardening method of Patent Document 1, a workpiece formed by compression molding a powder material containing iron, chromium, molybdenum and carbon is 1200 ° C. or higher which is higher than a general sintering temperature (about 1130 ° C.). And a quenching step of hardening the workpiece structure by quenching the workpiece to a temperature lower than the martensite transformation point after the sintering step.

  Here, in patent document 1, in order to re-compress (what is called sizing) the workpiece | work hardened | cured by the rapid cooling process, it adjusts so that the cooling rate of the workpiece | work in a rapid cooling process may be 1 degrees C / sec or less.

  That is, in Patent Document 1, in order to harden the structure of the workpiece, the workpiece is rapidly cooled from a high temperature of 1200 ° C. or higher to a temperature lower than the martensite transformation point, but the cooling rate when the workpiece is rapidly cooled is too large. Then, the workpiece is transformed into a structure having a lot of martensite, and the hardness of the workpiece becomes too high, so that subsequent sizing (recompression process of the workpiece) cannot be performed. Here, sizing is a process for correcting distortion of a workpiece caused by uneven cooling when the workpiece is rapidly cooled. For example, as shown in Patent Document 2, the workpiece is set in a mold, and the workpiece Is recompressed at a pressure of about 200 to 900 MPa to correct the distortion of the workpiece. In order to obtain a hardness that allows this sizing, in Patent Document 1, the cooling rate of the workpiece in the rapid cooling process is adjusted to be 1 ° C./second or less. At this time, the workpiece after rapid cooling has a structure mainly composed of bainite and partially mixed with martensite.

JP 2014-80642 A JP 2004-292840 A

  As described above, when sizing is performed after the workpiece is sintered, it is necessary to manufacture a mold for sizing, and thus there is a problem that the manufacturing cost of the sintered part is increased. On the other hand, if sizing after sintering the workpiece is omitted, distortion caused by uneven cooling when the workpiece is rapidly cooled cannot be removed, and high dimensional accuracy cannot be obtained.

  It is an object of the present invention to provide a sinter hardening method capable of producing a high hardness sintered part having extremely high dimensional accuracy without performing sizing after sintering.

A sintering hardening method according to an aspect of the present invention includes:
A sintering step in which a workpiece formed by compression molding a powder material containing iron, chromium, molybdenum, and carbon is heated to a temperature of 1100 ° C. or higher and lower than 1200 ° C., and sintered;
After the sintering step, a cooling and holding step for cooling and holding the workpiece at a temperature lower than 900 ° C. and higher than the austenite transformation point;
After the cooling and holding step, a gas quenching step of rapidly cooling the workpiece to a temperature lower than the martensite transformation point to martensite by blowing a cooling gas to the workpiece;
Is a sinter hardening method.

  According to the above, a high-hardness sintered part having extremely high dimensional accuracy can be manufactured without performing sizing after sintering.

It is a top view which shows the workpiece | work which sinter and harden | cure with the sintering hardening method concerning embodiment of this invention. It is sectional drawing along the II-II line of FIG. It is sectional drawing which shows the continuous sintering furnace used with the sintering hardening method concerning embodiment of this invention. It is sectional drawing which shows the conveyance state of the carbon tray which mounted the workpiece | work. It is a figure which shows typically the time change of the temperature of a workpiece | work when sintering and hardening of a workpiece | work are performed with the sintering hardening method which concerns on embodiment of this invention. High hardness obtained by the sintering hardening method according to the embodiment of the present invention, which sequentially undergoes a sintering step of 1100 to 1200 ° C., a cooling and holding step of A ₃ transformation point to 900 ° C., and a gas quenching step of 2 ° C./second or more. and sintered parts (measurement example 1), 1200 ° C. or more sintering process, a ₃ cooling holding step of transformation point to 900 ° C., sinter hardening of the order undergo comparative example and 2 ° C. / sec or more gas quench step It is a graph which shows the result of having measured the roundness of the outer periphery of a boss | hub part with the high-hardness sintered component (measurement example 2) obtained by the method, respectively. High hardness obtained by the sintering hardening method according to the embodiment of the present invention, which sequentially undergoes a sintering step of 1100 to 1200 ° C., a cooling and holding step of A ₃ transformation point to 900 ° C., and a gas quenching step of 2 ° C./second or more. and sintered parts (measurement example 1), 1200 ° C. or more sintering process, a ₃ cooling holding step of transformation point to 900 ° C., sinter hardening of the order undergo comparative example and 2 ° C. / sec or more gas quench step It is a graph which shows the result of having measured the flatness of the lower end surface of a flange part with the high-hardness sintered component (measurement example 2) obtained by the method, respectively.

[Description of Embodiment of the Present Invention]
(1) A sintering hardening method according to an aspect of the present invention includes:
A sintering step in which a workpiece formed by compression molding a powder material containing iron, chromium, molybdenum, and carbon is heated to a temperature of 1100 ° C. or higher and lower than 1200 ° C., and sintered;
After the sintering step, a cooling and holding step for cooling and holding the workpiece at a temperature lower than 900 ° C. and higher than the austenite transformation point;
After the cooling and holding step, a gas quenching step of rapidly cooling the workpiece to a temperature lower than the martensite transformation point to martensite by blowing a cooling gas to the workpiece;
Is a sinter hardening method.
In this case, since the workpiece is sintered at a temperature of less than 1200 ° C. in the sintering process, the shrinkage of the workpiece due to sintering can be suppressed to be extremely small as compared with the case where the workpiece is sintered at a temperature of 1200 ° C. or higher. It is possible to prevent a reduction in dimensional accuracy due to the contraction of the workpiece. In addition, since the work is once cooled and held at a temperature lower than 900 ° C. before the work is rapidly cooled, the occurrence of uneven cooling is suppressed as compared with the case where the work is rapidly cooled from a temperature of 1200 ° C. or higher. Can be cooled uniformly. Therefore, it is possible to manufacture a high-hardness sintered part having extremely high dimensional accuracy without performing sizing after sintering.
(2) As said powder material, 2-4 mass% chromium, 0.3-0.7 mass% molybdenum, 0.3-0.8 mass% carbon, and the remainder iron are contained. It is preferable to use one.
If the chromium content is 2% by mass or more, the work structure can be effectively martensitic, and if the chromium content is 4% by mass or less, the density of sintered parts obtained by sintering the work is ensured. Can do. If molybdenum is 0.3% by mass or more, the toughness of the sintered part obtained by sintering the workpiece can be effectively increased, and if molybdenum is 0.7% by mass or less, the material cost of the sintered part Can be kept within a practical range. If carbon is 0.3 mass% or more, the structure of the workpiece can be effectively martensitic, and if carbon is 0.8 mass% or less, the density of sintered parts obtained by sintering the workpiece Can be secured.
(3) The cooling rate of the workpiece in the gas quenching step is preferably 2 to 5 ° C./second.
If the workpiece cooling rate is 2 ° C / second or more, the structure of the workpiece can be effectively martensitic. If the workpiece cooling rate is 5 ° C / second or less, the occurrence of uneven cooling is suppressed, The work can be effectively and uniformly cooled.

[Details of the embodiment of the present invention]
Specific examples of the sintering hardening method according to the embodiment of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to a claim are included.

  FIG. 1 and FIG. 2 show a workpiece 1 that is sintered and hardened by a sintering hardening method according to an embodiment of the present invention. The workpiece 1 is obtained by compression molding a powder material with a mold. The powder material constituting the workpiece 1 contains 2 to 4% by mass of chromium, 0.3 to 0.7% by mass of molybdenum, 0.3 to 0.8% by mass of carbon, and the balance iron. What is used is used.

  Here, when chromium is 2% by mass or more, the structure of the work 1 can be effectively martensite in the gas quenching step described later, and when chromium is 4% by mass or less, the work 1 is sintered. The density of the obtained sintered part can be ensured. If the molybdenum content is 0.3 mass% or more, the toughness of the sintered part obtained by sintering the workpiece 1 can be effectively increased, and if the molybdenum content is 0.7 mass% or less, the material of the sintered part Costs can be kept within a practical range. If the carbon content is 0.3 mass% or more, the structure of the workpiece 1 can be effectively martensite in the gas quenching step described later, and if the carbon content is 0.8 mass% or less, the workpiece 1 is sintered. The density of the sintered parts obtained can be ensured.

  In this embodiment, the sintered part obtained by sintering the workpiece 1 is a synchro hub. The synchro hub is a component used in the synchro mechanism that absorbs the rotation difference when the gear is switched in the transmission. It has high strength that can withstand torque transmitted from the engine and motor, and wear resistance against sliding with surrounding components. Is required.

  The workpiece 1 includes an annular flange portion 2 that is horizontally disposed, and a cylindrical boss portion 3 that is provided so as to protrude in the axial direction from the upper end surface 2 a of the flange portion 2. A through hole 4 in the vertical direction is provided at the center of the boss portion 3. On the outer periphery of the boss portion 3, a plurality of tooth portions 5 are provided at equal intervals in the circumferential direction. Further, the boss portion 3 is provided with a plurality of notches 6 opened in the axial end surface 3a of the boss portion 3 at intervals in the circumferential direction. On the outer periphery of the flange portion 2, a plurality of tooth portions 7 are provided at equal intervals in the circumferential direction.

  FIG. 3 shows a continuous sintering furnace 10 used in the sintering hardening method according to the embodiment of the present invention. The continuous sintering furnace 10 includes a degassing chamber 11, a preheating chamber 12, a sintering chamber 13, a slow cooling chamber 14, a quenching chamber 15, and a cooling chamber 16 in order from the upstream side to the downstream side.

  Each of these chambers 11 to 16 is provided with a plurality of rollers 18 for moving the carbon tray 17 on which the work 1 (see FIG. 4) is placed from the upstream side to the downstream side. Each roller 18 is arranged at intervals along the moving direction of the workpiece 1 and is driven to rotate independently by a driving device (not shown). In the degassing chamber 11, the preheating chamber 12, the sintering chamber 13, and the slow cooling chamber 14, heaters 19 for heating the respective indoor atmospheres are provided.

  Between the degassing chamber 11, between the degassing chamber 11 and the preheating chamber 12, between the preheating chamber 12 and the sintering chamber 13, between the sintering chamber 13 and the slow cooling chamber 14, between the slow cooling chamber 14 and the quenching chamber 15. Shutters 20 to 26 are provided between the quenching chamber 15 and the cooling chamber 16 and at the exit of the cooling chamber 16. These shutters 20 to 26 are individually opened and closed. By installing these shutters 20 to 26, it is possible to make the temperature distribution in each of the chambers 11 to 16 uniform, and to improve the accuracy of temperature control of each of the chambers 11 to 16, and as a result, the dimensions of the sintered parts can be increased. The variation can be reduced.

  The quenching chamber 15 is connected with a quenching fan 27 that sends cooling gas into the chamber. The discharge port of the quenching fan 27 is connected to the upper part of the quenching chamber 15, and the suction port of the quenching fan 27 is connected to the lower part of the quenching chamber 15 so that the cooling gas circulates between the quenching chamber 15 and the quenching fan 27. It has become. The cooling gas is an inert gas (for example, nitrogen gas). A heat exchanger (not shown) for cooling the cooling gas is provided in the middle of the cooling gas path between the quenching fan 27 and the quenching chamber 15. The cooling gas discharged from the quenching fan 27 passes through the quenching chamber 15 from top to bottom, thereby quenching the workpiece 1 in the quenching chamber 15.

  The cooling chamber 16 is provided with a cooler 28 that cools the indoor atmosphere and a circulation fan 29 that circulates the indoor atmosphere.

  Next, a method for sintering the workpiece 1 will be described.

  First, as shown in FIG. 4, a plurality of ceramic plates 30 are placed on the carbon tray 17, and the workpiece 1 is placed on each ceramic plate 30. The ceramic plate 30 has a gas release hole 31 that vertically passes through a position corresponding to the through hole 4 of the workpiece 1. The carbon tray 17 also has a gas discharge hole 32 that vertically passes through a position corresponding to the through hole 4 of the workpiece 1. When the cooling gas blows into the through hole 4 of the work 1 in the later-described rapid cooling process, the gas escape hole 31 and the gas discharge hole 32 allow the cooling gas to escape downward, so that the work 1 is lifted by the wind pressure of the cooling gas. It is for preventing. In this manner, with the workpiece 1 placed on the carbon tray 17 via the ceramic plate 30, the workpiece 1 is fed into the continuous sintering furnace 10 shown in FIG.

<Degassing process>
When the carbon tray 17 on which the workpiece 1 is placed enters the degassing chamber 11, the workpiece 1 is heated until the temperature reaches about 500 to 700 ° C. (see times t _{1 to} t _{2 in} FIG. 5). At this time, the wax component and the like existing in the workpiece 1 are removed as gas. The gas generated from the work 1 is processed in an exhaust gas combustion furnace (not shown).

<Preheating process>
Next, the carbon tray 17 on which the workpiece 1 is placed moves from the degassing chamber 11 to the preheating chamber 12. In the preheating chamber 12, the workpiece 1 is heated to a temperature of about 800 to 1000 ° C. lower than the sintering temperature (time t _{2 to} t _{3 in} FIG. 5).

<Sintering process>
Thereafter, the carbon tray 17 on which the workpiece 1 is placed moves from the preheating chamber 12 to the sintering chamber 13. The sintering chamber 13 is temperature-controlled so that the atmospheric temperature in the chamber is maintained at 1100 ° C. or more and less than 1200 ° C. (preferably 1120 ° C. or more and less than 1180 ° C.). The carbon tray 17 on which the workpiece 1 is placed is conveyed by a roller 18 so as to stay in the sintering chamber 13 for about 15 to 30 minutes. Thereby, the workpiece 1 is heated and held until the sintering temperature reaches 1100 ° C. or higher and lower than 1200 ° C. (preferably 1120 ° C. or higher and lower than 1180 ° C.) (time t _{3 to} t _{4 in} FIG. 5). The powder material constituting the workpiece 1 is sintered and integrated.

<Cooling and holding process>
Thereafter, the carbon tray 17 on which the workpiece 1 is placed moves from the sintering chamber 13 to the slow cooling chamber 14. Slow cooling chamber 14, the atmosphere temperature in the room is lower than 900 ° C., and a temperature higher than the austenitic transformation point (the so-called _{A 3} transformation point) (e.g., a temperature range of 840-870 ° C.) so as to keep the The temperature is controlled. The carbon tray 17 on which the workpiece 1 is placed is conveyed by a roller 18 so as to stay in the slow cooling chamber 14 for about 10 to 20 minutes. As a result, the workpiece 1 is cooled and held at a temperature lower than 900 ° C. and higher than the austenite transformation point (time t _{4 to} t 5 in FIG. ₅ ).

<Gas quenching process>
Thereafter, the carbon tray 17 on which the workpiece 1 is placed moves from the slow cooling chamber 14 to the rapid cooling chamber 15. When the carbon tray 17 enters the quenching chamber 15, both the shutter 24 at the entrance and the shutter 25 at the exit of the quenching chamber 15 are closed, and in this state, the quenching fan 27 is operated to blow cooling gas onto the workpiece 1. Rapid cooling to a temperature (for example, a temperature range of 250 to 350 ° C.) lower than the martensitic transformation point (so-called Ms point) (time t _{5 to} t _{6 in} FIG. 5). The cooling rate of the workpiece 1 during the rapid cooling is 2 to 5 ° C./second (preferably 3 to 5 ° C./second). The workpiece 1 is quenched by this rapid cooling, and the structure of the workpiece 1 is transformed into martensite. By setting the cooling rate of the workpiece 1 at the time of rapid cooling to 2 ° C./second or more (preferably 3 ° C./second or more), the structure of the workpiece 1 can be effectively martensite and sintered with extremely high hardness. Parts can be obtained. By setting the cooling rate of the workpiece 1 at the time of rapid cooling to 5 ° C./second or less, the unevenness of cooling (the temperature distribution of the workpiece 1 becomes non-uniform when the workpiece 1 is rapidly cooled) is suppressed, and the workpiece 1 is effectively removed. It can cool uniformly. Here, the cooling rate of the workpiece 1 is an average cooling rate from when the workpiece 1 falls below 800 ° C. to 400 ° C.

<Cooling process>
Thereafter, the carbon tray 17 on which the workpiece 1 is placed moves from the quenching chamber 15 to the cooling chamber 16. In the cooling chamber 16, the work 1 is cooled to 0.99 ° C. or less of a low temperature (e.g., about 100 ° C.) (time _{t 6} onward in FIG. 5). When the cooling of the workpiece 1 in the cooling chamber 16 is completed, the carbon tray 17 on which the workpiece 1 is placed is discharged to a replacement chamber (not shown) adjacent to the downstream side of the cooling chamber 16.

  When the workpiece 1 is sintered and cured by the above-described sinter hardening method, the workpiece 1 is sintered at a temperature of less than 1200 ° C. in the sintering step, and therefore, the workpiece 1 is sintered at a temperature of 1200 ° C. or more In comparison, the shrinkage of the workpiece 1 due to sintering can be suppressed to be extremely small, and a reduction in dimensional accuracy due to the shrinkage of the workpiece 1 can be prevented. In addition, since the workpiece 1 is once cooled and held at a temperature lower than 900 ° C. before the workpiece 1 is rapidly cooled, the occurrence of uneven cooling is suppressed as compared with the case where the workpiece 1 is rapidly cooled from a temperature of 1200 ° C. or more. Thus, the workpiece 1 can be cooled uniformly. Therefore, it is possible to manufacture a high-hardness sintered part (work 1 in which the structure is hardened in the gas quenching process) having extremely high dimensional accuracy without performing sizing after sintering.

  The sintered part obtained by the above-mentioned sinter hardening method is different from the one in which only the surface of the work 1 is martensiticized by carburizing and quenching, and by rapidly cooling the work 1 after the sintering process of the work 1 Both the surface and the inside of the work 1 become martensite, and both the surface structure and the internal structure become a sintered part cured to the same extent.

In order to confirm the effect of the sintering hardening method of the embodiment of the present invention, a sintering step of 1100 to 1200 ° C., a cooling holding step of A ₃ transformation point to 900 ° C., and a gas quenching step of 2 ° C./second or more are sequentially performed. Through a high hardness sintered parts manufactured by sinter hardening method described above (measurement example 1), 1200 ° C. or more sintering process, a ₃ cooling holding step of transformation point to 900 ° C., 2 ° C. / sec or more gas quench A test for comparing the dimensional accuracy was performed with a high-hardness sintered part (Measurement Example 2) manufactured by a sintering hardening method according to a comparative example through the steps. 6 and 7 show the test results.

  The high-hardness sintered parts of Measurement Example 1 and Measurement Example 2 are both synchro hubs having the shapes shown in FIGS. 1 and 2, the boss portion 3 has an inner diameter of 68 mm, and the flange portion 2 has an outer diameter (of the tooth portion 7). The tip diameter is 82 mm.

  FIG. 6 shows the result of measuring the roundness of the outer periphery (tip of the tooth portion 5) of the upper end portion of the boss portion 3 shown in FIGS. As shown in FIG. 6, the roundness (average value) of the outer periphery of the boss 3 is significantly improved in the high hardness sintered part of Measurement Example 1 compared to the high hardness sintered part of Measurement Example 2. Can be confirmed. Moreover, the standard deviation of the roundness of the outer periphery of the boss part 3 is also kept low, and it can be confirmed that the dimensional accuracy of the boss part 3 is extremely stable.

  FIG. 7 shows the result of measuring the flatness of the lower end surface 2b of the flange portion 2 shown in FIG. Referring to FIG. 7, the flatness (average value) of the lower end surface 2b of the flange portion 2 is significantly improved in the high hardness sintered part of Measurement Example 1 compared to the high hardness sintered part of Measurement Example 2. Can be confirmed.

The metal structure, hardness, dimensional change rate, and density of the high-hardness sintered parts obtained in this test were as shown in the following table.

  In this table, the dimensional change rate is a percentage of the change in the outer diameter dimension of the flange portion 2 before and after the sinter hardening with respect to the outer diameter dimension of the flange portion 2 before the sinter hardening. The high-hardness sintered part of Measurement Example 2 manufactured by sintering the workpiece 1 at a temperature of 1200 ° C. or higher (for example, 1220 ° C.) is contracted in size with respect to the workpiece 1 before sintering. On the other hand, the high-hardness sintered part of Measurement Example 1 manufactured by sintering the workpiece 1 at a temperature of 1100 ° C. or more and less than 1200 ° C. (for example, 1130 ° C.) has a dimension relative to the workpiece 1 before sintering. Almost no change. Also from this, it can be confirmed that the high-hardness sintered part of Measurement Example 1 does not have a decrease in dimensional accuracy due to the shrinkage of the workpiece 1.

DESCRIPTION OF SYMBOLS 1 Work 2 Flange part 2a Upper end surface 3 Boss part 3a Axial end surface 4 Through-hole 5 Tooth part 6 Notch 7 Tooth part 10 Continuous sintering furnace 11 Degassing chamber 12 Preheating chamber 13 Sintering chamber 14 Slow cooling chamber 15 Quench chamber 16 Cooling chamber 17 Carbon tray 18 Roller 19 Heater 20 to 26 Shutter 27 Rapid cooling fan 28 Cooler 29 Circulating fan 30 Ceramic plate 31 Gas vent hole 32 Gas exhaust hole

Claims (3)

A sintering process in which a workpiece formed by compression molding a powder material containing iron, chromium, molybdenum, and carbon in a sintering chamber of a continuous sintering furnace is heated to a temperature of 1100 ° C. or higher and lower than 1200 ° C. for sintering. ,
After the sintering step, the temperature of the workpiece does not drop below the austenite transformation point by moving the tray on which the workpiece is placed in the slow cooling chamber of the continuous sintering furnace located downstream of the sintering chamber. A cooling and holding step that is performed continuously after the sintering step, and holds the workpiece at a temperature lower than 900 ° C. and higher than the austenite transformation point;
After the cooling and holding step, the tray is moved to the quenching chamber of the continuous sintering furnace located downstream of the slow cooling chamber, and the workpiece is blown with a cooling gas to make the workpiece more than the martensitic transformation point. A gas quenching process that rapidly cools to a low temperature and turns into martensite;
Sinter hardening method.   As said powder material, what contains 2-4 mass% chromium, 0.3-0.7 mass% molybdenum, 0.3-0.8 mass% carbon, and the remainder iron is used. The sinter hardening method according to claim 1.   The sintering hardening method of Claim 1 or 2 which made the cooling rate of the said workpiece | work in the said gas rapid cooling process 2-5 degree-C / sec.

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