JPH04371515A - Heat treatment method - Google Patents

Abstract

PURPOSE:To manufacture a carbon steel product having excellent wear resistance by developing a deep quench-hardened layer having uniform hardness at the time of quenching a surface by using high energy beam. CONSTITUTION:As a heat treatment method before quenching the carbon steel, after quenching the carbon steel, by tempering the steel at high temp., the structure of the carbon steel is changed into sorbitic structure dispersing fine and uniform carbide. At the time of quenching the carbon steel with the high energy beam starting from the above condition, the carbide is easily broken to carbon. As the broken carbon atoms are uniformly diffused on the surrounding, the crystal structure of the quenched part is changed into the structure hard and stable in energy with phase transformation. As a result, the wear resistance of surface of the carbon steel is improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for properly hardening steel containing carbon, that is, carbon steel, using a high-energy beam.

0002

2. Description of the Related Art Steel materials that are hardened with a high-energy beam such as a laser beam or an electron beam are carbon steels containing a carbon concentration of 0.4 to 0.6%. Unless otherwise specified by the customer, steel materials are delivered in a hot-rolled state without any particular heat treatment. If mechanical cutting is performed under such conditions, the material may become deformed later, and even after heat treatment such as quenching, the hardness may not increase or may vary, so appropriate pre-heat treatment is required. There are many cases.

Conventionally, when carbon steel is hardened using a high-energy beam, the steel is hardened as is without any preheat treatment, or normalization is performed as a preheat treatment. Normalizing is carried out for the purpose of fine-graining the crystal grains and removing non-uniformity in the distribution of carbides, and is generally carried out to bring the steel material to the A1 transformation point or A1 transformation point.
It refers to the operation of heating and holding at a temperature of 40 to 60°C above the cm transformation point for 1 to 3 hours, and then cooling in a quiet atmosphere.

[0004] Here, an example will be described in which a ball screw shaft is hardened as a workpiece made of carbon steel using a carbon dioxide gas laser beam, which is a typical high-energy beam, without performing preheat treatment as a conventional method. . The laser beam output at this time was 1000W, and the moving speed was 0.5m/mi.
It was carried out at n. FIG. 8 shows a microscopic microstructure photograph of a cross section of the quenched hardened layer on the surface of the ball screw shaft. The boundary line between the hardened part and the base metal part is clear. The base material is composed of a mixed structure of a ferrite structure that is soft and is not hardened, and a pearlite structure that is hardened, but the sizes of each structure are uneven and the degree of mixing is uneven. Furthermore, although the hardened portion has a martensitic structure, a ferrite structure is observed in some places. The hardness distribution curve of this hardened layer cross section is shown in FIG. The hardness of the hardened part is Hv540-615,
The average value was Hv575.

[0005] Next, an example will be described in which, as a conventional pre-hardening heat treatment method, normalizing is performed and then hardening is performed using a carbon dioxide gas laser beam under the same conditions as above. FIG. 10 is a microscopic structure photograph of a cross section of the quenched hardened layer. Compared to FIG. 8 in which no preheat treatment was performed, the ferrite structure and pearlite structure in the base material are fine and uniformly distributed. However, in the hardened part, ferrite structure still remains in the martensite structure. Therefore, as shown in the hardness distribution curve of FIG. 11, the hardness of the hardened portion was Hv520-635, with an average value of Hv580.

[0006]

[Problems to be Solved by the Invention] Conventionally, unless a customer specifies pre-heat treatment to a steel manufacturer, steel products are manufactured into various shapes and sizes by being repeatedly hot-rolled from an ingot shape after melting. After being shipped. When observed under a microscope, the cross-sectional metal structure of such a steel material consists of a ferrite phase and a pearlite phase. The size of the ferrite phase and pearlite phase differs not only inside and outside the steel material, but also on the surface, and there is density and density. The hardness of the ferrite phase is H
V200, and the pearlite phase is Hv250-300. It is the pearlite phase containing carbides that contributes to hardening. When the pearlite phase is quenched, it hardens to about Hv900, but the ferrite phase does not harden. For good hardening, it is better for the ferrite and pearlite phases to be finely distributed. If the pearlite phase is fine, the carbides contained therein will be thermally decomposed into carbon by a high-energy beam, and the carbon will be dispersed in the surrounding area. It is easily and uniformly diffused into the ferrite phase. Therefore, since the ferrite phase is also hardened, the hardness of the hardened layer becomes uniform and hard.

However, in the conventional method described above, if there is no preheat treatment specified and no preheat treatment is performed, the size of the ferrite phase and pearlite phase is large, so when irradiated for a short time like high energy beam hardening, the carbide becomes carbon. It is not decomposed and the carbon cannot fully diffuse into the ferrite phase. Therefore, since the ferrite phase hardly becomes hard, the quench-hardened layer consists of only the hardened pearlite phase. Therefore, the hardness varies widely and its value is low.

Furthermore, when normalizing is performed as a preheat treatment in the conventional method, the metal structure consisting of ferrite phase and pearlite phase becomes somewhat finer when observed microscopically. The pearlite phase contains a large amount of carbide, and carbon in the carbide diffuses into the surroundings within the pearlite phase due to high-energy beam heating, and when the high-energy beam heating is stopped, it undergoes a phase transformation and hardens due to self-cooling. Hardened pearlite phase has Hv90
It will be about 0. Further, during heating, some carbon in the carbide within the pearlite phase penetrates through the pearlite phase and diffuses into the adjacent ferrite phase. Therefore, the carbon concentration in the ferrite phase near the pearlite increases, and the ferrite phase is slightly quenched and hardened. However, the hardness of the ferrite phase is originally about Hv200, and it hardly hardens even when irradiated with a high-energy beam. Since the combination of such ferrite phase and pearlite phase becomes the quench-hardened layer, there is a problem that the hardness varies or is insufficient.

The present invention was made to solve these problems, and it is an object of the present invention to obtain a hardened layer with uniform hardness and depth when hardening carbon steel using a high-energy beam. The purpose of the present invention is to provide a heat treatment method that enables the following.

0010

[Means for Solving the Problems] In order to achieve this object, the heat treatment method of the present invention is characterized by subjecting carbon steel to quenching and high-temperature tempering treatment before quenching it using a high-energy beam. do.

[0011]

[Operation] According to the heat treatment method of the present invention as described above, when carbon steel is hardened with a high-energy beam, carbides are easily decomposed into carbon and uniformly diffused, so that the surface hardening layer of carbon steel is hardened. The hardness of is evened out and increased.

0012

[Embodiment] An embodiment embodying the heat treatment method of the present invention will be described below with reference to the drawings.

[0013] For example, SCM of JIS standard is used for carbon steel.
440 was used to process the ball screw shaft of precision mechanical parts. After the ball screw shaft is roughly machined to a size slightly larger than the final finished size, it is subjected to heat treatment in a heating furnace, which consists of a hardening process and a high-temperature tempering process.

FIG. 1 schematically shows a hardening apparatus 1 used in the present invention. The quenching device 1 has a heating heater 2 inside.
to control the internal atmospheric temperature. When an inert gas 3 such as nitrogen gas or argon gas is introduced into the apparatus, it is stirred by an atmosphere stirring fan 4 to obtain a uniform concentration. A ball screw shaft material 5, which is a workpiece, is placed on a conveyor 6 and heated by a heater 2. After being heated and maintained at a certain temperature, it moves on a conveyor 6, falls into a quenching tank 7, and is quenched. The quenching tank 7 is filled with quenching oil 8. Further, the temperature of the quenching oil 8 is controlled by a heater 9, and is stirred by an oil stirring device 10 so that the temperature is uniform. The ball screw shaft material 5 is received in a basket 11 submerged inside the quenching tank 7. The ball screw shaft material 5 is inserted into the basket 11 from the loading port.
The whole thing is taken out.

FIG. 2 schematically shows a high temperature tempering apparatus 20 of the present invention. The ball screw shaft material 5 hardened by the hardening device 1 is introduced through the charging port, brought into a vacuum state, and then heated by the heater 21. Once the temperature inside the device reaches the tempering temperature, it is maintained for a certain period of time. Subsequently, an inert gas 23 such as nitrogen gas or argon gas is flowed through the gas inlet pipe 22 to cool it. Once the ball screw shaft material 5 has cooled down sufficiently, it is taken out from the loading port.

Next, specific heat treatment will be explained using FIG. 3. The first quenching process as the pre-quenching heat treatment of the present invention is as follows. That is, in order to make the structure of the ball screw shaft material 5, which is a workpiece, into a completely uniform austenite state in the hardening device 1, the hardening device 1 is heated in a vacuum atmosphere or a nitrogen atmosphere to prevent oxidation. 30°C to 50°C above the transformation point, i.e. 820°C to 84°C
Heat to 0°C. After holding for about 90 minutes, it is immersed in oil at 80°C and quenched to harden martensite. Then the second
In the high-temperature tempering step, the hardened ball screw shaft material 5 is put into the high-temperature tempering device 20. After creating a vacuum state, the ball screw shaft material 5 is heated and maintained at a high temperature tempering temperature in the range of 300°C to 650°C for about 120 minutes, and then cooled by being blown with a high pressure inert gas 23 such as nitrogen or argon for cooling. be done. At this time, when the tempering temperature is low, the hardness of the ball screw shaft material 5 is high, and conversely, when the tempering temperature is high, the hardness of the ball screw shaft material 5 is low. FIG. 4 shows the relationship between tempering temperature and ball screw shaft material hardness. As a result, the ferrite phase and pearlite phase of the ball screw shaft material 5 are made much finer than when no preheat treatment is performed or when only normalization is performed, and the carbides contained in pearlite are finely distributed in sorbite. Change into an organization. As a result, when high-energy beam hardening is performed thereafter, the carbide is easily and uniformly destroyed, so that a deep hardened layer with uniform hardness can be obtained.

Here, after tempering at 480° C. in the high-temperature tempering step of the pre-quenching heat treatment of the present invention, the same conditions as in the conventional normalizing, that is, a beam output of 1000
A case will be described in which laser hardening is performed under the conditions of W and a beam moving speed of 0.5 m/min. FIG. 5 shows a microscopic structure photograph of a cross section of the laser-hardened layer on the surface of the ball screw shaft 5. The ferrite structure and pearlite structure in the base metal are both finely divided and changed to a sorbite structure, which is a very fine pearlite structure. Furthermore, the hardened portion shows a fine martensitic structure. The hardening depth was 0.5 mm. From the hardness distribution curve in Figure 6, the hardness of the hardened part is Hv680-700, with an average of Hv680-700.
It was v685.

[0018] In addition, in order to evaluate the wear resistance of the ball screw shaft 5 hardened with a carbon dioxide laser beam, this ball screw shaft 5 was attached to the machine tool in which it was actually installed and a 200-hour continuous durability test was conducted. . As shown in FIG. 7, the wear depth was 10 to 15 μm when normalized as a preheat treatment, and 3 to 5 μm when the hardening and high temperature tempering treatment of the present invention was performed.

After performing the hardening and high-temperature tempering treatment of the present invention, the laser-hardened ball screw shaft 5 can be used in cases where the conventional pre-hardening heat treatment is omitted or when only normalizing is performed as the pre-hardening heat treatment. Compared to the above, the hardness improved by about Hv100 in the hardened part and by about Hv30-70 in the base metal part. Also, regarding the quench hardening depth,
It became deeper by 0.2 to 0.3 mm. Furthermore, the wear resistance was improved by about 3 times.

[0020]

As is clear from the above explanation, in the heat treatment method of the present invention, carbon steel is subjected to quenching and high-temperature tempering before being quenched using a high-energy beam. As a result, the mixed structure of the ferrite structure and pearlite structure of the carbon steel is finely divided and turns into a very fine pearlite structure, that is, a sorbite structure. The sorbite structure is a structure in which carbides are uniformly and finely distributed, and during quenching, these carbides are destroyed and carbon tends to diffuse uniformly, making it easier for the sorbite structure to transform into a martensitic phase. Therefore, the quench-hardened layer is uniformly hard and has a large depth, which has the excellent effect of significantly improving wear resistance. Therefore, carbon steel products that are highly durable and have a long life can be manufactured.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a hardening apparatus used in the heat treatment method of this embodiment.

FIG. 2 is a diagram showing a high-temperature tempering apparatus used in the heat treatment method of this example.

FIG. 3 is a diagram showing each step of heat treatment in this example.

FIG. 4 is a diagram showing the relationship between tempering temperature and ball screw shaft material hardness.

FIG. 5 is a micrograph showing the metal structure of a cross section of the ball screw shaft surface portion hardened with a high-energy beam after the heat treatment of this example.

FIG. 6 is a diagram showing the cross-sectional hardness distribution of the ball screw shaft surface portion hardened with a high-energy beam after the heat treatment of this example.

FIG. 7 is a diagram showing the results of a continuous durability test of the ball screw shaft obtained by carrying out this example.

FIG. 8 is a micrograph showing the metal structure of a cross section of the surface of a ball screw shaft hardened by a conventional method.

FIG. 9 is a diagram showing the cross-sectional hardness distribution of a ball screw shaft surface portion hardened by a conventional method.

FIG. 10 is a micrograph showing the metal structure of a cross section of the surface of a ball screw shaft hardened by a conventional method.

FIG. 11 is a diagram showing the cross-sectional hardness distribution of a ball screw shaft surface portion hardened by a conventional method.

[Explanation of symbols]

1. Quenching equipment 4 Atmosphere stirring fan 5 Ball screw shaft 7 Hardened layer 10 Oil stirring device 20 High temperature tempering equipment 22 Gas inflow pipe 23 Inert gas

Claims (1)

[Claims] 1. A heat treatment method characterized by subjecting carbon steel to quenching and high-temperature tempering before quenching it using a high-energy beam.