Title:
MICROWAVE CARBURIZING FURNACE AND CARBURIZING METHOD
Kind Code:
A1


Abstract:
A microwave carburizing furnace comprising: a microwave heat source for conducting carburization in plasma generated in atmosphere without electrodes;
    • a microwave irradiating chamber into which the microwave is introduced;
    • a atmospheric gas supply mechanism for supplying atmospheric gas containing Ar and O2 into the irradiating chamber; and
    • a rapid quenching mechanism for quenching a specimen after carburizing treatment with a quenching medium;
    • wherein a steel material as the specimen and a carbon source necessary for carburization are placed in the carburizing chamber.



Inventors:
Taguchi, Masami (Hitachi, JP)
Baba, Noboru (Hitachiota, JP)
Sato, Motoyasu (Shiga, JP)
Matsubara, Akihiro (Mizunami, JP)
Takayama, Sadatsugu (Kasugai, JP)
Application Number:
11/829986
Publication Date:
01/31/2008
Filing Date:
07/30/2007
Primary Class:
Other Classes:
266/257
International Classes:
C23C8/22; C23C8/38
View Patent Images:



Primary Examiner:
WALCK, BRIAN D
Attorney, Agent or Firm:
ANTONELLI, TERRY, STOUT & KRAUS, LLP (Upper Marlboro, MD, US)
Claims:
What is claimed is:

1. A microwave carburizing furnace comprising: a carburizing chamber for placing a specimen of a steel material to be treated therein and a carbon source and for confining a atmosphere gas containing Ar and O2; a microwave heat source for generating a microwave; a guide for guiding the microwave into the carburizing chamber for conducting carburization of the specimen with plasma generated in the atmosphere; and an atmosphere gas supply mechanism for supplying the atmosphere gas into the carburizing chamber.

2. The microwave carburizing furnace according to claim 1, wherein the chamber has a port through which the carburized specimen is transferred to a quenching medium is a quenching container.

3. The microwave furnace according to claim 1, wherein the chamber is connected with a quenching container.

4. The microwave carburizing furnace according to claim 1, wherein a frequency of the microwave is 300 MHZ to 300 GHZ.

5. A microwave carburizing furnace comprising: a microwave heat source for conducting carburization in plasma generated in atmosphere comprising: a microwave irradiating chamber into which the microwave is introduced; a atmospheric gas supply mechanism for supplying atmospheric gas containing Ar and O2 into the irradiating chamber; and a rapid quenching mechanism for quenching a specimen after carburizing treatment with a quenching medium; wherein a steel material as the specimen and a carbon source necessary for carburization are placed in the carburizing chamber.

6. The microwave carburizing furnace according to claim 5, wherein a quenching room is provided next to the microwave irradiating chamber as the rapid quenching mechanism.

7. The microwave carburizing furnace according to claim 5, wherein a quenching gas supply mechanism for supplying gaseous quenching medium into the microwave irradiating chamber as the rapid quenching mechanism.

8. The microwave carburizing furnace according to claim 5, wherein at least one of CO and hydrocarbon gases is supplied into the irradiating chamber from the ambient gas supply mechanism in addition to Ar and O2.

9. The microwave carburizing furnace according to claim 5, wherein H2 is supplied into the irradiating chamber from the atmospheric gas supply mechanism in addition to Ar and O2.

10. The microwave carburizing furnace according to claim 5, wherein H2 is supplied into the irradiating chamber from the atmospheric gas supply mechanism in addition to Ar and O2.

11. The microwave carburizing furnace according to claim 5, wherein the specimen having a solid carbon source in the surface thereof is placed in the irradiating chamber.

12. The microwave carburizing furnace according to claim 5, wherein a frequency of the microwave is 1 to 30 GHz.

13. The microwave carburizing furnace according to claim 5, which is provided with the microwave irradiating mechanism that is built in the microwave irradiating furnace as the microwave irradiating chamber.

14. The microwave carburizing furnace according to claim 13, wherein the microwave irradiating furnace is provided with a fan for dispersing the microwave therein.

15. The microwave carburizing furnace according to claim 13, wherein the microwave irradiating furnace is provided with a susceptor therein.

16. The microwave carburizing furnace according to claim 5, wherein a part of the microwave irradiating furnace is provided with a window for introducing microwave.

17. The microwave carburizing furnace according to claim 5, wherein the carburizing chamber is provided with a specimen holder with a high thermal conductivity and a susceptor capable of having a carbon source whereby the specimen holder and the susceptor holds the specimen therebetween.

18. A method of carburizing a specimen made of steel material with a microwave as a heating source, which comprises: irradiating the specimen to be treated with the microwave in the presence of a carbon source and an atmospheric gas containing Ar and O2; generating plasma by reacting the carbon source and O2 to produce and excite CO thereby generating Ar plasma to produce carbon ions; and carburizing the specimen with the iron ions.

19. The method according to claim 18, which further comprises, after the carburizing the specimen, rapid-quenching the treated specimen with a quenching medium thereby annealing the specimen.

20. The method according to claim 18, wherein the frequency of the microwave is 300 MHZ to 300 GHZ.

21. The method of carburizing the specimen with the microwave according to claim 18, wherein the atmospheric gas is Ar, O2 and at least one of CO and hydrocarbon gas.

22. The method of carburizing the specimen with the microwave according to claim 18, wherein the atmospheric gas contains Ar, O2 and H2.

23. The method of carburizing the specimen with the microwave according to claim 18, wherein the atmospheric gas further contains H2.

24. The method of carburizing the specimen with the microwave according to claim 18, wherein the carbon source is a solid or gas, the solid being carbon powder and the gas being CO or hydrocarbon.

25. The method of carburizing the specimen with the microwave according to claim 18, wherein the specimen is a steel material having a crystal grains of 1 μm or more and a tensile strength of 1000 MPa or more.

26. The method of carburizing the specimen with the microwave according to claim 18, wherein a frequency of the microwave is 1 to 30 GHz.

27. The method of carburizing the specimen according to claim 18, wherein a selected portion of the specimen is in contact with the carbon source.

Description:

CLAIM OF PRIORITY

The present application claims priority from Japanese patent application serial No. 2006-208142, filed on Jul. 31, 2006, the content of which is hereby incorporated by reference into this application.

TECHNICAL FIELD

The present invention relates to a carburizing furnace with a microwave as a heat source and a method of carburizing steel material using the carburizing furnace.

BACKGROUND ART

Microwaves have been used for heating, drying or sterilizing foods, and used for curing polymer materials or sintering ceramics. Microwaves are defined in general as electro-magnetic waves having a frequency of 300 MHz to 300 GHz. In heating dielectric materials of polar molecules such as water, high polymers or ceramics, an electric field of the microwave plays an important role. On the other hand, it has been reported very recently that the waves can heat metal powder or metal lumps.

Study on irradiation of microwaves to metallic materials started around 1990's. Particularly, investigation on surface modification with microwaves has been vigorously conducted. Patent documents No. 1 to 4 are concerned with technologies on surface modification using microwaves. These publications disclose mainly apparatuses for generating plasma by means of microwaves. As a method for generating plasma by means of microwaves, there are disposed electrodes. Arrangements of the apparatuses are particularly important technical factors.

Patent document No. 5 discloses a plasma generating apparatus with no electrode; if the electrodes are disposed, the apparatus becomes very complicated. Applications of the microwaves to the surface modifications are mainly nitridation or filming, but carburization-annealing that needs a specific temperature pattern is not disclosed in the patent documents.

The conventional processes cover mainly solid carburization, gas carburization, plasma carburization, etc. Among these methods the plasma carburizing method is shorter in treatment time compared to other treatment methods. A surrounding atmosphere or ambient gas for the process should be an atmosphere of a reduced pressure so as to easily generate the plasma. Accordingly, the processes and apparatuses tend to become complicated.

Since the conventional plasma carburizing methods utilize conventional heaters, the methods have no advantages on time with respect to a temperature rise process being a dominant portion of the treatment time.

Patent document No. 1: Japanese Laid-Open 2005-235464

Patent document No. 2: Japanese Laid-Open 09-235686

Patent document No. 3: Japanese Laid-Open 2004-14631

Patent document No. 4: Japanese Laid-Open 2003-62452

Patent document No. 5: Japanese Laid-Open 2005-196980

As mentioned above, the conventional surface modification treatment methods with microwaves do not cover the carburizing treatment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a microwave carburizing furnace that uses microwaves in order to carry out carburizing treatment preferably under an atmospheric or normal pressure and to provide a method of carburizing steel materials with the microwave carburizing furnace.

The present invention provides a microwave carburizing furnace that employs electro-magnetic coil as a heating source for generating microwaves to conduct carburizing treatment under an atmospheric pressure, which comprises a microwave irradiating chamber into which the microwave is introduced; an ambient gas supply mechanism for supplying an ambient gas containing at least Ar and O2 to the microwave irradiating chamber; and a rapid quenching mechanism for quenching a specimen after carburizing treatment with a quenching medium, wherein the specimen made of steel material and a carbon source necessary for carburizing treatment are placed in the microwave irradiating chamber. The carburizing includes carburization in the specification. Carburizing mainly depends on selection of the atmosphere in the chamber.

Because the specimen made of steel material to be treated has a function of generating heat, the carburizing furnace does not need electrodes for generating plasma in the present invention.

Further, the present invention provides a microwave carburizing method for carburizing a specimen made of steel material using a microwave as a heating source, wherein the microwave is irradiated to the specimen in the presence of a carbon source and a surrounding gas containing Ar and O2 to generate Ar plasma by generating and exciting CO caused by the reaction between the carbon source and oxygen thereby to generate carbon ions to carburize the specimen, followed by rapid-quenching the treated specimen with a quenching medium in order to carry out annealing of the specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross sectional view of a multi-mode carburizing furnace of an embodiment according to the present invention.

FIG. 2 is a diagrammatic cross sectional view of a single mode carburizing furnace of another embodiment according to the present invention.

FIG. 3 is a graph showing a relationship between temperatures and microwave irradiation time using the apparatus shown in FIG. 2, wherein SCM415 was heated in a strong magnetic field in the absence of a susceptor.

FIG. 4 is a graph showing a relationship between a grain size of ultra fine crystal ferrite steel and hardness.

FIG. 5 is a diagrammatic view of the apparatus shown in FIG. 2, showing a status of a sample setting in the apparatus.

EXPLANATION OF REFERENCE NUMERALS

100; a microwave irradiating furnace, 101; a pipe for supplying atmospheric gas, 102; a pipe for supplying quenching gas, 103; a fan for dispersing microwave, 104; susceptor, 105; a rod for manipulation, 106; a nozzle for spraying the quenching medium, 107; a window for temperature measuring, 108; a quenching chamber, 109; a gate, 150; a sample, 200; a container, 201; a pipe for supplying the atmospheric gas, 202; a pipe for supplying the gas quenching medium, 203; a pipe for supplying liquid quenching medium, 204; a window, 205; a quenching chamber, 206; a gate, 208; specimen, 210; a specimen holder, 212; a susceptor with carbon source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The carburizing furnace of the present invention has a very simple structure and is free from electrodes for generating plasma in the furnace. Generation of plasma is done by chemical reaction and excitation of gas under irradiation of microwaves. In the present invention, the microwave should preferably have a frequency of 300 MHZ to 300 GHZ. In using the microwave, it is possible to carry out plasma generation on the sample and heat treatment simultaneously. In using high frequency waves, generation of plasma (glow discharge) on the sample and heat treatment are carried out separately.

According to embodiments of the present invention, there are provided the carburizing furnace that uses the microwave and capable of carrying out under the atmospheric pressure and the method of carburization of the steel material that can be carried out substantially under a normal pressure.

Since the conventional microwave irradiation apparatuses were not designed for carburization, they are not provided with a rapid quenching mechanism, which is necessary for carburizing and annealing treatment. The carburizing apparatus of the present invention is provided with a rapid quenching mechanism for annealing treatment. According to the carburizing furnace and carburizing method of the present invention, it is possible to drastically shorten the treating time, compared to the conventional method. Further, it is possible to conduct carburization, while preventing a temperature rise of the specimen to be treated, due to selective heating, which is peculiar to the microwave heating. Accordingly, it is possible to perform carburizing treatment without a change of the crystalline structure of ultra fine crystalline structure materials, which has a strong structure change with respect to temperature change.

The present invention is applied to carburizing treatment of steel materials. Carburized products of steel materials are produced to give the materials abrasion resistance. These materials have been used for constituting parts of plants such as electric power plants and chemical plants, constituting parts for electric power instruments, mechanical components for transportation such as railways, ships, land transportation, aviation, space, domestic electric appliances, mechanical parts for constructing machines, metal molds, parts for machining machines, medical parts, etc. The present invention can be applied to allover the fields to which the conventional carburized products are applied.

Since the microwave heating is capable of local-heating, compared to the conventional plasma carburizing method and since energy is concentrated to an object, it is possible to elevate temperatures within a very short time period. Further, plasma generated by irradiation of microwaves has such advantages as surface cleaning at the time of carburization and generation of carbon ions.

In the conventional plasma carburizing process, the plasma is generated under a reduced pressure by using electrodes. In the present invention, however, the plasma is generated under the atmospheric pressure by irradiating microwaves under controlling the atmospheric pressure without electrodes. As discussed earlier, patent document No. 1 et al discloses an apparatus for generating plasma under an atmospheric pressure by irradiating microwaves. Compared to the conventional apparatuses, the carburizing apparatus of the present invention is featured in that the microwave is a heating source, no electrodes are used, and the carburizing furnace is equipped with a rapid quenching mechanism immediately after carburizing treatment.

Metallic materials can be heated by microwaves. A magnetic field generated by the microwave plays an important role. Particularly, magnetic materials among metallic materials exhibit remarkable advantages. Since steel materials are a magnetic material, they are easily heated by microwaves. On the other hand, carbon atoms to be carburized are excited by an electric field. That is, microwaves with different phases, i.e. the magnetic field and electric field vibrate iron atoms and carbon atoms. By these actions, a diffusion speed of carbon atoms in the iron material under irradiation of microwave becomes faster than that of heating by a heater and, as a result, a carburizing time can be shortened, compared to the conventional method.

Furthermore, in case of microwaves, since any parts other than the specimens are not heated, it is possible to rapidly quench the specimens. And, the microwave carburizing furnace can shorten time of any steps including elevation of temperature, carburization, and quenching, compared to the conventional methods.

The microwaves have a frequency of 2.45 GHz, which is generated by a magnetron that has been widely used, and can be used for heating and carburizing steel materials. When a single irradiation mode for the microwaves is employed, it is preferable to select a frequency of about 1 GHz because an area of magnetic field becomes wide. Since the irradiation mode is the single mode, energy is focused to a small spot, which is suitable for rapid heating.

A publication “Rapid Heating with Microwave and Future Applications to Automobile Industries”, Materia Japan, vol. 45, No. 8, pp. 581-584 (2006) describes a microwave heating of metallic materials and applications to recent materials and ecological technologies. The publication exemplifies applications of microwave heating to production engineering of automobile industries that include sintering of ceramics, metals for sensors, actuators, and synthesis of polymers. It also discloses distributions of electromagnetic fields in the cavity or chamber of a single mode microwave oven (FIG. 6) and a double mode microwave oven (FIG. 7).

If the specimen to be treated is small sized in a small number, treatment with a single mode furnace enables the process a high speed so that depending on the size of the specimen and a quantity of products, there is a great advantage that it is possible to realize a remarkably high productivity.

On the other hand, if a frequency is about 30 GHz, oxidation of the metallic material at high temperatures is prevented. In carburizing treatment, oxidation of grain boundaries of the specimen is a serious problem. When the oxidation at high temperatures is prevented, high quality carburizing treatment can be done. From the above reasons, a frequency of the microwaves is preferably within a range of 1 to 30 GHz.

The specimen must be subjected to annealing treatment after carburization. The annealing treatment is a hardening treatment that utilizes martensitic transformation of iron, which is carried out by quenching steel material heated to a gamma-transformation temperature at a prescribed speed or higher. Whether the steel material is annealed or not is judged by a hardness of the steel material. If the hardness reaches 2 times or more of the hardness before the treatment, the steel has been annealed.

In ordinarily employed plasma carburizing treatment or gas carburizing treatment, quenching is generally conducted by introducing pressurized inert gas such as He into the furnace. In the microwave carburizing furnace of the present invention, the specimen is taken out from the opened furnace and handled immediately after heating, because only an area near the specimen is heated, but furnace walls etc are not heated, and because the specimen is handled under the atmospheric pressure.

If a quenching chamber is disposed close to the furnace, it is possible to conduct annealing by oil quenching. Further, if the quenching medium is water, it is possible to conduct quenching by introducing water directly into the furnace. Even if the inside of the furnace is wet with water, it is possible to immediately vaporize water by irradiating microwaves so that the furnace can be recovered to a condition where there is no problem for the treatment.

The microwave carburizing furnace of the present invention generates plasma by chemical reaction between the ambient gas molecules and excitation of the gas molecules, which leads to omission of electrodes.

The carburizing method of the present invention can be realized by placing steel material and a carbon source in a gas atmosphere containing a mixed gas of Ar and oxygen in a microwave furnace provided with a rapid quenching mechanism. When the microwave is irradiated to the steel material, plasma is easily generated even under the atmospheric pressure. Oxygen reacts with carbon to generate CO. Since CO is a polar molecule, it is excited when irradiated with the microwave.

The reaction between the carbon source and oxygen that generates CO and excitation of CO induces Ar plasma. When Ar becomes plasma, it sputters the surface of the steel material thereby to expose an active surface of the steel material, and it produces carbon ions, too. When carbon ions come into contact with the active surface of the steel material, carburization takes place.

CO has a role to chemically reduce the surface of the specimen and a role to supply carbon to the surface of the specimen as well. If an amount of CO generated by the reaction between the carbon source and oxygen is small, CO gas and/or hydrocarbon gas is supplied from outside the ambient gas of the furnace so that efficiency of carburization becomes better and a speed of carburization is made faster.

If H2 is added to the ambient gas, reduction of the surface of the specimen proceeds very quickly to thereby shorten the treatment time. Accordingly, the ambient gas containing Ar and O2, or containing Ar, O2 and at least one of CO, hydrocarbons and H2 is preferable. When H2 and hydrocarbons are used, a concentration of O2 should be as low as an explosion limit or less, which is difficult to control. Accordingly, the gas should preferably contain only Ar, O2 and CO.

The carbon source is supplied in a form of solid or gas. In case of the solid carbon source, it is preferable to coating or spray carbon powder on the specimen (steel material) to adhere it to the specimen or bring it into a close contact with the specimen. In case where gas is used as the carbon source, at least one of CO and hydrocarbons is preferably added to the ambient gas.

When the carbon source is adhered or contacted with the specimen of steel material, supply of carbon can proceed quickly. This is because CO gas generated by reaction of the carbon source and oxygen is effectively supplied to the surface of the specimen (steel material) and diffusion of atoms into the surface of the specimen proceeds by a direct contact between carbon atoms and the surface of the specimen. Particularly, under irradiation of the microwave, the carbon source is preferentially heated than the specimen so that the temperature of the specimen can be kept low.

If the treatment of the specimen is conducted while quenching the specimen, a temperature rise inside the specimen is controlled to a very low level; as a result, this process is particularly suitable for such a ultra fine crystalline steel material as being easily thermally changed or deteriorated in its structure or properties. The ultra fine crystalline steel material is one whose crystal grain size is 1 μm or less. The ultra fine crystalline steel material having the crystal grain size of 1 μm or less can be strengthened to a high strength of 1000 Mpa or more without losing its toughness. However, the ultra fine crystalline steel material tends to transform into an ordinary steel material if it is heated to a temperature zone where gamma transformation takes place to grow the crystal grains. According to the present invention, since the carburizing treatment on the specimen is carried out within a short period of time while keeping the temperature of the specimen low, the surface treatment can be carried out without loosing the mechanical properties of the specimen, while suppressing growth of the crystal grains of the ultra fine crystalline steel material.

Embodiment 1

FIG. 1 shows a diagrammatic cross sectional view of a multi-mode furnace of an embodiment of a microwave carburizing furnace of the present invention. The carburizing furnace shown in FIG. 1 is provided with a microwave irradiating furnace 100 in which a microwave irradiation function is built, an ambient gas supply pipe 101 for supplying ambient gas containing Ar and O2 into the irradiating chamber 100, and a quenching gas supply pipe 102. Inside the microwave irradiating chamber 100, there are disposed a microwave diffusion fan or stirrer 103 and a susceptor 104, a rod 105 of a manipulator (not shown) for moving a susceptor 104 and a quenching medium spray nozzle 106. The stirrer 103 is preferably made of a non-magnetic metallic material such as Al, SUS, etc. In the multi-mode type carburizing furnace shown in FIG. 1, reflection of the microwave is assisted by the stirrer to distribute electro-magnetic field randomly i.e. homogeneously in the furnace.

A carbon source is placed in the carburizing chamber; for example, carbon coating is formed on the surface of the specimen or a block or powder of carbon is set in the chamber.

The microwave generated by a magnetron 10 is guided through a window 30 formed in the wall of the chamber 100 by means of a waveguide 20 between the carburizing chamber 100 and a magnetron 10.

In case of the multi-mode furnace, the microwave is present randomly or homogeneously in the furnace. An microwave oven is an example of the multimode furnace. The distribution of the microwave in the furnace is not completely random or homogeneous, but there may be strong portions and weak portions of magnetic fields in the furnace. Thus, the stirrer is used to alleviate the non-homogeneity of the microwave distribution.

On the other hand, in a single mode type microwave furnace shown in FIG. 2, microwave is introduced from a direction shown by an arrow into a furnace or chamber 200 so that standing waves shown by a dotted line 220 are generated regularly. In the standing waves, there are strong magnetic fields and weak magnetic fields as shown in FIG. 2. The size and dimension of the chamber 200, which surrounds the sample 150 are designed so that the sample is covered within a standing wave. When the single mode furnace is used, effects of electric fields or magnetic fields are easily recognized. Because the electric field and magnetic field locally concentrate, it is possible to create a higher energy region.

A window 107 in FIG. 1 for measuring temperatures is formed in a part of a furnace wall of the microwave irradiating chamber 100. A quenching chamber 108 is disposed next to the microwave irradiating furnace 100 wherein the microwave irradiating furnace and the quenching chamber 108 are communicated through a gate 109. The carburizing chamber 100 and the quenching chamber 108 are normally shut off until the treated specimen is to be quenched. The gate has a mechanism in association with an output system of microwave. The output of the microwave is off before the gate is opened. Accordingly, it is possible to charge desired liquid in the quenching chamber 108.

Further, the output of the microwave is off when the quenching medium is sprayed. As a rapid quenching mechanism for conducting annealing after carburization, one of the quenching gas supply means and the quenching chamber is enough, but in this embodiment, both of them are provided so that any of them are selected on demand.

Using the carburizing furnace shown in FIG. 1, carburizing tests were conducted. A frequency of the microwave was 2.45 GHz. A specimen 150 to which carburizing treatment is applied was steel material SCM104, whose composition was similar to ISO: 18CrMo4. Carbon powder was coated on the surface of SCM415. The susceptor 104 made of carbon was disposed to cover the specimen.

A manipulator (not shown) for the susceptor 104 in FIG. 1 lifts up and down the manipulator rod 105. The susceptor is preferably made of a material such as SiC or C that absorbs electro-magnetic wave or of a good hot insulation such as alumina or mullite.

Part of the microwave excites the carbon susceptor and carbon coating; radiation heat from the heated susceptor and coating assists elevation of temperatures of the specimen. An ambient mixed gas containing 90% Ar ands 10% O2 by volume was flown through the furnace. Carburization conditions are: temperature: 900° C. and retention time: 3 minutes.

In the above setting, microwave of the maximum output of 6 kW was irradiated to the specimen. The temperature of the specimen arrived at a target temperature in 10 minutes. During the microwave irradiation, generation of plasma was observed allover the inside of the furnace.

After the temperature reached the target temperature, it was maintained for a predetermined time; then, the specimen 150 was dropped into the quenching chamber 108. Though the specimen 150 was dropped into the quenching chamber, He gas may be blown from a quenching medium spray nozzle 106 to the specimen so as to conduct annealing. When quenching medium is blown from the quenching medium spray nozzle, the susceptor is moved so as to directly blow the He gas towards the specimen.

If the sample to be treated shows a strong electro-magnetic absorption, the susceptor 104 ma be omitted. The energy of the microwave can be concentrated on the sample.

On the other hand, according to a condition of the sample (powder, granular or bulk material, dense or loose, etc), there may be formed portions where heating effects are weak. In this case, it is preferable to uses the susceptor. That is, the susceptor is placed only in the vicinity of the sample.

Hardness of the specimens in the vicinity of their surfaces after the annealing treatment is shown in Table 1. An increase in hardness in a depth of about 20 μm s from the surface was observed after only a three minutes treatment, which means the specimens were carburized and annealed.

In this process, a necessary time for setting the specimens to taking out them from the furnace was about 15 minutes. In the conventional plasma carburization or gas carburization, it takes several hours from the setting to taking out the specimen from the furnace. That is, the present invention performs the carburizing treatment within a very short time of period.

TABLE 1
Distance from the
surfaceVickers Hardness (HV)
(μm s)Water quenchingHe gas quenching
0755725
20720697
40711705
60710692
80692672
100651633
120543555
140508525
160459463
180395416
200351368
220283317
240268242
260290252
280281282

Embodiment 2

As another embodiment of the present invention, a single mode carburizing furnace is shown inn FIG. 2. The carburizing furnace shown in FIG. 2 is provided with a carburizing chamber 200, which is capable of being evacuated before ambient gas is supplied to the furnace. The chamber 200 is provided with a an ambient gas supply pipe 201 for supplying an ambient gas containing Ar and O2 thereinto, a gas quenching medium supply pipe 202 and liquid quenching medium supply pipe 203.

There is disposed a window 204 made of alumina for introducing microwave MW into the chamber.

A quenching chamber 205 is disposed next to the chamber 200; the carburizing chamber 200 and the quenching chamber 205 are communicated through a gate 206.

In case of the single mode, unlike the multimode furnace, its volume is small and a strong magnetic field zone can be formed so that a quicker heating is possible. In FIG. 3, there is shown a relationship between temperatures and microwave irradiation time wherein SCM415 was heated in the strong magnetic field without using the susceptor.

In the first embodiment shown in FIG. 1, the susceptor 104 absorbs the microwaves to transfer them to the specimen surrounded by the susceptor 104 so that the specimen is heated. On the other hand, in the second embodiment shown in FIG. 2, the specimen 150 is subjected to irradiation with microwaves through the window 204. The wall of the vessel 200 are made of materials that do not absorb the microwaves.

An output of the microwave was 2 kW. It is seen that a heating speed of 100° C./min. or more was realized without the susceptor. In the single mode furnace, it is possible to heat the iron base material to 900° C. within 5 minutes with the susceptor even in case of an output of 3 kW of microwave.

Embodiment 3

Carburization and annealing of the ultra fine crystalline ferrite steel was conducted using the single mode furnace shown in FIG. 2. A relationship between a crystal grain size and hardness of the ultra fine crystalline steel is shown in FIG. 4. The relationship between the crystal grain size and the hardness is expressed by Hall-Petch relation; the hardness of the ultra fine crystalline ferrite steel is exhibited by strengthening it by making the crystal grains fine. A hardness of the ultra fine crystal ferrite steel used for carburizing treatment was about 400 HV.

An arrangement of the specimen setting in the embodiment shown in FIG. 2 is shown in FIG. 5, which shows a selective carburizing method. That is, only the portion of the specimen 208 near the susceptor 212, which is in close contact with the specimen 208 and contains a carbon source, is selectively carburized. The rear face of the specimen 208 was in a close contact with a specimen holder 210 (i.e. cooling block) made of cupper, which is placed on the wall 230 of the quenching chamber 205 filled with liquefied nitrogen. The specimen holder 210 prevents temperature rise of the specimen during carburization. An ambient gas was a mixed gas of 90% Ar and 10% O2 by volume; a carburization temperature was 900° C., and a carburization time was 20 minutes. After the temperature of the specimen reached the target temperature, the temperature was maintained for a predetermined time and the output of microwave was shut down. Annealing was carried out by quenching it with liquefied nitrogen by means of the specimen holder made of copper.

Hardness distribution in the vicinity of the surface of the specimen is shown in Table 2. It has been revealed that an increase in hardness until a thickness of about 500 μm from the surface was observed, which carburization and annealing were carried out. The hardness of the base material of the ultra fine crystalline ferrite steel material did hardly lower, compared to that of the material before the carburizing treatment wherein growth of crystal grains inside the base material was suppressed.

TABLE 2
Distance from the surfaceVicker's hardness after
(μm)carburizing treatment (HV)
0755
50747
100751
150721
200722
250717
300697
350682
400631
450579
500494
550429
600374
650439
700413
750420
800415
850408
900416
950407
1000410