PROCESS OF AND APPARATUS FOR HARDENING STEEL SURFACE

FIELD OF THE INVENTION

The present invention relates to a process and an apparatus for hardening steel surface for forming, in a short time, a steel material of a deep hardening depth, provided with a compound layer. BACKGROUND OF THE INVENTION

Conventionally, steels and cast irons used for components for machine structural use are surface-hardened by nitriding, soft nitriding, carburizing, high-frequency hardening, etc. for the purpose of improving their mechanical strength such as friction property, abrasion resistance, fatigue strength, etc. It is known that among those, the steel materials processed by nitriding are good in friction property, abrasion resistance and heat resistance, however, if compared to those processed by high-frequency hardening or carburizing, their hardening depths are shallow, and more improvement in contact strength, fatigue strength, etc. is required. To solve this problem, there are known, combined heat treatment processes of nitriding/soft nitriding and high-frequency hardening, to improve the contact strength, etc. by providing a deep hardening depth, by performing high-frequency hardening after nitriding or soft nitriding.

One example of a known combined heat treatment process is a process of manufacturing components for machine structural use of good mechanical strength, and the process processes a steel of a composition by soft-nitriding under conditions providing a nitriding layer of a depth of 150 μιη or higher, and then performs high-frequency hardening on the steel under conditions austenitizing the nitriding layer (Patent Document 1). The process of the Patent Document 1 can manufacture components for machine structural use of good mechanical properties, such as contact strength, bending strength, torsional strength, by means of temper softening resistance and cracking resistance of a martensite formed by quenching austenite containing nitrogen and carbon.

However, although the process of the Patent Document 1 tries to improve contact strength and fatigue strength by means of advantageous effects of the nitrogen diffusion layer, that is temper softening resistance and cracking resistance, by forming a martensite structure containing nitrogen obtained through quenching, however, the compound layer formed by nitriding is not utilized. Furthermore, in high-frequency hardening after nitriding, austenitizing is performed at a temperature of 900-1200°C, in that respect, the formed compound layer is a compound of iron and nitrogen, and therefore, if re-heated to 650°C or higher, it will be decomposed due to oxidation and nitrogen in the compound layer is released at the topmost surface as a nitrogen gas and diffused inside thereof, and as a result, the compound layer is lost. This has been reported for a long time (Non-Patent Document 1). In that respect, there is another combined heat-treatment process for treating a quenched steel material, which performs high-frequency hardening after nitriding, while leaving an effective compound layer on the surface of the steel material, by heating the surface of the steel material to 350°C-600°C to nitride, to form a compound layer on the surface of the steel material and diffusing nitrogen in the surface part of the steel material covered by the compound layer, and then performs high-frequency hardening under an ammonia gas atmosphere, inert-gas atmosphere, reducing gas atmosphere, mixed-gas atmosphere of these gases, or hypoxic atmosphere, or under vacuum (Patent Document 2).

The process of the Patent Document 2 forms, by nitriding, a compound layer and a nitrogen diffusion layer on the surface part of the steel material, and high-frequency hardening is performed under an inert-gas, etc. atmosphere so that the compound layer is not decomposed due to oxidation, to form a steel material of a deep hardening depth, provided with a compound layer.

There is another art to leave an effective compound layer on the surface of the steel material, which is a combined heat treatment process that performs high-frequency hardening after nitriding, and includes a step, after nitriding and before high-frequency hardening, of forming a 0.1- to 5-μηι thick oxidation layer at 600°C or lower, on the surface side of the compound layer formed by nitriding (Patent Document 3).

The process of the Patent Document 3 preliminarily forms a dense oxidation layer on the compound layer, and the oxidation layer works as a protection film for preventing oxidation degradation of the compound layer, caused in high-frequency hardening, to evenly leave the compound layer obtained after high-frequency hardening.

Furthermore, there is another surface processing process in which a compact of cast iron is nitrided at a processing temperature of 580-590°C, and then, heated and kept to the Al transformation point or higher but not higher than the eutectic temperature of cast iron, and then, quenched and kept in a heat bath of a temperature, and the nitrogen diffusion layer and the base material formed by nitriding isothermally transformed (Patent Document 4). The process of the Patent Document 4 can form on the surface of a compact of cast iron a compound layer, and a nitrogen diffusion layer and a base material, which is a bainite structure transformed from the matrix under the compound layer.

[Background Art]

[Patent Document]

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 7-90364

[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2011-32536 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2012-062494 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2000-337410 [Non-Patent Document 1] "Netsu-shori (heat treatment)", p. 206, 4th issue, vol. 16, 1976 EMBODIMENT OF THE INVENTION

The processes of the Patent Documents 2 and 3 can form by combined heat treatments a steel material of a deep hardening depth, provided with a compound layer, however, if these processes perform nitriding as salt bath nitriding, gas soft nitriding, gas nitriding, etc., the nitriding is processed at a heating temperature of 350°C-600°C under a controlled nitriding potential. Therefore, to form a compound layer of a required thickness of e.g. 5 μπι or thicker, it takes at least 1 hr and most likely 2-4 hrs, and the whole process including the preprocess of degreasing and preheating and the postprocess of washing takes at least 2-5 hrs.

In that respect, the nitriding rate of iron depends on the nitriding potential, and if the nitriding potential is increased, the rate of increase in nitrogen concentration increases as well, and therefore, the compound layer is formed faster. However, in the processes of the Patent Document 2 and 3, if nitriding is performed at a high nitriding potential, the formed compound layer may be broken, cracked, etc. after cooling, and therefore, the nitriding potential is controlled so that it does not become unnecessarily high.

Generally, the nitrogen concentration in a compound layer becomes higher toward the surface of the steel material, and its phase changes from the γ phase to the ε phase to the ζ phase from the innermost boundary with the base material to the topmost surface. In that respect, a compound layer with no broken pieces, crack, etc. is mainly formed of the ε phase or a mixture of the ε phase and the γ phase. However, as shown in a phase diagram of Fe-N of Fig. 2, the ε phase contains 6 to 11 wt% nitrogen, however, if the ε phase contains about more than 9wt% nitrogen, the compound layer is brittle and easily broken, and so is the compound layer of the ζ phase containing 1 lwt% nitrogen

In other words, in the processes of the cited references 2 and 3, if nitriding is performed at a high nitriding potential, the nitrogen concentration in the compound layer increases too much along with the increase in the nitriding potential, and the ε phase and the ζ phase of about more than 9 wt% are formed in a part or the whole of the compound layer, and as a result, the compound layer is broken or cracked due to the stress induced during cooling. Therefore, the processes of the cited references 2 and 3 cannot perform nitriding in a short time by increasing the nitriding potential.

Furthermore, the nitriding rate of iron depends on the processing temperature, and if the processing temperature is increased, the nitriding rate becomes faster. However, in the processes of the Patent Document 2 and 3, if nitriding is performed on a steel material in the temperature range of 600°C-650°C, the rate of nitrogen diffusion from the surface becomes faster than that inside, decreasing the nitrogen concentration in the compound layer, and its hardness and the hardness of the inner diffusion layer, and therefore, the formed compound layer is poor in friction property, abrasion strength, heat resistance, and accordingly fatigue strength.

Furthermore, if nitriding is performed on a steel material in the temperature range of 600°C-650°C, it reaches the austenitic region as shown in the phase diagram of Fe-N of Fig. 2, and therefore, an austenite structure containing nitrogen is formed directly under the compound layer, and the austenite structure containing nitrogen is transformed to a braunite structure by slow cooling. The braunite structure has a low hardness, and decreases the mechanical strength. Therefore, the processes of the cited references 2 and 3 cannot perform nitriding in a short time by increasing the processing temperature.

Therefore, the processes of the Patent Document 2 and 3 take a long time to finish the series of process, as there is limit on shortening the nitriding time, without being able to employ a high nitriding potential and a high processing temperature in nitriding. Furthermore, although the surface processing process of the Patent Document 4 can form on the surface of a compact of cast iron a compound layer, and a nitrogen diffusion layer and a base material which is a bainite structure transformed from the matrix under the compound layer, the nitriding is performed at a nitriding temperature of 580-590°C and there is limit on increasing the nitriding potential, and therefore, like the processes of the Patent Document 2 and 3, the series of process takes time. As a result of earnest consideration on performing the surface-hardening process on a steel material in a short time, with the performance equal to or better than that of the existing combined heat treatments, the inventors achieved the present invention, upon finding that even if a compound layer of high nitrogen concentration, e.g. a compound layer of the ε phase of more than 9-wt% nitrogen, or a compound layer of a nitrogen concentration high enough to form ζ phase of 11 wt%„ as a result of performing nitriding on the steel material at a high nitriding potential and a high processing temperature, by making use of the fact that if the steel material on which nitriding has been performed is heated to 600 degrees Celsius or higher, the compound layer is thermally decomposed, releasing nitrogen outside and diffusing in the steel material, if the nitrogen concentration in the compound layer is decreased, a steel material of a deep hardening depth, provided with a compound layer of the layerund layertra, or of the yerund layertration in the trogen ou

The present invention was created upon consideration of the above-described matters, and one of its objects is to provide a process and an apparatus for hardening steel surface, for forming in a short time a steel material of a deep hardening depth, provided with a compound layer.

[Solution to Problem]

To solve the afore-mentioned problems, a process for hardening steel surface of the present invention comprises: a nitriding step for heating a steel material in a nitriding-gas atmosphere of an ammonia-gas content of 20-100 vol% and at 592-650°C by high-frequency induction heating, to form on the surface of the steel material a compound layer of high nitrogen concentration including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration; and a quenching step for heating the steel material, which has been processed by the nitriding step, by high-frequency induction heating at a temperature and under an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases, or under vacuum, and then quenching, to form on the surface of the steel material a compound layer of the ε phase of 6- to 9-wt% nitrogen concentration, or of the ε phase and the γ ^' phase of 6- to 9-wt% nitrogen concentration; wherein the temperature of the steel material which has been processed by the nitriding step is kept to 350°C or higher, until the quenching step is started (claim 1).

In the present invention, a compound layer of high nitrogen concentration means such a compound layer a part or the whole of which includes a compound layer containing more than 9-wt% nitrogen, for example, one of the nore than 9 which includes a h-wt% nitrogen, or one of a nitrogen concentration high enough to create the ntil the quenching step is started (claim l).as atmosphere, a means one that is enough to precipitate the to create the ntil , and in other words, more than 1 l-wt% nitrogen content in the compound layer.

For the above-described structure, the nitriding step only needs to form a compound layer of high nitrogen concentration including, in a part or the whole thereof, a compound layer of more than 9-wt% nitrogen concentration, and therefore, nitriding is performed in a short time since it becomes possible to employ a high nitriding potential formed under process conditions heating a steel material in a nitriding-gas atmosphere of an ammonia gas content of 20-100 vol%, at a temperature in the range of 592-650°C by high-frequency induction heating. Furthermore, by keeping the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, the compound layer of high nitrogen concentration including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration is kept from being cracked or broken. Furthermore, by performing the quenching step for heating the steel material, which has been processed by the nitriding step, under an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases, or under vacuum, to a temperature by high-frequency induction heating, and for then quenching, nitrogen in the compound layer of high nitrogen concentration, including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration, is released outside and diffused in the steel material, reducing the nitrogen concentration in the compound layer, to form the steel material provided with a compound layer of the ε phase of 6- to 9-wt% nitrogen concentration, or of the ε phase and the γ ^' phase of 6- to 9-wt% nitrogen concentration, and on its surface part a hardened layer including a fine martensite structure containing nitrogen, providing a deep effective hardening depth. In this case, before the quenching step, a second evacuating step for keeping the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, and exhausting the nitriding gas to produce a vacuum process atmosphere may be provided (claim 2).

The above-described structure makes it possible to keep the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, and to produce a vacuum atmosphere in the quenching step to prevent decomposition of the compound layer due to oxidation.

In this case, before the quenching step, a replacement step for keeping the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, and exhausting the nitriding gas to produce an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases may be provided (claim 3).

The above-described structure makes it possible to keep the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, and to produce an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases in the quenching step to prevent decomposition of the compound layer due to oxidation.

In this case, it is preferable if the nitriding step includes a nitriding-gas supplying step for transforming the process atmosphere to a nitriding-gas atmosphere, and a heating step for then heating the steel material in the nitriding-gas atmosphere by high-frequency induction heating (claim 4).

If the nitriding gas contains an ammonia gas, the ammonia produces a nitride as a result of N, which is obtained as a result of decomposition of a chemical equation NH3< » N]+3/2H2, diffusing from the surface of the steel material. By heating the steel material by high-frequency induction heating, it becomes possible to heat only the steel material, without heating the furnace, jigs and components. Therefore, thermal decomposition reactions of ammonia decomposition, 2NH3< >N2+3H2, rarely happen on the surfaces of the furnace, jigs and components. Therefore, the nitriding potential around the surface of the steel material is increased, allowing for speed-up of the increase in nitrogen concentration of the compound layer, and therefore, nitriding is performed in a shorter time. Furthermore, ammonia used in nitriding can be conserved.

Preferably, the nitriding step includes, before the nitriding-gas supplying step, an evacuating step for producing a vacuum process atmosphere, and in the evacuating step, the process atmosphere is a vacuum of 0.01-10.0 Torr, and after the nitriding-gas supplying step, the process atmosphere is 100-760 Torr (claims 5 and 6).

For the above-described structure, in the nitriding step, oxidation of the surface of the steel material is prevented. Furthermore, by producing a process atmosphere of 100-760 Torr after the nitriding-gas supplying step, the concentration of the nitriding gas in the process atmosphere becomes appropriate.

In this case, in the heating step, the steel material is preferably heated by high-frequency induction heating, while an airflow is generated in the direction of the steel material (claim 7). The above-described structure makes it possible to remove hydrogen and nitrogen, formed as a result of ammonia decomposition, from the surface of the steel material, allowing for supplying ammonia to the vicinity of the surface of the steel material at all times, and therefore, nitriding can be performed in a shorter time. In this case, in the nitriding step, the steel material may be heated by high-frequency induction heating for 1200 seconds or shorter, and its final heating temperature will reach the range of 600-650°C (claim 8).

Furthermore, in the quenching step, the steel material may be heated for 5 seconds or shorter, and its final heating temperature will reach the range of 750-860°C (claim 9). An apparatus of the invention for hardening steel surface is to perform the process for hardening steel surface of claim 1 , and the apparatus nitriding and quenching a steel material comprises: a furnace for charging the steel material; a nitriding-gas feeding unit for supplying, to the furnace, a nitriding gas containing an ammonia gas of 20-100 vol%; a heating unit for heating, in the nitriding and quenching, the steel material in the furnace to a temperature by high-frequency induction heating; an exhaust unit for exhausting gas from the furnace to produce a vacuum atmosphere; a cooling unit for cooling the steel material in the furnace; and a control unit for controlling the nitriding-gas feeding unit and the heating unit, to perform nitriding in which the steel material is heated at 592-650°C to form, on the surface of the steel material, a compound layer of high nitrogen concentration including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration; and then controlling the exhaust unit to keep, while producing a vacuum in the furnace, the temperature of the steel material, which has been processed by the nitriding, to 350°C or higher until the quenching is started, and then performing the quenching by controlling the heating unit and the cooling unit to form on the surface of the steel material a compound layer of the ε phase of 6- to 9- wt% nitrogen concentration, or of the ε phase and the γ ^' phase of 6- to 9-wt% nitrogen concentration (claim 10).

An apparatus of the invention for hardening steel surface is to perform the process for hardening steel surface of claim 1 , and the apparatus nitriding and quenching a steel material comprises: a furnace for charging the steel material; a nitriding-gas feeding unit for supplying, to the furnace, a nitriding gas containing an ammonia gas of 20-100 vol%; a heating unit for heating, in the nitriding and quenching, the steel material in the furnace to a temperature by high-frequency induction heating; an inert-gas feeding unit for supplying to the furnace an inert gas, a reducing gas, or a mixed gas of both; an exhaust unit for exhausting gas from the furnace; a cooling unit for cooling the steel material in the furnace; and a control unit for controlling the nitriding-gas feeding unit and the heating unit, to perform nitriding in which the steel material is heated at 592-650°C to form, on the surface of the steel material, a compound layer of high nitrogen concentration including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration; and then controlling the inert- gas feeding unit and the exhaust unit to keep the temperature of the steel material, which has been processed by the nitriding, to 350°C or higher until the quenching is started, and to produce an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases in the furnace, and then performing the quenching by controlling the heating unit and the cooling unit to form on the surface of the steel material a compound layer of the ε phase of 6- to 9-wt% nitrogen concentration, or of the ε phase and the and γ' phase of 6- to 9- wt% nitrogen concentration (claim 1 1).

Preferably, in the nitriding, the control unit controls the exhaust unit to produce a vacuum process atmosphere before the nitriding-gas feeding unit supplies the nitriding gas (claim 12). In this case, preferably, the control unit controls the exhaust unit to transform the process atmosphere to a vacuum of 0.01-10.0 Torr, before the nitriding-gas feeding unit supplies the nitriding gas, and to a process atmosphere of 100-760 Torr after the nitriding-gas feeding unit supplies the nitriding gas (claim 13). In this case, preferably, a blower unit for generating an airflow in the direction of the steel material in the furnace is provided, wherein in the nitriding, the control unit controls the blower unit to generate an airflow in the direction of the steel material (claim 14).

In the nitriding, the control unit may control the heating unit to heat the steel material for 1200 seconds or shorter, to a final heating temperature in the range of 600-650°C (claim 15).

In the quenching, the control unit may control the heating unit to heat the steel material for 5 seconds or shorter, to a final heating temperature in the range of 750-860°C (claim 16). A temperature sensor for measuring the temperature of the steel material in the furnace may be provided; wherein the control unit may control the heating unit based on the information from the temperature sensor, to keep the temperature of the steel material which has been processed by the nitriding to 350°C or higher until the quenching is started (claim 17). [Advantageous Effects of Invention]

With the present invention, the nitriding step only needs to form a compound layer of high nitrogen concentration including, in a part or the whole thereof, a compound layer of more than 9-wt% nitrogen concentration, and therefore, nitriding is performed in a short time since it becomes possible to employ a high nitriding potential formed under process conditions heating a steel material in a nitri ding-gas atmosphere of an ammonia gas content of 20-100 vol%, at a temperature in the range of 592-650°C by high-frequency induction heating. Furthermore, by keeping the temperature of the steel material, which has been processed by the nitriding step, to 350°C or higher until the quenching step is started, the compound layer of high nitrogen concentration, including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration, is kept from being cracked or broken. Furthermore, by performing the quenching step for heating the steel material, which has been processed by the nitriding step, under an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases, or under vacuum, to a temperature by high-frequency induction heating, and for then quenching, nitrogen in the compound layer of high nitrogen concentration, including in a part or the whole thereof a compound layer of more than 9-wt% nitrogen concentration, is released outside and diffused in the steel material, reducing the nitrogen concentration in the compound layer, to form the steel material provided with a compound layer of the ε phase of 6- to 9-wt% nitrogen concentration, or of the ε phase and the γ ^' phase of 6- to 9-wt% nitrogen concentration, and on its surface part a hardened layer including a fine martensite structure containing nitrogen, providing a deep effective hardening depth. In other words, the process and apparatus of the invention for hardening steel surface can form, in a short time, a steel material of a deep hardening depth, provided with a compound layer of the ε phase of 6- to 9-wt% nitrogen concentration, or the ε phase and the γ' phase of 6- to 9-wt% nitrogen concentration. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 illustrates a sequence of processing temperatures of a steel material of a first embodiment of the present invention.

Fig. 2 shows a Fe-N binary phase diagram.

Fig. 3 illustrates a schematic cross-sectional view of the first embodiment of the apparatus for hardening steel surface of the present invention.

Fig. 4 shows a flowchart of the steps of the first embodiment of the process for hardening steel surface.

Fig. 5 shows an optical micrograph illustrating the cross-sectional of the steel material (Embodiment 1), processed by the first embodiment of the process for hardening steel surface. Fig. 6 shows hardness profile on the cross-section of the specimen after the Embodiment 1 , obtained from the results of the cross-sectional hardness measurement of the Embodiment 1. Fig. 7 illustrates a sequence of processing temperatures of a steel material of a second embodiment of the present invention.

Fig. 8 illustrates a schematic cross-sectional view of the second embodiment of the apparatus for hardening steel surface of the present invention.

Fig. 9 shows a flowchart of the steps of the second embodiment of the process for hardening steel surface.

Fig. 10 illustrates a schematic cross-sectional view of the third embodiment of the apparatus for hardening steel surface of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Now, a first embodiment of a process and an apparatus for hardening steel surface of the present invention will be explained with reference to the drawings. As shown in Fig. 3, the apparatus for hardening the surface of a steel material W of the invention comprises: a furnace 1 for charging the steel material W; a nitriding-gas feeding unit 10 for feeding a nitriding gas to the furnace 1 ; a heating unit 20 for heating, in nitriding and quenching, the steel material W in the furnace to a temperature by high-frequency induction heating; an exhaust unit 30 for exhausting the gas inside the furnace 1 to produce a vacuum atmosphere in the furnace 1 ; a cooling unit 40 for cooling the steel material W in the furnace 1 ; and a control unit 100. As shown in Fig. 3, the furnace 1 includes a furnace body 2 almost in a hollow tetrahedral shape, and one side surface of the furnace 1 is provided with a door (not illustrated) for putting in/taking out the steel material W from the furnace body 2. Also, a stand 3 for holding the steel material W is provided on the inner bottom surface of the furnace body 2. The inside of the furnace 1 of the above-mentioned structure is air tight, and it withstands high pressure and high temperature.

As shown in Fig. 3, the nitriding-gas feeding unit 10 includes a nitriding-gas supply source 1 1 for storing a nitriding gas by means of a high-pressure gas cylinder, a nitrogen-gas supply line 12, connected to one surface of the furnace body 2, for connecting the nitriding-gas supply source 11 and the inside of the furnace 1, and an on-off valve VI attached to the nitrogen-gas supply line 12 for adjusting the flow rate. As shown in Fig. 3, the heating unit 20 includes an induction heating coil 21 provided around the stand 3 in the furnace 1 , a high-frequency oscillator 22 provided outside the furnace 1 and connected with the induction heating coil 21 through one side surface of the furnace body 2. The induction heating coil 21 is connected with the high-frequency oscillator 22 provided outside the furnace 1, and provides high-frequency electricity for heating a substance to a desired temperature.

As shown in Fig. 3, the exhaust unit 30 includes an exhaust apparatus 31 , an exhaust tube 32, connected to one surface of the furnace 1, for connecting the exhaust apparatus 31 and the furnace 1, and an on-off valve V2 attached to the exhaust tube 32. The exhaust tube 32 includes an air introducing tube 33 connected thereto for introducing air into the furnace 1, and the air introducing tube 33 includes an on-off valve V3.

As shown in Fig. 3, the cooling unit 40 includes a coolant supply source 41 for storing coolant, a nozzle 42 provided in the furnace 1 to face the stand 3, a coolant supply line 43 for connecting the coolant supply source 41 and the nozzle 42, and an on-off valve V4 attached to the coolant supply line 43. As shown in Fig. 3, the control unit 100 is a computer provided with an arithmetic processing unit including a microprocessor such as a CPU and peripheral circuits, and includes a program storage unit (not illustrated) for storing an execution program, etc. for executing the surface-hardening process, a storage unit (not illustrated) for storing data that relate to setup nitriding temperatures, etc., and an input unit (not illustrated) for the operator to input and set parameters such as processing temperatures, processing times, etc.

As shown in Fig. 3, the control unit 100 is electrically connected to the on-off valves VI- V4,the exhaust apparatus 31 and the high-frequency oscillator 22, and based on control signals from the control unit 100, opening/closing operation, heating operation, exhaust operation, etc. are performed.

[Embodiment 1] Now, how the steel material W is processed by the above-explained first embodiment of the apparatus for hardening steel surface is explained. Fig. 4 is a flowchart that illustrates the steps of a surface-hardening process by the first embodiment of the apparatus for hardening steel surface, and the steps are carried out in the order shown by the arrow. The steel material W, for example, an S45C steel material with 25 mm in diameter and 30 mm in length which was refined is surface-hardened after its surface is degreased. Note that the steel material W to which the present invention is applied includes carbon steel, low-alloy steel, medium-alloy steel, high-alloy steel, cast iron, etc., and is not limited to any specific steels. For the sake of cost savings, preferable materials are carbon steel, low-alloy steel, etc. For example, preferable carbon steels are carbon steels for machine structural use (S20C- S58C), and preferable low-alloy steels are nickel-chromium steels (SNC236-836), nickel- chromium-molybdenum steels (SNCM220-815), chromium-molybdenum steels (SCM415- 445, 822), chromium steels (SCr415-445), manganese steels for machine structural use (SMn420-443), manganese-chromium steels (SMnC420,443), etc.

Firstly, as shown in Figs. 1 and 4, the steel material W that has been preprocessed by degreasing, etc. is processed by a nitriding step (Step HI). The nitriding step HI includes an evacuating step (Step SI), a nitriding-gas supplying step (Step S2), and a heating step (Step S3), in which the steel material W is heated in a nitriding-gas atmosphere to form a compound layer of high nitrogen concentration, and to form a nitrogen diffusion layer beneath the compound layer on the surface part of the steel material W.

The operator places the steel material W to be surface-hardened on the stand 3 in the furnace 1, and then operates the control unit 100 to begin the process. Firstly, as shown in Figs. 1 and 4, in the nitriding step HI, the evacuating step SI is performed to produce a vacuum process atmosphere.

The control unit 100 runs a program based on the data input by the operator through the input unit, to activate the exhaust apparatus 31 of the exhaust unit 30 and open the on-off valve V2, to produce a vacuum process atmosphere. Here, the vacuum in the furnace 1 is reduced to 0.1 Torr by activating the exhaust apparatus 31 for 10 seconds. After activating the exhaust apparatus 31 for 10 seconds, the control unit 100 stops the exhaust apparatus 31 and closes the on-off valve V2. As described above, by performing the evacuating step SI to produce a vacuum process atmosphere before the nitriding-gas supplying step S2, oxidation of the surface of the steel material W in the nitriding step is prevented. Note that in this embodiment, the vacuum is 0.1 Torr, however, in the present invention, it is better if the vacuum in the furnace 1 in the evacuating step SI is 0.01-10.0 Torr, or preferably 0.1-1.0 Torr.

Then, as shown in Figs. 1 and 4, the nitriding-gas supplying step S2 for transforming the process atmosphere to a nitriding-gas atmosphere is performed. The control unit 100 opens the on-off valve VI of the nitriding-gas feeding unit 10, and supplies a nitriding gas to the furnace 1 at a preset flow rate of 50 Torr/s.

In this case, the nitriding gas contains a 100% ammonia gas by volume. In the present invention, it is better if the content of the ammonia gas in the nitriding gas is 20-100 volume percent (vol%), or preferably 80-100 vol%. This is because, if the concentration is less than 20 vol%, the nitriding potential is too low to form a compound layer of high nitrogen concentration in a short time. If the content of the ammonia gas in the nitriding gas is 20-100 vol%, the nitriding potential is high enough to speed up the increase in the nitrogen concentration of the compound layer, and therefore, it becomes possible to form, in a short time, a compound layer of high nitrogen concentration on the surface of the steel material W. Note that in this embodiment, the nitriding gas contains a 100% ammonia gas by volume, however, the nitriding gas may be a mixture of ammonia gas and carbon dioxide gas.

When the nitriding-gas supplying step S2 is started, the nitriding gas is supplied to the furnace 1 for a preset time of 10 seconds, and then the control unit 100 closes the on-off valve VI of the nitriding-gas feeding unit 10.

When the nitriding-gas supplying step S2 is finished, the pressure in the furnace 1 is 500 Torr. In this embodiment, the vacuum is 500 Torr, however, in the present invention, it is better if the vacuum in the furnace 1 in the heating step S3 is 100-760 Torr, or preferably, 500-760 Torr. In that way, the concentration of the nitriding gas in the process atmosphere becomes appropriate.

Then, the heating step S3 is performed as shown in Figs. 1 and 4, and as a result, the steel material W is heated under the nitriding-gas atmosphere at a temperature Tl in the austenitic region, to form a compound layer of a nitrogen concentration that is high enough to supply nitrogen to yFe to saturation or more to create the γ ^' phase, and then, supply nitrogen the γ ^' phase to saturation or more to create the ε phase, and then, supply nitrogen to the ε phase to saturation or more to create the ζ phase Theontrol unit 100 controls the high-frequency oscillator 22 to supply high-frequency electricity to the induction heating coil 21 , to heat up the steel material W. The control unit 100 controls the high-frequency oscillator 22 to a preset temperature and a time.

In this case, the control unit 100 controls the high-frequency oscillator 22 to heat up at a preset temperature of 650°C, which is obtained in 2 seconds, and for a preset time of 300 seconds. In this embodiment, the heating temperature is 650°C, however, in the present invention, as shown in the phase diagram of the Fe-N system of Fig. 2, the temperature only needs to be the temperature Tl of 592-650°C in the austenitic region, which can form a compound layer of a nitrogen concentration that is high enough to supply nitrogen to yFe to saturation or more to create the γ ^' phase, and then, supply nitrogen to the is high enough to supply nitrogen to yFe to saturation or more to create the γ ^' phase, and then, supply nitrogen to the is high enough to supply nitrogen to the ε phase to saturation or more to form the ζ phase, and the processing temperature Tl is preferably 600-650°C, or even more preferably 640-650°C.

As shown in the phase diagram of the Fe-N system of Fig. 2, the ferrite region (aFe) contains up to 0.4 at% nitrogen, while the austenitic region (yFe) contains up to 10.3 at%. To increase the nitrogen concentration of the steel material W, the processing temperature Tl of 592°C or higher is required in the heating step S3. In this embodiment, nitriding is performed in a temperature range of 592°C or higher, unlike the conventional nitriding temperature of 580°C or lower. By performing the nitriding at a temperature of 592°C or higher, the amount of nitrogen in the steel material W is increased, which increases the reaction rate of nitriding which is a type of a chemical reaction, and since the nitriding can be performed at a high nitriding potential as described above, as a result, a compound layer of high nitrogen concentration is formed in a short time.

If the processing temperature Tl in the heating step S3 is over 650°C, in an ammonia gas of an equivalent nitrogen pressure of 760 Torr or lower, de-nitrogenation in which a part of the once-reacted nitrogen in the surface of the steel material W is released to be in counterpoise with the equivalent nitrogen pressure becomes obvious, and therefore, a compound layer of high nitrogen concentration cannot be formed in an efficient way. In that respect, since the present invention also uses a low-pressure ammonia gas, it is impossible to effectively form a compound layer. For that reason, the processing temperature Tl in the heating step S3 is set to be 592-650°C. Note that the temperatures in the austenitic region vary depending on the compositions of the base material, and therefore, the processing temperature may be appropriately changed by setting parameters of the control unit 100 depending on the phase diagram of the base material. In this case, the heating unit 20 heats up the steel material W to 650°C in 2 seconds, and then keeps the 650°C for 300 seconds. In other words, the heating time of the steel material is 302 seconds. In this embodiment, the heating is performed for 302 seconds, however, in the present invention, it only needs to be 1200 seconds or shorter, or preferably 2-1200 seconds, or more preferably 300 seconds. This is because, if it is shorter than 2 seconds, even though a compound layer is formed, its thickness is too thin, and if it is longer than 1200 seconds, the thickness of the compound layer becomes too thick as a result of too much nitriding reaction.

After the preset time of 300 seconds has passed, the control unit 100 controls the high- frequency oscillator 22 to stop the supply of high-frequency electricity to the induction heating coil 21. When the heating step S3 is finished, a compound layer of high nitrogen concentration is formed on the surface of the steel material W, and on its surface part covered by the compound layer, a nitrogen diffusion layer is formed. In this case, the compound layer formed on the surface of the steel material W including in its whole (layer) of a compound layer that contains more than 1 l-wt% nitrogen, which is high enough to form the material W including ininvention, a compound layer of high nitrogen concentration means such a compound layer a part or the whole of which includes a compound layer containing more than 9-wt% nitrogen, for example, one that includes a compound layer of the ε phase containing more than 9-wt% nitrogen, or one whose nitrogen concentration is high enough to create the ζ phase. Here, the concentration enough to create the ζ phase means one in a range that is enough to precipitate the ζ phase when the compound layer is cooled, and in other words, more than l l-wt% nitrogen content in the compound layer.

In this embodiment, the nitrogen concentration on the formed compound layer is high enough to create the ζ phase in the surface of the steel material W, however, in the present invention, the concentration of nitrogen in the compound layer only needs to be over 9 wt%, and for example, a compound layer of the the ε phase containing more than 9-wt% nitrogen may be formed. If the compound layer contains 9-wt% or lower nitrogen, the nitrogen concentration in the compound layer after the quenching step H2 becomes too low, which as a result makes it difficult to form a compound layer of a desired nitrogen concentration, or in other words, a compound layer of the the ε phase of 6-9 wt%, or one of the ε phase and the γ ^' phase of 6-9 wt%. On the other hand, there is no upper limit of the nitrogen concentration in the compound layer, a compound layer of a desired nitrogen concentration can be formed by adjusting the heating temperature, heating time, etc. in the second heating step S5 of the quenching step H2.

In this embodiment, the whole (layer) of the compound layer formed on the surface of the steel material W contains more than l l-wt% nitrogen, however, in the present invention, the formed compound layer only needs to contain, in a part thereof, a compound layer of high nitrogen concentration, that is more than 9 wt%, of nitrogen. Generally, in a compound layer, the concentration of nitrogen becomes higher from the area next to the boundary with the inner base material toward the topmost surface. Therefore, the compound layer of high nitrogen concentration only needs to contain, in the topmost layer (in a part) of the compound layer, more than 9-wt% nitrogen.

As described above, nitriding can be performed in the austenitic region by heating the steel material W at a temperature in the austenitic region, to form a compound layer of nitrogen concentration that is high enough to supply nitrogen the yFe to saturation or more to create the γ' phase, and then supply nitrogen the γ' phase to saturation or more to create the ε phase, and then supply nitrogen to the ε phase to saturation or more to create the ζ phase, and therefore, the nitrogen amount in the steel material W is increased, and as a result, a compound layer of high nitrogen concentration is formed on the surface of the steel material W in a short time.

In the heating step S3, the steel material W is heated by high-frequency induction heating, and therefore, thermal decomposition reactions of ammonia, 2NH3position, thermal decomposition reactions of ammonian heatingel mcomponents. Therefore, the nitriding potential around the surface of the steel material W is increased, which speeds up the increase in nitrogen concentration of the compound layer, and therefore, nitriding is performed in a shorter time.

With the above, the nitriding step HI is finished. Now, as shown in Figs. 1 and 4, before the quenching step H2, a second evacuating step (Step S4) for keeping the temperature of the steel material W to 350°C or higher and producing a vacuum process atmosphere by exhausting the nitriding gas is started. The control unit 100 activates the exhaust apparatus 31 of the exhaust unit 30 and opens the on-off valve V2 to exhaust the ammonia gas, to produce a vacuum process atmosphere. The second evacuating step S4 takes 20 seconds. At that time, the vacuum in the furnace 1 is reduced to 0.1 Torr by activating the exhaust apparatus 31 for 20 seconds. After activating the exhaust apparatus 31 for 20 seconds, the control unit 100 stops the exhaust apparatus 31 and closes the on-off valve V2.

In this case, during execution of the second evacuating step S4, the temperature T2 of the steel material W must be kept to 350°C or higher. This is because, if it becomes lower than 350°C, the compound layer of high nitrogen concentration may be cracked or broken due to the stress induced during cooling. Therefore, the second evacuating step S4 must be finished before the temperature of the steel material W becomes lower than 350°C. In this embodiment, the second evacuating step S4 is finished when the temperature T2 of the steel material W becomes 550°C.

As described above, by keeping the temperature of the steel material W, which has been processed by the nitriding step HI, to 350°C or higher until the quenching step H2 is started, the compound layer of high nitrogen concentration is kept from being cracked or broken due to the stress induced during cooling.

The second evacuating step S4 makes it possible to keep the temperature of the steel material W, which has been processed by the nitriding step HI, to 350°C or higher until the quenching step H2 is started, and to produce a vacuum atmosphere in the quenching step H2 to prevent decomposition of the compound layer due to oxidation.

Then, as shown in Figs. 1 and 4, in a quenching step (Step H2), the steel material W which has been processed by the nitriding step HI is quenched after it is heated in a vacuum by high-frequency induction heating at a temperature T3. The quenching step H2 includes a second heating step (Step S5) and a cooling step (Step S6). As a result of execution of the second evacuating step S, the temperature of the steel material W is kept to 550°C, while the pressure in the furnace 1 is reduced to 0.1 Torr. The control unit 100 starts the, by controlling the high-frequency oscillator 22 to supply high- frequency electricity to the induction heating coil 21, to heat the steel material W.

In this case, the control unit 100 controls the high-frequency oscillator 22 to perform heating at a preset temperature of 800°C, which is obtained in 1 second. In this embodiment, heating is performed at the processing temperature T3 of 800°C, however in the present invention, the temperature only needs to be 750-860°C, and preferably the processing temperature T3 is 800-850°C.

In that respect, if the temperature of a deep part of the steel material W, where not enough nitrogen is supplied in its depth direction, becomes lower than 750°C, the area is not fully austenitized and cannot be quenched well. On the other hand, if the heating temperature is over 860°C, the compound layer becomes broken during quenching, and excess residual austenite is likely to be formed in the martensite structure directly under the compound layer.

The heating time in this embodiment is 1 second, however, in the present invention, it only needs to be 5 seconds or shorter, and preferably 1 second. If it is 1 second or shorter, despite nitrogen diffusion in the compound layer, a deep part of the steel material W, where not enough nitrogen is supplied in its depth direction, is not fully austenitized, and therefore not quenched well. On the other hand, if it is over 5 seconds, nitrogen in the compound layer is diffused, causing the compound layer to be lost. After the preset time of 1 second has passed, the control unit 100 finishes the second heating step S5 by controlling the high-frequency oscillator 22 to stop the supply of high- frequency electricity to the induction heating coil 21, and starts a cooling step S6 for opening the on-off valve V4 of the cooling unit 40 and spraying water as coolant into the steel material W, from the nozzle 42 that faces the stand 3. The cooling step S6 takes 2 seconds.

As soon as starting the cooling step S6, the control unit 100 opens the on-off valves V2,V3 of the exhaust unit 30 to bring the inside of the furnace 1 back to the atmospheric pressure. With that, the quenching step H2 is finished. The operator opens the door of the furnace 1 and takes the steel material W out from the furnace 1. It takes 3 seconds for the pressure inside the furnace to be increased to the atmospheric pressure. As described above, the quenching step H2 for heating by high-frequency induction heating to the temperature T3 under vacuum, and then quenching, releases nitrogen in the compound layer of high nitrogen concentration outside and diffuses in the steel material W, allowing for reduction in the nitrogen concentration in the compound layer, to form the steel material W of a deep effective hardening depth, provided with a compound layer of the the ε phase of 6-9 wt% , or of the ε phase and the γ ^' phase of 6-9 wt%, and on its surface part, a hardened layer including a fine martensite structure containing nitrogen

As shown in Fig. 1, the time it took for each process described above is: the evacuating step SI = 20 seconds; the nitriding-gas supplying step S2 = 10 seconds; the heating step S3 = 302 seconds; the second evacuating step S4 = 20 seconds; the second heating step S5 = 1 second; and the cooling step S6 = 2 seconds (the pressure increase performed at the same time: 3 seconds), which totals 356 seconds.

In the present invention, the compound layer of high nitrogen concentration only needs to be formed in the nitriding step HI, which allows for employment of high nitriding potential, and therefore, nitriding is performed in a short time. Furthermore, by keeping the temperature T2 of the steel material W, which has been processed by the nitriding step HI, to 350°C or higher until the quenching step H2 is started, the compound layer of high nitrogen concentration is kept from being cracked or broken due to the stress induced during cooling.

In the quenching step H2, the steel material W which has been processed by the nitriding step HI is heated, under vacuum, to the temperature T3 by high-frequency induction heating, and then quenched, and as a result, nitrogen in the compound layer of high nitrogen concentration is released outside and diffused in the steel material W, reducing the nitrogen concentration in the compound layer, to form the steel material W of a deep effective hardening depth, which is provided with a compound layer the ε phase of 6- to 9-wt% nitrogen concentration, or one of the ε phase and the γ' phase of 6- to 9-wt%nitrogen concentration, and on its surface part, a hardened layer including a fine martensite structure containing nitrogen.

In other words, the process and apparatus for hardening the surface of a steel material W of this invention can form, in a short time (356 seconds), a steel material W of a deep hardening depth, provided with a compound layer.

Evaluation tests discussed below are performed on the steel material W formed in the above-explained way.

Firstly, it was confirmed that there was no broken pieces, cracks, etc. in the compound layer on the surface of the steel material W of the embodiment 1, even after the quenching step H2. Then, the hardness of the center of the tested surface was measured by a micro- Vickers hardness tester. The surface hardness of the steel material W was 750 Hv.

Then, the steel material W was cut by a microcutter and then embedded into a resin for observation of the microstructure on the cross section of the steel material W, and as a result, a micrograph of Fig. 5 was obtained. The micrograph proves that a ΙΟ-μπι-thick compound layer was formed on the surface of the steel material W. It also proves that there was a layer of austenite containing a high nitrogen content, directly under the compound layer, but no braunite, and its hardness was confirmed to be 720 Hv.

Those experimental results prove that the 10-rmecompound layer was formed with no cracks, etc. on its surface, and that the surface hardness of the steel material W is 750 Hv, and therefore, they confirm that on the steel material W of the embodiment 1, a compound layer of the ε phase of 6-9 wt%, or one of the ε phase and the γ' phase of 6-9 wt% was formed

Then, the hardness profile in a cross-section of the embedded sample was measured by a micro-Vickers hardness tester. Fig. 6 illustrates hardness profile in the cross-section. In the distribution of cross-sectional hardness, the Vickers hardness at a depth 0.1 mm from the surface of the steel material W was 700 Hv. A depth of the Vickers hardness of 550 Hv, which provides an effective hardened layer, was 0.6 mm from the surface. These results confirm a high enough hardness throughout from the surface to an adequate distance in the cross-section of the embedded sample.

The steel material W processed by the present invention was confirmed to have good friction property, high abrasion property, and high heat resistance provided by the compound layer on the topmost surface, and high contact strength, high fatigue strength, and a deep hardened depth provided by the fine martensite structure containing nitrogen.

As described above, the apparatus for hardening the surface of the steel material W of the present invention can form a compound layer on the steel material W in a short time, and provide thereto a deep hardening depth. Therefore, it can be retrofitted to machine production lines of components, which require nitriding, to produce finished products in a series of process. Therefore, unlike the conventional manufacturing that processes a massive amount of products in a furnace, the present invention does not require huge man-hours for product management including production-site management, etc., bookkeeping, delivery-date management, shipment, etc., and allows for an increased production efficiency and a significant reduction in the cost.

The steel material W formed by the process and apparatus for hardening the surface of a steel material W of this invention is suited for the use under highly-loaded/high surface- pressure environment. The shapes of the steel material W or the types of components are not limited, and their examples are shafts, gears, cams, valve lifters, plungers, etc., including transmission-related components and powertrain components for automobiles or construction machinery.

[Second Embodiment]

In the quenching step H2 of the first embodiment, the steel material W is heated under vacuum to the temperature T3 by high-frequency induction heating and then quenched, however, before quenched in the quenching step H2, the steel material W may be heated by high-frequency induction heating, under an inert-gas atmosphere, a reducing gas atmosphere, or a mixed-gas atmosphere of both gases. In this case, as shown in Figs. 7 and 9, instead of the second evacuating step S4 in the first embodiment, a replacement step P4 in which the temperature T2 of the steel material W is kept to 350°C or higher, and the process atmosphere is transformed to an inert-gas atmosphere, a reducing gas atmosphere or a mixed-gas atmosphere of both gases (hereinafter, referred to as "inert-gas, etc. atmosphere") may be performed, before the quenching step H2a.

As shown in Fig. 8, the second embodiment of the apparatus for hardening the surface of the steel material W comprises; the furnace 1 for charging the steel material W; the nitriding- gas feeding unit 10 for supplying a nitriding gas to the furnace 1 ; the heating unit 20 for, in nitriding and quenching, heating the steel material W in the furnace 1 to a temperature by high-frequency induction heating; the exhaust unit 30 for exhausting gas from the furnace 1; the cooling unit 40 for cooling the steel material W in the furnace 1 ; the control unit 100; and an inert-gas, etc. feeding unit 50 (hereinafter, referred to as "inert-gas feeding unit 50") for supplying to the furnace 1 an inert gas, a reducing gas, or a mixed gas of both.

As shown in Fig. 8, the inert-gas feeding unit 50 includes an inert-gas, etc. supply source 51 for storing an inert-gas, etc. by a high-pressure gas cylinder; an inert-gas, etc. supply line 52, connected to one surface of the furnace 1 for connecting the inert-gas, etc. supply source 51 and the furnace 1 ; and an on-off valve V5 attached to the inert-gas, etc. supply line 52 for adjusting the flow rate.

As shown in Fig. 8, the control unit 100 is electrically connected to the on-off valves VI- V5,the exhaust apparatus 31 and the high-frequency oscillator 22, and based on control signals from the control unit 100, opening/closing operation, heating operation, exhaust operation, etc. are performed.

Note that in the second embodiment, the rest of the structure is the same as that of the first embodiment, and therefore, the same elements are denote by the same reference numbers and explanations thereof are omitted. Now, how the steel material W is processed by the above-explained second embodiment of the apparatus for hardening steel surface is explained. Fig. 9 is a flowchart that illustrates the steps of a surface-hardening process by the second embodiment of the apparatus for hardening steel surface, and the steps are carried out in the order shown by the arrow.

As shown in Figs. 7 and 9, after preprocessed by degreasing, etc., the steel material W is now processed by a nitriding step (Step HI a). Like in the first embodiment, the nitriding step HI a is performed from an evacuating step (Step PI) to a nitriding-gas supplying step (Step P2) to a heating step (Step P3).

Then, as shown in Figs. 7 and 9, before the quenching step H2a is started, the replacement step (Step P4) is performed to keep the temperature T2 of the steel material W to 350°C or higher, and exhaust the nitriding gas to transform the process atmosphere to an inert-gas, etc. atmosphere. The control unit 100 opens the on-off valve V5 of the inert-gas feeding unit 50 to supply an inert-gas, etc. to the furnace 1 by a preset flow rate of 50 Torr, and activates and opens, respectively, the exhaust apparatus 1 of the exhaust unit 30 and the on-off valve V2 to exhaust the ammonia gas from the furnace 1. In the second embodiment, the inert-gas, etc. is an argon gas. The replacement step P4 takes 10 seconds. After the preset time of 10 seconds has passed, the control unit 100 stops the exhaust apparatus 31 and closes the on-off valve V2, while closing the on-off valve V5 of the inert- gas feeding unit 50.

In this case, during the replacement step P4, the temperature T2 of the steel material W must be kept to 350°C or higher. This is because, if it becomes 350°C or lower, the compound layer of high nitrogen concentration may be cracked or broken due to the stress induced during cooling. Therefore, the replacement step P4 must be finished before the temperature of the steel material W becomes 350°C or lower. In this embodiment, the replacement step P4 is finished when the temperature T2 of the steel material W becomes 570°C.

As described above, by keeping the temperature of the steel material W, which has been processed by the nitriding step HI a, to 350°C or higher until the quenching step H2a is started, the compound layer of high nitrogen concentration is kept from being cracked or broken due to the stress induced during cooling.

The replacement step P4 makes it possible to keep the temperature of the steel material W, which has been processed by the nitriding step HI a, to 350°C or higher until the quenching step H2a is started, and to produce an inert-gas, etc. atmosphere in the quenching step H2a to prevent decomposition of the compound layer due to oxidation.

Then, as shown in Figs. 7 and 9, in a quench quenching step (Step H2a), the steel material W which has been processed by the nitriding step HI a is quenched after it is heated under an inert-gas, etc. atmosphere by high-frequency induction heating at the temperature T3. The quenching step H2a includes a second heating step (Step P5) and a cooling step (Step P6). As a result of running the replacement step P4, the temperature of the steel material W is kept to 570°C, and an argon-gas atmosphere is produced in the furnace 1. The control unit 100 starts the second heating step P5, by controlling the high-frequency oscillator 22 to supply high- frequency electricity to the induction heating coil 21, to heat the steel material W.

In this case, the control unit 100 controls the high-frequency oscillator 22 to perform heating at a preset temperature of 800 ^P C in a preset time of 1 second. In this embodiment, heating is performed at the processing temperature T3 of 800°C, however in the present invention, the temperature only needs to be 750-860°C, and preferably the processing temperature T3 is 800-850°C.

In that respect, if the temperature of a deep part of the steel material W, where not enough nitrogen is supplied in its depth direction, becomes lower than 750°C, the area is not fully austenitized and cannot be quenched well. On the other hand, if the heating temperature is over 860°C, the compound layer becomes broken during quenching, and excess residual austenite is likely to be formed in the martensite structure directly under the compound layer. The heating time in this embodiment is 1 second, however, in the present invention, it only needs to be 5 seconds or shorter, and preferably 1 second. If it is 1 second or shorter, despite nitrogen diffusion in the compound layer, a deep part of the steel material W, where not enough nitrogen is supplied in its depth direction, is not fully austenitized, and therefore not quenched well. On the other hand, if it is over 5 seconds, nitrogen in the compound layer is diffused, causing the compound layer to be lost. After the preset time of 1 second has passed, the control unit 100 finishes the second heating step P5 by controlling the high-frequency oscillator 22 to stop the supply of high- frequency electricity to the induction heating coil 21 , and starts a cooling step P6 for opening the on-off valve V4 of the cooling unit 40 and spraying water as coolant into the steel material W, from the nozzle 42 that faces the stand 3. The cooling step P6 takes 2 seconds.

As soon as starting the cooling step P6, the control unit 100 opens the on-off valves V2,V3 of the exhaust unit 30 to bring the furnace 1 back to the atmospheric pressure. With that, the quenching step H2 is finished. It takes 1 second for the pressure inside the furnace to be increased to the atmospheric pressure.

As described above, the quenching step H2a for heating under an inert-gas, etc. atmosphere to the temperature T3 by high-frequency induction heating, and then quenching, releases nitrogen in the compound layer of high nitrogen concentration outside and diffuses in the steel material W, allowing for reduction in the nitrogen concentration in the compound layer, to form the steel material W of a deep effective hardening depth, provided with a compound layer of the ε phase of 6-9 wt% , or of the ε phase and the γ' phase of 6-9 wt%, and on its surface part, a hardened layer including a fine martensite structure containing nitrogen

The time it took for each process described above is: the evacuating step PI = 20 seconds; the nitriding-gas supplying step P2 = 10 seconds; the heating step P3 = 302 seconds; the replacement step P4 = 10 seconds; the second heating step P5 = 1 second; and the cooling step P6 = 2 seconds (the pressure increase performed at the same time: 3 seconds), which totals 345 seconds. In the second embodiment of the apparatus and process for hardening the surface of the steel material W, the compound layer of high nitrogen concentration only needs to be formed in the nitriding step HI a, which allows for employment of high nitriding potential, and therefore, nitriding is performed in a short time. Furthermore, by keeping the temperature T2 of the steel material W, which has been processed by the nitriding step HI a, to 350°C or higher until the quenching step H2a is started, the compound layer of high nitrogen concentration is kept from being cracked or broken due to the stress induced during cooling.

In the quenching step H2a, the steel material W which has been processed by the nitriding step HI a is heated, under an inert-gas, etc. atmosphere, to the temperature T3 by high- frequency induction heating, and then quenched, and as a result, nitrogen in the compound layer of high nitrogen concentration is released outside and diffused in the steel material W, reducing the nitrogen concentration in the compound layer, to form the steel material W of a deep effective hardening depth, which is provided with a compound layer of the the ε phase of 6- to 9-wt% nitrogen concentration, or one of the ε phase and the γ ^' phase of 6- to 9-wt% nitrogen concentration, and on its surface part, a hardened layer including a fine martensite structure containing nitrogen.

In other words, the second embodiment of the process and apparatus for hardening the surface of a steel material W can form, in a short time (345 seconds), a steel material W of a deep hardening depth, provided with a compound layer. Note that in the second embodiment described above, the inert-gas, etc. used is an argon gas, however, it may be an inert gas, a reducing gas, or a mixed gas of both. Examples of the reducing gas are oil gases such as hydrogen, propane, butane, converted gases of those, alcohols, esters, ketones, etc. Examples of the inert gases are neutral gases such as nitrogen, argon, etc. and mixed-gases of those. In such an atmosphere, in the quenching step H2a of the second embodiment, oxidation of the compound layer is well prevented.

[Third Embodiment]

As shown in Fig. 10, the third embodiment of the apparatus for hardening the surface of the steel material W comprises, in addition to the structure of the first embodiment of the apparatus for hardening the surface of the steel material W, a temperature sensor 7 for measuring the temperature of the steel material W in the furnace; a blower unit 60 for generating an airflow in the furnace toward the steel material W; and the control unit 100. As shown in Fig. 10, the temperature sensor 7 is disposed on an inner sidewall of the furnace 1 , and measures the temperature of the surface of the steel material W in the furnace 1.

As shown in Fig. 10, the blower unit 60 includes a plurality of blades 61 disposed around the stand 3 to be concentric with the stand 3, and rotation thereof generates an airflow in the direction of the arrow, that is, toward the steel material W. As shown in Fig. 10, the control unit 100 is electrically connected to the on-off valves VI- V4,the exhaust apparatus 31, the high-frequency oscillator 22, the temperature sensor 7, and the blower unit 60, and based on control signals from the control unit 100, opening/closing operation, heating operation, exhaust operation, etc. are performed. Note that in the third embodiment, the rest of the structure is the same as that of the first embodiment, and therefore, the same elements are denote by the same reference numbers and explanations thereof are omitted. In the third embodiment of the apparatus for hardening steel surface, the steel material W is processed by the same way as it is in the first embodiment. In the third embodiment of the apparatus for hardening the surface of the steel material W, the control unit 100 controls the heating unit 20 based on the information from the temperature sensor 7, to heat the steel material W to a temperature. The temperature sensor 7 measures the temperature of the steel material W at all times and sends signals to the control unit 100, and the control unit 100 receives the detection signal detected by the temperature sensor 7 and controls the high-frequency oscillator 22 based on .

In this case, during execution of the second evacuating step S4 in the first embodiment, the temperature T2 of the steel material W must be kept to 350°C or higher, and the control unit 100 receives the detection signal, to set the temperature of the steel material W to 400°C, detected by the temperature sensor 7, and controls the heating unit 20 to heat the steel material W, and keep the temperature of the steel material W which has been processed by the nitriding step HI to 350°C or higher, until quenching is started. With this structure, the compound layer of high nitrogen concentration is kept from being cracked or broken due to the stress induced during cooling.

As soon as the heating step S3 of the first embodiment is started, the control unit 100 controls the blower unit 60 to rotate the plurality of blades 61, and generates an airflow in the direction of the arrow of Fig. 10, that is, toward the steel material W to remove hydrogen and nitrogen, formed as a result of ammonia decomposition, from the surface of the steel material W, to supply ammonia around the surface of the steel material W at all times. As soon as the heating step S3 is finished, the control unit 100 stops the rotation of the blades 61 by controlling the blower unit 60.

In the third embodiment of the process and apparatus for hardening the surface of a steel material W described above, in addition to the advantageous effects of the first embodiment of the process and apparatus for hardening the surface of a steel material W, in the heating step S3, the steel material W is heated by high-frequency induction heating, while an airflow is generated in the direction of the steel material W, and therefore, it becomes possible to remove hydrogen and nitrogen formed as a result of ammonia decomposition from the surface of the steel material W, allowing for supplying ammonia to the vicinity of the surface of the steel material W at all times, and therefore, nitriding can be performed in a short time.

Therefore, the third embodiment of the process and apparatus for hardening the surface of a steel material W can form, in an even shorter time, the steel material W of a deep hardening depth, provided with a compound layer. Note that in addition to the first embodiment of the apparatus for hardening the surface, the third embodiment of the apparatus for hardening the surface of the steel material W comprises the temperature sensor 7 and the blower unit 60, however, it may comprise the second embodiment of the apparatus for hardening the surface of the steel material W, together with the temperature sensor 7 and the blower unit 60. This structure also provides the same advantageous effects as those described above. The present invention is not limited to the above-described embodiments nor examples, and the claimed art should include various changes made without departing from the scope of the invention.

[Explanation of Reference Numerals]

W steel material

HI, H2a nitriding step

H2, H2a quenching step

SI, PI evacuating step

S2, P2 nitriding-gas supplying step

S4 second evacuating step

P4 replacement step

1 furnace

7 temperature sensor

10 nitriding-gas feeding unit

20 heating unit

30 exhaust unit

40 cooling unit

50 inert-gas feeding unit

60 blower unit

100 control unit

BEST MODE OF THE INVENTION

As mentioned in the section of Detailed Description of the Invention.