JPH0874027A - Carburizing method - Google Patents

Abstract

(57) [Summary] [Purpose] To prevent the occurrence of thermal strain in the material to be processed when it is locally heated, to eliminate the need for post-processing and to shorten the processing cycle. [Composition] A process of immersing a material to be treated in oil, a step of locally heating a carburized portion of the material to be treated, and a step of carburizing by supplying a carburizing agent to the carburized portion. Characterize. The heat transferred to the non-heated portion due to heat conduction is taken by the surrounding oil, and the cooling efficiency of the non-heated portion is high, so that the temperature rise of the material to be treated is prevented and the occurrence of thermal strain is prevented.
Further, the atmosphere of the material to be treated does not need to be a carburizing gas atmosphere, and steps such as evacuation are not required and the treatment cycle is shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for carburizing metal parts.

[0002]

2. Description of the Related Art Carburizing treatment is widely used as a treatment for increasing surface hardness and abrasion resistance. This carburizing is a process of heating killed steel or low alloy steel to 900 to 1000 ° C. and exposing it to an atmosphere containing CO to diffuse and infiltrate C from the surface. Solid carburizing, liquid carburizing, gas carburizing Broadly divided.

In a general carburizing method, it is known that the carburizing rate becomes slower as the chromium content of the material to be treated increases. Therefore, when trying to carburize high chromium alloy steel, there is a problem that the carburizing process takes a long time and the grain boundary oxidation of chromium occurs in the gas carburizing method. Therefore, as a method for carburizing even high chromium alloy steel in a short time, Japanese Patent Laid-Open No. 60-36656 discloses that a carburized portion of a material to be treated is locally heated by high frequency heating in a carburizing gas atmosphere for carburizing. A method is disclosed. According to this carburizing method, since it is possible to locally heat and carburize at 1000 to 1200 ° C., the diffusion rate is accelerated, the processing time is shortened, and grain boundary oxidation is prevented.

[0004]

By the way, in order to improve the high temperature wear characteristics and the corrosion resistance of the valve face of an engine valve, it has been considered to apply carburizing treatment only to the valve face. In this case, it is effective to apply the method disclosed in the above publication and locally heat only the valve face to carry out the carburizing treatment.

However, when the carburizing method disclosed in the above publication is applied to the carburizing treatment of the valve face, even if only the valve face is locally subjected to high frequency heating, the temperature of the shaft portion also rises due to heat conduction and thermal distortion occurs. Alternatively, the hardness may be reduced due to the decomposition of the soft nitriding layer. Further, when the valve is distorted, it is necessary to correct it by post-processing such as cutting, but in that case, the carburized layer formed at the corners is cut.

Further, the carburizing method disclosed in the above publication requires a gas replacement step involving evacuation in order to create a carburizing gas atmosphere, and has a problem that the processing cycle is long. The present invention has been made in view of such circumstances, and an object of the present invention is to prevent the occurrence of thermal strain of a material to be processed when locally heated, to eliminate the need for post-processing and to shorten the processing cycle. And

[0007]

A carburizing method of the present invention for solving the above-mentioned problems comprises a step of immersing a material to be treated in oil, a step of locally heating a carburized portion of the material to be treated,
And a step of carburizing by supplying a carburizing agent to a carburized portion.

[0008]

In the carburizing method of the present invention, the material to be treated is locally heated in oil to be carburized. Therefore, since the non-heated portion is in contact with the oil, the heat transferred to the non-heated portion by heat conduction is deprived of by the oil, and the cooling efficiency of the non-heated portion is high.
Therefore, the temperature rise of the material to be treated is prevented and the occurrence of thermal strain is prevented.

Since the carburizing agent is supplied in the oil, the portion to be treated is first in contact with the oil, and the supply of the carburizing agent causes the surface of the treated portion to be replaced with the oil and the carburizing agent. The treated portion is contacted with the carburizing agent to be carburized. Therefore, the atmosphere of the material to be treated does not need to be a carburizing gas atmosphere, and steps such as evacuation are not required and the treatment cycle is shortened.

[0010]

【Example】

[Specific Example of the Invention] In the carburizing method of the present invention, since the cooling efficiency of the non-heated portion is high, thermal strain is unlikely to occur. Therefore, since the local heating temperature can be increased, not only killed steel and low alloy steel but also high alloy austenitic steel, which has been difficult to carburize in the past, can be used as the material to be treated.

Examples of the oil used in the present invention include quenching oil and light oil. Since these oils are thermally decomposed at the heating temperature during carburization to generate carburizing gas, carburization is further promoted and can be suitably used. Further, as the carburizing agent, a liquid or gas carburizing agent which is hardly dissolved in the oil used can be used. For example, if a liquid carburizing agent having a specific gravity almost equal to that of oil is used, the carburizing agent can be held at the position of the carburized portion for a long time, and the yield of the carburizing agent is further improved.

In order to locally heat the material to be treated, it is desirable to use high frequency induction heating as in the conventional case. Moreover, about 1000-1200 degreeC is recommended as a heating temperature. If the temperature is less than 1000 ° C, the carburizing time will be long.
This is because if the temperature exceeds 1200 ° C., the crystal grains may be significantly coarsened, or in the case of heat-resistant austenitic steel, the solution temperature may be exceeded. [Examples] Hereinafter, specific examples will be described. In this embodiment, carburizing is performed on the valve face surface of the exhaust valve of the engine. (Embodiment 1) FIG. 1 is a schematic structural explanatory view of a carburizing apparatus used in this embodiment. This carburizing apparatus includes a container 1 filled with quenching oil 10, a high frequency heating coil 2 arranged in the container 1, and a high frequency oscillator 2 connected to the high frequency heating coil 2.
0, a coil-shaped nozzle 3 coaxially arranged below the high-frequency heating coil 2 in the container 1, and a carburizing gas supply device 30 connected to the nozzle 3.

Above the carburizing device, there is arranged a holding device 4 comprising a drive shaft 40 connected to a drive device (not shown) and a chuck 42 connected to the drive shaft 40 via a bearing 41. The shaft 50 of the valve 5
Is held. The holding device 4 is configured to be vertically movable, and the valve 5 is entirely separated from the oil 10 and the processing position where the valve face 51 is located in the high frequency heating coil 2 by immersing almost the entire valve 5 in the oil 10. It can be stopped at two positions, the take-out position.

At the processing position, the carburizing gas blown out from the nozzle 3 flows upward along the valve face 51. Now, the shaft portion 50 of the valve 5 is held on the chuck 42, and the entire holding device 4 is lowered,
The valve 5 is rotationally driven at 30 rpm as the processing position where the valve face 51 is located inside the high frequency heating coil 2.

The valve 5 is a SUH having the composition shown in Table 1.
The valve face 51 is formed from 35 steel by forging and machining, and the valve face 51 is previously ground after soft nitriding.

[0016]

[Table 1] Next, the high frequency oscillator 20 is driven to apply a high frequency voltage to the high frequency heating coil 2. This allows the valve face 51
Is heated to 1100 ° C. At the same time, the carburizing gas supply device 30 is driven to supply the carburizing gas in which propane and nitrogen are mixed at a molar ratio of 1:12 to the coil-shaped nozzle 3.

The carburizing gas is sprayed from the nozzle 3 toward the surface of the rotating valve face 51 at a flow rate of 1 L / min, and replaces the oil on the surface of the valve face 51, as shown by the arrow in FIG. 51 flows upwards along the surface. As a result, the surface of the valve face 51 heated to 1100 ° C. is constantly in contact with the carburizing gas while the carburizing gas is being sprayed, and the surface of the valve face 51 is carburized.

Then, after carrying out carburizing treatment at 1100 ° C. for 5 minutes, the supply of carburizing gas and the application of high frequency voltage are stopped, the holding device 4 is raised and the valve 5 is pulled up from the oil 10, cut and polished. Then, the tissue inward from the surface was examined by an optical microscope. As a result, a carburized layer having a depth of 0.3 mm was uniformly formed on the valve face 51. During the carburizing process, the change in the amount of shake with reference to the shaft portion 50 of the valve face 51 was measured, and the result is shown in FIG. (Other Examples) Three levels of heating temperature of the valve face 51 by high-frequency heating were taken between 600 ° C and 1200 ° C, and carburizing treatment was performed in the same manner as in Example 1. Then, during high frequency heating, changes in the shake amount with respect to the shaft portion 50 of the valve face 51 were measured, and the results are shown in FIG. (Comparative Example) Without using the container 1, the holding device 4 holding the valve 5 and the high-frequency heating coil 2 were placed in a closed container, and after evacuation, the same carburizing gas as in the example was introduced into the closed container. And the valve face 5 by the high frequency heating coil 2
The heating temperature of 1 was set to 4 levels between 600 ° C. and 1200 ° C., and the valve face 51 was carburized for 5 minutes in a carburizing gas atmosphere.

The valve face 51 is used during high frequency heating.
The change in the shake amount with respect to the shaft portion 50 was measured, and the results are shown in FIG. (Evaluation) From FIG. 2, in the example carburized by the treatment method of the present invention, the change in the shake amount was extremely small even when heated at a high temperature, but in the comparative example, the shake amount at the heating temperature of 1100 ° C. or higher. It is clear that there is a large change and that thermal strain occurs.

That is, it is apparent that the heat distortion of the valve can be prevented by carburizing while heating in oil because the cooling efficiency is high. Since the above-mentioned thermal strain occurs, the valve face has conventionally been subjected to a plating process using a stellite material (cobalt-based alloy). This deposit processing costs not only the material cost but also the processing equipment cost,
In addition, since the machining after the plating was required and the yield was low, the treatment method was extremely expensive.

However, according to the carburizing method of this embodiment,
Since the material cost is low and the man-hour is remarkably reduced, the processing cost can be remarkably reduced as compared with the conventional deposit processing.

[0022]

That is, according to the carburizing method of the present invention, the non-heated portion is efficiently cooled by oil,
Even if locally heated to a high temperature of 1000 ° C. or higher, thermal strain is prevented, and post-processing after carburization becomes unnecessary. Therefore, even if it is a material such as a high chromium alloy, which has been difficult to carburize in the past, it is possible to easily carburize only a necessary portion with a small number of steps.

Further, since it is not necessary to perform gas replacement by evacuation to create a carburizing gas atmosphere, the processing cycle can be shortened and the required amount of carburizing agent can be reduced as compared with the conventional case. Therefore, the processing cost can be reduced.
Further, in the conventional heating in a carburizing gas atmosphere, the temperature of the carburized portion rises steadily when the input power for heating is constant, so that it is difficult to control the power. However, according to the carburizing method of the present invention, since the cooling efficiency is high, the heating temperature is constant at a temperature at which the cooling and the input electric power are balanced during heating, so that the heating temperature can be easily controlled.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of a carburizing device used in an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between heating temperature and shake amount.

[Explanation of symbols]

1: Container 2: High frequency heating coil
3: Nozzle 4: Holding device 5: Valve (material to be treated) 1
0: Oil 30: Carburizing gas supply device 51: Valve face (carburized portion)

Claims (1)

[Claims] 1. A step of immersing a material to be treated in oil, a step of locally heating a carburized portion of the material to be treated, and a step of supplying a carburizing agent to the carburized portion for carburizing, A carburizing method comprising: