JPS61117268A - Vacuum carburization method of steel material parts - Google Patents

Abstract

PURPOSE:To decarburize the projecting parts of excessive carburization and to execute uniform carburization of steel material parts in a vacuum carburization method consisting of a heating period, carburizing period and diffusing period by forming a reduced pressure decarburizing atmosphere in the diffusing period. CONSTITUTION:The steel material parts are heated to about 850-1,040 deg.C in a vacuum kept under about 0.5-0.01Torr and when the uniform heat is attained, a gaseous mixture composed of gaseous N2 and gaseous C3H8 is fed into the furnace and the carburization treatment is executed while the furnace pressure is maintained under about 300-600Torr. The furnace pressure is again reduced to about 0.5-0.01Torr after the prescribed time and the decarburizable atmosphere gas such as gaseous CO2 is fed into the furnace for the prescribed time until about 300-600Torr furnace pressure is attained. The inside of the furnace is then evacuated to a vacuum and the diffusion and decarburization are repeated. The furnace inside is again evacuated to a vacuum upon completion of the diffusion and decarburization treatment and thereafter gaseous N2 is supplied into the furnace and the steel material parts are cooled (to about 600 deg.C) by which the parts are subjected to a normalizing treatment. The inside of the furnace is further evacuated to a vacuum and the steel material parts are reheated; thereafter the gaseous N2 is supplied into the furnace to restore the pressure and the parts are subjected to hardening.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for vacuum carburizing steel parts.

(Prior art and its problems) Vacuum carburizing, which is disclosed in Japanese Patent Publication No. 54-31976, has conventionally been used as a method for forming a suitable hardened resin on the surface of steel parts.

In this vacuum carburizing method, depending on the shape of the steel parts to be treated, there may be deep carburized parts and shallow carburized parts.
It is a well-known fact that non-uniform carburization occurs.

This is because the diffusion of infiltrated carbon differs depending on the shape of the steel component, and for example, carburization becomes deeper in protrusions (edges).

In other words, in vacuum carburizing, the saturation value adjustment method is used to control the surface carbon percentage, so at the end of carburizing, the entire surface will be excessively carburized.

Then, the permeated carbon is adjusted to a predetermined percentage by diffusing into the copper parts during the diffusion period, but at the protrusions, the diffusion into the steel parts interferes with each other, hindering the diffusion, and eventually the protrusions This is because some parts remain in an excessively carburized state.

As a result of various studies, the inventors of the present invention have found that although the diffusion of carbon into the interior of the steel component at the protruding portion is inhibited as described above, the diffusion to the outside occurs more quickly than at the flat portion.

(Means to Solve the Problem) Therefore, the present invention has been made in view of the above-mentioned facts, and a decarburizing atmosphere gas is supplied during the diffusion period to form a decarburizing atmosphere, giving priority to excessive carburization of the protrusions. By decarburizing the steel surface, the carbon percentage between the protrusion and the flat surface is made uniform at the end of the diffusion period.

(Example) Next, the vacuum carburizing method of the present invention will be explained with reference to the drawings which are one example.

FIG. 1 shows a carburizing cycle illustrating the carburizing method according to the present invention.

That is, the steel parts are, for example, 05 to 0.0 ITo.
It is heated to the carburizing temperature of 850-1040°C in a vacuum of rr. After uniform heating is achieved, a carburizing gas, N, a mixture of gas and C3H8 gas, is fed into the furnace to maintain the furnace pressure at 300 to 600 Torr to carburize the steel parts. . After performing carburizing treatment for a predetermined time, the inside of the furnace was evacuated again to 0.5 to 0.01 Torr, and CO,
Decarburizing atmospheric gas such as gas is applied for a predetermined period of time at a furnace pressure of 300~
The power is supplied so that it becomes 600 Torr. After that, it is evacuated and the same operation is repeated to perform diffusion and decarburization.

When the diffusion and decarburization process is completed as described above, the inside of the furnace is evacuated again, and then N and gas are supplied to cool the steel parts (to 600°C) and normalize them, and then,
The inside of the furnace is evacuated to reheat the steel parts, and after reheating, N and gas are supplied into the furnace to restore pressure and then the steel parts are quenched.

Note that for materials that do not have to worry about coarsening of crystal grains, normalization after carburizing and diffusion is not performed, but the material is cooled in a furnace to the quenching temperature.

By the way, if the target surface carbon content is 0.9% (Fig. 2 A), the carbon content on the protrusion surface will be 1.5% in the conventional carburizing method in which diffusion is carried out under vacuum (Fig. 2 A). According to the method of the present invention, a decarburizing atmosphere gas such as CO2 is supplied into the furnace during the diffusion period to perform diffusion and decarburization, so excess carbon on the protrusions is decarburized and carbon on the surface of the protrusions is removed. The amount decreased to 1.0% (Figure 2 C).

In the above embodiment, the decarburizing atmosphere gas was supplied into the furnace in a pulsed manner to form an intermittent reduced pressure decarburizing atmosphere, but continuous supply may also be used.
02 gas alone or a mixture of C2 gas and N gas may be used.

(Effects) As is clear from the above explanation, according to the vacuum carburizing method of the present invention, a decarburizing atmosphere gas is supplied into the furnace during the diffusion period to form a reduced pressure decarburizing atmosphere. It is possible to decarburize the protrusions that have formed on the surface and uniformly carburize the steel parts.

Moreover, due to the presence of a decarburizing atmosphere, the soot adhering to the inside of the furnace is also decarburized, reducing the amount of adhesion, eliminating the need for burnout treatment, and even if burnout treatment is performed, the number of times it must be performed This has the effect of significantly reducing the

[Brief explanation of drawings]

FIG. 1 is a carburizing cycle diagram of the vacuum carburizing method for steel parts according to the present invention, and FIG. 2 is a graph showing the relationship between the carburizing depth of a protrusion and its carbon content in the method of the present invention and the conventional method.

Claims (3)

[Claims] (1) In a vacuum carburizing method consisting of a heating period in which the temperature is raised under vacuum and reduced pressure, a carburizing period in which carbon is permeated by heating and holding in a reduced pressure carburizing gas atmosphere, and a diffusion period in which the permeated carbon is diffused inside. . A vacuum carburizing method for steel parts, characterized in that a reduced pressure decarburizing atmosphere is formed during the diffusion period. (2) The decarburization atmosphere is CO_2 gas alone or C
The vacuum carburizing method for steel parts according to claim 1, wherein the gas is a mixture of O_2 gas and N_2 gas. (3) The first aspect of claim 1, wherein the decarburization atmosphere is formed intermittently or continuously.
The vacuum carburizing method for steel parts according to item 1 or 2.