JPS5952705B2 - Vacuum carburizing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a vacuum carburizing method, particularly to a method for carburizing metal materials using carbon generated as organic carburizing gas is decomposed in a furnace.

BACKGROUND ART When pyrolyzing organic gases such as methane gas (CH4), propane, butane, etc. in a furnace and carburizing metal members placed in the furnace, the following method has conventionally been used.
In other words, the material to be carburized is placed in a vacuum furnace and heated, while an organic liquid (in some cases, this gas is further thermally decomposed) is dripped into the furnace and gasified to carburize the inside of the furnace. This is a method of carburizing while maintaining the temperature. In this case, in order to control the amount of carburization, the gas in the furnace is sampled and the carbon potential is measured with an infrared analyzer, and the carbon potential in the furnace is determined to be at a specified level. This is done by adjusting the amount of organic liquid supplied to maintain the same value. However, in this method, the gas tends to accumulate in the shape of the objects to be treated, the contact areas between the objects, or the holes in the objects, so it is necessary to newly supply the gas from the outside. Replacement with the incoming gas is also not sufficient.

Therefore, the carbon potential value is maintained at a predetermined value only in areas where gas exchange is successful, and the targeted carburizing treatment is applied to all the workpieces placed in the furnace. It was difficult to rub. The purpose of this invention is to provide a new vacuum carburizing method that solves these problems. In a method of carburizing a metal member by causing a carburizing reaction with the carbon generated by the carburizing process, a step of filling the above-mentioned furnace with fresh carburizing gas, and a step of detecting a change in the gas pressure in the furnace as the carburizing gas is decomposed. 5. When the detected gas pressure in the furnace exceeds a predetermined value, a series of steps for uniformly evacuating the inside of the furnace is repeatedly performed.

Next, the present invention will be further described in detail based on FIG. 1, which is a diagram illustrating the main parts of one example, and FIG. 2, which illustrates the time course of carburizing treatment in a furnace.

In FIG. 1, 1 is a vacuum furnace, 2 is a heating furnace, 3 is a workpiece, 4 is a heater, 5 is a control unit for the heater 4, 6 is a carburizing gas reservoir, 7 is a vacuum pumping section, and 8 is a Cooling gas reservoir, 9
, 10, 11 are on-off valves; 12 is an oil reservoir for quenching;
13 is a pressure detector that detects the pressure inside the heating furnace 2.
In FIG. 2, a polygonal line m indicates a change in temperature within the heating furnace 2 over time, and indicates that the temperature increases as it goes upward.

A polygonal line n indicates a change over time in the degree of vacuum within the heating furnace 2, and indicates that the lower the polygonal line n goes, the higher the degree of vacuum becomes. Period T,,t,,t. Each indicates the time during which carburizing is actually performed. Sections A to G indicate each stage of the carburizing process. Section A is the evacuation period of the vacuum furnace 1, section B is the heating period for heating the workpiece 3 to reach the carburizing temperature, and section C is mainly the The carburizing period is the carburizing process, section D is the diffusion period during the diffusion process, section E is the cooling period, section F is the reheating period of the workpiece 3, and section G is the oil cooling period for quenching. First, when the workpiece 3 is placed in the heating furnace 2 in the vacuum furnace 1, the on-off valve 10 is opened and the vacuum exhaust system 7 pumps the vacuum furnace 1 and the heating furnace 2.
The inside is evacuated (corresponding to the evacuation period A in FIG. 2).

When this evacuation ends, the workpiece 3 is first heated by the heater 4, and the temperature of the workpiece 3 is maintained at T by the control unit 5 (during the heating period B in FIG. Equivalent to). Then, the on-off valve 9 is opened and carburizing gas (methane gas in this embodiment) starts to be filled into the heating furnace 2 from the carburizing gas reservoir 6. When the gas pressure inside the heating furnace 2 reaches P, the carburizing gas is filled. The carburizing reaction is started. As the carburizing reaction progresses, carbon gas among the decomposed methane gas (CH.→C+2H,) is consumed in the reaction, but hydrogen gas remains, so as the reaction progresses, the pressure inside the heating furnace 2 decreases. It gradually rises (in other words, the degree of vacuum decreases). An increase in pressure means a shortage of methane gas necessary for the reaction, and when the pressure inside the heating furnace 2 reaches P, the on-off valve 10 is opened to evacuate the inside of the heating furnace 2 (the carburizing process is Of course, the carburizing reaction is repeated in the same way as before (corresponding to the carburizing period in Figure 2) after filling with fresh methane gas until the gas pressure reaches P. The number of repetitions is determined depending on the type and amount of the object 3 to be treated and the degree of carburization. The actual net carburizing time is determined by the gas pressure in the heating furnace 2 being P and P. This period is the sum of the sections between the two (periods T,, t,, t, in FIG. 2). When the carburization (reaction) is completed, the on-off valve 10 is opened again, the vacuum is evacuated, and the surface carbon concentration of the object to be treated 3 is diffused for a certain period of time in a vacuum (corresponding to the diffusion period D in Fig. 2). . Thereafter, the heating of the heating furnace 2 is stopped and the workpiece 3 is cooled to a temperature below the transformation point to refine the crystal grains (corresponding to the cooling period E in FIG. 2). Finally, the workpiece 3 is reheated and taken out into the vacuum furnace 1, nitrogen gas is introduced into the vacuum furnace 1, and the workpiece is put into the oil reservoir 12 and hardened. As described in detail above, this invention allows methane gas in the furnace to be controlled within a certain pressure range (
By repeatedly using only a certain carbon potential range), carburizing can be carried out within a certain carbon potential range regardless of the surface area of the workpiece, and at the same time, the used gas is exhausted and new gas is introduced into the corner. Since the carburizing agent is supplied up to each point, it is possible to uniformly carburize the contact areas between the objects to be treated or the pores of the objects to be treated.

Furthermore, in conventional carburizing furnaces that detect the carbon concentration in the gas and supply gas at any time, it is necessary to install an atmosphere stirring device in the furnace in order to keep the carbon potential in the furnace constant. It is not necessary in . As described above, the effects of the present invention are remarkable, and the accuracy of carburizing treatment of objects to be treated can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a main part of an embodiment of the present invention, and FIG. 2 is a diagram for explaining the time course of carburizing treatment in a furnace. 1 ... Vacuum furnace, 2 ... Heating furnace, 3
...Product to be treated, 4 ...Heating heater, 5 ...Control section, 6 ...Carburizing gas reservoir, 7 ...Vacuum Exhaust system, 8...Cooling gas reservoir, 9, 10, 11... Opening/closing valve, 12.
...Oil sump.

Claims (1)

[Scope of Claims] 1. A carburizing method for carburizing a metal member by causing a carburizing reaction with carbon generated as a result of the decomposition of carburizing gas supplied into an evacuated furnace, comprising the following (1) to ( 3)
A vacuum carburizing method characterized by sequentially repeating the following steps. (1) Filling the furnace with fresh carburizing gas. (2) A process to detect changes in gas pressure in the furnace due to decomposition of carburizing gas. (3) When the detected gas pressure in the furnace exceeds a predetermined value, the inside of the furnace is evacuated.