JPH01212748A - Rapid carburizing treatment for steel - Google Patents

Abstract

PURPOSE:To subject a steel to rapid carburizing treatment to a sufficient surface carbon content by repeating, plural times, a treatment in which case-hardening steel parts are carburized to a proper limit by means of heating and the flow of carburizing gas and further carbon is diffused in an inert gas or in vacuum. CONSTITUTION:Carburizing is carried out by placing parts made of case- hardening steel into a furnace to apply heating to the above parts and also allowing a carburizing gas, such as C3H8, to flow. At this time, carbon is allowed to enter into solid solution to the austenite solid solution limit of the steel at the carburizing temp., and carburizing is limited to a degree precipitating no coarse carbide. Subsequently, the carburizing gas in the furnace is exhausted and, by introducing an inert gas or maintaining the inside of the furnace in vacuum, the above carbon existing in the form of solid solution is diffused. Further, the above carburizing treatment and diffusion treatment are repeated twice or more, followed by the required heat treatment such as quench-and-temper treatment. By this method, carburizing is carried out in a shorter time than conventional one as a whole and sufficient surface carbon content is secured in the course of the above procedure to produce carburizing effects.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a carburizing method for parts made of wA.

[Prior Art] For example, some mechanical structural parts constituting automobiles, construction machinery, etc. are required to have fatigue strength and surface wear resistance.

To meet this demand, in many cases parts are manufactured from case-hardened steel and the surface is hardened by carburizing.

Currently, the mainstream carburizing method for mechanical structural parts is gas carburizing. Due to the demand for cost reduction, carburizing methods with high carbon potential such as vacuum carburizing method and ion carburizing method have recently been implemented so that carburizing treatment can be carried out in a short time.

However, when carburizing is performed using a carburizing method with a high carbon potential, the C concentration in the surface layer of steel parts exceeds the austenite solid solubility limit, producing coarse mesh-like carbides, which not only deteriorate physical properties but also pose a risk of reducing fatigue strength. There is. On the other hand, if one attempts to secure the minimum amount of carbon to increase the carburizing effect by 1q, the reality is that the formation of coarse carbides in the surface layer is unavoidable. Therefore, a step is required to dissolve the coarse carbide into the austenite structure again.

The time required for re-solid solutionization is generally 3 times the time required for carburization.
The total processing time is at least four times the carburizing time. Therefore, even if a carburizing method with a high carbon potential is used to shorten the time required to supply the amount of carbon necessary to obtain the carburizing effect, if the time required for the disappearance of coarse carbides cannot be shortened, the total processing time will be reduced. It does not serve the purpose of shortening it.

[Problems to be Solved by the Invention] The purpose of the present invention is to provide a technology for carburizing case-hardened steel parts that can be carried out in a shorter time than conventional methods, while ensuring a sufficient amount of surface carbon to achieve the carburizing effect. An object of the present invention is to provide a method for carburizing steel.

[Means for Solving the Problems] The rapid carburizing method for steel of the present invention involves heating parts made of case-hardened steel in a furnace, passing carburizing gas (A), and carburizing the steel at the carburizing temperature. Carburizing to dissolve carbon up to the austenite solid solubility limit, and (B) exhausting the carburizing gas in the furnace and injecting inert gas or keeping it in vacuum to diffuse carbon are repeated two or more times. , followed by heat treatment.

Carbon is dissolved in steel up to the austenite solid solution limit. - It includes not only a state in which carbide has almost dissolved into solid solution but has not yet precipitated, but also a state in which even if a small amount of carbide precipitates, it is so fine that it is easily re-dissolved in the subsequent diffusion process.

Case hardening steels to be treated in the present invention include various alloy steels such as chromium steel, nickel-chromium steel, chromium-molybdenum steel, and nickel-chromium-molybdenum steel.

The heat treatment may be performed by quenching and tempering, which has been performed in the conventional surface hardening treatment by carburizing.

Carburizing is carried out by vacuum carburizing or ionic carburizing, preferably the latter. In the case of using the ion carburizing method, it is preferable to perform glow discharge in an Ar and H2 atmosphere for a while to clean the surface of the component, as shown in the examples described later, prior to carburizing.

[Function] In the carburizing treatment of the present invention, carbon is not carburized into the surface layer of the part all at once as in the conventional method, but carburization is performed to the extent that coarse carbides are not precipitated, and the diffusion of carbon is thereby prevented. By repeating this process twice or more, the long treatment time required for solid solution of coarse carbides can be omitted, and the time required for carburizing treatment can be shortened as a whole.

The number of repetitions of carburizing and diffusion can be determined by determining the time required for carburizing based on the type of case hardening steel, treatment temperature, required carburizing method, etc., and selecting an appropriate number of times accordingly. Of course, it is disadvantageous to increase the number of times of diffusion by making one carburizing time shorter than necessary. Considering the required depth of the carburized layer and the limit of the amount of carburization, the maximum number of times would be 30 times.

[Example] Alloys having compositions NQ1 to NQ3 shown in Table 1 were melted.

A test piece of each material was placed in an ion carburizing furnace, and the inside of the furnace was heated to 1O-2.
The pressure was reduced to Torr, heated to 880°C, Ar and H
2 was supplied to adjust the furnace pressure to 2 Torr. The test piece was used as a cathode, and a DC voltage of 300 V was applied between it and the anode to cause glow discharge. As a pretreatment, the surface was cleaned with Ar and To1 ions for 20 minutes, and then Ar and H2 were exhausted. Subsequently, carburizing was performed using C3H8 as a carburizing gas, and one of the following treatments A to C was performed in combination with exhausting the gas and diffusion to maintain the temperature.

Treatment A (comparative example) Carburizing was performed for 2 hours and then diffusion was performed for 4 hours (total 6 hours) Treatment B (present invention) Carburizing was performed for 40 minutes and then diffusion was performed for 80 minutes, and this was repeated three times ( 6 hours in total) Treatment C (comparative example) After carburizing for 2 hours, re-solution treatment for 7 hours (9 hours in total) Using the above carburizing-diffusion method, holding at 830°C for 30 minutes and then oil cooling. and tempering, which was held at 160° C. for 2 hours and then allowed to cool.

The surface hardness, effective hardened layer depth, and rotary bending fatigue strength of each test piece after treatment were measured, and the presence or absence of coarse carbides was examined. The results are shown in Table 2.

Here, the effective hardened layer depth refers to the distance from the surface of the test piece to the point where the hardness is 550 Hv on the graph by measuring the hardness from the end of the cross section of the test piece and drawing a graph corresponding to the depth.

The rotational bending fatigue strength was measured under the following conditions using a smooth test piece with a parallel portion having a diameter of 8 shoes.

Rotation ri 10 million times Rotation speed 3,500 rl) m Temperature Room temperature As can be seen from the results in Table 2, the grains subjected to treatment A have coarse carbides in the grain boundaries and have low rotational bending fatigue strength. The quality is decreasing. On the other hand, in the specimens subjected to treatment B according to the present invention, no coarse carbides were observed, and the rotational bending fatigue strength was approximately three times that of treatment A.

On the other hand, no coarse carbides were observed in the case of treatment C, and the rotational bending fatigue strength was about 1.5 times that of treatment A, but treatment C required 1.5 times the treatment time of treatment B. There is.

Table 1 Remaining Fe Table 2 [Effects of the Invention] The rapid carburizing method of the present invention has the advantages of the ion carburizing method, which has a high carbon potential, that is, the ability to quickly carburize the desired amount of carbon. This method succeeded in avoiding the problem caused by the high carbon potential, that is, the decrease in fatigue strength caused by the formation of coarse carbides.

In other words, it is a technology that can produce products with properties equivalent to those obtained by conventional carburizing methods more quickly than conventional technologies. Needless to say, shortening processing time contributes to cost reduction.

Patent applicant: Daido Steel Co., Ltd. Agent: Patent attorney, Souo Suga

Claims (1)

[Claims] Parts made of case-hardened steel are placed in a furnace and heated; (A) carburizing by passing carburizing gas to dissolve carbon up to the austenite solid solubility limit of the steel at the carburizing temperature; B) Exhaust the carburizing gas in the furnace and introduce an inert gas or keep it in a vacuum to diffuse carbon.
A method for rapid carburizing treatment of steel, characterized by repeating the process several times or more, followed by heat treatment.