JP4092729B2 - Bolt and heat treatment method thereof - Google Patents

Description

  The present invention relates to a bolt used for a joint that requires disassembly and the heat treatment method thereof.

Bolt tightening structures are used in joints that require disassembly, such as joints between pipes and pipes, joints between pipes and other equipment, container lids, valve and pump lids, and various other parts.
In many cases, fluid is present inside these devices, and a method of preventing leakage from a joint or a lid by tightening with a strong bolt is used.
However, if the bolts are tightened strongly, the threads will collapse and stick, often making it impossible to detach.

Therefore, bolts and nuts that require strong tightening are tightened by applying machine oil, grease, solid lubricant, anti-sticking agent, or the like.
However, even if these treatments are performed, a phenomenon occurs in which the bolt and the nut are fixed at some rate. When the bolts are fixed, the bolts are cut or the nuts are cut and removed to remove the fixed bolts and insert new bolts.

When complete countermeasures are required, a method is used in which surface treatment is applied to the thread portions of bolts and nuts, and solid lubricant is applied in combination.
As the surface treatment in this case, chrome plating, nickel plating, Teflon (registered trademark) coating, nitrosulphurizing treatment or the like is applied.

  As a method for preventing the sticking of the screw, among the above-mentioned conventional examples, the nitrosulphurizing treatment is the best, but it is clear that this treatment deteriorates the Charpy impact characteristics, and the general spread is braked. Yes.

Bolts and nuts with a diameter of 25 mm or more that are tightened at normal / low temperature or used at normal / low temperature require Charpy impact characteristics (absorbed energy and lateral bulge) of the material from the viewpoint of preventing brittle fracture. Is done. For example, according to “Ministry of Economy, Trade and Industry Notification No.501: Technical Standards for Structures for Nuclear Power Generation Facilities”, when the impact test is performed at a temperature below the minimum operating temperature, the lateral bulge of three test pieces And the absorbed energy meets the requirements shown in the table below. The “minimum use temperature” refers to the minimum temperature at which the operation of the device (stress load) is allowed from the viewpoint of material strength.

  However, when the nitronitriding treatment is performed, the temperature (minimum use temperature) that satisfies these requirements increases. If the minimum operating temperature is equal to or higher than room temperature, a brittle bolt may cause brittle fracture if tightened with a strong force. Therefore, before the bolt is tightened, it is necessary to preheat the bolt to a temperature that does not cause brittle fracture. However, in practice, this surface treatment may not be applicable.

  In view of the above circumstances, an object of the present invention is to prevent brittle fracture of a bolt.

Means for solving the above-mentioned problems are as follows .

The present invention is a heat treatment method comprising a step of quenching a bolt, a step of tempering the quenched bolt, and a step of nitronitriding the tempered bolt; In order to recover the impact characteristics of the bolt deteriorated by the treatment, the bolt subjected to the nitronitriding treatment is the same as the tempering temperature in the tempering step, a range up to a temperature 20 ° C. higher than the tempering temperature, or Heating in a range up to a temperature lower by 20 ° C. than the tempering temperature, holding the time for allowing the impurities precipitated during the nitronitriding treatment to be dissolved again in the matrix , and then performing rapid heat treatment by rapid cooling. And a heat treatment method for the bolt.

The present invention is the method for treating a bolt, wherein the post-heat treatment temperature is 550 ° C to 700 ° C.

The present invention is a heat treatment method comprising a step of quenching a bolt, a step of tempering the quenched bolt, and a step of nitronitriding the tempered bolt; In order to recover the impact characteristics of the bolt deteriorated by the treatment, the bolt subjected to the nitronitriding treatment is the same as the tempering temperature in the tempering step, a range up to a temperature 20 ° C. higher than the tempering temperature, or Heating in a range up to 20 ° C. lower than the tempering temperature, holding for a time that allows the impurities precipitated during the nitronitriding treatment to be dissolved again in the matrix , and then performing rapid heat treatment by rapid cooling The bolt is characterized in that it is heat treated by a heat treatment method for a bolt.

The effects of the present invention are as follows.

  According to the present invention, the minimum use temperature does not increase because the bolt subjected to the nitrosulphurizing treatment is post-heat treated. Therefore, even if the bolt is tightened with a strong force, there is no possibility that brittle fracture will occur.

An embodiment of the present invention will be described with reference to FIG.
Usually, in a heat treatment of a bolt or the like that requires high strength, for example, a tensile strength of 600 MPa to 1200 MPa, a quenching treatment is first performed. In this quenching process, as shown in FIG. 1 (A), heating the bolt to quenching temperature (austenitizing temperature) T _Q, after a predetermined time holding, quenching C. In this state, the bolt has a martensite structure and is hard and brittle.

Following this, the bolt is tempered. In this tempering process, as shown in FIG. 1 (B), the bolt is heated to a tempering process temperature T _T , held for a certain time, and then cooled C1 (almost rapidly in the case of alloy steel). By performing this treatment, the martensite is tempered and becomes a structure in which small carbides are dispersed in the ferrite, and the tensile strength and the proof stress are slightly lowered as compared with the quenched state, but it has high toughness. Ordinary bolts are used in this state.

On the other hand, as shown in FIG. 1C, in the nitronitriding treatment, after the quenching and tempering treatment, the bolt is put in a vacuum furnace and heated to a nitronitriding treatment temperature T _S , and plasma of nitrogen and sulfur is obtained. creating a state atmosphere, after a certain time holding, slow cooling C ₂ to in a furnace. As a result, nitrogen and sulfur diffuse to the extreme surface of the steel, hardening the surface and improving the lubrication performance.

This treatment temperature T _S is usually about 50 ° C. lower than the tempering treatment temperature T _T so as not to lower the mechanical strength (tensile strength, yield strength) determined by the already quenched and tempering treatment. Will be implemented. Therefore, the strength (tensile strength, yield strength) during tempering can be maintained except for the pole surface.

However, according to our experiment, the impact characteristics (Charpy absorption energy and lateral bulge amount) were deteriorated by this nitrosulphurizing treatment. To eliminate this phenomenon, the result of various experiments, immersion after sulfonitriding treatment, post heat treatment, i.e., as shown in FIG. 1 (D), was heated to near the tempering temperature T _T, subsequently quenching C It has been found that it is effective to perform the process ₃ .

This embrittlement and recovery mechanism is considered as follows.
The solubility of impurities in α-iron is lower than the tempering temperature T _{T at the} nitronitriding temperature T _S , and the supersaturated impurities (or their compounds) are precipitated in the grains and at the grain boundaries. Along with this, it is considered that the crystal grains are difficult to beat and the grain boundaries are embrittled. Therefore, it is considered that the impact characteristics are deteriorated (the lateral bulge amount at low temperature and normal temperature and the absorbed energy are reduced).

On the other hand, in the post-heat treatment, the post-heat treatment temperature T _P is substantially equal to the tempering treatment temperature T _T , so that the solubility of impurities in α-iron is increased in the same manner as in tempering, and the precipitated impurities are again in the matrix. It is considered that the impact characteristics are also recovered because the melting characteristics of the crystal grains and the strength of the grain boundaries are recovered.

Next, the reason why the post-heat treatment temperature level is determined will be described.
The properties of the hardened steel change as follows in relation to the subsequent heating (tempering) temperature.
(1) When the tempering temperature is low: Tensile strength and proof stress are slightly lowered from the quenched state, and elongation, drawing and impact properties are slightly increased.
(2) When the tempering temperature is high: Tensile strength and yield strength are slightly decreased from the quenched state, and elongation, drawing, and impact characteristics are greatly improved.

Therefore, if the post-heat treatment temperature T _p is significantly higher than the tempering treatment temperature T _T , the tensile strength necessary for the bolt may be impaired. On the other hand, if the post-heat treatment temperature is low, the toughness is not sufficiently recovered. Therefore, basically, the post-heat treatment temperature T _P = tempering treatment temperature T _T
And set.

However, the temperature during the heat treatment is not necessarily uniform in the furnace, and a variation of about 20 ° C. can be considered. Therefore, when the tensile strength and proof stress of the bolt are very important, it is better to set the temperature so as not to exceed the tempering temperature even if the temperature variation of the post heat treatment is taken into consideration. In this case, post-heat treatment temperature T _P <tempering treatment temperature T _T
And set.
As the heat treatment temperature T _P After this time, for example, equal to 700 ° C., or it lower temperature is selected.

On the other hand, although there is room in terms of tensile strength, it is used at a low temperature, so if you want to ensure high toughness, the post-heat treatment temperature T _P should be lower than the tempering treatment temperature T _T , considering temperature variations and errors. It is considered better not to. In this case, post-heat treatment temperature T _P > tempering treatment temperature T _T
And set.
As the heat treatment temperature T _P After this time, for example, equal to 550 ° C., or, even higher temperatures are selected.

Next, the effect when the level of the post heat treatment temperature is determined will be described.
There are the following three effects.
(A) When the post-heat treatment temperature T _P = the tempering treatment temperature T _T : Tensile strength and impact characteristics almost equal to the quenched state can be obtained.
(B) When the post-heat treatment temperature T _P <the tempering treatment temperature T _T : The tensile strength in the quenched and tempered state can be reliably ensured.
(C) Post-heat treatment temperature T _P > Tempering treatment temperature T _T : Impact characteristics equal to or better than those in the quenched and tempered state can be secured.

An embodiment of the present invention will be described with reference to FIGS.
A tempering process is performed after the quenching process of the bolt 1.
As shown in FIG. 2, the bolt 1 is made of a material JIS SCM 435, a nominal diameter of a screw: M48, a bolt length L = 300 mm, a diameter d = 48 mm, and a thread length m = 100 mm.

Quenching process:
In this step, as shown in FIG. 1 (A), heating the bolt 1 until quenching temperature T _Q, after a predetermined time holding performs rapid cooling C. The quenching treatment temperature _TQ is 860 ° C., and the holding time is 2 hours.

Tempering process:
In this step, as shown in FIG. 1 (B), the bolt 1 is heated to the tempering treatment temperature T _T, and after holding for a certain period of time, rapid cooling C1 is performed. The tempering treatment temperature T _T is 580 ° C., and the holding time is 1.8 hours.

  After the tempering treatment, the Charpy impact test of the bolt 1 was conducted, and the lateral bulge amount was obtained. As shown by the □ in the graph of FIG. (See the Ministry of Economy, Trade and Industry Notification No. 501).

Nitronitriding process:
In this step, as shown in FIG. 1C, the bolt 1 is heated to the nitronitriding temperature T _S , held for a certain time, and then cooled in the furnace C ₂ . The temperature T _S is 520 ° C., and the holding time is 10 hours.

  After the nitronitriding treatment, the Charpy impact test of the bolt 1 is performed and the lateral bulge amount is obtained. As shown by a circle in the graph of FIG. 3, the temperature satisfying the standard value of 0.65 mm is about 35 ° C. It was. This value was not good for chromium molybdenum steel. From this, it can be determined that the impact characteristics have been degraded by the nitronitriding treatment.

Post heat treatment process:
Therefore, in order to recover the impact properties, was tried the various heat treatments, post heat treatment, i.e., immersion and heating at the post heat treatment temperature T _P after sulfonitriding treatment, after a certain time holding, it is effective to quench C3 I found out. Thereafter, the heat treatment temperature T _P is 580 ° C., and the holding time is 1.8 hours.

  After the post-heat treatment, the Charpy impact test of the bolt 1 was performed, and the lateral bulge amount was obtained. As shown by the Δ mark in the graph of FIG. 3, the temperature satisfying the standard value of 0.65 mm was almost 0 ° C. . As described above, the post-heat treatment was performed until the impact characteristics after quenching and tempering were approximately equal.

  The embodiments of the present invention are not limited to the above. For example, instead of performing a quenching process, a tempering process, a nitronitriding process, and a post heat treatment process, a quenching process, a nitronitriding process, A post heat treatment step may be used. In this method, the tempering step is omitted, but the effects of both are almost the same.

BRIEF DESCRIPTION OF THE DRAWINGS It is a conceptual diagram which shows embodiment (Example) of this invention, FIG. 1 (A) is a quenching process state, FIG. 1 (B) is a tempering process state, FIG. 1 (D) indicates a post-heat treatment state. It is a front view which shows the Example of this invention. It is a figure which shows the relationship between the test temperature about the Charpy impact test result in a present Example, and a lateral bulge amount.

Explanation of symbols

1 Bolt T _Q Quenching temperature T _T Tempering temperature T _S Nitronitriding temperature T _P Heat treatment temperature

Claims (3)

A heat treatment method comprising: quenching a bolt; tempering the quenched bolt; and nitronitriding the tempered bolt;
In order to recover the impact characteristics of the bolt deteriorated by the nitronitriding treatment, the bolt subjected to the nitronitriding treatment is the same as the tempering treatment temperature in the tempering step, up to a temperature 20 ° C. higher than the tempering temperature. Heat treatment in a range, or a range up to a temperature 20 ° C. lower than the tempering temperature, hold the time for allowing the impurities precipitated during the nitronitriding treatment to be dissolved again in the matrix, and then rapidly cool by post-heat treatment A method for heat-treating a bolt. 2. The bolt heat treatment method according to claim 1, wherein the post heat treatment temperature is 550 ° C. to 700 ° C. A bolt that is heat-treated by the method for heat-treating a bolt according to claim 1 .

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