JP7052976B1 - Manufacturing method of rod-shaped fastener - Google Patents

Abstract

PROBLEM TO BE SOLVED: To disclose a technique capable of manufacturing a class 12.9 high-tensile bolt specified by JIS with high quality. SOLUTION: SCM435 or an equivalent steel material is used as a material. The vacuum carburizing method is used for the heat treatment, but the amount of acetylene introduced is adjusted so that hydrogen does not become excessive. The carburizing time is shorter than that of normal hydrogen-free carburizing, and the carburizing time is about 1 minute to several minutes. On the other hand, diffusion takes 20 to 40 times longer than the carburizing time. The tempering temperature is preferably 430 to 440 ° C. The hardness of the surface layer can be increased to the required hardness while retaining the toughness characteristic of SCM435 up to the surface layer. This makes it possible to provide a high-tensile bolt having excellent delayed fracture resistance and improved reliability. [Selection diagram] Fig. 2

Description

The present invention relates to a method for manufacturing a rod-shaped fastener such as a bolt or a post-installed anchor. The bolts also include screws and machine screws. Post-construction anchors include expandable anchors for attaching various members to concrete, screw anchors that are drilled in the construction section and screwed into pilot holes, self-drilling anchors that are screwed into the ALC, and bottoms that are drilled in the construction section. Various types such as stud type anchors (blind anchors) that are fixed to the holes by utilizing the expansion action can be mentioned.

Bolts are elemental components used in various industrial fields, but the required characteristics vary greatly depending on the application. Therefore, in JIS (B1051), for example, the strength of steel bolts is classified from 4.6 to 12.9. The highest class 12.9 has a tensile strength of 1220 N / mm ² or more and is called a high-tensile bolt, and is a necessity especially in the construction field.

Class 12.9 high-tensile bolts are required to have workability and toughness in the material itself, as well as high quenching characteristics. Therefore, among the chromium molybdenum steels, SCM435 (or SCM435H) is often used as a material.

By the way, not only bolts but also steel products are often heat-treated for hardening. Heat treatment is roughly divided into carburizing and carburizing, and carburizing is further divided into vacuum carburizing performed using a vacuum furnace and gas carburizing performed under atmospheric pressure. In any case, hydrocarbon gas such as acetylene gas and propane gas is used for carburizing, and conventionally, a large amount of hydrocarbon gas is filled in the furnace to secure the contact amount of carbon molecules with the work. However, the problem of hydrogen brittleness (the problem of delayed fracture) appeared in the carburized products as if it were destined.

As a cause of hydrogen embrittlement, the idea that hydrogen itself has an adverse effect on steel has been common knowledge, and various measures against hydrogen embrittlement such as removing hydrogen during tempering have been proposed. There was a fact that even if it was removed, the phenomenon of delayed fracture with the same fracture surface as before occurred.

Therefore, the inventors of the present application question the common wisdom that hydrogen itself is the cause of brittleness, and the voids (vacancy) generated in the steel material due to the intrusion of hydrogen may be the trigger for fracture. Based on the hypothesis, the correctness of the hypothesis was demonstrated by an experiment of carburizing in the absence of voids. Then, this result was disclosed in Patent Documents 1 and 2.

Japanese Patent No. 6058846 WO2018 / 105693 Gazette

A major feature of Patent Documents 1 and 2 is that a hydrocarbon gas such as acetylene is used as a carbon source as in the conventional case, and the same vacuum furnace as the conventional one is used as it is to prevent (or suppress) hydrogen embrittlement. Being able to manufacture products is therefore highly realistic and can make a significant contribution to the industry.

On the other hand, as mentioned above, high-tensile steel is used in various fields including the construction industry, but SCM435, which is the main material of high-tensile steel, is a medium carbon-containing (alloy) steel and can be cured only by quenching. Therefore, in the past, there was no idea of actively carburizing high-tensile bolts when manufacturing them, and tempering was performed to adjust the hardness by quenching and tempering.

Therefore, the phenomenon of delayed fracture due to hydrogen embrittlement should not appear in high-tensile steel made of tempered steel. The phenomenon of delayed fracture, which is presumed to be caused by brittleness, appeared. In terms of quality, there was a problem that the tempered high-tensile bolts could only obtain the same fatigue strength as carburized products.

It can be said that this problem is caused by the tempering method. That is, it is necessary to heat in a heating furnace for tempering, but conventionally, an atmosphere furnace using hydrocarbon gas such as a modification furnace has been used as a heating furnace, and the work is done by heating (by oxidation). Since the heating is performed while supplying a large amount of hydrocarbon gas so that decarburization does not occur, the hydrogen separated from the hydrocarbon gas invades the surface layer of the work and creates innumerable voids (vacancy). It can be said that this void prevented the improvement of the fatigue strength.

The problems of the conventional product are as described above, but since the techniques of Patent Documents 1 and 2 can prevent hydrogen embrittlement, if Patent Documents 1 and 2 are applied to high-tensile bolts, the quality should be significantly improved. Therefore, the inventors of the present application tried carburizing the SCM435 material by the methods of Patent Documents 1 and 2.

As a result, no fracture phenomenon due to hydrogen embrittlement was observed, but there were cases where problems with impact resistance were observed.

The present invention has been made to improve such a situation, and does not disclose a technique for improving Patent Documents 1 and 2 to obtain a high-quality rod-shaped fastener.

When the methods of Patent Documents 1 and 2 are applied to a bolt, a thick carburized layer having no voids can be formed on the surface layer of the bolt, but the inventors of the present application have found that this thick carburized layer rather reduces the toughness of the bolt. I speculated that this might be the case, and repeated research and experiments to reach the present invention.

That is, the present invention is intended for manufacturing methods of rod-shaped fasteners such as bolts and anchors, and these typical configurations are specified in each claim. Of these, the invention of claim 1 has, as a basic configuration,
"Preparation steps to prepare rod-shaped intermediates processed with S CM435 , SCM440, S35C, S45C, or other medium carbon-containing steel containing 0.30 to 0.45% carbon, and use of hydrocarbon gas. A hardening step in which the surface layer of the intermediate product is carburized and then quenched by the vacuum carburizing method, and a tempering step in which the hardness of the intermediate product is lowered . "
Includes.

And in the above basic configuration
"The carburizing treatment by the vacuum carburizing method is the same amount or the same amount as the reference gas amount, which is the amount of the hydrocarbon gas necessary and sufficient for evenly adhering carbon to the surface of the intermediate product. The carburizing step includes a carburizing step of introducing a small amount of hydrocarbon gas into a furnace and a diffusion step of stopping the introduction of the hydrocarbon and continuing heating after the carburizing step . The diffusion step is carried out for 0.5 to 4 minutes and 30 seconds including the introduction stop time when the introduction of the hydrocarbon gas is intermittently carried out, and the diffusion step is carried out for 20 to 40 times as long as the carburizing step.
Moreover, the tempering is performed in the range of 420 to 450 ° C. "
It has the feature.

In the present invention, other alloy steels containing molybdenum can also be used. Other steel grades similar to SCM435 include S35C, S45C and the like. Medium carbon steel is generally classified as a steel type having a carbon content of 0.25 to 0.55%, but in the present invention, the carbon content is preferably 0.3 to 0.45%.

As a specific example of the invention of claim 1, claim 2
"In the treatment by the vacuum carburizing method, the carburizing step is carried out for 1 minute , the diffusion step is carried out for 30 minutes , and the tempering is carried out in the range of 430 to 440 ° C."
It is configured as.

The invention of claim 3 also relates to a method for producing a rod-shaped fastener.
" A method for producing a rod-shaped fastener made of S CM435 , SCM440, S35C, S45C, or another medium carbon-containing steel material containing 0.30 to 0.45% of carbon.
In forming a hardened portion on the surface layer by hardening treatment by a vacuum carburizing method and then quenching and tempering.
The hardness is changed so that the hardness is higher on the surface side than on the deep side, and the surface hardness is maintained in the range of Hv385 to 435, and the hydrogen storage amount immediately after tempering is maintained at 0.05 ppm or less. Is set to
It is configured as. The rod-shaped fastener of claim 3 can be easily realized by the method of claims 1 and 2. The hardness of Hv385 to 435 is the same as the hardness range of class 12.9 high-tensile bolts specified by JIS.

Although the hydrogen storage amount is specified in claim 3, it is possible to measure the hydrogen storage amount, but it cannot be said that it is easy. Therefore, it is convenient if the mechanical properties caused by the hydrogen storage amount can be replaced by some simple test method, and it is also meaningful as a method for specifying the technical range.

Regarding this point, in claim 4, since the difference in the hydrogen storage amount clearly appears in the numerical value of the fatigue intensity, the numerical value of the fatigue strength test is adopted as an alternative means for detecting the hydrogen storage amount. That is, in the invention of claim 4,
"The stress range that does not break even if a load is repeatedly applied to the fatigue test 5 million times or more is set to a fatigue strength of 800 MPa or more."
It is configured as. In a rod-shaped fastener made of a medium carbon steel grade such as SCM435, the combination of the hardness specified in claim 3 and the fatigue strength specified in claim 4 is the quality that can be obtained for the first time in the present invention, and is the quality that can be obtained for the first time in the present invention, such as gas tempering and gas carburizing. It is a quality (numerical value) that cannot be reached by.

In the present invention, since the invasion of hydrogen into the surface layer of the rod-shaped fastener is remarkably suppressed, the phenomenon of delayed fracture caused by the voids can be significantly suppressed, and the hardness can be increased and the tensile strength can be improved. Therefore, a class 12.9 high-tensile bolt specified by JIS can be easily provided.

The feature of the present invention is that the surface layer of the rod-shaped fastener does not have a thick carburized layer but a hardened portion in which carbon is diffused. In this way, carbon is appropriately contained in the surface layer. Due to the diffusion, the surface layer does not become too hard and the toughness is maintained. Therefore, it is possible to provide a rod-shaped fastener that has high strength against impact and bending and can significantly improve tensile strength.

Further, SCM435, which is a material suitable for the present invention, is a steel type (alloy steel) used for structural materials, fasteners, etc., and has high tensile strength and excellent hardenability. By drawing out the characteristics, it is possible to secure sufficient toughness while ensuring high strength, and therefore, compared to conventional high-tensile bolts, reliability (fatigue strength and impact resistance) is significantly improved while maintaining strength equal to or higher than that of conventional high-tensile bolts. Can be improved.

(A) is a conceptual diagram of a vacuum carburizing facility, and (B) is a schematic diagram of a process. (A) is a table showing the amount of hydrogen storage, and (B) is a graph and a table showing the relationship between hardness and depth. (A) is a table showing a Charpy impact value, and (B) is a graph showing a tensile test result. It is a graph which shows the fatigue test result.

Next, an embodiment of the present invention will be described with reference to the drawings. This embodiment uses the apparatus disclosed in Patent Documents 1 and 2. Therefore, although the description may overlap with Patent Documents 1 and 2, it will be described just in case.

(1). Outline of equipment ・ Basic treatment pattern As shown in Fig. 1 (A), the vacuum carburizing equipment is a processing furnace (carburizing chamber) in which the gas inlet 2 and the gas discharge port 3 are opened inside as the main elements. It has a cooling chamber 5 adjacent to the processing furnace 1 via a door 4, and a quenching chamber 6 arranged below the cooling chamber 5. In the quenching chamber 6, a quenching tank 7 in which a cooling liquid (water system or oil) is stored is arranged. Although not shown, the processing furnace 1 and the cooling chamber 5 are provided with doors for taking in and out the work W.

A hydrogen carbonate gas cylinder 8 is connected to the gas introduction port 2 via a gas introduction pipe 9, and a first valve 10 is arranged in the middle of the gas introduction pipe 9. The cylinder 8 is filled with a hydrocarbon gas as a carbon source. In this embodiment, acetylene is used as the hydrocarbon gas. The gas discharge port 3 is connected to the vacuum pump 12 via the first discharge pipe 11, and a second valve 13 is interposed in the middle of the first discharge pipe 11. The first discharge pipe 11 is also connected to the cooling chamber 5 via the second discharge pipe 14, and the third valve 15 is arranged in the middle of the second discharge pipe 14.

A large number of heat transfer heaters 16 are arranged inside the processing furnace 1, and each heater 16 is connected to a power source 17. Further, the carburizing equipment is provided with a hydrogen concentration sensor 18 as a means for directly detecting the hydrogen concentration. The hydrogen concentration sensor 18 detects the thermal conductivity of the gas in the processing furnace 1, and is provided in the bypass pipe 19 provided in the first discharge pipe 11. An oxygen sensor 20 faces the processing furnace 1.

The vacuum carburizing equipment has a control device (control means) 21. The control device 21 may be a set with the vacuum carburizing equipment, or may be replaced with a personal computer. In any case, it has a memory, a monitor, an input means, and the like. The valves 10, 13, 15, the heater power supply 17, the hydrogen concentration sensor 18, and the oxygen sensor 20 are electrically connected to the control device 21. Although not shown, the processing equipment is provided with a digital temperature sensor and a vacuum gauge, which are also connected to the control device 21.

Next, the vacuum carburizing treatment pattern will be described with reference to FIG. 1 (B). The surface treatment work includes a heating process in which the work W is placed in the processing furnace 1 to raise the temperature inside the furnace, a soaking process in which the work W is uniformly heated while maintaining the set temperature, and acetylene is applied to the processing furnace 1 in a vacuum atmosphere. A carburizing process that infiltrates the introduced and decomposed carbon into the work W, a diffusion process that stops the introduction of acetylene and diffuses carbon in the surface layer of the work W, and gas cooling that is transferred to the cooling chamber 5 and cooled with nitrogen gas. Or, it has a quenching step of putting it in a quenching liquid and quenching it.
In the normal carburizing step, the carburizing (T4-T2) time is taken, but the carburizing time (T3-T2) of the present embodiment is shorter than the normal carburizing time (T4-T2) (for example, a fraction to a fraction). One tenth of the time).

The carburizing temperature is the same as before. Although it varies depending on the material of the work W and the like, it is, for example, about 750 to 1100 ° C. The degree of vacuum of the processing furnace 1 is not significantly different from that of the conventional one, and the opening degree of the second valve 13 is automatically controlled so as to be maintained at, for example, about 4 to 10 Torr.

In the present embodiment, as disclosed in Patent Document 1, it is divided into an initial step of first introducing a certain amount of acetylene and a continuous step of introducing acetylene after a certain period of time. However, as the carburizing process, a method of continuously introducing acetylene for a certain period of time without providing an initial process can also be adopted.

In the initial step, a certain amount of acetylene is continuously introduced into the processing furnace 1 for a predetermined time (for example, several tens of seconds). On the other hand, in the continuation step, the amount of acetylene introduced is basically controlled with the hydrogen concentration as a variable, and is also corrected by the oxygen partial pressure.

When shifting from the initial process to the continuous process, and when the introduction of acetylene is cut in the continuous process, the partial pressure of acetylene inside the processing furnace is lower than that in the initial process, and this state. However, the processing furnace 1 is maintained in a predetermined vacuum atmosphere (the degree of vacuum is lower than when acetylene is introduced).

(2). Control mode In this embodiment, the amount of H ₂ (actual amount of hydrogen) produced by the decomposition reaction of acetylene introduced in the initial step on the work surface is measured, and this and the known work surface area are measured. The total area of the work is calculated by comparing with the data of the amount of H2 produced ₍ reference hydrogen amount), and the amount of acetylene introduced is set based on this calculated value.
That is, using a sufficient amount of acetylene required for carburizing the work as a reference value, acetylene in a reference value amount is continuously introduced into the furnace over 1 minute in the continuation process. However, depending on the required quality, it is possible to introduce a hydrocarbon in an amount smaller than the reference value or in a larger amount than the reference value. The total time between the initial process and the continuous process can be 1 to 2 minutes.

The initial process and the continuation process are as described in Patent Documents 1 and 2, but will be described just in case. In the initial step, the introduction of acetylene causes the hydrogen concentration to rise sharply from zero and the rate of increase to drop sharply. Therefore, the range in which the rate of increase is significantly reduced is set as the stabilized state, and the average value in the stabilized state is set as the reference hydrogen concentration ρ0 in the initial step.

The surface area (total surface area) of the work W is calculated from the reference hydrogen concentration ρ0. As a preparation for this, multiple types of samples whose areas are known in advance are produced, and each sample is exposed to hydrocarbon gas to detect the hydrogen concentration. As a result, the relationship between the absolute value of the surface area of the work W and the hydrogen concentration is created as a map (correspondence table).

Conceptually speaking, a large number of samples such as 1 m ² sample, 2 m ² sample, 3 m ² sample, 4 m ² sample, 5 m ² sample, etc. are manufactured, and each of them is used as an initial step. Record the hydrogen concentration (ρ1, ρ2, ρ3 ...) Generated when carburized under the same conditions, and create a relational expression (map) between the surface area and the hydrogen concentration.

Then, the average value ρ0 of the hydrogen concentration detected from the actual work W is compared with the reference hydrogen concentration ρ1, ρ2, ρ3 ... Of each sample, and the actual average value ρ0 is the closest reference hydrogen concentration ρ1. , Ρ2, ρ3 ... Is selected, and the total surface area of the sample to which this reference hydrogen concentration belongs is set as the total surface area of the work W, and is set in the case of the total surface area based on the set total surface area. The saturated hydrogen concentration is set as the saturated hydrogen concentration of the actual work W, and the target hydrogen concentration (control upper limit value) is set based on this.

The lower limit can be set arbitrarily. For example, with reference to the reference hydrogen concentration ρ1, ρ2, ρ3 ..., the target hydrogen concentration ρ3', which is the control upper limit value, is 0.9 times the reference hydrogen concentration ρ1, ρ2, ρ3 ..., And the lower limit value is 0. It can be multiplied by .85. It is also possible to set the reference hydrogen concentration ρ1, ρ2, ρ3 ... As the upper limit value (reference hydrogen concentration = target hydrogen concentration). It is also possible to make the target hydrogen concentration (upper limit value) and lower limit value different depending on the material and shape of the work W. The reference hydrogen concentration can also be referred to as a saturated hydrogen concentration.

When a predetermined amount of acetylene is introduced in a predetermined time in the initial step, acetylene is decomposed to generate hydrogen, but the amount of hydrogen generated is directly proportional to the surface area of the work W. Further, since the inside of the processing furnace 1 has a vacuum atmosphere, acetylene comes into contact with the surface of the work W evenly if a certain amount is maintained as an initial process. Therefore, in the initial step, the separated carbon is evenly adhered to the surface of the work W. Therefore, in the initial process, regardless of the size of the surface area of the work W, the amount of acetylene (reference gas amount) necessary and sufficient for evenly adhering carbon to the entire surface of the work W and the reference gas amount are supported. Hydrocarbon gas can be detected.

The hydrogen concentration ρ in the furnace changes from moment to moment, but the amount of acetylene introduced is controlled so that ρ is maintained between ρ3'and ρ3 ″. That is, the actual hydrogen concentration ρ is When the saturated hydrogen concentration (upper limit value ρ3') is approached, the amount of acetylene introduced is reduced, and when the actual hydrogen concentration ρ approaches the lower limit value ρ3 ″, the amount of acetylene introduced is increased to bring the actual hydrogen concentration ρ into a predetermined range. Keep at (level).

As a result, it is possible to prevent excessive generation of hydrogen in the processing furnace 1 and prevent or significantly suppress the infiltration of hydrogen into the work W. That is, carburizing can be performed using acetylene (hydrocarbon gas) in a state where the harm of hydrogen is eliminated. The upper limit value Q3'and the lower limit value Q3 ″ corresponding to the saturated hydrogen concentration are set for the introduction amount Q of acetylene. Since the quality required for the product varies, acetylene depends on the required quality. It is also possible to prioritize efficiency by introducing more hydrocarbon gas such as, etc. than the standard gas amount.

If the total area of the work is known, it is possible to omit the initial process. Further, when the same process is repeated, the initial process can be omitted. However, since the environment inside the furnace changes, it is preferable to perform an initial process as appropriate to correct the numerical value.

In Patent Documents 1 and 2, after the diffusion step, a step of gradually lowering the temperature to a predetermined temperature and then a heat equalizing step of maintaining the predetermined temperature are provided. However, in the present embodiment, the temperature lowering step and the subsequent soaking step are provided. The step is omitted, and the quenching (quenching) is performed immediately after the diffusion step. Oil was used for quenching, but water quenching is also possible.

In the present embodiment, the carburizing step (continuation step) can be completed in a short time of, for example, about 1 minute. Therefore, the change in hardness is gradual, and the difference between the core hardness and the surface hardness (difference between the maximum hardness and the minimum hardness) is small. When obtaining JIS 12.9 high-tensile bolts, the hardness is set to be within the specified range. Since the diffusion step is performed in a vacuum atmosphere, decarburization due to oxidation does not occur, but if necessary, nitrogen gas may be introduced into the furnace in the diffusion step.

(3). Evaluation Figure 2 and below show the evaluation. FIG. 2A shows the numerical value of the amount of hydrogen occluded in the surface layer of the work immediately after the heat treatment. Each sample was partially excised using the same rod-shaped sample as in the tensile test to form a test piece, and hydrogen was vacuum-sucked while raising the temperature from room temperature to 300 ° C. to measure the total amount of hydrogen.

Five samples were produced in each case, and the average value was used as the inspection value. The gas carburizing of the comparative example is carburized at 860 ° C. for 55 minutes using an atmospheric furnace. The normal vacuum carburized product of the comparative example is an example of Patent Document 1, in which an initial step of 1 minute is followed by a continuous step for 30 minutes (the same applies hereinafter). From FIG. 2A, it can be understood that the examples of this embodiment and the embodiment of Patent Document 1 have a significantly small occlusion of hydrogen.

FIG. 2B shows the relationship between depth and hardness. Samples a to c are examples of the present invention, and other samples (d to k) are comparative examples. The gas-treated product of the comparative example is oil-quenched at 870 ° C. and then tempered at 430 ° C. for 100 minutes. From this graph, the carburized product usually has a high surface hardness and the change in hardness is clear, the gas-treated product does not have a high hardness, and there is almost no difference in hardness depending on the depth. Then, it can be understood that the surface hardness is higher than that of the gas carburized product and the change in hardness is small.

Further, since the embodiment of the present application and the normal vacuum carburized product are originally intended for carburizing, the hardness increases as the depth becomes thinner, and the tempering temperature and the hardness have the opposite relationship. It can be understood that the hardness decreases as the tempering temperature increases. The hardness of the tempered product at 430 ° C. in this example is within the range of class 12.9 specified by JIS. Therefore, the hardness of class 12.9 is particularly high by tempering at about 430 to 440 ° C. It can be said that the conditions can be met.

The hardness of sample d is high at a depth of 0.5 mm, which seems to be a measurement error. In addition, it is presumed that the gas-treated product was not originally treated for carburizing, but there is no clear correlation between depth and hardness.

FIG. 3A shows the result of the Charpy test. The material of each sample was SCM435, and tempering was performed at 440 ° C. As a sample, a test piece according to JIS Z2242 was used.

The diffusion time was 30 minutes for Example A, 60 minutes for B, and 90 minutes for C. According to the JIS B1051 regulations, the impact value is 15J or more, but all the samples greatly clear the JIS regulations. As for the embodiment of the present application, the condition of high-tensile bolt can be largely cleared by reducing the amount of carburizing.

In FIG. 3A, it can be understood that the hardness of each sample tends to increase from the core side toward the surface, but this means that the deeper the hardening, the larger the carburizing amount. Means. The hardness does not always change in a single tone, and there are cases where it changes while having irregularities, but this is presumed to be due to the effect of hardness measurement errors and variations in the chemical composition of the substrate.

FIG. 3B shows the result of the tensile test. In the test, five round bar samples were used, measured according to JIS regulations, and the average value was graphed. From this test result, it can be understood that the examples of the present application satisfy the tensile strength specified in JIS class 12.9. Further, in FIG. 3B, no clear yield point appears in both the examples of the present application and the comparative examples, but since 1260 MPa, which is 90% of the maximum value, is within the elastic deformation region, the original load is released by 90%. It can be said that the condition of class 12.9 to return to is cleared.

FIG. 4 shows a fatigue test (relationship between the stress range and the number of repeated fractures). The test was carried out in accordance with JIS Z2273. In FIG. 4, the embodiment of the present application and the gas-conditioned product show a slightly similar tendency, which is presumed to be due to the fact that the material is SCM435 in both cases.

It should be noted here that Comparative Example a has a maximum stress range of 760 MPa that does not break at 5 million times or more, but in the embodiment of the present application, it has fatigue strength that does not break at 5 million times or more even at 840 MPa. From this, it can be understood that the product according to the present invention has much higher fatigue strength than the gas carburized product due to the hydrogen storage amount shown in FIG. 2 (A).

Although the embodiments of the present invention have been described above, the present invention can be embodied in various ways. As the material, other chrome molybdenum steel such as SCM440 can be used in addition to SCM435. Needless to say, SCM435 also includes SCM435H.

JIS B1051 defines 12.9 as the highest class of high-tensile bolts, but according to the present invention, bolts with higher strength such as 13.9 and 14.9 and excellent fatigue strength are classified. It can be said that it is also possible to make it. In this case, the upper limit of hardness may be higher than the Hv435 currently defined in class 12.9.
The invention of the present application can also be applied to a class lower than 12.9 among the classes defined by JIS (B1501). In this case as well, the fatigue strength can be significantly improved while ensuring the required strength.

The present invention can be embodied in the heat treatment of a rod-shaped fastener. Therefore, it can be used industrially.

W work 1 Processing furnace 2 Gas inlet 3 Gas discharge port 5 Cooling room 6 Quenching room 8 Hydrocarbon gas cylinder (acetylene cylinder)
12 Vacuum pump (vacuum source)
18 Hydrogen concentration sensor 20 Oxygen sensor 21 Control device

Claims (4)

A preparatory step of preparing rod-shaped intermediates processed with S CM435 , SCM440, S35C, S45C, or other medium carbon-containing steel containing 0.30 to 0.45% carbon, and hydrocarbon gas were used. It includes a hardening step of carburizing the surface layer of the intermediate product by a vacuum carburizing method and then quenching, and a tempering step of lowering the hardness of the intermediate product.
The carburizing treatment by the vacuum carburizing method is the same as or smaller than the standard gas amount, which is the amount of the hydrocarbon gas necessary and sufficient for evenly adhering carbon to the surface of the intermediate product. The carburizing step includes a carburizing step of introducing an amount of hydrocarbon gas into a furnace and a diffusion step of stopping the introduction of the hydrocarbon and continuing heating after the carburizing step . The introduction stop time is 0.5 to 4 minutes and 30 seconds including the introduction stop time when the introduction of hydrogen gas is performed intermittently, and the diffusion step is performed 20 to 40 times as long as the carburizing step.
Moreover, the tempering is performed in the range of 420 to 450 ° C., which is a method for producing a rod-shaped fastener. In the treatment by the vacuum carburizing method, the carburizing step is carried out for 1 minute , the diffusion step is carried out for 30 minutes , and the tempering is carried out in the range of 430 to 440 ° C.
The method for producing a rod-shaped fastener according to claim 1. A method for producing a rod-shaped fastener made of S CM435 , SCM440, S35C, S45C, or another medium carbon-containing steel material containing 0.30 to 0.45% of carbon.
In forming a hardened portion on the surface layer by hardening treatment by a vacuum carburizing method and then quenching and tempering.
The hardness is changed so that the hardness is higher on the surface side than on the deep side, and the surface hardness is maintained in the range of Hv385 to 435, and the hydrogen storage amount immediately after tempering is maintained at 0.05 ppm or less. Is set to
Manufacturing method of rod-shaped fasteners.
The stress range that does not break even if a load is repeatedly applied to the fatigue test 5 million times or more is set to a fatigue strength of 800 MPa or more.
The method for producing a rod-shaped fastener according to claim 3.

Images