JP7289728B2 - Nitrided material manufacturing method and nitrided material - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a nitrided material in which a sintered body of a composite material of titanium or a titanium alloy and ceramics is partly nitrided, and to a nitrided material produced by the production method.

The present applicant has proposed that a sleeve used for die casting of non-ferrous metals such as aluminum, magnesium, zinc, tin, lead, and alloys thereof has a double structure of an outer cylinder and an inner cylinder, and is fitted into a steel outer cylinder. It has been proposed and implemented to form the inner cylinder from a sintered body of a composite material of titanium or a titanium alloy and ceramics (hereinafter referred to as "TC composite material") (see, for example, Patent Document 1).

In the past, general sleeves were made of steel such as SKD61, but since non-ferrous metals tend to react with iron, steel sleeves are prone to erosion due to contact with the molten metal to be filled, and have a short service life. I had a problem. Also, since steel has a high thermal conductivity, the temperature of the molten metal supplied into the sleeve tends to drop. If the temperature of the molten metal supplied into the sleeve drops in the sleeve before it reaches the cavity and solidified pieces are generated, defects such as peeling are likely to occur at those portions in the molded product, and the mechanical strength is reduced. There is a problem of lowering

On the other hand, the TC composite material used for the inner cylinder has low reactivity with non-ferrous metals, and is therefore excellent in erosion resistance. In addition, while SKD61 has a high thermal conductivity of 35.6 W/mK, TC composite material has a very low thermal conductivity of 7.4 W/mK, which is excellent in heat retention. It has the advantage that the temperature of the metal does not easily drop. Furthermore, when the inner cylinder is formed only from ceramics, although it is possible to improve the resistance to corrosion and heat retention, ceramics, which is a brittle material, has the disadvantage of low impact resistance. Since it is a composite material of metal and ceramics, it has the advantage of being excellent in impact resistance.

However, although the TC composite material has many advantages such as excellent erosion resistance, heat retention and impact resistance, it has the disadvantage of low hardness. In die casting, the plunger tip is inserted from one end of the sleeve and slides in the sleeve in the axial direction. If the hardness is low, the sliding of the plunger tip will wear the inner peripheral surface. When the inner peripheral surface of the inner cylinder is worn, a gap is formed between it and the plunger tip, and molten metal may leak from the gap. For this reason, conventional steel sleeves use steel that has undergone surface hardening treatments such as quenching and nitriding, but the TC composite material has a lower hardness than the hardened steel.

Therefore, the present applicant proposes nitriding an inner cylinder made of a TC composite material by heating it in an atmosphere containing nitrogen to form a nitrided layer on the inner peripheral surface of the inner cylinder. (See Patent Document 2). The nitrided layer is not only harder than TC composites, but it is also much harder than hardened steel, which makes it possible to increase the wear resistance of the inner cylinder of the sleeve.

However, in the conventional manufacturing method of heating the TC composite material in a nitrogen-containing atmosphere, the fact is that only the very surface of the TC composite material can be nitrided. If the nitrided layer is formed only on the very surface layer, the nitrided layer may be lost when processing is performed to increase the dimensional accuracy in order to bring the inner peripheral surface of the inner cylinder into close contact with the plunger tip. In addition, when the nitride layer is formed only on the very surface layer, due to the large difference in hardness at the boundary between the surface layer and the inner layer, it tends to peel off due to contact with the sliding plunger tip. There is

Regarding the nitriding of steel, the higher the heating temperature and the longer the heating time in an atmosphere containing nitrogen, the easier it is to form a nitrided layer. There is a problem of deformation.

JP-A-3-142053 JP-A-4-224066

Therefore, in view of the above circumstances, the present invention provides a method for producing a nitrided material that can increase hardness by nitriding to a deep layer while suppressing deformation of a sintered body of a composite material of titanium or a titanium alloy and ceramics. and to provide a nitride material produced by the production method.

In order to solve the above problems, a method for producing a nitride material according to the present invention (hereinafter sometimes simply referred to as "manufacturing method") comprises:
"A heating step of heating a sintered body of a composite material of titanium or a titanium alloy and ceramics at a temperature of 700 ° C to 1200 ° C in an atmosphere containing nitrogen gas,
After the heating step, a slow cooling step of cooling the sintered body at a temperature decreasing rate of 50°C/h to 100°C/h until the temperature of the atmosphere reaches at least 500°C.

Hereinafter, the nitrogen-containing gas may be referred to as "nitriding gas", and the nitrogen-containing gas atmosphere may be referred to as "nitriding gas atmosphere".

In this manufacturing method, a sintered body (TC composite material) of a composite material of titanium or a titanium alloy and ceramics is heated in a nitriding gas atmosphere, and then slowly cooled until the temperature of the atmosphere reaches at least 500°C. It is. Conventionally, in the nitriding treatment of steel materials, in anticipation of hardening by "quenching", the steel materials are sometimes heated in a nitriding gas atmosphere and then quenched. The inventors of the present invention did not easily follow the nitriding treatment for such steel materials, and as a result of continuing their studies, it was found that slow cooling after heating at a high temperature is effective in nitriding TC composite materials. It has been found. The reason for this is considered as follows.

That is, when the heating is stopped after the heating process at a high temperature, the temperature of the sintered body decreases, and this temperature decrease is thought to progress from the outer surface of the sintered body toward the inside. Therefore, even if nitrogen gas remains in the atmosphere around the sintered body during the temperature drop process, the nitrogen cannot permeate and diffuse from the outer surface whose temperature has dropped early. On the other hand, in the present manufacturing method, the temperature drop rate is set to 50° C./h to 100° C./h, which is considerably smaller than the temperature drop rate due to natural cooling. Therefore, in order to control the temperature drop rate, heating is also performed during the temperature drop process. In other words, it is in a state of "heating even while the temperature is falling". Therefore, even in the process of lowering the temperature, the nitrogen gas in the atmosphere around the sintered body penetrates from the outer surface of the heated sintered body and diffuses into the inside, so that the deep layer can be nitrided.

Therefore, according to the present production method, since the sintered body can be nitrided to a deep layer without heating the sintered body at an excessively high temperature for a long time in a nitriding gas atmosphere, deformation due to excessive heating can be suppressed and the nitriding can be performed. It is possible to produce a nitrided material with increased hardness.

In addition to the above configuration, the method for producing a nitride material according to the present invention includes:
It is possible that "the slow cooling step is performed in an atmosphere in which nitrogen gas is constantly supplied".

In this configuration, the supply of nitrogen gas is continued even in the process of lowering the temperature. As a result, the sintered body is under the gas pressure of nitrogen gas even while the temperature is lowered, and nitrogen can easily permeate and diffuse into the interior of the sintered body as if it were press-fitted from the surface of the sintered body.

In addition, if the nitrogen gas supply is stopped after the heating process, the nitrogen concentration in the atmosphere around the sintered body decreases, and the nitrogen that has once permeated the inside of the sintered body may escape to the outside. There is On the other hand, in this configuration, the nitrogen concentration in the atmosphere around the sintered body is maintained by continuing to supply nitrogen gas even during the temperature drop, so that the nitrogen that has once permeated the inside of the sintered body escapes to the outside. is prevented.

In addition to the above configuration, the method for producing a nitride material according to the present invention includes:
"The composite material contains nickel."

By including nickel in the TC composite material, the sintered body of the TC composite material can be densified. For example, when 0.1 to 10 parts by weight of nickel is added to 100 parts by weight of a TC composite material that does not contain nickel, the apparent porosity of the sintered body measured by the Archimedes method is 0.07%. ~0.5%, which is very small. On the other hand, since nickel does not form a solid solution with nitrogen and does not form a nitride, the inclusion of nickel inhibits the nitriding of the TC composite material.

As described above, since the porosity is extremely small, there are almost no passages for permeation of the nitriding gas, and in addition, even if the TC composite material contains nickel, which is a hindrance to nitriding, as will be described in detail later, In this manufacturing method in which the slow cooling step is performed after the heating step, deep layers can be nitrided.

Next, the nitride material according to the present invention is
"A nitrided material having a nitride layer formed by nitriding a sintered body of a composite material on the surface of a base material that is a sintered body of a composite material of titanium or a titanium alloy and ceramics,
The composite material contains nickel,
While the Vickers hardness of the base material is 360HV to 420HV,
The nitride layer has a Vickers hardness of 640 HV to 1300 HV and a thickness of at least 1.4 mm."

This is a configuration for a nitride material that can be manufactured for the first time by the above-described manufacturing method. Conventionally, when a TC composite material is nitrided following the nitriding treatment of steel, a nitrided layer having a hardness higher than that of the base material could be formed only up to a depth of about 0.1 mm from the surface. Furthermore, when the TC composite material contains nickel, as described above, the porosity is extremely small, so there are almost no passages for the nitriding gas to permeate. hard to be According to the manufacturing method described above, a nitrided layer having a hardness as high as 1.5 to 3.6 times that of the base material while containing nickel, which acts adversely on the formation of the nitrided layer, is produced at least 1.5 times. A nitrided material of this configuration having a thickness of 4 mm can be obtained.

Next, the nitride material according to the present invention, instead of the above configuration,
"A nitrided material having a nitride layer formed by nitriding a sintered body of a composite material on the surface of a base material that is a sintered body of a composite material of titanium or a titanium alloy and ceramics,
The composite material contains nickel,
While the Vickers hardness of the base material is 360HV to 420HV,
The nitride layer has a Vickers hardness of 1080 HV to 1300 HV with a thickness of at least 1.8 mm.

As will be described later in detail, this is the structure of the nitrided material obtained when the temperature drop rate is set to the lowest rate of 50° C./h in the above manufacturing method. It has a nitrided layer having a hardness as high as 2.6 to 3.6 times that of the base material in spite of being contained, with a thickness as large as at least 1.8 mm.

As described above, according to the present invention, there is provided a method for producing a nitrided material capable of suppressing deformation of a sintered body of a composite material of titanium or a titanium alloy and ceramics and increasing hardness by nitriding to a deep layer. And, a nitride material manufactured by the manufacturing method can be provided.

10 is a graph comparing hardness distribution in the depth direction with comparative example R for the nitrided materials of Examples E1 and E2.

Specific embodiments of the present invention are described below. The manufacturing method of the present embodiment includes a heating step of heating at a temperature of 700 ° C. to 1200 ° C. in an atmosphere containing nitrogen gas, and after this heating step, the temperature of the atmosphere reaches at least 500 ° C. and a slow cooling step of cooling the sintered body at a temperature drop rate of h to 100° C./h.

More specifically, in the heating step, the TC composite material (sintered body) is placed in a heating furnace and heated from room temperature to a target temperature within the range of 700°C to 1200°C. At this time, the nitriding gas is introduced into the heating furnace to fill the processing space with the nitriding gas before the temperature of the processing space reaches the target temperature. Examples of the nitriding gas include 100% nitrogen gas, mixed gas of nitrogen and inert gas, and mixed gas of ammonia gas and nitrogen gas generated by decomposition thereof. Nitrogen gas is continuously supplied into the heating furnace, and the surplus nitrogen gas is discharged to the outside of the heating furnace.

When the temperature in the processing space of the heating furnace reaches the target temperature, the target temperature is maintained for a certain period of time while continuing to supply the nitriding gas into the heating furnace. With the heating in this step, nitrogen permeates and diffuses toward the inside of the sintered body, forming a nitride layer. The nitride layer is a layer of a compound of titanium and nitrogen, or a layer in which nitrogen diffuses between crystal lattices. Here, the holding time in the heating step can be 1 hour to 5 hours, and preferably 3 hours or more in order to sufficiently progress the permeation and diffusion of nitrogen.

In the slow cooling step, the temperature of the atmosphere in the heating furnace is lowered at a rate of 50° C./h to 100° C./h. In order to achieve a temperature drop rate of 50° C./h to 100° C./h, it is necessary to continue heating the atmosphere in the heating furnace with a certain amount of power while aiming at cooling in the slow cooling step. Further, the supply of the nitriding gas into the heating furnace is continued even during the slow cooling process.

In the slow cooling step, since heating is continued to control the temperature drop rate, the temperature of the outer surface of the sintered body does not drop rapidly, and the slow cooling step following the heating step Penetration diffusion of nitrogen into the inside of the sintered body progresses. That is, according to the present embodiment, the permeation and diffusion of nitrogen into the sintered body progresses even in the temperature-lowering process in which the permeation and diffusion of nitrogen did not occur in the conventional art.

If the temperature drop rate in the slow cooling step is higher than 100° C./h, the temperature of the outer surface of the sintered body drops early, making it difficult for nitrogen to diffuse and permeate. In order to sufficiently progress the diffusion and permeation of nitrogen, the lower the temperature drop rate, the better. However, considering the work efficiency that is easy to implement in the industrial world, the lower limit of the temperature drop rate is set to 50 ° C./h. .

Here, the heating step and slow cooling step can be performed under normal pressure, but can also be performed under pressure. By performing the nitriding under pressure, the nitriding gas can be easily injected deep into the sintered body, and the nitriding can proceed more efficiently. If the pressurization condition is two to three times the normal pressure, the gas can be effectively injected at a pressure that is easy to handle. The pressure in the heating step and the slow cooling step may be the same or different.

The TC composite material can contain molybdenum as a metal in addition to titanium. By including molybdenum, the erosion resistance of the TC composite material to molten non-ferrous metals can be enhanced. In this case, the content of molybdenum in the TC composite material is preferably 10 to 50 parts by weight of molybdenum atoms, more preferably 20 to 45 parts by weight, per 100 parts by weight of titanium atoms.

The proportion of ceramics in the TC composite material is desirably 1-15 parts by weight, more desirably 3-10 parts by weight, per 100 parts by weight of metal atoms. When the TC composite material contains molybdenum in addition to titanium as a metal, "100 parts by weight of metal atoms" referred to herein is the sum of the parts by weight of titanium atoms and molybdenum atoms. By setting such a ratio, it is possible to make use of the advantages of ceramics and compensate for the drawbacks of ceramics, which is a brittle material, with the toughness of metal.

The inventors of the present invention have learned from past studies that the nitriding of a sintered body of a TC composite material is affected by the porosity, and that if the porosity of the sintered body is low, the nitriding is difficult to progress. It is considered that this is because the open pores serve as passages through which the nitrogen-containing gas penetrates into the sintered body. However, if the porosity is high, the mechanical strength of the sintered body is reduced, such as cracks extending along the pores. Therefore, when the sintered body of the TC composite material is densified in order to increase the mechanical strength, it becomes difficult to nitride to a deep layer, and there is a problem that the nitrided layer is formed only on the surface layer.

In contrast, according to the manufacturing method of the present embodiment, even a sintered body densified by adding nickel as a TC composite material can be nitrided to a deep layer. It is thought that this is because nitrogen permeation diffusion can be progressed even during the cooling process in which nitrogen permeation diffusion did not occur in the past, and thus nitrogen permeation diffusion can be continued for a long period of time, albeit slowly. Thus, by using a sintered body densified by the addition of nickel as a TC composite material, it is possible to obtain both the effect of increasing mechanical strength by densification and the effect of increasing hardness by nitriding. .

The densification of the TC composite material by the addition of nickel will be explained more specifically. For example, a TC composite material (sample S0) and samples S1 to S12 in which nickel was added to the TC composite material of sample S0 at different ratios. Here, the addition ratio of nickel is expressed in parts by weight of nickel atoms with respect to 100 parts by weight of the TC composite material excluding nickel.

As can be seen from Table 1, by adding nickel in the range of at least 0.1 parts by weight to 10 parts by weight of nickel atoms with respect to 100 parts by weight of the TC composite material excluding nickel, the apparent porosity of the TC composite material can be a very dense sintered body with a content of 0.07% to 0.5%.

Actually, a TC composite material (sintered body) was produced and nitrided by the production method of the present embodiment. The sintered body of the TC composite material was produced by powder metallurgy, in which a molded body formed from a raw material mixture of titanium powder, molybdenum powder, silicon carbide powder, and nickel powder was fired in a non-oxidizing atmosphere. When the apparent porosity of the obtained sintered body was measured by the Archimedes method according to JIS R2205, it was 0.5% and was very dense. That is, the obtained sintered body is a TC composite material sintered body densified by the addition of nickel.

This sintered body was placed in a heating furnace, heated to a target temperature of 700° C. to 1200° C. at a predetermined temperature rising rate, and subjected to a heating step of heating at the target temperature for a certain period of time. Thereafter, Example E1 was subjected to a slow cooling step at a temperature lowering rate of 50° C./h, and Example E2 was subjected to a slow cooling step at a temperature lowering rate of 100° C./h. In the heating furnace used in this example, when the temperature inside the furnace is naturally cooled (furnace cooling) after heating to 1200° C., the temperature drop rate is 250° C./h to 350° C./h. Therefore, heating was continued during cooling in order to perform a slow cooling step at 50°C/h or 100°C/h. For comparison, Comparative Example R was a sample which was heated under the same conditions as above, subjected to the heating process under the same conditions, and then allowed to cool naturally (furnace cooling) without slow cooling.

In any of Example E1, Example E2, and Comparative Example R, no deformation was observed in the shape of the sample when the temperature was lowered to room temperature.

For each of the samples of Example E1, Example E2, and Comparative Example R heated at a heating temperature of 1200° C., the hardness distribution in the depth direction was measured. Hardness was measured using a micro Vickers hardness tester under a load of 1 kgf. The results are also shown in FIG.

In Comparative Example R, the hardness was almost constant at 360 HV to 420 HV within the depth range of 0.1 mm to 1.8 mm regardless of the depth. This hardness was comparable to the hardness of a sample which is the same TC composite material used in Examples and Comparative Examples and which was not subjected to nitriding treatment. In other words, the hardness of Comparative Example R is the hardness of the base material in each sample after nitriding treatment. Although illustration is omitted, the hardness of the surface (depth of 0 mm) of Comparative Example R was about 520 HV. In other words, when natural cooling is performed after the heating process, the nitride layer is formed only on the very surface. Such a nitrided layer only on the surface layer is lost due to processing at the time of commercialization, or is lost due to peeling or abrasion in the initial stage of use.

On the other hand, both Example E1 and Example E2 show higher hardness than Comparative Example R in the entire range of measured depths, and it is considered that nitriding progresses to a depth of at least 1.8 mm. rice field. Considering Example E1 and Example E2 together, a nitrided layer having a hardness of 530HV to 1300HV is formed to a depth of at least 1.8mm, and a nitrided layer having a hardness of 640HV to 1300HV is formed to a depth of at least 1.4mm. A nitride layer having a hardness of 800HV to 1300HV is formed to a depth of at least 1.2 mm.

In Example E1, in which the temperature drop rate is as low as 50° C./h, a nitrided layer having a very high hardness of 1080 HV to 1300 HV is formed to a depth of at least 1.8 mm. The thickness of the nitride layer formed by natural cooling without slow cooling is less than 0.1 mm, and the hardness thereof is about 1.2 to 1.4 times the hardness of the base material. In view of this, it is very significant that a nitrided layer with high hardness can be formed to a deep layer.

In addition, the width of the wear marks was measured using an Okoshi rapid wear tester for each of the samples of Example E1, Example E2, and Comparative Example R with a heating temperature of 1000°C. As a result, as shown in Table 2, Example E1 and Example E2, which are slowly cooled, have a smaller wear scar width than Comparative Example R, which is naturally cooled. The width of the wear scar was even smaller in the case of . The width of the wear scar is considered to be an index of wear resistance, and when combined with the hardness measurement results described above, a nitrided layer with high hardness can be formed to a deep layer by using a slow cooling process as the cooling process. , it was thought that the wear resistance could be improved.

As described above, according to the manufacturing method of the present embodiment, the hardness can be increased by nitriding the TC composite material to a deep layer while suppressing deformation due to excessive heating. In particular, since the apparent porosity is extremely small at 0.5%, it is a sintered body to which nickel, which hinders nitridation, is added, in addition to having almost no voids that serve as passages for nitrogen to permeate. However, according to the manufacturing method of the present embodiment, the hardness can be increased by nitriding to a deep layer.

As described above, the present invention has been described with reference to preferred embodiments, but the present invention is not limited to the above embodiments, and as shown below, various improvements can be made without departing from the scope of the present invention. and design changes are possible.

For example, in the above, the case where the present invention is applied to nitriding when the TC composite material is used for the inner cylinder of a die-casting sleeve is illustrated, but the present invention is not limited to this, and the TC composite material used for other applications is nitrided. Of course, the present invention can also be applied when

In the above examples, silicon carbide (SiC) was used as _a raw material for the TC composite material, but the type of ceramics is not limited to this. can do.

Claims (4)

a heating step of heating a sintered body of a composite material of titanium or a titanium alloy and ceramics at a temperature of 700° C. to 1200° C. in an atmosphere containing nitrogen gas;
After the heating step, a slow cooling step of cooling the sintered body at a temperature decrease rate of 50 ° C./h to 100 ° C./h until the temperature of the atmosphere reaches at least 500 ° C. ,
Including nickel in the composite material
A method for producing a nitride material, characterized by: 2. The method of manufacturing a nitrided material according to claim 1, wherein said slow cooling step is performed in an atmosphere in which nitrogen gas is constantly supplied. A nitrided material having a nitrided layer formed by nitriding a sintered body of a composite material on the surface of a base material that is a sintered body of a composite material of titanium or a titanium alloy and ceramics,
The composite material contains nickel,
While the Vickers hardness of the base material is 360HV to 420HV,
The nitride layer has a Vickers hardness of 1080HV to 1300HV and a thickness of at least 1.8 mm.
A nitride material characterized by: A nitrided material having a nitrided layer formed by nitriding a sintered body of a composite material on the surface of a base material that is a sintered body of a composite material of titanium or a titanium alloy and ceramics,
The composite material contains nickel,
While the Vickers hardness of the base material is 360HV to 420HV,
A nitrided material characterized in that the nitrided layer has a Vickers hardness of 640HV to 1300HV and has a thickness of at least 1.4 mm.

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