CN111344429A

CN111344429A - Iron-based sintered alloy material and method for producing same

Info

Publication number: CN111344429A
Application number: CN201880072656.0A
Authority: CN
Inventors: 大守洋
Original assignee: Hitachi Chemical Co Ltd
Current assignee: Lishennoco Co ltd
Priority date: 2017-11-10
Filing date: 2018-11-09
Publication date: 2020-06-26
Anticipated expiration: 2038-11-09
Also published as: JP2019085634A; CN115595530A; WO2019093480A1; US20230211413A1; CN111344429B; JP7167428B2; US20200331068A1

Abstract

The iron-based sintered alloy material has a hardened layer exhibiting a martensite phase in which nitrogen is supersaturated and solid-dissolved on the surface. The iron-based sintered alloy material may contain one or more of chromium, copper, molybdenum, manganese, and nickel. The method for manufacturing the iron-based sintered alloy material comprises the following steps: the iron-based sintered gold base material containing carbon is subjected to nitriding treatment by heating to a nitriding temperature of 590 ℃ or higher in an atmosphere containing ammonia gas, and is quenched by quenching.

Description

Iron-based sintered alloy material and method for producing same

Technical Field

The present invention relates to an iron-based sintered alloy material having improved strength by hardening the surface thereof, and a method for producing the same.

Background

Conventionally, a metal material has been surface-treated by a chemical hardening method in order to impart material characteristics such as wear resistance and fatigue resistance required for machine parts and the like. The chemical hardening method is a method of forming a hardened layer on the surface of a material by allowing a hardening component to act on the surface, and there are various treatment methods such as carburizing treatment, nitriding treatment, carbonitriding treatment, sulfurizing-nitriding treatment, and boronizing treatment. Carburizing treatment is a hardening method that has been carried out since a long time ago and is widely used, but has a problem of large strain due to quenching carried out as heat treatment after carburizing.

On the other hand, the nitriding treatment by the precipitation hardening by nitride can be performed at a lower heating temperature than the carburizing treatment, and can reduce the thermal strain, but has a problem that the treatment time is long and the hardened layer is thin. In addition, since nitrides are brittle even if they are hard, they have a problem in strength. On the other hand, the nitriding treatment by the solid solution diffusion of nitrogen does not depend on the formation of nitrides, and therefore, the problem caused by brittleness can be avoided, and the thermal strain is reduced as compared with the carburizing treatment. However, nitriding still has the disadvantages of long treatment time, shallow hardened layer, and the like. For example, patent document 1 below describes a surface layer hardening treatment method for hardening the surface of a metal material, and discloses that the vickers hardness of the metal material from the surface to a depth of 78 μm is increased by 5% or more by nitriding the metal material. The hardened layer of this depth was obtained by a treatment for 12 hours.

Documents of the prior art

Patent document

Patent document 1: japanese re-publication of patent No. WO2014/104085

Disclosure of Invention

Problems to be solved by the invention

Austenite into which nitrogen has been diffused by infiltration undergoes martensitic transformation when quenching is performed, and the hardness is significantly increased. That is, the hardened layer on the surface is formed by performing such a rapid cooling heat treatment. The austenitizing temperature of Fe-N system is lower than that of Fe-C system, and the strain due to heat treatment can be reduced by nitriding treatment as compared with carburizing treatment. However, as described above, in the conventional nitriding treatment, it is difficult to form a sufficiently thick hardened layer on the surface. In order to efficiently and inexpensively provide a machine component or the like excellent in wear resistance and the like, it is necessary to achieve surface hardening which is small in strain due to heat treatment and capable of forming a thick hardened layer in a short time, and to improve material characteristics of a metal material constituting the machine component or the like.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a metal material having an improved strength by a hardened layer at an effective and low cost, and to provide a high-quality product with high accuracy.

Means for solving the problems

The present inventors have studied a chemical hardening method of a metal material in order to solve the above problems, and have found that a metal material having a hardened layer formed on the surface thereof suitably can be obtained by nitriding and quenching an iron-based sintered alloy, and have realized a technique capable of providing a mechanical component such as a sprocket and various members excellent in wear resistance and fatigue strength.

According to one embodiment of the present invention, an iron-based sintered alloy material has a hardened layer exhibiting a martensite phase in which nitrogen is supersaturated and solid-dissolved on the surface.

The iron-based sintered alloy material may contain 0.1 to 1.0 mass% of carbon. Further, the alloy may contain one or more alloying components selected from the group consisting of chromium, copper, molybdenum, manganese, and nickel. The alloying component is any one of 0.15-4.5 mass% of chromium, 0.2-4.5 mass% of copper, 0.1-2.0 mass% of molybdenum, 0.1-3.0 mass% of manganese and 0.2-4.5 mass% of nickel. The hardened layer contributes to an increase in the surface fatigue strength by increasing the depth from the surface to 100 μm or more.

In addition, according to one embodiment of the present invention, a method for producing an iron-based sintered alloy material includes: forming an iron-based mixed powder containing carbon powder into a green compact having a desired shape, heating the green compact to 1000 to 1300 ℃ in a non-oxidizing atmosphere, sintering the green compact to obtain an iron-based sintered gold base material, heating the iron-based sintered gold base material to a nitriding temperature of 590 ℃ or higher in an atmosphere containing ammonia gas, nitriding the iron-based sintered gold base material, and quenching the nitrided iron-based sintered gold base material by rapid cooling.

The quenching is effective for suppressing thermal strain if it is performed at a quenching temperature lower than the nitriding temperature. After the quenching, the steel sheet is further heated to 100 to 200 ℃ and tempered, and is effective in eliminating stress and martensitic transformation of retained austenite. The carbon powder contained in the iron-based mixed powder may be 0.1 to 1.2 mass% of graphite powder. The iron-based mixed powder may further contain one or more alloying components selected from the group consisting of chromium, copper, molybdenum, manganese, and nickel. The iron-based mixed powder preferably contains at least one alloying component selected from the group consisting of 0.15 to 4.5 mass% of chromium, 0.2 to 4.5 mass% of copper, 0.1 to 2.0 mass% of molybdenum, 0.1 to 3.0 mass% of manganese, and 0.2 to 4.5 mass% of nickel.

Effects of the invention

According to the present invention, the characteristics such as fatigue strength and wear resistance of the iron-based sintered alloy material having the hardened layer formed on the surface thereof are improved, and various products such as machine parts can be provided with high accuracy and at low cost by reducing thermal strain.

Drawings

Fig. 1 is an SEM image of a metal structure of a cross section of a surface-hardened iron-based sintered alloy material, wherein (a) shows an SEM image of hardening by carburizing and quenching, (b) shows an SEM image of hardening by carburizing and nitriding and quenching, and (c) shows an SEM image of hardening by nitriding and quenching.

Detailed Description

An iron-based sintered alloy material is a sintered material having an alloy composition containing iron as a main component, and is obtained by compression-molding a powder containing iron as a main component into a desired shape to obtain a green compact, and heat-sintering the obtained green compact. The final sintered body is formed into a product made of an iron-based sintered alloy by, for example, forming the product into a net shape or a near net shape of the target product during the forming. The sintered material is a porous material having pores, and the iron-based sintered alloy material of the present invention is also a porous material having pores at a porosity corresponding to the powder compact density at the time of molding. Since the sintered material is used as a product by performing processing such as shaping (サイジング) and coining (コイニング) on the sintered material as necessary, the surface of the sintered material is densified in this case. Iron and steel materials provided in the form of molten materials, cast materials, forged materials, and the like are widely used as raw materials for constituting machine parts, structural members, and the like, and iron-based sintered alloy materials having the same alloy composition are also suitable for various parts and members. Therefore, it is very useful to produce an iron-based sintered alloy material exhibiting excellent material characteristics at low cost and efficiently, and by realizing an iron-based sintered alloy material in which the material characteristics are improved by surface hardening treatment, it is possible to provide a power transmission component, a mechanical device component, and the like with high quality.

The iron-based sintered alloy material in the present invention is a case hardened material obtained by nitriding and quenching a sintered alloy base material having the same alloy composition as that of a steel material, that is, a carbon-containing iron alloy composition. By nitriding, nitrogen permeates and diffuses from the alloy surface, and austenite in which nitrogen is dissolved is generated. The steel sheet is quenched to cause martensitic transformation, thereby forming a hardened layer of martensite in which nitrogen is supersaturated and solid-dissolved. The nitrogen-diffused layer can be formed at a depth of about 100 μm or more from the outermost surface in a short period of time, and the hardened layer thus formed is effective for improving the surface fatigue strength. By extending the nitriding treatment time, the hardened layer can be formed more deeply. The increase in hardness of the surface portion by the hardened layer contributes to improvement in strength and wear resistance. Since the iron-based sintered gold base material is a porous material, the nitrogen permeation and diffusion can be performed not only on the outer surface of the sintered gold base material but also in the pores. Therefore, the hardened layer formed by nitriding quenching reaches the inner surface of the pore, that is, the deep part of the sintered alloy, and an effect similar to the formation of a deep hardened layer can be obtained. The depth of the hardened layer formed by nitriding quenching of the molten material is usually about 50 μm, whereas the depth of the hardened layer in the sintered alloy material is likely to reach about 200 μm. In the present invention, depending on the content of nitrogen, the nitride may be dispersed in the martensite phase, but the dispersion of the nitride to a certain extent is allowable, and the function as the hardened layer is not impaired.

The austenitizing temperature of Fe-C is about 727 ℃ and the austenitizing temperature of Fe-N is about 590 ℃ lower by 130 ℃ or more than that, so that the nitriding treatment can be performed at a temperature lower by 100 ℃ or more than that of the carburizing treatment. Therefore, the quenching temperature after the nitriding treatment may be set to a temperature lower than the carburizing and quenching temperature. Therefore, the thermal strain can be significantly reduced as compared to carburizing and quenching. Further, since the eutectoid point (2.35 mass% N) of Fe-N system is higher in element content than the eutectoid point (0.77 mass% C) of Fe-C system, the austenitizing temperature increases in the range (0.77 mass% or more) where the carbon amount increases from the eutectoid point in Fe-C system, whereas the austenitizing temperature decreases as the nitrogen amount increases (but up to 2.35 mass%) in the same range of Fe-N system. That is, in the nitriding treatment, not only nitrogen can be solid-dissolved at a low temperature, but also the amount of solid-dissolved nitrogen can be increased as compared with the carburizing treatment.

The iron-based sintered alloy material of the present invention is explained below. The iron-based sintered alloy material of the present invention is an iron-based sintered alloy material in which a hardened layer, that is, a surface hardened layer, is formed on the surface of a sintered alloy base material composed of an iron-based alloy containing carbon. Therefore, the main part thereof is composed of an iron-based sintered alloy containing carbon, and the surface hardened layer produced by nitriding quenching exhibits a martensite phase in which nitrogen is supersaturated and dissolved. The iron-based sintered alloy substrate is composed of an iron-based sintered alloy as follows.

< iron-based sintered alloy containing carbon >

The iron-based sintered alloy material before surface hardening is made of an iron-based sintered alloy containing carbon, and the iron-based sintered alloy material after surface hardening has the same alloy composition for the portions other than the hardened layer. The alloy composition is a carbon-containing iron alloy composition and comprises steel compositions such as carbon steel, low alloy steel and high alloy steel. Examples of the alloy steel include, but are not limited to, chromium steel, nickel-chromium-molybdenum steel, nickel-molybdenum steel, manganese steel, and manganese-molybdenum steel. An iron-based sintered alloy having a hardened surface can be obtained by nitriding and quenching the iron-based sintered alloy substrate having the above composition to form a hardened layer.

The production of the iron-based sintered alloy base material before hardening is described below, but it is needless to say that an iron-based sintered alloy product having such a composition is commercially available and subjected to nitriding quenching to harden the surface thereof. Nitriding quenching of the iron-based sintered gold base material will be described later.

< production of iron-based sintered alloy substrate >

The raw material powder used for producing the iron-based sintered gold base material is a mixed powder containing carbon powder and iron as a main component (iron-based mixed powder), and depending on the target alloy composition, for example, an alloying component such as chromium (Cr), copper (Cu), molybdenum (Mo), manganese (Mn), nickel (Ni), aluminum (Al), vanadium (V), titanium (Ti), or silicon (Si) may be optionally blended. The alloying components may be blended in the raw material powder in the form of an alloy powder with iron, or may be blended in the form of a single powder. Chromium and molybdenum are particularly effective components for improving the hardness and mechanical properties of the material. An iron-based sintered alloy having the same composition as the alloy steel such as the chromium steel and the nickel-molybdenum steel can be obtained from a raw material powder containing one or more of chromium, copper, molybdenum, manganese, and nickel. When any or all of the above alloying components are blended, the contents of the alloying components in the iron-based sintered alloy are preferably, respectively, chromium: 0.15 to 4.5 mass%, copper: 0.2 to 4.5 mass%, molybdenum: 0.1 to 2.0 mass%, manganese: 0.1 to 3.0 mass%, nickel: 0.2 to 4.5 mass%.

When the graphite powder having an average particle diameter of about 1 to 40 μm is used as the carbon powder used for preparing the raw material powder, the graphite powder is favorably diffused into the matrix. When the single powder or alloy powder for iron and alloying components has an average particle diameter of about 1 to 300 μm, preferably about 45 to 150 μm, the powder is preferably excellent in compressibility during molding and easy to produce and handle. Powders for the respective components are blended in a ratio corresponding to the composition of the target iron-based sintered alloy and uniformly mixed, and the obtained mixed powder is used as a raw material powder for molding a green compact. In consideration of the scattering portion (emanation), the ratio of the graphite powder in the raw material powder for preparing the iron-based sintered gold substrate having a carbon content of 0.1 to 1.0 mass% may be about 0.1 to 1.2 mass%. If a powder lubricant such as a stearate is appropriately added as necessary, the compressibility of the raw material powder is improved.

In the molding of the raw material powder, the raw material powder is put into a cavity having a desired shape, and the raw material powder is compressed by a press machine to be molded into a green compact. The molding pressure may be appropriately set in accordance with the density required for the target product, and may be set within a range of about 250 to 800MPa in general.

The green compact obtained by the molding is heated and sintered to obtain a green compact having a density of 6.0 to 7.6Mg/m³A sintered body, i.e., an iron-based sintered gold substrate. The sintering temperature is set to an appropriate temperature according to the composition of the iron-based sintered alloy, and generally, may be set to a temperature in the range of about 1000 to 1300 ℃. Since the sintering alloy is oxidized if the sintering environment is oxidizing, the sintering is performed in an "environment non-oxidizing to the green compact", that is, an environment in which the green compact is not oxidized. Specifically, the sintering may be performed under reduced pressure or in a non-oxidizing atmosphere of an inert gas such as argon or nitrogen. In the case of a green compact containing neither chromium nor molybdenum, an endothermic conversion gas (endothermic change ガス) may be used in the sintering atmosphere since it does not cause oxidation of the green compact. That is, depending on the composition of the green compact, the endothermic conversion gas may be "an environment non-oxidizing to the green compact". The atmosphere gas containing hydrogen has an advantage of reducing oxygen on the surface of the powder and promoting sintering. It is preferable to use an atmosphere having a low dew point. The iron-based sintered gold base material is obtained by such heating and sintering,the furnace was cooled to recover the iron-based sintered alloy base material.

< quenching by nitriding >

The nitriding quenching is performed by contacting the iron-based sintered alloy base material with a gas for nitriding, and therefore, it is necessary to adjust the atmosphere conditions of the iron-based sintered alloy base material. Therefore, before and after the iron-based sintered alloy base material recovered from the sintering furnace is introduced into the quenching furnace, the atmosphere conditions in the quenching furnace are adjusted as follows.

As the atmosphere adjustment before nitriding quenching, the inside of the furnace was evacuated and pressure recovery (nitrogen substitution) was performed with nitrogen gas to sufficiently remove oxygen. The iron-based sintered alloy substrate was placed in a furnace whose atmosphere was adjusted in this manner. Then, the vacuum is again applied, and a low pressure of about 50Pa is maintained for about 10 to 30 minutes, preferably about 20 minutes. Thereby, the residual gas in the pores of the iron-based sintered gold base material was removed. Further, the pressure is recovered by nitrogen gas, and heating is started to raise the temperature in the furnace to the nitriding temperature. Nitriding temperature is higher than or equal to austenitizing temperature, namely higher than or equal to 590 ℃, and nitriding is carried out at the temperature of 590-900 ℃. In view of nitriding speed and thermal strain, a temperature range of about 650 to 800 ℃ is preferable. The time for carrying the iron-based sintered alloy substrate into the furnace and performing the evacuation and the temperature rise may be less than or equal to about 1 hour.

After the temperature in the furnace reaches the nitriding temperature, the temperature is maintained and kept still for 10 to 30 minutes, preferably for about 20 minutes, so that the temperature of the iron-based sintered alloy substrate is equal on the whole. Then, a nitriding gas is supplied into the furnace to start a nitriding treatment.

The nitriding treatment is performed by bringing a nitriding gas into contact with the iron-based sintered alloy base material. As the gas for nitriding, nitriding is performed in an atmosphere containing ammonia and nitrogen using a gas containing ammonia. Nitriding can also be performed using a mixed gas of ammonia and hydrogen, and such a gas can also be used. Ammonia gas becomes unstable when heated and thermally decomposes into nitrogen molecules and hydrogen molecules. If steel is present, atomic nitrogen and hydrogen are generated only on the surface of hot steel due to the catalytic action thereof, and active atomic nitrogen permeates and diffuses into the steel. On the surface of the iron-based sintered gold base material heated to an austenitizing temperature (about 590 ℃) or higher, active atomic nitrogen penetrates into the alloy, and at the same time, nitrogen is dissolved to cause diffusion (nitriding), and the surface layer portion assumes an Fe — N austenite phase. Since nitriding proceeds with the decomposition reaction as described above, it is preferable to use a mixed gas of ammonia gas and nitrogen gas in a ratio of 1:2 as the nitriding gas. The speed of progression of nitriding depends on the nitrogen concentration, and the solid solubility limit of nitrogen in the Fe-N system is about 2.8 mass% N, which is larger than the solid solubility limit of carbon in the Fe-C system (2.1 mass% C). The nitriding treatment may be performed for about 30 to 180 minutes, preferably about 120 to 180 minutes, whereby a hardened layer having a depth of about 100 μm or more can be formed. The depth of nitrogen solid solution varies depending on the treatment conditions, and by extending the nitriding treatment time, nitrogen can be diffused more deeply, and the depth of the hardened layer obtained after quenching increases. The nitriding time is preferably set so that a hardened layer having a depth of about 200 μm or more can be formed.

The Fe — N austenite phase is transformed into martensite (nitrogen martensite) supersaturated with nitrogen and solid-dissolved by quenching, and forms a hardened layer having high hardness and fatigue strength. Therefore, by quenching the nitrided iron-based sintered gold substrate, a hardened layer can be formed on the surface. The quenching temperature may be 640 to 800 ℃ as long as it is not lower than the austenitizing temperature, but may be set to a temperature lower than the nitriding temperature in order to reduce thermal strain. Therefore, the quenching temperature is preferably set to about 640 to 720 ℃, more preferably to 660 to 700 ℃, and the temperature in the furnace can be lowered to the quenching temperature after the nitriding treatment. In this case, from the viewpoint of suppressing thermal strain, it is desirable to avoid a rapid temperature drop, and the rate of temperature drop may be set to about 0.6 to 1.0 ℃/min, preferably about 0.8 ℃/min.

After the temperature in the furnace reaches the quenching temperature, the temperature is maintained at the quenching temperature for about 10 to 30 minutes, preferably about 20 minutes, so that the temperature of the iron-based sintered alloy substrate is equalized as a whole. Then, the supply of the nitriding gas is stopped, and quenching is performed using a quenching liquid or gas, whereby the surface layer portion is hardened by martensitic transformation of the austenite phase, and quenching is performed. The quenching liquid can be oil or water, and oil quenching using oil with the temperature of 40-150 ℃ is preferred. As the gas, an inert gas such as nitrogen or argon is preferably used. Cooling is carried out until the temperature of the iron-based sintered alloy substrate is less than or equal to 50 ℃.

The iron-based sintered alloy material obtained by quenching has a hardened layer exhibiting a martensite phase in which nitrogen is supersaturated and solid-dissolved on the surface. In the hardened layer on the surface, nitrogen is dissolved in solid solution and the concentration increases. Since the iron-based sintered alloy material is cooled from a quenching temperature lower than the carburizing and quenching temperature, the thermal strain is smaller than that of the carburizing and quenching material. When the iron-based sintered alloy material is tempered, the residual austenite can be transformed into martensite to stabilize the structure and impart flexibility while further removing stress. The tempering is suitably low temperature tempering capable of preventing embrittlement, and the tempering temperature may be set to about 100 to 200 ℃, preferably about 150 to 200 ℃. The heating time for tempering may be about 1 hour, and the tempering may be performed in any of an air (air) atmosphere, a nitrogen atmosphere, and a reducing atmosphere.

The iron-based sintered alloy material obtained as described above has increased hardness by forming a hardened layer on the surface by nitriding quenching, and contributes to improvement of the surface fatigue strength by forming the hardened layer to a depth of 100 μm or more. Specifically, an iron-based sintered alloy base material having a hardness (Vickers hardness) of about 100 to 350Hv is subjected to nitriding quenching, whereby the hardness at a distance of 0.1mm from the surface is increased to about 800Hv or more. By such case hardening, an iron-based sintered alloy material having improved wear resistance and reduced wear amount can be provided.

In the present invention, depending on the austenitizing temperature, the nitriding temperature is lower than the carburizing temperature, and the quenching may be performed at a low temperature. In the above-described manufacturing method, the thermal strain after quenching is further reduced by setting the quenching temperature to be lower than the nitriding temperature. Therefore, the thermal strain in the obtained iron-based sintered alloy material is halved as compared with the thermal strain at the time of carburizing and quenching, and therefore, the dimensional accuracy of the product can be significantly improved.

Thus, by nitriding quenching, a surface-hardened iron-based sintered alloy material can be produced with high dimensional accuracy, and an iron-based sintered alloy product excellent in surface fatigue strength and wear resistance can be provided which is suitable for machine parts and structural members. In machine parts and the like, depending on the application field, the required accuracy and quality may vary, and the iron-based sintered alloy material may be subjected to nitriding quenching after being subjected to machining such as shaping, coining, rolling, and the like as appropriate. Even if such processing is performed, pores remain in the densified surface layer, and nitriding proceeds. In this case, the porous iron-based sintered alloy material can be provided as a product in which the surface is densified. Alloy compositions (numerical values representing composition ratios are mass%) of excellent materials which can be provided by carrying out the present invention are exemplified below.

(Fe-C series sintered alloy)

Iron materials contain a trace amount of unavoidable impurities due to the manufacturing method, and carbon steels also contain a trace amount (less than 1%) of manganese and the like. Carbon steel, which is an iron-carbon alloy containing carbon in an amount of about 0.02 to 2%, has toughness and can be used for the production of automobile parts, structural parts for mechanical devices, and the like, but because of its low hardness, various parts having improved durability can be provided by performing surface hardening by nitriding quenching. The durability of an Fe-C sintered alloy having the same alloy composition as that of a carbon steel can be improved by surface hardening by nitriding quenching, and for example, by applying the technique of the present invention to a sintered alloy material having the same alloy composition as that of a carbon steel for machine structures (S45C in JIS), a carbon tool steel (SK 95 in JIS) having a carbon content of 0.45%, or the like, the carbon tool steel (SK 95 in JIS) having a carbon content of 0.9 to 1.0%, or the like, a machine part, a tool, or the like of an Fe-C sintered alloy having excellent durability can be provided.

(Fe-Cr-C series sintered alloy)

Chromium steels (SCr 435, SCr440, SCr445, etc. in JIS standard), stainless steels (SUS 420, etc. in JIS standard), high-carbon chromium bearing steels (SUJ 2 in JIS standard), etc. contain chromium in the range of 0.15 to 4.5%, carbon in the range of 0.2 to 1.0%, and manganese as an unavoidable impurity. Further, the chromium molybdenum steel (SCM 435, SCM440, etc. of JIS standard) contains about 0.9 to 1.2% of chromium, about 0.1 to 0.2% of molybdenum, about 0.35 to 0.5% of carbon, and inevitable impurities, and is a material having high strength, and thus can be used as a structural material. The effectiveness of nitriding quenching of Fe-Cr-C alloys is high, and the effect is improved if chromium nitride is dispersed in the hardened layer. Therefore, the durability of the Fe-Cr-C sintered alloy can be improved by surface hardening by nitriding quenching, and the technique of the present invention can be applied to a sintered alloy material having the same alloy composition as the steel material, thereby providing a machine part, a tool, or the like of the Fe-C sintered alloy having excellent durability.

(Fe-Cu-C sintered alloy)

The copper steel contains about 0.2 to 4.5% of copper, about 0.4 to 1.0% of carbon, and inevitable impurities. Can be used as general structural material. When the technique of the present invention is applied to a sintered alloy material having the same alloy composition as that of such a steel material, it is possible to provide a Fe — Cu — C-based sintered alloy having excellent durability as a general structural material or the like.

(Fe-Ni-Mo-C series sintered alloy)

The nickel-molybdenum steel contains 0.2 to 5.0% of nickel, 0.1 to 2.0% of molybdenum, 0.2 to 1.0% of carbon, and unavoidable impurities. The composition is a composition to which toughness and wear resistance are imparted by nickel and wear resistance is imparted by molybdenum. Since nickel and molybdenum improve hardenability and suppress softening during tempering, when the technique of the present invention is applied to a sintered alloy material having the same alloy composition as the steel material, the iron-based sintered alloy material having a hardened layer formed thereon exhibits very high hardness.

(Fe-Mn-Mo-C series sintered alloy)

The manganese-molybdenum steel contains 0.1-3.0% of manganese, 0.1-2.0% of molybdenum, and 0.2-1.0% of carbon, and has a high tensile strength.

The composition is one in which toughness and wear resistance are imparted by manganese and wear resistance is imparted by molybdenum. Since molybdenum suppresses softening during tempering, when the technique of the present invention is applied to a sintered alloy material having the same alloy composition as the steel material, the iron-based sintered alloy material having a hardened layer formed thereon exhibits a very high hardness.

Example 1

(sample 1)

Graphite powder was mixed with Fe-Cr-Mo-Mn alloy powder and uniformly mixed to prepare a powder having an overall composition (mass%) of Cr: 0.5%, Mo: 0.2%, Mn: 0.2%, C: 0.5% and the balance iron. Using this raw material powder, the following molding and sintering were performed.

A mold having a circular cavity with an outer diameter of 50mm, an inner diameter of 30mm and a length of 6mm was prepared, and a raw material powder was put into the cavity and compressed by a press to form a green compact with a density of 7.2Mg/m³To a degree of dust compaction. The compact was placed in a sintering furnace, heated to 1200 ℃ in a mixed gas atmosphere of 90% nitrogen and 10% hydrogen, and the temperature in the furnace was reduced after sintering for 60 minutes, to obtain an iron-based sintered gold substrate of sample 1. In addition, as for the density, the powder compact was immersed in the liquid rust preventive oil according to the archimedes method to measure the weight change at room temperature, and determined based on the obtained weight change.

(sample 2)

Copper powder, graphite powder and a forming lubricant were mixed with iron powder to prepare a powder composed of Cu: 1.5%, C: 0.6% and the balance iron, and using this raw material powder, a green compact was produced in the same manner as in sample 1, and this green compact was placed in a sintering furnace, heated to 1130 ℃ in a mixed gas atmosphere of 90% nitrogen and 10% hydrogen, and after sintering for 60 minutes, the furnace temperature was lowered to obtain an iron-based sintered gold substrate of sample 2.

(sample 3)

Nickel powder, graphite powder and a forming lubricant were mixed with Fe — Mo alloy powder to prepare a Fe — Mo alloy powder containing Mo: 1.5%, Ni: 2.0%, C: the same operation as in sample 1 was repeated except that 0.5% and the balance of raw material powder made of iron were used, and the iron-based sintered gold substrate of sample 3 was obtained.

(sample 4)

Fe-Mn alloy powder, copper powder, graphite powder and a forming lubricant are mixed into Fe-Mo alloy powder, and the mixture is prepared by mixing the following components in percentage by weight: 1.3%, Mo: 0.5%, Cu: 1.0%, C: the same operation as in sample 1 was repeated except that 0.5% and the balance of raw material powder made of iron were used, and the iron-based sintered gold substrate of sample 4 was obtained.

(nitriding quenching)

Each of the iron-based sintered alloy base materials of samples 1 to 4 was subjected to a size matching process (sample length after processing: 5.6mm), and then subjected to nitriding quenching and tempering as follows. In the following operations, the nitriding temperature was 780 ℃ (samples 1 to 3) or 740 ℃ (sample 4), the quenching temperature was 700 ℃, and the tempering temperature was 180 ℃.

After evacuation of the furnace of the hot wall type nitriding quenching furnace, nitrogen gas was supplied for pressure recovery, and the iron-based sintered alloy substrate was placed in the furnace, and after evacuation for 20 minutes, nitrogen gas was supplied for pressure recovery. The furnace is heated and the temperature is raised to the nitriding temperature for about 40 minutes. After reaching the nitriding temperature, the temperature was maintained and left for 20 minutes. Then, a mixed gas of ammonia gas and nitrogen gas (flow ratio: 1/2) was used as a nitriding gas, and the nitriding gas was started to be supplied and brought into contact with the iron-based sintered alloy substrate to carry out nitriding treatment. After nitriding treatment was continued for 180 minutes, the temperature in the furnace was lowered to the quenching temperature at a cooling rate of 0.8 ℃/minute, and the temperature was maintained for 20 minutes. Then, the supply of the nitriding gas was stopped, and the iron-based sintered alloy substrate was quenched using oil of 65 ℃ as a quenching liquid, thereby quenching the substrate.

Further, the iron-based sintered alloy material surface-hardened by quenching was tempered by heating at a tempering temperature for 60 minutes in a furnace in an atmospheric atmosphere, and then, the heating was stopped and the iron-based sintered alloy material was recovered by natural cooling.

(carburizing and quenching)

The same procedure as nitriding quenching was repeated except that a gas carburizing agent (gas containing carbon monoxide and hydrocarbon) was used instead of the gas for nitriding, the heating temperature was changed from the nitriding temperature to the carburizing temperature, and quenching was performed at the carburizing temperature without lowering the temperature after the carburizing treatment. The same tempering was performed to obtain an iron-based sintered alloy material obtained by carburizing and quenching the iron-based sintered alloy base material of samples 1 to 4. Note that the carburizing temperature was set to 850 ℃ (samples 1, 3, and 4) or 900 ℃ (sample 2).

(measurement of hardness)

For each of samples 1 to 4, the hardness (H) of the iron-based sintered alloy material after surface hardening was measured using a Rockwell hardness tester (ARK-F1000, manufactured by AKASHI Co., Ltd.)_RA) In that respect The measurement was performed at room temperature with a conical diamond indenter under a load of 60kgf (588N), and the value was obtained as an average value of 5-point measurements. Further, the hardness (Vickers hardness H) at a depth of 0.1mm from the surface of the cross section of the iron-based sintered alloy material subjected to the etching treatment with a 5% nital etchant was measured (load: 0.98N) using a micro hardness measuring apparatus (HM-200, manufactured by Sanfeng Co., Ltd.)_v) The value was obtained as an average of the 5-point measurement values. The results are shown in Table 1.

[ Table 1]

As is clear from table 1, in any of samples 1 to 4, the hardness at a depth of 1mm in the iron-based sintered alloy base material obtained by nitriding quenching was significantly improved as compared with the case of carburizing quenching, and the case hardening was appropriately completed.

Further, in sample 2, the hardness at a depth of 1.0mm from the surface was measured in the cross section of the iron-based sintered alloy material, and the result was 700H at the time of nitriding quenching_v610H in carburizing and quenching_v. From this, it is considered that the penetration diffusion of nitrogen upon nitriding quenching reaches a depth of approximately 1 mm.

Example 2

A mold having a cavity for forming sprocket teeth for a variable phase system having a roller outer bus diameter of 94.425mm was prepared. The alloy powder is mixed with Fe-Mo-Ni alloy powder, graphite powder and a forming lubricant to prepare a composite material (mass percent) consisting of Mo: 0.55%, Ni: 0.55%, C: a mixed powder of 0.25% and the balance of iron and inevitable impurities was used as a raw material powder, and a base material made of an iron-based sintered alloy having a sprocket tooth shape was produced in the same manner as in example 1. Then, the teeth are subjected to rolling treatment to densify the outermost surface of the teeth.

Using the above base material, nitriding quenching or carburizing quenching was performed in the same manner as in example 1 to obtain an iron-based sintered alloy material having a sprocket tooth shape with a hardened layer formed on the surface. Wherein the nitriding temperature is set to 700 ℃ and the carburizing temperature is set to 900 ℃.

Further, the above base material was subjected to carbonitriding quenching to obtain an iron-based sintered alloy material having a sprocket tooth shape with a hardened layer formed on the surface thereof. Carbonitriding quenching is performed by repeating the same operation as the above-described carbonitriding quenching except that a gaseous carburizing agent containing ammonia gas is used as an atmospheric gas for carbonitriding in place of the gaseous carburizing agent in the above-described carbonitriding quenching, and the heating temperature is changed from the carburizing temperature to the carbonitriding temperature (780 ℃).

The metal structures of the cross sections of the 3 alloy materials obtained above were photographed, and the obtained SEM images are shown in fig. 1. In fig. 1, (a) is an SEM image obtained by carburizing and quenching, (b) is an SEM image obtained by carburizing and nitriding quenching, and (c) is an SEM image obtained by nitriding and quenching. Further, the hardness at 0.1mm from the surface of the cross section of the alloy material was measured, and the hardness at 0.1mm from the surface was 680H_v(carburizing and quenching) 680H_v(carbonitriding quenching) 700H_v(nitriding quenching). Further, 7.0g/cm was measured by a three-ball pitting test³Fatigue strength of face pressure (temperature: room temperature, rotation speed: 600 min)^-1And (3) using oil: MTF-III, ball material: SUJ-2), the surface compressive fatigue strengths were 2.35GPa (carburizing and quenching), 2.35GPa (carburizing and nitriding quenching), and 2.40GPa (nitriding and quenching).

Further, the strain of the sprocket tooth shape was evaluated based on the strain measurement as the ellipse amount obtained by the strain analysis, and the average values of the ellipse amounts were 156 μm (carburizing and quenching), 119 μm (carburizing and nitriding and quenching), and 60 μm (nitriding and quenching). It is understood that the strain due to nitriding quenching is reduced to about 40% of the strain at the time of carburizing and quenching.

The disclosure of the present application is related to the subject matter described in japanese patent application No. 2017-217064, filed on 11/10/2017, the entire disclosure of which is incorporated herein by reference.

It should be noted that various modifications and changes may be made to the above-described embodiments in addition to those already described without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and changes are intended to be included within the scope of the appended claims.

Industrial applicability of the invention

Since a hardened layer obtained by nitriding can be formed at an appropriate depth on the surface of an iron-based sintered alloy, and a sintered member having excellent hardness, wear resistance, and surface fatigue strength can be provided with high dimensional accuracy, the sintered alloy is suitable for various machine parts requiring durability, such as sprockets, gears, rollers, and shafts of motors, and can contribute to the spread of products by improving quality and reducing manufacturing cost.

Claims

1. An iron-based sintered alloy material contains carbon and has a hardened layer exhibiting a martensite phase in which nitrogen is supersaturated and solid-dissolved on the surface.

2. The iron-based sintered alloy material according to claim 1, which contains 0.1 to 1.0 mass% of carbon.

3. The iron-based sintered alloy material according to claim 1 or 2, further containing one or more alloying components selected from the group consisting of chromium, copper, molybdenum, manganese, and nickel.

4. The iron-based sintered alloy material according to claim 1 or 2, further comprising at least one alloying component selected from the group consisting of 0.15 to 4.5 mass% of chromium, 0.2 to 4.5 mass% of copper, 0.1 to 2.0 mass% of molybdenum, 0.1 to 3.0 mass% of manganese, and 0.2 to 4.5 mass% of nickel.

5. The iron-based sintered alloy material according to any one of claims 1 to 4, wherein a depth of the hardened layer from the surface is 100 μm or more.

6. A method for manufacturing an iron-based sintered alloy material,

an iron-based mixed powder containing a carbon powder is formed into a compact of a desired shape,

heating the green compact to 1000-1300 ℃ in a non-oxidizing environment to sinter the green compact to obtain an iron-based sintered alloy substrate,

heating the iron-based sintered alloy base material to a nitriding temperature of 590 ℃ or higher in an atmosphere containing ammonia gas to perform nitriding treatment,

quenching the iron-based sintered alloy base material subjected to the nitriding treatment by rapid cooling.

7. The method for producing an iron-based sintered alloy material according to claim 6, wherein the quenching is performed at a quenching temperature lower than the nitriding temperature.

8. The method for producing an iron-based sintered alloy material according to claim 6 or 7, further heating the alloy material to 100 to 200 ℃ after the quenching, and tempering the alloy material.

9. The method for producing an iron-based sintered alloy material according to any one of claims 6 to 8, wherein the carbon powder contained in the iron-based mixed powder is 0.1 to 1.2 mass% of graphite powder.

10. The method for producing an iron-based sintered alloy material according to any one of claims 6 to 9, wherein the iron-based mixed powder further contains one or more alloying components selected from the group consisting of chromium, copper, molybdenum, manganese, and nickel.

11. The method for producing an iron-based sintered alloy material according to any one of claims 6 to 9, wherein the iron-based mixed powder further contains at least one alloying component selected from the group consisting of 0.15 to 4.5 mass% of chromium, 0.2 to 4.5 mass% of copper, 0.1 to 2.0 mass% of molybdenum, 0.1 to 3.0 mass% of manganese, and 0.2 to 4.5 mass% of nickel.