CN100455698C - Surface carbonitrided stainless steel part excellent in wear resistance and manufacturing method thereof - Google Patents

Abstract

The present invention is a stainless steel part. The stainless steel part is a part hardened by carbonitriding the surface of austenitic stainless steel containing 3 to 20% by mass of Mn. The Vickers hardness of the surface hardness is 1350HV or The depth of the hardened part above that and 1000HV or more from the surface is 10 μm or more, especially when it is applied to parts that must have sliding and wear resistance, the durability time can be greatly improved. In addition, its manufacturing method is only heating in a gas atmosphere, and a large number of parts can be processed at the same time. Accordingly, it can be applied in a wide range of fields as stainless steel parts requiring wear resistance.

Description

Surface carbonitrided stainless steel part excellent in wear resistance and manufacturing method thereof

technical field

The present invention relates to austenitic stainless steel components case-hardened for improved wear resistance and methods for their manufacture.

Background technique

Austenitic stainless steels are effectively used in food machinery, chemical-related machinery and plants, automobile engines, and various other fields requiring corrosion resistance. Among them, there are many uses that simultaneously require wear resistance such as sliding mechanical parts such as shafts, valves, and gears. In order to improve such wear resistance, hardening by heat treatment such as quenching and surface hardening such as carburizing or carbonitriding are often used for carbon steel, alloy steel, or tool steel for machine structures.

In the case of austenitic stainless steel, hardening cannot be achieved by quenching, and sufficient hardening cannot be obtained by carburizing. In particular, when the surface is hardened, wet plating such as hard chromium plating and hardening by PVD (physical vapor deposition) are performed. Coating of the texture layer or surface nitriding, etc. However, coatings such as plating and PVD have a problem of difficulty in adhesion to the substrate of the coating, and it is difficult to say that they can be used stably enough especially when the surface pressure is high.

On the contrary, surface nitriding treatment is a method of hardening by infiltrating nitrogen from the surface. Since there is an oxide film in stainless steel containing a lot of chromium, it is generally difficult to nitriding treatment. However, hydrochloric acid treatment and halide treatment are used. Even nitriding becomes easy by ion nitriding treatment and the like, and is often used as a surface hardening method for austenitic stainless steel.

Surface hardening of austenitic stainless steel can also improve fatigue strength, but it is more necessary for the purpose of improving wear resistance. Improvement in wear resistance can suppress wear of sliding surfaces in sliding parts, improve durability, and also have the effect of reducing wear of grinding and cutting tools or suppressing surface damage of stainless steel parts.

Nitriding treatment does not require quenching like carburizing treatment, and the surface can be hardened by treatment at a relatively low temperature. However, there is an optimum temperature for high surface hardness. If the hardened layer is thickened at this temperature, it will take a long time for nitriding. If the temperature is raised, the depth of hardening can be increased, but the obtained surface hardness is low.

In addition, in sliding parts and the like, in order to improve wear resistance, the higher the surface hardness, the better, but when the hardness is increased and the temperature is lowered, a hard layer such as a compound layer is formed on the surface layer. Also, this layer is fragile and does not necessarily improve abrasion resistance.

Thus, although the method by nitriding is important as a surface hardening method for austenitic stainless steel, it cannot necessarily be said that it can be sufficiently satisfied.

Contents of the invention

An object of the present invention is to provide an austenitic stainless steel part having a high surface hardness and a sufficient hardness in the part immediately below, which is used for a sliding part and the like, and a manufacturing method thereof.

The present inventors conducted various studies on improving the properties of case-hardened austenitic stainless steel by gas nitriding. As the surface nitriding method of stainless steel, there is also the ion nitriding method, but since this method is carried out under reduced pressure, the processing speed is slow and the shape is also limited. On the contrary, it can be considered that gas nitriding can process a large amount at one time and is suitable for mass production.

Furthermore, steels with various compositions related to the surface hardening by gas nitriding of austenitic stainless steels were prepared, and various influences of composition and treatment conditions on the surface hardness and hardening depth by gas nitriding were studied.

In this case, use a furnace that can isolate the air, introduce a fluoride-containing gas containing 10 volume % NF ₃ and the rest is N ₂ , keep it for 30 minutes to activate the surface, and then inject a nitriding gas containing NH ₃ to change various Temperature and time, carry out nitriding treatment.

Generally, the surface hardness of nitriding steel is about 1200-1300HV, and a hard compound layer is formed on the surface according to the treatment conditions, but it cannot be used because it is brittle. However, in investigations using such austenitic stainless steel, it was found that the Vickers hardness of the surface hardness exceeded 1350HV.

It can be confirmed that this hard surface layer has sufficient toughness instead of the brittle compound layer conventionally obtained in steel for machine structural use and ferritic stainless steel, etc. In addition, it was prepared as a sample for wear test, and the wear resistance was investigated and observed. Sex, very good.

Therefore, when the hard surface steel parts obtained in this way were examined and observed in detail, the following findings were found.

(a) The compound layer can be seen on the surface layer.

(b) is an austenitic stainless steel containing a lot of Mn.

(c) The atmospheric gas used for the nitriding treatment contains carburizing gas such as RX gas in addition to NH ₃ .

In the case of steel for machine structural use and ferritic stainless steel, a compound layer may also appear on the surface layer depending on the processing conditions. This compound layer is considered to be a layer formed by nitriding Fe, Cr, etc. due to the progress of nitriding, an increase in the concentration of active nitrogen generated by decomposition of NH ₃ on the surface.

However, in the case of austenitic stainless steel, such a compound layer does not usually appear. This is considered to be because the solubility of nitrogen in the austenite phase is much higher than that in the ferrite phase, and the surface concentration of nitrogen is difficult to increase due to the diffusion of nitrogen into the interior of the steel, so a compound layer is not formed.

In addition, in the case of austenitic stainless steel containing a lot of Mn, a compound layer appears when treated in a carburizing atmosphere other than nitriding, and the sample treated in this way exhibits excellent wear resistance.

The reason why such results are obtained is not necessarily clear, but when considered together with the facts of (a), (b) and (c) above, the following assumptions can be made.

First, hardening by nitriding treatment performed at a lower temperature than normal carburizing can be obtained by the formation of fine nitride precipitates and the increase of solid-solution nitrogen. In the case of mechanical structural steel and ferritic stainless steel, since it is a ferrite phase, the solubility of nitrogen is small, and although it is easy to form a compound layer, the concentration of nitrogen in the part directly below it cannot be increased, so the hardness decreases. The difference in hardness is large.

Therefore, since the hard and brittle compound layer cannot be fully maintained, and the hard compound layer is easily broken under a small stress, only the brittleness is prominent, and the hard compound layer cannot be fully utilized.

On the contrary, in the case of austenitic stainless steel, the solubility of nitrogen in the austenite phase is much larger than that in the ferrite phase. And it can be considered that the presence of a compound layer in austenitic steel containing more Mn may be due to less Ni in one aspect.

The purpose of containing a lot of Mn in austenitic stainless steel is to suppress the use of high-priced Ni. When there is a lot of Mn, the Ni content must be low. Ni generally hinders nitriding, and since there is little Ni, infiltration of nitrogen and infiltration of carbon become easy. Therefore, it is presumed that the nitrogen concentration near the surface during nitriding is much higher than that in the case of containing less Mn and containing more Ni.

In addition, when there is a carburizing gas containing CO and CH ₄ such as RX gas in the nitriding atmosphere, carburizing also proceeds at the same time, and the nitrogen dissolved in the steel is different from when the concentration increases due to the presence of solid solution carbon. With the same effect, compounds are easily formed on the surface. Furthermore, the portion immediately below the compound layer of the austenite phase contains more solid-dissolved nitrogen and solid-dissolved carbon than in the case of the ferrite phase because of its high solubility.

In this way, because it is an austenite phase, the solid solution concentration of nitrogen is high, and because Ni is low, the infiltration of nitrogen can be actively carried out, and the compound layer is easily formed through the infiltration of carbon above it. In addition, the solid solution concentration is high, and more Fine carbides and nitrides are formed, whereby the hardness of the portion directly under the compound can be greatly increased.

It is considered that the compound layer can supplement its brittleness by maintaining the lower layer with sufficient strength, and as a result, it may become a surface reinforcement layer excellent in wear resistance. In order to improve the wear resistance, it is important that the surface hardness is high and that there is a hardened layer of an appropriate thickness with an intermediate hardness between the substrate and the hard layer on the surface.

Furthermore, various investigations on the composition of the steel, nitriding treatment conditions, various characteristics of the obtained surface-hardened parts, etc. were further carried out for austenitic stainless steel containing a large amount of Mn. Based on the results of the investigation, the limit of which such effects can be obtained has been clarified, and the present invention has been completed. The gist of the present invention is as follows.

(1) A surface carbonitrided stainless steel part excellent in wear resistance is a part obtained by carbonitriding and hardening the surface of austenitic stainless steel containing 3 to 20% by mass of Mn, wherein the surface The Vickers hardness is 1350 HV or more, and the depth of the hardened layer as 1000 HV or more is 10 μm or more.

(2) A method for producing a surface carbonitrided stainless steel part excellent in wear resistance according to the above (1), characterized in that an austenitic stainless steel part containing 3 to 20% by mass of Mn formed into a desired shape is formed in the After surface activation treatment in an atmosphere containing halogen gas or halide gas, carbonitriding treatment at 430-600°C is carried out in an atmosphere containing NH ₃ and carburizing gas.

Detailed ways

The steel used as the component of the present invention is austenitic stainless steel containing 3 to 20% by mass of Mn. The reason why the content of Mn is made 3% by mass or more is that in the case of austenitic steel, when the Mn is low, the Ni content increases, and the case hardening hardness by nitriding cannot be sufficiently high. In addition, since the corrosion resistance of austenitic stainless steel is poor when the Mn content is large, it can only be taken up to 20% by mass even if it is large.

Such steel includes, for example, SUS201, SUS202, SUS304J2, SUH35, SUH36, etc. of JIS standards. Compositions other than Mn are not particularly limited as long as they fall within the scope of austenitic stainless steel. Since sufficient surface hardness may not be obtained, it is preferable that the Ni content is smaller than the Mn content if possible.

The surface hardness after carbonitriding is taken to be 1350HV or above. This is because in the case of less than 1350HV, sufficiently high wear resistance cannot be obtained. Also together with the surface hardness, the depth of the hardened layer whose hardness is 1000 HV or more is taken to be 10 μm or more.

This is because, when the hardness of the hardened layer directly below the surface compound layer is lower than 1000HV and the depth of the hardened layer is lower than 10μm, in either case, not only the hardness of the surface will be lower than 1350HV, but also the hardness of the surface will be reduced. The compound layer becomes brittle and wear resistance becomes poor.

Austenitic stainless steel parts whose surface is hardened to the above-mentioned state cannot be obtained only by nitriding treatment, and must be produced by carbonitriding treatment. For surface nitriding of austenitic stainless steel, there is a method of heating the surface in an atmosphere containing halogen gas or halide gas to activate the surface, and then introducing a nitriding gas containing NH ₃ , but in the present invention, the halogen Or the halide method is used as the basis for carbonitriding treatment.

First, using a sealable heating container, for example, in an atmosphere gas containing 0.5 to 20% by volume of a halogen gas such as F ₂ , Cl ₂ , HCl or NF ₃ or a halide gas, and the rest being nitrogen, hydrogen or an inert gas, etc., The surface is activated by heating at 200 to 550° C. for 10 minutes to 3 hours.

After surface activation, an atmosphere gas containing a mixed gas of NH ₃ for nitriding and CO or CH ₄ for carbonization is formed, and carbon nitrogen is carried out by heating in the temperature range of 430 to 600°C for 20 minutes or more processing.

The atmospheric gas for the carbonitriding treatment contains 10 to 95% by volume of NH ₃ , and 5 to 30% by volume of either or both of CO or CH ₄ . The reason why NH ₃ is set to 10 volume % or more is because if it is smaller than this value, nitriding cannot be sufficiently performed and a hardened layer cannot be obtained. For the purpose of only nitriding, NH ₃ can also be set to 100 volume % , but because carbonizing gas must be used sometimes, even if there is more, it can only be taken up to 95% by volume.

For carbonization, one or both of CO and CH ₄ must be 5% by volume or more. However, since gas is generated when the ratio of this gas is too high, even if it is large, it can only be taken up to 30% by volume.

The atmospheric gas for carbonitriding treatment is not particularly limited as long as it is sufficient for nitriding and carbonizing, contains NH ₃ , CO, and CH ₄ as mentioned above, and other components are inert gases, hydrogen, nitrogen, or other hydrocarbon gases. . In addition, for example, a gas satisfying the above-mentioned composition range, such as mixing NH ₃ and RX gas, may be used, or a gas for carbonitriding treatment may be a mixture of NH ₃ and a conventionally used carburizing gas.

When the carbonitriding temperature is lower than 430° C., the surface hardness of 1350 HV or higher cannot be obtained, and generation of a hardened layer of 1000 HV or higher is insufficient. This is due to the progress of nitriding, but the insufficient progress of carbonization. When the temperature is 430°C or higher, the surface hardness and hardened layer can be obtained, but when the temperature exceeds 600°C, not only the surface hardness exceeding 1350HV cannot be obtained, but also the corrosion resistance as stainless steel decreases.

In addition, when the carbonitriding treatment time is less than 20 minutes, the compound layer on the surface cannot be obtained, and the surface hardness of 1350HV or more cannot be obtained. The treatment time is not particularly limited as long as it is 20 minutes or more, and if the time is longer, the thickness of the hardened layer having a hardness of 1000 HV or more becomes thick. However, since the wear resistance is hardly improved but the corrosion resistance is deteriorated, it is preferable to set it to 50 hours even if the time is long.

As specific examples of wear-resistant stainless steel parts that are effective when the present invention is applied, engine valves, compressor shafts, compressor blades, piston rings, bearing balls, micro motor shafts, motor shafts, etc. are examples of sliding mechanical parts. Examples of fluid wear-resistant parts include mesh filters, nozzles, valves, piping joints, pressure reducing valves, pumps, and the like. In addition, there are bolts, nuts, wood screws, and self-tapping screws as connection parts, and there are dressing tools, cutting saws, wire saws, saws, and drills among tools, and they can also be applied to extrusion dies, die-casting dies, and injection molding. metal mold.

(Example)

Stainless steel with the composition shown in Table 1 was used for cutting to produce a disc-shaped sample with a diameter of 35 mm and a thickness of 10 mm. When it is used as a rotating sample for the Armsra wear test, the peripheral surface of the disc is mirror-polished to remove the edges. This sample was heated to 300°C, then heated and held in an atmosphere gas containing NF ₃ , and subjected to nitriding or carbonitriding treatment to harden the surface. Table 2 shows the atmosphere gas, temperature, and treatment time during surface nitriding.

For the sample after surface hardening, the surface hardness is measured with a Vickers hardness (HV0.1) with a test force of 0.9806N, and the hardness distribution of the cross-section is measured with a Vickers hardness (HV0.05) with a test force of 0.4903N. The dents after the surface hardness measurement were observed with a 100-magnification optical microscope, and when defects and cracks were recognizable, it was judged as poor brittleness.

The Amsra wear test is a two-cylinder type rolling wear test. A load of 150 kg is applied to a cylindrical metal (SKH52) surface with a diameter of 35 mm and 50 mm, and the above-mentioned sample is rotated so that the sliding parts are in the same direction. For the circumferential surface, under the condition of no lubrication, the sliding speed was taken as 0.12m/sec, and the specific wear amount [mg/(m·sec)] was obtained.

Table 1

^* indicates that it is outside the scope specified by the present invention.

The results are shown together in Table 2. The samples of Test Nos. 1 to 3, which used austenitic stainless steel with sufficiently high Mn and were subjected to carbonitriding as the surface hardening treatment, showed low and excellent specific wear, and it can be estimated that This is due to having a high surface hardness exceeding 1400HV and a sufficiently thick hardened layer depth of 1000HV or more.

Table 2

^* indicates that it is outside the scope specified by the present invention.

In contrast, in Test No. 6, even austenitic stainless steel with a high Mn content had insufficient surface hardness and was brittle because the surface hardening treatment was not carbonitriding. The treatment temperature of Test No. 5 was low, 1000HV or The depth of hardening above it is as shallow as 7 μm. In addition, in Test No. 4, even though the surface hardening treatment was carbonitrided, the surface hardness was lower than 1300 HV because the steel had a low Mn content. In the case of the test numbers 4 to 6, all of the specific wear amounts were large, showing results inferior to those of the test numbers 1 to 3.

As described above, for various mechanical parts using austenitic stainless steel, especially by applying to parts that must have sliding and wear resistance, the durability time can be greatly improved.

According to the surface carbonitrided stainless steel part of the present invention and its manufacturing method, by making the Vickers hardness of the surface hardness 1350HV or more and making the depth of the hardened part at 1000HV or more from the surface to be 10 μm or more, especially It is suitable for parts that must have sliding and wear resistance, and can greatly increase its durability time. In addition, its manufacturing method is only heating in a gas atmosphere, so a large number of parts can be processed at the same time. Therefore, it can be used as stainless steel parts requiring wear resistance, specifically, sliding machine parts, fluid wear-resistant parts, connecting parts, and tools, and can be applied to a wide range of fields.

Claims (2)

1. A surface carbonitrided stainless steel part excellent in wear resistance is a part made by carbonitriding and hardening the surface of an austenitic stainless steel containing 3 to 20% by mass of Mn, wherein the hardness of the surface is The Vickers hardness is 1350 HV or more, and the depth of the hardened portion of 1000 HV or more from the surface is 10 μm or more. 2. A method of manufacturing a surface carbonitrided stainless steel part having excellent wear resistance according to claim 1, wherein the austenitic stainless steel part containing 3 to 20% by mass of Mn formed into a desired shape After the surface activation treatment is carried out in the atmosphere gas containing halogen gas or halide gas, in the atmosphere gas containing 10-95 volume % NH ₃ and the rest is carburizing gas, carry out 20 minutes to 50 hours at 430-600 ° C carbonitriding treatment.