JP3685322B2 - Method for nitriding maraging steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitriding method for maraging steel.
[0002]
[Prior art]
Conventionally, nitriding treatment is known as a method for improving the fatigue strength of steel. However, steel containing a lot of alloy elements has a stable surface state, and it has been difficult to perform uniform nitriding treatment. 18% Ni maraging steel and precipitation hardening stainless steel (17-4PH) used as tough steels have strengths close to the highest as steel, but further improvement in strength is desired for the purpose of weight reduction. Rarely, the strength of these tough steels is expected to be improved by nitriding as a method. However, in order to strengthen the toughened steel, it contains many alloy elements, and the surface is a dense oxide film composed of various alloy elements. Since it is covered, it is difficult to perform uniform nitriding.
[0003]
Conventionally, as a nitriding method for maraging steel, for example, a method of removing an oxide film using hydrogen chloride or phosphoric acid, a method of performing a nitronitriding treatment in a nitriding atmosphere to which hydrogen sulfide is added, and a fluorine compound are used. A method of performing nitriding after reducing the oxide film on the surface, a method of performing nitriding in a salt bath, and the like have been proposed to improve fatigue strength.
[0004]
[Problems to be solved by the invention]
Although each of the above proposals has improved fatigue strength, problems remain in quality stability, workability, economy, and pollution control. For example, a method of removing an oxide film using hydrogen chloride or phosphoric acid has a quality problem that it is difficult to obtain a uniform nitride layer, and a method of performing nitrosulphurizing treatment in a nitriding atmosphere to which hydrogen sulfide is added is However, because it uses highly toxic hydrogen sulfide, there is a problem in workability such as handling and management. The method of nitriding after reducing the oxide film on the surface with a fluorine compound is applied to the surface of maraging steel. There are problems such as the remaining of fluoride and the cost of nitriding after the fluorination treatment, and the cost is increased. The method of nitriding treatment in a salt bath uses wastewater treatment because it uses cyanide. Problems remained in pollution measures and workability.
[0005]
In addition, maraging steel is a precipitation hardening type alloy steel, and the aging effect progresses simultaneously with the nitriding treatment, and the mechanical properties change. It is necessary to confirm the selection of conditions.
[0006]
In view of the above circumstances, an object of the present invention is to improve quality stability, workability, economy, etc. in nitriding treatment of maraging steel, and at the same time improve fatigue strength and extend fatigue life.
[0007]
[Means for solving the problems]
In view of the above circumstances, Claim 1 of the present invention is that, in an atmosphere gas in which an organic chloride is added to a nitriding atmosphere, the treatment temperature is equal to or lower than the aging precipitation temperature of the workpiece, and the soaking time is 15 to 60 minutes. This is a nitriding method of maraging steel that performs nitriding within the range.
[0008]
According to the first aspect, the organic chloride is vaporized in the furnace and decomposed into hydrogen chloride and carbon, and the hydrogen chloride generated by the decomposition removes the object to be treated, that is, the oxide film of the maraging steel. The surface is activated, NH ₃ gas in the atmosphere is adsorbed on the activated maraging steel surface, the nitriding reaction is promoted, the hardness is increased, and a deep nitrided layer is obtained. In addition, it is possible to provide a nitrided article that has improved fatigue strength against tensile, bending, and unbending stress, and has extended fatigue life.
[0009]
In the present invention, NH ₃ gas, NH ₃ gas + N ₂ gas, NH ₃ gas + RX gas, or the like is used as the nitriding atmosphere.
[0012]
According to one embodiment of the present invention, in an atmosphere gas in which an organic chloride is added to a nitriding atmosphere, the treatment temperature is equal to or lower than the aging precipitation temperature of the object to be treated, that is, maraging steel, and the soaking time is 15 This is a maraging steel nitriding method in which nitriding is performed within a range of ˜60 minutes without forming a fragile white compound layer on the surface of the workpiece.
[0013]
According to the second aspect , the increase in hardness and deep nitrided layer obtained by the accelerated nitriding reaction improves the fatigue strength against tensile, bending and unbending stresses and extends the fatigue life. By not forming a fragile compound layer (white layer) on the surface of the object, it is possible to provide a nitrided product that further improves fatigue strength and further extends the fatigue life.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described below.
[0018]
(Example)
The maraging steel was subjected to solution treatment at 800 to 900 ° C., and then subjected to aging treatment at 480 ° C. for 3 hours. This solution treatment and aging treatment are preferably performed in vacuum (about 1.33 × 10 ⁻¹ Pa≈10 ⁻³ Torr) in order to prevent oxidation of the maraging steel surface. Subsequently, the maraging steel that had undergone the solution treatment and the aging treatment was subjected to nitriding treatment in a nitriding furnace heated to 450 ° C. below the aging precipitation temperature.
[0019]
The nitriding treatment is performed as a nitriding atmosphere with a mixed gas of NH ₃ gas 40% + N ₂ gas 60%, and after the maraging steel is charged into the nitriding furnace, the furnace temperature returns to 450 ° C. As a film removing agent, methylene chloride, an organic chloride, was added into the furnace at a rate of 90 ml per 1 m ³ of the furnace volume.
[0020]
The soaking time at 450 ° C. was carried out in the range of 15 to 120 minutes, and after soaking, it was transferred to a cooling chamber and cooled in an N ₂ gas atmosphere.
[0021]
(Comparative example)
Unlike the embodiment of the present invention, the nitriding atmosphere, the soaking temperature, the soaking time, and the cooling step are nitrided in the same manner as in the above embodiment of the present invention without adding organic chloride methylene chloride in the furnace. Processed.
[0022]
The result of measuring the cross-sectional hardness of the maraging steel processed by the Example of the present invention and the comparative example is shown in FIG. According to FIG. 1, in the case of the comparative example, it was confirmed that the hardness on the surface was slightly increased by the nitriding treatment, but a nitride layer having a sufficient depth could not be obtained.
[0023]
In contrast to the comparative example, in the case of the example of the present invention, the hardness at the surface increased to 839 Hv, and the depth of the nitrided layer reached 0.04 mm.
[0024]
In the embodiment of the present invention, as described above, the addition of methylene chloride, an organic chloride, vaporizes the added methylene chloride in the furnace and decomposes it into hydrogen chloride and carbon. The surface is activated by removing the oxide film, and NH ₃ gas in the atmospheric gas is adsorbed on the activated maraging steel surface to promote the nitriding reaction.
[0025]
When the amount of methylene chloride added is less than 30 ml per 1 m ^{3 of} the furnace volume, the surface of the maraging steel is not sufficiently activated, causing uneven nitriding and nitriding loss, and If the amount of methylene added exceeds 100 ml per 1 m ^{3 of the} furnace volume, the amount of free carbon in the atmosphere increases and soot adheres to the surface of the maraging steel and the furnace, so the amount of methylene chloride added is It was confirmed that 30 to 100 ml per 1 m ^{3 of} the furnace volume was appropriate.
[0026]
Next, in the examples of the present invention, FIG. 3 shows the fatigue test results of maraging steel when the soaking time of the nitriding treatment is within 15 to 60 minutes and when it exceeds 60 minutes. . The fatigue test was conducted for tensile strength, bending and unbending strength using an endless belt made of maraging steel and using a belt pulley type fatigue tester shown in FIG.
[0027]
That is, in FIG. 2, 1 is a main pulley, 2 is a driven pulley, the endless belt 3 is stretched between the driven pulley 2 and the main pulley 1, and the tensile load applied to each belt is set to 4, The number of revolutions at which each belt causes fatigue failure is evaluated by adjusting 903N (= 500 kgf) and rotating the main pulley 1 at 1,000 revolutions per minute. As reference data, the fatigue life of an endless belt not subjected to nitriding was plotted with a soaking time of 0 minutes.
[0028]
Considering the data of FIG. 3, the fatigue life of the endless belt is not subjected to nitriding treatment, whereas a which was 10 ⁴ times, the fatigue life of the endless belt which has been subjected to nitriding treatment in the scope of the embodiments of the present invention Was 10 ⁶ to 10 ⁷ times, and it was found that the fatigue life was dramatically improved. However, it has been found that when the soaking time is extended as compared with the embodiment of the present invention, the fatigue life is decreased.
[0029]
FIG. 4 shows the relationship between the processing condition range of the embodiment of the present invention and the nitride layer depth. That is, in the range of the embodiment of the present invention, the depth of the nitrided layer was 20 to 38 μm, and it was found that 20 to 45% of the nitrided layer was obtained with respect to 1/2 of the thickness of the endless belt. did.
[0030]
Considering the result of the nitrided layer obtained in the embodiment of the present invention together with the relationship between the soaking time and the fatigue life of FIG. 3, the depth of the nitrided layer with respect to 1/2 of the thickness of the endless belt is It was confirmed that time management so as not to exceed the range of 20 to 45% is an effective means for maximizing the fatigue life of the endless belt.
[0031]
That is, it was confirmed that when the depth of the nitride layer is shallower than 20%, the fatigue life of the endless belt tends to vary, and when it is deeper than 45%, the fatigue life itself is shortened.
[0032]
FIG. 5 shows the relationship between the processing condition range of the embodiment of the present invention and the presence or absence of white layer formation. That is, it was found that only the nitride layer can be obtained without forming a white layer (fragile compound layer) on the surface of the object to be processed within the scope of the examples of the present invention.
[0033]
Considering the result of the presence or absence of the white layer formation obtained in the embodiment of the present invention together with the relationship between the soaking time and the fatigue life of FIG. 3, the fatigue life is reduced by the formation of the white layer, It was confirmed that not forming the layer is an effective means for improving the fatigue life of the endless belt.
[0034]
Further, in the examples of the present invention, the nitriding treatment of the maraging steel has been described. However, it can be applied to the nitriding treatment of a tough steel such as a precipitation hardening stainless steel other than the maraging steel. Has been confirmed experimentally.
[0035]
【The invention's effect】
According to the nitriding method of the present invention, it is possible to effectively perform nitriding treatment of maraging steel, improving the stability of quality, workability, economy, etc. in nitriding treatment of the maraging steel, and pulling In addition, the fatigue strength against bending and unbending stress can be improved, and the life of the fatigue life can be extended.
[Brief description of the drawings]
FIG. 1 is a cross-sectional hardness measurement diagram of an example of the present invention and a comparative example.
FIG. 2 is a principle diagram of a belt pulley type fatigue tester.
FIG. 3 is a measurement diagram showing test results of a burning time and a fatigue life.
FIG. 4 is an explanatory diagram showing the relationship between the heating time and the depth of a nitride layer in an embodiment of the present invention.
FIG. 5 is an explanatory diagram showing a relationship between a processing condition range and whether or not a white layer is formed in an embodiment of the present invention.

Claims (2)

Nitriding of maraging steel characterized in that nitriding treatment is performed in an atmosphere gas in which organic chloride is added to a nitriding atmosphere, with the treatment temperature being equal to or lower than the aging precipitation temperature and the soaking time within a range of 15 to 60 minutes. Processing method. A brittle white compound layer is formed on the surface of the object to be processed in a nitriding atmosphere with an organic chloride added and the processing temperature not higher than the aging precipitation temperature and the soaking time within the range of 15 to 60 minutes. A nitriding treatment method for maraging steel, characterized in that the nitriding treatment is performed without the nitriding treatment.

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