JPH04268015A - Treatment of surface on steel parts - Google Patents

Abstract

PURPOSE:To secure structural stability on surface treating layer without lowering productivity and to improve stability of performance. CONSTITUTION:Gas softening nitriding treatment is executed at eutectoid temp. or higher of Fe-N and successively, isothermal treatment is executed at the above eutectoid temp. or lower and >=450 deg.C. After that, by cooling this to the room temp., Fe-N eutectoid layer formed at between surface nitride layer and matrix, is structurally stabilized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for surface treatment, particularly nitrocarburizing, of steel parts.

0002

[Prior Art] For example, a viscous plate (inner plate, outer plate), which is a component of a viscous joint,
Metal-to-metal contact has come to occur in the so-called hump torque mode, which generates a large torque. Therefore, in such viscous joints, the viscous plate is subjected to soft nitriding treatment to increase its wear resistance and increase the number of hump torque generation cycles (hump durability). We are trying to improve the quality of our products.

By the way, although there have been various methods for soft nitriding treatment, recently gas soft nitriding treatment has been widely adopted from the viewpoint of non-pollution. This gas nitrocarburizing treatment is
This is done by introducing ammonia gas and endothermic metamorphic gas into a furnace maintained above the eutectoid temperature of Fe-N, but recently, in order to avoid the hassle of producing metamorphic gas and increase productivity, vacuum Vacuum nitrocarburizing, which is performed by directly introducing nitriding gas such as ammonia gas (NH3), carbon dioxide gas (CO2), or nitrogen gas (N2) into the furnace, is increasingly being adopted (for example, JP-A-62 -270761
(Japanese Patent Application Laid-Open No. 63-255355, etc.).

FIG. 9 shows a thermal cycle generally employed in the vacuum soft nitriding process. After loading the workpiece into a vacuum furnace, the furnace is evacuated until the burn-off temperature T0 is reached. (300 to 400°C) to perform thermal vaporization cleaning (burn-off) on the processed item,
Next, the nitrocarburizing temperature T1 (
600 to 650 °C) and introduces nitriding gas to carry out soft nitriding. After the soft nitriding process, the nitriding gas is discharged from the furnace while a cooling gas (usually N2
gas) is introduced to forcefully cool it down to room temperature.

[0005]

By the way, when the above-mentioned viscous plate is subjected to vacuum nitrocarburizing treatment, it has often been experienced that large variations occur in the hump durability of the viscous joint. As a result of various studies on the causes of this variation, the present inventors found that this kind of hump durability is due to the Fe formed between the surface nitride layer and the base.
It is clear that the hump durability depends on the structural stability of the -N eutectoid layer, and that the structural stability also depends on the cooling rate after nitrocarburizing, and that if this cooling rate is too large, the hump durability decreases. became. Therefore, in order to suppress variations in hump durability, it is sufficient to set the cooling rate of the workpiece (viscous plate) in the cooling process after the soft nitriding treatment to a slow value. There was a problem in that it was extremely difficult to uniformly control the large variations depending on the flow rate and velocity of the cooling gas introduced into the furnace, the number (thermal capacity) and position of the viscous plates charged into the furnace, etc. .

[0006] Furthermore, after the soft nitriding process, the introduction of cooling gas into the furnace is stopped, and after the soft nitriding process, the furnace is allowed to cool (
The above structural stability can be ensured by slow cooling), but in this case, cooling takes a long time, significantly extending the cycle time, and inevitably leading to a significant drop in productivity. .

The present invention has been made to solve the above-mentioned conventional problems, and its purpose is to ensure the structural stability of the surface treatment layer without significantly reducing productivity; The object of the present invention is to provide a surface treatment method for steel parts that greatly contributes to stabilizing performance.

[0008]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention performs gas nitrocarburizing treatment at a temperature above the eutectoid temperature of Fe-N, and then performs a gas nitrocarburizing treatment at a temperature below the eutectoid temperature at 450°C.
It is characterized in that it is subjected to constant temperature treatment at the above temperature and then cooled to room temperature.

In the present invention, the gas soft nitriding treatment is preferably carried out using a vacuum furnace, and in this case, a mixed gas of NH3, CO2, N2, etc. is used as the nitriding gas. Additionally, the constant temperature treatment described above may be performed in a separate furnace from the gas soft nitriding process, but if a vacuum furnace is used for the gas soft nitriding process, it is recommended that the process be performed in the same vacuum furnace in order to reduce equipment investment costs. It is desirable to do so. In addition, when performing gas soft nitriding, it is necessary to remove impurities such as oil and moisture from steel parts in advance, but if a vacuum furnace is used for gas soft nitriding, the same purpose is to reduce equipment investment costs. It is desirable to perform heating vaporization cleaning in the same vacuum furnace. Further, in the present invention, the time period for the constant temperature treatment is such that a structurally stable Fe--N eutectoid layer can be obtained, and approximately 30 to 120 minutes is sufficient for this time period.

0010

[Operation] In the method for surface treatment of steel parts constructed as described above, surface nitrides are treated by isothermal treatment at a temperature below the eutectoid temperature of Fe-N and above 450 °C following gas soft nitriding treatment. Fe formed between the layer and the base
The -N eutectoid layer can be structurally stabilized in a manner similar to that obtained by extremely slow cooling rate after nitrocarburizing.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

In this example, a series of surface treatments were performed using a vacuum furnace on the viscous plate (JIS SPCC) of the above-mentioned viscous joint. Figure 1 shows the thermal cycle, including burn-off, gas nitrocarburizing,
It consists of constant temperature treatment and cooling steps. In the burn-off process, after the workpiece (viscous plate) is loaded into a vacuum furnace, the furnace is immediately evacuated, the inside of the furnace is heated to a burn-off temperature T0 (300 to 400°C), and the temperature is maintained for a certain period of time. Due to this vacuum heating and holding, moisture and fats and oils adhering to the viscous plate are rapidly vaporized and discharged from the inside of the furnace to the outside. After completing this bar-off process, the inside of the furnace is heated to a soft nitriding temperature T1 (600 to 650 °C) that is higher than the Fe-N eutectoid temperature (approximately 590 °C),
Subsequently, NH3 gas, CO2 gas, and N2 gas are introduced at predetermined ratios. Then, due to the reaction of these nitriding gases, nascent nitrogen (N) and carbon (C) are generated, which invade the surface of the viscous plate, causing nitriding and carburization to occur simultaneously.

[0013] After the above gas soft-nitriding process is completed, the introduction of NH3 gas, CO2 gas, and N2 gas into the furnace is stopped, and at the same time, the set temperature in the furnace is set to below the Fe-N eutectoid temperature and above 450°C. Lower the constant temperature treatment temperature to T2,
Hold at that temperature for a predetermined time (30 to 120 minutes). Note that when lowering the furnace temperature from the soft-nitriding temperature T1 to the constant temperature treatment temperature T2, N2 gas may be introduced into the furnace, and by introducing this N2 gas, the furnace temperature quickly drops to the constant temperature treatment temperature. do. Then, through this constant temperature treatment, Fe − is formed between the surface nitride layer and the base.
The N eutectoid layer becomes structurally stable. After the constant temperature treatment process is completed, the viscous plate is taken out of the vacuum furnace and allowed to cool to room temperature in the atmosphere, thereby completing the series of surface treatments.

[0014] Here, as the above constant temperature treatment condition, 500
The viscous plate obtained by selecting ×1 hour was assembled into a viscous joint, and a hump durability test was conducted. For comparison, the surface treated with the conventional thermal cycle shown in Figure 9 (conventional method (1)) and the surface treated with furnace cooling after gas nitrocarburizing (
A similar hump durability test was also conducted on the samples subjected to gradual cooling (conventional method (2)). As a result, as shown in Fig. 2, the viscous joint incorporating the viscous plate obtained by the method of the present invention is different from that of the viscous joint that has been surface-treated using a conventional thermal cycle (
It was found that the variation in the number of hump durability was much smaller than that of the conventional method (1)), and it maintained a high level similar to that of the one that was slowly cooled after gas nitrocarburizing (conventional method (2)).

[0015] Furthermore, as the above constant temperature treatment conditions, A:40
0 x 1 hour, B: 450 x 1 hour, C: 500 x
The surface layer portion of each sample material subjected to surface treatment was selected for 1 hour and D: 550 x 1 hour and was observed using a microscope. For comparison, the surface layer portions of E: surface treated with the conventional heat cycle shown in Figure 9 and F: furnace cooled after gas nitrocarburizing were similarly observed using a microscope. . For microscopic observation, 5% of each sample material was
Corrosion was performed using a nitric acid alcohol solution (Nital). As a result, as shown in FIGS. 3 to 8, a nitride layer a was formed on the surface of each specimen from the surface to the base c.
and Fe--N eutectoid layer b are continuously formed.

[0016] Therefore, the Fe-N eutectoid layer b was formed in the test materials B, C, and D, which were subjected to constant temperature treatment within the scope of the present invention.
Sample material F, which was furnace-cooled after soft-nitriding treatment, appeared black due to corrosion, while sample material A, which was subjected to constant temperature treatment outside the scope of the present invention, and specimen material E, which was surface-treated by conventional thermal cycles. It appears white. The fact that the Fe-N eutectoid layer b appears black means that the structure is in a stable equilibrium state, and therefore, as described above, the hump durability of the viscous plate obtained by the method of the present invention is lower than that of the conventional one. It is presumed that this structural stability is responsible for the high level of stability compared to the viscous plate obtained by this method.

[0017]

Effects of the Invention As described above in detail, according to the method for surface treatment of steel parts according to the present invention, surface nitrides can be removed by performing a predetermined constant temperature treatment following gas soft nitriding treatment. The Fe--N eutectoid layer formed between the layer and the matrix becomes stable and greatly contributes to stabilizing the performance of the obtained parts. Moreover, the time required for constant temperature treatment is short, and productivity can be greatly improved compared to the case where the cooling rate after soft nitriding is extremely slow.

[Brief explanation of the drawing]

FIG. 1 is a graph showing a thermal cycle of surface treatment according to the present invention.

FIG. 2 is a graph showing the hump durability of a viscous joint incorporating a viscous plate obtained by the method of the present invention in comparison with that of a viscous joint incorporating a viscous plate obtained by a conventional method.

FIG. 3 is a micrograph showing the metallographic structure of a sample material subjected to constant temperature treatment outside the scope of the present invention after gas nitrocarburizing treatment.

FIG. 4 is a micrograph showing the metal structure of a sample material subjected to constant temperature treatment within the scope of the present invention after gas nitrocarburizing treatment.

FIG. 5 is a micrograph showing the metal structure of a sample material subjected to constant temperature treatment within the scope of the present invention after gas nitrocarburizing treatment.

FIG. 6 is a micrograph showing the metal structure of a sample material subjected to constant temperature treatment within the scope of the present invention after gas nitrocarburizing treatment.

FIG. 7 is a micrograph showing the metal structure of a sample material that was forcibly cooled after gas nitrocarburizing treatment.

FIG. 8 is a micrograph showing the metallographic structure of a test material slowly cooled after gas soft-nitriding treatment.

FIG. 9 is a graph showing a thermal cycle of conventional surface treatment. a Nitride layer b Tapered part c Fe-N eutectoid layer 14 Base

Claims (1)

[Claims] [Claim 1] After performing gas soft nitriding treatment at a temperature higher than the eutectoid temperature of Fe-N, the material is subsequently treated at a temperature lower than the eutectoid temperature at 450°C.
A method for surface treatment of steel parts, characterized by carrying out constant temperature treatment at a temperature of ℃ or higher, and then cooling to room temperature.