JPH0440425B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a salt bath nitriding method that significantly improves the mechanical properties of iron-based structural parts, and more specifically to the eutectoid transformation temperature that has been widely used to date.
Instead of nitriding by holding in a molten salt bath held at a temperature of 590°C or less for about 90 minutes, nitriding is performed at a temperature of 590°C to 650°C, which is above the eutectoid temperature.
This is followed by a control method that shortens the heat treatment time and improves mechanical properties by decomposing the austenite layer immediately below the compound layer on the surface by placing it in a salt bath at 250 to 550°C. [Prior Art] It is well known that iron-based structural parts are carbonitrided in a nitriding salt bath in which an alkali metal cyanate as a source of nitrogen and carbon and an alkali metal carbonate as a subcomponent are mixed and melted. This is the method. In this case, generally, ferrite (α-Fe) in the iron-nitrogen phase diagram is The nitriding process is performed at a temperature below the eutectoid point of iron nitride (Fe ₄ N). When nitriding is carried out in molten salt at a eutectoid temperature of 590°C or lower, for example, in a typical heat treatment process at 580°C for 60 to 120 minutes, the surface of the treated iron-based parts will have 6 intruded and diffused. A compound layer with non-alloying properties containing ~11% nitrogen and about 1% carbon is typically
A diffusion layer with a thickness of 0.3 to 0.6 mm is formed in which approximately 0.1% nitrogen is further diffused from directly below the layer toward the core of the base material. The treated parts have good sliding properties, abrasion resistance, seizing and galling resistance, and corrosion resistance due to the compound layer formed on the outermost surface, and the diffusion layer allows for bending and corrosion resistance.
It is possible to improve torsional fatigue strength, and it does not cause metallurgical phase changes, but only dimensional changes due to expansion due to intruded and diffused elements occur.
This method has the advantage that it is easy to maintain component precision before and after processing. [Problem to be solved by the invention] On the other hand, when performing salt bath nitriding treatment at a temperature of 590°C or higher, the same performance can be achieved in a shorter treatment time compared to the conventional method in which nitriding treatment is performed at a temperature of 590°C or lower. It has been found that a compound layer and a diffusion layer having a compound layer and a diffusion layer can provide a very effective heat treatment method in terms of improving workability due to the increased nitriding ability obtained. A soft austenite phase (γ phase) containing nitrogen and carbon as a solid solution forms between the diffusion layer and the strength decreases compared to the properties of the nitrided layer obtained when treated at temperatures below 590°C. The problem arises. Moreover, this austenite phase is unstable and undergoes phase transformation with the passage of time and temperature changes, forming iron nitride (mainly Fe ₄ N), iron carbide (Fe ₃ C), and ferrite (α-
Decomposes into Fe). If this phase transformation is incomplete,
Until now, it has not been used for nitriding purposes because the mechanical properties obtained are unstable and at a low level.
There are no examples of practical use at processing temperatures of 590°C or higher. As an exception, the applicant's own prior application
No. 21222 discloses a salt bath nitriding method characterized by setting the treatment temperature in a nitriding salt bath with a low cyanide content to 600°C to 650°C, but the steel types targeted by this invention are This is limited to high alloy steels such as stainless steel and heat-resistant steel, in which case a conventional treatment temperature of 590°C or lower will only result in a non-uniform nitrided layer, but by increasing the temperature to 600°C to 650°C, The gist of the invention is to obtain a uniform nitrided layer, and there is no mention of a method for solving the problem of austenite phase formation and the resulting decrease in strength, which is solved by the invention. In addition, Japanese Patent Publication No. 59-12727 is entitled "Salt Bath for Quenching Steel and Iron Parts," and the treatment is carried out in a nitriding salt bath containing little cyanide and high cyanate, or in other salt baths containing cyanide. The invention relates to a salt bath for rapidly cooling parts made of steel and iron. [Means for Solving the Problems] As a result of intensive research to solve the above problems, the present inventors set the temperature conditions for salt bath nitriding in an appropriate range of 590°C or higher to increase nitriding efficiency. The resulting soft austenite intermediate layer is completely decomposed by nitriding followed by salt bath cooling to cause a phase transformation of γ → α + Fe ₃ C + Fe ₄ N, increasing the mechanical strength of the treated parts. It has been found that this method can be significantly improved compared to the conventional method. To describe the method of the present invention more specifically, iron structural parts are subjected to salt bath nitriding treatment at a constant temperature of 590°C to 650°C, and then heated to 250°C to 550°C, preferably 300°C to 500°C. Cool and maintain for several minutes to several tens of minutes in a molten salt maintained at a constant temperature of °C. As a result, the soft austenite phase generated between the compound layer and the diffusion layer obtained by nitriding treatment can be completely decomposed, significantly shortening the heat treatment time, and the generated decomposed phase significantly reduces the mechanical strength of the part. It will help you improve. The temperature of the nitriding treatment according to the present invention is preferably 650°C or lower, taking into account the fact that the alkali cyanate is prevented from being consumed by thermal decomposition and that a porous layer in the resulting compound layer increases at a temperature of 650°C or higher. It is. It is effective to set the temperature of the molten salt in the range of 250℃ to 550℃ for cooling the iron-based structural parts after nitriding, but the time required for austenite decomposition is specified to be within 60 minutes to achieve sufficient production efficiency. 300℃〜
500°C is appropriate (see Examples 3 and 4) [Example] Example 1 Using a rotating bending fatigue test piece (notch shape factor α _K = 2.45) made of S15C steel in the shape shown in Figure 1. Comparison of the fatigue strength obtained using an Ono rotary bending fatigue tester revealed that the specimens treated according to the present invention had higher strength than the specimens treated by the conventional method. The chemical composition of S15C used in this example is shown below.

[Table] C Si Mn P 0.15 0.21 0.41 0.012 S C 0.013 0.01 Example 2 φ10 made of S15C steel (same as Example 1)
When we measured the amount of change in the diameter of a mm round bar before and after treatment, we found that the ratio of the thickness of the compound layer generated and the amount of increase in diameter after nitriding of the conventionally treated specimen and the treatment method of the present invention is as follows. were equivalent.

[Table] Example 3 A specimen made of S20C steel was nitrided at 650°C for 90 minutes, then cooled in molten salt at 400°C, heated for 1 minute, 3 minutes,
After holding for 5 and 15 minutes, the compound layer was cooled with water, and the austenite layer formed between the compound layer and the diffusion layer was decomposed using an X-ray diffraction device (Rigaku CN4036A2, Geiger Flex) from Rigaku Denki Kogyo Co., Ltd. R.A.D.
-B) and the photograph of the cross-sectional structure taken with an optical microscope were as follows (see Figures 2 and 3). The chemical composition of the S20C steel material used in this example is as follows. C Si Mn P 0.20 0.21 0.45 0.019 S C 0.020 0.01 From this result, it was confirmed that when the cooling bath temperature is 400°C, 15 minutes is sufficient for holding the nitrided iron-based structural parts. When specimens made of materials SK5 and SCM440 were subjected to the same treatment as in this example and tested, exactly the same results as in this example were obtained. Example 4 A specimen made from the above S15C steel was heated at 650°C x 90°C.
After the nitriding treatment, it was cooled in a molten salt that was heated from 200°C to 550°C in 50°C increments and held for up to 420 minutes. As a result, at 200°C, a slight decomposition of austenite was observed after 420 minutes, but most of it remained as it was. On the other hand, at 250℃, 240
minutes, 60 minutes at 300℃, 15 minutes at 350-450℃, 500
At ℃, austenite decomposed after 30 minutes. At 550°C, the processing time becomes longer at 240 minutes, and anything longer than this will not contribute to time reduction. Photos 1 to 6 in Figure 4 are 50℃ from 200℃ to 500℃.
The results show the decomposition results of austenite after 15 minutes have elapsed. See Figure 3 for a photo at 400℃. Example 5 After nitriding an S20C steel specimen with the same composition as in Example 3 under the nitriding conditions shown in Table 3, a commercially available salt bath containing melted hydroxides, carbonates, and nitrates at 400°C as main components was applied. The mixture was immediately cooled down, held for 15 minutes, and then cooled with water. These specimens were subjected to a 5% salt spray test in accordance with JIS Z2371, and their corrosion resistance was compared. Table 3 shows the elapsed time until spot rusting occurs.

[Table] [Effects of the Invention] Since the present invention is configured as described above, the following effects can be expected. Ferrite, which undergoes salt bath nitriding treatment at a higher temperature than conventional
By exceeding the eutectoid temperature of iron nitride of 590°C and performing the treatment in the range of 590°C to 650°C, mechanical strength and corrosion resistance equivalent to or higher than those obtained by conventional methods can be achieved with a shorter processing time compared to conventional methods. A compound layer and a diffusion layer having the following properties can be obtained, and the processing time can be shortened, making it possible to significantly improve workability. In the salt bath nitriding of the present invention, an austenite layer is formed between the compound layer on the surface and the internal diffusion layer, resulting in a decrease in strength and unstable performance of the treated member. By cooling and maintaining the salt bath after bath nitriding, the austenite phase undergoes rapid phase transformation and decomposes into iron nitride, ferrite, and iron carbide, forming a highly hard and metallurgically stable layer, which becomes a tough layer that backs up the compound layer. It acts as a layer and improves the mechanical strength of steel-based structural parts. Even with the processing method of the present invention, the stability of the dimensional accuracy of the processed parts is equivalent to that of the conventional method.

[Brief explanation of drawings]

Figure 1A is a dimensional diagram of the rotary bending fatigue test piece used in Example 1, Figure 1B is an enlarged dimensional diagram of the notch B in Figure 1A, and Figures 2 and 3 are respectively , are optical micrographs showing X-ray diffraction diagrams and cross-sectional metal structures corresponding to four types of cooling conditions applied after the nitriding treatment in Example 3, and FIG. 4 shows the austenite phase under each temperature condition described in Example 4. 1 is an optical micrograph of a cross-sectional metal structure showing the decomposition state of .

Claims (1)

[Claims] 1. After performing nitriding treatment in a molten salt maintained at a temperature of 590°C to 650°C, immediately cooled and maintained in a molten salt maintained at a temperature of 250°C to 550°C, The austenite phase formed directly under the compound layer is decomposed and α _Fe +
A salt bath nitriding method for iron-based structural parts characterized by Fe ₄ N + Fe ₃ C. 2. The salt bath nitriding method for iron-based structural parts according to claim 1, wherein the temperature of the molten salt for cooling and holding is 300°C to 500°C.