JPH0214408B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a surface hardening method that enables induction hardening while performing thermal refining during the surface hardening process of small gears (module 3 or less) to obtain a hardening pattern that follows the tooth profile. Regarding the law.

Examples of induction hardening methods for small gears include single-shot hardening of all teeth and single-shot hardening of all teeth with preheating.

When a small gear is surface hardened by single-shot hardening of all teeth, as shown in Figure 1A, since the teeth are small, not only the surface of the toothed part 11 but the entire surface is heated and hardened, resulting in deep hardening (Fig. 12 indicates the cured area).

In addition, preheating and one-shot hardening of all teeth is a method in which the teeth are preheated to 500 to 600°C and then heated and hardened in a short period of time at high output. According to this method, even small gears can be As shown in FIG. 2, the surface of the tooth portion 11 can be hardened along the profile 11a. That is, it is contour hardening.

Regarding the relationship between the induction hardening pattern of small gears and the tooth base fatigue strength, contour hardened gears have a fatigue strength of 120 to 150% compared to sub hardened gears. In addition, high-frequency contour hardened gears are more durable than carburized contour hardened gears.
It has a fatigue strength of 100-120%.

Therefore, induction hardened gears have high fatigue strength (hereinafter referred to as
To obtain high strength (high strength), it is necessary to perform contour hardening along the tooth profile.

In any of the above quenching methods, in order to ensure induction hardenability, the small gear must be subjected to tempering heat treatment such as quenching, tempering, or normalizing in the raw material state before the quenching process. is commonly practiced. Normally, medium to high carbon steel used for induction hardening has a high hardness after heat treatment, which causes tool wear during turning or gear cutting work (hereinafter referred to as gear cutting work, etc.) performed immediately before surface treatment. Case hardened steel (low carbon alloy steel such as SCM420, SCr420, etc.) has a long tool life.
It had the disadvantage of being very short compared to .

Softening annealing is also performed as a pretreatment to facilitate gear cutting work on gear rough material, but in this case, if the gear is hardened using the conventional preheating induction hardening heat cycle, the resulting small gear The hardened structure at the bottom of the tooth became incomplete, and when the heating temperature during induction hardening was increased to prevent this, the above-mentioned sub-hardening occurred, which was undesirable.

The present invention is aimed at solving the drawbacks of conventional methods. It performs softening annealing as a refining heat treatment before the induction hardening process, improves workability in cutting processes such as gear cutting, prevents wear and tear on tools, and The present invention provides a surface hardening method capable of obtaining a high-strength small gear having contour hardening along the surface.

That is, the surface hardening method for small gears of the present invention is a surface hardening method for small gears in which a small gear is subjected to tempering heat treatment in a rough material state, then cutting and induction hardening, and softening annealing is performed as the tempering heat treatment. The teeth of the gear are A3 or Acm using high frequency.
It is characterized by preheating to a temperature above the Acm point, cooling to a temperature below the A3 or Acm point, and then reheating and quenching at high output for a short time.

The method of the present invention will be explained below in comparison with a conventional method.

The small gear to which the method of the present invention is applied has a module of 3 or less, especially 1.0 to 3.0, an outer diameter of 10 to 250 mm, and a tooth width of 5.
~30 mm shapes are suitable and made of medium to high carbon steel such as S45C, S50C, S58C, etc.

As the refining heat treatment, softening annealing is performed to facilitate subsequent gear cutting work, and this is carried out, for example, by holding at 800 to 850°C for 30 minutes to 1 hour and then slow cooling.

Gear cutting of small gears is carried out using a hob tool in a conventional manner.

In order to harden the small gear obtained in the above-mentioned process, induction hardening has conventionally been carried out using, for example, a thermal cycle shown in FIG. i.e. about 100K
Using a high frequency of Hz, a) heat to a temperature of 500-600°C at a preheating rate of 0.3-100°C/sec, b) hold at this temperature for 5-30 seconds, and c) austenite at high power and short time. (800-1000°C) and d) rapid cooling.

However, although the hardening pattern of the small gear obtained by the above method is a contour hardening in which only the tooth surface is hardened, this method uses hardening and tempering as refining heat treatment. During gear cutting, the hardness of the gear was too high, resulting in severe wear and tear on the tool.

When softening annealing is performed as tempering heat treatment to prevent the tool wear mentioned above, if induction hardening is performed using the heat cycle shown in Figure 2, the hardened structure of the tooth bottom of the resulting gear will be incomplete. . In order to avoid this, the induction hardening after softening annealing was carried out according to the thermal cycle of the hardening method shown in Figure 2, but the reheating temperature in (c) was set to a higher temperature, but this did not result in the tooth being hardened in its entirety. It becomes hardened.

The induction hardening method of the present invention is carried out as shown in Fig. 3, in which the gear obtained by softening annealing and gear cutting is heated at a heating rate of (1) 0.5 to 140°C/sec. Preheat (2) to a specified temperature of A3 or Acm point or above, i.e. 800 to 1000℃, and (3) 1.0 to 150℃/sec.
After cooling to 500-650℃ by air cooling at a speed of
(4) Reheat to 850-950°C in a short time, and (5) rapidly cool with a coolant to perform quenching.

In the above method, a high-frequency induction heating device with a frequency of 30 to 200 KHz is used, (1) preheating is performed using high-frequency power density 0.1-1.5kW/ ^cm2 , and (4) quenching heating is performed using high-frequency power. Density 2.0~15.0kW/cm ² hours 0.3~
Do it in 1.0 seconds.

The A3 or Acm point varies depending on the material of the small gear, but is approximately 800 to 1000°C when medium to high carbon steel such as S50C, S58C, etc. is used.

The cooling method is not particularly limited, but in the case of (3), air cooling is carried out, and in the case of (5), rapid cooling is carried out by spraying a coolant such as water-soluble quenching fluid or fresh water.

In the graphs of FIGS. 2 and 3 above, the dashed-dotted line represents the A3 or Acm point.

Hereinafter, the method of the present invention will be explained in more detail using Examples and Comparative Examples.

Comparative Example 1 A small gear (module 2.55, outer diameter
85.4mm, tooth width 16mm, material S58C) was induction hardened according to the thermal cycle shown in the graph of Figure 2.

That is, (a) using a high-frequency oscillator with a capacity of 150 kW and a frequency of 100 KHz (manufactured by JEOL Ltd., model JEH-150D), the temperature was raised to 580°C at a high-frequency power density of 0.40 kW/ ^cm2 , and (b) the temperature was raised to 580°C. After holding at temperature for 30 seconds, (c) after reheating for 0.6 seconds at high frequency power density 3.9kW/ ^cm2 ;
(d) The quenching fluid was sprayed and cooled to perform quenching.

The resulting hardening pattern of the gear is
As shown in the figure, it is contour hardened, and the hardened structure of the bottom of the tooth is a completely hardened structure as shown in FIG. In the figure, 11 indicates a tooth portion, and 12 indicates a hardened portion.

However, the tool life used in gear cutting, a pre-induction hardening process, is 160 pieces, which is much shorter than the 400 pieces used for case hardening steel.

Comparative Example 2 After holding at 850℃ for 1 hour, softening annealing was carried out under slow cooling conditions,
A small gear obtained by gear cutting (Comparative Example 1)
(having the same dimensions and shape as shown in ) were quenched in the same thermal cycle as in Comparative Example 1. The hardening pattern of the obtained small gear is shown in FIG. The quenching pattern is contour quenching as in Comparative Example 1, but the quenched structure at the tooth bottom is an incompletely quenched structure of ferrite + troostite + martensite, as shown in Figure 7. ing. Also, the life of the machining tool at this time was 350 pieces.

Comparative Example 3 In order to change the incompletely quenched structure in Comparative Example 2 to a completely quenched structure, induction hardening is performed at elevated temperature. That is, a small gear that was soft-annealed and gear-cut under the same conditions as Comparative Example 2 was preheated to 600°C, held for 30 seconds, cooled to 580°C, and then reheated to 950°C in a short time. Quenching liquid (fresh water)
is injected for rapid cooling.

The hardened structure of the obtained small gear becomes a completely hardened structure as shown in FIG. 9, but the hardened pattern becomes almost partially hardened as shown in FIG. The life of the processing tool is the same as in Comparative Example 2.

Example 1 Softening annealing is performed under the same conditions as in Comparative Example 2, and gear cutting is performed to obtain a small gear (S50C) having the same dimensions and shape as Comparative Example 1. This small gear is subjected to induction hardening using a heat cycle shown in FIG.

That is, using a high frequency of 100KHz, high frequency power density of 0.40kW/cm ² and preheating rate of 21.6℃/sec.
Preheat to 950℃, and after reaching that temperature, immediately cool it at a cooling rate of 6.7℃/sec to the specified temperature (550℃).
℃), it is reheated to 900℃ within 1.0 seconds using a high-frequency power density of 2.8kW/ ^cm2 , and after reaching this temperature, quenching fluid (fresh water) is injected to rapidly cool it and quench it. I did this.

The hardening pattern of the obtained gear was contour hardening along the tooth surface, as shown in Fig. 10, and the hardening structure of the tooth bottom was completely hardened, as shown in Fig. 11. Obtained.

The life of the gear cutting tool was 350 teeth, which is almost the same as that of carburized material.

As is clear from the above description, the surface hardening method for small gears of the present invention performs pretreatment by softening annealing, so the hardness of small gears during gear cutting can be kept low, and wear and tear on processing tools is significantly reduced. This increases the tool life. Moreover, since it is easy to process, work efficiency is improved. In addition, when induction hardening small gears that have been subjected to gear cutting after softening annealing, the preheating temperature is higher than that of conventional gears, and then the gears are heated and hardened again. The hardened pattern is obtained along the profile of the tooth portion, and the hardened structure at the bottom of the tooth is also good, making it possible to obtain a small gear with high strength.

[Brief explanation of drawings]

Figure 1A is a cross-sectional view showing the teeth of a gear that has been sub-hardened, Figure 1B is a cross-sectional view showing the tooth that has been contour-hardened, and Figure 2 shows the thermal cycle of conventional induction hardening. The graph shown in Figure 3 is a graph showing the thermal cycle of induction hardening in the method of the present invention, Nos. 4, 6, and 8.
The figures are schematic cross-sectional views showing the hardened state of the teeth of small gears obtained in Comparative Examples 1, 2, and 3, respectively.
Figures 7 and 9 are micrographs showing the hardened structure of the tooth bottoms of the small gears obtained in Comparative Examples 1, 2, and 3, respectively, and Figure 10 is the hardened state of the teeth of the small gears obtained in Example 1. FIG. 11 is a micrograph showing the hardened structure of the tooth bottom of the small gear obtained in Example 1. In the figure, 11... tooth portion, 11a... tooth surface, 12...
...hardened part.

Claims (1)

[Claims] 1. After subjecting the small gear to heat treatment in the state of rough material,
In the surface hardening method for small gears, which involves cutting and induction hardening, high frequency is used to harden the teeth of the gears to A3 or Acm.
A method for surface hardening small gears, which comprises preheating to a temperature above the Acm point, cooling to a temperature below the A3 or Acm point, and then reheating and quenching at high output for a short time.