JPS58204122A - Surface hardening method of steel material - Google Patents

Abstract

PURPOSE:To increase the hardening penetration of a steel material by increasing the primary electric power for the purpose of heating with time from the start or mid-way of heating. CONSTITUTION:The surface of a steel material is subjected to induction heating followed by quick cooling and is thereby hardened. The primary electric power (primary current X primary voltage X load power factor) for the purpose of heating is increased with time from the start or mid-way of heating in this stage. Therefore, the cooling rate after the heating in the surface layer of the steel material increases and the hardening penetration of the steel material increases. As a result, the reduction in the electric power for heating is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for surface hardening steel materials by induction heating.

When surface hardening is applied to a predetermined portion of a steel material, for example, a shaft-shaped steel material, as shown in FIG. A high-frequency current is passed through the heating coil 2 to raise the temperature of the surface layer of a predetermined portion of the steel material 1 to a predetermined temperature required for hardening, and then the steel material 1 is moved (raised in the figure) and the heating coil 2 is heated. By positioning the predetermined portion in a cooling ring 3 which is disposed close to the heating coil 2 (in the figure, placing it above the heating coil 2), and cooling the predetermined portion by injecting water into the cooling ring 3, the predetermined portion of the steel plate 1 is cooled. The method is to apply surface hardening to the surface.

In such a surface hardening method, the induction heating conditions are usually such that the primary voltage is constant; however, when heating the steel material 1 under such conditions, the steel material 1 has a surface close to its outer surface. Heat is generated from the layer, the temperature rises, and when the temperature continues to rise and the temperature exceeds the Curie point, the magnetic coupling between the heating coil 2 and the steel material 1 becomes weaker, so the load power factor decreases. Therefore, even if the steel material 1 is heated by induction at a constant primary voltage, the electric power contributing to the temperature rise of the steel material 1 gradually decreases.
It becomes warmer and hotter over time. In addition, when the temperature of the surface layer of the steel material 1 exceeds the Curie point, the induced current tends to flow in the inner part that does not exceed the Curie point.
The inside of the steel material 1 will gradually become heated. Therefore, while the temperature of the surface layer of the steel material 1 is heated to the predetermined temperature required for quenching, parts deeper than the surface of the steel material 1 reach a considerably high temperature. Even if heat is removed by water cooling, a heat flow is generated from the inside of the steel material 1 to the outer surface due to thermal conduction, and the cooling rate in the surface layer of the steel material 1 is slow, and the surface hardness of the steel material 1 is low and the hardening depth is also deep. do .

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The present invention was made in view of the above circumstances, and an object of the present invention is to increase the cooling rate after heating in the surface layer of the steel material and deepen the quench hardening depth of the steel material in a method for surface hardening of steel materials. shall be.

The surface hardening method for steel materials according to the present invention is a method in which the surface of the steel material is induction heated and then rapidly cooled to subject the surface hardening to the steel material.
- next voltage x load power factor) is increased over time from the start of heating or from the middle of heating.

Next, a detailed description will be given based on drawings showing the results of an investigation conducted to limit the conditions related to the method of the present invention.

Figure 2 (a), (b), and (c) show the primary power pattern when induction heating is performed to surface harden a predetermined part of a steel material using the method shown in Figure 1, so that the maximum value is 1. This is a normalized graph, with time plotted on the horizontal axis and primary force on the vertical axis. In the negative power pattern of (a), the primary power is set to a maximum value at the start of heating and gradually decreases, and is kept constant in the second half of heating! D, the -order power pattern in (b) keeps the primary power constant during heating, and the -order power pattern in (C) keeps the primary power at approximately 30% of the maximum value in the first half of heating. The power is kept constant during the second half of heating.
The temperature is gradually increased to reach the maximum value at the end of heating. (b
) is close to the conventional conditions, and (c
) are excellent conditions according to the present invention, as will be described later.

Figure 3 (a) (b) (C) is shown in Figure 2 (a) (b) (
C) The temperature distribution of the steel material at the end of heating when heated with each of the primary power patterns shown in each case is shown, with the horizontal axis representing the depth from the surface of the steel material9 and the vertical axis representing the temperature of the steel material □ Ru. If a steel material is heated under the condition shown in Figure C (b), that is, the condition in which the primary power during heating is always constant, or the condition in (a), that is, the condition that the primary power is reduced over time, from the surface of the steel material to the inside. While the temperature gradient to
It can be seen that when the steel material is heated under the condition (C), that is, the primary power during heating is increased over time from the middle of heating, the temperature gradient from the surface to the inside of the steel material becomes steep. This is because after the surface layer of the steel material is heated by induction heating and its temperature exceeds a single point, an induced current flows more easily to the inside of the steel material, which becomes more likely to be heated, and that heat is transferred to the steel material. By conducting to the surface layer,
The tendency of the temperature gradient from the surface to the inside of the steel material to be relaxed,
This is because it can be avoided under the condition (c), that is, by increasing the primary power during heating over time to rapidly heat the surface of the steel material.

Figure 4 (a) (b) (c) is shown in Figure 2 (a) (1:
,) The horizontal axis represents the change in temperature of each part at different depths from the surface of the steel material over time during the cooling process after heating is completed when heating is performed using the primary power pattern shown in (c). , temperature is shown on the vertical axis. In the figure,
The solid line is the part of the steel surface, the dashed line is the part 211 m deep from the surface, the one-dot chain line is the part 4 fins deep from the surface, the two-dot chain line is the part 6 am deep from the surface, and the three-dot chain line is the surface The data are shown for a depth of s mm from . Note that (a), (b), and (Q) in FIG. 4 correspond to (a), Cb), and (C) in FIG. 2, respectively. For example, from 8oO℃ to 35℃ at a depth of 4 fins from the steel surface.
The cooling time at 0℃1 is 11.5 seconds in case (a), (b
), it is 10.8 seconds, and in case (C), it is 7.5 seconds. This result also shows that if the primary power of the heating tank is increased over time from the middle of heating as in condition (C), the cooling rate of the surface layer of the steel material after heating can be increased, and the quenching of the steel material can be increased. It can be seen that the curing depth can be increased.

Next, examples of the method of the present invention will be described. Figure 5 shows how carbon steel for machine structures (rxs standard:
538c), the results of surface hardening applied to a predetermined portion of a round bar with an outer diameter of 110 mm are shown in comparison with the conventional method.

The figure shows the hardness distribution of the steel material, with the horizontal axis representing the depth from the surface of the steel material, and the vertical axis representing the Vickers hardness. Each case is shown. Examining the depth of the part where the Vickers hardness is 400 from the figure, it can be seen that it is approximately 111 m deeper in the case of the method of the present invention than in the case of the conventional method. Since this depth is usually around 4 mm, the method of the present invention increases the depth to about 1111 m, which has a great effect on improving the strength of the steel material.

In addition, as a method for increasing the primary power during heating in the method of the present invention, the temperature of the surface of the steel material is kept constant until it reaches the single point, and then it is increased over time after the temperature reaches the single point, that is, the second method. A heating method in which the power is changed under the conditions shown in Figure (c) at the time when the surface temperature of the steel material reaches the single point is most preferable in terms of uniform heating of the steel material and stable quality after quenching. However, it is also effective to increase the primary power over time from the start of heating. Further, as a method of increasing the primary power during heating, a method of increasing it stepwise may be used instead of a method of increasing it continuously as shown in FIG. 2(c).

As described in detail above, the present invention, when surface hardening a steel material, increases the primary power for heating from the start of heating or from the middle of heating over time, so that the surface layer of the steel material can be heated. Since the cooling rate can be increased, it is possible to deepen the hardening depth of the steel material, and as a result, it is also possible to reduce heating power. Therefore, the present invention makes it possible to improve the strength of steel materials when they are used after surface hardening, thereby increasing the use of steel materials for mechanical structures and the like.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing a state in which a steel material is surface hardened, and Fig. 2 (a), (b), and (c) are graphs showing heating conditions when induction heating is applied to surface hardening a steel material. , 3rd
Figures (a), (b), and (C) are graphs showing the temperature distribution of the steel material at the end of heating when the steel material is induction heated.
Figures (a), (b), and (c) are graphs showing changes in steel material temperature over time during the cooling process □ after induction heating of the steel material, and Fig. 5 is a graph showing the effect of the method of the present invention. 1... Steel material 2... Heating coil 3... Cooling ring time (枦j) Book 1 calculation 23 calculation 5
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Claims (1)

[Claims] 1. A method of surface hardening a steel material by induction heating and then rapid cooling, characterized in that the primary power for heating is increased over time from the start of heating or from the middle of heating. Method for surface hardening of steel materials.