JPS5852428A - Heat treatment for improving stress of shaft - Google Patents

Abstract

PURPOSE:To improve the stresses of a steel shaft through generation of residual compressive stresses on the surface of the shaft by heating the shaft in a temp. range in which the metallic structure thereof does not change then cooling the surface of the shaft quickly. CONSTITUTION:A steel shaft 1 is heated by a high frequency heating source connected to a heating coil 2 provided on the outside circumferential part in the required part of said shaft 1 and a conductor 3 via said coil. The heating temp. is controlled by using a temp. sensor 5 provided on the surface of the shaft 1 and is heated in a temp. range in which the metallic structure of the shaft 1 does not change, that is, to a suitable temp. lower than the hardening temp., in such a way that the shaft is soaked down to certain depth from the surface layer. Thereafter, a cooling element 6 is moved to the heated part and a coolant 7 is blown through a pump 10, a pipe 9 and a valve 8, whereby said part is cooled quickly. Then, the stresses of the shaft 1 are improved without oxidation, deformation, change in metallic structure of the shaft 1. The above- mentioned heat treatment method for improving stresses is particularly suited for treatments for the purpose of preventing corrosion and damage owing to the residual stresses in the sealing parts of pump shafts.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to stress improving heat treatment of shafts, and more particularly to a stress improving heat treatment method of shafts suitable for use in pump shafts of atomic couplants and the like.

Heat treatment conventionally performed for the purpose of stress relief includes stress annealing applied to welded structures.

This heat treatment involves heating to 650C for 1 to 2 hours per thickness of the member, followed by slow cooling. Disadvantages of stress annealing include oxidation of the surface of the component, deformation, and long processing times.

Carbon steel as a heat treatment performed for the purpose of surface hardening.

Surface hardening 1, such as carburizing and induction hardening, is available for machine parts made of low alloy steel. Since compressive residual stress is generated on the surface by these surface hardening and quenching, it is thought that it can be used as a stress improving heat treatment. However, in this heat treatment, the heating temperature is about 800 C, which is even higher than stress relief annealing, so oxidation and deformation become even greater.

In addition, changes in metallographic structure are inevitable, which is a drawback from the perspective of stress relief only.

Particularly in the shaft seal portion of the pump shaft, if the fluid within the pump is corrosive, residual stress on the machined surface may accelerate corrosion damage. Therefore, in order to prevent corrosion damage in the shaft seal, it is effective to reduce the tensile residual stress caused by machining or to actively apply compressive residual stress to the machined surface.

However, if the stress improvement treatment method such as stress annealing or surface hardening described above is applied to the 0 shaft sealing part of the pump shaft, oxidation, deformation, changes in metal structure, etc. will occur, and the function as a shaft sealing part will not be fulfilled. It becomes impossible.

An object of the present invention is to provide a stress-improving heat treatment method for a shaft that can impart compressive residual stress to the surface of the shaft without causing oxidation, deformation, changes in metallographic structure, or the like.

The present invention involves heating a steel shaft to a temperature range at which its metallographic structure does not change, that is, an appropriate temperature below the quenching temperature, and then rapidly cooling the shaft surface to bring the shaft surface into tensile yield. Plastic deformation is compressed by cooling contraction of the shaft center, and compressive residual stress is generated on the shaft surface when completely cooled.

Hereinafter, the present invention will be explained in more detail based on examples.

Figures 1-3 and 1-2 schematically illustrate the principle by which compressive residual stress is generated on the shaft surface in the stress-improving heat treatment method of the present invention.

In Figure 1 (b), the shaft is first heated at a temperature below the quenching temperature, e.g. 300 to 600C, using an appropriate means such as an electric furnace or flame.
The changes in surface temperature, core temperature, and temperature difference between the surface and the 6th part are shown when the surface is rapidly cooled after soaking at a temperature of . In Figure 1 (4), when the surface is rapidly cooled from point A, the surface changes from A to B and then to C.
1, and the temperature of part 6 changes from A to B2 to C2. The temperature difference between inside and outside changes as shown by the broken line, and shows the maximum temperature "degree difference Δ'I" max between bB2 and B1. The stress-strain behavior at this time is as shown in FIG. 1 (to), and tensile strain occurs on the surface. The tensile strain is maximum when ΔT reaches the maximum Δ'["m3'X, and ΔTmax is 200 to 3000.
At this point, tensile yield occurs and the tensile plastic strain ε reaches point B1.
B1-ey is generated. When the temperature further decreases, the tensile strain on the surface decreases due to the shrinkage of 6 parts. Cool completely 01
When the point is reached, the tensile plastic strain gB, -ey generated on the surface
is compressed and compressive residual stress occurs. On the contrary, tensile residual stress occurs in the 6th part.

In general, the load stress acting on a member is high on the surface of the member, and damage and destruction due to fatigue, stress corrosion, corrosion fatigue, etc. occur on the surface, so applying compressive residual stress to the surface prevents damage and destruction on the surface of the member. It is extremely effective in preventing.

Further, in normal quenching, the material is rapidly cooled from a high temperature above the AC transformation point, whereas in normal stress annealing, the material is heated and held at about 650C and then slowly cooled, which is different from the stress improvement heat treatment method of the present invention.

In the present invention, since it is carried out at a lower temperature than normal hardening or stress annealing, oxidation of the surface and change in one dimension shape are small. Ordinary stress annealing utilizes the fact that increasing the temperature lowers the material's yield point and relaxes tensile and compressive residual stresses. That is, since it relies only on stress relaxation, it is not possible to actively apply compressive residual stress.

FIG. 2 shows an embodiment in which stress improvement heat treatment of the shaft is performed using a high-frequency induction heating device. FIG. 3 shows an example of a shaft, which has a stepped portion 32 that forms a shaft seal along with a smooth shaft surface 31.

In this embodiment, a method for applying compressive residual stress to the shaft seal part will be explained particularly from the viewpoint of strength.

In FIG. 2, a heating coil 2 is provided on the outer periphery of the stepped portion of the axis l, which is omitted in the drawing, and the heating coil 2 is connected to a high frequency heating source 4 via a conductor 3. A temperature sensor 5 is attached to the shaft surface.

Used for heating temperature control. With this configuration, the stepped portion of the shaft 1 is heated. At this time, it is desirable that a certain depth range from the surface layer of the shaft 1 be uniformly heated as much as possible. for example,
It is preferable to heat with a low heating power, or to heat intermittently instead of continuously. When heated in this manner, the temperature distribution within the shaft is relatively uniform even though it is high frequency heating, as shown in Figure 4 (2). From this state.

Move the cooler 6 shown in Fig. 2 to the heating section and pump 8° tube 9.
Then, the pulp 10 is rapidly cooled with a coolant 7 blown through the pulp 10. Immediately after the start of cooling, tensile yield occurs at the surface and compressive stress occurs just below the surface, as shown in Figure 4 (2). When it is further cooled and completely cooled, it becomes compressive residual stress at the surface and tensile residual stress just below the surface, as shown in Figure 4 (2). Focusing on the surface layer, the behavior of temperature, strain, and stress follows the same course as explained in FIG. 1.

FIG. 5 shows another embodiment of the present invention, in which a direct energization method is applied as the heating method for the shaft 1.

In FIG. 5, electricity is supplied from the heating source 12 to the electrodes 11A and 11B installed around the stepped portion of the shaft 1, heating is controlled by the temperature sensor 5, and after the stepped portion reaches a predetermined temperature, As in the case of FIG. 2, rapid cooling is performed using the coolant 7.

In this embodiment, uniform heating can be achieved compared to high frequency heating.

In the embodiments shown in FIGS. 2 and 5, the heat treatment range can be arbitrarily adjusted by changing the coil width and the electrode spacing, so that desired shapes such as stepped portions forming shaft seals and other discontinuous portions can be formed. It has the advantage of being able to process only a range. In addition, since the heating temperature is low and the processing time is short, about several minutes, oxidation of the shaft surface is even more slight, and deformation of the stepped portion and change in the metal structure do not occur, so it is particularly effective for shaft seals.

As described above, according to the present invention, the shaft surface is hardened.

Compressive residual stress can be applied to the shaft surface by effectively utilizing the thermal strain on the shaft surface and inside during heating and cooling without causing any structural changes.

Furthermore, since the present invention does not involve quench hardening, there is no reduction in toughness.

[Brief explanation of drawings]

Figure 1 schematically shows the principle of the present invention.
4) is a temperature-time diagram, FIG. 1 (to) is a stress-strain diagram, FIG. 2 is an explanatory diagram showing an example of the present invention, FIG. FIG. 4 is an explanatory diagram showing the temperature distribution and stress distribution of the axial cross section in the embodiment shown in FIG.
FIG. 5 is an explanatory diagram showing another example of the present invention. 1...Shaft, 2...Heating coil, 5...Temperature sensor, 6...Tree 1. Fig. 3 (B) Fig. 3 Fig. 4- Fig. 5

Claims (1)

[Claims] 1. A stress improvement heat treatment method for a shaft, which is characterized by heating a steel shaft in a temperature range in which its metallographic structure does not change, and then rapidly cooling the shaft surface. 2. The stress improving heat treatment method for a shaft according to claim 1, wherein the steel shaft is a pump shaft. 3. Claim 2 characterized by a shaft sealing portion of the pump shaft
Shaft stress improvement heat treatment method described in section. 4. Claim 1, in which heat treatment is performed by a heating means that heats the entire circumference of the shaft in the circumferential direction and locally in the axial direction.
4. The shaft stress improvement heat treatment method according to any one of items 1 to 3.