JPH04365811A - Austempering heat treatment for flat spring - Google Patents

Abstract

PURPOSE:To improve the microstructure of a flat spring and to reduce set-up time in austempering heat treatment. CONSTITUTION:The austempering heat treatment for a flat spring consists of a process wherein a flat spring stock is subjected to low temp. austempering treatment at the prescribed temp. 300 deg.C to 320 deg.C and then to tempering. By low temp. austempering treatment, cooling capacity can be increased and microstructure can be improved, and further, the sheet thickness capable of austempering treatment can be increased. Moreover, the necessity of changing austempering treatment temp. according to sheet thickness can be obviated, and as a result, set-up loss can be removed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a heat treatment method including austempering treatment for thin plate springs.

0002

[Prior Art] Fastener products (industrial fastening parts), which are thin plate springs, have conventionally been heated to about 830°C, then put into a salt solution, and then rapidly cooled to 300°C-400°C. By doing this, it is austempered and hardened. The austempering temperature is varied within the range of 300°C to 400°C depending on the target hardness of the target part.

0003

[Problems to be Solved by the Invention] FIG. 5 shows the relationship between austempering temperature and hardness for each plate thickness of JISSK5, which is a main steel type for typical thin plate springs. The target hardness of thin plate springs is generally HRC40-50, and depending on the target hardness, the austempering temperature can be changed from around 300°C to 400°C.
It is changed to around C.

[0004] Figure 6 shows the material when the austempering temperature is changed from around 300°C to around 400°C.
The average cooling rate from 800°C to 450°C when rapidly cooling from 30°C to the austempering temperature is shown. As can be seen from FIG. 6, the average cooling rate decreases linearly as the austempering temperature (temperature of the salt solution in the salt bath) increases. That is, as the austempering temperature increases, the cooling ability decreases.

FIG. 7 shows a TTT diagram of SAE1095, which is equivalent to SK5, and the pearlite nose near 550°C is on the extremely short time side (approximately 1 sec), and the critical cooling rate that does not reach the pearlite nose is 200°C/sec. It turns out that this is all.

From FIGS. 5 to 7, it can be seen that the conventional austempering process has the following problems. (b) As shown in Figure 6, the austempering temperature is 4.
When the temperature rises to around 00°C, the cooling rate decreases, resulting in the pearlite nose shown in Figure 7, and the center of the thickness of the material becomes an incomplete austempered structure with a mixture of bainite and pearlite structures, resulting in poor mechanical properties. This poses a problem, such as a decrease in ductility. (b) As the austempering temperature increases, the cooling ability decreases, and in particular, when the thickness of the plate is 2.0 mm or more, it becomes difficult to rapidly cool down to the center of the plate thickness, and the plate thickness that can be austempered becomes small. (c) As shown in FIG. 5, it is necessary to change the austempering temperature in accordance with the target hardness and in consideration of the plate thickness. Salt baths have a large heat capacity, so it takes a lot of time to change the temperature (for example, it can only change a few °C/hour).
, the setup loss is large.

The objects of the present invention are (1) to improve the cooling capacity during austempering, thereby improving the microstructure and increasing the thickness of the plate that can be austempered;
) The aim is to eliminate setup losses by allowing the austempering temperature to remain constant.

[0008]

According to the present invention, the above object is achieved by providing an austempering heat treatment method for thin plate springs, which includes the following steps. 30 thin plate spring materials
A low-temperature austempering process at a constant temperature of 0°C to 320°C, and a subsequent tempering process.

[0009]

[Operation] In the method of the present invention, the spring material of the thin plate spring is
The material is heated to 0°C and placed in a salt bath containing a salt solution at 300°C to 320°C to perform austempering treatment at a constant low temperature, followed by tempering.

[0010] Unlike conventional methods, the austempering temperature is not changed in the range of 300°C to 400°C depending on the plate thickness.
Austempering at a constant and low temperature of 300°C-320°C. Therefore, as is clear from FIG. 6, the average cooling rate increases and the cooling capacity increases.

Due to the increased cooling capacity, when rapidly cooling from 830° C. to the austempering temperature, the proportion of pearlite nose in FIG. 7 is reduced, the quality of the microstructure is improved, and the material can be austempered. The thickness can be further increased from the conventional maximum of 2.0 mm.

In addition, since the austempering temperature is kept constant regardless of the thickness of the material, the temperature of the salt solution is changed from 300°C to 400°C every time the thickness of the material changes, unlike conventional methods.
There is no need to change to C, eliminating setup loss.

Further, the hardness is made hard by austempering at a low temperature of 300° C. to 320° C., and the hardness is adjusted by subsequent tempering.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a heat treatment pattern of an austempering heat treatment method for a thin plate spring according to an embodiment of the present invention. Thin spring material (JIS, SK5, or SAE1095
etc.) at 3 points AC (76
0°C) or higher, for example 830°C (section of curve 1), then put it in a salt bath and heat it to 300°C-3.
After rapid cooling (curve 3) to a low and constant austempering temperature of 20° C. (curve 2), the material is removed from the salt bath and brought to room temperature before being tempered (curve 4). In this case, even if the material plate thickness changes, the austempering temperature (salt solution temperature) remains constant and does not need to be changed.

The method of the present invention differs from the conventional method in the following points. First, the austempering temperature is kept constant without changing depending on the material plate thickness, and the austempering temperature is set to 3 on the low temperature side.
00°C-320°C. This results in
As shown in Figure 6, the cooling capacity increases, and as shown in Figure 5,
For plates with a thickness of 2.5 mm or less, the target hardness (for example, H
RC40-50) will be heat treated to make it harder.

[0016] Furthermore, tempering is performed after the austempering treatment, thereby combining austempering and tempering. In the conventional method, there is no idea of combining austempering and tempering, and there is no idea of weakening the hardness produced by austempering by tempering. In the present invention, the grain is made hard by austempering, and the hardness is adjusted to the target hardness by tempering. By adjusting the hardness through tempering, the austempering temperature can be kept low and constant.

FIG. 3 shows the relationship between tempering temperature and hardness after low-temperature austempering. 300°C in Figure 3
means that it remains austempered, and 375°C or higher indicates the tempering temperature. Tempering temperature is 375
By selecting a temperature of -475°C, the plate thickness is 1.5
It can be seen that the austempered material with -3.0 mm hardness is adjusted to the target hardness HRC40-50. While maintaining the good physical properties of a fully austempered structure,
Only the hardness is adjusted by tempering. Therefore, compared to the conventional method, the toughness is improved with the same hardness.

FIG. 4 shows the ductility characteristics of a thin plate spring heat-treated according to the present invention and a thin plate spring heat-treated according to the conventional method. However, this is a case where the plate thickness t is 2.5 mm. The thin plate spring made by the method of the present invention has a complete bainite structure and has a uniform structure, so high elongation can be obtained, whereas the thin plate spring made by the conventional method has a pearlite and bainite structure in which pearlite is mixed. Therefore, the elongation is lower than that of the thin plate spring produced by the method of the present invention. As described above, the thin plate spring produced by the method of the present invention has an improved microstructure and improved ductility and toughness.

Comparing the method of the present invention with the conventional method from the viewpoint of working time, the method of the present invention is characterized in that the temperature of the salt bath does not need to be changed depending on the thickness of the material. Salt baths have a large heat capacity, and when the thickness of the material changes, it takes a lot of time to change the temperature of the salt solution.
The method of the present invention eliminates such setup loss.

[0020]

[Effects of the Invention] According to the present invention, since the thin plate spring is heat treated by austempering at a low and constant temperature and then tempering, (1) the cooling capacity is increased by the low temperature austempering treatment, and micro It is possible to improve the structure and increase the thickness of the plate that can be austempered. (2) Furthermore, there is no need to change the austempering temperature, so it is possible to eliminate setup losses.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a heat treatment pattern of an austempering heat treatment method for a thin plate spring according to an embodiment of the present invention.

FIG. 2 is a diagram showing a heat treatment pattern of a conventional austempering heat treatment method for a thin plate spring.

FIG. 3 is a diagram showing the relationship between tempering temperature and hardness after low-temperature austempering in the method of the present invention.

FIG. 4 is a diagram showing the relationship between elongation and tensile strength of thin plate springs according to the method of the present invention and a conventional method.

FIG. 5 is a diagram showing the relationship between austempering temperature and hardness.

FIG. 6 is a diagram showing the relationship between austempering temperature and average cooling rate.

FIG. 7 is a T, T, T diagram of SAE1095.

[Explanation of symbols]

1 Heating process 2 Austempering temperature process 3 Rapid cooling process 4 Tempering process

Claims (1)

[Claims] [Claim 1] The thin plate spring material is heated at 300°C-320°
An austempering heat treatment method for thin plate springs, which comprises performing low-temperature austempering treatment at a constant temperature of C and then tempering.