JPH0142326B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an austempering method for improving the toughness and fatigue strength of spheroidal graphite cast iron.

(Prior art) Conventionally, austempering treatment for converting the base structure into bainite in order to improve the toughness of spheroidal graphite cast iron has been disclosed in U.S. Pat. Disclosed.

In addition, with the austempered treatment mentioned above,
When the surface hardness is low and the fatigue strength is insufficient, as seen in the previous example above, there is a technology that applies processing strain to the cast iron surface by shot peening to increase the surface hardness and improve the fatigue strength. be.
Other methods for improving the fatigue strength of austempered spheroidal graphite cast iron include rolling processing, inducing transformation of retained austenite into martensite during cutting, and deep cooling treatment of retained austenite to martensite. There are methods that harden the surface by carburizing, etc., but all of these methods require a separate treatment process from the austempering process, which makes the process complicated and the overall process time longer. There are problems such as the need for processing equipment for this purpose.

(Object of the Invention) In view of the above circumstances, the present invention provides a method for austempering cast iron in which a series of treatments to improve fatigue strength are performed during austempering, without using surface hardening methods such as shot peening. The purpose is to provide

(Structure of the Invention) The austempering method of the present invention involves heating and holding spheroidal graphite cast iron in the austenitizing temperature range of 810 to 980°C for at least 5 seconds or more, and then removing the iron from this state.
In the austempering process, which involves rapid cooling to a temperature of 220 to 460°C, holding constant temperature at this temperature to perform bainite transformation, and then cooling, once from the start of bainite transformation to the completion of bainite transformation,
It is characterized by rapidly cooling to a temperature below the Ms point and converting only the surface layer to martensite to improve fatigue strength.

The austenitization process described above involves heating and holding for 5 seconds or more in the temperature range of 810 to 980℃, but if the temperature is
If the temperature is less than 810℃, sufficient austenitization will not be obtained, and if the temperature exceeds 980℃, the crystal grains will become coarse and the strength will decrease.If the temperature is less than 5 seconds, only the surface will become austenite due to induction hardening. Even if austempering is performed, it is difficult to obtain sufficient and uniform austenitization, so it is carried out within the above range.

The constant temperature treatment for bainite transformation is performed by immersing and quenching the austenitized spheroidal graphite cast iron in a constant temperature salt bath, and the constant temperature is set at 220 to 460°C. This treatment temperature is such that when spheroidal graphite cast iron is rapidly cooled or quenched to a temperature lower than 220℃, it undergoes martensitic transformation and expansion becomes large, resulting in quench cracking during quenching, or dimensional changes if the shape is complex. growing. In addition, at temperatures exceeding 460°C, toughness decreases due to the formation of pearlite or carbides.

Next, in the middle of the austempering process, from the start of bainite transformation to the completion of bainite transformation, the bainite transformation is rapidly cooled to below the Ms point (martensitic transformation start temperature) by water cooling or the like. It is preferable to do it immediately after because it is more effective. Before the start of bainite metamorphosis
When rapidly cooled below the Ms point, untransformed austenite quickly transforms into martensite, making it difficult to control cooling to obtain toughness and compressive stress. On the other hand, after bainite transformation is completed, this base structure becomes bainite and retained austenite, and since retained austenite is a metastable structure, it is necessary to maintain the temperature at -80°C or lower than the transformation temperature for a long time, and processing is required. It takes longer.

In addition, the reason why only the surface part is rapidly cooled to a temperature below the Ms point is that by cooling the untransformed austenite in the process of bainitic transformation to a temperature below the Ms point, this untransformed austenite is converted to martensite, and this The purpose is to apply compressive stress to a desired depth through expansion due to transformation, and as a result, to obtain particularly high fatigue strength as well as toughness, at least the desired depth from the surface must reach the Ms point or below. Suitable cooling methods include cooling the spheroidal graphite cast iron undergoing bainitic transformation in boiling water, impact air, mist, or an atmosphere at 0° C. or lower for a short time (preferably within 10 minutes).

Furthermore, after cooling below the above Ms point,
It is preferable to carry out constant temperature treatment at the bainite transformation temperature for about 5 minutes to 8 hours. That is, after the untransformed austenite at a desired depth from the surface is turned into martensite by rapid cooling to below the Ms point, the untransformed austenite at this point is turned into bainite by subsequent constant temperature treatment. 5
A constant temperature treatment for more than 8 hours can simultaneously improve toughness and fatigue strength, whereas a treatment for more than 8 hours can cause a decrease in toughness and an increased loss of thermal energy. That is, it is preferable that the isothermal treatment for bainite transformation lasts for at least 5 minutes after cooling below the Ms point, and does not exceed 8 hours in total.

Cooling after bainite transformation is performed by natural cooling or the like. Bainite start characteristic curve (Bs) of S-curve of spheroidal graphite cast iron (indicates the transformation status of austenite at each temperature below the _A1 transformation point, where the vertical axis is temperature and the horizontal axis is time) where austenite begins to transform into bainite. The bainite finish characteristic curve (Bf), where bainite transformation is completed, shifts to the left (short time side) in the case of non-alloys, and to the right (short time side) in the case of high alloys containing Mn, Cu, Mo, Ni, etc. Shift to the long-term side). That is,
In the case of non-alloys, the hardenability decreases, but on the other hand, the time to start bainite transformation becomes earlier and the time to end bainite transformation also becomes earlier. Therefore, in the case of a non-alloy, the bainite base, that is, untransformed austenite, is completely converted into bainite in a short time. On the other hand, in the case of a high alloy, the initiation and completion of bainitization of untransformed austenite takes a longer time than in the case of a non-alloy. When unalloyed or highly alloyed spheroidal graphite cast iron is cooled after bainite transformation, the untransformed austenite has already been transformed into bainite, so the structure remains unchanged even by natural cooling to below the Ms point. You can obtain something that does not change and satisfies the required characteristics. In addition, since the above-mentioned natural cooling increases the time loss until the next step, the cooling after bainite transformation is carried out by washing with water or hot water, etc. Even if the cooling rate is accelerated, the untransformed austenite becomes bainite. Since the process has been completed, martensite does not occur, there is no structural change, and a product that satisfies the required properties can be obtained.

Spheroidal graphite cast iron that has been austempered including cooling to below the Ms point as described above is treated so that the residual compressive stress within at least 50μ of the cast iron surface has a value of 10 to 120 Kgf/mm ² . Improvement in fatigue strength can be obtained by creating a compressive stress of at least 10 Kgf/mm ² or ^more , and from this value, stress in the tensile direction cannot significantly improve fatigue strength. In order to obtain a compressive stress that exceeds the above, cracks will occur due to thermal strain and transformation strain during cooling below the Ms point.

Particularly, in areas subject to repeated stress concentration, cracks occur due to surface fatigue, and compressive stress near the surface acts effectively.
Preferably, a compressive stress of mm ² remains.

The chemical composition of the spheroidal graphite cast iron in the present invention is applicable to, for example, transmission gears and shifts of automobile parts.
It is used for sliding members, etc., and is able to generate compressive stress that can exhibit sufficient fatigue strength and obtain wear resistance by using a non-alloy material with low hardenability. In addition, when the wall thickness of the casting is large, 0.2
~0.9%Mn, 0.4~1.5%Cu, 0.03~4.0%Mo, 0.2
~3.5% Ni is used in combination of two or more types.

Further, when the austempering method of the present invention is used in combination with conventionally known surface hardening methods such as roll processing and shot peening, even more stable and higher fatigue strength can be obtained.

(Effects of the Invention) According to the present invention, the fatigue strength of spheroidal graphite cast iron after austempering can be improved by leaving compressive stress in the surface layer through this austempering and a series of rapid cooling treatments. Therefore, the process can be easily carried out in a short time, and can be carried out with simple equipment, with the aim of improving processing efficiency.

(Example) Examples of the present invention will be described below.

Example 1 This example shows an example of austempering treatment of a crankshaft using spheroidal graphite cast iron.

The chemical composition of the spheroidal graphite cast iron used is as shown below.

C Si Mn P S Cu Mo Ni Mg 3.53 2.45 0.29 0.021 0.007 0.83
0.08 0.48 0.042 The heat pattern of the austempering treatment performed is shown in Figure 1. That is, spheroidal graphite cast iron is heated and kept at an austenitizing temperature of 890°C for 2.5 hours, then immersed in a constant temperature salt bath of 380°C to start bainitizing, and after 30 minutes it is heated to an austenitizing temperature of 100°C below the Ms point.
After being immersed in boiling water for about 3 seconds, it was placed in an electric furnace at 380°C to proceed with bainite formation, held for 90 minutes, and then cooled in water.

The material with the above chemical composition is austempered using the heat pattern shown in Figure 1, and the pin diameter is
51mm, journal diameter 60mm, crank radius 43mm, pin and journal fillet R3.0 after machining, the results of a plane bending fatigue test are shown in Figure 2 along with a comparative example (conventional austempering only). .

In this plane bending fatigue test, one end of the crankshaft is fixed, a bending moment is applied to the other end, and the number of times N of repeated bending is measured until the sample breaks under various bending moments. In addition, the comparative example is one in which spheroidal graphite cast iron having the same composition as above was simply austempered. That is, after austenitization at 890°C x 2.5 hours, bainite formation at 380°C x 2 hours, and then air cooling.

As is clear from FIG. 2, the crankshaft according to the present invention has increased bending fatigue strength compared to that of the comparative example.

Furthermore, the constant temperature for bainitization in the heat pattern shown in Figure 1 was changed to two types, 250℃ and 370℃, and the other samples were treated in the same manner (diameter
Figure 3 shows the results of determining the distance from the surface (45 mm, length 100 mm) and the distribution of residual compressive stress.
In FIG. 3, it can be seen that the specimen with a bainitic temperature of 250°C of characteristic I has a larger residual compressive stress near the surface and has a higher fatigue strength than the specimen of characteristic I of 370°C.

Example 2 This example shows an example of austempering treatment of a connecting rod using spheroidal graphite cast iron.

The chemical composition of the spheroidal graphite cast iron used is as shown below.

C Si Mn P S Cu Mg 3.46 2.17 0.23 0.027 0.006 0.73 0.041 The heat pattern of the austempering treatment performed is shown in FIG. Heating spheroidal graphite cast iron to 870℃
After being held at the austenitizing temperature for 2 hours,
Bainite formation is started by immersion in a constant temperature salt bath at 375℃, and after 25 minutes, immersion in hot water at 90 to 100℃ below the Ms point for about 2 seconds, and then charged into an electric furnace at 375℃ to form bainite. The mixture was allowed to heat up, held for 85 minutes, and then cooled in air.

A material with the above chemical composition was austempered using the heat pattern shown in Figure 4 to obtain a connecting rod material with a length of 80 mm from the large end to the small end. The results of the test are shown in FIG. 5 together with a comparative example (as-cast).

In this plane bending fatigue test, one end of the connecting rod material is fixed, a load is applied to the other end via an arm of a predetermined length, a bending moment is applied, and the specimen is repeatedly bent under various applied loads until it breaks. The number of times N is measured. Further, in the comparative example, spheroidal graphite cast iron having the same composition as above was simply cast, and no austempering treatment was performed.

As is clear from FIG. 5, the bending fatigue strength of the conrod according to the present invention is significantly increased compared to that of the comparative example.

Example 3 This example shows an example of austempering treatment of a gear using spheroidal graphite cast iron.

The chemical composition of the spheroidal graphite cast iron used is as shown below.

C Si Mn P S Cu Mo Mg 3.62 2.53 0.30 0.031 0.006 0.72 0.08 0.042 The heat pattern of the austempering treatment performed is shown in FIG. Heating spheroidal graphite cast iron to 890℃
After being held at the austenitizing temperature for 2 hours,
Begin bainite formation by immersing it in a constant temperature salt bath at 270℃, and after 60 minutes, immerse it in a salt bath at 180℃ below the Ms point for about 15 seconds, and then charging it into an electric furnace at 270℃ to convert it into bainite. The mixture is kept for 60 minutes, left to cool, and then washed with hot water.

Materials with the above chemical composition are
3.850, 77 teeth, module 2.25, outer diameter 200mm, austempered with the heat pattern shown in Figure 6, and after applying compressive stress at least to the tooth bottom, it was processed using a power circulation gear testing machine. The results of the fatigue test are shown in FIG. 7 together with a comparative example (conventional austempering only).

In this fatigue test, the number of repeated engagements N until the sample breaks was measured under a load torque of 15.2 kgm. In addition, the comparative example is one in which spheroidal graphite cast iron having the same composition as above was simply austempered. That is, after austenitization at 890°C x 2 hours, bainite formation at 270°C x 2 hours, and then air cooling.

As is clear from FIG. 7, the fatigue strength of the helical gear of the present invention is increased compared to that of the comparative example.

Furthermore, the metallographic structure of spheroidal graphite cast iron, which has the same chemical composition as that used in the gear described above, was austempered using the heat pattern shown in Fig. 8 using an optical microscope as shown in Figs. 9 and 10. The metallographic structure is shown in the photograph.

In the austempering treatment, spheroidal graphite cast iron is heated and kept at an austenitizing temperature of 890℃ for 1.5 hours, then immersed in a constant temperature salt bath of 230℃ to start bainitization, and after 30 minutes, the temperature reaches 30℃ below the Ms point. ℃
After immersing it in water for about 2 seconds, it was placed in an electric furnace at 230°C to proceed with bainite formation, held for 90 minutes, cooled in air, and washed with water.

Figure 9 shows the structure near the surface, which was rapidly cooled to a temperature below the Ms point. It is composed of a mixture of bainite and white martensite produced by rapid cooling below the Ms point. This martensite imparts compressive stress and improves fatigue strength.

In contrast, Figure 10 shows the internal structure that has not been affected by rapid cooling below the Ms point, where the black spherical bodies are graphite, and the matrix surrounding this graphite is acicular, which is formed by bainite transformation. It is homogeneously composed of bainite. This bainite imparts toughness.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the heat pattern of the austempering method for the crankshaft according to the first embodiment of the present invention, and FIG. 2 is a comparative example showing the results of the plane bending fatigue test of the crankshaft according to the first embodiment. 3 is a graph showing the depth from the sample surface and the distribution state of compressive stress when the bainitic temperature is changed, and FIG. 4 is a graph showing the austempering treatment method for con rod according to the second embodiment of the present invention. FIG. 5 is a graph showing the results of the plane bending fatigue test of the connecting rod according to the second embodiment together with a comparative example. FIG. 6 is a graph showing the austempering of the gear according to the third embodiment of the present invention. An explanatory diagram showing the heat pattern of the processing method, Fig. 7 is a graph showing the results of the fatigue test of the gear according to the third example together with a comparative example, and Fig. 8 is the heat pattern of the austempering processing method applied for metal structure photography. An explanatory diagram showing the pattern, FIG. 9 is an optical micrograph showing an example of the metal structure near the surface of the sample after austempering treatment, and FIG. 10 is an example of the metal structure inside the sample after austempering treatment. It is an optical microscope photograph shown.

Claims (1)

[Claims] 1 Heat and hold spheroidal graphite cast iron in the austenitizing temperature range of 810 to 980°C for at least 5 seconds, rapidly cool from this state to a temperature of 220 to 460°C, maintain constant temperature at this temperature to perform bainite transformation, and then cool. In the austempering treatment, from the start of the bainite transformation to the completion of the bainite transformation, the austenitization of cast iron is rapidly cooled to a temperature below the MS point, and only the surface layer is turned into martensite to improve fatigue strength. Tempering method.