JPS6179718A - Method for austempering cast iron - Google Patents

Abstract

PURPOSE:To improve the toughness and fatigue strength of spheroidal graphite cast iron by quenching the cast iron once to the Ms point or below to form martensite only in the surface layer before bainite transformation is completed in bainite transformation subjecting the cast iron to beat treatment, quenching and isothermal holding. CONSTITUTION:Spheroidal graphite cast iron is heated at a temp. in the austenitizing temp. range of 810-980 deg.C for >=5sec, and it is quenched to 220-460 deg.C and held at the temp. to carry out bainite transformation. The cast iron is then cooled. Thus, the cast iron is austempered. During the bainite transformation, the cast iron is quenched once to the Ms point or below to convert untransformed austenite into martensite. By expansion caused by the transformation, compressive stress is produced up to the desired depth. By this treatment, high fatigue strength as well as high toughness is provided to the cast iron.

Description

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

(Prior art) Traditionally, austempering treatment, which converts the matrix structure into bainite, has been used to improve the toughness of spheroidal graphite cast iron.
T 4,222,793j3, Japanese Patent Publication No. 57-1932
No. 0 Mochima Sho 5f> -13H53@ and others.

In addition, when the above austempered material has low surface hardness and lacks fatigue strength, shot peening is used to apply processing strain to the cast iron surface to increase surface hardness and improve fatigue strength. There are techniques to improve it.

Other methods for improving the fatigue strength of A-stempered spheroidal graphite cast iron include rolling 1"0 machining, inducing transformation of retained austenite into martensite during cutting, and deep cooling treatment of retained austenite. There are methods that convert the material to martensite, and methods that harden the surface through carburization, etc., but all of these methods require a separate treatment process from the austempering process, making the process complicated and reducing the overall processing time. As the length increases, there are problems such as the need for a processing film width 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 for improving fatigue strength are performed during austempering, without using surface hardening methods such as shot peening. The purpose is to

(Structure of the Invention) The austempering method of the present invention is a method for treating spheroidal graphite cast iron with a
Heat and hold for at least 5 seconds in the A-stenitizing temperature range of 10 to 980°C, and from this state
In an austempering treatment in which the material is rapidly cooled to a temperature of °C, kept constant at this temperature to undergo bainite transformation, and then cooled, the material is rapidly cooled once to a temperature below the Ms point from the start of the bainite transformation to the completion of the bainite transformation, It is characterized by improving fatigue strength by making only the surface layer martensite.

The above-mentioned Austety 1-ization involves heating and holding in a temperature range of 810 to 980°C for 5 seconds or more, but when the temperature is 810°C
If the temperature is less than 980℃, sufficient austenitization cannot be obtained.
If the temperature exceeds 5 seconds, the crystal grains become coarse and the strength decreases, and if it is shorter than 5 seconds, only the surface part will be austenitized by induction hardening, and even if local austempering treatment is performed, sufficient and uniform austenitization will not be achieved. Since it is difficult to obtain, it is carried out within the above range.

The constant humidity treatment for bainite transformation is performed by immersing and quenching the austenitized spheroidal graphite vI iron in a constant temperature salt bath, and maintaining the constant temperature at 220 to 460 degrees Celsius.
Set to . This treatment temperature is 220℃ for spheroidal graphite cast iron.
When rapidly cooled to a lower temperature, that is, quenched, martensitic transformation occurs and expansion becomes large, resulting in quench cracking during quenching, or large dimensional changes when the shape is complex.

Furthermore, at temperatures exceeding 460'C, the 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 baikite transformation, the bainite transformation is rapidly cooled to below the M s point (martensite transformation FF3 starting temperature) by water cooling or the like. It is preferable to carry out the process immediately after the start of transformation because it is highly effective.If the process is rapidly cooled to below the Ms point before the start of bainite transformation, the transformation of untransformed austenite to martensite is rapid;
Cold IJI control for toughness and compressive stress I'/ is in-house. On the other hand, after the bainite transformation is completed, this austenite becomes beyphyte and the remaining ILIi austenite, and since the retained austenite has a metastable structure, it is difficult to maintain the temperature at -80°C below the transformation temperature for a long time. There is a need, ff1T! [! Time gets better.

In addition, rapidly cooling only the surface part to a temperature below the Ms point means that the untransformed austenite 1~ in the process of bainite transformation is
By cooling the untransformed austenite to a temperature below the s point, the untransformed austenite becomes martensite, and the expansion caused by this transformation applies compressive stress to a desired depth, resulting in particularly high fatigue strength as well as toughness. Therefore, at least a desired depth from the surface must reach the Ms point or below. This cooling is preferably performed by 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).

Further, after cooling to the above-mentioned 1vls point or less, it is preferable to perform constant temperature treatment to 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. A constant temperature treatment for 5 minutes or more can improve toughness and fatigue strength at the same time, whereas a treatment for more than 8 hours will cause a decrease in toughness and an increased loss of thermal energy. That is, the isothermal treatment for vapyite transformation is preferably carried out for at least 5 minutes after cooling below MsJ, and does not exceed 8 hours in total.

Cooling after bainite transformation is performed by natural cooling or the like. At that time, S +Il+ glands of spheroidal graphite & 8 iron migrate to the left side in the case of unalloyed, 1vln, Cu, Mo.

In the case of a high alloy having 3 N1 etc., it shifts to the right side.

If it shifts to the left, 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, the conversion of bainite ground, that is, untransformed austenite, to bainite is completed in a short time. On the other hand, in the case of high alloys, on the contrary, it takes a long time. In this phenomenon, a product satisfying the required characteristics can also be obtained by natural cooling after cooling below the Ms point. Furthermore, since natural cooling results in a large time loss in the next step, cooling after bainite transformation may be carried out by washing with water or hot water.

Spheroidal graphite cast iron that has been subjected to austempering treatment including cold IJ to below the Ms point as described above has a compressive stress of 10 to 120
It is processed in a manner having one shift of kQ r/ff1m2. The improvement in fatigue strength is at least 10kOf,' mm2
The stress in the tensile direction cannot significantly improve the fatigue strength, and in order to obtain a compressive stress exceeding 120 kgf/mm2, the stress must be below the Ms point. Cracks will occur due to thermal strain and transformation strain at 6111.

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 /mm2 remains.

The chemical composition of the spheroidal graphite vJ iron in the present invention is that used, for example, in automobile parts such as transmission gears, shifts, moving parts, etc., and exhibits sufficient fatigue strength due to the non-alloy material with low hardenability. It is possible to form compressive stress and obtain wear resistance. In addition, for large casting butchers, 0.2~0.9%Mn, QJ~l, 5% due to the need to ensure hardenability.
CLJ, 0.03-4.0% MO, 0.2-3.5
%N1 is used in combination of two or more types.

Furthermore, the present invention austemper ff1l! ! By combining this method with conventionally known surface hardening methods such as roll processing and shot peening, even more stable and high fatigue strength can be achieved.
It is something that can be admired.

(Effects of the Invention) According to the present invention, the fatigue strength of spheroidal graphite cast iron after austempering can be improved by using this A-Stenbar relief and a series of rapid cooling 1t! Since the film is obtained by leaving compressive stress in the surface layer using a process, the process can be easily carried out in a short time, improving processing efficiency, and can be carried out with a simple film width. .

(Example) The present invention will be explained in detail 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.

CSi Mn P S3.53
2,45 0-29 0.021 0.007C
u Mo Ni MgO9830,08
0,480,042 The heat pattern of the austempering treatment performed is shown in FIG. Zunawara heated spheroidal graphite cast iron to 890℃.
After holding for 2.5 hours at the austenitizing temperature of 38
It is immersed in a constant temperature salt bath at 0°C to start bainite formation, and after 30 minutes it is immersed in boiling water at 100°C below the Ms point for about 3 seconds, and then charged into an electric furnace at 380°C to form bainite. After 90 minutes of cooling, the mixture is cooled with water.

The material with the above chemical composition was austempered using the heat pattern shown in Figure 1, machined into pin diameter 51 mm, journal diameter 5 Q mm, crank radius 43 mm, pin J3 and journal fillet R3,0, and then flat bent. The results of the fatigue test are shown in FIG. 2 together 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 austenitizing at 890°C x 2.5)-1r, bainizing at 380°C x 2 hours, and then air cooling.

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

Furthermore, the constant temperature for bainitization in the heat pattern shown in Figure 1 was changed to 250°C and 370°C, and the distance from the surface of the sample (diameter 45+nm, length 100mm) was otherwise treated in the same manner. Figure 3 shows the results of determining the distribution of residual compressive stress.

In FIG. 3, it can be seen that the specimen with a bainitic temperature of 250° C. (characteristic (2)) has a larger residual compressive stress in the vicinity of the surface and has a higher fatigue strength than the specimen (characteristic (2) at 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.

CSi Mn P 3.46 2.17 0.23 0.02
73 CLI MO 0,0060,730,041 The heat pattern of the austempering treatment performed is shown in FIG. Spheroidal graphite cast iron was heated and held at an austenitizing temperature of 870°C for 2 hours, then immersed in a tHW salt bath at 375°C to start bainitizing, and after 25 minutes M
After being immersed in hot water at 90 to 100° C. below the s point for about 2 seconds, it is placed in an electric furnace at 375° C. to proceed with bainite formation, held for 85 minutes, and then air cooled.

The material with the above chemical composition was austempered using the heat pattern shown in Figure 4, and a connecting rod (A
1; I, the results of a plane bending fatigue test on this are not shown in FIG. 5 together with comparative examples (casting and shelling).

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, and a bending member is applied, and the test is performed until the specimen breaks under various applied loads. This is a result of measuring 1 IIN of repeated bending. In addition, the comparative example is one in which spheroidal graphite cast iron having the same composition as the one described above was simply manufactured, but was not subjected to austempering treatment.

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

Actual fM 1 row 3 This example shows an example of austempering treatment of g) 7- using spherical black #i1 Si iron.

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

CSi Mn P 3.62 2.53 0.30 0.03
1S Cu Mo ~1g06
006 0.72 0.08 0,042
The heat pattern of the 7j austempering process was
As shown in the figure. Spheroidal graphite cast iron is heated and kept at an austenitizing temperature of 890°C for 2 hours, then immersed in a constant temperature salt bath of 270°C to start bainitizing, and after 60 minutes, it is soaked in a salt bath of 180°C below the Ms point. After being immersed in the water for about 15 seconds, it is placed in an electric furnace at 270°C to proceed with bainite formation, held for 60 minutes, left to cool, and then washed with hot water.

A material with the above chemical composition is used with a gear ratio of 3.85.
0, number of teeth 17, module 2.25, outer diameter 200m
After processing the helical gear A7- made of +++, austempering it with the heat pattern shown in Figure 6, and applying compressive stress to at least the tooth bottom, we compared the results of a fatigue test using a power circulation gear testing machine. It is shown in FIG. 7 along with an example (conventional austempering only).

In this fatigue test, the number of repeated engagements N until the sample breaks after a load torque of 15.2 kgm was measured. 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 austenitizing at 890°C x 21-1 r, bainizing 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 state of metal coarse weave when spheroidal graphite cast iron made of the same chemical composition as that used for the gear was austempered using the heat pattern shown in FIG. 8 was shown in FIGS. 9 and 10. This is shown in the metal structure photograph taken with a microscope.

Austempering treatment involves heating spheroidal graphite cast iron to a temperature of 890
After holding at an austenitizing temperature of 'C for 1.5 hours, it was immersed in a constant temperature salt bath at 230°C to start bainitization, and 30 minutes later, it was immersed in water at 30'C below the Ms point for about 2 seconds. Thereafter, it is placed in an electric furnace at 230°C to proceed with bainite formation, held for 90 minutes, cooled in air, and then 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.

On the other hand, Figure 10 shows the internal structure that has not been affected by rapid cooling below the Ms point. It is homogeneously composed of bainite. This bainite provides toughness.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the heat pattern of the austempering method for the crankshaft in the first embodiment of the present invention, and FIG. 2 is a comparative example of the results of the plane bending fatigue test of the crankshaft according to the first embodiment. Figure 3 is a graph showing the depth from the sample surface and the distribution state of compressive stress when the bainitization temperature is changed. An explanatory diagram showing the heat pattern of the treatment method, FIG. 5 is a graph showing the results of a plane bending fatigue test of connecting rods according to the second actual fU example together with a comparative example, and FIG. Fig. 7 is a graph showing the results of the fatigue test of the gear according to the third embodiment along with comparison 19+, Fig. 8 is for metallographic photographs. An explanatory diagram showing the heat pattern of the austempering method applied to the sample, Fig. 9 is an optical micrograph showing an example of the metal structure near the surface of the sample after the austempering process, and Fig. 10 shows the austempering process. This is an optical micrograph showing an example of the internal metallographic structure of the sample. Fig. 1 Fig. 2 Closing l!lφKyosaka N Fig. 3 Pasig 50 distance 5 back (pm) Fig. 4 Fig. 5 Figure 6 Figure 7

Claims (1)

[Claims] (1) Spheroidal graphite cast iron is heated and held in the austenitizing temperature range of 810 to 980°C for at least 5 seconds, then rapidly cooled from this state to a temperature of 220 to 460°C, kept constant at this temperature to undergo bainite transformation, and then In the cooling austempering treatment, the cast iron is rapidly cooled to a temperature below the Ms point from the start of bainite transformation to the completion of bainite transformation, and only the surface layer is turned into martensite to improve fatigue strength. Austempering method.