JPS5976817A - Heat-treating method of abrasion resistance spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To obtain abrasion resistance spheroidal graphite cast iron which secures high toughness and is provided with high abrasion resistance and fatigure strength by heating and holding only the surface part which requires abrasion resistance, of parts cast by ferrite type spheroidal graphite cast iron, and thereafter, air-cooling it, and subsequently, hardening the whole parts. CONSTITUTION:Only the surface part of parts cast by ferrite type spheroidal graphite cast iron whose pearlite area rate is <=30% is heated and held at 900- 1,000 deg.C for five seconds - five minutes. As a result, the base ground of said surface is converted to austenite. Subsequently, said surface which is heated and held is air-cooled, and the austenite is converted to pearlite. Thereafter, said whole parts are brought to hardening or austempering treatment, and the pearlite is transformed to a single structure of martensite (at the time of hardening) or bainite (at the time of austempering treatment) which does not contain ferrite. In this way, abrasion resistance and fatigue strength of the surface of said parts are improved.

Description

[Detailed description of the invention] It is related to.

Conventionally, spheroidal graphite cast iron has been widely used to house parts such as gears and sprockets. It is desirable that such power transmission parts have high toughness as a whole. Therefore, as shown in, for example, JP-A-52-62116 and JP-A-56-130453, the above requirements can be met by using cast iron toughened by forming a fine mixed structure for the matrix.

On the other hand, since the tooth surfaces on the surfaces of parts such as gears and sprockets strike against the tooth surfaces of other parts, it is desirable that these tooth surfaces have high wear resistance. However, cast iron, which has a fine mixed structure as described above, contains ferrite in its structure, so it has poor wear resistance and also does not have high fatigue strength because ferrite acts as the starting point for fatigue cracks. When used, insufficient wear resistance and fatigue strength were unavoidable.

The present invention has been made in view of the above circumstances, and while ensuring high toughness as a ferrite base as a whole (allowing the inclusion of a small amount of pearlite of 30% or less), desired parts of the surface are made of martensite or The present invention provides a heat treatment method for producing wear-resistant spheroidal graphite cast iron having high wear resistance and fatigue strength as a bainitic structure.

The heat treatment method for wear-resistant spheroidal graphite cast iron of the present invention is to treat only the surface areas that require wear resistance of parts cast from ferritic spheroidal graphite cast iron with a pearlite surface ratio of 30% or less. It is characterized in that it is heated and held at .degree. C. for 5 seconds to 5 minutes, then cooled in air, and then the entire part is quenched or austempered.

In the present invention, the material is ferritic cast iron with a pearlite area ratio of 30% or less, because the pearlite content is 30%.
%, it already has the characteristics of ferritic cast iron? This is because it is undesirable because it damages the area and lacks toughness.

The above ferritic cast iron material? 5 at 900-1000°C
By heating and holding for up to 5 minutes, the matrix on the surface becomes austenite, and at the same time, the diffusion solid solution of carbon from graphite to this austenite is promoted, and by subsequent air cooling, the austenite becomes pearlite.

When quenching or austempering is then performed, the pearlite formed on the surface transforms into a single structure of martensite (during quenching) or bainite (during austempering) that does not contain ferrite, improving the wear resistance of the surface. This improves fatigue strength. On the other hand, since the internal base is mainly composed of ferrite, it is not affected much by the above heat treatment, and most of the ferrite remains. When the pearlite is hardened or austempered, it transforms into martensite (during hardening) or bainite (during austempering), resulting in a two-phase structure with the ferrite, which maintains high toughness without loss. Become something to do.

Note that if the above heat treatment is performed at a temperature below 900°C, the rate of solid solution of carbon from graphite is slow (the process takes a long time).On the other hand, if the heat treatment is performed at a temperature exceeding 10506C, there is a risk that the material will melt locally. In addition, it is not preferable because it deteriorates the quality structurally.Also, heating and holding for 5 seconds to 5 minutes is required for more than 5 seconds to fully dissolve and diffuse the carbon into the austenite. On the other hand, if the heating time exceeds 5 minutes, the crystal grains will become coarse and the high temperature at the surface will be transmitted to the inside, which is not preferable.

Examples of the present invention will be described below.

[First Example] 0 Sample composition C: 3.72, Si: 2.75, M
n: 0.23.

P: 0.03, S: 0.0]5,
Mg: 0.044.

(more than % by weight), remaining Fc (FCD45) ○ Sample shape 20 x 25 x 40 mm O pearlite area ratio 0 ratio: Complete ferrite base (annealing the above sample to complete ferrite) 0 heat treatment
Process ■ A heat-treated sample for pearlitization of the surface area was placed on a surface plate and heated with an oxygen/acetylene burner to raise the temperature to 930-950℃ in about 30 seconds. Sample A was held at the temperature for 90 seconds and then cooled in air. Sample B was held at the temperature for 30 seconds and then cooled in air.

The cross-sectional hardness of these samples A and B after the heat treatment is shown in FIG. In addition, a photograph of the structure of the above sample after heat treatment is shown in Fig. 5 (magnification: 100x).

The microstructure photograph in FIG. 5 shows sample A at zero depth from the surface.
This is a photograph of the bottom surface after 5 degrees of cutting, and it can be seen that it has a very fine pearlite structure.

(2) Heat treatment for toughening Samples A and B, which had been subjected to the heat treatment described in (1) above, were quenched in a furnace under the following three different conditions.

(Heat treatment conditions) (After heating at 11,850°C for 32 minutes, water quenching (2,183
After heating at 0°C for 30 minutes, water quenching (3183,0'CX
After heating for 60 minutes and water quenching, the cross-sectional hardness of the sample after the heat treatment is shown in FIG. 2, and the cross-sectional hardness of sample B is shown in FIG.

As clearly shown in FIGS. 2 and 3, the surface portion of each sample has sufficient hardness. In addition, the above-mentioned quenching place +
l (11')
Figure 7 shows a photograph of the internal structure (the magnification is 40
0 times). As shown in FIG. 6, the surface portion, which had a pearlite structure before quenching, now has a martensitic structure. On the other hand, the interior, which had a ferrite structure before quenching, becomes a two-phase structure in which a whitish-gray ferrite structure and blackish-gray island-like martensite are mixed, as shown in the photograph in FIG. In addition, the large black dot-shaped portions in FIGS. 6 and 7 are graphite.

Hereinafter, the temperature conditions for heat treatment for toughening will be described in (1) above. As can be seen from the well-known equilibrium phase diagram of the F''e-C system, the A, transformation temperature is lower for the Fe-Fe5C system than P.
e - lower than that of the C system.

Therefore, when spheroidal graphite cast iron containing pearlite and ferrite is heated, pearlite quickly transforms into austenite. On the other hand, ferrite transforms into austenite only after carbon from graphite diffuses, but this diffusion of carbon from graphite progresses along ferrite grain boundaries, so austenitization also progresses slowly along grain boundaries. (Refer to the heating transformation characteristics diagram of pearlite and ferrite in Figure 10). Therefore, ■ The heat treatment temperature for toughness ratio should be higher than A1 change, 4 points, but 850
If the temperature exceeds 750°C, carbon diffusion becomes rapid (and it is difficult to set the heat treatment time to obtain an appropriate austenitization rate.
If it is less than C, the austenitization of ferrite does not progress,
The interior cannot have the above-mentioned two-phase structure.

[Second Example] 0 Sample composition C: 3.55, Si: 2.86, M
n: 0.24 P: 0.04, S: 0.012
．． Mg: 0.038 (more than weight %), remainder rlFe
(PCI) 45) 0 Sample shape Same as the first example 0 Pearlite area ratio 5% (others are ferrite) 0 Heat treatment ■ Heat treatment for pearlitizing the surface area Same as the first example. However, the heating holding time is only 90 seconds.

The cross-sectional hardness distribution of the sample after the heat treatment is almost the same as that in the second example. However, the internal hardness was a little high at Hv (Vickers hardness) -170.

■　Heat treatment for toughening The sample subjected to the heat treatment described in ■ above is Austempered under certain conditions.

(1) After heating at 830°C for 60 minutes, hold at 250°C for 2 hours (2) After heating at 830°C for 60 minutes, at 350°GX2
The cross-sectional hardness of the sample after the time-holding heat treatment is shown in Figure 4.

As in the first embodiment, the hardness of the surface portion is sufficiently high. Further, FIG. 8 shows a photograph of the surface structure of the sample subjected to the austempering treatment (1), and FIG. 9 shows a photograph of the inside structure (magnification: 400x in each case). As shown in FIG. 8, the surface portion, which had a pearlite structure before the austempering process, now has a bainite structure. On the other hand, the interior, which was a ferrite structure before the austempering process, becomes a two-phase structure in which a whitish-gray ferrite structure and a blackish-gray island-like bainite structure are mixed, as shown in the photograph in FIG.

As explained in detail below, the heat treatment method of the present invention forms a single structure of martensite (during quenching) or bainite (during austempering) on the surface of spheroidal graphite cast iron, and ferrite and bainite are formed inside. It forms a two-phase structure consisting of martensite (during quenching) or bainite (during austempering), and therefore, according to the heat treatment method of the present invention, the surface portion has particularly excellent wear resistance and fatigue strength, Overall, wear-resistant spheroidal graphite cast iron with high toughness is obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the cross-sectional hardness of the sample after the surface pearlitization treatment according to the first embodiment of the present invention. Figure 2 is a graph showing the heating holding time of the surface pearlitization treatment for 90 minutes.
A graph showing the cross-sectional hardness of the samples subjected to the heat treatment according to the first and third examples with 0 seconds as the heating holding time of the above-mentioned six-sided pearlitization treatment and 30 seconds. Fig. 4 is a graph showing the cross-sectional hardness of the sample heat-treated according to the second embodiment of the present invention, and Fig. 5 is a graph showing the cross-sectional hardness of the sample heat-treated according to the second embodiment of the present invention. 6 and 7 are micrographs showing the surface structure and internal structure of the sample subjected to the heat treatment according to the first embodiment, respectively. 9 and 9 are micrographs showing the surface structure and internal structure of the sample heat-treated according to the second embodiment, respectively, and FIG. 10 shows the heating transformation characteristics of pearlite and ferrite according to the present invention. 1 - It is a graph. Figure 2 @ Table Ilb Force et al.
/m1 3rd drawing surface

Claims (1)

[Claims] Parts cast from ferritic spheroidal graphite cast iron with a pearlite area ratio of 30 inches or less are heated and held at 900 to 1000°C for 5 seconds to 5 minutes, and then air cooled, A heat treatment method for wear-resistant spheroidal graphite cast iron, characterized in that the entire part is then quenched or austempered.