JPS6053736B2 - Heat-resistant spheroidal graphite cast iron - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to heat-resistant spheroidal graphite cast iron for use in turbine components, which has excellent oxidation resistance and yield strength.

Conventionally, heat-resistant spheroidal graphite cast irons that require this kind of oxidation resistance include those shown in Table 1 as exhaust gas turbine housing materials, for example.

In other words, in the conventional case, as shown in D-2 and D-field in Table 1, austenitic high nickel spheroidal graphite cast iron (Niresto) containing a large amount of Ni was often used, and other base materials were used. High-silicon spheroidal graphite cast iron, which has a ferrite structure, was sometimes used.

However, such conventional heat-resistant spheroidal graphite cast iron has the disadvantage that high nickel spheroidal graphite cast iron is expensive, and high silicon spheroidal graphite cast iron lacks oxidation resistance at high temperatures. There was a problem in that the conventional heat-resistant spheroidal graphite cast iron shown in Table 1 could not satisfy both price and performance.

The present invention has been made in view of these conventional problems, and aims to provide heat-resistant spheroidal graphite cast iron that has excellent oxidation resistance at high temperatures, has excellent yield strength, and is inexpensive. .

The heat-resistant spheroidal graphite cast iron according to the present invention has a C:1 by weight%.
．． 7-3.4%, Si: 3.5-5.5%, Mn: 0.
6% or less, Cr: 0.1-0.7%, MO: 0.3-0
．． 9%, the balance essentially consisting of Fe, Mg, Ca, Ce
It is characterized in that it is spheroidized using a graphite spheroidizing agent such as, and the base structure is mainly composed of ferrite. The reasons for limiting the component range (wt%) of the heat-resistant spheroidal graphite cast iron according to the present invention will be explained below.

C (carbon): 3.4% or less If the C content exceeds 3.4%, the amount of graphite crystallized will be excessive and the strength and toughness will decrease, so the C content should be 3.4% or less.

However, C is the main alloying element of cast iron and is generally used in a predetermined amount.
In most cases, if the content is less than about 1.7%, the amount of graphite crystallized will be small, and the fluidity of the molten metal will decrease, making it impossible to exhibit the characteristics of cast iron. Good. Si (silicon): 3.5 to 5.5% Si is generally added to cast iron for graphitization treatment, but in the present invention, in addition to that, Si is added for the purpose of improving oxidation resistance. The content is higher than normal.

That is, in order to investigate the influence of Si content on improving oxidation resistance, spheroidal graphite cast iron FCD4O (Si: 2.90
%), and the Si content was further increased to 3.28% and 4.86%.
%, 5.56%, 600℃ and 700℃
When the relationship between the retention time and the maximum oxide scale thickness was determined, the results shown in FIGS. 1 and 2 were obtained. As is clear from the figure, the resistance. Regarding oxidation, the higher the Si content, the greater the effect. However, if the Si content increases, the elongation at room temperature will decrease and the material will become brittle, so the Si content is set in the range of 3.5 to 5.5%. Mn (manganese): 0.6% or less Mn is an element that has a desulfurization effect, but since a high content stabilizes pearlite, the upper limit is set to 0.6%.

Cr (chromium): 0.1 to 0.7% Cr is an element that contributes to strengthening the ferrite base and improving oxidation resistance, but if it is less than 0.1, the effect is not sufficient, so 0.1% or more Contain.

By increasing the content, the effect of reinforcing the ferrite base becomes greater, so 0.25% is more desirable.
or more. However, if it exceeds 0.7%, carbides are generated, deteriorating mechanical properties and impairing workability, so the upper limit is set at 0.7%. MO (molybdenum):
Like Cr, 0.3 to 0.9% MO is added for the purpose of improving high temperature strength by strengthening the ferrite base. Therefore, for that purpose, the content is 0.3% or more. However, if the content exceeds 0.9%, free carbides are generated and deteriorate the mechanical properties, so the upper limit is set at 0.9%.
%. In addition, too much P (phosphorus) content will impair workability such as ductility, so it is desirable to keep it below 0.1%, and too much S (sulfur) content will inhibit the spheroidization of graphite. , preferably 0.1% or less.

As the spheroidizing agent, Mg (magnesium), Ca (
Calcium), Ce (cerium), etc. can be used, but for example, if the Mg content is too large, cementite will be stabilized, so it is desirable that the upper limit is 0.1% or less.

Example 1 The mechanical properties and oxidation resistance of spheroidal graphite cast iron having the chemical composition shown in Table 2 were investigated.

Among these, the present invention material NO. As shown in FIG. 3, 930°C x 2. After heating for a period of time, it is furnace cooled (F.C.) and then air cooled (A.C.) from 300°C to undergo ferritic annealing.The structure is mainly composed of ferrite, with a small amount of pearlite containing spheroidized cementite and carbides. It is a ferritic terrestrial graphite cast iron.

Figure 5 shows No. This is a micrograph of l. Also, comparative material N
O. 3 is high nickel spheroidal graphite cast iron, which is the comparative material No.
4 is ferritic terrestrial graphite cast iron. The test conditions for mechanical properties were: gauge length of the tensile test piece was 5', parallel length was 7'.
The diameter of the parallel part was 7'', the strain rate was 20%/min, and the test was conducted in accordance with JIS Z224l. Further, the oxidation resistance test was conducted by determining the maximum oxide scale thickness after being held in the atmosphere for 20 hours at each test temperature. As shown in FIG. 4, the present invention material No. l,2 is the test temperature 750
℃, the expensive comparative material NO. Compared to Comparative Material No. 3, it shows good oxidation resistance with no darkening at all, and even at higher test temperatures. It has considerably better oxidation resistance than No. 4, and the results are satisfactory in terms of both oxidation resistance and price.

Furthermore, as shown in Table 2, fairly good results were obtained in the mechanical property tests. Furthermore, as is also clear from Table 2, the material of the present invention is significantly superior to the comparative material in terms of tensile strength and yield strength, and also in terms of elongation.
%, it was confirmed that it has sufficient performance even when used as a material for a turbine housing.
As explained above, the heat-resistant spheroidal graphite cast iron for turpin members according to the present invention has a C: 1.7 to 3 in weight percent.
．． 4%, Si: 3.5-5.5%, Mn: 0.6% or less, Cr: 0.1-0.7%, MO: 0.3-0.9%,
Spheroidization treatment agent: 0.1% or less, the balance is substantially composed of Fe, the base structure is mainly ferrite, and graphite is spheroidized, so it has good oxidation resistance and excellent yield strength. It is a heat-resistant spheroidal graphite cast iron for turbine parts that is inexpensive, and is particularly suitable as a material for turbine parts that are required to have excellent oxidation resistance and proof strength. be.

[Brief explanation of drawings]

Figures 1 and 2 show the temperature at 600°C and 700°C, respectively, when the Si content of spheroidal graphite cast iron FCD4O was changed.
Graph showing the relationship between test time and maximum oxide scale thickness at ℃, Figure 3 is an explanatory diagram of ferritization annealing conditions in the example of the present invention, Figure 4 is the oxidation resistance test investigated in the example of the present invention A graph showing an example of the relationship between temperature and maximum oxide scale thickness, and FIG. 5 is a micrograph (10 mm) of spheroidal graphite cast iron according to an example of the present invention.

Claims (1)

[Claims] 1% by weight, C: 1.7~3.4%, Si: 3.5~
5.5%, Mn: 0.6% or less, Cr: 0.1 to 0.7
%, Mo: 0.3-0.9%, Spheroidizing agent: 0.1%
Hereinafter, a heat-resistant spheroidal graphite cast iron for use in turbine components, which has excellent oxidation resistance and yield strength, is characterized in that the balance is substantially made of Fe, the matrix structure is mainly ferrite, and the black edges are spheroidal.