GB2147007A

GB2147007A - Spheroidal graphite ferrite cast iron

Info

Publication number: GB2147007A
Application number: GB08424106A
Authority: GB
Inventors: Yoshikazu Fukuhara; Shinichi Ohama
Original assignee: IHI Corp
Current assignee: IHI Corp
Priority date: 1983-09-27
Filing date: 1984-09-24
Publication date: 1985-05-01
Also published as: JPS6070162A; JPS6250546B2; IT1176831B; DE3432525C2; IT8422871A1; GB8424106D0; CH660754A5; IT8422871A0; GB2147007B; FR2552447B1; DE3432525A1; FR2552447A1

Abstract

Heat resistant spheroidal graphite ferrite cast iron suitable for fabricating mechanical parts such as turbochargers, exhaust manifolds comprises 2.6-3.8% of C, 3-4.2% of Si, less than 0.5% of Mn, less than 0.1% of P, less than 0.03% of S, less than 0.6% of Mo and 0.02-0.15% of Mg plus rare earth elements, the mean ferrite grain size being less than 25 microns. Such cast iron when repeatedly subjected to high temperatures has high resistance to oxidation at the blue brittleness temperature range, a high degree of toughness and a high fracture resistance. <IMAGE>

Description

SPECIFICATION Spheroidal graphite ferrite cast iron The present invention relates to spheroidal graphite ferrite cast iron and more particularly heat-resistant spheroidal graphite ferrite cast iron which exhibits a high degree of toughness in the blue brittleness temperature range.

Spheroidal graphite cast iron (such as that equivalent to Japanese standard JIS FCD 40) has been widely used as ferrite cast iron for the fabrication of, for example, turbine casings of turbochargers and exhaust manifolds, but it has been found that ferrite cast iron has insufficient heat resistance (especially resistance to oxidation) as the exhaust gas temperature rises as high as 800"C.

To overcome this problem, the use of cast iron with a high silicon content which has high resistance to oxidation has been proposed, but as compared with the conventional spheroidal graphite cast iron, cast iron with a high silicon content has the drawback that repetitive heating weakens it so that cracks are propagated during use. One of the major reasons is that the fracture elongation or elongation to fracture is as little as 2 to 3% at the so-called blue brittleness temperature range between 300 and 400"C.

Therefore use of cast iron having a high silicon content containing 4% of silicon (Si) and 1% of molybdenum (Mo) has been proposed. Compositions as herein expressed are in terms of percent by weight unless otherwise stated. Cast iron with a high silicon content has been used in the fabrication of some casings of thurbochargers for automobiles and exhaust manifolds; but it has been found that if only the chemical composition is specified, the fracture elongation maybe unacceptably small at temperatures between 300 and 400"C and it is difficult to obtain a fracture elongation higher than 5%.

In view of the above, it is an object of the present invention to provide spheroidal graphite ferrite cast iron which has a high fracture elongation at the blue brittleness temperature range and which is capable of withstanding repeated heating.

According to the present invention a spheroidal graphite ferrite cast iron comprises 2.6 to 2.8% carbon (C), 3 to 4.2% silicon (Si), 0 to 0.6% molybedenum (Mo), and between 0.02 and 0.15% of mgnesium (Mg), one or more rare earth elements or a combination thereof, the average ferrite grain size being less than 25 microns.

All percentages are expressed by weight.

Preferably the cast iron comprises 0 to 0.5% manganese (Mn), preferably 0 to 0.1% phosphorus (P), and preferably 0 to 0.03% sulphur (S). Some rare earth element or elements are preferably present as well as some magnesium. The magnesium and rare earth elements act as spheroidizing agents. Particularly good results are obtained if the silicon content is between 3.0 and 4.0%.

Various materials are usually present at least as impurities. For this reason the following ranges are to be found: Mo - 0.01 to 0.6%; Mn - 0.01 to 0.5%; S - 0.005 to 0.03% and P - 0.005 to 0.1%.

It is preferred that the molybdenum content is between 0.05 and 0.5% molybdenum, and more preferably between 0.1 and 0.4%.

Thus according to the present invention a particular preferred spheroidal graphite ferrite cast iron comprises 2.6 to 3.8% of carbon (C), 3 to 4.2% of silicon (Si), 0 to 0.5% of manganese (Mn), 0 to 0.1% of phosphorus (P), O to 0.3% of sulphur (S), 0 to 0.6% of molybdenum (Mo) and 0.02 to 0.15% of magnesium (Mg) plus rare earth elements, the average ferrite grain size less than 25 microns.

The invention may be put into practice in various ways but one composition of spheroidal graphite ferrite cast iron composition will now be described by way of example, with reference to the accompanying drawings and examples, in which: Figure 1 is a graph showing the relationship between the silicon content in spheroidal graphite ferrite cast iron and the resistance to oxidation; Figure 2 is a graph showing the relationship between the silicon content in spheroidal graphite ferrite cast iron and the fracture elongation; Figure 3 is a graph showing the relationship between the molybdenum content of spheroidal graphite ferrite cast iron and the fracture elongtioh; and Figure 4 is a graph showing the relationship between the ferrite grain size and the fracture elongation.

The chemical composition of a heat-resistant, spheroidal graphite ferrite cast iron in accordance with the present invention will be described below with reference to Figures 1 to 4 which were obtained from extensive studies and experiments conducted by the inventors.

Silicon has the highest resistance to oxidation. Figure 1 shows one example of the experiments conducted by the inventors showing the relationship between silicon content in spheroidal graphite ferrite cast iron and resistance to oxidation. In Figure 1, plotted along the ordinate is the ratio, expressed as a percentge, of the thickness of an unaffected portion and the original thickness obtained by the microscopic inspection of a cross section view of each sample after it has been heated from room temperture to 8000C times. it is seen from Figure 1 that if the silicon content is higher than 3.5%, the variation in thickness becomes very much smaller. In practice if it is assumed that the thickness of the unaffected portion must be higher than 85%, the silicon content must be higher than 3%.

Figure 2 shows the relationship between the silicon content and the elongation to fracture, the solid line showing results at 400"C and the dot dash line showing results at 300"C. It is seen from Figure 2 that if the silicon content is higher than 3%, the elongation to fracture is decreased in the temperature range of 300 to 400"C and that if the silicon content is higher than 3.9%, the fracture elongation decreases to a very low value (minimum value). The upper dashed line shows the results at room temperature (RT). a silicon content above 3% produces thickness variation of an unaffected portion higher than 85%.To produce a fracture elongation higher than 5% at 400"C together with a thickness variation above 85% the silicon content must be between 3 and 4.2%, preferably 3 to 4%, together with a suitable amount of carbon and molybdenum present and a suitably small ferrite grain size. These points are discussed below.

Figure 3 shows the relationship between the molybdenum content in a cast iron whose silicon content is such that the fracture elongation at 400"C would be less than 5% if there were no molybdenum present on one hand and the fracture elongation in percent on the other hand. The upper dashed line shows results at room temperature (RT). It is seen from Figure 3 that the fracture elongation is much improved ie. much higher if the molybdenum content is between 0.05 and 0.5%, the fracture elongation increasing above 5%.

However, because of the effect obtained by decreasing the ferrite grain size, the molybdenum content can be extended up to 0.6 without adversely affecting the fracture elongation. However, the carbon content in cast iron is generally high so that carbide tends to be produced and consequently toughness is decreased.

Therefore, according to the present invention, the molybdenum must be less than 0.6%, that is between 0 and 0.6%.

If the carbon content is less than 2.6%, the amount of spheroidal graphite is insufficient so that it is difficult to reduce the particle size of the crystals sufficiently. Moreover, castability is adversely affected. On the other hand, if the carbon content is higher than 3.8%, the graphite particles are enlarged, resulting in a decrease in toughness and an increase in the amount of dross after casting. Therefore according to the present invention, the carbon conent must be 2.6 to 3.8%.

If the manganese content is in excess of 0.5%, pearlite tends to be produced and the fracture elongation is adversely affected. That is, it becomes difficult to attain the fracture elongation of higher than 5%. Therefore the upper limit of the manganese content is preferably 0.5%, that is between 0 and 0.5%.

If the phosphorous content is higher than 0.1%, segregation tends to occur at boundaries with a resultant decrease in fracture elongation. Therefore it becomes very difficult or impossible to attain a fracture elongation higher than 5%. As a result, according to the present invention, the phosphorus content is preferably less than 0.1% (between 0 and 0.1%) as in conventional cast iron.

Sulphur tends to cause segregation at boundaries and adversely effects the spheroidization of graphite. If the sulphur content is high, it becomes difficult to attain the elongation of higher than 5%. therefore, the sulphur content is preferably less than 0.03% that is between 0 and 0.03%, as in conventional cast iron.

According to the present invention, the remainder, that is, magnesium, one or more rare earth elements, or a combination thereof is 0.02 to 0.15%. If their content is less than 0.02%, the graphite does not usually become sufficiently spherical. If their content is less than 0.02%, the graphite does not usually become sufficiently spherical. If their content is higher than 0.15%, substantially similar effects can be obtained, but excessive amounts of manganese oxides tend to be produced. It is desirable that magnesium is not the only agent used for obtaining spheroidal cast iron and that in additoon to magnesium one or more rare earth elements are used so that spheroidal graphite is finely divided and dispersed.

Figure 4 shows the relationship between the mean or average ferrite grain size obtained from spheroidal cast iron comprising the above preferred composition of 2.6 to 3.8% of carbon, 3 to 4.2% of silicon and less than 0.6% of molybdenum in accordance with the testing method specified in JIS G 0552 and the fracture elongation at 300 to 400"C (lower curve) and at room temperature (upper curve). It is seen from figure 4 that the smaller the mean ferrite grain size, the higher the fracture elongation becomes. It follows therefore that in order to obtain a fracture elongation of higher than 5% with the above-described chemical composition, the mean ferrite grain size must be less than 25 microns.

In order to decrease the ferrite grain size, conventional heat treatment methods may be used in addition to spheroidizing agents including for example rare earth elements so that spheroidal graphite can be finely divided and dispersed.

The results of comparison tests between the cast iron in accordance with the present invention and a conventional cast iron will be described. Table 1 shows the chemical composition of typical test pieces; Table 2 shows mechanical properties; and Table 3 shows the resistance to oxidation, ferrite grain sizes and heat treatment conditions. Test pieces 1 to 3 were made from cast iron in accordance with the present invention and their silicon content was increased as specified. Magnesium plus rare earth elements (RE) were used as spheroidizing agent. Test pieces 4 to 6 were made of conventional cast iron. Test piece 4 has the highest molybdenum content; test piece 5 has the highest silicon content and has a large grain size; and test piece 6 has the lowest silicon content among the test pieces which are equivalent to JIS spheroidal graphite cast iron.Test piece 6 shows a high fracture elongation at 400"C but is inferior in resistance to oxidation to the cast iron in accordance with the present invention. Test piece 6 contains magnesium alone as spheroidizing agent.

TABLE 1 No. C Si Mn Mo Mg + RE 1 3.27 3.21 0.15 0.29 0.06 2 3.55 3.52 0.11 0.32 0.10 3 3.32 4.13 0.15 0.32 0.06 4 2.84 3.55 0.35 1.10 Mg 0.04 5 3.00 4.01 0.30 - Mg 0.04 6 3.42 2.45 0.25 - Mg 0.05 In examples 1 to 3: P = 0.04-0.06,S = 0.01 - 0.02 In examples 4 to 6: P = 0.01 - 0.06, S = 0.01 - 0.02 TABLE 2 0.02% Tensile Elongation {%) No. Resistance strength Room 400"C kg flmm2 kg flmm2 temperature 1 37.8 (370) 50.4 (494) 12.7 16.4 2 37.8 (370) 51.6 (506) 19.1 17.1 3 49.5 (485) 62.5 (613) 17.3 15.0 4 47.8 (468) 55.4 (543) 10.0 5.3 5 45.2 (443) 57.8 (566) 18.9 3.0 6 26.2 (258) 41.4 (403) 25.8 15.0 Values in parentheses are MPa.

TABLE 3 No. Oxidation Ferrite Heat Treatment Character- grain is tic size (variation {pom) in thickness) r%) 1 88.7 20 750"C x 3h, Furnace Cooling (FC) 2 92.8 16 750"C x 3h, Furnace Cooling (FC) 3 94.0 18 750"C x 3h, Furnace Cooling (FC) 4 - 35 920"C x 10h, FC and 800"C x 2h,FC 5 - 32 950 C x 5h,FC 6 79.8 23 750"C x 3h, FC From the above tables, it is seen that the spheroidal cast iron test pieces 1,2 and 3 in acordance with the present invention have fine ferrite crystal grains, are excellent in resistance to oxidation and have the highest fracture elongation at 400"C.

Test pieces 4 and 5 satisfy the chemical composition defined by the present invention, but the fracture elongation at 400"C is remarkably small because of large grain size.

Test piece No. 6, which is equivalent to JIS FCD 40, has a high fracture elongation at 400"C, but is inferior in resistance to oxidation to the cast iron in accordance with the present invention.

As described above, the spheroidal cast iron in acordance with the present invention has a silicon content higher than JIS spheroidal cast iron, but less than the so-called high silicon content cast iron. However, the cast iron in accordance with the present invention has exellent resistance to oxidation and ductility or toughness is increased because the grain size is small. Moreover, the cast iron of the present invention has a high fracture elongation at the blue brittleness temperature range. Therefore when it is used to fabricate mechanical parts such as turbochargers, exhaust manifolds or the like which are repeatedly subjected to high temperatures, oxidation and cracking can be substantially obviated or prevented and a long service life provided.

Claims

1. A spheroidal graphite ferrite cast iron comprising 2.6 to 3.8% carbon (C), 3 to 4.2% silicon (Si), 0 to 0.6% molybdenum (Mo), and between 0.02 and 0.15% of magnesium (Mg), one or more rare earth elements, or a combination thereof, the average ferrite grain size being less than 25 microns.

2. A cast iron as claimed in claim 1 in which there is 0 to 0.5% manganese (Mn).

3. A cast iron as claimed in Claim or 2 in which there is0 to 0.1% phosphorus (P).

4. A cast iron as claimed in Claim 1,2 or 3 in which there is0 to 0.03% sulphur (S).

5. A cast iron as claimed in any one of the preceding claims in which there is between 3.0 and 4.0% silicon.

6. A cast iron as claimed in any one of the preceding claims in which there is between 0.05 and 0.5% molybdenum (Mo).

7. A cast iron as claimed in claim 6 in which there is between 0.1 and 0.4% molybdenum.

8. A cast iron as claimed in any one of the preceding claims in which a rare earth element is present as well as magnesium.

9. A cast iron composition substantially as specifically described herein with reference to test pieces 1,2 or3.