Method of producing austempered spheroidal graphite cast iron body

Abstract

 57 An excellently austempered spheroidal graphite cast iron body, which may be a thick-wall body, is obtained by using a spheroidal graphite cast iron composition consisting essentially of 3.0-4.0 wt% of C, 1.5-3.0 wt% of Si, 0.3-0.8 wt% of Mn, 0.3-2.0 wt% of Cu, 0.005-0.2 wt% of a spheroidizing agent such as Mg, Ca or Ce, 0-0.1 wt% of Mo, 0-0.3 wt% of Ni and the balance of Fe, and by accomplishing the cooling of the cast iron body from the austenite stabilizing temperature to the bainite transformation temperature at an adequate cooling rate so as to avoid precipitation of pearlite, preferably at a cooling rate between 10°C/sec and 0.64°C/sec. Best results are obtained by performing the cooling in a fluidized bed furnace.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to a spheroidal graphite cast iron, and more particularly to a method of producing an austempered spheroidal graphite cast iron body and the product of the same.

[0002] Much interest has been attached to austempering of spheroidal graphite cast iron as a prospective measure for obtaining an inexpensive and lightweight material which can be used in place of steels for various machine elements in motor vehicles, ships, civil or agricultural machines, steel manufacturing facilities and so forth. This is because austempered spheroidal graphite cast irons, which can be called bainitic spheroidal graphite cast irons, have a remarkably better combination of tensile strength and elongation or toughness than conventional ferritic or pearlitic spheroidal graphite cast irons. For example, a success was reported in replacing forged and carburized steel gears for automobiles by austempered spheroidal graphite cast iron gears with advantages in many respects such as a reduction in the total production cost, reduction in weight and reduction of noise owing to high damping rate of the new material.

[0003] In practice, however, austempering of spheroidal graphite cast irons has found only limited applications to small-sized and thin-wall parts because it is not easy to accomplish ideal austempering or utilization of high bainite by heat treatment of ordinary spheroidal graphite cast irons. The fundamental reason for the difficulty in austempering spheroidal graphite cast irons of the popular compositions is that the stability of supercooled austenite is insufficient. Therefore, it is required for accomplishment of' proper austempering that the cast iron be cooled from the austenizing temperature to the bainite transformation temperature at a sufficiently high rate of temperature reduction. In the cases of thick-wall parts, however, it is difficult to realize such a high rate of cooling in every portion of each casting, and therefore much pearlite precipitates from supercooled austenite during the cooling process. In some cases a salt bath is used to accomplish rapid cooling. However, in the current industries the use of salt baths is generally unwelcome because of troublesomeness of the operation, lowness of productivity and the use of pollutive materials. Furthermore, cooling in a slat bath tends to produce greater strains in the treated castings than in the case of air cooling.

[0004] As a typical measure for solving the above described problem, it is known to use a spheroidal graphite cast iron containing considerable amounts of Mo and/or Ni, as shown in, e.g., Japanese patent application publication No. 47-19496 (1972) and Japanese patent applications primary publication Nos. 50-127823 (1975) and 54-133420 (1979). The addition of Mo and/or Ni is effective in stabilizing austenite, and accordingly it becomes possible to accomplish proper austempering of the spheroidal graphite cast iron without suffering from precipitation of pearlite even when the cooling from the austenizing temperature to the bainite transformation temperature is performed at a relatively low cooling rate by air cooling. The employment of a relatively low cooling rate offers an additional advantage that less strains are produced in the treated castings.

[0005] However, the addition of considerable amounts of very expensive Mo and Ni to a spheroidal graphite cast iron results in a considerable rise in the material cost and, therefore, is not widely practicable.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a method of easily and economically producing a properly austempered spheroidal graphite cast iron body which has good mechanical properties, the method being fully practicable even when the cast iron body is a thick-wall member.

[0007] In another aspect, it is an object of the invention to provide a properly austempered spheroidal graphite cast iron body which has good mechanical properties and is fairly low in the material cost.

[0008] A method according to the invention for producing an austempered spheroidal graphite cast iron body comprises the steps of (a) casting a spheroidal graphite composition, which consists of 3.0 to 4.0 wt% of C, 1.5 to 3.0 wt% of Si, 0.005 to 0.2 wt% of a spheroidizing agent, 0.3 to 0.8 wt% of Mn, 0.3 to 2.0 wt% of Cu, 0 to 0.1 wt% of Mo, 0 to 0.3 wt% of Ni and the balance of Fe and inevitable impurities, into a desirably shaped body, and (b) austempering the cast iron body by first keeping the cast iron body at a first temperature at which austenite is stabilized, cooling the cast iron body from the first temperature to a second temperature at which transformation of austenite into bainite takes place at such a cooling rate that the cooling is accomplished substantially without precipitation of pearlite, keeping the cast iron body at said second temperature to accomplish isothermal transformation of austenite into bainite, and thereafter quenching the cast iron body.

[0009] In this method it is preferred that the cooling rate mentioned at the step (b) is in the range from 10°C/sec to 0.64°C/sec, and it is also preferred to perform the cooling from the austenite stabilizing temperature to the bainite transformation temperature in a fluidized bed furnace.

[0010] An important feature of the present invention is the coexistence of 0.3-0.8 wt% of Mn and 0.3-2.0 wt% of Cu in the spheroidal graphite cast iron. We have discovered that by using such amounts of Mn and Cu jointly it is possible to realize sufficient stability of supercooled austenite at the stage of austempering without the need of adding large amounts of Mo and/or Ni to the spheroidal graphite cast iron composition. Accordingly, the cooling from the austenite stabilizing temperature to the bainite transformation temperature can be done at a relatively low cooling rate without suffering form precipitation of pearlite. The above specified range of the cooling rate, from 10°C/sec to 0.64°C/sec, is preferred from the viewpoints of surely preventing precipitation of pearlite and minimizing the strains produced in the cast iron body by the austempering heat treatment. In many cases the cooling can be accomplished by air cooling. In the cases of thick-wall cast iron bodies it is possible to use a salt bath or a metal bath to realize a desired cooling rate even in the interior portions of the cast iron bodies. However, a fluidized bed furnace is preferred to salt or metal baths firstly because the former does not involve the problems mentioned hereinbefore in respect of salt baths and also because the austempered products have better mechanical properties when the cooling is performed in a fluidized bed furnace. By using a fluidized bed furnace, it is possible to apply the present invention to very thick-wall parts such as automotive crankshafts or to accomplish austempering of thick-wall parts so as to fully utilize high bainite.

[0011] In the present invention a suitable range of the austenite stabilizing temperature is from about 800°C to about 1000⁰C, and preferably from 850 to 950°C, A suitable range of the bainite forming temperature is from about 200°C to about 400°C.

[0012] The present invention has made it practicable to use an inexpensive spheroidal graphite cast iron in place of expensive high molybdenum and/or high nickel spheroidal graphite cast irons for producing thick-wall machine parts that are high in both strength and toughness or machine parts that utilize high bainite. This invention is applicable to a wide variety of structural machine parts such as gears, joints, cylinders, casings, drums, forks, crankshafts, rocker arms, cylinder rings and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 

Fig. 1 is a plan view of a test piece of spheroidal graphite cast iron;

Fig. 2 is a graph showing the result of an experiment about the relationship between the cooling rate in an austempering heat treatment of a spheroidal graphite cast iron and strains in the treated cast iron;

Fig. 3 is a longitudinal sectional view of a test piece of spheroidal graphite cast iron;

Fig. 4 is an explanatory illustration of an experimental fluidized bed furnace used for an austempering heat treatment of the test piece of Fig. 3;

Fig. 5 is a graph showing the influence of the content of Mn and/or Cu in a spheroidal graphite cast iron on the stability of supercooled austenite in the cast iron; and Figs. 6, 7 and 8 show three kinds of test pieces

for testing mechanical properties of a spheroidal graphite cast iron in side elevational views, respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0014] In the present invention the composition of a spheroidal graphite cast iron must be as specified hereinbefore. The effects of the respective alloying elements and the reasons for the limitations on the amounts of the respective elements are as follows. Throughout the following description, the amounts of the elements in the cast irons are given in percentages by weight.

(1) Carbon

[0015] Carbon is an alloying element indispensable to cast irons. In the spheroidal graphite cast irons according to the invention the content of C is limited to a maximum of 4.0% because the presence of more than 4.0% of C together with Si tends to cause crystallization of graphite as primary crystal, which is unfavorable for the strength and toughness of the cast iron. If the content of C is too low, the cast iron composition becomes inferior in castability. Therefore, the minimum content of C is set at 3.0%.

(2) Silicon

[0016] Silicon is an alloying element that plays an important role in accomplishing graphitization in the cast iron. If the content of Si is too low the cast iron composition is unsatisfactory in castability and also in the degree of graphitization. On the other hand, if the content of Si is too high the cast iron becomes insufficient in elongation and unfavorably brittle. In view of such tendencies, the content of Si in a spheroidal graphite cast iron according to the invention is limited within the range from 1.5 to 3.0%.

(3) Manganese

[0017] In the present invention, manganese is used as an alloying element that makes an important contribution to enhancement of the stability of supercooled austenite in the spheroidal graphite cast iron. The minimum content of Mn is set at 0.3% because the expected effect is insufficient if the Mn content is less than 0.3%. The content of Mn is limtted to a maximum of 0.8% because the addition of a larger amount of Mn is liable to cause lowering of the strength and toughness of the cast iron.

(4) Copper

[0018] Copper has the effect of improving the susceptibility of the spheroidal graphite cast iron to heat treatment. The minimum content of Cu is set at 0.3% because the expected effect is insufficient if the Cu content is less than 0.3%. On the other hand, the presence of a relatively large amount of Cu offers a difficulty to the spheroidization of graphite, and the addition of an unnecessarily large amount of Cu results in lowering of the tensile strength and impact resistance of the spheroidal graphite cast iron. Therefore, the content of Cu is limited to a maximum of 2.0%.

(5) Spheroidizing Agent

[0019] For a spheroidal graphite cast iron according to the invention, a spheroidizing agent can freely be selected from well known spheroidizing elements such as Mg, Ca and Ce, though we prefer Mg to other spheroidizing elements. To realize good spheroidization of graphite without adversely affecting the mechanical properties of the cast iron, a suitable range of the content of Mg or an alternative spheroidizing agent in a cast iron composition in this invention is from 0.005% to 0.2%.

(6) Molybdenum

[0020] In the present invention, molybdenum is an optional alloying element which is effective in enhancing the stability of supercooled austenite in the cast iron. The content of Mo is limited to a maximum of 0.1% because the addition of a larger amount of Mo tends to result in lowering of the tensile strength and toughness of the cast iron and also because Mo is an expensive material.

(7) Nickel

[0021] In the present invention, nickel too is an optional alloying element effective in enhancing the stability of supercooled austenite in the cast iron. The content of Ni is limited to a maximum of 0.3% because when the content of Ni is more than 0.3% there arises a tendency for Ni to suppress bainite transformation rather than pearlite transformation and, therefore, it takes a longer time to accomplish transformation into bainite with little degradation of the mechanical properties of the austempered cast iron, and also because Ni is an expensive material.

[0022] In practice, it is inevitable and permissible that a spheroidal graphite cast iron according to the invention contains very small amounts of impurities besides the above described essential and optional alloying elements and Fe. Typical examples of such impurities are phosphorus and sulfur. It is desirable that the content of P is below 0.1% because a higher content of P is detrimental to the workability of the cast iron. Also it is desirable that the content of S is below 0.1% because a higher content of S is obstructive to the spheroidization of graphite.

[0023] In the austempering heat treatment according to the invention, a matter of great importance is the rate of cooling from the austenization temperature to a temperature at which bainite forms. The following is a description of an experiment we conducted in the course of completing the present invention to examine the influence of the cooling rate on the strains in the austempered specimens of a spheroidal graphite cast iron.

EXPERIMENT 1

[0024] In this experiment, use was made of a spheroidal graphite cast iron composition consisting of 3.6% of C, 2.6% of Si, 0.8% of Mn, 2.0% of Cu, 0.10% of Mo, 0.29% of Ni, 0.04% of Mg, 0.05% of Cr, 0.02% of P, 0.007% of S and the balance of Fe.

[0025] The cast iron composition was cast and machined into test pieces of an annular shape as shown in Fig. 1. Each test piece 10 was 8 mm in outer diameter, 6 mm in inner diameter and 2 mm in thickness and was formed with a cut 12 of which the width D was 2 mm. On every test piece 10 the width D of the cut 12 was measured to an accuracy of the order of 0.001 mm.

[0026] The'test pieces were individually subjected to an austempering heat treatment by using a transformation point measuring apparatus, which was capable of heating the test piece to a desired temperature in a vacuum atmosphere by means of a high frequency heating coil and then cooling the heated test piece at a variably prescribed rate by blowing nitrogen gas or hydrogen gas against the test piece. Each test piece was first heated up to 900°C for stabilization of austenite and kept at this temperature for 2 hr. After that the test piece was cooled down to 300°C at a constant cooling rate which was selectively prescribed within the range from 0.5°C/sec to 30°C/sec, and kept at 300°C for 2 hr. After that the apparatus was disconnected from the power source to allow the test piece to cool down to room temperature.

[0027] After the heat treatment, the width D of the cut 12 of each test piece 10 was measured to an accuracy of the order of 0.001 mm to find a change in the cut width D produced by the heat treatment as the absolute value of the difference between the width D before heat treatment and the width D after heat treatment. This dimensional change is attributed to a strain produced by the heat treatment and the expansion of the test piece by reason of a change in the cast iron structure by transformation. Fig. 2 shows the result of this experiment. In the graph of Fig. 2, the magnitudes of the change in the cut width D are relative values compared with the value of the dimensional change at the cooling rate of 0.8°C/sec, which was a very low cooling rate close to a critical rate below which the precipitation of pearlite is probable.

[0028] As can be seen in Fig. 2, the strain produced by the heat treatment and represented by the dimensional change of the test piece was sufficiently small when the cooling rate was not higher than 10°C/sec. However, when the cooling rate was lower than 0.64°C/sec a considerable increase in the dimensional change was observed. This is not because of augmentation of the strain produced by the heat treatment, and this is attributed to the precipitation of some pearlite.

[0029] In the austempering heat treatment according to the invention, it is preferred to accomplish the cooling from an austenite stabilizing temperature to a bainite transformation temperature at a cooling rate in the range from 10°C/sec to 0.64°C/sec with a view to minimizing the strain attributed to the heat treatment. An optimum cooling rate is variable within this range depending on the composition of the spheroidal graphite cast iron.

[0030] When austempering relatively small-sized castings it is not difficult to realize a desirable cooling rate within the above specified range. However, difficulties arise when austempering thick-wall castings because in the interior regions of thick-wall castings subjected to air cooling the rate of cooling is likely to be below the lower boundary of the above specified range. If a salt bath is used for the cooling to avoid such a problem of air cooling, it is likely that the cooling rate exceeds the upper boundary of the above specified range in the surface regions of the castings.

[0031] We have experimentally confirmed that the rate of cooling in the austempering heat treatment can surely be kept within the preferred range, even when treating considerably thick-wall castings, by performing the cooling in a fluidized bed furnace. The following is a description of an example of our experiments in this regard.

EXPERIMENT 2

[0032] In this experiment, the spheroidal graphite cast iron composition described in Experiment 1 was cast and machined into test pieces of a cylindrical shape as shown in Fig. 3. The length L and outer diameter D of the test piece 20 were varied so as to obtain four differently sized test pieces 20 as shown in Table 1. Each test piece 20 was formed with a central bore 22 of a small diameter, and a Pt-Pt13%Rh thermocouple (not shown) was inserted into the hore 22 and spot-welded to the test piece at the bottom 24 of the bore 22.

[0033] Fig. 4 shows a fluidized bed furnace 30 used in this experiment. The body of the furnace 30 was a cylindrical metal container 32 which was 610 mm in inner diameter and 600 mm in depth. In a lower section the container 32 was provided with a gas diffusion plate 34 of a porous or sintered metal plate, and the space above the gas diffusion plate 34 was filled with an alumina powder 36 which was employed as a heat transferring medium. In this case the alumina powder 36 consisted of particles that passed through 80-mesh screen. In general it is suitable to use alumina (or an alternative material) particles that pass through 60-mesh screen but do not pass through 100-mesh screen in a fluidized bed furnace for use in the present invention. A gas feed pipe 38 is connected to the container 32 at the bottom section to blow an inactive gas such as nitrogen gas into the bed of the alumina powder 36 through the gas diffusion plate 34. The furnace 30 has electric heaters 40 disposed circumferentially of the container 32. The alumina powder 36 is heated by the heaters 40 and is forcibly fluidized by the action of the gas flowing upward through the diffusion plate 34 and, therefore, provides a uniformly heated fluidized bed. The flow rate of the gas through the pipe 38 is suitably regulated according to the capacity of the furnace 30 and the specified heating temperature. In this experiment, nitrogen gas was supplied at a rate of 250 liters/min.

[0034] In the experiment, each test piece 20 was placed in a supporting basket 42 made of a stainless steel and first heated in a separate electric furance which was maintained at a constant temperature of 900°C. The test pieces 20 was kept in the electric furnace for 4 hr after the interior temperature of the test piece 20 reached 900°C. Then, the basket 42 containing the heated test piece 20 was quickly transferred into the fluidized bed furnace 30 in which the heating medium 36 had already been heated to 300°C. The basket 42 was kept in position such that the test piece 20 is located in a central region of the heating medium 36, and the temperature in the interior of each test piece 20 was continuously recorded to examine the rate of drop in the temperature.

[0035] For each of the four types of test pieces 20 tested in this experiment, the rate of cooling from 900°C to 300°C was as shown in Table 2. For comparison, the test pieces 20 of the types A and D were cooled in a salt bath. In this case, each sample was kept at 900°C for 4 hr and then put into a salt bath which was maintained at about 300°C. The cooling rates measured in this case are also shown in Table 2.

[0036] The experimental results in Table 2 demonstrate that by using a fluidized bed furnace the cooling of castings of a spheroidal graphite cast iron according to the invention, which are as thick as 10-70 mm, from 900°C to 300⁰C can be accomplished at a desirable cooling rate.

EXAMPLES 1-6

[0037] In these examples, spheroidal graphite cast iron compositions according to the invention were produced by adding variable amounts of Mn, Cu, Mo and Ni to a fundamental composition which was an example of commonly used spheroidal graphite cast iron compositions. The fundamental cast iron composition consisted of 3.6% of C, 2.6% of Si, 0.04% of Mg, 0.05% of Cr, 0.02% of P, 0.007% of S and the balance of Fe. As shown in Table 3, the content of Mn was varied within the range from 0.4 to 0.8% and the content of Cu within the range from 0.4 to 2.0%, while the content of Mo was varied within the range from 0.00 to 0.10% and the content of Ni within the range from 0.00 to 0.29%.

[0038] Each of the spheroidal graphite cast iron compositions of Examples 1-6 was cast and machined into test pieces and then austempered by first heating at 900°C for 4 hr, then cooling to 250°C in a salt bath, thereafter keeping at 250°C for 2 hr and then quenching in water. On the thus treated test pieces, the tensile strength and Charpy impact value were measured by the standard methods. The results are shown in Table 3.

[0039] Furthermore, the degree of stability of supercooled austenite was examined in every example by using a transformation expansion-shrinkage measuring apparatus to obtain a TTT (time-temperature-transformation) diagram, also called an isothermal transformation diagram. In this test, each sample was austenized by heating at 900°C for 15 min and then cooled. Table 3 contains the values of the latent period t_i of a pearlite nose which appeared in the TTT diagram. It can be said that as the value of t_i is larger austenite is more stable and the transformation into pearlite is less probable. Supplemental to this experiment, the same measurement was made on separately prepared spheroidal graphite cast iron compositions. These compositions were prepared by adding 0.01% of Mo and 0.20% of Ni to the aforementioned fundamental composition and further adding Mn and/or Cu in various amounts as shown in Fig. 5.

REFERENCES 1-7

[0040] For comparison, the spheroidal graphite cast iron compositions of Examples 1-6 were modified by greatly increasing the contents of Mo and/or Ni and also by varying the contents of Mn and/or Cu, as shown in Table 3. Reference 3 can be regarded as the spheroidal graphite cast iron shown in the Japanese specification No. 54-133420 mentioned hereinbefore. The tests described in Examples 1-6 were made on the samples of References 1-7 too. The results are contained in Table 3.

[0041] In the TTT diagram obtained in the above described experiment, the position of the bainite transformation line was also checked. It was confirmed that when the content of Ni was more than 0.3% the transformation into bainite required a longer time than in the other cases. For instance, the latent period for bainite transformation at 250°C was measured to be about 800 see in the case of Example 6 (Ni 0.29%) and about 2000 sec in the case of Reference 5 (Ni 0.48%).

[0042] The experimental results shown in Table 3 and Fig. 5 indicate or demonstrate that when the content of Mo exceeds 0.1% or when the content of Mn exceeds 0.8% the austempered castings become slightly lower in tensile strength and/or impact value, that when the content of Cu exceeds 2.0% lowering of the tensile strength and impact value is inevitable probably because some difficulty arises in the spheroidization of graphite, and that when the contents of Mn and Cu are respectively 0.3-0.8% and 0.3-2.0% as specified in this invention it is possible to greatly improve the stability of supercooled austenite in the spheroidal graphite cast irons containing not more than 0.1% of Mo and/or not more than 0.3% of Ni to a level comparable to, or even above the level in the high molybdenum or high nickel spheroidal graphite cast irons.

EXAMPLE 7

[0043] A spheroidal graphite cast iron composition consisting of 3.6% of C, 2.6% of Si, 0.04% of Mg, 0.6% of Mn, 1.5% of Cu, 0.05% of Mo, 0.20% of Ni, 0.05% of Cr, 0.02% of P, 0.007% of S and the balance of Fe was cast and machined into three kinds of test pieces of the shapes shown in Figs. 6, 7 and 8, respectively.

[0044] The test piece 50 of Fig. 6 was for a tensile test. This test piece 50 had a diameter of 7 mm in its cylindrical middle portion, and the gauge length was 40 mm. The test piece 60 of Fig. 7 was for Charpy impact test. This test piece 60 was 10 x 10 mm square by 55 mm long and had a slit-like cut 62 which was 3 mm in depth. The test piece 70 of Fig. 8 was for fatigue test by a rotary bending tester. This test piece 70 was 200 mm long and had a diameter of 16 mm in both end portions, which were each 70 mm long, and a reduced diameter of 8 mm in a cylindrical middle portion.

[0045] The three kinds of cast iron test pieces 50, 60, 70 were respectively divided into two groups in order to perform austempering of these test pieces by two different methods. The test pieces of one group were kept immersed for 4 hr in a chloride salt bath which was maintained at 900°C, then immersed in a nitrate salt bath maintained at 300°C and kept in that state for 2 hr, and then quenched in water.

[0046] The test pieces of the other group were first heated at 900⁰C for 4 hr in a nitrogen gas atmosphere in an electric furnace, then transferred into the fluidized bed furnace 30 of Fig. 4 in which the heating medium 36 was maintained at 300°C and kept therein for 5 min, then transferred into an electric furnace filled with nitrogen gas and maintained at 300⁰C and kept therein for 1 hr and 55 min, and thereafter quenched in water.

[0047] The test pieces 50, 60, 70 austempered by either of these two methods were subjected to a tensile test, Charpy impact test or a rotary bending test as a fatigue test. Table 4 shows the results of the tests. In Table 4, the fatigue strength refers to the maximum stress the test piece 70 could endure for 10⁶ times of bending stress cycles without breaking.

[0048] The test results in Table 4 demonstrate that in the austempering heat treatment according to the invention, better results with respect to the mechanical properties of the treated castings can be obtained by using a fluidized bed furnace for the cooling to form bainite instead of a salt bath.

Claims

1. A method of producing an austempered spheroidal graphite cast iron body, the method comprising the steps of:

(a) casting a spheroidal graphite cast iron composition, which consists of 3.0 to 4.0 wt% of C, 1.5 to 3.0 wt% of Si, 0.005 to 0.2 wt% of a spheroidizing agent, 0.3 to 0.8 wt% of Mn, 0.3 to 2.0 wt% of Cu, 0 to 0.1 wt% of Mo, 0 to 0.3 wt% of Ni and the balance of Fe and inevitable impurities, into a desirably shaped body; and

: (b) austempering the cast iron body by first keeping the cast iron body at a first temperature at which austenite is stabilized, cooling the cast iron body from said first temperature to a second temperature at which transformation of austenite into bainite takes place at such a cooling rate that the cooling is accomplished substantially without precipitation of pearlite, keeping the cast iron body at said second temperature to accomplish isothermal transformation of austenite into bainite, and thereafter quenching the cast iron body.

2. A method according to Claim 2, wherein said cooling rate is in the range from 10°C/sec to 0.64°C/sec.

3. A method according to Claim 2, wherein the cooling at step (b) from said first temperature to said second temperature is performed in a fluidized bed furnace.

4. A method according to Claim 1, wherein said first temperature is in the range from about 800 C to about 1000°C and said second temperature is in the range from about 200°C to about 400°C.

5. A method according to Claim 1, wherein said spheroidizing agent comprises an element selected from the group consisting of Mg, Ca and Ce.

6. An austempered spheroidal graphite cast iron body produced by a method according to Claim 1.

Drawing