Ductile iron and method for manufacturing ductile iron for engineering components requiring strength and toughness

The invention relates to ductile iron according to the preamble of claim 1.

The invention relates also to a method according to the preamble of claim 9.

Typical metallic initial raw materials of ductile iron known from prior art are the by-product parts of cutting carbon-steel plate intended for engineering but unsuitable for it because of their form, that is, steel scrap, correspondingly parts remaining from sheet-metal cutting packaged to sheet-metal packages, that is, package scrap. In addition, pig iron produced in other iron production processes is used. For performing casting, a die needs casting gates and heads which are removed from the piece after casting and are used as foundry returns. Raw materials are usually melted in cupola or induction melting furnaces. For specifying the composition, alloys, such as carburiser and different ferro-alloys, are added to the melt.

In these, the typical (for example, in Valuraudat, Valuterakset, MET: Raaka- ainekasikirja II) chemical compositions for durable ductile irons EN-GJS 600-900 (Standard EN 1563) of prior-art applications are

Carbon 3. 8-3 .2%

Silicon 1. 8-2 .8%

Manganese 0. 1-0 .8%

Magnesium 0.040-0.065%

An essential melting treatment of ductile iron is so-called spheroidisation, in which the prerequisites of carbon precipitation process are changed in order the

graphite precipitating from iron to be formed into spheres, and which is realised either as a ladle treatment or inside a die by using magnesium-rich and/or cerium- rich ferro-silicon alloy or nickel-magnesium. Magnesium reacts with sulphur and other impurities, which promotes the growth of graphite as spherical instead of lamellar.

Another ductile-iron melting treatment is inoculation in which typically grained ferro-silicon alloy is added to the melt for increasing grains of crystallisation. The aim of inoculation is to make the casting structure more fine-grained.

Unequal solidification and cooling create internal stresses to the casting structure of ductile iron. Usually, a stress-relief annealing in the temperature of over 600°C is performed for engineering components for demanding applications. If one wishes to balance the internal microstructure differences of the material, a pearlitising treatment is performed in which the temperature of the piece is increased to 880-940°C and it is cooled with the furnace to a desired pearlitising temperature in the range of 750-860 ⁰ C from which the piece is cooled in air to room temperature.

In paper-machine components, one uses currently common structural materials, such as grey cast iron EN-GJL 150-350, ductile iron EN-GJS 370-900 (EN 1563), EN-GJS 800-1400 (EN- 1564), constructional steels and quenched and tempered steels. The same materials are used in other similar engineering applications.

The most important disadvantages of current materials are the following:

- No sufficient strength and toughness properties are acquired with grey cast iron (strengths below 350 kN/rnm and elongation around

1%)

- With ductile irons in accordance with the standard, one has to make too heavy and large-dimensioned components in order to gain sufficient structural rigidity, and they have no sufficient

fatigue resistance even according to standards. No values of ultimate elongation are defined for more durable grades for over 60-mm walls. In addition, the values of mechanical properties described by the standard are not realised in the inner parts of thick-walled pieces.

- Parts of quenched and tempered steel have to be manufactured by forging, whereby the die and forging costs are high and the ample machining requirement further increases manufacturing costs. The strength and ultimate elongation are on target level, but the lack of freedom of design and the more disadvantageous notch sensitivity compared to ductile iron decrease the actual practical fatigue resistance.

- In castable quenched and tempered steels, the difficulty of achieving internal tightness increases the risk of internal flaws, which has to be considered in the dimensioning of components as greater accuracy, which increases costs.

- Austempered ductile iron ADI (typically), that is, EN-GJS-800-8 (EN-1564), achieves sufficient mechanical properties in a tension bar cast separately, but its machining with chip removal is extremely difficult because of its hardness, and manufacturing ADI in over 1,000-kg weighing and 200-mm walled pieces is technically and economically difficult. Mechanical values in accordance with standards are defined with a separately cast test bar. - In heat treatments belonging to the manufacturing processes of quenched and tempered steels and ADI, one typically uses either oil- or salt-bath furnaces the emissions and waste of which are harmful to the environment.

Paper-machine components are exposed to extreme mechanical and wearing stress. The rates of both paper machines and paper finishing machines have

doubled and tripled in the last decades. For example, the web rates of finishing machines have increased from the rate of 600 m/min to the rate of more than 2,000 m/min and the development is ongoing.

Even higher tensile strengths, limit of proportionality, elongation and fatigue resistance are required of machine components, such as bearing chocks, thrust blocks, spindles and idler levers. Material properties through the piece in walls of different thickness have to be as uniform as possible.

All this has to be accomplished with a material the manufacturing method of which gives a full freedom of design for minimising moving masses and the machinability of which is relatively good with conventional methods. The manufacturing method of tools required for manufacturing a blank, such as models and dies, is as simple and short as possible for achieving short project times. Many times, the manufacturing times of forging-product dies stretch to months, and in free and die forging, the freedom of design is in practice more limited compared to casting. Producing wooden, plastic or similar model equipment required by sand casting requires essentially shorter times.

No material similar to the one described above is known from prior art, and the object of the invention is to achieve a material essentially meeting the above properties and requirements in which material the disadvantages of materials known from prior art from the viewpoint of demanding machine components, for example, paper- or board-machine components, have been eliminated or at least minimised.

The object of the invention is to create a new grade of ductile iron which has new, exceptional combinations of material properties.

The object of the invention is also to achieve a manufacturing process with which said properties for ductile iron may be accomplished.

To achieve the afore-mentioned objects and those that come out later, ductile iron according to the invention is mainly characterised in what is presented in the characterising part of claim 1.

A method according to the invention is mainly characterised in what is presented in the characterising part of claim 9.

According to the invention, a new grade of ductile iron is achieved for use in, for example paper- or board-machine components, which ductile iron has exceptional mechanical properties and which is more economical and environmentally- friendly to manufacture compared to other materials of corresponding strength properties.

An extremely good grade of ductile iron for paper- or board-machine components has been achieved according to the invention which ductile iron is technically and economically most cost-effectively austempered ductile iron cast as sand-mould casting

- the castability of which is good, whereby freedom of design is accomplished and the casting is tight,

- the ultimate tensile strength, limit of proportionality and fatigue resistance of which are on the level of quenched and tempered steel, and the mechanical values from test bars cut from a thick- walled piece give at least the same values as individually-cast test bars with ADI ductile irons, and

- the economical machinability of which is not difficult with modern technology,

- the machining with chip removal of which is technically and economically good, as its hardness is principally below 300 HB,

the possibly required heat treatment is realised without oil or salt bath with controllable advantageous air, water or water mist added with desired additives harmless to the environment, and it is environmentally friendly because re-usable material may also be used.

A blank manufactured of the grade of ductile iron according to the invention is more accurate dimensionally compared to free-forging from the viewpoint of machining, whereby cost savings are achieved. The best mechanical properties are achieved with ductile iron having a new, exceptionally advantageous combination of properties with a more economical and environment-friendly manner compared to alternative engineering materials.

In the combination of mechanical properties of the grade of ductile iron according to the invention:

- the limit of proportionality is adjustable even through a thick- walled (over 150 mm) piece wall from over 500 N/mm ² to 750 N/mm ² by varying the casting process and heat-treatment process, and at the same time, - the elongation is adjustable by varying the casting process and heat-treatment process from 4% to 8%, and

- the ultimate tensile strength is even adjustable to 1,000 N/mm at the same time, the fatigue limit in rotational bending is adjustable to the range of 300-420 kN/mm ² - the hardness of the piece through the piece is principally 260-300

HB

- with a SharpV bar in accordance with the impact strength standard 4-9 J / mm ² -2O ⁰ C

The grade of ductile iron according to the invention is fine grain ductile iron (FGDI) which differs from current ductile irons in that:

- the metal matrix comprises of fine-lamellar pearlite and of finegrained pearlitic-ferritic or bainitic-ferritic structure, the lamella gap of which pearlite and bainite is below 1,0 μxa. (typically below 0,4 μm), and of ferrite which is in pearlitic quality on grain boundaries as desired zones and in the bainitic structure as a needle-shaped or comb-shaped structure in the vicinity of grain boundaries and interlocked in bainite,

- there is less than 1% of carbides in the structure,

- no retained austenite usually occur in the structure itself, - the spheroidisation level of graphite spheres is over 85% (typically over 95%) and the sphere density is typically 150-400 pcs/mm ² in the cutting surface of the sample depending on wall thickness

In the manufacturing process of fine grain ductile iron in accordance with the method according to the invention:

With the choice of raw material basis and alloying, a composition is achieved in which a first prerequisite is created for fine-grained microstructure, most important alloying and trace elements are Si, Mn, S, Cu, Ni, Mo, Mg, Ti and Ce. - Additionally, Nb, La, Bi, Sn and B advantageously affect the finegrained structure.

- In melting treatments, that is, in the spheroidisation of ductile iron and in the inoculation creating cores of crystallisation, great sphere density and small inherited austenitic grain size in casting condition are created, which creates a second basis for the finegrained structure.

In heat treatment, the slow increase of temperature (50°C/h) is performed traditionally in order to avoid stresses and warpings. Austenising is performed in an atmosphere furnace in the temperature of 880-960 ⁰ C.

- After austenising, the piece is brought to the temperature range of 840-760°C, whereby the heat content of the piece decreases thus assisting quick quenching, at the same time, one is able to adjust the ferrite around pearlite and bainite required for achieving excellent toughness and fatigue resistance.

- The quenching performed after this is performed in a controlled way according to the composition with such a rate that sufficiently fine pearlitic structure is achieved in both pearlitic and ferritic- pearlitic quality (temperature range of 800-600°C in time, for example, 3-15 minutes depending on the composition, piece size, wall thickness and desired properties, for example, strength) and in bainitic-ferritic structure, a dense structure of ferritic needles and extremely fine bainitic structure are achieved (less than 5 minutes). Quick cooling is advantageous for the properties of fine grain ductile iron.

- The choice and control of the correct austensite decomposition temperature and holding time (austempering) is the fourth basic principle in achieving a fine-grained matrix. Pearlitic structure is mainly developed in the lower temperatures of the pearlitic region. The structure of bainitic grade totally in the bainitic region.

Typically 290-550°C, but depending on the composition may be 100-600 ⁰ C. The decomposition of austenite in low temperatures in slower than in higher temperatures, the result of which is a finegrained matrix. - The complete decomposition of austenite, stresslessness of the structure and good machinability are ensured with tempering into which the piece is tempered slowly from the austempering or pearlitisation temperature to the tempering temperature between 520-720°C depending on the composition, piece thickness and desired strength.

- At the end of heat treatment, the piece is cooled to room temperature in a controlled way (desirably, for example 50°C/h) to room temperature.

A controllably controlled heat treatment method described in patent application FI-20011954 is used advantageously in manufacturing the grade of ductile iron according to the invention.

A grade of ductile iron is achieved according to the invention the properties of which may be varied controllably by adjusting the composition, manufacturing process and heat treatment.

The material notes of fine grain ductile iron (FGDI) according to the invention are, inter alia, in accordance with the ultimate tensile strength and adapting the standard EN-1563:

GJS-FGDI-P-850/500-5C-HB280-H which is a pearlitic grade, GJS-FGDI-BF-850/600-5C-HB280-H which is a bainitic-pearlitic grade, GJS-FGDI-BF-750/600-7C-HB250-H which is a bainitic-ferritic grade, GJS-FGDI-PF-850/500-7C-HB280-H which is a pearlitic-ferritic grade, and GJS-FGDI-P-900/550-4C-HB290-H which is a totally pearlitic grade.

Properties achieved with fine grain ductile iron (FGDI) according to the invention in a piece the wall thickness of which is, for example, 30 ... 500 mm

Rm (tensile strength) 450 ... 1,000 N/rnm ² from a tension bar taken from the piece

RpO. ,2 (limit of proportionality) 500 ... 750 N/mm ² from a tension bar taken from the piece

A5 (ultimate elongation) 10 ... 4% from a tension bar taken from the piece £ (fatigue resistance) 300 ... 420 N/mm ² from a tension bar taken from the piece

Hardness 250 ... 320 HB from a tension bar taken from the piece

The hardness of the grade of ductile iron according to the invention is through the piece in the range of 250 ... 320 HB and no hard phases occur significantly whereby machinability is good through the piece.

The fatigue resistance of the grade of ductile iron according to the invention is defined as rotational-bending fatigue resistance. A positive fact in the comparison one should consider is that fine grain ductile iron (FGDI) has low notch sensitivity compared to steels.

With the fine grain ductile iron (FGDI) according to the invention, because of its even microstructure after casting and the further balancing effect of heat treatment, the structure is equal thought the piece, and if desired, one may realise a functional structure, that is, different surface and inner parts, with the manufacturing process (if required, microstructure may also be controlled). With casting-technical solutions, such as the placing of cooling irons and controlling of die filling, the best properties and lowest risks are located to the most demanding regions in the piece.

This is advantageously ensured with a computer-assisted simulation of the filling and solidification of casting in advance, by which one is able to ensure beforehand the success of manufacturing process, whereby time and material savings are achieved.

According to an advantageous embodiment of the invention, the control of a casting event is designed with a computer-assisted simulation in which the filling of the die with melt and the solidification and cooling of melt may be ensured. Then, no internal porosity nor slag inclusions are created in the casting piece to weaken the material properties and to cause surface flaws which would act as starting points of fracture. Computer-assisted simulation a test of heat-treatment programme in which the simulation of heat treatment ensures the accuracy of heat

treatment; the suitability of heat treatment is authenticated quality risks especially in short-run and single-piece production.

The manufacturing process of ductile iron according to the invention differs from the normal manufacturing processes of ductile iron in many respects. The most significant differences will now be described.

Usually pig iron sold for manufacturing ductile iron is considered the best metallic melting material. In the manufacturing according to the invention, it is used as little as possible because its most common commercial grades are the by-products of manufacturing titanium oxide, that is, white paint pigment, and they include too much harmful metals, such as titanium, chromium, vanadinium, molybdenum and phosphorous. In addition, purest steel grades and purest alloys are used in the manufacturing according to the invention. Using re-usable steel-plate material is more environmentally friendly and cost-effective. Trace elements outside common foundry analyser accuracy, such as niobium, bismuth, lanthanum etc., play an essential part.

In the method according to the invention, in melting, the melting temperature is maintained precisely below 1520 ⁰ C and loading is performed so that the melting time is as short as possible. In order to prevent oxidation, it is advantageous to use shielding gas or vacuum treatment in shielding gas in the melting for eliminating possible casting flaws.

Magnesium treatment may be implemented, inter alia:

- as a ladle treatment with a magnesium-rich treatment agent in shielding gas or without shielding gas,

- with pure magnesium in a converter,

- with magnesium alloy in chamber treatment, - in in-die treatment.

In order to achieve a fine-grained microstructure after solidification, an addition of solidification cores, that is, inoculation is performed before casting or in connection with it, for example, with ferro-silicon alloy. A recommended grain size of the structure which is described by graphite sphere density, is typically 400 ... 150 spheres/mm ² . Sphere density is also added by increasing metallic die parts, that is, moulds to the die casting surface which moulds have good thermal conductivity and which speed up cooling and solidification.

The chemical composition of the material according to the invention depends on the desired combination of properties and wall thickness and typically it is the following:

typically advantageously

Carbon weight % 3.60-2.80 3.40-2.90 Silicon weight % 1.30-2.60 1.30-1.90

Magnesium weight % 0.065-0.025 0.055-0.035

Manganese weight % 0.80-0.10 0.40-0.20

Copper weight % 0.10-1.60 0.60-1.20

Nickel weight % 0.20-2.00 0.40-0.80 Phosphorus weight % 0.030-0.005 0.001-0.005

Sulphur weight % 0.003-0.010 0.005-0.008

Slight amounts of chromium (Cr), titanium (Ti), vanadium (V), wolfram (W), molybdenum (Mo), cobalt (Co), lead (Pb), tin (Sn), cerium (Ce) and bismuth (Bi).

In the method according to the invention, in heat treatment, the material is first austenised in 880-960 ⁰ C for at least 1 hour + 1 hour/25 mm of wall thickness. The object is to solute all carbides and other phases anomalous from the matrix to austenite. As a continuation, in atmosphere furnace is performed a decrease of temperature in a controlled way and a balancing to the level of 800-850 ⁰ C, after which a quick controlled cooling is performed to the temperature of 650-100°C,

after a temperature balancing of 2 ... 4 hours performed in which the temperature of the piece is maintained in chosen temperature, if the tempering is in the same temperature as when quenched, or is increased to the tempering temperature 55O...75O°C depending on the composition. In the tempering temperature, the pieces are kept for 2...8 hours after which they are cooled to room temperature. Most suitably, shielding gas is used throughout the heat treatment.

For the microstructure of the ductile iron according to the invention, three basic structures are obtained depending on the choice of heat treatment: - fine-lamellar totally pearlitic microstructure with graphite spheres

(multi-phase medium quenching)

- needle-shaped ferrite and fine-lamellar bainite with graphite spheres (to quickly quenching medium)

- ferrite and fine-lamellar pearlite with graphite spheres (quick cooling in atmosphere).

The control of the microstructure takes place by means of the composition and the heat treatment in a controlled way, whereby the desired combination of mechanical properties may be achieved for each piece.

According to certain advantageous additional features, the pieces manufactured from the material according to the invention are simulated in order to predict the filling of the die, the solidification of the melt, the sphere density of graphite and possible carbide phases and hardness and to define the composition with a computer-assisted simulating programme into which the material values are programmed. The heat-treatment programme required by mechanical properties is checked with a computer-assisted simulation beforehand. The purpose of using simulation is to prevent flawed products in order to gain material and cost savings and to realise the manufacturing schedule in a planned way. These advantages are emphasised in single-piece and short-run production. After casting, the tightness of casting is checked with an ultra sonic testing or an X-ray examination.

Hardness is measured with a Brinell meter. The chemical composition is checked in the casting from piece-specific melt. For each piece, cast-on test bars are cast for tensile tests and batch-specifically a comparison piece which represents the actual casting and from which the results of destructive testing are defined. This sample is correspondingly simulated piece of a test piece for ensuring sample match. According to further advantageous additional features, in the process both casting event and the mechanical properties of the material are modelled and computer-assistedly simulated before finishing the final casting process.

When manufacturing machine components from the fine grain ductile iron (FGDI) according to the invention, one is able to use castable iron material giving sufficiently good mechanical properties, whereby one is able to choose the form and dimensioning of the machine component freely for the purpose and is not dependent on the limitations, for example, of a forging or machining method. Accurate dimensioning enables the optimisation of material use, which is meaningful for minimising the whole energy consumption of the manufacturing, transport and use of the machine. At the same time with the dimensioning of models and tools, one is also able to manufacture the machine components of lower load factors by using the lowest values of the strength of the material according to the invention and the highest values of the material according to the invention for more demanding components. Components and machines manufactured of a stronger, more tenacious, more fatigue-durable and easily machinable material are smaller, for example, the diameter of a heavy-duty bearing chock is 500 mm when manufactured of the material according to the invention, and if being of ductile iron according to the standard it would be 650 mm. For a heavy-duty roll the diameter of which is 500mm, the bearing chock weighs 500 kg when manufactured of fine grain ductile iron (FGDI) according to the invention, if made of cast iron according to the standard, it weighs 800-900 kg.

Because of the casting process used in the method according to the invention, one is able to come extremely close to the final dimensioning, which decreases the requirement of machining compared to forging products. At the same time, the consumption of material and energy are minimised.

Fine grain ductile irons (FGDI) according to the invention an exceptional combination of properties, that is, high strength and high limit of proportionality supplemented with good elongation and fatigue resistance, when also machinability is good. Usually in ductile irons, the strength is achieved with such alloying and heat treatment which weaken toughness and machinability. The fine grain ductile iron (FGDI) according to the invention has good fatigue resistance in a sample taken even from a thick piece. This is a result of the manufacturing process of the fine grain ductile iron (FGDI) according to the invention which process creates prerequisites for the structure not including phases weakening it and flaw regions. Such solidification conditions are created in the casting process that the casting structure is produced dense, homogeneous and unoriented.

All variation of the strength properties of ductile iron according to the invention in relation to wall thickness and from one piece to other is essentially smaller than that of ductile irons in accordance with the standard, whereby the safety coefficient of structural dimensioning may be decreased.

A special feature of the heat-treatment process used advantageously in connection with the method according to the invention is that with quick cooling, the microstructure becomes fine-grained and does not include weakening phases or flaw regions. With a quenching rate, tempering temperature and holding time chosen according to the desired properties, one is able to adjust the final microstructure and material properties in accordance with it. Usually, heat- treatment processes including accurate and quick coolings require the use of oil or salt bath. In connection with the invention, an advantageously controlled special heat-treatment process is used in which one achieves sufficiently quick, controlled

cooling rates with air and a medium added to it. Because the cooling agent is advantageously air and water with added natural particles, no emissions to the environment occur. When using oil, there is a risk of groundwater pollution, and the further treatment of salt-bath salts creates a potential environmental threat. The quenching and heating phases of the heat treatment are short and temperature differences reasonable, and the piece is not cooled to room temperature but a major part of the heat content remains in the piece through the whole heat- treatment process. No materials, except for possible water vapour, are transferred to the environment from the heat treatment.

Compared to other materials suitable for the same purpose, the raw material base of ductile iron according to the invention consists mainly of re-usable materials, its melting temperatures are lower compared to steels, the energy requirement is lower compared to forging, and the piece is cast very close to the final dimensions whereby the material waste from the blank is small. As a casting material, the material according to the invention may be totally re-used: inside foundry, the casting systems are utilised immediately in the melting of next castings, and the foundry buys the components manufactured of the material according to the invention when removed for re-usable raw material. The intermediate products of casting, such as casting gates, are re-used, die material is recycled, machining chips are re-melted, and the end-product is re-usable iron material.

The following tables 1-6 show some results obtained with the ductile iron according to the invention.

Table 1

Casting piece 600 kg

Tension bars cut from side of casting from Table A, s = 150 mm in the point in question

Atmosphere quenching (air quenching)

Piece ID Tensile strength Limit of Ultimate elongation Rm proportionality A MPa Re MPa

TABLE A

K671 842 492 5.7

K672 858 508 6.1

K673 847 501 5.1

K691 827 480 5.7

K692 852 496 6.1

K623 842 492 6.4

TABLE B

STANDARD SFS-EN 1563+A1 TEST BAR BLANKS

Ks 51 873 510 7

Ks 56 873 512 7

Ks 58 855 511 6.5

Ks 60 865 496 7.0

Ks 62 873 520 6.5

Ks 64 863 508 7.0

Ks 67 859 506 7.0

Ks 69 877 517 6.5

Table B shows the values of standard test bars (SFS-EN 1563+A1) cast in connection with the casting of different pieces.

Table 2

Heavy piece mass around 1,700 kg

The test bars cut from sides of different pieces, cut piece located S=IOO and ,S=SOO mm from the wall-region side.

Air quenching

Table 3

Cored casting piece mass 600 kg. Quick cooling to medium (air quenching). Test bars cut from side of piece wall.

Table 4a

Cored casting piece mass 700 kg. Quick quenching (water as medium). Test bars cut from inside the piece.

Table 4b

Cored casting piece mass 700 kg. Quick quenching (water as medium). Test bars cut from inside the piece.

Table shows mechanical properties in the different parts of the piece.

Table 5

Heavy casting cut from inside the piece mass 1300 kg Quick quenching (water as medium).

Properties of the same piece in different thicknesses of the wall. The variation of properties is relatively small.

Table 6

Rotational fatigue test (test bars cut from different parts of piece)

Test bar Diameter Total Stress

Mark mm (reading) MPa

External surface

1 6.740 144 104 627 323

6 6.740 183 306 743 333

10 6.740 46 368 -754 357

14 6.740 76 338 376 357

18 6.740 259 344 245 333

22 6.740 2427 656 352

26 6.740 1 481 978 357

30 6.740 97 530 904 357

Core side

5 6.740 44 323 676 333

9 6.740 493 647 333

13 6.740 19 047 785 333

17 6.740 20493 782 333

21 6.740 16 667 061 333

25 6.740 326 086 333

29 6.740 7 585 424 333

The invention will now be described in more detail with reference to the figures of the accompanying drawing, to the details of which the invention is, however, by no means intended to be narrowly confined.

Figure 1 schematically shows a manufacturing flow chart of cast iron GJS 800 known from prior art.

Figure 2 schematically shows a manufacturing flow chart of fine grain ductile iron (FGDI) according to the invention.

Figure 3 schematically shows a flow chart of the URVA 850 heat treatment of fine grain ductile iron (FGDI) according to the invention.

Figure 4 schematically shows a process chart of the simulation of heat treatment for a multiform piece.

Figures 5A and 5B show the cooling rate of the piece in different points of the piece related to the simulation of the heat treatment of the piece.

Figure 6 shows a picture of a typical microstructure of the grade of ductile iron according to the invention in a 100-fold enlargement.

Figure 7 shows a 400-fold enlargement of Figure 6.

Figure 8 shows a 700-fold enlargement corresponding Figure 6.

Figure 9 shows a picture of a typical microstructure of cast-iron material in accordance with prior art in a 100-fold enlargement.

Figure 10 shows a 700-fold enlargement corresponding Figure 9.

Figure 1 schematically shows a manufacturing flow chart of prior-art material, GJS-800. As it is evident from Figure 1, processes known from prior art use steel scrap, pig iron, re-used materials and carburisers ferro-silicon and alloys as raw material, and if required, a spectrometer analysis is performed. Melting takes place as induction melting 11, after which in melt treatments, in the spheroidisation phase 12, a spheroidising alloy is added to the melt. After this follows inoculation 13, in which an inoculant is added to the melt, after which the melt is cast in the casting phase 14 after a possible spectrometer analysis and casting temperature is monitored. After this, the casting is shaken out in the shake-out phase 15, after which take place the secondary operations 16: surface cleaning, removal of extra castings and grinding. After this, quality control is performed as ultra sonic testing 17 and a possible tensile test 18 is performed.

Figure 2 schematically shows a flow chart of the manufacturing process of the grade of ductile iron according to the invention. Steel scrap, pig iron, re-used materials, carburisers, ferro-silicon and alloys are used as raw material, after

which a spectrometer analysis is performed if required. Melting is performed as induction melting 21, and in the melt treatment phase, in spheroidisation 22, a spheroidising alloy is added. After spheroidisation 22 follows the adding of an inoculant and inoculation 23, after which a spectrometer analysis is performed and casting temperature is checked, after which the melt is cast in the casting phase 24. After this follows the shake out of castings in the shake-out phase 25, and after that the secondary operations of castings 26: surface cleaning, removal of extra castings and grinding. Quality control is performed as ultra sonic testing 27. After this, heat treatment is performed as URVA 850 heat treatment 28 and quality control is performed with a tensile test 29, after which follows machining and checking of external cracks (Magna Flux).

Figure 3 schematically shows an advantageous embodiment of the URVA 850 heat treatment of the grade of ductile iron according to the invention. In the heat treatment, first the temperature of the piece is heated to around 900°C, in which temperature pieces are kept for a certain period of time, after which the temperature is lowered to around 800°C, in which the temperature is balanced for a short period of time, after which the material is quickly cooled to 400°C, in which temperature the pieces are kept for a certain period of time, after which the piece is re-heated to temperature around 580°C, in which temperature the pieces are kept for a certain period of time, after which the piece is cooled with desired cooling rate. The URVA 850 heat treatment is described in more detail, inter alia, in patent application FI-20011954.

Figure 4 schematically shows a simulation process chart of the heat treatment for multiform pieces. On the basis of material properties 31, the desired microstructure 32 (temperature curves) and the thermal properties of materials 33 are defined, which data are input to the heat- treatment simulation 30. In addition, data on desired material properties 34 and data on the geometry of the piece are input to the heat-treatment simulation 30. On the basis of this, one defines the target heat-treatment programme 38, based on which furnace control parameters

39 are defined on the basis of both the target heat-treatment programme 38 and furnace parameters 40. On the basis of the furnace parameters 40, one also defines the transfer of heat into the environment 36 which is taken into consideration in the heat-treatment simulation 30.

Figures 5A and 5B show the simulation of the heat treatment and the cooling rate of the piece in different points of the piece. Figure 5A shows temperatures T as a function of time t, in which heat-treatment curves, in the item with reference S, occur the pearlitic and bainitic regions transferred with doping which are dependent on material additives. References Pj and P2 refer to the points P ₁ ja P ₂ of the sample piece in Figure 5B from which points the affecting factors are heat flows Q to different sides of the piece depending on the form of the piece.

It is evident from Figures 4, 5A and 5B that, with the simulation of the cooling rate of the piece, one is able to calculate beforehand the microstructure achieved with the heat treatment in the different parts of the piece. In the method, one is able to define a heat-treatment curve beforehand based on the desired properties of the piece. The heat-treatment simulation may be implemented, for example, with the commercial Finite Element Methods (FEM) software. The simulation of heat treatment is also possible with a suitably modified casting simulation software.

Figure 6 shows a picture of a typical microstructure of the grade of ductile iron according to the invention. The enlargement of the picture is 100-fold. The picture shows the graphite spheres of the microstructure on pearlitic base, and according to the invention, this is fine-grained pearlite. The picture also shows ferlite on grain boundaries, which increases the toughness of the material.

Figure 7 shows a 400-fold enlargement of Figure 6 of fine grain ductile iron (FGDI) according to the invention. The figure shows graphite spheres on pearlitic base 2 and ferrite on grain boundaries which is marked with reference numeral 1.

Figure 8 shows a 700-fold enlargement of fine grain ductile iron (FGDI) according to the invention which figure illustrates the fine-grained lamellar structure in which the lamellar distance is <0,4 μm. The grain size is 10-20 μm.

Figure 9 shows a picture of a typical microstracture of prior-art material which picture shows graphite spheres on pearlitic base, and around 1-2% of carbides which are designated with the reference A and the graphite spheres are designated with reference B. This structure does not typically include ferrite.

Figure 10 schematically shows a 700-fold enlargement of a material known from prior art which enlargement illustrates a typical pearlitic structure of a good- quality material. The grain size is typically 50-100 μm.

The invention was described above only referring to some of its advantageous embodiments, to the details of which the invention is, however, by no means intended to be narrowly confined.