Solidification Mechanism of Ductile Iron

SOLIDIFICATION MECHANISM DUCTILE IRON

ir G Henderieckx Gietech BV April 2008 1

DUCTILE IRON

SOLIDIFICATION MECHANISM

ir G HENDERIECKX GIETECH BV



CONTENT 1. INTRODUCTION 2. TARGET 3. INFLUENCES 4. MECHANISME 5. ROLE OF MAGNESIUM 6. ROLE OF RARE EARTHS 7. TESTING 8. CONCLUSION ADDITION: IRON CARBON DIAGRAM ADDITION: DENDRITIC SOLIDIFICATION

1. INTRODUCTION There are a lot of theories about the solidification of ductile iron; all of them do explain some of the results but not all. A fairly new theory, using the sulphide inclusions, does explain most of the results. The target solidification mechanism will be set and explained. The main influences on the solidification of ductile iron are the presence of sulfur and oxygen, the presence of nuclei and the cooling rate. Some actions, performed by purpose, with particular products can influence the result. The role of the magnesium and rare earths will be considered separately. For the foundries, it is important that they can predict the solidification before they pour the casting. This must be done by a test, indicating if the result will be or will be more or less or will not be according to the required solidification mechanism. The solidification will be considered perfect as the results shows a ductile iron with very good mechanical properties, not suffering of a high tendency for shrinkage or white solidification.



2. TARGET The target for the solidification is, getting:

1. solidification with a fairly low metal pressure on the mould, resulting in a casting with no or controlled shrinkage

2. casting with no or very few difference in structure and graphite segregation

throughout the casting

3. casting with the required structure

4. casting with an equal or better than required appearance

5. casting with high (more than required) mechanical properties concerning tensile and shock resistance, even at low temperatures if required.

The metal pressure that forms during the segregation of the graphite must be controlled by the mould strength, which means that the mould cavity should not (or nearly not) increase in volume. The resulting shrinkage, taken in account riserless or with riser pouring, should be not existing (can even be an expansion) or very low shrinkage, meeting the customer requirements all over the casting. If the requirements from the customer do differ concerning location on the casting, so the shrinkage, if any, may also differ to meet the requirements. The casting should have an equal structure (mainly referring to the mechanical strength) and graphite segregation, indicated by number of nodules and the nodularity (mainly referring to the ductility, especially at low temperatures).



3. INFLUENCES The following items do influence the result:

1. chemical composition 2. presence and amount of nuclei 3. type of nuclei 4. cooling rate 5. mould strength conditions (with or without risers).

3.1 Chemical composition

Depending in the chemical composition, the iron will solidify hypo-, hyper- or eutectic, which gives different results concerning metal pressure on the mould and on the graphite segregation.

3.2 Presence and amount of nuclei

If no nuclei are present, the graphite can, theoretically, not segregate. There is a series of elements promoting the graphite segregation: Si, P, Al, Ni, Co, Cu, Ca, Ba, Sr (bold indicated are interesting for iron). Another group is delaying the graphite segregation: V, Cr, Mn, Mo, W, N, Mg, RE (bold indicated are interesting for iron). But also the amount is very important because an increasing amount of nuclei leads to an increasing tendency to segregate for the graphite. But not all nuclei lead to a free graphite nodule. The size of nuclei is mostly about 0,5 to 2,0 µm and that of nodules is mostly 10 to 80 µm.

3.3 Type of nuclei

The nuclei should be similar crystal lattice as graphite. Graphite will segregate immediately meeting nuclei with equal crystal lattice. An increasing lattice disregistry leads to an increasing undercooling (ΔT). Nuclei with other crystal structure need an intermediate to come closer to the crystal structure of graphite.

Also the interfacial surface energy barrier is important. For MgS and MgO, this energy barrier is too large. There is a need for an outer silicate shell. But complex components like MgSiO3 and Mg2SiO4 are not good enough. Others like CaO.SiO2 decrease the barrier sufficiently to form free graphite. Some figures of disregistry:



Material component Lattice Material component Lattice disregistry disregistry CaO.SiO2 7,5 % CaO.Al2O3.2SiO2 3,7 % BaO.Al2O3.2SiO2 7,1 % SrO.SiO2 3,5 % SrO.Al2O3.2SiO2 6,2 % BaO.SiO2 1,5 %

These components are good for forming suitable nuclei. The core of such nuclei will be mostly: MgS, CaS, CeS and SrS. The surrounding material can also be, although not frequently met, Al2O3 or TiO2. The elements Mg, Ca, Ce, Sr and Ba are oxide and sulphide promoters; but the sulfides will be formed first. 3.4 Cooling rate

If the metal cools with a high rate, the theoretical Fe-C diagram will not be valid because the formation of graphite (segregation) needs time. An increasing cooling rate will lead to graphite segregation on much lower temperatures as indicated by the diagram, will lead to an increasing undercooling. This tends to white solidification and less nodule count.

3.5 Mould strength conditions

The metal will become pressurised during the graphite segregation. The amount of pressure will depend on the amount of graphite segregating (depending on the chemical composition) and the mould strength (must try to resist this pressure) and the situation riser-riserless, which can (with risers) or cannot (riserless) absorb the expanding metal.



4. MECHANISM 4.1 Metal volume behaviour 4.2 Solidifying metal (austenite) behaviour 4.3 Free graphite 4.4 Fading effect 4.5 Mould behaviour 4.1 Metal volume behaviour Metal is poured in the mould cavity, having a temperature Tmould at the end of the pouring. This temperature can be characterised as follows: Tpouring > Tmould > Tliquidus If we consider now that the metal is present in an open, very strong container, the solidification concerning volume will happen as indicated in next figure.

The metal volume will decrease from Tmould (point 0) to Tliquidus (point A) with a volume shrinking rate of 1,5 – 1,6 % / 100 °C. Up to the Tliquidus (point A), there is only liquid metal present. From the Tliquidus temperature (point A) on, some graphite will start segregating. The segregating graphite has a larger volume (density 2,2 g/cm³) as the liquid (density 6,9 g/cm³) and will first decrease the volume shrinkage of the metal, then stop it (volume lost due the decreasing temperature is equal to the volume increase from the segregated graphite) (point B) and then will even be able to expand the metal (volume lost due the decreasing temperature is smaller than the volume increase



from the segregated graphite) (area C). At the end of the solidification, nearly all graphite did segregate and the volume shrinkage of the metal is again larger than the small volume expansion due to still few residual graphite segregation, the metal volume stops increasing and at the end, will even decrease a little (is not shown in the figure). The solidified metal volume will decrease with decreasing temperature (area D). An increasing amount of “late graphite segregation” will decrease the secondary shrinkage (see figure). The amount of metal expansion is depending on the chemical composition, especially the carbon equivalent (whereas Ceq = % C + % Si / 3 + % P / 3). The solidification is called “hypo-eutectic” if Ceq < 4,33 and “eutectic” if Ceq = 4,33 and “hyper-eutectic” if Ceq > 4,33. 4.2 Solidifying metal (austenite) behaviour This will be studied by the Fe-Ceq diagram, which is shown in next figure. The upper line is indicating the Tl (liquidus temperature) temperature, which is the lowest for the eutectic iron. Between the Tl-line and the horizontal (eutectic temperature) line, there is liquid and solid metal. In the theoretical condition, below the eutectic temperature, there is only solid material. The solidification is different depending on the Ceq and can be hyper-, hypo- or eutectic type.



Hyper-eutectic This is the case for a metal with Ceq > 4,33 (see figure).

The graphite nucleates in the liquid and starts growing (will continue even in the solid material). The chemical composition of the liquid will have a decreasing carbon content. The nodules will be very large (an increasing Ceq will lead to an increasing graphite size). Some other graphite segregation (eutectic graphite) starts when the eutectic composition is reached and these nodules will be small in size.

The solidified surface layer (when the mould cavity is filled, due to the contact with the cold mould wall) will be very thin and the graphite in the liquid will pushes the metal back or in the ingates (pouring system) or in the riser or vents in a large amount. Or, if there are no risers and the ingates and vents are closed (blocked), the thin solidified surface layer will be pushed outside and there will be a large mould wall movement. The increase of the mould cavity leads to a decrease of the metal pressure.

The volume increase (due to the graphite segregation at the eutectic temperature) will not be able to offset the liquid shrinkage, because of the presence of these mould wall movement. This type of solidifying can mostly be seen or in the riser (pushed up metal, cauliflowering riser) or in casting swelling (very low linear shrinkage). Hypo-eutectic This is the case for metal with Ceq < 4,33 (see figure).

The liquid will start solidifying when it reaches the Tl temperature. The line is also called the austenite liquidus line. The dendrites (solidified metal, austenitic structure) will growth in the liquid. The chemical composition of the liquid metal will increase concerning the carbon content and also Ceq. The thick and strong solidified surface layer will resist the metal expansion when the graphite segregates, together with the solidifying of the residual liquid, which leads to less mould wall movement. But with the hypo-eutectic type of solidification, there is less graphite to segregate (Ceq < 4,33; see figure in Chapter Mechanism) and to compensate the shrinkage of the liquid metal.

The nodules will be small and the number is depending on the number of nodules and to a lesser degree to the type of nuclei.

Eutectic This is the case for metal with Ceq = 4,33 (see figure).

The liquid metal will start solidifying when it reaches the eutectic temperature. The solidification starts equal time for forming dendrites and segregate graphite. The amount of graphite (eutectic graphite) is very suitable for the compensation of the liquid metal shrinkage.



The nodules will be small and the number is depending on the number of nodules and to a lesser degree to the type of nuclei.

But whatever the type of solidifying was, the result is solid iron and free graphite. The more the solidification was hypo-eutectic (lower Ceq), the more dendritic (thicker layer) the surface layer will be. The higher the Ceq (hyper-eutectic solidification), the less dendritic and the thinner the layer will be. For eutectic solidifying iron, theoretically there is no layer but in practice there is always a surface layer. The type of solidification also has an influence on the structure. The structure can be predicted by the CCT-diagrams, which are available for every material (and chemical composition), taking in account the cooling rate. If there is a lot of dendritic solidification, in this layer mainly elements with a low segregation tendency will be in, like silicon, nickel and copper. Hereby something about segregation:

The ranking, segregation factor, is as follows, stating that a figure < 1,00 indicates an element that goes to the “first freezing” liquid and a figure > 1,00 indicates an element that segregates to the “last freezing” liquid. The elements with a factor < 1,00 (see table below) will segregate inside the eutectic cell, the other (factor > 1,0) outside the eutectic cell, intercellular area.

Element Mo Ti V Cr Mn P Factor 25,3 25,0 13,2 11,6 1,7 – 3,5 2,0

Element Si Co Ni Cu Factor 0,7 0,4 0,3 0,1

The higher the factor, the more important the cooling time. A slow cooling (up to solidification), will increase the segregation.

In the last solidifying metal, mostly the center part of the section, there will be a lot of elements with a high segregation factor like manganese, phosphorous, chromium, molybdenum…. The solidified metal is first austenitic, which will transform (at temperature lower as Ac3) to ferrite, pearlite, bainite or martensite depending on the chemical composition and the cooling rate. 4.3 Free graphite There are two factors to consider:

• first: nodularity (nodules, compacted or flakes or other degenerated free graphite shapes) and

• second: number of nodules. The number of nodules, also called NC (nodule count), does also influence the free graphite size, which on his turn has an influence on the nodularity.



The free graphite will segregate if there are suitable nuclei, which will be of 2 types: oxides like SiO2, MgO, CeO and sulfides like MgS, CeS. The nuclei that have a similar crystal type will be used very quickly for segregation graphite. This is the case with SiO2. Some others like MgS have a different crystal lattice and will only initiate graphite segregation after SiO2 surrounds it (lower surface energy barrier). The extra step (surrounding the MgS nuclei) will take some more time for the start of graphite segregation. Mostly the size of the graphite will be smaller (in case of MgS) because the time to growth is shorter. See figure below. This indicates that there can be “early” and “late” graphite.

CeS act similar like MgS but is more dangerous due to the fact that free Ce (cerium) will degenerate the graphite to chunky graphite. The smaller the free graphite is (size of nodules), the more nodules there will be because the total amount of free graphite is equal (C-content – 2 %). Small graphite will have a higher nodularity than larger graphite due to the irregular growth that will exist due to gravity, metal flow streaming and presence of other nuclei… This indicates that “late” graphite will be smaller and have a higher nodularity. The higher the nodularity, the higher the tensile strength, the elongation and the shock resistance (especially at low temperature) will be. During solidification of ductile iron, there will be some metal pressure due to the segregation of free graphite. The amount of pressure is depending on the amount of free graphite. In case of hypo-eutectic (Ceq < 4,33) and eutectic (Ceq = 4,33) composition, the total amount of free graphite will be equal. In case of hyper-eutectic (Ceq > 4,33) composition, the amount of free graphite will be somewhat higher, which means that the pressure will be higher.

MgS graphiteSiO2 slag

First step Second step Third step

Fourth step

100% 75%NODULARITY



If this pressure is mainly build in the beginning of the solidification and the surface solidified metal layer (including the dendrites in case of hypo-eutectic composition) is not yet thick, the mould will expand (mould wall movement) or a lot of iron will be pushed back in the riser or in the ingates or in the vents if the riserneck or ingate or vent is not yet solidified. Castings with a high mould wall movement will be difficult to produce without shrinkage.

For eutectic melting, graphite is segregating and austenite is formed at the same time. Because the density of the ductile iron melt is about 6,90 g/cm3 and the one from graphite about 2,2 g/cm3 and austenite 7,8 g/cm3, the carbon will float up and the austenite will go down (if there is a lot of time involved). If the graphite is formed more lately, the surface layer will be thick enough to resist the metal pressure. This indicates that late graphite will lead to no or small mould wall movement, give a higher change for casting without shrinkage and has less tendency for graphite flotation.

HYPO-EUTECTIC SOLIDIFICATION

Liquid metal

Solidified layer

Mould

Pressure of liquid metal

HYPER-EUTECTIC SOLIDIFICATION

Liquid metal

Solidified layer

Mould

Pressure of liquid

Free graphite Smaller pressure of liquid



4.4 Fading effect Fading means that the number of nuclei is decreasing as time passes as well as the Mg content. This has an influence on the number of nodules as well as their nodularity. Mg fading This is possible by the contact with air, which contains oxygen. The residual Mg can react with this oxygen depending on its disposure to the air. If the ladle is not very well covered, the fading will be higher compared to a non-covered ladle. The fading will also be more if the surface to volume ratio of the ladle is increasing. Mg + O ---> MgO

Another point is that Mg can attack the SiO2 and split it so that it is lost as nuclei. 2 Mg + SiO2 ----> Si + 2MgO If the magnesium-sulphide slag is still on the metal, due to the reaction with SiO2, the sulfur can re-enter the metal and more MgO is formed.

2 MgS + SiO2 ----> Si + 2MgO + S A few good rules for Mg fading are:

* The amount of Mg decrease has, due to oxidation, an average rate of 0,0013 % per minute, depending on the temperature. * The correct formula is:

% Mg-loss = (t / 1000) x (T / 1450)2 t : time in minute T : temperature in °C * Other tests did reveal: 0,0007 % / min as the metal surface is covered very well 0,003 % / min in a non-covered metal surface situation.

MgO

MgSSlag



When the residual Mg content decrease below 0,03 %, non-spheroidal graphite will be formed (more compacted or worm graphite). The number of nodules is not specially influenced. See picture. Nuclei fading This is also called “inoculant fading”, which is a less correct designation. As already mentioned, the nuclei are mainly SiO2 and the non-solvable ones like MgS, CeS… There is always fading! Only the fading rate can be different. Fading is depending on the type of inoculant, temperature and time to solidification. The fading will happen in 3 different ways:

1. Solving of the nuclei (SiO2) These nuclei will dissolve by forming Si and CO, depending on the equilibrium temperature for this reaction.

Below the line: Si + O2 + C à SiO2 + C

Above the line: SiO2 + 2 C à Si + 2 CO



2. Transforming to other products by reaction

2 Mg + SiO2 ----> Si + 2MgO (1 nuclei disappears, SiO2) 2 MgS + SiO2 ----> Si + 2MgO + S (2 nuclei disappear, MgS and SiO2)

3. Floating to the surface

Especially the non-soluble MgS, CeS…will float to the surface. The speed of floating will depend on the size of the nuclei (mostly 0,5 – 2,0 µm), the temperature and the amount of them. An increasing number of nuclei present will increase the possibility that they touch each other and click together (double dimension). This are nuclei that coarsen, ending up in floating slag.

It is accepted that after 10 minutes the effect of any inoculation is disappeared! This figure shows that the number of inclusions (most of them can act as nuclei) does decrease drastically during the first 5 minutes (about 75 %). After 10 minutes, only 15 % of the initially present inclusions are still present in the metal. This is the total effect of dissolving and floating up.

INCLUSIONS / mm³ X 10.000

0

1

2

3

4

5

0 10 20 30

HOLDING TIME (min)

NU

MB

ER



4.5 Mould behaviour The mould material will increase in volume and decrease the mould cavity a little because it cannot expand to the outside (O to A). Then, when graphite segregation is starting, the mould will expand because of the pressure of the liquid metal. The mould will expand less than the metal because there will be a pressure build up in the metal (A to B). At the end, when the metal mostly has a small (or more depending on the quality of the metal) decrease (secondary shrinkage), the mould will keep the equal volume as before (B to C). See figure. The amount is expanding is depending on:

1. Ceq of the metal 2. Mould strength 3. “Early” or “late” graphite segregation.

This expansion is mostly the cause that hyper-eutectic irons do have shrinkage.

O A B C

100

TIME0

VOLUME

MOULD

METAL



5. ROLE OF MAGNESIUM There are quite some elements that retard the graphite segregation. The elements are: V, Cr, Mn, Mo, W, N, Mg and RE (rare earths). The elements that involve ductile iron are Mn (also segregates very much) and N (nitrogen). Nitrogen is no problem for ductile iron because the Mg-reaction did remove it nearly all. The element Mg does work properly and is always present in the metal with an amount of 0,03 till 0,06 % (residual magnesium). The Mg retards the graphite segregation that much that this segregation (seen in the Temperature-Fe-Ceq diagram) does only start above Ceq = 4,40 %. This is interesting because the eutectic solidification is now possible over a range (4,33 till 4,40) in stet of a point (4,33). More details are given in Addition: Iron Carbon diagram. The Mg-addition can be calculated. The most common formular is: % Mg = (0,75 * Sinitial + Mgresidual) / η η: efficiency of treatment Another indication is given by next figure: The blue curve is necessary to neutralise the S-content. The dark green colour gives the recommended minimum. The orange colour has some safety. The red colour takes in account the extra fading due to temperature (> 1450 °C) or time (> 12 minutes).

Mg addition compared to initial S

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

0 0,01 0,02 0,03 0,04 0,05 0,06Initial S (%)

Des

ired

resi

dual

Mg

(%)

Neutralise SRecommended minimumWith some safetyHigher temperature & time



The residual Mg-content is not equal in the complete ladle volume. Some of the Mg will enter the metal, some other will form MgS and MgO. These combinations do not solve in metal and will float to the surface depending on their size and the temperature of the metal. On top of the metal, there will be a lot of large-sized Mg-products. This slag includes MgS, MgO, Mg.SiO2… A calm (non violent) reaction, mostly present if the Mg-content in the noduliser is less than 5 %, will produce more small reaction products, which will float up with a much lower rate than the coarse inclusions. An increasing amount of the Mg-content will lead to a more violent reaction. Pay attention for wire-nodulising will Mg-filled wire. The MgS, and in a lesser degree MgO and MgN, can act as nuclei that will promote late graphite segregation (which is the target). If the surface (floated up) slag is not removed properly, it can be that other reactions happen (see Mg fading) and S re-enter the metal. 6. ROLE OF RARE EARTHS The RE consists mostly for 40 % of Ce (cerium). This cerium is especially active in ductile iron and:

1. compensates the deteriorating effects of As, Pb, Ti… (restore nodularity) 2. combines with S to form CeS, which also acts as nuclei (also CeO) 3. deteriorates the nodules in thick wall castings to “chunky graphite” if not

balanced with the presence of As, Pb, Ti… 4. chill formation in thin section castings.

This indicates that for thin wall ductile iron castings, there are few problems. But for thick wall castings, the Ce-content must be properly balanced with the presence of As, Pb, Ti… to avoid degenerated (chunky) graphite. The formed CeS can act as nuclei and promote the late graphite segregation. But it must be balanced with Mg because it can remove MgS and MgO to form CeS and CeO.



7. TESTING It is very important to find tests to monitor and control the solidification. There are 2 types of tests: the tests before pouring and the other after pouring. 1. After pouring It is possible to check the microstructure of the metal after solidification. It is very important that the test sample is removed from a test bar with equal (at least similar) section size because the presence of graphite and structure depends on the section size. A microstructure sample reveals:

• structure: ferrite, pearlite… Chemistry and cooling rate • nodule count Metal quality and inoculation • nodularity Mg-content (lesser degree RE-content) • early or late graphite Presence of proper nuclei • graphite flotation Metal chemistry, especially Ceq • chunky graphite Presence of Ce (lesser degree As, Pb, Ti…) • flake graphite Presence Ti • other degenerated graphite Presence of S in the mould material at the casting-mould surface.

The problem is that these tests reveal what the foundry did incorrect. No action can be performed anymore to improve the quality.



Graphite flotation

2. Before pouring Before pouring, after the nodulising and the first inoculation, there is an excellent opportunity to look at the cooling curve. As already known, the amount of undercooling does indicate if the metal has sufficient nuclei, ready for the initiating of graphite segregation. This stays an important item. But a second feature is coming. The shape of the curve is indicating if the graphite is segregating earlier or later. During the graphite segregation, energy is given back to the metal and the metal temperature increase (or will decrease less: cooling minus extra heating energy). The amount of energy and temperature increase does increase with an increasing amount of graphite segregation.



If the graphite is segregating in an early stage, the curve has more or less shape A. Due to the fact that the graphite is segregated for a large part at the end of the solidification, the curve bends very quickly down. If the graphite is segregating late, the curve is more or less horizontal (shape B) until the Ts temperature (all metal is solidified). As a consequence, the undercooling for late graphite will be small (curve B: < 1 °C) compared to early graphite (curve A: 4 °C).



8. CONCLUSION The solidification of ductile iron is strongly depending on the chemistry and number of nuclei, present at the time of solidification. Although a lot of theories about this solidification are running, the one with nuclei (including non soluble inclusions like MgS) seems to explain a lot. A good quality ductile iron solidifies in a way that the maximum mechanical properties are valid together with the lowest tendency to form porosity inside the casting. This is most present if the chemistry is valid for an eutectic solidification (Ceq = 4,33), more or less good for hypo-eutectic solidification (Ceq < 4,33 but close to it) and the least for chemistry valid for hypo-eutectic solidification (Ceq > 4,33). The presence of non soluble inclusions, like MgS, does promote the late graphite segregation, which is much better than the early graphite segregation. Late graphite is small, has a high nodularity and a high nodule count. The quality of the solidification can be predicted with the evaluation of the cooling curve.



ADDITION: IRON CARBON DIAGRAM Every foundry knows about the theoretical Ion-Carbon diagram, which indicates the condition of the ferrous alloys depending on temperature and percentage of carbon. But it is clear that more elements are involved in iron and steel. So the elements silicon, phosphorus, sulfur and manganese are nearly all the time involved. For this reason the Iron-Carbon equivalent diagram is mostly used, especially for iron. The carbon equivalent is: Ceq = % C + % Si / 3 + % P / 3. The example is given on next page. It is important to have an idea what will happen during the cooling of the liquid iron. Depending on the Ceq, the solidification (cooling) is classified as hypo-eutectic (Ceq < 4,3), eutectic (Ceq = 4,3) or hyper-eutectic (Ceq > 4,3). Once the metal is solidified (austenitic material and free graphite), the austenitic material will transform to ferrite, pearlite, bainite and or martensite depending on the cooling rate. The graphite is not transforming anymore. Some of the graphite, present in austenite will segregate and form new free graphite particles or increase the size of existing ones. Cementite will remain like this. The best solidification is the eutectic one because it has the lowest tendency to shrinkage. This is very important to get a high quality (concerning material section) casting without the use of extensive risers. The hyper-eutectic solidification has a high tendency to form graphite flotation (ductile or nodular iron) or “kish graphite” (gray or flake iron) because the graphite segregates in the liquid and float to the top surface. The hypo-eutectic iron has a higher tendency to form shrinkage in thick wall castings and hot spots in all types of castings. In practice the solidification will not happen according to the diagram because there will be some undercooling. These undercooling does shift the eutectic point more to the right (higher Ceq). These undercooling is increased or decreased by the type of elements present in the iron. Some elements have a large decreasing influence like Sn, SB and Mg. Other elements have a smaller decreasing influence like Ni, Cu, Co, Al, Pt and Mn. Other elements like Si, Ti, V and Cr decrease the undercooling and Mo, W and P do shift the eutectic temperature to higher temperatures. All this is indicated in figure Iron – Carbon equivalent modified diagram.



Figure: Iron – Carbon equivalent diagram



Figure : Modified iron – Carbon equivalent diagram Group Elements 1 Cr, V, Ti, Si 2 Sn, Sb, Mg 3 Mo, W, P 4 Ni, Cu, Co, Al, Pt, Mn The initially eutectic temperature can, due to the quick cooling and or presence of elements, be decreased from 1130 °C to about 1095 °C. This means for the solidification that:

1. the graphite segregation starts at a lower temperature 2. the eutectic solidifying zone expands from a point Ceq 4,33 to a range Ceq

of 4,33 to about 4,50 3. there will be cementite (Fe3C) formation (less free graphite) and this

cementite will increase the shrinkage of the solidifying metal. The undercooling must be kept under control to avoid cementite formation. The cooling is difficult to control. The influencing elements can be used for this purpose. The 2 important for ductile iron are:

• Mg if it is kept below 0,06 %. It has a high cementite promoting effect that can be seen in the graphitising factor:

K = C * {Si – 0,2 *(Mn – 1,7 * S – 0,3) – 1,2 * Cr – 0,4 * Mo – 2 * V – 8 * Mg + 0,1 * P + 0,4 * Ni + 0,5 * Al +0,4 * Ti + 0,2 * Cu

• Mn should not be too low because it helps to increase the eutectic solidification temperature.

For ductile iron with a Mg content from 0,05 to 0,055 %, the eutectic range can be between 4,33 and 4,40 without problems. For ductile iron, used for low temperature applications, the manganese content should not be too low because decreasing this content means decrease the eutectic carbon equivalent zone. The Mn content must be low for meeting the shock resistance requirements (at -20 to -40 °C) and this means that this type of metal will be more difficult to control for eutectic solidification.

Primary freezingEutectic freezing

1

2

3

4

Ceq

Temperature (°C)

1130

1095

4,33 4,50

MODIFIED IRON - CARBON EQUIVALENT DIAGRAM



ADDENDUM: DENDRITIC SOLIDIFICATION When the temperature of the liquid metal has dropped sufficiently below its freezing point, stable aggregates or nuclei appear spontaneously at various points in the liquid. These nuclei, which have now solidified, act as centers for further crystallization. As cooling continues, more atoms tend to freeze, and they may attach themselves to already existing nuclei or form new nuclei of their own. Each nucleus grows by the attraction of atoms from the liquid into its space lattice. Crystal growth continues in three dimensions, the atoms attaching themselves in certain preferred directions, usually along the axes of a crystal. This gives rise to a characteristic treelike structure which is called dendrite.



Since each nucleus is formed by chance, the crystal axes are pointed at random and the dendrites will grow in different directions in each crystal. Finally, as the amount of liquid decreases, the gaps between the arms of the dendrite will be filled and the growth of the dendrite will be mutually obstructed by that of its neighbours. This leads to a very irregular external shape. The crystals found in all commercial metals are commonly called grains because of this variation in external shape. The area along which crystals meet, known as the grain boundary, is a region of mismatch. The boundaries are formed by materials that are not part of a lattice, such as impurities, which do not show a specific grain pattern. This leads to a non-crystalline (amorphous) structure at the grain boundary with the atoms irregularly spaced. Since the last liquid to solidify is generally along the grain boundaries, there tends to be a higher concentration of impurity atoms in that area.

Formation of dendrites in a molten metal. Dendrites observed at a magnification of 250 x.



This type of solidification is very sensitive for micro-porosities, which appears between the dendrite arms. These locations are difficult to reach for the feeding metal of risers. See next figure.

Solidification Mechanism of Ductile Iron

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