Chill sensitiveness and thermal analysis parameters relationship in hypo-eutectic , Ca and CaLa inoculated commercial grey cast irons

Previous experiments shown a specific distribution of Al, La and Ca on the section of complex (Mn,X)S compounds, found as major nucleation sites for graphite flakes in low-S cast irons (< 0.03%S), and a possible contribution of La to improve their capacity to nucleate graphite, avoiding carbides formation. In the present work, standard thermal [cooling curves] investigations are undertaken to explore Ca and La-Ca bearing FeSi alloys inoculation effects [10 measurements for each inoculant], in 3.7 – 3.8%CE and optimum S and Mn relationship [0.046 – 0.056%S, (%Mn) x (%S) = 0.024 – 0.029]. Representative temperatures on the cooling curves and under-cooling degrees referring to the meta-stable eutectic temperatures are determined and correlated with the chill [carbides/graphite formation sensitiveness], in different solidification conditions [cooling modulus, wedge shape castings, resin sand mould]. Supplementary addition of La to Ca-bearing inoculants has limited, but specific benefits in these cast irons: lower eutectic recalescence and of the maximum recalescence rate, higher GRF1 and lower GRF2 graphitizing factors and lower value of the first derivative at the end of solidification. Consequently, it results a premise for lower shrinkage sensitiveness and lower chill (carbides) sensitiveness, especially at the highest solidification cooling rate (thin wall castings).


INTRODUCTION
Cast iron continues to be the most produced metallic material in the world foundry industry [cca 70% of the total, more than 75 million tons castings each year], including different graphite morphologies, such as lamellar, nodular (spheroidal), vermicular (compacted) or temper carbon, with grey (lamellar graphite) cast iron on the first place (cca 48% rate). Reducing necessity of the metal and energy consumption led to re-design of the metal parts, and as result, thin wall grey iron castings (less than 5mm wall thickness) are more and more produced, especially in the automotive industry. On the other hand, melting procedures of this cast iron diversified, as furnace type, energy form and metallurgical treatments, resulting a large range of iron melt temperature [1300 -1600 o C] and sulphur content [0.015 -0.15%S). Transition from cupola furnace melting [moderate overheating and usually more than 0.08%S content] to electrically melting, especially as induction furnaces, typically at higher melt overheating and lower sulphur content characterizes the actual world cast iron industry.
It was found that for commercial grey cast irons, solidified in foundry conditions, complex manganese sulphides, in (Mn,X)S system, act as major graphite nucleation sites for graphite. [1] Higher iron melt temperature (>1500 o C) and lower sulphur content (<0.03%S), typically for electric induction furnace melting are non-favourable conditions for (Mn,X)S formation, and, as a result, solidification will occur at excessive under-cooling referring to equilibrium eutectic temperature. In these conditions, solidification pattern will be characterized by higher sensitiveness to free carbides and/or under-cooled graphite morphologies formation, both of them at negative effects on the mechanical properties of iron castings.
Three stage lamellar graphite formation in commercial cast irons, illustrated by the authors in previous papers [1 -5] pointed out the importance of three groups of active elements, involved in this process. Strong oxide forming elements, such as Al or/and Zr, promote low size (< 3m) first compounds, which will act as nucleation sites for (Mn,X)S later formation, usually up to 10m size (both Mn and S content is important), major nucleants for graphite. The third group of elements, such as Ca, Ba, Sr, Ce, La etc, will act in the first or/and the second stage, to sustain their formation or/and to increase their capacity to nucleate graphite, such as the decreasing of the characteristic parameter of the lattice relationship, plane mismatch. The smaller the mismatch of the two substances, the stronger the nucleation potential between them.
The nucleation of graphite on MnS-type particles is also confirmed by microstructure simulations [6]; role of Mn/S ratio [7]; silica-rich oxide bi-films to behave as substrates on which oxy-sulphide particles form [8]; especially for > 0.02%S in grey iron. [9] In previous papers [10,11] it is illustrated that for critical solidification conditions, such as at lower sulphur content [0.018 -0.026%S, (%Mn) x (%S) = 0.008 -0.015] and more than 1500 o C overheating temperature in the induction furnace, grey iron castings are sensitiveness to carbides and under-cooled graphite formation. In these conditions, La-bearing FeSi alloys act as effective inoculants, with better efficiency comparing to the conventional commercial inoculants, as Ca-FeSi system.
The main objective of the present work is to apply the thermal [cooling curves] investigations to explore the effects of supplementary addition of La to commercial Cabearing FeSi inoculant on solidification parameters, in electrical melted hypo-eutectic grey cast iron (3.7 -3.8% carbon equivalent) with optimum level of sulphur content [0.046 -0.056%S, (%Mn) x (%S) = 0.024 -0.029] and with or without anti-graphitizing action after inoculation.

EXPERIMENTAL PROCEDURE
Two heats of experimental cast irons are obtained in graphite crucible electric induction furnace [10kg, 8000Hz], with cast iron scrap as metallic charge material, in similar conditions as chemical composition (3.76 -3.77% carbon equivalent), thermal regime (1550 o C overheating and metallurgical treatments (ladle inoculation, 0.25wt.% alloy addition, 0.2 -0.8mm size grains) (Fig. 1). The iron melt was tapped into the pouring ladle, with inoculating alloy added in the metal stream, during the ladle filling, at 1426 -1429 o C inoculation temperature. Two inoculating systems are considered: commercial Ca-bearing FeSi alloy (wt.%: 0.75Ca, 1.25Al, 75Si, bal Fe) and Ca, La-bearing FeSi alloy (wt.%: 1.2Ca, 0.96Al, 4.0La, 67Si, bal Fe). The two tested inoculants are similarly as Ca and Al content, while supplementary La addition to commercial Ca-bearing inoculant makes the difference.
Un-inoculated and inoculated iron melts have been poured in standard ceramic cups (7.3mm cooling modulus) including a thermocouple for thermal (cooling curve) analysis of the solidification process. Cooling modulus is the ratio between the volume and total surface area of castings, and it expresses the capacity to transfer the heat from casting through mould media outwardly. Lower cooling modulus value leads to higher solidification cooling rate, with important effects on the eutectic and eutectoid structure formation and characteristics. The used cooling modulus for ceramic cup is similarly with 30mm diameter standard bar casting, typically used for grey cast irons quality evaluation.
Appropriate pouring temperature (1369 -1375 o C) allows a similar thermal regime as mould behaviour. For both inoculation variants, similar 14 ceramic cups were used: one for un-inoculated iron (1 -UI); ten for inoculated irons [2 -11 cups]; other three cups [12 -14], also filled with inoculated irons, included Te, at known strong anti-graphitizing factor.
The two experimental heats, as un-inoculated and inoculated irons have been also poured in wedge type casting, type W1, W2 and W3, according to ASTM A 367, in resin sand mould media, to evaluate the chill tendency, the sensitiveness to form free carbides instead of graphite, respectively. The standard size and cooling conditions are illustrated by Figure 2 [12, 13].

Wedge
No.

Chemical Composition
Tables 1, 2 and 3 show the chemical composition of the two experimental heats, as Ca-FeSi inoculation (Heat 1) and Ca, La-FeSi inoculation (Heat 2). According to the Table 1 Table 2 summarizes the content of some active elements in the final inoculated cast irons, which could play an important role in heterogeneous graphite nucleation. Lower content of typical oxide forming elements, acting in the first stage of graphite nucleation (micro-oxide formation), such as < 0.004%Al, < 0.0007%Zr and < 0.015%Ti leads to difficulties in the initiation of graphite formation. On the other hand, less than 0.008%N content does not favour the participation of nitrides in graphite nucleation. As inoculating elements (Mg, Ca, La, Ce), it is visible the contribution of Ca, La-FeSi inoculation in La   Minor elements content (Table 3) is kept at a low level and close content in the two experimental heats. Mn, Si and some minor elements have important role in pearlite forming sensitiveness (Px factor), according to Equation 3. [14] In the experimental conditions, Px = 4.5 -4.8, illustrating a predominant pearlite amount formation.
Px =3 (%Mn) -2.65 (%Si -2) + 7.75 (%Cu) + 90 (%Sn) + 357 (%Pb) + 333 (%Bi) + 20.1 (%As) + 9.60Cr + 71.7 (%Sb) According to the obtained chemical composition, the experimental heats are characterized by a medium hypo-eutectic position on the Fe-C, good ratio as manganese and sulphur content, and low level of active and minor elements. Stable (Tst) and metastable (Tmst) eutectic temperatures could be calculated depending on the chemical composition of the cast irons (Si as a major influencing factor) or could be measured, by inciting to graphitic solidification by over-inoculation (Tst measurement) or to carbidic solidification by addition of a strong anti-graphitizing agent, such as Te or S (Tmst measurement). In the present work, a Te addition in inoculated cast irons is applied, and the real Tmst is measured on the cooling curve (Tmst = TEU = TER, no recalescence]. It is important to be noticed that solidification above Tmst leads to graphite formation, while at solidification under Tmst only carbide could be formed. Two solidification events are the most important for the structure characteristics and the integrity of iron castings: T1 and T3. T1 parameter illustrates the position of the lowest eutectic temperature comparing to the metastable eutectic temperature; negative value of this parameter shows the occurrence of free carbides in the cast iron structure, in the specific solidification cooling rate. A more negative level of T1 parameter brings about higher sensitiveness to carbide nucleation, instead of graphite formation. Carbides are characterized by the highest hardness in the cast iron structure, leading to the lowest machinability of castings, negative affecting their cost. Also, free carbides presence leads to lower mechanical properties and fracture incidence, especially in impact conditions. Contrary, for positive position of T1 parameter carbides are not formed, being favourable conditions for graphite formation. On the other hand, it is also important the position of T1 parameter above the zero level: positive, but lower values means that TEU is closed to Tmst, so graphite will be formed, instead of carbide, but at higher eutectic undercooling. Lower T1 value, higher incidence of under-cooled graphite morphology, such as D-type ASTM, favouring ferrite formation and decreasing of the mechanical properties and wear resistance. Figure 5 shows the level of the T1 parameter for un-inoculated and inoculated cast irons, with the two systems of alloys, with and without Te-addition after inoculation. The solidification behaviour of the experimental cast irons at the lowest eutectic temperature is highly depending on the status of the iron melt (with or without inoculation) and anti-graphitizing action after inoculation, but it is less depending on the inoculant system.

Thermal (Cooling Curve) Analysis
For both inoculants, the iron melt inoculation before solidification leads to visible improvement of the cast iron quality, as T1 increased from negative value (-15 o C) up to positive values (17 -20 o C). In this way, inoculation suppresses the free carbides formation and promoted favourable conditions for graphite nucleation, in the applied solidification parameters: ceramic mould media and casting at 7.3mm cooling modulus.
Te addition into the ceramic cup, before iron pouring, contributes to important decreasing of inoculated cast irons T1 values, up to -2.5 ─ +2.5 o C, respectively. As result, carbides formation was promoted and solidification was moved to carbidic system. For the same addition rate, the inoculant type, as La supplementary addition to Ca-bearing FeSi alloy does not appear to have an important influence for normal solidification. At lower scale, La appears to have a limited beneficial effect if a strong antigraphitizing factor will act after inoculation.
Complex chemical composition, typical for commercial grey cast iron (more than 30 elements presence, in a large range of content, see Tables 1-3) favours a segregation process during solidification, between primary austenite and eutectic cells, on the one hand, and especially inside or outside of eutectic cells (in the space between them).
A lot of elements have a limited solubility in the solidified iron comparing to the iron melt, so they will be concentrated in the last solidified iron part, in the last available space, i.e. between eutectic cells, such as C, P, Mn, Mo, V, Cr etc. This last solidified iron melt, usually at a lower temperature than the metastable eutectic temperature in commercial cast irons, will include different phases, such as free carbides, phosphides, steadite etc. On the other hand, these last solidified liquid iron micro-volumes will form micro-shrinkages (holes, contraction defects), without possibility to avoid them, by feeding with liquid. The inter-eutectic cells defects, including segregated phases and micro-shrinkages, have negative effects on the quality of iron castings.
T3 parameter offers information on the sensitivity to formation phase and the presence of contraction defects at the end of solidification, with inter-eutectic cells distribution. Generally, for grey cast iron solidification, T3 parameter has negative values, as usually the temperature at the end of solidification (TES) is lower than metastable eutectic temperature (Tmst). Lower (more negative) T3 values, higher the sensitiveness to structure and contraction defects at the end of solidification. Figure 6 shows the obtained values for T3 parameter for the tested cast irons. Non-inoculated cast iron (UI) solidified at a very low temperature, as TES is more than 40 o C below Tmst (T3 = -42.5 o C). Inoculation significantly affects the last part of the solidification process, as this treatment decreases the under-cooling at this moment up to T3 = -5 …-15 o C, much more for Ca-FeSi inoculated cast irons. Anti-graphitizing treatment of the inoculated irons (Te addition before solidification) drastically affects the end of solidification, illustrating by T3 increasing up to -40…-47 o C.
A relationship between the under-cooling at the end of solidification (T3) and in the first part of eutectic reaction, corresponding to the lowest eutectic temperature (T1) is shown in Figure 7. Both under-cooling parameters have negative values, corresponding to the highest under-cooling on the entire solidification process for basic iron (non-inoculated iron), which consumed below metastable eutectic temperature, promoting carbides instead of graphite (including the area between eutectic cells) and contraction defects between eutectic cells.
Inoculation affects both considered under-cooling parameters, resulting T3 no less than -17 o C and T1 more than 17 o C. Higher T1 level (lower under-cooling at the beginning of eutectic reaction), lower under-cooling at the end of solidification (less negative values), for both inoculating systems, with a good relationship between them.
Te addition, as anti-graphitizing treatment of the iron melt after inoculation, leads to the intermediary level of the under-cooling parameter T1 (close to zero or just below it), but to restore the highest under-cooling at the end of solidification, similarly to the noninoculated cast irons.
The position of TEU and TER temperatures (visible especially as zero point level on the first derivative of the cooling curve) as the lowest and the highest (recalescence) eutectic temperatures, are important not only comparing to the stable (Tst) and metastable (Tmst) Fig. 6. Under-cooling (T3 = TES -Tmst) at the end of solidification (TES temperature) comparing to metastable eutectic temperature (Tmst) [1: non-inoculation; 2 -11: inoculation; 12 -14: inoculation + Te addition] Fig. 7. Relationship between the under-cooling at the end of solidification (T3) and the under-cooling in the first part of the eutectic reaction, corresponding to the lowest eutectic temperature (T1) of non-inoculated (UI) and inoculated cast irons eutectic temperatures, expressed by eutectic under-cooling parameters but also as a difference between them.
The heat delivered by eutectic cells, including austenite plus graphite, contributes to temperature increasing during eutectic reaction, so the TER parameter (the maximum temperature in eutectic reaction) is generally visibly higher compared to TEU. Known as eutectic recalescence, Tr = TER -TEU parameter is important especially for the integrity of iron castings, produced in soft mould media, such as a green sand mould.
Expressing the level of eutectic (graphite + austenite) precipitation during the eutectic reaction, recalescence is also a measure of the resulted graphitic force applied on the mould walls, resulting an enlargement of the mould cavity and of the shrinkage (holes in the casting) during solidification, respectively. Too high amount of graphite precipitated during the first stage of eutectic reaction means a deficit in graphite precipitation at the end of eutectic reaction. Consequently, there is not enough available graphite to compensate the contraction process and to avoid micro-shrinkage, respectively. For this reason, in many industrial cases it is important to obtain a reduced level of the eutectic recalescence Tr. One of the important criteria as quality of an inoculant is its capacity to lead to a reduced eutectic recalescence after this treatment, usually applied to avoid free carbides formation and under-cooled graphite occurrence in grey cast irons. Figure 8 compares the values of eutectic recalescence (Tr, o C, on the cooling curve) and the maximum rate of recalescence (TEM, °C/s, on the first derivative of the cooling curve) of the two inoculated cast irons. It is visible that Ca,La-FeSi inoculation leads to lower values for recalescence (Tr = 3…5 o C) and the maximum rate of recalescence (TEM = 0.18…0.24 o C/s), comparing to commercial Ca-FeSi alloy (Tr = 5…6 o C, TEM = 0.26….0.34 o C/s), at 25-30% as average. As strong anti-graphitizing treatment, Te treatment leads to graphite avoidance, so all of carbon content, non-dissolved in austenite formed carbides. In this condition, TEU = TER, so not recalescence appears on the cooling curve.
Graphitization process [graphite formation during solidification] could also be characterized by Graphitic Factors, GRF1 and GRF2 [15] and the value of the first derivative at the end of solidification FDES, obtained by measurement on the cooling curve and its first derivative.
GRF1 refers to graphite formation immediately after the attending of the maximum recalescence (TER) (Fig. 3) and it is expressed by registered time for 15 o C temperature decreasing after TER point [higher GRF1 means that the nucleation and the growth of eutectic occur in longer times, resulting a higher graphite amount formation].
According to Figures 9 and 10, inoculation visibly contributed to increasing the capacity of cast iron to form graphite (instead carbides) during solidification, as GRF1 parameter increased for both inoculating systems with 70-85%, as average from 25.5 level in non-inoculated iron up to 43.7 level for Ca-FeSi inoculation and 47.4 level for Ca,La-FeSi inoculation, respectively. For all of the ten measurements, Ca,La-FeSi inoculation leads to higher graphitizing factor GRF1 than it was obtained for Ca-FeSi inoculation (43.6 -50.5 comparing to 40.7 -46.0 range). As it was expected, Te addition after inoculation cancelled the graphitizing effect of this metallurgical treatment, so GRF1 factor returned to the same level such as without inoculation (20-30).
Graphitizing factor GRF2 and the value of the first derivative at the end of solidification (FDES) illustrate the behaviour of cast iron at the end of solidification (Fig. 3). GRF2 indirectly expresses the thermal conductivity and it is illustrated by the angle of the first derivative at TES, calculated by the use of FDES parameter. Low values of FDES [more negative] and GRF2 are related to a high thermal conductivity, which means a higher amount of graphite precipitated at the end of solidification, and, as a result, cast iron is more resistant to micro-shrinkage formation. This part of graphite is an important favourable factor to  decrease the sensitiveness of iron castings to contraction defects formation. [16,17] Lower levels of GRF2 and FDES factors mean a higher cast iron quality. In this case, inoculation decreased the GRF2 values from 31 up to 16 -20 range, for both inoculants, with Ca,La-FeSi as performance: 16.7-21.8 (18.1 as average) versus 17.0-21.8 (18.9 as average).
Tellurium addition after inoculation increased the GRF2 factor up to the level even above of the non-inoculated iron (30 -45). Similarly behaviour was recorded also as FDES parameter. If non-inoculated cast iron is characterized by FDES = -2.30 o C/s, inoculation moves this parameter to -3.3 ….-3.6 o C/s, while Te addition after inoculation leads to intermediary range (-2.8…-3.0 o C/s).
It can be seen that the performance of Ca,La-FeSi alloy, comparing to Ca-FeSi inoculant is higher for GRF1 evaluation comparing to GRF2 or FDES, not only after inoculation, but also (at a reduced power) if a strong anti-graphitizing factor acts after inoculation (Te addition).

Chill tendency
Chill (carbide formation sensitiveness) is evaluated by simultaneous pouring of typical test castings (W1, W2, W3), defined by ASTM A 367 (Fig. 2). The wedge shape of these castings leads to a strong variation of the solidification cooling rate, from the base (the lowest level) up to the apex (the maximum level). As Fig. 2b shows, the portion nearest the apex, entirely free of grey spots, is designated as the clear chill zone (Wc), while the portion from the end of the clear zone to the location where the last spot of cementite or white iron is visible is designated the mottled zone. The region from the junction of grey fracture to the first appearance of chilled iron is designated the total chill (Wt). In the present work the relative total chill [RTC = 100 (Wt / B), %] is considered, where B is the maximum width of the test wedge.
There are used W1, W2 and W3 wedge samples defined by Fig. 2a, typically used to evaluate the solidification characteristics of thin wall castings. In order to have a more accuracy in evaluation of the obtained results, each casting is measured as size characteristics, including B-parameter, volume (V) and total area (A), to obtain the real values for cooling modulus and relative total chill, respectively. Previous experiments pointed out that these measurements are necessary to have a more realistic evaluation on the obtained results. Figure 11 a shows the obtained results for inoculated cast irons, as relative total chill (RTC), influenced by the solidification cooling rate, expressed by the cooling modulus of casting (higher cooling modulus, lower cooling rate) and by inoculating system (Ca,La-FeSi versus Ca-FeSi). As normally expected, the decreasing of the solidification cooling rate by increasing of the cooling modulus leads to the decreasing of the carbides formation incidence, expressed by decreasing the relative total chill. It is true for both inoculating systems, with Ca,La-FeSi performance (cca 20% more efficient). The superior inoculating power of Ca,La-FeSi alloy is more visible for W1 type wedge sample, for the highest solidification cooling rate, respectively. This behaviour is very important for thin wall castings category (less than 5mm wall thickness), more and more produced in the world foundry industry, especially for the automotive industry. Figure 11 b illustrates the importance of inoculation in chill tendency control and also the relationship between the chill tendency of grey cast iron and the eutectic under-cooling T1, referring to the lowest eutectic temperature TEU, comparing to the metastable eutectic temperature Tmst.

CONCLUSIONS
The evaluation of the effects of supplementary La addition to commercial Ca-FeSi inoculants on the quality of electrically melted, hypo-eutectic (3.7 -3.8% carbon equivalent) grey cast iron at optimum Mn and S relationship is recorded by thermal (cooling curve) analysis (10 measurements) and chill tendency test and the following conclusions can be drawn: 1) In experimental conditions, the solidification behaviour of cast irons at the lowest eutectic temperature (T1) is strongly depending on the status of the iron melt (with or without inoculation) and anti-graphitizing action after inoculation, but it is less depending on the inoculant system. 2) Inoculation significant beneficial affected the last part of the solidification process (T3), much more for Ca-FeSi inoculated cast irons. 3) Te addition, as anti-graphitizing treatment of the iron melt after inoculation, leads to the intermediary level of the under-cooling parameter T1 (close to zero or just below it), but to restore the highest under-cooling at the end of solidification T3, similarly to the non-inoculated cast irons. 4) Higher T1 level (lower under-cooling at the beginning of eutectic reaction), lower under-cooling at the end of solidification T3 (less negative values), for both inoculating systems, with a good relationship between them. 5) Ca,La-FeSi inoculation leads to lower values for eutectic recalescence Tr (3…5 o C), comparing to commercial Ca-FeSi alloy (5…6 o C), at 25% as an average, favourable especially for soft mould use. 6) For all of the ten measurements, Ca,La-FeSi inoculation leads to a higher graphitizing factor GRF1 than it was obtained for Ca-FeSi inoculation (47.4 comparing to 43.7 as average), expressing a higher capacity to form graphite immediately after attending the maximum eutectic temperature (recalescence). 7) Inoculation decreases the graphitizing factor GRF2 (from 31 up to 16 -20) and the value of the first derivative at the end of solidification FDES (from -2.3 up to -3.3-3.6 o C/s), for both inoculants, with Ca,La-FeSi as a performance in the improving of the cast iron quality. 8) The decreasing of the solidification cooling rate by increasing of the cooling modulus leads to the decreasing of the carbides formation incidence for the both inoculating systems, with Ca,La-FeSi performance (cca 20% more efficient). The superior inoculating power of Ca,La-FeSi alloy is more visible for W1 type wedge sample, for the highest solidification cooling rate, respectively. 9) A good relationship was obtained between the chill tendency of grey cast iron and the eutectic under-cooling T1, referring to the lowest eutectic temperature TEU, comparing to the metastable eutectic temperature Tmst.