new approach to process control in cast iron …...new approach to process control in cast iron...

New approach to process control in cast iron foundries throughthermal analysis

ANDREA ZONATOa1

1ProService Srl, Massanzago (PD) ITALY, www.proservicetech.it , www.itaca8.com

Abstract

The continuous increase in the quality demand from final customers and the continuous need of reduction forthe costs of production have further increased the importance of an effective control system of the productionprocess in cast iron foundries. The modern thermal analysis systems, like for example ITACA MeltDeck andITACA8, can generate an enormous contribution, allowing to reduce and manage the variance in the meltingshop. In the first part of this work it will be described the role of thermal analysis in the foundries, brieflyexplaining how it integrates in the actual control processes and showing the main reasons making it an essentialpart of a modern productive system. In the second part four testimonies will be presented, written by foundriesthat have implemented these tools in their control system, reporting an evaluation on the results obtained.

The transformation of the foundries in com-plex industrial systems with high productiv-ity started in the ’50s and continued until the

’70s. Since the beginning, it raised problems relatedwith the quality control of big quantities of castingsthat theoretically should not have differed very muchamong them, if they were produced in the same pro-duction batch and even if they were part of differentproductions.In reality any productive plant, especially if very com-plex or articulate, has a certain degree of variabilitythat cannot be eliminated. This variability is oftenconstituted by the sum of more single effects derivingfrom causes that are almost always difficult to be con-trolled or impossible to be cancelled, even if they canoften appear as obvious.

Thus, it is understandable the high interest that al-ways has been shown by the foundries for any toolor technique contributing to reduce this variability, ordirectly, or making some information available andusable to correct in real time the productive process.

The introduction of spectrometersin foundry

The shop of foundry that maybe more than the othershas suffered big problems of process stability is themelting one. This is due to the big variety of usedmaterials that are coming from the external and tothe complex nature of the phenomena involved in theoperations of melting of the charge, treatment out of

aTechnical and sales engineer, e-mail: [email protected]

1 E-journal • Issue 01-2014

furnace and pouring in the moulds.The metallurgists rapidly discovered how the simpledefinition of a fixed charge recipe was not enoughto contain the variability in an adequate level, evenif were used only materials coming from the samesupplier.In the castings were often detected relevant variationsin the chemical composition, whose impacts in termsof variability in quality of the production will notbe here treated, since there is wide literature alreadyavailable on this subject. This problem appeared fora long time as not solvable, because the only avail-able analysis technique was the so-called “wet anal-ysis” determination, an analysis method requiringsome highly skilled personnel, as well as being quitecomplex, long and expensive. These characteristicsimpeached any possibility of its use as control tool,directly usable during the production.Starting from the ’80s, the first optical emission spec-trometers (OES, optical emission spectroscopy) dif-fused in foundry, often simply called “quantometers”.The introduction of this tool represented a real revo-lution concerning the process control: in fact, it waspossible to determine in real time the chemical com-position of the liquid metal, with accessible operatingcosts and without using highly skilled personnel.Even with some limits and defects, especially in thefirst implementations, this tool allowed an enormousreduction of the process variability, insomuch as itbecame its linchpin. To this day, a lot of foundriesentirely base their process control on this tool.

Second revolution in process con-trol: thermal analysis

Thermal analysis is a technique of analysis in realtime made through the direct measurement of thetemperature trend of a small sample of liquid metalduring its solidification (Fig. 1).The first studies concerning this technique of researchgo back to the end of the ’60s and the first measure-ment systems started to appear in the foundries start-ing from the second half of the ’70s. Regardless ofthe high number of information that can be found outfrom a single acquisition, the first measurement toolsaimed only at the determination of the contents ofCarbon, Silicon and at a direct estimation of CarbonEquivalent. Thus, this technique was seen as a toolalternative to the spectrometer, with the only target ofmonitoring the chemical composition of the metal.Before describing the further data that it is possible toobtain from a thermal analysis, as well as the contentof some elements, it is important to observe how thesetwo tools measure only apparently the same physicalquantities. In fact, the optical emission spectrometersbase their functioning on the analysis of the light radi-ation emitted exciting a small portion of the materialto be analyzed. The mainly used technique of exci-tation employs an electric discharge vaporizing themetal in the interested zone. Consequently, for eachchemical element that is present the total quantity willbe measured, independently from the state in which itis.

Figure 1: Example of solidification curve of a cast iron


Nevertheless, there are wide studies proving how ageneric element can be present into the metal in dif-ferent shapes (when it is in the liquid state or after itssolidification). In fact, it can be:

• dissolved in the liquid or in the matrix (as Sili-con, or part of Carbon, for example);

• combined with other elements like Oxygen, Sul-fur, Silicon, etc, in the form of chemical com-pound (oxide, disulfide, silicate, etc.);

• present as undissolved aggregate (particles ofgraphite, recarburizers, ferroalloys or inocu-lants that have not been solubilized and dis-solved).

On the opposite, thermal analysis does not searchthe specific presence of the different elements, but itdirectly analyzes the solidification process from thethermo-physical point of view. During the solidifica-tion of a cast iron, there are various reactions that start,develop and end, and each one of them generates theproduction or subtraction of heat, significantly influ-encing the dynamics of the process. Analyzing thetrend of the temperature over the time, it is possible toidentify these single reactions and the correspondingparameters (as time and temperature of starting andend, for example).Observing the influence of the different elements fromthe solidification point of view, the state under whichthey are present is fundamental. Considering for ex-ample the influence of Carbon, from the Fe-C diagramit is known that if its concentration is lower, higherwill be:

• the temperature where the metal starts solidify-ing;

• the quantity of primary austenite that forms dur-ing the solidification;

• the mechanical properties of the casting, forexample the ultimate strengthen (in particularfor lamellar irons);

• the shrinkage that the liquid iron will developduring the solidification.

Nevertheless, considering all the Carbon present inthe alloy, the quantity that effectively influences theprocess of solidification in the way described above,is the fraction that has been really solubilized in theiron, that is commonly called “free”, “not combined”or “active”. The thermal analysis systems estimatethe Carbon content basing on the variations that itgenerates in the critical phases of solidification. Asconsequence of what here described, they detect onlythe active fraction on the total that is present. Anyway,the eventual combined or not dissolved fraction caninfluence the solidification from a physical point ofview, even if not from a chemical one.At this point, it is understandable the reason why thesetwo tools often supply discordant results, that in somecases the metallurgists ascribe to failures of one toolor the other one: both supply a measurement of thequantity of a certain element, but in one case is thetotal quantity that present considering all the shapes,in the other one is only the active fraction.This difference is very important for the process con-trol. For example, supposing to produce a nodulareutectic cast iron, but at the end of the treatment theiron is hypo-eutectic (too low Equivalent Carbon)because of a lack of Carbon. The metallurgist willcalculate and add to the ladle the quantity of graphitethat is necessary to restore the correct value of Ceq.Nevertheless, it is not possible to determine a priorithe part of this addition that will be correctly solubi-lized, contributing to the increase of Ceq. Thus, itcould happen that the cast iron will not really reachthe eutectic condition, despite the Carbon content de-tected by the spectrometer (that will probably resultas coherent with the executed addition). On the otherhand, the thermal analysis will correctly detect thereal state of the cast iron, less hypo-eutectic as theprevious one, but not yet eutectic.Nevertheless, it is important to remark that thermalanalysis has not to be seen as a replacing tool of thespectrometer, but as a complementary tool: in fact,there are some cases where a chemical element couldbe present in too low content to significantly influencethe curve of solidification, but enough to change some


specific properties of the casting (for example, con-sider the impact of very low contents of Phosphoruson the resilience in the so-called “low temperaturecast irons”, that is not easily detectable from the ther-mal analysis curves).

Further evolution: beyond the sim-ple chemical approach

Up to this point, the process control has been analyzedin function of the chemical composition. Even if it isan important requirement, anyway it is not enough toguarantee the constancy in quality of production.Their formation can follow two ways: as spontaneousaggregation of groups of molecules (homogeneousnucleation) or as aggregation of molecules aroundan existing nucleus (heterogeneous nucleation). Thesecond way is that one definitely prevalent and thestarting centers of aggregation can be of many differ-ent types: impurities not dissolved coming from thematerials of charge, minuscule lining particles erodedby the siding of the furnace, compounds expresslyintroduced for this target (as preconditioners or inoc-ulants), or the “not active” fractions of the chemicalelements that are present in the cast iron as mentionedabove (the combined fractions and the not dissolvedones). The ability of the cast iron to nucleate grains,lamellas or nodules is called “nucleative potential”and by its nature, it is not directly quantifiable.The number of these germs and the speed of growingaround them for the different phases are two funda-mental parameters heavily influencing the continuingof solidification (with consequent influence on theformation of metallurgical defects) and the final char-acteristics of the castings (in terms of microstructureand of mechanical properties).The importance of the nucleative potential and itsalmost complete independence from chemical com-position can be easily observed in foundry. Probably,the most simple and at the same time effective exam-ple concerns the effects of in-stream inoculation (oralternatively, in mould). This operation consists in theaddition of a very low quantity of inoculant (usuallybetween 0.05% and 0.2%) during the last phase of theproduction process, that is the pouring of the cast ironin the mould.

On a chemical point of view, the inoculants and pre-conditioners are usually composed for 75% by Sili-con and, in function of the product, they can containother elements in contents that rarely arrive at 10%and are typically between 1% and 5%. An in-streaminoculation at 0.1% will generate a restrained varia-tion of chemical composition: an increase of 0.07%in Silicon and an almost negligible increase of theother elements (between 0.0001% and 0.0005%), of-ten lower than the spectrometers resolution. Never-theless, against these almost negligible variations inchemical composition, this operation generates somemacroscopic changes in the cast iron. It is a verycommon experience to detect hardnesses excessivelyhigh or problems of carbides formation in castingsthat have been poured without a correct in-stream orin-mould inoculation, because of problems in the pro-ductive process. Moreover, this operation can enor-mously modify the way of solidification of the metal,when it is executed on cast irons with low nucleativepotential: a strongly hypo-eutectic cast iron can ar-rive to have a fully eutectic solidification after a lowinoculation, with consequences that can impact evenon the feeding system of castings that was fitted inthe moulds. Similar effects can be verified in everypart of the productive process, even if not directlyobservable, if the nucleative potential of liquid metalis not correct.It is certainly realizable the difficulty in the estimationof this parameter in absolute value, that is an opera-tion almost impossible, because it is based only on thevariation in chemical composition. Nevertheless, it isanyway possible to monitor it, analyzing the results ofthe curve of solidification and, if needed, to interveneon the productive process to restore it at an adequatevalue.The development of computerized calculus systemshas allowed an enormous evolution of thermal anal-ysis systems: a modern software, as for example isITACA MeltDeck (to control the base metal) andITACA8 (to control the final cast iron), can au-tonomously execute the analysis of the solidificationcurve, the calculus of the main results and their inter-preting, in order to supply simple indications for theoperators.The last versions of ITACA have, in addition to themain thermal results, some indications about the risk


of formation of the main defects (from carbides toshrinkage defects), as well as information concerningthe grade of generated graphite expansion, the shapeof graphite or the estimation of the main mechanicalproperties, even keeping simple their use.Through the use of a last generation thermal analysissystem, now the foundries can implement an effectivecontrol system of variability: through the combinationof thermal and spectrometer analysis, it is possible to

Figure 2: ITACA MeltDeck thermal analysis unit

stabilize the quality of the metal in all the phases ofthe process, from the preparation of the base iron tothe final pouring in the moulds, passing through theeventual monitoring of the holding furnaces and thecontrol before and after the treatment in ladle.The metallurgists can identify any deviation as regardsthe various quality targets in real time, and make thenecessary corrections in the needed process phase, al-lowing as well the optimization of the used additivesand ferroalloys.

Figure 3: ITACA8 thermal analysis software


The experience of foundries

As you can easily guess, the main concerns of thefoundries when they are going to plan an investmentfocus primarily in a few simple aspects. What arethe benefits that can be achieved by this investment?Benefits are that my organization (as a set of peopleand skills) can get to achieve? In how much time?We decided to collect the testimonies of fourfoundries, our customers, which differ in their typeof production and turnover, by sending them a briefquestionnaire right about these aspects.Here are the results of the questionnaire, introducedby a brief description of the customers who kindlygave their testimony.

VonRoll casting (emmenbrucke) sa

VonRoll casting is a swiss foundry group produc-ing gray and ductile iron: the materials range fromthe standard grades to special alloys such as SiMo,Ni-Resist and austempered ductile iron (ADI). Von-Roll casting is producing castings for a wide rangeof market segments: the biggest segment representsthe engines with exhaust manifolds, cylinder heads,turbochargers, etc., followed by vehicles (railway andtrucks) with all kinds of safety parts, then the ma-chine, textile and energy industry. The foundry of Em-menbrucke, produces castings from 2 to 200 kg witha moulding line from HWS. The melting shop hasfour induction melting furnaces (medium frequency)and one holding furnace.

�Are you using ITACA system as aprocess control tool?�

�Yes we are using the system as a tool forprocess control in addition to the verifica-tion of samples for new alloys and testsfor the improvement of the inoculationprocess.�

�Have you obtained any benefit fromthe use of ITACA? What are these

benefits? How much time has ittaken?�

�Yes we did. We achieved a more stableprocess since the introduction of ITACA.There is still some work to do but weare on the right path. We could alsoreduce the amount of inoculant in theladle by optimizing the inoculation pro-cess to save cost. We have realized thatwe are sometimes hypereutectic for cer-tain alloys. With the help of the thermalanalysis we could reduce the carbon andsilicon content to get eutectic. Further-more we could reduce problems causedby porosity.�

�What is the level of complexity of thesystem according to the workers?�

�Most of our workers in the melting shopunderstand how to influence the differ-ent temperatures and points in the iron-carbon phase diagram. We are still train-ing the workers at the moulding line togive them the know-how to understandand use the system.�

Fonderie Glisenti

Foundry specializes in the production of castings ofhigh and medium series, small and medium size (upto 70 kg) in ductile iron. The melting shop consists of5 induction melting furnaces. The production is donein a horizontal moulding line for the industry of trac-tors, earthmoving machines and industrial vehicles. Avertical molding line allows to produce medium andhigh series for the automotive industry.


�Yes, we will put it in the Control Plansoon.�


�Have you obtained any benefit fromthe use of ITACA? What are thesebenefits?�

�Yes we did. Yes we did. Constance ofliquid type at the pouring and customiza-tion of thermal analysis in function of thecasting, in relation to the defects. Weuse ITACA MeltDeck as a system to sta-bilize, quickly and easily, the final ironand keep it in acceptable ranges definedfor each casting or castings families. Onsome castings, this heavy control of thebase and final iron has allowed us to re-duce the porosity, returning within theparameters defined by the client.�

�How much time has it taken?�

�In general terms, we can say that start-ing from the installation of ITACA Melt-Deck ,which took place two years ago,we got benefits. In the last year we haveinserted the operation into the procedureand we have seen benefits also in termsof documents.�


�After quickly gained acquaintance, thesystem now appears easy to use both inthe execution of trials that in reading theresults.�

INFUN FOR

Infun For is a leading company in Europe for the man-ufacturing of mechanical parts and high quality andsafety components for the auotmotive industry. Thetypical production mainly consists of engine parts,braking systems, transmission and suspension sys-tems and includes both gray irons and nodular ironscastings.


�Yes, the systematic control of all melt-ings in the furnaces and of all the ladleson pouring has been included in the pro-cess control for ductile iron, while we arestudying how to apply it for the gray castiron.�

�Have you obtained any benefit fromthe use of ITACA? What are thesebenefits? How much time has ittaken?�

�Yes. In few months the systematic con-trol with thermal analysis has allowedus to control and/or eliminate some vari-ables, allowing us to restrict the range ofHipoEuteticHiper, with important bene-fits on defects. At this time we are focus-ing on improving the yield metal/stirrup,made possible with the stabilization ofthe final iron.�


�The curves and some parameters con-trolled by the operators in the pouringline are very well defined and clear, al-lowing the interpretation of the analysisalready at a quick glance. Other param-eters, if necessary, always well-defined,are studied more carefully by other oper-ators better prepared.�

Atlantis Foundries

Atlantis Foundries (Pty) Ltd produces automotivecastings for both the passenger and commercial ve-hicle industries. In addition, the company machinescylinder blocks and crankshafts for automotive appli-cations. The company is a wholly owned subsidiaryof Mercedes-Benz South Africa, belonging to the


Daimler AG group, and was established on 12 March1979.The Atlantis Foundries plant is situated in At-lantis, South Africa, approximately 50 km north ofCape Town.


�Yes.�

�Have you obtained any benefit fromthe use of ITACA? What are thesebenefits? How much time has it

taken?�

�Yes, we have obtained reduced metal-lurgical defects at our customer (about90% reduction), during the time of threemonths.�


�They found it easy to use as comparedto the classical wedge test.�


new approach to process control in cast iron …...new approach to process control in cast iron...

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