VES SSN (18 Volv

- - ! - I I - .

FOUNDRY GfNEERING -1 y;* 'E

Published quarterb as the organ of ihe r u u r ~ v r y kurnmisSm of !he Palish Academy of Scbncss

Fading of inoculation effects in ductile iron E. FraSB'*, M. G6rnyn

" AGH - University of Science and TechnoIogy, Reyrnonta 23,30-059 Cracow, PaIand 'contact e-mail: edfras@agh.cdu.pl

Otrzymano 2 1.03.2008; zaakceptowano do druku 03.04.2008

Abstract In work i t has bccn shown rcsults or invcsligations of influcncc of rime Iapsed form inoculation proccss on graphitc nucleation potential rcprcscntcd by: numbcr o f graphitc nodulcs N and N,, maximum undercooling AT,, during solidification o f gmphile eutcct ic. abmlutc chilling tcndcncy CT and critical casting diametct dh. undcr which cementite euteclic occur (so-callcd chills). Morcovcr i t has hccn cstima~cd raic o f changc of N and N, AT,,,. CT and dk,. Also, i t has bccn provcd that altcr onc minutc sincc rhc momcnt of inocuIation proccss nhout 35% of prnphttc nucIeation potenrial is tost. by 40% chitking tendency, by 70% incrcascs maximum undcrcmling for graphitc ci~tccric and by nearly 40% caging diameter has to bc incrcascd in ordcr to avoid chills.

Key words: Ductilc imn; Inoculation; Graphite n d u t c count: Chilling tcndcncy.

1. Introduction

Duc~ile iron is a modcm construction maleriat and offer widc rangc of mechanical pmpcrtics, simul~ancously exhibits good wcar rcsisrance and dumping capacity. Onc of thc production s tay of ductilc iron is its inocittntion, which consist in change in physicochcmical state of liquid imn. Primary and original cffccr o f inociit;rlinn proccss is radical increase in nuclcarion porential of gmphitc and thcrcforc also graphi~e nodulc count. As a conscqucncc o f highcr graphitc nodulc count is dccrcasc in dcgrcc of undcrcwling during solidification of graphirc cubx~ic [I]. scprcg;ttion o f iilloying clcmcnts [2] and chilling rcndcncy as wcll as fmction of ccmcnritc cutcctic i n casting 111 and also incrcasc in ferrite Fraction in casting [3] and preshinkagc expansion of cast imn [4] (so callcd secondary cffccts). inoculation cffccts arc timc dcpcndcnt Iapscd rrom thc momcnt o f inoculan~ dissotut ion in liquid ha! h.

Thc aim nf this work i s rhc diagnosis o f connections bctwccn timc lapscd from rhc insrant of inoculation and selectcd its effects in ductile imn.

Ductilc iron was obtrincd in an clcctric induction ittrnacc of medium frequency and with 15 kg capacity. The raw matcrinl were pig iron. stccl scrap and rcgulas fcm-silicon Fc-Si75%. After prchcating to 1480 "C sphcroidization and inoculation pmesscs wcrc rnadc in crucihlc o f furnace using Fc-Si-9Mg master alloy and inoculant Foundrysil (Elkcm) in amounts nf t ,O and 0.8 % with rctalion to bath wcight. rcspcctivcly.

Aftcr dissolarion o f inoculnnt in timc ~n~crva l of 0,4: 1.1 ; 2.3; 3.1 : 6.0; 7.3 and 10,O minu~cs liqitid imn was poured into sand moulds in order to obtain mds with dismctcr OF 15 rnm and length o f 100 mm. Chemical composition clctcrrnincd hy "wet analysis" was as follows: C = 3.7 %, Si = 2.7%); Mn = 0.03 'Fh; P = 0,028; S = 0.01 %. Fmm castines samplcs wcrc rakcn Tor rnctnltagraphic cxarninations in order ro dctcrrninc graphitc nodulc counl N pcr 1 mm' using quantitative irnagc anal yzcr Lcica QWin, Ahcrwards. on thc basis on Wiencck /5] equation graphitc nodule counl pcr 1 mm' wcrc calculntcd.

whcrc: f, - graphitc rraction in duailc imn.

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3. Results of investigations and their T ~ C innucncc of tim kapsal from inoculation ~ ~ C C S S on graphitc ndu lc counls N and N, can dc drown by Ihe following

analysis rcgrcssion equations:

Results of influcncc of zimc lapscd from inoculation process on graphitc nodulc counts N and N, arc summarized in Tablc S and are graphically shown in Fig, la

for f,= 0.12

=2,9 l 0 ' (~ -17 '4 ] '~ t1/3

Tahlc I . Influcncc o f t imc Iapscd from inoculation process on pwphitc nalulc counts in ductilc iron

Morcovcr. on thc basis on cqua~ion (2) rate of changc of graphitc nodulc count N with reIation lo time can be calculated as

Time lapsed from inoculalion 0.4 process t. min Graphite nodule count. NF, rnm*' 478

Granhitc nodule count. N,. cm 3.1 1 o7

Results of these calculations according to equation (4) is show in Fig. I b, (ram which i t can be concluded that the highcst change of graphite nodulc count N is obscrvcd in a first minuzc aftcr inaculat ion pmccss.

In foundry practicc chilling tendency of different types of cast irons arc charactctized on thc basis on comparison of cemcn~i~c cutec~ic traaion in standard castings (more often in wedge shape castings). The hieher ccmcntirc fraction the higher chilling tcndcncy of c ~ q t iron. From such comparison difference in chilling Icndency of various ~ypcs of cast iron can bc estimated but absolutc cbilSing tcndcncy CT values lor given imns cannot bc derived, It is well known that chilEing tendcncy dctcminc its utilization in divcrsc practical applications. In particular cast iron with high chilling lendcncy has susceptibility to form extremely hard and thus difficult to rnachinc zoncs of white or rnottlcd iron in castings. Convcrsety when the aim is to obtain whirc or monlcd iron a rclativety small chilling tendcncy will favour thc formation of gray Iron. This in turn lcads lo low hnrdncss and poor wcar properties in the as-cast cornponcnts.

Absolute chilling tendcncy of cast iron is cbaractcrizcd by the Following cquation 113:

Whcre: D - diffusion coeficient of carbon in auslcnitc, ATw - tcmpcrature range ot the graphite euzcctic sotidification, AT* = T, - T,; (T, - Graphitc eutectic equilibrium tcrnpcraturc. T, - solidilicarion tcrnperatuse for ~ h c cementite eutectic), P -

1.1

354 1.9 lo7

solubility coefficient of carhon in mnstirucnts in ductilc iron structure.

Table 2. Data Tor calculalions

2-3

247 1.1 10'

After combining equations (3) and (5) thc function CT = F(t) can bc obtaincd that is absotutc chilling tendcncy as a runclion or timc lapscd from inoculation proccss.

t CT = 0.07 (6)

( 5 1 2 - 1 7 4 t " ~ ) ~ ~ ~ p ~ ~ ~ $

3.1 ------

182 7.1 10'

Pouring ternpcrature

84 A R C H I V E S of FOUNDRY ENGINEERING Volume 8. Spec ia l I s s u e 112008, 83-88

T,, = 1380 : "C

6.0

102 2.5 10"

7.3

69 1.6 10"

54 1.1 10''

Super4tmcuhted stata

Super-bdated state

2 4 G 10 I t

t. min t d n

Super-inoculated state

0 1 3 4 5

t, min t, mln

t, min

Fig. I .Influence of time lapsed from the moment of inoculation pmccss on: a) graphite nodule count, b) rate o f change o f graphite nodule count, c) absolute chilling tendency, d) rate of change of absolute chilling tendcncy, c) maximum degree of undercoaling of graphite eutectic. f) rate of change of maximum degree of undercoofing of graphite eutectic. g) critical casting diameter bclow which there are

chilIs. h) rate of change o f casting diameter below which therc arc chiEls

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Integration of cquation (G) with respect to time gives rate of wherc: changc or absoIutc chilling tcndcncy of ductile iron. T, I4 = In=

Functions (6) and (7) artcs taking into account Ihcrmophisical data given in Tablc 2 arc show in Figs. Ec,d. Thc IblIowing conclusion can be drown. namely change of absolutc chitling tcndcncy of ductilc iron is thc highcst up to onc minute from the momcnt of thc inoculation process.

From theoretical analysis of the solidification process of ductile iron result [ I ] the following relationship bctwcen maximum degrcc of undcrcooling of graphite euteczic and graphitc nodutc count Nv.

Fig.

t, min

c - specific hmt of cast iron, T,, - pouring temperature, a- rnatcrial mould ability to absorb thc hcat, L, - latcnt hcat of graphite cutcctic. M -casting modulus, d -casting diameter.

After combining equations (3) and (8) for a given casting diameter a relationship cm be found between maximum degrcc OF undercooling and time lapsed from inoculation proccss of ductilc iron !hat is the function in rhc form of AT, = f(t).

! C) ;

0 2 4 6 8 10 2 4 8 10

f min t, min 2. Influence of iimc lapscd from inoculation process on change (expressed in percent): a) graphitc nodule count, b) chilling tendency.

C) maximum dcgrce of undercooling, d) critical casting diameter below which here chills can bc prcscnt

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Plot of this function for a rod with diameter equal 15 rnm and for data taken from Tablc 2 is show in Fig. Ic.

Ratc of changc of maximum dcgrec of undercooIing is obtaincd aftcr diffcrcntiatinp of rhc cquaiion (12) in rclalion to timc

Plot of this equation (13) after taking into consideration data from Table 2 are show in Fig. 1 f.

From investigations I ] result that critical casting diarnctcs dk, b l o w which them are chiIl present in casting can bc described by thc equation

After substituting cquation (6) into equation (14) and after different iaring in rcspcct 10 timc ralc of changc of crirical casting diameter is nchievcd

Equations (15). and ( 1 6) for data taken from Table 2 arc show in Figs. 1g.h.

4. Summary

Basic effect of ~ h c inaculation process is incrcac in graphile nuclcatian potential. I t is well known rhnt from each gmphirc nuclci in ductilc iron ariscs onc singlc graphite nodulc and thus the measure of thc nuclcation potcntial can hc graphitc nodulc count. From cxpcrimcnt rcsult khat thc highest gnphi~c nodulc count (the highest nucleation ptcntial) is achicvcd whilc i n ~ u l a n t is comptetely dissolved in a bath. At that rnomcnt bath is in super-inoculated statc.

As the time go sincc the momcnt of inoculant dissolution graphite nucleation ptential is lowring. Ratc of chang of this potcntial is the highest in the first minutc Iapscd from inocutation process, Accordingly also secondary inoculation cffccts arc changing that ase absolute and reldivc chilling tcndency. dcgm of undcrcmling of graphitc cutectic and critical casting diarnctcr below which chills can occur. From calcularions which m shown in Pig. 2 result that aftcr onc minute sincc thc moment of inoculation process 35% of graphite nuclea~lon ptenziaI is lost (Fig. 25). by 40 % increases chilling tendency (Fig. 2b), by 70% incrcascs maximum undcrcooling for graphite eutccric (Fig. 2c) and by nearly 409 casting diarnctcr must bc increased in order to avoid chills (Fig. 2d).

This work was donc within starutory rcscarch at AGSI No. 11.11.170.318.

Literature

[ I ] FwS E., Gdrny M., Lopez H.F.: Eutcc~ic Ccll and Ndulc Count in Cast Iron, Intcrnational Stccl and Imn Japan Institute, ISU ., vol. 47 (2007) No. 2,269.

121 LesouEt G, Castro M., h a z e J.: Solidification of SphcroidaE Gnphitc Cast Iron: 111 Microsegrcgation related effccrs. Actn Materialia. 1999, vol. 47,3779.

[3] Vcnugopalan D.: A Kinetic Model of thc y + a + Gr Eutccloid Transformation in Spheroidal Graphitc Casr Iron. Metallurgical and Materials Transactions. 1990. vol. 2 1 A. 913.

I41 F r d E., Lopez E I.: Generntion of International Pressure During Solidification of Eulcclic Cast Iron. AFS Transactions. 2994. vol. 102, 597.

(51 Wiencck K., RyS.: Thc cs~ima~ion of Fc3C particle densi~y in steel by simple counting mcasurcmcnts madc in plan sccrions. Materials and Engineering, 199R. vnl. 3,396.

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