U. Klančnik1, J. Habjan1, M. Drobne1, P. Mrvar2, J. Medved2 1 Valji, d.o.o., Železarska cesta 3, Štore, Slovenija / Slovenia
2Univerza v Ljubljani, Naravoslovnotehniška fakulteta, Aškerčeva 12, Ljubljana, Slovenija / Slovenia
Karakterizacija modificirane indefinitne litine za izdelavo delovne plasti valjev
Characterization of a modified indefinite chill cast iron working layer in rolls
Povzetek
Pri proizvodnji valjev je vedno prisotna potreba po razvoju obrabno odpornejših zlitin ter optimizaciji že obstoječih. Med procesom izdelave litih valjev le-ti niso podvrženi procesom termo-mehanske obdelave, zato je ugodna lita struktura ključnega pomena za doseganje želenih končnih lastnosti delovnih plasti valjev. Dodajanje heterogenih nukleacijskih kali v talino oz. modificiranje je pogosta praksa za doseganje boljših mehanskih lastnosti zlitin. Drobnozrnate visoko taljive ferozlitine ustrezajo pogoju za uspešno modifikacijo taline ter posledično udrobnitev matrice.
V raziskovalnem delu je predstavljen učinek modificiranja taline s ferovanadijem in ferovolframom na mikrostrukturo indefinitno strjene litine, ki je razširjena pri izdelavi delovnih plasti valjev za končna ogrodja. Modifikator je bil dodan v obliki prahu v curek med izpustom iz peči. Modificirana talina je bila nato centrifugalno ulita v predgreto cilindrično jekleno kokilo. S plašča valja so bili odvzeti vzorci za metalografsko preiskavo in karakterizacijo faz. Opravljena je bila preiskava z optičnim mikroskopom z oceno deleža posameznih faz ter z vrstičnim elektronskim mikroskopom opremljenim z EDS analizatorjem.
Izmerjena je bila trdota delovne plasti, ki je pokazala do 5% višje trdote od primerljivih kvalitet brez modifikacije. S predstavljeno modifikacijo je bila dosežena raven trdot primerljiva z višje legiranimi indefinitnimi litinami, hkrati pa se delež cementita ni povečal. EDS analiza je pokazala prisotnost precipitatov na osnovi vanadija ter volframa, ki v primerljivih kvalitetah brez modifikacije niso bili dokazani.
Ključne besede: indefinitna litina, modifikacija, ferozlitine, abrazij-ska obrabna odpornost
Summary
In roll production there is an ever present need for new alloy development and optimization of alloys already in use. The goal is to improve wear resistance while maintaining the level of ductility and other mechanical properties. During the production of cast rolls, the rolls are not subjected to thermo-mechanical processing. Thus an optimal as-cast microstructure is crucial. Addition of heterogeneous nucleation sites into the melt (modification) is a widely used, cost efficient method of improving as-cast mechanical properties of alloys. Powder ferroalloys with a high melting point are an excellent candidate for modification additions. The effect of melt modification using ferrovanadium and ferrotungsten on as-cast microstructure of indefinite cast iron has been studied. Modifying agents were added in powder form before casting. The modified melt was centrifugally cast in a preheated cylindrical steel mould. Samples were taken directly from the working layer of the roll for
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metallographic investigation and phase characterization using optical and scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The hardness of the modified working layer showed up to 5% higher values compared to non-modified alloys of similar grade. In fact, the hardness values of the modified indefinite alloy reached those of its more high-alloyed counterparts, while maintaining a lower percentage of cementite. EDS analysis showed the presence of V and W-based precipitates, which have not been confirmed in the non-modified alloys.
Key words: indefinite cast iron, melt modification, ferroalloys, abrasive wear resistance
1 Uvod
1 Introduction
Modifikacija taline je razširjena in splošno uporabljana metoda v livarski industriji, namenjena izboljšanju mehanskih lastnosti. Po nukleacijski teoriji, kot sta jo predstavila Volmer in Webber [1], je prosta Gibbsova energija potrebna za homogeno nukleacijo definirana kot:
AG
hom
_ 16WsL
3A G,
2 »
(1)
kjer je ysL površinska energija na stiku trdno-tekoče in AGv gonilna sila strjevanja. V primeru, ko so talini dodana heterogena nukleacijska mesta (HNM), se enačba 1 spremeni:
mu =
(2)
Melt modification is a standard procedure used in foundries for material improvement. Based on the nucleation theory proposed by Volmer and Webber [1], the free energy barrier, AGhom*, for homogeneous nucleation is given as:
AG
hom

3A G,
2 »
(1)
where ysL is the surface energy of solidliquid interface and AGv is the driving force for solidification. Where heterogeneous nucleation sites (HNS) are implemented, the free energy barrier, AGhet*, takes the form of:

(2)
kjer f(&) predstavlja funkcijo kota omočenja Q med kaljo in preostalo talino. Vrednost funkcije f(Q) je vedno < 1, torej se Gibbsova prosta energija zmanjša v primerjavi s homogeno nukleacijo (En. 1) [1], kar pomeni, da so HNM energetsko ugodnejša za stabilno nukleacijo.
Cilj modifikacije taline z visoko taljivimi ferozlitinami, ki se običajno uporabljajo pri legiranju, je izboljšati abrazijsko obrabno odpornost ter druge mehanske lastnosti z vnosom HNM. Inkubacijski čas (t.j. čas, ki je potreben za raztapljanje) visoko taljivih ferozlitin je daljši kot pri ferozlitinah z
where f(Q) is a function of the contact angle Q between the embrio and the surrounding liquid. As the function f(Q) in Eq. 2 is always < 1, the energy barrier for stable atom deposition lowers compared to homogeneous nucleation (Eq. 1) [1].
This makes HNS energetically favourable for the start of solidification. As HNS implemented into the metal melt are often equally dispersed throughout the volume of the melt, the effect of the modification is grain refinement and improvement of abrasive wear resistance, ductility and other mechanical properties.
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nižjim tališčem (npr. ferosilicij, feromangan ipd). Kumar in Chandra [2] sta predstavila model raztapljanja visoko taljivih ferozlitin v talini jekla ali železove litine. Ob vnosu modifikatorja v talino se okoli delca ustvari trden plašč. Plašč se kmalu spet raztali, kar izpostavi jedro modifikatorja talini. Kinetika raztapljanja visoko taljivega modifikatorja temelji na prenosu mase (difuzija) in ne na prenosu toplote (raztapljanje). Ker je prenos mase za velikostni razred počasnejši od prenosa toplote, se inkubacijski čas temu ustrezno podaljša v primerjavi z nizko taljivimi modifikatorji.
V primeru, ko je čas med modifikacijo in ulivanjem krajši od inkubacijskega časa, se v talino vnesejo HNM v kolikor imajo leta ustrezne fizikalne lastnosti (npr. podobne mrežne parametre, majhen kota omočenja 0, itd.). V kolikor temu pogoju ni zadoščeno, delci ne delujejo kot HNM in ostanejo ujeti v materialu. Wragg et al. [3] je v svojem delu talini dodajal visoko taljive (Ti,W)C delce, ki so ostali enakomerno razporejeni tako znotraj matrice, kot v karbidih. Natezna trdnost Rm indefinitne litine z dodatkom omenjenih karbidov je dosegla do 50% višje vrednosti od nemodificiranih litin [3].
Po drugi strani, če je čas med modifikacijo in ulivanjem le nekoliko daljši od inkubacijskega časa modifikatorja, v talini ostanejo mesta z lokalno nehomogeno kemijsko sestavo. Na teh mestih je omogočen nastanek drobnih karbidnih faz, v kolikor je delež karbidotvornih elementov (V ali W) dovolj visok. Tako nastale karbidne faze so predvidoma drobnejše in enakomerno razporejene, kar bi izboljšalo trdoto in obrabno odpornost materiala.
2 Eksperimentalna metodologija
Talini sta bila kot modifikatorja dodana ferovanadij (FeV) in ferovolfram (FeW).
By modifying the melt with high-melting ferroalloys that are mainly used during the process of alloying, the goal was to improve abrasive wear resistance of the metal through the addition of HNS. The dissolution time of high-melting ferroalloys is longer than with low-melting ferroalloys such as ferrosilicon, ferromanganese etc. The model proposed by Kumar and Chandra [2] acknowledges a different path in the dissolution of high-melting ferroalloys in a cast iron or steel bath. Immediately after immersion a metal shell forms around the modifier. After the shell has melted away the modifier interior is exposed to the melt. The solution kinetics of a high-melting modifier is governed by mass-transfer through the liquid-solid boundary layer rather than by heat-transfer kinetics. As mass-transfer kinetics is one order of magnitude slower than heat-transfer kinetics, dissolution time is sufficiently prolonged in comparison to low-melting modifiers.
In the case when modification to casting time is shorter than the dissolution time of the modifier, dispersed HNS are left within the melt if the modifying agent chosen has the appropriate physical properties (e.g. similar lattice parameters, small value of 0, etc.) [1]. However, if the modifying agent does not act as a HNS, undissolved modifier particles could remain trapped in the solidified metal matrix. The work of Wragg et al. [3] suggests that the addition of high-melting (Ti,W)C particles indeed resulted in said particles being uniformly distributed within the matrix as well as in the transformation carbides. The ultimate tensile strength Rm of an indefinite chill cast iron with the addition of said carbide particles had increased up to approx. 50% [3].
On the other hand, when modification to casting time is just slightly longer than the modifier dissolution time, localized sites of
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Delež visoko taljivega elementa v ferozlitini je znašal 76,5 mas.% za V in 76,9 mas.% za W. Dodana sta bila v obliki prahu z granulacijo od 1 do 5 mm.
Iz jeklenih odpadkov, krožnega materiala, grodlja, ferozlitin in elektrolitsko čistega Ni je bila pripravljena indefinitna litina. Modifikator je bil dodan v curek med izpustom iz peči v vmesno ponovco, da je bila dosežena optimalna razporejenost delcev znotraj taline. Modificirana talina je bila horizontalno centrifugalno ulita v jekleno kokilo. Izvedena je bila preliminarna preprosta termična analiza tako pred kot po modifikaciji. Ko je bil plašč strjen, se je centrifugalni stroj zaustavil, kokilo pa se je dvignilo v pokončen položaj. V votli plašč je bilo gravitacijsko ulito jedro iz sive litine s kroglastim grafitom, ki se je spojilo s plaščem v kompozitni valj.
Z zunanje delovne plasti so bili odvzeti vzorci za metalografsko analizo. Vzorci so bili pripravljeni po standardnem metalografskem postopku brušenja in poliranja ter jedkani v 2% Nitalu za identifikacijo mikrostrukture. Metalografska analiza je bila opravljena z optičnim mikroskopom Olympus BX51M s kamero Olympus DP-12. Ocena mikrostrukture je bila opravljena s programom JMicroVision v.1.2.7. Vzorci so bili analizirani z vrstičnim elektronskim mikroskopom (SEM) JEOL JSM-6390LV, mikrokemijska sestava konstituentov pa je bila določena z INCA Oxford 7582 energijsko disperzivnim spektrometrom (EDS). Trdota vzorcev je bila izmerjena na Emco Rockwell merilniku trdote (EN ISO 6508 [4], ASTM E-18 [5]).
Abrazijska obrabna odpornost je bila testirana s po meri narejenim tribometrom po metodi »valjček-plošča«. Naprava je bila načrtovana in izdelana po standardu ASTM G99-04 [6]. Po standardu so bili prav tako pripravljeni vzorci ter sama izvedba meritve. Površino plošče je predstavljal 120
inhomogeneities in chemical composition are ensured throughout the melt. This would enable the formation of small carbide phases where the composition of the carbide-forming elements, such as V or W, is sufficient. The carbide phases formed in such a manner would likely be small and evenly dispersed, thus enhancing the hardness and wear resistance of the material.
2 Experimental Methodology
Ferrovanadium (FeV) and ferrotungsten (FeW) powder alloys were used as modifying agents with the composition of the higher-melting element of 76.5% and 76.9 % (mass fraction), respectively. The powders were ground to a size distribution between 1 and 5 mm in diameter.
An indefinite chill cast iron alloy was prepared in an induction furnace from steel and cast iron scrap, pig iron, ferroalloys and electrolytically pure Ni. The modifying agents were added to the melt during the discharge of the melt into the intermediate ladle so that maximal dispersion of the agents throughout the volume of the melt was achieved. The modified melt was centrifugally cast in a horizontal steel mould to form a hollow cylinder. A preliminary simple thermal analysis of the material was conducted before and after modification (results not shown). After the working layer has completely solidified, the mould on the centrifugal casting machine was stopped and erected into a vertical position then a ductile cast iron core was statically cast into the working layer cylinder to form a composite roll.
Samples were taken from the working layer for metallographic analysis. The samples were prepared with a standard metallographic procedure of grinding and
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^m brusni papir iz SiC. Hitrost preizkusa je znašala 0,25 m/s z 1000 m drsne poti in 10 N obremenitve. Test je bil opravljen na zraku (T = 21 °C; ~ 55 % relativna vlažnost zraka) brez uporabe lubrikanta (hlajenje s curkom zraka). Testirani vzorci so bili predhodno toplotno obdelani.
Delovni plašč modificiran s FeV in FeW (oznaka M-CIN) je bil primerjan s tremi primerljivimi indefinitnimi litinami, ki se uporabljajo za izdelavo delovnih plasti valjev; prva nemodificirana (oznaka CIN), druga nizko legirana z V do 0,5 mas.% (oznaka CIND) in tretja visoko legirana z V do 1,5 mas.% (oznaka CINA). Kemijske sestave zlitin so podane v Tabeli 1. Način priprave in testiranja je bil enak za vse štiri kvalitete in je opisan zgoraj.
3 Rezultati in diskusija
Kemijska sestava analiziranih indefinitnih litin je predstavljena v Tabeli 1.
Slika 1 prikazuje primerjavo mikrostruktur zlitin M-CIN in CIN.
Mikrostrukturi obeh vzorcev sestavljajo: primarni transformirani avstenitni dendriti, prosti grafit in cementit, tako sekundarni kot v obliki ledeburita. V primeru M-CIN je na pogled opazno večje število manjših evtektskih celic v meddendritnem prostoru. Usmerjenost ledeburita je v primerjanih zlitinah različna, kar je povezano z udrobnitvijo strukture in kinetiko evtektskega strjevanja. Po ugotovitvah v literaturi, je
polishing and etched in 2% Nital to reveal the microstructure. Optical microscopy was conducted using an Olympus BX51M optical microscope equipped with an Olympus DP-12 camera. The microstructure was evaluated using JMicroVision v.1.2.7 software. Micro chemical analysis of the phase constituents was done with a JEOL JSM-6390LV scanning electron microscope (SEM) equipped with an INCA Oxford 7582 energy dispersive spectrometer (EDS). The hardness of the samples was measured using an Emco test Rockwell hardness tester (EN ISO 6508 [4], ASTM E-18 [5]).
The resistance to abrasive wear was tested using a custom made pin-on-disk tribometer. The design of the machine as well as sample preparation and the measurement itself was done according to ASTM G99-04 standard [6]. The disk was comprised of a 120 ^m SiC grinding paper. The testing speed was 0,25 m/s with a 1000 m sliding distance and a testing load of 10 N. The test was conducted under normal atmosphere (T = 21 °C; ~55 % relative air humidity) without the use of a lubricant (cooled with pressurized air). The test was conducted on heat treated specimens. The working layer modified with FeV and FeW (labelled M-CIN) was compared to three similar indefinite chill cast irons already used for roll working layers; one without modification (labelled CIN), the second micro-alloyed with V in the amount of 0,5% mass fraction (labelled CIND) and
Tabela 1. Nominalna kemijska sestava analiziranih indefinitnih litin (v mas.%)
Table 1. Nominal chemical composition of the analysed indefinite chill alloys (mass fraction in %).
	C	Si	Mn	Cr	Ni	Mo	V	W	Fe
CIN	3,1-3,2	0,8-0,9	0,6-0,8	1,6-1,7	4,2-4,3	0,3	0,03	0,01	Bal.
M-CIN	3,1-3,2	0,8-0,9	0,6-0,8	1,6-1,7	4,2-4,3	0,3	0,17	0,08	Bal.
CIND	3,1-3,2	0,8-0,9	0,6-0,8	1,6-1,7	4,2-4,3	0,3	0,20-0,50	0,01	Bal.
CINA	3,1-3,2	1,1-1,3	0,9-1,0	1,6-1,7	4,2-4,3	0,3	1,50-1,60	0,01	Bal.
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Slika 1. Mikrostruktura zlitin a) M-CIN in b) CIN v litem stanju
Figure 1. As-cast metallographic image of: a) M-CIN alloy and b) CIN alloy
morfologija evtektika med postopkom strjevanja ali ploščate oblike, ali v obliki zvezne cementitne faze s posameznimi otočki avstenita, karjeneposredno povezano s stabilnostjo fronte strjevanja [7]. Sklepamo lahko, da je dodatek HNM povzročil začetek hkratnega strjevanja večjega števila avstenitnih dendritov iz taline. Večja gostota avstenitnih dendritov je posledično zmanjšala razpoložljiv meddendritni prostor, ki je na voljo za strjevanje ledeburita. Na
0 20 40 60 80 100 110 140 Premer primarnih dendrilrttli vej / diameter of primary dendrite arms (^rnj
Slika 2. Normalna porazdelitev premerov primarnih dendritnih vej v zlitinah M-CIN in CIN
Figure 2. Normal distribution of primary dendrite arm diameter measurements done on a sample of M-CIN and CIN
the third high-alloyed with V up to 1,5% (labelled CINA). Chemical compositions of the analysed alloys are given in Table 1. The method of production and testing method was the same as described for all presented alloys.
3 Results and Discussion
Chemical compositions of the analysed alloys are presented in Table 1.
Figure 1 represents comparative metallographic images of M-CIN and CIN alloys.
Both of the comparative as-cast micrographs (Figure 1) show a similar microstructure composed of primary transformed austenite dendrites, free graphite flakes and cementite, both secondary and in the form of ledeburite. In the case of M-CIN however, one can notice a larger number of smaller eutectic cells extending into the interdendritic area. The orientation of eutectic cells between the compared qualities is different. This is related to structure refinement and kinetics of eutectic solidification. According to
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ta način je bila spremenjena usmeritev strjevanja ledeburita.
Meritve, ki so bile opravljene na transformiranih avstenitnih dendritih, dodatno potrjujejo tezo o udrobnitvi strukture. Na Sliki 2 so predstavljeni rezultati meritev premerov vej primarnih dendritov. Rezultati so predstavljeni v obliki normalne (Gaussove) porazdelitve z upoštevano statistično napako.
Porazdelitvena diagrama na Sliki 2 kažeta, da sta tako povprečen premer dendritnih vej kot raztros okoli povprečne vrednosti (statistična standardna deviacija) v primeru M-CIN manjša kot pri nemodificirani CIN. To nakazuje na bolj enakomerno mikrostrukturo, ki je posledica udrobnitve strukture.
Rezultati SEM analize modificirane M-CIN zlitine so prikazani na Sliki 3.
EDS analiza konstituenta na Sliki 3a potrjuje povišan delež elementov V in Nb. Prav tako je analiza potrdila visok delež Mo in W v svetlem konstituentu na Sliki 3b. Obe karbidni fazi sta se pojavili znotraj matrice. Ker tako Nb kot Mo v osnovni
observations, during eutectic solidification the morphology changes either into plate eutectic or a continuous cementite phase with dispersed austenite inclusions depending on the stability of the growth front [7]. We can assume that the addition of HNS has refined the structure in such a way, that multiple dendrites of austenite started to grow from the liquid almost simultaneously. This in turn minimized the space available in the interdendritic regions where ledeburite starts to solidify thus changing the colonies' orientation.
Another result that supports the fact the structure was refined is found in the measurements done on the transformed austenite dendrites. The results of the measured diameters of primary dendrite arms are scattered around an average value and are presented in Figure 2 in a normal distribution.
The results shown in Figure 2 indicate that the average dendrite arm diameter as well as the scatter ratio (statistic standard deviation) is slightly smaller in the case of modified M-CIN. This confirms a more
IS^Pt
WE7nux
a »¡c;T
" 111 ,1, ' L i--. I IIW-. " ill «I -. Il-L ' :(»>( I
Slika 3. Mikrostrukturi M-CIN zlitine; temen (a) in svetel (b) konstituent prikazujeta fazo na osnovi modifikatorja
Figure 3. SEM micrograph of the M-CIN alloy; the dark (a) and light (b) constituents show a modifier-based phase
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Livarski vestnik, letnik 63, št. 4/2016
zlitini nastopata le v sledovih, prav tako pa nista prisotna v FeV in FeW, visok izmerjen delež v konstituentih nakazuje na difuzijski proces, ki se je zgodil pred oz. med strjevanjem. S tem, ko je delež V oz. W v okolici modifikatorja narasel sta z Nb oz. Mo iz okolice tvorila karbidni fazi. Rezultati raziskav [8, 9] kažejo, da ima Nb zelo visoko afiniteto do C in pogosto tvori MC (M = Nb,V) karbidno fazo. Tako lahko z dodajanjem visoko taljivih ferozlitin v obliki modifikacije taline dosežemo izločevalno utrjevanje matrice. SEM preiskave nemodificirane CIN kvalitete do objave tega raziskovalnega dela niso pokazale prisotnosti drugih karbidnih faz z izjemo cementita. Po drugi strani pa so rezultati pokazali, da imata tako nizko legirana CIND, kot visoko legirana CINA kvaliteta prisotne V karbide.
Slika 4 prikazuje korelacijo med izmerjeno trdoto in deležem cementita v preiskovanih zlitinah.
61
52 -—-1—*—*—*-\-—-—---*—|-—-1---f-
25	27	29	31	33	35	3?	35
delci ccmcntita / ccmendte rwic [%)
Slika 4. Diagram trdote (HRC) v odvisnosti od deleža cementita za analizirane zlitine
Figure 4. Measured hardness (HRC) in relation to cementite ratio of the tested alloys
Območja, ki označujejo posamezen niz meritev, prikazujejo povprečno območje pojavnosti za posamezno kvaliteto. Rezultati na Sliki 4 kažejo, da je trdota zlitine M-CIN
uniform microstructure which is a direct result of structure refinement.
The results of SEM analysis done on the M-CIN alloy are shown in Figure 3.
EDS analysis done on the constituent shown in Figure 3a indicate a higher presence of elements V and Nb. EDS results also show a high value of elements Mo and W in the light constituent shown in Figure 3b. Both of the carbide phases are present inside the transformed austenitic matrix. Since neither Nb nor Mo were additionally alloyed into the metal or were present in the modifier, both appeared as trace elements. The high percentage measured inside the phase constituents suggests a diffusional process that occurred before or during solidification. As the value of V and W around the added modifier agents in the melt increased sufficiently, the surrounding residual Nb and Mo interacted to form completely separated carbides during solidification. Nb has been known to have a high affinity for C and form composite Nb-V based MC carbides [8, 9]. This indicates that the method of adding a high-melting ferroalloy as a modifier can result in precipitation hardening of the matrix. The SEM investigations conducted thus far on non-modified CIN quality had not proven the existence of any carbide phases other than cementite. However this is not the case with micro-alloyed CIND and high-alloyed CINA; both qualities were proven to have V-bearing carbide phases.
Figure 4 represents a correlation between the hardness and the cementite ratio of the tested alloys.
The circles around each set of measurements represent an averaged area of occurrence. The results show that the hardness of M-CIN is up to 5% higher than the CIN and CIND alloys and is comparable to that of the high-alloyed CINA quality. However the ratio of cementite is lower
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za približno 5% višja kot pri zlitinah CIN in CIND ter je primerljiva s kvaliteto CINA. V primerjavi s slednjo je delež cementita pri zlitini M-CIN nižji, kar nakazuje na izboljšanje mehanskih lastnosti v smislu duktilnosti, odpornosti na toplotne razpoke in korozijske odpornosti (predmet nadaljnjih preiskav).
Trdota ni edino merilo za izboljšanje mehanskih lastnosti delovne plasti valja. Ena najpreprostejših in najpogosteje uporabljanih metod je abrazijsko obrabni test. Rezultati le-tega so prikazani na Sliki 5.
Slika 5. Rezultati abrazijsko obrabnega testa »valjček-plošča« merjenega v izgubi volumna (podano v %)
Figure 5. Results of the pin-on-disk abrasive wear test measured in volume loss (given in %)
S Slike 5 je razvidno, da je abrazijsko najbolj odporna visoko legirana zlitina CINA, kar pomeni, da ima izmed testiranih zlitin najmanjšo izgubo materiala. Odpornost na abrazijsko obrabo z oddaljenostjo od površine še naraste. Ostale zlitine, vključno z modificirano M-CIN, imajo podobno obrabno odpornost. Meritve opravljene na zlitini CIN so močneje raztresene v odvisnosti od globine delovne plasti medtem, ko so meritve za CIND kvaliteto razmeroma enakomerne. Modificirana M-CIN ima
indicating better mechanical properties in terms of ductility, hot cracking and corrosion resistance which remain the subject of further investigation.
Hardness is not the only reference for roll material improvement. One of most commonly used and simple ways of testing working layer material is through the well accepted abrasive wear test, the results of which are presented in Figure 5.
As is shown in Figure 5, the high-alloyed CINA quality is most resistant to abrasive wear meaning it has the least amount of volume loss. Its resistance increases throughout the depth of the working layer. All of the other qualities - modified M-CIN included, - have shown a similar resistance to abrasive wear. However, as the measurements for CIN are rather scattered, CIND remained mostly constant throughout the depth of the layer. The modified M-CIN shows somewhat less resistance to wear on the surface, but this changes rather rapidly. At approximately 10 mm from the roll surface M-CIN proves to be superior both to CIN and CIND with a steady trend of increasing wear resistance.
4 Conclusions
The benefits of melt modification using high-melting ferroalloys were studied in the presented paper. With the use of different testing methods the effect of HNS addition into an indefinite chill cast iron was proven to be structure refinement, higher hardness per cementite ratio and a somewhat better abrasive wear resistance in comparison to non-modified CIN and micro-alloyed CIND qualities. The high-alloyed CINA remained the top abrasive-wear resistant quality among the analysed group.
According to SEM and EDS analysis, the method of melt modification during the
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nekoliko nižjo obrabno obstojnost tik ob površini. Pri globini delovne plasti pribl. 10 mm pa pokaže boljšo obrabno odpornost kot CIN in CIND zlitini z nakazanim trendom naraščajoče odpornosti.
4 Zaključki
V raziskovalnem delu so predstavljeni učinki modifikacije taline z visoko taljivimi ferozlitinami. Z uporabo različnih testnih metod se je pokazal učinek dodajanja HNM indefinitni litini kot udrobnitev strukture, višja trdota na dosežen delež cementita ter nekoliko izboljšana obrabna odpornost v primerjavi z nemodificiranim CIN in nizko legiranim CIND. Izmed analiziranih kvalitet ima najboljšo abrazijsko odpornost visoko legiran CINA.
SEM in EDS analizi sta pokazali, da modifikacija taline v curek med izpustom v ponovco spodbudi nastajanje utrjevalnih karbidnih faz na lokaliziranih mestih, kjer se delež visoko taljivega elementa zadostno poveča. Neraztaljenih delcev, ki bi bili ujeti v materialu, nismo potrdili.
Količina porabljenih ferozlitin med postopkom modifikacije M-CIN zlitine je od 2 do 3,5-krat nižja od količine potrebne za legiranje CIND in CINA zlitine. Cena dodanega FeV in FeW se je tako znižala do 40 % glede na trenutne cene ferozlitin [10]. V preiskanem primeru smo z modifikacijo taline dosegli nižji strošek izdelave indefinitnega valja ter hkrati enake mehanske lastnosti kot pri nizko legiranih kvalitetah. Raziskava znova potrjuje pozitivne učinke modifikacije kot stroškovno učinkovitega načina izboljšanja že obstoječih zlitin v smislu boljše operativnosti in obstojnosti.
discharge into the ladle is in favour of forming harder carbide based phases in regions rich in high-melting alloying elements instead of agent particles remaining trapped inside the matrix.
Regarding cost efficiency of melt modification, the quantity of ferroalloys needed for modification of M-CIN is up to 2 and 3,5 times lower than the quantity needed for alloying CIND and CINA qualities, respectively. The estimated price of FeV and FeW additions is thus up to 40% lower according to actual ferroalloy prices [10]. In the studied case, melt modification reduced the production cost of an indefinite chill roll while reaching the same mechanical properties as those of low-alloyed qualities. This study proves once again that the method of melt modification is indeed a cost-efficient way of improving existing alloys in terms of better performance and sustainability.
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5 Viri / References
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