A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 55 2010 Issue 3

M. CHOLEWA∗, T. WRÓBEL∗, S. TENEROWICZ∗∗, T. SZUTER∗

DIFFUSION PHENOMENA BETWEEN ALLOY STEEL AND GRAY CAST IRON IN LAYERED BIMETALLIC CASTING

ZJAWISKA DYFUZYJNE POMIĘDZY STALĄ STOPOWĄ I ŻELIWEM SZARYM W BIMETALICZNYM ODLEWIEWARSTWOWYM

The work presents bimetallic layered casting technology in configuration: the upper layer of high chromium steelor chrome-nickel steel with layer made of cast iron casting. In presented technology of layered bimetallic castings steelelements with a thickness of 1.5 and 5mm. It was used a process of overlay of a surface layer, directly during castingprocess so-called mould cavity preparation method. Steel elements were placed in sand mould just before pouring theliquid gray cast iron. Verification of bimetallic castings was based on ultrasonic non-destructive testing and examinationof the structure and selected utility properties.

Keywords: Casting, Bimetal, Grey cast iron, Steel, Austenite, Ferrite

W pracy przedstawiono technologię bimetalicznych odlewów warstwowych w konfiguracji: warstwa wierzchnia nabazie stali wysokochromowej lub chromowo-niklowej z podłożem w postaci odlewu z żeliwa szarego. W przedstawionejtechnologii bimetalicznych odlewów warstwowych użyto do uszlachetnienia warstwy wierzchniej elementów stalowycho grubości 1,5 i 5mm, które w wyniku zastosowania metody nakładania warstw bezpośrednio w procesie odlewaniatzw. metoda preparowania wnęki formy, zostały umieszczone w formach piaskowych bezpośrednio przed zalaniemciekłym żeliwem szarym. Weryfikację wykonanych bimetalicznych odlewów warstwowych przeprowadzono w oparciuo nieniszczące badania ultradźwiękowe oraz badania struktury i wybranych własności użytkowych.

1. Introduction

In the engineering industry noticeable is a grow-ing demand for castings with special properties suchas abrasive wear resistance, corrosion resistance atroom or elevated temperature. Elements of this typeoften carried out entirely from expensive and hardto reach materials like of Ni, Co, Ti, or others. Inmany cases the requirements for high performanceproperties affect only the working surface of thecasting. Especially if wear of an element leads toits destruction through exceeded the allowable maindimension decrease.

Among many methods for producing metalliccoatings on materials for specific performance prop-erties to be mentioned is casting technology calledmethod of mould cavity preparation in which theelement which is the working surface layer of thecasting is placed in a form immediately before pour-ing molten metal. This technology is the most eco-nomical way of enrichment the surface of castings,both ferrous and nonferrous alloys, as it allows theproduction of layer elements directly in the process

of casting [1, 2]. Therefore, this technology can pro-vide significant competition for the commonly usedwelding technologies and thermal spraying [3÷5],because in addition to economic advantages do notgenerate opportunities for the development of cracksin the heat affected zone, which arises as a result ofmaking layer by welding method. The idea of theproposed technology of layer casting was taken fromthe relevant mining industry method of manufactureof composite layers of surface based on Fe-Cr-Cgranularity inserts and placed in a form immedi-ately before pouring molten metal. Obtained in thisway working surface layers have a high hardness andmetal-mineral wear resistance [1, 2]. Another, alsohas common features with the proposed technologyis layered bimetallic castings cast steel- chromiumalloy cast iron, in which part of the casting is madeof cast steel plate is designed to allow connection bywelding to another part, and part is made of cast ironto ensure high hardness and abrasive wear resistance[6, 7]. However, a significant economic limitationof this method is the need to preheat the cast steel

∗ SILESIAN UNIVERSITY OF TECHNOLOGY, FOUNDRY DEPARTMENT, TOWAROWA STR. 7, 44-100 GLIWICE, POLAND∗∗ EXPERIMENTAL-IMPLEMENTION FOUNDRY DIVISION, TOWAROWA STR. 7, 44-100 GLIWICE, POLAND

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component placed mould. This treatment is carriedout by two-stage form pouring with the liquid castiron (Fig. 1).

Fig. 1. Technology of bimetallic layer castings with use of twogating systems: 1 – cast iron layer, 2 – cavity (preheater) 3 –steel sheet, 4,5 – gating[6]

In the first stage mould cavity beneath a caststeel plate is filled with liquid metal for preheat ofcast steel element. Formed in this way, layer aftercleaning cast iron is separated from the casting. Inthe second stage mould cavity over cast steel plate isfilled, in which the liquid metal forms a layered con-necting with the plate creating a layered bimetalliccasting. Restrictions resulting from the applicationof preheating on the first reduce yield and the in-ability to make high mass and stiffness of the cast-ing. According to the authors those problems canbe eliminated by applying appropriate cover activa-tion, applied on steel surface in contact directly withliquid metal.

Next method of bimetallic casting technology isbased on a skeleton steel structure filled with castiron. The advantages of this method include largesurfaces of contact of both materials and the abilityto carry out several connections anywhere in casting[8].

In addition, attention should be paid to themethod of producing bimetallic castings casted ina continuous process. In this method, the castingprocess is carried out using two independent cru-cibles, from which two streams of molten metal isintroduced to the crystallizer equipped with a spe-cial barrier that allows a combination of both ma-terials connect in the bimetal. However, this inno-vative technology is so far limited only to selectednon-ferrous alloys such as Al-Zn, Al or Al-Sn-Pb[9].

2. Range of research

For the researches, the casting with a rectangu-lar shape and dimensions of 125×105×40 mm was

used. In sand moulds steel sheet X8Cr X10CrNi18-8 13 of 1.5 and 5mm thick was placed withcover activation. No procedure of preheating wasapplied (Fig. 2). Then casted low-alloy cast ironEN-GJL-XCr1 with temperature 1450◦C. In effectbimetallic layer casting was obtained (Fig. 3).

Fig. 2. Section of the sand mould ready to be poured by liquidalloy

Fig. 3. View of cast iron test-castings with the surface enrichedby alloy steel plate – 1

The quality of the bimetal layer casting wasevaluated on ultrasonic NDT made using the DIO562 flaw detector by Starmans Elektronics. Nextmetallographic examination of macro-and micro-scopic was carried out. Metallographic specimensetched in the reagent Mi19Fe containing [13]: 3 g offerric chloride, 10cm3 hydrochloric acid and 90cm3

of ethanol. Qualitative microstructure research wasconducted on the Nikon light microscope and elec-tron scanning InspectF equipped with EDS system.

The study was extended to phase X-ray structureanalysis carried out on the X-ray diffractometer

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Fig. 4. View of cross-section of bimetallic low-alloy cast iron casting with enriched surface by austenitic steel plate about thickness1,5mm (a) and 5mm (b)

XPertPro by Panalytical with the angular range 2θfrom 35◦ to 125◦ in steps of 0.05◦ at the time ofcounting 5s. Using filtered X-rays from the lampwith a cobalt anode. Phase identification was basedon data from the database International Centre forDiffraction Date ICDD. Samples for investigationswere cut from the layer castings with surface en-richment of chromium-nickel steel plate after onecycle operation of abrasion of the metal – mineralaccording to the guidelines of ASTM G 65 – 00, tocheck whether there is a possibility of increasing theresistance to wear by plastic deformation induced bytransformation austenite to martensite (γ → α′).

In addition, measurements of macro-and microhardness test were made using appropriately MIC2Krautrkramer-Branson’s and FM 700’s Future-Tech.

3. Results

On the basis of non-destructive ultrasonic test-ing it was found that the entire sample bimetalliccasting surface (head placed on the side of the metalsheet) the bottom echo was larger than the echo ofthe transition zone, which indicates a good diffusionconnection.

These results are confirmed in Fig. 4 that showsan example of the cross-section of the bimetalliccasting. Furthermore, it was found that in the caseof thin sheet metal that is 1.5 mm thick stainlesssteel, both X8Cr13 and X10CrNi18-8 a deforma-tion occurs that is due to the low heat capacity thatgenerates stress. As a result, a part of the liquidiron poured on the top of the plate, causes its par-tial dissolution (Fig. 4) and determines the lack ofusefulness of this type of castings for industrial ap-plications. The described phenomenon does not ap-ply to cast steel sheets with a thickness of 5 mm(Fig. 4b), which allows to obtain the correct shapeof the surface layer.

Figures 5 and 6 presents the results of metal-lographic microscopic examination. As a result of

C diffusion phenomena in the direction from ironto steel metal sheet and to a lesser extent Cr in theopposite direction, transition zone has been formedat the junction of cast iron – steel. Shaped transi-tion zone, structurally different from the iron andsteel plates used, has the character of diffusion indetermining the quality of a combination of bothbi-metallic composite components. In addition, theformation of microstructure transition zone and ad-jacent areas is affected by the heating temperatureof steel, because of the iron poured to form. For thetemperature of pouring iron 1450◦C, the temperatureTs at the border of the contact liquid metal – steelplate was fixed on the basis of dependence [11]:

Ts =

√λz · cz · ρz · Tz +

√λs · cs · ρs · Ts√

λz · cz · ρz +√λs · cs · ρs

(1)

λz, λs – coefficient of thermal conductivity, re-spectively for the liquid iron and steel, W/(m·K),

cz, cs – appropriately for the heat of molten ironand steel, J/(kg·K),

ρz, ρs – mass density, adequate for the liquidiron and steel sheet, kg/m3,

Tz – the temperature of molten cast iron ◦C,Ts – temperature of the sheet metal which is for

the ferrite stainless steel X8Cr13, about 870◦C, andfor austenitic stainless steel, about 950◦C.

Effect of diffusion of the basic components ofalloys in combination with a high temperature heat-ing of steel sheet, and its effects an cast iron duringits solidification like chill, decide on the formationof microstructure in the area diverse combination ofboth materials.

In the case of bimetallic layer casting X8Cr13steel – high chromium iron EN-GJL-XCr1, theanalysis carried out on the metal sheet side allows todistinguish the zones shown in Fig. 5 and outlinedin Tab. 1.

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Fig. 5. Microstructure of bimetallic layer casting in configuration: surface layer on the basis of X8Cr 13 ferritic plate of the thicknessabout 5 mm on low-alloy cast iron EN-GJL-XCr1 base – etching Mi19Fe

TABLE 1Microstructure characteristic of bimetallic layer casting in

configuration X8Cr 13 steel – low-alloy cast ironEN-GJL-XCr1

Zonenumber*

Microstructurecomponent

MicrohardnessµHV

1 Ferrite (α) 205

2 Martensite (α′) 365

3Martensitewith carbides(Fe, Cr)xCy

α′ = 410,(Fe, Cr)xCy = 900

4 Pearlite 300

5Hard spots-pearlite,ledeburite with carbides(Fe, Cr)3C

Matrix = 420,(Fe, Cr)3C = 800

6Flake graphite inpearlite matrix Matrix = 300

* – zone designation according to fig. 5

In the surface layer of this type bimetallic layercasting a single phase ferritic structure was obtainedwhich provides high corrosion resistance also at el-evated temperatures.

As a result of carbon diffusion in the directionfrom iron to steel in the outer area of the metalsheet (zone 2) the concentrations of carbon increas-es above 0.1%, which, combined with the high tem-perature heating of this area leads to the structure

of the solid solution γ, which during cooling of thecasting with a not to high speed transforms in tomartensitic. The ratio of martensite to the amountof ferrite increases towards the border bimetallic.

In the last zone (no. 3) on the side of steel sheet,as evidenced by preserved the original orientationof crystal grains, as a result of intensive carburizingfrom iron martensite occurs with iron and chromiumcarbides.

The first zone (no. 4) on the side of cast ironcreates perlite. This zone is combined with the previ-ous one, no.3 non-linear connections border createsperlite. The presence of pearlite results from the de-pletion of this zone in the carbon, which preventsthe emergence of phases i.e. graphite or cementite,typical for cast iron.

Then, in zone 5, as a result of high-speed solidi-fication of molten metal at a concentration of carbonappropriate for cast iron, hard spot area is consistedof iron and chromium carbides, probably in the formof cementite (Fe, Cr)3C.

Last zone 6 consists of a typical structure forthe iron poured into molds such as flake graphite ina pearlitic matrix.

In the case of bimetal layer casting steel compo-sition of high chromium X10CrNi 18-8 – low alloycast iron EN-GJL-XCr1, the analysis carried out onthe side of steel sheet allows to distinguish the zonesshown in Fig. 6 and outlined in Tab. 2.

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Fig. 6. Microstructure of bimetallic layer casting in configuration: surface layer on the basis of X10CrNi 18-8 austenitic steel plateabout thickness g = 5 mm on low-alloy cast iron EN-GJL-XCr1 base – etching Mi19Fe

TABLE 2Microstructure characteristic of bimetallic layer casting in

configuration X10CrNi 18-8 steel - low-alloy cast ironEN-GJL-XCr1

Zonenumber*

Microstructurecomponent

MicrohardnessµHV

1 Austenite (γ) 220

2Austenite withcarbides Cr23C6

330

3 Ferrite and austenite 275

4 Martensite 350

5Hard spots-pearlite,ledeburite with carbides(Fe, Cr)7C3

Osnowa = 430,(Fe, Cr)7C3 = 1100

6Flake graphite inpearlite matrix Matrix = 300

* – zone designation according to fig. 6

The surface layer of this type bimetal layer cast-ing obtained single phase austenitic structure, whichis similar like the ferritic layer provides high resis-tance to corrosion, also at elevated temperatures.

However, compared to a purely ferritic(150HV), austenitic structure has a greater resistanceto abrasive wear. The increase in hardness of austen-ite (from 220HV to 400HV) in operating conditionsoccurs as a result inducted of plastic deformationmartensitic transformation γ → α′.

The presence of martensite Fe′α confirms car-ried out a qualitative X-ray phase analysis (Fig. 7).Martensite is increasing hardness and wear resis-

tance of Feγ austenitic steel surface layer of layerbimetallic casting.

Fig. 7. X-ray diffraction of bimetallic casting with enriched sur-face by austenitic steel plate after one cycle exploitation in con-ditions of abrasive action of type metal-mineral

The result of heating the external area of thesteel sheet from molten cast iron to a temperatureabove 900◦C, in this area (zone 2) is dissolvedcarbon interstitial diffusion in austenite, which mi-grates from the whole volume of γ phase to thegrain boundaries and in combination with vacancychromium diffusion from near border grains areasCr23C6 chromium carbides are formed. In additionbesides presence of this type of carbides on theborders, they occur in the central areas of austenitegrains. The presence of carbides Cr23C6 result ina decrease of corrosion resistance in this area, as

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Fig. 8. Distribution of C, Fe, Si, Cr and Ni in transition zone between austenitic steel cast steel and low-alloy cast iron in bimetalliclayer casting: a) structure of research area, b) pointwise microanalysis of chemical composition in point “A”, c) pointwise microanalysisof chemical composition in point “B”, d) pointwise microanalysis of chemical composition in point “C”

illustrated by the effects of etching metallographicspecimens appear explicitly in those areas.

Another, distinct in structural terms is typicaltransition zone connecting the areas typical of steelon one side and iron on the other. Taking into ac-count the complexity of the microstructure in thiszone, the phase composition was based on the mi-croanalysis of chemical composition (Fig. 8) andpredicting the presence of components based on theanalysis done in THERMOCALC. On this basis,the area marked 3 is characterized by a duplex mi-crostructure consisting of austenite and ferrite.

High quality of both materials connection inthis type of bimetallic casting provides the diffusenature, which illustrates the phenomenon of pene-tration of strips of martensite formed in the impov-erished in carbon cast iron layer (zone 4) in thetransition zone (zone 3). Furthermore, it was foundthat depending on local concentrations of carbon andother elements, as well as local cooling rate in zone4, in place of perlite martensite occurs (Fig. 9). How-ever, this does not affect the reduction of connectionquality in layered bimetallic casting.

Fig. 9. Microstructure of joint zone in bimetallic layer cast-ing in configuration: austenitic steel plate – low-alloy cast iron.In direction from surface is visible following: α + γ, pearlite,ledeburite + pearlite – etching Mi19Fe

Then, in zone 5, as a result of high-speed solidi-fication of molten metal at a concentration of carbonappropriate for iron, whitened area is composed ofiron and chromium carbides (Fig. 10).

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Fig. 10. Superficial distribution of Cr in transition zone be-tween austenitic steel and low-alloy cast iron in bimetallic layercasting. Structure of research area from Fig.8a

Last Zone 6 consists of a typical structure forthe iron that is poured into molds flake graphite ina pearlitic matrix.

4. Conclusion

Based on analysis of research results was foundthat:

1. Casting technology based on mould cav-ity preparation method allows to obtain a lay-ered bimetallic casting in high chromium steel orchrome-nickel steel with gray iron configuration,free from defects especially in the sensitive area ofa connection of both materials. Obtained permanentconnection sheet steel with a cast iron is character-ized with diffusion, which is determined primarilyby diffusion of carbon in the direction from cast ironto steel.

2. Application of thin sheets 1.5 mm thick,resulting in their deformation during the casting,which disqualifies a casting for industrial applica-tions. Much better results were obtained as a resultof the application sheet with greater thickness, i.e.5 mm.

3. Made of layered bimetallic casting can workin conditions demanding from the surface layer ofhigh heat resistance and / or resistance to corrosionin environments such as industrial water. In addi-

tion, if the configuration with the austenitic steel ispossible to obtain high resistance to abrasive wear.

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Received: 10 February 2010.