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Kolokoltsev V.M., Petrochenko E.V.
STRUCTURE FEATURES AND PROPERTIES OF HIGH-ALLOY WHITE IRONS
Abstract. In this paper the regularities of structure formation, mechanical properties and wearability of chrome-vanadium white cast irons, depending on chemical composition, cooling conditions during solidification have been investigated.
The relation of wearability of chrome vanadium white cast irons with the morphology of carbide phase, types of binary and ternary eutectics, phase and chemical composition of metallic matrix of castings in abrasive wearing-out has been established.
The reasoning of the influence of structure formation features in chrome vanadium white cast irons on the mechanical and special features, forming during crystallization and the following effects of abrasive ambient are represented.
Cast iron classification according to metallic matrix structure, eutectic type and quantity, eutectic morphology, phase morphology, forming any eutectic, is suggested.
Keywords: structure, eutectic compositions, phase structure, phase composition, microhardness, wearability, straining martensit-ic transformation, classification according to white cast iron morphology.
White cast iron is widely used as a material for tools and machinery parts, which undergo intensive wear and oxidation. It was traditionally attributed to the class of fragile and low-strength materials and this fact significantly limited the area of its use. The progress in the field of white irons alloying, achieved in recent years, has significantly changed the ideas about their properties and possible applications.
Modern white cast irons are complex multi-component alloys, different in structure and specific properties. They are a separate group of industrial cast irons, which composite structure is being formed during solidification. It is the group that determines the specific properties of white cast irons in the as-cast state.
Despite the literature readings abundance concerning the composition optimization of complex alloyed white irons of functional purpose, the effect of alloying elements on the crystallization processes and structure formation, mechanical and special (heat resistance, durability) properties of cast irons has not been considerably and systematically investigated. Especially, it concerns the formation conditions of various eutectic and carbide phase, containing some carbide-forming elements in iron composition.
In this paper the regularities of structure formation, mechanical and specific properties of chrome-vanadium white cast irons, depending on the chemical composition, cooling conditions during solidification have been investigated.
The selection of alloying structure and varying limits of alloying elements and carbon content largely determines the metallic matrix morphology, quantity, carbide phase type and eutectic, and, consequently, the alloy properties in whole and is settled by the following statements.
The most wear resistant, in accordance with Charpy principle, are cast irons requiring the complete inversion of phase location. It means the most solid structured constituents should lie in the form of isolated impurities, but the most viscous constituents should form a solid matrix, that provides not only high wear-resistant properties, but also strength, toughness, resistance to thermal cycling, etc.
Such phase arrangement inversion in the structure of austenitic-carbide eutectic can be achieved in high
chrome cast iron alloying with more than 3% vanadium. Carbide is the branched phase but austenite or its transformation products are the matrix phase, being in general a cast composite.
Carbon is the main regulator of the carbide phase, which determines the properties of the present irons. Carbon addition of 3.2-3.6% provides the M7C3 carbides formation, which improves cast iron wear resistance. Carbon addition of less than 3.2% leads to the primary aus-tenite quantity increasing. Carbon addition of more than 3.2% leads to the reducing of alloying elements content in the solid solution, and to the disruption of the cast structure uniformity at the expense of large branched carbides precipitation. Both negatively affect on cast iron properties.
Chromium can partially replace the iron atoms in the iron carbide (Fe,Cr)3C or it can form chromium carbides, in which the part of chromium atoms is substituted by iron: trigonal (Cr,Fe)7C3 and cubic (Cr,Fe)23C6. In a-iron chromium has unlimited solvency, in y-iron chrome dissolves to 14% Cr. Chromium carbides have significantly higher hardness than chromium alloyed cementite, and it promotes cast iron durability and mechanical properties.
Cementite carbides in iron form a hard framing of ledeburite eutectic. Criteria of fragile destruction for such carbide hard framing are achieved earlier than for eutectic with carbides (Cr,Fe)7C3. Cast irons with carbides (Cr,Fe)7C3 (chromium content in the iron exceeds 11-14%) are of maximum wear resistance due to increased micro-hardness of these carbides and eutectic branched morphology. Carbide (Cr,Fe)23C6 microhardness is 2000 MPa below than carbide (Cr,Fe)7C3 microhardness. It is more crisp and intent to crushing through the process of coupling with abrasive particles that helps to reduce cast iron durability.
To form complex carbides (Cr,Fe)7C3, giving maximum cast iron durability, chromium levels range from 14,0-20,0% is required. When the chromium content is less than 14%, the formation of carbides (Cr,Fe)7C3 along with carbides (Fe,Cr)3C is possible reducing cast iron durability. When the chromium content is more than 20.0%, large and fragile carbides (Cr,Fe)23C6 appear in the iron structure, resulting in wear-resistant properties reducing.
Vanadium in the range of 3.0-9.0% forms special VC carbides with carbon of high microhardness (~ HV 3000). Moreover, two types of eutectic are formed in the iron struc-
ture: double austenitic-vanadium-carbide and triple austenit-ic-chrome-vanadium-carbide, which, being the composite strengtheners, greatly increase cast iron durability.
Investigations were carried out on Fe - C - Cr - V alloys, containing 2.6-3.2% C, 14,0-20,0% Cr, 3,0-9,0% V. The amount of silicon and manganese in the experiment alloys was at the permanent level: Si (0,4-0,6%); Mn (0,4-0,6%). The experimental alloys were melted in the induction furnace IST-006 with the basic lining. The cooling rate influence on the structure and wear resistance during crystallization was studied on the iron samples, poured into dry and wet sand and clayed molds (SCM) and casting mold [1, 2].
The chemical composition of the samples was determined by an emission spectrometer «Bird» and by a spectrometer OBLF QSG 750 GOST 18895-97.
The structure and phase composition of cast irons were examined with the help of metallographic and x-ray methods. X-ray imaging was carried out on a DRON-UM1 dif-fractometer (in the cobalt Ka radiation). The diffractometer was connected to a PC. Phase analysis was carried out with the help of XRAYAN program and PDF database.
Quantitative metallographic analysis, automated processing of microhardness measurement results were performed using a Thixomet PRO image analyzer. Micro-hardness measurement was carried out using a PMT-3 according to GOST 9450-76.
Micro X-ray spectrum studies of phase components in alloys were carried out using scanning electron microscopes «JEOL» JSM-6460 LV, «TESCAN VEGA II XMU», «Camscan» with micro X-ray spectrum analyzers.
Comparative tests of alloy and cast iron wear resistance in rubbing with semifixed abrasive particles were carried out according to GOST 23.208-79. Wear-out was performed by abrasive particles of various hardness (elec-trocorundum and periclase), allowing to define various mechanisms of wear-out. Testing corundum, whose hardness (20-22 hPa) is comparable with vanadium carbide hardness and exceeds chromium carbide hardness and martensite-austenite matrix, the main mechanism of surface destruction is microcutting. Periclase hardness (10-12 hPa) is lower than vanadium carbide hardness and close to chromium carbide and metallic matrix hardness, therefore coupling iron with periclase plastic ousting is the main wear-out mechanism.
Phase composition of chromium-vanadium irons in as-cast state presents a-phase (martensite), y-phase (austenite), vanadium carbide (VC), chromium carbide (Fe, Cr, V)7C3. The combination of these phases provides two binary eutec-tics y + VC, y + (Fe, Cr, V) 7C3 and triple eutectic y + (Fe, Cr, V) 7C3 + VC while crystallizing. The coexistence of carbides of different forms and types is determined by iron composition and its crystallizing conditions.
Carbide and metallic matrix composition is variable and depends on chemical composition of the alloy and cooling rate during solidification. Carbides (Fe, Cr, V)7C3 contain 26,0-48,0% iron, 41,0-52,0% chromium, 9,0-22,0% vanadium, vanadium carbide dissolves iron partially (up to 2,0-5,0%), chromium dissolves iron some more - (8,0-16,0%) [3].
The determination of vanadium carbide volume quantity was performed on unpickled sections. The determination of the chromium carbide amount and size, the volume
quantity of eutectics and their dispersity was carried out on sections after pickling.
Depending on iron composition the following types of alloy structures are formed (structural classes) [4]:
1 - hypoeutectic, consisting of excessive austenite dendrites (or products of its decomposition) and triple eutectics y + (Fe, Cr, V) 7C3+ VC;
2 - structure, consisting of two eutectics y + VC (spherulitic form) and y + (Fe, Cr, V) 7C3 + VC;
3 - structure, consisting of two eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC;
4 - structure, consisting of pre-eutectic VC carbides and eutectic y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC;
5 - structure, consisting of excessive VC carbides (or carbides (Fe, Cr, V) 7C3) and eutectics y + VC, y + (Fe, Cr, V) 7C3 + V (Fig. 1).
Eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC are socket-shaped in cross section, and fan-shaped in longitudinal section.
Composition, structure and carbide phase properties depend on the ratio of chromium and vanadium in cast irons. When the content of carbon and alloying elements is excessive, massive branched dendrites of primary vanadium carbides are formed (see Fig. 1).
The chromium increasing in the alloy causes vanadium content reducing in the composition of carbides VC and (Fe, Cr, V) 7C3. It manifests in microhardness decreasing of vanadium carbide from 22 to 18 GPa and complex chromium carbides from 16 to 10 GPa. The increasing of vanadium and carbon concentration in the alloy reduces iron content in carbides and increases chromium and vanadium content. As a result, carbide (Fe, Cr, V) 7C3 microhardness rises up to 16-17 GPa.
The cooling rate increasing leads to the following change in carbides composition: reduces chromium content from 10 to 8% in VC carbide, increases iron content from 37 to 47% and reduces chromium content from 51 to 41°% in complex carbide (Fe, Cr, V) 7C3. As a result, the alloying level of metallic matrix increases.
Carbide phase volume in eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC is 28-36%, in eutectic A + VC the amount of carbides is less - 10-15%. The difference in the eutectic structure determines their different properties.
Eutectic compositions are crystallized within the temperature range and have variable phase composition (Table 1 ), different density (by changing the amount of carbides in the eutectic, intercarbide distance) and carbide phase dispersion, depending on alloy chemical composition and cooling rate during solidification. Eutectic type and proportion in the structure of iron also depend on the alloy composition and cooling conditions, determining cast iron mechanical properties and wear resistance in wearing-out by different hardness abrasive.
Table 1
The influence of cooling conditions on the amount of martensite qa, austenite qy, complex chromium carbides q1and vanadium q2, %
Dry SCM Wet SCM Chill mold
qa qv q1 q2 qa qv q1 q2 qa qv q1 q2
67,4 3,5 27,6 1,4 48,1 8,4 40,4 2,1 19,0 31,61 51,1 3,9
3 a
3 b
4
4
Fig. 1. Types of chrome-vanadium iron structures: 1 - A dendrites and eutectic Y + (Fe, Cr, V) 7C3 + VC; 2 - eutectics y + VC and y + (Fe, Cr, V) 7C3 + VC; 3 - eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC (a - dry SCM; b - chill); 4 - excessive VC carbides and eutectics y + VC (fibrous form), Y + (Fe, Cr, V) 7C3 + VC; 5 - excessive VC carbides and eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC
The metallic matrix consists of austenite and martensite, the ratio of these phases depends on the chemical composition of metallic matrix, which is defined by alloy composition and casting mould type. In chill casting chromium and vanadium content in the matrix increases, that causes the increasing of austenite proportion in the structure.
Different structural types are formed in cast irons of the following compositions, %: type 1 - 2,6 C 14-20 Cr; 3 V and 3,2 C; 14 Cr; 3 V type 2 - 2,6 C; 14 Cr; 9 V; 2,6 C 14-20 Cr; 9 V; 3 type - 3,2 C; 20 Cr 3 V; 4 type -3,2 C; 14 Cr; V 9 and 2.9 C, 17 Cr , 6 V; 5 type-3,2 C; 20 Cr; V 9.
The features of formation of structure and properties of different structural types (classes) cast irons have been studied. There is one eutectic y + (Fe, Cr, V) 7C3 + V in cast iron structure of the first structural class. Complex (Fe, Cr, V) 7C3 carbide is the predominant phase in the ternary eutectic.
3% vanadium content is sufficient concentration when it is not only in the solid solution and is a part of complex carbide (Cr, Fe) 7C3, but forms separate VC carbides in the form close to spherical. The maximum size of carbides is 2, 5-6, 8 microns; the average size is 1.0-2.8 microns. Vanadium carbide is on chromium eutectic carbides.
With increasing chromium, carbon content and cooling rate, the volume fraction (from 58,8,0 to 27.6%) and primary austenite dendrites sizes (the average size from 13.7 to 28.0 mm) decline, the dispersion and the volume fraction of aus-tenite--chromium carbide eutectic increase (Fig. 2a, b). Eutectic microhardness changes slightly 6.0-6.8 GPa. Hardness and wear resistance increase.
The cooling rate increasing causes martensite quantity reduction, but amount of austenite, at the same time, increases. This can be explained by matrix chemical composition changing: chromium content increases and iron content decreases, vanadium content changes slightly.
The increasing of chromium concentration in the alloy is accompanied by chromium content from 10% to 15% increasing in the metallic matrix of aus-tenite--chromium carbide eutectic and reducing iron concentration from 87 to 80%. This causes temperature reducing at the martensite start Ms and results in quantity reduction. Chromium content in
but va-
martensite
chromium carbides increases from 40% to 50%, nadium content reduces. Matrix microhardness decreases from 7.1 to 4.4 GPa. Chromium carbide microhardness decreases from 15.1 to 13.9 MPa.
a
b
Fig.2. Microstructure of the 1st type chrome-vanadium cast irons, poured into dry SCM (a) and chill mold (b), x500
1
2
The phase composition of cast iron samples, containing 3.2% C, 14% Cr, 3% V, depending on the cooling conditions is shown in Table 1.
Durability corundum is small and increases along with the growing of metallic matrix microhardness and the amount of cast irons carbide phase volume. Durability of chrome-vanadium cast irons containing periclase insignificantly depends on carbide phase volume and cast iron hardness, but depends on austenite quantity and its meta-stability towards straining martensitic transformation. Met-astable austenite, being transformed into strain martensite during the wearing, strengthens the surface and improves durability (Table 2).
High chrome-vanadium cast iron durability under conditions of plastic push-off mechanism wear-out is due to surface layers hardening because of phase transformations and phase straining hardening.
Table 2
The influence of cooling conditions on the amount of transformed austenite qVp, Ha/Hb ratio and wear resistance
in corundum Kc and periclase Kp
Mould type qYp,% Ha/Hb' corundum Ha/Hb periclase Kc Kp
Dry SCM 0 1,3 1,1 4,2 17,6
Wet SCM 0 1,7 1,3 4,9 43,8
Chill mold 23,0-25,0 2,2 2,0 5,4 108,0
* Hb m Ha- microhardness of the metallic matrix before and after wear
The absence of straining martensitic transformation in cast irons, poured into dry and wet SCM, can be explained by a large amount of cooling martensite in the metallic matrix structure. Strain martensite formation during cast iron wear-out with corundum and periclase is relieved in the structure with metastable austenite predominance.
The features of the 2nd and 3d class structure formation. The structure of the 2nd class is formed in cast irons, containing 2.6% C; 14-20% Cr and 9% V; the structure of the 3d class is formed in cast iron composition,0/»: 3.2 C, 20 Cr; 3 V. Cast iron structure consists of two eutectics (refer to Fig. 1, 2 and 3). There is less amount of carbide phase in eutectic y + VC than in eutectics y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3+ VC.
With the help of X-ray mapping concentrated eutectic irregularities, determining their structure and properties, were revealed (Fig. 3).
With chromium content of 14% in the alloy the metallic matrix chemical composition in vanadium eutectic of irons of the 2nd structural class is,//: 5,79 V; 11,4 Cr and 82,0 Fe; ternary eutectic matrix composition is,%: 1,7 V; 13 9 and Cr 83,3 Fe.
The increasing of chromium content in the alloy up to 20,0% alters phase composition. Vanadium content reduces to 2.7% , but chromium content increases to 12.5% in metallic matrix of eutectic y + VC. The amount of vanadium and chromium in eutectic matrix y + (Fe, Cr, V) 7C3 + VC increases up to 7.1 and 14.9%. That concerns not only with chromium quantity increasing in the alloy, but also with changes of volume fraction and vanadium and chromium carbide content. The difference in eutectic matrix compositions is shown in their properties: eutectic metallic matrix microhardness y + (Fe, Cr, V) 7C3 + VC is under 1200-2000 MPa. Thus, changing the quantitative ratio of eutectics with different properties it is possible to obtain various properties of the alloy in whole.
The volume fraction and size of binary eutectic vary depending on carbon and alloying elements content and cooling conditions. With increasing chromium content and the cooling rate the volume fraction of ternary eutec-tic, dispersion eutectic y + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC increase (see Fig. 1, 3a and 3b ).
In increasing cooling rate the density and dispersion of eutectic y + VC increase (the number of vanadium carbides increases from 2705 up to 16410 1/mm2, interparti-cle distance decreases from 62 to 21 microns and carbide size decreases from 6.5 to 2.8 microns).
Cooling martensite 72,2-90,0% prevails in cast iron structure of 2nd and 3d classes, filled in dry and wet SCM.
Wear resistance of cast irons with corundum, poured into SCM is 4,5-10,1 units, with periclase it is 10,8-28,7 units. Wear resistance of cast irons of the 2nd and 3d structural classes is higher than wear resistance of cast irons of the 1st type. Wear resistance of cast irons of the 2nd class is higher because of the presence of 7,5-9,1% vanadium carbides in the structure. Wear resistance of cast irons of the 3d class is higher because of the presence 30,2-72,3% complex chromium carbides. In the cast irons of the 1st class volume ratio of vanadium carbides is 0,3-5,2%, 20,0-51,5% of chromium carbides.
In pouring into a mold the austenite proportion in iron-structure increases, straining martensitic transformation takes place with corundum and periclase wearing-out, resulting in significantly hardened surface (microhardness increases in 1.5-2.0 times). Wear resistance with corundum increases up to 9,1-13,0 units and in periclase up to 19,8-60,6 units.
There are dendrites of excessive vanadium carbides in the structure of irons of the 4th and 5th structural classes. The structure of irons of the 4th class consists of excessive VC carbides and eutectic y + VC and y + (Fe, Cr, V) 7C3 + +VC; of 5th class of excessive VC carbides and eutectic y+ + (Fe, Cr, V) 7C3 and y + (Fe, Cr, V) 7C3 + VC (see Fig. 1, 4, 5). The structure of the 4th type is formed in cast irons with the following composition, %: 3 2 C; 14 Cr; V and 2.9 C 9, 17 Cr; 6 V; the structure of the 5 type is formed in cast irons with the following composition, %: 3,2 C; 20 Cr; V 9.
Using X-ray mapping the element distribution between iron structural components of the 4th and 5th structural classes was detected (Fig. 4, 5).
Fig. 4. Electron micrograph of iron of the 4th structural class and element distribution in structural components
V Ka1 0 Ka1_2
Fig. 5. Electron micrograph of cast iron of the 5th structural class and element distribution in structural components of cast iron with eutectics y + VC and y + (Fe, Cr, V) 7C3 + VC
With cooling rate increasing volume fraction and sizes of vanadium carbides, binary eutectics decrease, the volume fraction of ternary eutectic increases. For example, in cast irons of the 4th class in casting into dry SCM volume fractions of structural components are as follows: 11.2% of vanadium carbides, 53.8% binary eutectic, 34.9% triple eutectic. In pouring into the mold the volumetric proportions of excessive VC carbides, eutectic y + VC and Y + (Fe, Cr, V) 7C3 + VC are 9,3; 38,5 and 51,7%, accordingly.
In the case of high cooling speed (chill casting) the nature of excessive phase changes in the alloy structure containing 2.9% C; 17% Cr; 6% V. Complex carbide (Fe, Cr, V) 7C3 (Fig. 6b) becomes the excessive phase instead of vanadium carbide (Fig. 6a).
Wear resistance of cast irons of the 4th, 5th structural
classes is 8,0-14,0 units with corundum, 33,0-99,0 units with periclase.
As a result of analysis of the chemical composition and cooling condition effect on the types of chrome-vanadium iron structures in the studied concentrated intervals laws of morphology of excessive phases, eutectic composition and metallic matrix were established, which allowed to offer the classification according to the following criteria:
- according to the metallic matrix type: mainly mar-tensitic and martensitic- austenite;
- according to the eutectic type:
• with eutectic y + carbides of M7C3 type;
• with eutectic y + carbides of MC type, such as VC;
• with eutectic y + M7C3 and MC, example (Fe, Cr)7C3 and VC, etc.;
- according to the number of eutectics and their constitutive phases:
• cast irons with double and ternary eutectics (y+MC and Y+MC+ M3C; y +MC and y +MC+ M7C3, y + M7C3 and y +MC+ M7C3);
• cast irons with two double and ternary eutectics (Y +M3C, y + M7C3, y+ M7C3 +MC) and others;
- according to the eutectic morphology:
• spherulitic shape eutectic y + VC;
Fig. 6. Micrographs of irons of the 4th structural class and spectrograms of composition of excessive vanadium carbides (a) and chromium (b), x1000
• eutectics y + (Fe, Cr,) 7C3 and y + (Fe, Cr, V) 7C3+ +VC, having a socket shape in cross-sectional view, and a fan shape in longitudinal section;
- according to the phase morphology, forming eu-tectic:
• branched (fibrous (y + VC));
• compact (grained (y + VC));
• rod (y + Cr7C3).
References
1. Kolokoltsev V.M., Petrochenko E.V., Molochkov P.A. Complex effect on the structure of wear-resistant white cast irons in order to improve operational stability of castings Vestnik Magnitogorskogo gosudar-stvennogo tehnicheskogo universiteta im. G.I. Nosova. [Vestnik of No-sov Magnitogorsk State Technical Univeersity]. 2004, no 4, pp. 23-29.
2. Kolokoltsev V.M., Petrochenko E.V., Molochkov P.A. Structure and wear resistance of chrome-vanadium cast irons. Izvestiya vuzov. Chernaya metallurgia. [Izvestiya VUZ. Ferrous metallurgy]. 2004, no 7, pp. 25-28.
3. Petrochenko E.V., Valishina T.S. The influence of the chemical composition, solidification conditions and heat treatment modes on microstructure features, mechanical properties and special properties of chrome vanadium white cast irons. Izvestiya vuzov. Chernaya metallurgia. [Izvestiya VUZ. Ferrous metallurgy]. 2009, no 2, pp. 39-42.
4. Petrochenko E.V. The relationship of the chemical composition, structure and properties of complex-alloyed white irons in the as-cast condition. Izvestiya vuzov. Chernaya metallurgia [Izvestiya VUZ. Ferrous metallurgy]. 2012, no 3, pp. 51-55.
