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Vdovin K.N., Gorlenko D.A., Zavalischin A.N.
STRUCTURE CHANGES OF CHROMIUM-NICKEL INDEFINITE CAST IRONS IN HEATING
Abstract. The paper studies the processes taking place in chromium-nickel indefinite cast iron heating. Intervals of these processes and their changes depending on the heating rate were defined.
Keywords: indefinite cast iron, abstraction, phase transformations, carbides, carbon, austenite.
Chromium - nickel cast irons are assigned to the class of alloy, wear resistant cast irons. Long run service ability without damage and with minimal wear is the main requirement for this group of irons [1].
There are considerable shrinkage and thermal stresses in the castings after solidification and cooling due to the production large mass and alloy cast iron low thermal conductivity, which are supplemented by phase strain hardening y ^ a transformation. Therefore, the product usage in the as-cast condition is not desirable, in view of possible product destruction. Long (for about six months) maturing in storehouses is one of the internal stress relieving methods. This is accompanied by a 30% stress relaxation. That is enough to increase service durability and preserve production hardness and, consequently, durability. It is not always economically effectually, because large areas are required. Tempering is the replacement of natural maturing, wherein the relaxation processes are much faster and required time varies from several hours to several days, depending on the product shape and mas-siveness. There is no common opinion concerning the tempering temperature. High temperature promotes more rapid processes and a 70% residual stress relieving, but wherein there is a significant decrease in hardness.
Ageing lower temperature results in only a 30 ... 50% stress relieving, wherein the hardness and wear resistance are less reduced, but the possibility of early cracking increases [2, 3]. Therefore, it is necessary to choose the optimal ratio between the structure, that determines product properties, and stresses by selecting the required tempering temperature.
Therefore, the aim of this work was to study the structure formation and its properties in cast iron during the heating after crystallization.
The studies were carried out on iron samples; iron chemical composition is shown in Table 1.
Table 1
White iron chemical composition, %
C Si Mn S P Cr Ni
3.05-3.20 0.7-1.0 0.75-0.95 0.015 0.045 1.5-1.85 4.0-4.6
Samples were selected from the castings after crystallization. The microstructure was studied with the help of a light microscope MEIJI 2700 at 50- to1000-fold magnification using an image analyzer Thixomet PRO. The processes, taking place during cast iron heating were determined with the help of a thermal analyzer NETZSCH STA 449 F3 Jupiter. The sample heating rate was from 5
to 30°C / min. The average microhardness of carbide and metal base stock was defined using a PMT-3 device, with 100 g load. To fix high-temperature structure, samples were cooled in water; thereby the secondary phase precipitation and austenite disintegration were mitigated.
Derivatograms exhibits some specific peaks, obtained in a 5°C / min heating rate, corresponding to the structural and phase transformations being accompanied by thermal effects (Fig. 1).
Fig. 1. Derivatogram, obtained in cast iron heating
The first reaction to the heating is observed when the temperature is in the range of 250-280°C, herewith with heating rate increasing, the range upper limit shifts to higher temperatures. The second peak at a 5°C / min heating rate occurs when the temperature is in the range of 420 ... 540°C.
With heating speed rising, reaction temperature range increases, along with the simultaneous temperature rising proportionally to the heating rate within the interval boundaries.
The third reaction to the heating is observed when the temperature is in the range of 700 ... 740°C and the temperature at the beginning and at the end does not significantly depend on the heating rate. The fourth reaction
Structure changes of chromium-nickel indefinite cast irons in heating
takes place at temperatures above 830°C, and the starting temperature increases with the heating rate rising.
Exothermic peak at a temperature range of 260 to 280°C agrees with the secondary carbides releasing (Fig. 2a, b).
Endothermic reaction occurring in the temperature range of 350 to 550°C is in agreement with it (Fig. 1). The solid graphite mass is proportional to the ageing time and heat temperature.
Average length,
10 15 20 25
Heating rate, °C/ min
Volume fraction^
Average length,|im
Fig. 2. The iron microstructure in as cast state (a), x200 after tempered at 300°C, 15 minutes ageing and quenched (b), x200 volume ratio and carbides average length dependence upon the heating rate (c)
The temperature reduction decreases the diffusion rate, making carbides precipitation difficult, and the abstraction process almost completely stops when the temperature is below 180°C. The most intense secondary carbides precipitation occurs at 10 ... 15°C / min heating rates (Fig. 2 c). The average microhardness of new extracted carbides increases from 810 to 1490 HV units.
Further heating speed rising produces a decrease in secondary carbides precipitation density, wherein the heating rate does not affect on the secondary carbides size. The average carbides microhardness decreases due to the «cast» carbides prevalence (Table 2).
Table 2
The base stock and carbides hardness and microhardness dependence on the heating rate
Heating rate, °C/ min Hardness HRC The average microhardness, HV
base stock carbides
5 53,6 540...570 810.990
10 52,1 520...550 830.1440
15 54,5 490.550 980.1490
20 54,0 440.480 860.1100
30 53,6 440.490 830.1090
Temperature rising mitigates carbides precipitation and is replaced by a more active process of fine flaky graphite particles precipitation (Fig. 3a, b).
Fig. 3. The cast iron microstructure in as-cast condition (a) (not pickled.) X500; tempered at 550°C, 30 ageing and quenched (b), (not pickled.) x500; graphite nodules volume mass and average diameter dependence upon the heating rate (c)
The precipitation temperature range increasing results in the graphite quantity increasing with the heating rate rising to 15°C / min, and their average size is reduced, because of new impurities formation. With further heating rate increasing, the graphite quantity is reduced due to the precipitation time decreasing, and the average graphite size grows at the expense of the precipitation on the existing impurities (Fig. 3 c).
With ageing time increasing, temperature ranges of carbides and graphite precipitation collide and their simultaneous formation becomes possible. Therefore, it is possible to vary the required ratio of these phases by the heating rate and ageing time variation.
Endothermic reaction at the temperature of 720°C agrees with a-y phase transformation, since other transformations and precipitations are mitigated by high heating rates. The subsequent high cooling rate stabilizes aus-tenite and allows austenite and carbides, formed during crystallization, to be observed in the structure.
Consequently, in heating above 720°C, the cast iron structure is represented by austenite and carbides (Fig. 4), which begin to dissolve at the temperature of about 830°C, that is in agreement with endothermic reaction in the curve (see Fig. 1).
b
b
a
a
c
c
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Vestnik of Nosov Magnitogorsk State Technical University 2013. №5.
Fig.4. The cast microstructure after heating to 740°C, 10 minutes ageing and cooled in water, x100
With heating rate increasing, the temperature of carbides dissolving starting slightly rises and with the increasing of austenitizing temperature and ageing time, carbides volume fraction decreases from 30.5 to 16.0% (Fig. 5).
b
Fig. 5. Cast iron microstructure, x100: a - as-cast, b - after heating up to 1100 0C, 60 min ageing and cooling in water
Along with the carbides fraction reducing (Fig. 6 a) their microhardness decreases, resulting from elements redistribution between the eutectic carbides and the base stock (Fig. 6 b).
After 5 minutes ageing at the temperature of 900°C, the average microhardness falls from 1050 to 1000 HV units, at a 30 minutes ageing it reduces to 840 VH units, and it does not change at the further ageing. Ageing at the temperatures 1000 and 1100°C during one hour leads to the similar reduction of the average microhardness to 660 HV units.
Conclusions
1. Four thermal effects taking place during the process of indefinite chromium - nickel pig iron heating in the temperature range of 20 to 1100°C are observed in the derivatograms.
Ageing time, in in
a
1050
20 40
Ageing time, min b
Fig.6. Volume fraction (a) and average microhardness (b) of carbides, depending on the temperature and ageing time
2. The first effect at the temperatures of 250 to 280°C matches to the secondary carbides precipitation. The graphite releasing takes place at the temperatures of 420 to 540°C. The third thermal effect is observed at the temperatures of 700 to 740°C and is connected with a-y transformation. The latter effect occurs at the temperatures above 830°C and agrees with carbides dissolving.
3. With heating rate increasing, temperatures at the beginning and at the end of thermal effects shift. Temperature at the end of carbide precipitation shifts to the range of higher temperatures; temperature and range of graphite separation boundaries increase, the temperature of a-y transformation is not significantly dependent on the heating rate, the temperature at the beginning of carbides dissolving increases proportionally to the heating rate.
4. In changing heating rate and temperature it is possible to control the phase relationship in indefinite chromium-nickel cast iron, which allows to adjust the tempering mode in order to obtain the required structures and properties.
References
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2. Vdovin K.N., Gimaletdinov R.H., Kolokoltsev V.M. and others. Prokatnye valki. [Forming rolls]. Magnitogorsk, 2005, 543 p.
3. Vdovin K.N., Zaitseva A.A. Heat processing effect on roll-foundry iron, modified with boron. Vestnik Magnitogorskogo gosudarstvennogo tehnich-eskogo universiteta im. G.I. Nosova. [Vestnik of Nosov Magnitogorsk State Technical Univeersity]. 2011, no 4, pp. 13-15.
