Effects of Si on microstructure and phase transformation at elevated temperatures in ferritic white cast irons

    Effects of Si on microstructure and phase transformation at elevated temperatures in ferritic white cast irons A. Wiengmoon, J.T.H. Pearce, S. Nusen, T. Chairuangsri PII: DOI: Reference:

S1044-5803(16)30275-3 doi: 10.1016/j.matchar.2016.08.025 MTL 8359

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

29 April 2016 17 August 2016 23 August 2016

Please cite this article as: Wiengmoon A, Pearce JTH, Nusen S, Chairuangsri T, Effects of Si on microstructure and phase transformation at elevated temperatures in ferritic white cast irons, Materials Characterization (2016), doi: 10.1016/j.matchar.2016.08.025

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ACCEPTED MANUSCRIPT Effects of Si on Microstructure and Phase Transformation at Elevated Temperatures

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in Ferritic White Cast Irons

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Wiengmoon A1*, Pearce JTH2, Nusen S3 and Chairuangsri T3

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Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand.

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Panyapiwat Institute of Management, Nonthaburi 11120, Thailand.

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Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.

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*Corresponding author: Wiengmoon A, Tel: +66-55-96-3561 Fax: +66-55-96-3501 E-mail address:

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[email protected]

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Abstract

The effects of Si on microstructure and phase transformation at elevated temperature

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of ferritic 31wt.%Cr-1.1wt.%C white cast irons with up to 3wt.%Si have been studied. Applications of these irons include parts requiring heat resistance at elevated temperature. The

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irons were produced by sand casting. The microstructure in as-cast condition and after being subjected to high temperature (700 to 1000 oC) was investigated by light microscopy, X-ray diffraction, and electron microscopy. The results revealed that the as-cast microstructure consisted mainly of primary ferrite dendrites and eutectic (ferrite+M7C3). Si promotes M7C3to-M23C6 transformation in the irons subjected to transformation at elevated temperature, but no sigma phase was found. The extent of M7C3-to-M23C6 transformation increases proportional to the increasing transformation temperature, holding time and Si content in the irons. For the iron with 1.0wt.%Si content after holding at elevated temperatures, martensite was also found, which could be attributed to carbon accretion effects in eutectic ferrite. Si was incorporated in M23C6 such that M23C6 containing Si can show darker contrast under SEM-

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ACCEPTED MANUSCRIPT BEI as compared to M7C3; this is the opposite to what has been observed for the cases of typical M23C6 and M23C6 containing Mo or W. The results obtained are important to

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understand the change in properties of ferritic, high chromium irons containing Si subjected to

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elevated temperature.

Keywords : Ferritic white cast iron, Phase transformation, Electron microscopy, Silicon

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1. Introduction

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From the foundry viewpoint, the main reason for adding Si to high chromium cast irons is to control melt oxidation and to increase fluidity. Si forms a thin tenacious layer of

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SiO2 on the surface of the melt hindering the formation of Cr2O3 and thus providing some

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degree of protection against further oxidation. Si has also been reported to make furnace slag less viscous and easier to handle on removal prior to tapping [1]. Less than 0.4wt.%Si in high

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chromium iron melts can lead to difficulties arising from viscous slag [2]. Regarding possible effects on microstructure, Si reduces the hardenability of high

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chromium cast irons and Si contents higher than about 0.4wt.% can promote the formation of pearlite [1-2]. Recently, Jacuinde et al. [3] studied the effects of up to 5wt.% Si on solidification of 16.8wt.%Cr-2.56wt.%C white cast irons (Cr/C ratio about 6.6). Up to 3wt.%Si was found to increase the volume fraction of eutectic carbides and promote austenite-to-martensite transformation during cooling after solidification, whereas over 3wt.%Si promoted structural changes from hypoeutectic to hypereutectic and from austeniticmartensitic to pearlitic-ferritic matrices. Si reduces the C solubility in austenite and raises the martensite start (Ms) temperature and thus the as-cast hardness [1]. The effects of Si on the nature and morphology of eutectic carbides in high chromium cast irons are not fully understood. Si additions were reported [4-5] to result in fewer and

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ACCEPTED MANUSCRIPT slightly larger eutectic carbides which was attributed to the possible inhibition of carbide nucleation by Si. However, in contrast, other works [3,6] have shown that increasing the Si

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content refines eutectic carbides and reduces their connectivity due to the effect of Si on the

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growth behaviour of eutectic carbides. Jacuinde and Rainforth [7] studied the effects of up to

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5wt.% Si on wear of 16.8wt.%Cr-2.56wt.%C white cast irons. The iron with 2wt.%Si gave the best wear resistance due to the finer eutectic carbides in its microstructure, whereas the iron with 5wt.% Si exhibited the worst performance due to the formation of pearlitic matrix.

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During eutectic solidification of irons with compositions in the primary austenite

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phase field, Si tends to be rejected by M7C3 eutectic carbides to segregate in eutectic austenite and hence remains in solution in the matrix [1]. However, it has recently been reported that a

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significant amount of Si can be present in M6C carbides, although it is mostly rejected from

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eutectic M7C3 carbides [8-9]. Boyes [10] examined the effects of (1-2)wt.%Si on the structural changes during holding at elevated temperatures of (33-34)wt.%Cr irons whose compositions

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lie in the primary ferrite phase field. An increase in hardness and loss in toughness of the irons were attributed to the formation of sigma phase and precipitation of secondary carbides in the

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ferrite matrix.

In the present study, the effects of Si on phase transformation during exposure at elevated temperatures in ferritic 31wt.%Cr-1.1wt.%C white cast irons with relatively high Cr/C ratio of about 28 were of interest. Such irons are used in applications that require heat resistance at elevated temperatures. The irons were produced by conventional sand casting. The microstructures in the as-cast condition and after exposure at varying times to elevated temperatures were investigated by light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Phase identification was performed by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and energy-dispersive Xray spectroscopy (EDS) in SEM and TEM.

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ACCEPTED MANUSCRIPT 2. Materials and Methods

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2.1 Production of test materials

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High chromium cast irons were prepared by melting in a magnesia crucible, using an

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electric resistance furnace that could be rapidly heated up to temperatures exceeding 1,500 oC using silicon-carbide rods. The metal was cast into a sand mold to produce cylindrical test bars with a diameter of 25 mm and length of 300 mm. The chemical compositions of the irons

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as determined by spark emission spectrometry are shown in Table 1.

2.2 Phase stability testing at elevated temperatures

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The samples prepared for the phase stability test were taken as sections, 5 mm in

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thickness, cut at 50 mm from the top of the cast test bars. Phase stability testing was carried

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out at 700, 800, 1000 oC for 2, 4 and 8 day holding times in air.

2.3 Characterization

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Metallographic specimens were prepared by conventional means: grinding on silicon carbide papers to 1000 grits and progressive polishing using 6, 3 and 1 m diamond paste. Non-etched specimens were used for phase identification by a Philips X’pert diffractometer with a Cu K X-ray source set to cover a 2 range of 30-130 degrees and to record data at 0.04 degrees steps at a speed of 0.004 degrees/min. Microstructures were investigated using an Olympus BX60M light microscope, a LEO 1455VP scanning electron microscope equipped with an EDAX EDS detector and a JEM JEOL2010 transmission electron microscope equipped with an Oxford, Inca EDS detector, and operated at 200 kV. For LM, metallographic etching was done using a solution containing 5 g of sodium hydroxide and 5 g of potassium ferricyanide in 100 ml distilled water. For SEM, specimens were examined 4

ACCEPTED MANUSCRIPT without and with etching as appropriate. For TEM, thin foils were prepared by twin-jet electropolishing at 15 V and -15 oC using a solution containing 10% perchloric acid and 30%

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of 2-butoxyethanol in ethanol as the electrolyte.

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3. Results and Discussion

3.1 XRD analysis

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Fig. 1 shows examples of XRD patterns obtained for 0.3Si, 1Si, 2Si and 3Si irons, in

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the as-cast condition and after being subjected to high temperature holding treatment. In the as-cast condition, the phases detected by XRD are ferrite and M7C3 (Fig. 1(a)) in all irons with

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different Si content. The absence of 200ferrite peak in the 2Si iron can be attributed to texture

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effects due to growth directions during solidification. After holding at high temperatures in the range of 700 to 1000 oC, M23C6 was additionally detected by XRD as a transformation

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product in all irons, e.g. as shown in Fig. 1(b) for the case of the irons subjected to transformation at 1000 oC for 8 days. The observed peaks from M23C6 are included: 422 at 2

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= 41.629o, 511 at 2 = 44.283, 440 at 2 = 48.449o, 531 at 2 = 50.823o, 822 at 2 = 75.972o and 751 at 2 = 77.829o. The absence of some ferrite peaks in the alloys with higher Si content including 200ferrite, 211ferrite and 220ferrite can be attributed to texture effects.

3.2 Overall microstructure from LM The overall microstructures, as revealed by LM, of all irons in the as-cast condition are shown in Fig. 2. Microstructural constituents in the as-cast condition as seen from LM are dendritic primary ferrite (), eutectic (ferrite+M7C3) and a dark-contrast phase (later identified as Si-rich).

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ACCEPTED MANUSCRIPT Effects of Si on microstructure and phase transformation at elevated temperature are shown in Fig. 3, in which LM images taken from the irons subjected to transformation at 800 o

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C for 8 days are given as examples. At higher magnification in Figs. 3(b to h), it can be seen

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that the eutectic M7C3 carbide has transformed to another phase (later identified as M23C6).

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The two types of carbide can be easily distinguished under the light microscope by their colors after etching i.e. the remaining eutectic M7C3 appears in white color, whereas the phase formed by transformation shows a blue or brown color. For a particular transformation

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temperature and time, the extent of M7C3-to-M23C6 transformation increases proportional to

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the increasing Si content. As later evidenced by SEM-EDS, Si incorporated in M23C6, but not in M7C3. Si addition higher than 1wt.% should provide more Si atoms for substituting M sites

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or even C sites in M23C6 structure and hence promotes M7C3-to-M23C6 transformation.

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Secondary precipitation of fine particles in the ferrite dendrites after exposure at elevated temperatures was also found e.g. in Fig. 3(b) as reported previously by Boyes [10], but this

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precipitation was not extensive.

The effects of holding temperature and time on the extent of M7C3-to-M23C6

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transformation are shown in Fig. 4, where LM images taken from the 2Si iron containing 2.175wt.%Si are given as examples. The extent of M7C3-to-M23C6 transformation increases in proportion to the increasing holding temperature and time.

3.3 SEM investigation on eutectic structure and M7C3-to-M23C6 transformation Fig. 5 shows backscattered electron images (BEIs) in SEM revealing the M7C3-toM23C6 transformation in eutectic structure of the 3Si iron at 800 oC and 1000 oC for 8 days. A core-shell structure in which the M7C3 core is surrounded by the M23C6 shell is clearly observed after 8 days at 800 oC (Fig. 5(a)). After 8 days at 1000 oC, nearly all of the eutectic

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ACCEPTED MANUSCRIPT M7C3 has been transformed to M23C6 (Fig. 5(b)). The darkest phase in both Figs. 5(a and b) was later identified as Si-rich.

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Even though the two carbides (M7C3 and M23C6) can be distinguished using the

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backscattered electron mode in SEM as seen in Fig. 5 (the carbide showing brighter contrast is

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M7C3, while the one with darker contrast is M23C6), it is very important to note that this is opposite to what has been reported in our previous works for the case of M7C3 and M23C6 in 30wt.%Cr-2.3wt.%C [11] and 27wt.%Cr-2.6wt.%C containing Mo or W [8-9], in which

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M23C6 showed brighter contrast due to relatively high Fe/Cr, Mo/Cr and W/Cr atm% ratios

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compared to that in the M7C3. In the present work, the irons contain higher Si content, which is a lighter element than Cr and Fe. Examples of SEM-EDS spectra from the M7C3 and M23C6

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in the 3Si iron held at 800 oC for 8 days are given in Fig. 6. It can be seen that the M7C3

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contains almost no Si, which is consistent with the results in previous work [8-9]. The M23C6 possesses higher Si/Cr atm% ratio than M7C3 such that the average atomic weight of M23C6

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containing Si can be lower than that of M7C3. Hence, the M23C6 appears darker under SEMBEI as compared to M7C3 in the irons containing Si. From the characterization viewpoint,

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care must then be taken in interpretation of SEM-BEI imaging of M7C3 and M23C6 in high chromium cast irons with different chemical composition since opposite contrast effects can occur. The Mo peaks in Fig. 6 are from impurities in the charge materials. In Figs. 5 and 6, the M23C6/ferrite boundaries are seen to have a wavy form suggesting that the M23C6 also grew into the adjacent ferrite matrix, the formation of which may occur as: M7C3 + ferrite  M23C6.

3.4 TEM investigation of the eutectic structure and M7C3-to-M23C6 transformation Fig. 7 shows a bright-field (BF) TEM image and corresponding selected area diffraction patterns (SADPs) confirming the presence of eutectic M7C3 carbide and ferrite

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ACCEPTED MANUSCRIPT matrix in the as-cast structure of the 1Si iron. For comparison, Fig. 8 shows a BF-TEM image and corresponding SADPs for the 1Si iron held at 800 oC for 4 days, during which the eutectic

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M7C3 has partially transformed to M23C6 forming a duplex core-shell structure. Faulting

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contrast can be the unique characteristic identifying that the core structure is M7C3 [12-14].

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Fig. 9 shows a BF-TEM image and corresponding SADP for the eutectic structure, in which the original eutectic M7C3 has been fully transformed to multi-grained M23C6 in the 1Si iron after 4 days at 1000 oC. It is important to note that the formation of sigma phase as found in

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(33-34)wt.%Cr irons with (1-2)wt.%Si subjected to transformation at elevated temperature

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[10] has not been observed in the present study.

In the 1Si iron subjected to transformation at elevated temperatures, TEM also

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revealed the presence of martensite () adjacent to M23C6 in some areas as shown for

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example in Fig. 10. The results from TEM-EDS in Figs. 10(d and e) and the tabulated atm% of elements in martensite and ferrite revealed a higher carbon content in martensite than that

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in the adjacent ferrite. The explanation for the formation of martensite only in the 1Si iron is two-fold. Firstly, addition of 1wt.%Si content may encourage carbon segregation during

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solidification to eutectic ferrite. Secondly, because M23C6 contains less carbon than M7C3, M7C3-to-M23C6 transformation in eutectic carbides may consume silicon and repel carbon into the adjacent eutectic ferrite. Consequently, such carbon accretion in eutectic ferrite allows partial transformation of eutectic ferrite to austenite during exposure at 1000 oC and the austenite later transformed to martensite during cooling. From the isoplethal diagrams reported by Bungardt et al. [15], partial transformation of ferrite to austenite would be possible at 1000 oC for the iron with 30wt.%Cr-2wt.%C, which is comparable to the composition of the 1Si iron in the present experiment. However, such carbon accretion in eutectic ferrite was not severe in the 0.35Si iron such that the eutectic ferrite remained stable in this iron, whereas higher Si content (over 1wt.%Si) provided higher amount of Si retained

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ACCEPTED MANUSCRIPT in the adjacent eutectic ferrite and in turn stabilized the adjacent eutectic ferrite in the 2Si and 3Si irons.

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Regarding the Si-rich phase, TEM-EDS results in Fig. 11 revealed high content of

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silicon and other elements together with oxygen. Corresponding selected area electron

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diffraction in Fig. 11(b) revealed that this Si-rich phase is amorphous. Therefore, it can be proposed that the Si-rich phase in the present study can be amorphous inclusions from slag.

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4. Conclusions

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4.1. The as-cast microstructure of ferritic 31wt.%Cr-1.1wt.%C white cast irons with 0.35wt.%Si up to 2.96wt.%Si consist mainly of primary ferrite dendrites and eutectic

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(ferrite+M7C3).

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4.2 After holding at elevated temperature, M7C3-to-M23C6 transformation was observed in the irons, but no sigma phase was found. The extent of M7C3-to-M23C6 transformation

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increases proportional to the increasing transformation temperature, holding time and Si content in the irons.

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4.3 For the iron with 1.0wt.%Si content after holding at elevated temperatures, martensite was also found, which could be attributed to carbon accretion effects in eutectic ferrite. 4.4 Si incorporated in M23C6 so that M23C6 containing Si can possess darker contrast under SEM-BEI as compared to M7C3. This is the opposite to what is normally observed for these two carbides in irons with lower Si content or where the irons contain Mo or W.

Acknowledgment The authors are grateful for funding support from the Thailand Research Fund (TRF) under Grant No. MRG5480285, Naresuan University, and the Materials Science Research Center, Faculty of Science, Chiang Mai University.

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References

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[1] Laird G, Gundlach R, Rohrig K. Abrasion-resistant cast iron handbook. USA: 2000.

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[2] Davis JR. Cast irons. ASM Specialty Handbook. ASM International;1996.

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[3] Jacuinde AB, Rainforth WM, Mejia I. The role of silicon in the solidification of high-Cr cast irons. Metall Mater Trans A 2013;44A:856-872.

[4] Laird G, Powell GLF. Solidification and solid-state transformation mechanisms in Si

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alloyed high-chromium white cast irons. Met Trans A 1993;24(4):981-988.

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[5] Powell GLF, Carlson RA, Randle V. The effect of Si on the relationship between orientation and carbide morphology in high chromium white irons. J Mater Sci

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1994;29:4889-4896.

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[6] Powell G, Randle V. The effect of Si on the relationship between orientation and carbide morphology in high chromium white irons. J Mater Sci 1997;32:561-565.

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[7] Jacuinde AB, Rainforth WM. The wear behavior of high-chromium white cast irons as a function of silicon and mischmetal content. Wear 2001;250:449-461.

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[8] Imurai S, Thanachayanont C, Pearce JTH, Tsuda K, Chairuangsri T. Effects of Mo on microstructure of as-cast 28wt.%Cr-2.6wt.%C-(0-10)wt.%Mo irons. Mater Charact 2014;90:99-112.

[9] Imurai S, Thanachayanont C, Pearce JTH, Tsuda K, Chairuangsri T. Effects of W on microstructure of as-cast 28wt.%Cr-2.6wt.%C-(0-10)wt.%W irons. Mater Charact 2015;99:52–60. [10] Boyes JW. High-chromium cast irons for use at elevated temperature. Iron and Steel 1966;39:102-109.

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ACCEPTED MANUSCRIPT [11] Wiengmoon A, Chairuangsri T, Brown A, Brydson R, Edmonds DV, Pearce JTH. Microstructural and crystallographical study of carbides in 30wt.%Cr cast irons. Acta

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Mater 2005;53:4143-4154.

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[12] Dudzinski, W, Morniroli JP, Gantois M. Stacking faults in chromium, iron and

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vanadium mixed carbides of the type M7C3, J Mater Sci 1980;15:1387-1400. [13] Pearce JTH, Elwell DWL. Duplex nature of eutectic carbides in heat treated 30% chromium cast iron. J Mat Sci Lett 1986;5:1063-1064.

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[14] Inoue A, Masumoto T. Carbide reactions (M3CM7C3M23C6M6C) during

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tempering of rapidly solidified high carbon Cr-W and Cr-Mo steels, Met Trans A 1980;11A:730-747.

[15] Bungardt K, Kunze E, Horn E. Investigation of structure of the iron-chromium-carbon

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system. Arch Eisenhüttenwes 1958;29:193-203.

Irons

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Table 1 Chemical composition of the irons.

Element (wt.%)

Cr

C

Si

Mn

P

S

Ni

Mo

Cu

Fe

0.3Si

31.59

1.184

0.354

0.252

0.014

0.0019

0.220

0.004

0.013

Bal.

1Si

31.01

1.173

1.035

0.202

0.015

0.0013

0.216

0.005

0.014

Bal.

2Si

31.94

1.140

2.175

0.265

0.015

0.0012

0.215

0.004

0.016

Bal.

3Si

31.54

1.040

2.960

0.223

0.018

0.0052

0.215

0.004

0.017

Bal.

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ACCEPTED MANUSCRIPT Fig. 1 XRD patterns of 31wt.%Cr-1.1wt%.C with 0.3, 1, 2 and 3wt%Si: (a) as-cast and

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(b) subjected to holding for 8 days at 1000 oC.

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Fig. 2 LM images show primary ferrite dendrites, eutectic structure and a Si-rich phase (dark

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contrast) in the as-cast irons: (a) 0.3Si, (b) 1Si, (c) 2Si and (d) 3Si.

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Fig. 3 LM images taken at low and high magnifications show effects of Si content on the magnitude of M7C3-to-M23C6 transformation after holding at 800 oC for 8 days: (a,b) 0.3Si,

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(c,d) 1Si, (e,f) 2Si and (g,h) 3Si.

Fig. 4 LM images taken at low and high magnifications show effects of holding temperature and time on the magnitude of M7C3-to-M23C6 transformation in the 2Si iron: 700 oC, 800 oC

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and 1000 oC for 2, 4 and 8 days.

Fig. 5 SEM-BEIs show M7C3-to-M23C6 transformation in the 3Si iron for 8 days at: (a) 800 o

C and (b) 1000 oC.

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ACCEPTED MANUSCRIPT Fig. 6 (a-b) SEM-BEIs show M7C3-to-M23C6 transformation in the 3Si iron for 8 days at 800 o

C and (c-d) SEM-EDS spectra from the points marked (1) and (2) in (b), respectively. The

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phase with the brighter contrast (1) is M7C3, while that with the darker contrast (2) is M23C6.

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Fig. 7 (a) A BF-TEM image of eutectic M7C3 carbide and ferrite matrix in the as-cast 1Si

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iron, (b-c) corresponding SADPs from M7C3 and ferrite.

Fig. 8 (a) A BF-TEM image of the 1Si iron transformed at 800 oC for 4 days shows a duplex

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core-shell structure in eutectic carbides. The core area with faulting contrast is M7C3 surrounded by multi-grains of M23C6. (b) and (c) are corresponding SADPs from M7C3 and

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and M23C6, respectively.

Fig. 9 (a) A BF-TEM image shows an area where eutectic M7C3 has fully transformed to multi-grains M23C6 in the 1Si iron at 1000 oC for 4 days. (b) is a corresponding SADP from M23C6.

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Fig. 10 (a) A BF-TEM image shows grains of M23C6, martensite () and ferrite () in the 1Si

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iron after transformation at 1000 oC for 4 days. (b-c) and (d-e) are corresponding SADPs and

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TEM-EDS spectra from  and respectively.

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Fig. 11 TEM results from the Si-rich phase in eutectic structure of the as-cast 3Si iron : (a)

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BF-TEM, (b) Corresponding SADP from the Si-rich phase showing amorphous characteristics

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and (c) Corresponding TEM-EDS spectrum from the Si-rich phase.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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ACCEPTED MANUSCRIPT Table 1 Chemical composition of the irons.

Iron grade

Element (wt.%) Cr

C

0.3Si

31.59

1Si

Si

P

S

Ni

Mo

Cu

Fe

1.184 0.354

0.252

0.014 0.0019 0.220

0.004

0.013

Bal.

31.01

1.173 1.035

0.202

0.015 0.0013 0.216

0.005

0.014

Bal.

2Si

31.94

1.140 2.175

0.265

0.015 0.0012 0.215

0.004

0.016

Bal.

3Si

31.54

1.040 2.960

0.223

0.018 0.0052 0.215

0.004

0.017

Bal.

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Mn

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