Materials Letters 70 (2012) 7–10

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Materials Letters

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Microstructural evolution of high manganese steel solidified undersuperhigh pressure

Bo Han, Sujun Wu ⁎School of Materials Science and Engineering, Beihang University, Beijing, China

⁎ Corresponding author. Tel.: +86 10 82316326; fax:E-mail addresses: hmeng521@126.com (B. Han), wu

0167-577X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.matlet.2011.11.057

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 June 2011Accepted 13 August 2011Available online 27 November 2011

Keywords:High pressureHigh manganese steelSolidificationMicrostructureCarbide

The microstructure of high manganese steel solidified under 6 GPa was examined by using OM, FEGSEM andTEM for analysis of influences of high pressure on phase transformation and the morphology of carbides. Theresults indicate that the microstructure consisting of complete equiaxed dendrites, the mean dendrite armspacing of which was ~18 μm, was remarkably refined under 6 GPa. The change of the morphology of car-bides from needle-like and rhombic carbide (M3C) to nodulized cubic carbide (M23C6) was observed byTEM, which is associated with the undercooling and distribution of trace elements of manganese. The diam-eter of spheroidal carbides is approximately between 150 nm and 200nm.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Solidification microstructure is a complex function of the rate ofsolidification, temperature gradients, composition and several mate-rial characteristics [1]. When taking pressure into consideration, so-lidification microstructure will become more complex. Due to somelimitations of theory and technology, however, the investigation ofpressure on solidification has not been popular [2].

Solidification of metals under superhigh pressure can result in fineand non-equilibrium microstructures [3]. Significant changes canoccur in solid solubility of alloy elements, solid–liquid interface fea-ture and formation of new phases [4]. For example, the transitionfrom γ-phase to ε-phase was observed in the iron under superhighpressure [5]. Diamond can be made from graphite at high tempera-ture and high pressure [6]. Some investigations for the effects of pres-sure on the nucleation process and solidification microstructure oflight alloys, such as Al–Zn, Al–Mg, Al–Si, Al–Ni–Y and Al–Ge havebeen carried out [4,7–9]. Yu et al. [4] studied the non-equilibriummicrostructure of Al–Si alloy formed under superhigh pressure, andindicated that high pressure can significantly enhance the solidsolubility of Si and Al in primary phase. Straumal et al. [7] reportedthat nanograined structure of binary Al–Zn and Al–Mg alloys can beformed under high pressure torsion. Jie et al. [8] showed that highpressure can change the amount of β−Al3Mg2 and a new way toprepare supersaturated Al(Mg) solid solution has been provided. Xustudied the microstructure of Al–Ni–Y [2] and Al–Ge [9] alloys

+86 10 82317108.sj@buaa.edu.cn (S. Wu).

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solidified under high pressure and found that the grains and thesecondary phases are greatly refined. The change of the eutectic Gegrowth mode from facet to no-facet was observed using TEM in theAl–Ge alloy solidified under 5 GPa. Above-mentioned analysis ismainly about eutectic or hyper-eutectic alloy with low meltingpoint. However, the effects of pressure on the solidification of metalwith high melting point, such as ferrous metals, are seldom-reported.

High manganese steel has been widely used in industry due to itshigh wear-resistance. During normal solidification, however, den-drites can easily grow to columnar crystals owing to wide solidifica-tion temperature range, low thermal conductivity and solidificationvelocity, which will reduce the toughness of the steel. Solidificationunder high pressure may have some effects on the microstructureand properties of steels. In this work, microstructural evolution andcarbide formation were investigated for high manganese steel solidi-fied under a pressure of 6 GPa.

2. Method

The chemical composition (wt.%) of the commercial high manga-nese steel used in this work is 1.25C, 16Mn, 0.4Si, 0.0025S, 0.0025P,and Fe (bal.). The sample is a cylinder of φ5×5 mm which waspacked in BN flux during experiment. The sample was heated to1750 °C for 15 min to melt and then cooled down to room tempera-ture at a cooling rate of 25 °C/s through circulated cooling agent. Aconstant pressure of 6 GPa was applied to the cylindrical sample dur-ing heating and cooling. The pressure calibration was carried out atroom temperature by means of the known pressure-induced phasetransitions of Bi and ZnTe at 2.55, 7.7, 9.6 and 12.0 GPa. The cell tem-perature under 6 GPa was measured up to 1750 °C by WRe3–WRe25

8 B. Han, S. Wu / Materials Letters 70 (2012) 7–10

thermocouple. To compare microstructural evolution, water toughen-ing heat treatment was carried out for the sample.

The observation of microstructure was carried out using opticalmicroscope and a Hitachi S-3400 field emission gun scanning electronmicroscopy (FEGSEM) operated at 20 kV equipped with an energydispersive spectrometer (EDS). The phase constituent was analyzedon a D/max-2500/PC X-ray diffractometer (XRD) with CuKα radiationand a JEM-2100 transmission electron microscope (TEM) operated at200 kV.

3. Results and discussions

As is well known, the microstructure of the as-cast high manga-nese steel possesses austenitic matrix and discontiguous net-like car-bides distributed in the interface between phases. Significant changesoccurred, however, in the microstructure of the high manganese steelsolidified under high pressure.

The microstructure of high manganese steel solidified under 6 GPais given in Fig. 1a, which shows a distinctive solidification microstruc-ture consisting of fine equiaxed dendrites. It can be seen that the re-fined equiaxed dendrites have complete dendritic arms, in whichthe length of the second dendrite arms is much smaller than thelong major primary dendrites. Since the equiaxed dendrites were sur-rounded by subcooled liquids and the latent heat can hardly be re-leased during solidification, some second dendrite arms weretherefore formed as rod-like structure. It was found that the mechan-ical properties of various cast alloys and wrought material produced

Fig. 1. Optical micrographs of high manganese steel under different states. (a) Originalstructure of the sample solidified under 6 GPa; (b) Water toughened structure of thesample solidified under 6 GPa.

from cast ingots strongly depend on dendrite arm spacing [10]. Themean dendrite arm spacing of equiaxed dendrites formed under6 GPa pressure is ~18 μm, which is greatly less than that (about170 μm) solidified under normal pressure.

According to Frenkel [11], when the pressure is close to GPa, thediffusing coefficient of solute, DL, is as follows:

DL ¼ D0 exp −PV=RTð Þ ð1Þ

where, D0 is the diffusing coefficient of solute at normal pressure, P ispressure, R is gas constant, T is the temperature of moltenmetal (K), Vis initial volume of a liquid. It can be found that diffusing coefficient ofsolute reduces markedly under high pressure.

The constitutional supercooling criterion under normal pressure isexpressed as follows [12].

GL=νð Þ ≥ mL � C0=DLð Þ � 1−k0ð Þ=k0½ �: ð2Þ

Where, GL is the actual temperature gradient, ν is the solidificationrate, mL is the slope of liquid line, C0 is the original alloy concentra-tion, k0 is the equilibrium partition coefficient, DL is the diffusing co-efficient of solute in liquid. In Eq. (2), the left side representsexternal conditions, whereas the right hand side is determined bythe parameters of alloys.

It can be seen, from Eqs. (1) and (2), that when the applied pres-sure increases, the tendency of constitutional supercooling in thesolid–liquid interface increases due to the decrease of DL. Underhigh pressure, the crystalloids of the alloy can grow along three-dimensions to form dendrites in the supercooled liquid metals andthe dendrite arm length is limited because of the reduction of the dif-fusing coefficient. A small DL can result in the formation of a stablesolute-rich layer in front of the liquid–solid interface, and a relativelywide range of constitutional supercooling. When the constitutionalsupercooling is bigger than the supercooling degree of nucleation,new nucleus can be formed, which will further limit the growth ofthe second dendrite arms. As a result, equiaxed dendrites becomethe main characteristic of the solidification microstructure of metalssolidified under high pressure, as shown in Fig. 1a. Jie [8] has reportedthat the amount of β phase decreases and the microstructure is re-fined with increasing pressure. The grain sizes of the high manganesesteel solidified under 6 GPa are also much smaller than that undernormal pressure.

To reveal the effect of the treatment on the microstructure ofthe steel concerned, the specimen was heated to 1050 °C for30 min and then quenched in water. An optical micrograph illus-trating the microstructure of the water toughened specimen solid-ified under 6 GPa is presented in Fig. 1b. It can be seen that thedendrites become short and incomplete. Metallographic analysisshowed that the treatment has no significant influence on the den-drite arm spacing of equiaxed dendrites and there were no grainboundaries of austenite after the specimen subjected to austeniti-zation at 1050 °C (Fig. 1b), which indicates that the mechanismof nucleation and growth under high pressure is different fromthe classical nucleation theory.

The FEGSEM image of the morphology and the EDS compositionalanalysis of equiaxed dendrites were shown in Fig. 2a and b. The re-sults showed that the average concentration of Mn in the interfacialarea between the equiaxed dendrites is much higher than that inother sections, twice of the content in primary dendrite. During thesolidification, redistribution of Mn in the austenite will occur. Mn gra-dients may be present especially in the austenite where diffusion ismuch slower and the diffusing coefficient of solute elements will befurther decreased under 6 GPa pressure, which may be the reason ofthe high Mn concentration in the late solidified interfacial area.

The mean microhardness value of the interfacial area is ~845 HV,and the hardness of the dendrites is ~230 HV under 6 GPa, which is

b

a

Fig. 2. Typical FE-SEM image of high manganese steel solidified under 6 GPa (a) anddistribution of Mn in different sections (b).

Fig. 3. X-ray diffraction of high manganese steel under different states. (a) Solidifiedunder normal pressure; (b) Solidified under 6 GPa; (c) Water toughened after solidifi-cation under 6 GPa.

a

b

1 2[2,2,4]; [0,1,1]; [1,2,3];Z Z Zγ = = =

23 6M C

Fig. 4. TEMmicrograph of spheroidal carbides in high manganese steel (a) and the cor-responding SAD pattern (b).

9B. Han, S. Wu / Materials Letters 70 (2012) 7–10

higher than that of the specimen solidified under normal pressure.The high hardness of the interfacial area may be attributed to thehigh content of the alloying element Mn or its compounds, whichmay play a role in the improvement of the wear resistance of thesteel.

Fig. 3 shows the result of X-ray diffraction patterns of the highmanganese steel specimens under different states. It can be foundthat a new carbide phase exists in the specimens solidified under6 GPa, different from the M3C carbide formed in the steel solidifiedunder normal pressure. The carbide has been identified as M23C6

through calibration. The main peak of the γ-Fe phase shifts to thehigh angles, indicating the lattice parameter changed due to highpressure. The M23C6 phase disappeared after water tougheningtreatment.

Fig. 4 demonstrates the TEM microstructure and the correspond-ing selected area diffraction (SAD) pattern of the carbide in high man-ganese steel solidified under 6 GPa. The diffraction pattern shows thatit is complex cubic carbide (M23C6) with lattice parametera=10.333 nm. As shown in Fig. 4a, it is a nodular shape carbide dif-ferent from the needle-like and network-like carbides in high manga-nese steel solidified under normal pressure. The size of these finecarbides is about 150 nm–200nm. The constitutional undercoolingunder high pressure is useful to carbide nucleation. Since the proba-bilities of obtaining carbon in different orientations are equivalent,the carbides therefore grow into a spherical shape. Generally, modifi-cation is used to change the morphology of carbides from network-like to spherical shape which can improve toughness and wearresistance of the high manganese steel. The results of XRD and TEMshowed, however, that the spheroidal M23C6 phase can be obtainedunder the condition of high pressure solidification, which may havethe similar beneficial effect on the wear resistance of the steel,without modification.

10 B. Han, S. Wu / Materials Letters 70 (2012) 7–10

4. Conclusion

Effect of high pressure on the solidification microstructure of highmanganese steel and the nature of carbides were studied. Main con-clusions were drawn as follows:

(1). During the solidification process of high manganese steelunder 6 GPa, a distinctive solidified microstructure consistingof equiaxed dendrite was obtained, with a mean dendritearm spacing of ~18 μm.

(2). There are no austenitic grain boundaries in the microstructureof high manganese steel solidified under 6 GPa and that afterwater toughening treatment, which indicates that the mecha-nism of nucleation and growth under high pressure is differentfrom the classical nucleation theory.

(3). The mean hardness of the interfacial area in the microstructuresolidified under 6 GPa is much higher than that of the den-drites, which may be beneficial to the wear resistance.

(4). Complex cubic spheroidal fine carbides (M23C6) formed in thesteel solidified under 6 GPa.

Acknowledgments

The authors are grateful to the School of Earth and Space Sciences,Peking University, and the School of Materials Science and Engineer-ing, Beihang University, for the support of this research and for theprovision of laboratory facilities.

References

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