鐵錳鋁合金鋼的M23C6碳化物研究

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行政院國家科學委員會專題研究計畫成果報告

鐵錳鋁合金鋼的 M23C6 碳化物研究研究成果報告(精簡版)

計畫類別：個別型

計畫編號： NSC 97-2221-E-011-009-

執行期間： 97 年 08 月 01 日至 98 年 07 月 31 日執行單位：國立臺灣科技大學機械工程系

計畫主持人：鄭偉鈞

處理方式：本計畫可公開查詢

中華民國 98 年 11 月 19 日

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M

23

C

6

carbide in high manganese steel

Wei-Chun Cheng*

Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei, 106, TAIWAN

*Corresponding author

E-mail address:

[email protected]

Tel: 886-2-27376241.

Abstract

We found the first experimental evidence that M23C6carbide exists in a high manganese steel. The composition of the steel is Fe-20.0 Mn-0.5 C (wt%). The M23C6carbide has a complex FCC crystal structure, and its lattice constant is about 1.057 nm. The M23C6carbide precipitates on the FCC grain boundary, and exhibits a cubic to cubic orientation relationship with one of its neighboring austenitic grains, i.e., [001]_C

// [001]

_and (200)_C

// (200)

_.

Key words: M23C6carbide, high manganese steel, cubic to cubic orientation relationship, ternary alloy

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Introduction

For phase transformations of steels from austenite to ferrite during cooling from high temperatures, Widmanstatten side-plate, massive, and martensitic phases formed in the austenitic matrix [1]. In contrast to the phase transformations of the steels during cooling, ferrite to austenite phase transformations have been found in Fe-Mn-Al alloys. Product phases such as Widmanstatten side-plate [2], massive [3], and 18R type martensitic phases [4] have also been observed in the ferritic matrix of the alloys.

In the isothermal phase transformations of low alloy steels and stainless steels at low temperatures, various carbides, such as MC, M3C, M5C2, M7C3 and M23C6, have been found to precipitate in the steels [5-12]. The crystal structures of the carbides are as follows: MC is cubic (B1), M3C is orthorhombic (D011), M5C2 is monoclinic (C2/c), M₇C₃ is complex hexagonal (P31C), and M₂₃C₆ is complex FCC (D8₄) [13]. The M₃C carbide, named cementite, with an othorombic crystal structure is the well-known carbide in steels. Cementite forms in carbon steels on tempering between 523 and 973 K [1]. The M₂₃C₆carbide has a complex FCC crystal structure.

The M₂₃C₆ carbide precipitates on either austenitic or ferritic grain boundaries of the low alloy steels and stainless steels, and exhibits a cubic to cubic orientation relationship with one of its neighboring grains [6-10].

For the Fe-Mn-Al-C alloys undergoing phase transformations at low temperatures, -carbide [14-17] and M23C6 carbide [18] have been observed to precipitate in the parent grains of the alloys. The-carbide with a L12crystal structure could coherently precipitate in the austenitic matrix of the alloy after being quenched from high temperature. The mechanism for the precipitation of the coherent-carbide in the austenitic matrix during cooling could attribute to the Spinodal decomposition [15,16]. The M23C6 carbide was found once in a Fe-26.6 Mn-8.8 Al-0.61 C (wt%) alloy comprising a dual phase structure of austenite and ferrite [18]. The M23C6

carbide formed on the austenite-ferrite interphase interface. It seems that the M23C6

carbide nucleated on the γ-α interphase boundary, exhibited a cubic to cubic orientation relationship with the austenite grain, i.e., [001]C//[001]γand (200)C//(200)γ

where the subscript c refers to the M₂₃C₆ carbide andto FCC matrix, and grew into the ferrite grain without any orientation relationships.

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M₃C, M₅C₂, M₇C₃and M₂₃C₆ carbides exist in the binary Mn-C alloy system [19]. Lamellae of M3C embedded in ferrite are the essential part of the pearlite in the high manganese steels [20-22]. The M₂₃C₆carbide was found in the binary Mn-C and quaternary Fe-Mn-Al-C systems [18]. No literature has thus far indicated that M₂₃C₆ carbide is a constituent phase in the ternary Fe-Mn-C alloy system. From the above observation, the M₂₃C₆carbide should exist in the ternary Fe-Mn-C system. Therefore, the paper reports the existence of M₂₃C₆carbide in a ternary high manganese steel.

Experimental procedures

Slabs with a composition of Fe-20.0 Mn-0.5 C (wt%) were initially prepared by induction melting. The commercial 1020 steel, carbon, and electrolytic manganese were melted together and cast into 3-kg ingots. After being homogenized at 1473 K for 4 h under a protective argon atmosphere, the ingots were hot forged, cold rolled to plates with a thickness of 2 mm, and cut into a dimension of 10 x 15 mm. The steel plates were solution heat treated at 1373 K for 1 h in a protective argon atmosphere, and quenched into water at room temperature. The as-quenched specimens were aged at 873 K for 100 h.

Samples were sectioned, mechanically polished and etched in a 5% nital solution for optical microscopic observation (OM). Some of the samples were also examined by X-ray diffraction (XRD) in a RIGAKU DMAX-B X-ray diffractometer operated at a maximum power of 12 kW. Samples used for observation by transmission electron microscope (TEM) were mechanically polished into thin foils about 100 m in thickness, and then electro-polished using a twin jet polisher in a 10% HClO4 and 90% CH3COOH solution. The TEM samples were examined in a JEOL JEM 2010 transmission electron microscope operated at 200 kV.

Results and discussion

Figure 1(a) shows an optical micrograph (OM) of the high manganese steel after the solution heat treatment at 1373 K for 1 h. Martensites with irregularly long and thin morphology distributed uniformly in the parent grains. The XRD analysis of the steel with the same heat treatment as that of Fig. 1(a) is shown in Fig. 1(b). The

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XRD analysis illustrates the coexistence of FCC and HCP phases in the high manganese steel. Some peaks with which Miller indexes are underlined come from the FCC matrix and FCC martensite, and the others are from the HCP martensite. The high-temperature phase of the steel at 1373 K is austenite. The austenite phase has been reserved in the as-quenched condition. The martensites formed in the austenitic grains during cooling from high temperature. We verified that the martensites are composed of not only the well-known HCP -martensite, but also FCC micro-twin [23].

Fig. 2(a) displays the OM of the steel after being solution heat treated at 1373 K and aged at 873 K for 100 h. The morphology of the austenitic grains is similar to that of the steel in the as-quenched condition, except on some parts of the grain boundaries. Precipitates have nucleated and grown on some of the austenitic grain boundaries as the arrows indicate the locations of the M23C6 carbide, for example, in Fig. 2(a). The XRD analysis of the alloy with the same aging process as that of Fig.

2(a) is shown in Fig. 2(b). Due to lack of sufficient amount of the precipitate, no peaks coming from the precipitate are revealed in the XRD analysis of Fig. 2(b). Thus, we performed the TEM study in order to identify the crystal structure of the grain boundary precipitate.

Figures 3(a) and (b) show the TEM analysis of the high manganese steel with the same heat treatment as Fig. 2(a). Fig. 3(a) is the TEM bright-field image which reveals the precipitates growing along the grain boundary. The precipitates are marked with C, the austenitic grains with,and the grain boundary with GB as shown in Fig.

3(a). Fig. 3(b) is the selected area diffraction pattern (SAD) covering with both of the precipitate on the upper-middle region and its neighboring austenitic grain at left-side.

Tilting the specimen to some other zone axes of the precipitate, we confirmed that the grain boundary precipitate is M₂₃C₆ carbide. The crystal structure of the M₂₃C₆ carbide belongs to a complex FCC. The lattice parameter of the M₂₃C₆ carbide is approximately equal to 1.057 nm. The zone axes of the M23C6 carbide and FCC matrix are along the [100] direction in the SAD of Fig. 3(b). The Miller indexes of the FCC matrix are underlined to distinguish them from those of M23C6carbide. From the SAD of Fig. 3(b), the cubic to cubic orientation relationship exists between the M23C6

carbide and FCC matrix, i.e., [001]C//[001]γand (200)C//(200)γ. The compositions of the M23C6carbide and austenitic matrix were examined by the EDS equipped in TEM.

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The concentrations of the constituent elements, Fe-Mn, without carbon in a 100%

basis are Fe65.5-Mn34.6 in the M23C6 carbide and Fe78.2-Mn21.8 in the austenitic grain, respectively. Thus, the M₂₃C₆ carbide contains more manganese than that of the austenitic matrix. The result is consistent with the Mn-C binary phase diagram showing that the M₂₃C₆ carbide occupies at high manganese portion of the phase diagram [19].

We have observed the M₂₃C₆ carbide precipitates on the austenitic grain boundary, and it exhibits the cube to cube orientation relationship with one of its neighboring austenitic grains. The orientation relationship is [001]C//[001]γand (200)_C//(200)γ. The M₂₃C₆ carbide was frequently found to exhibit the cubic to cubic orientation relationship with the austenite phase in various alloys [6-10]. The orientation relationship is also consistent with the Lin’s report in the Fe-Mn-Al-C alloy [18]. The original parent phase of the Fe-Mn-Al-C alloy is composed of austenite and ferrite at 1323 K. The M23C6carbide precipitated on the austenite-ferrite interphase boundary of the alloy during the isothermal aging at 823 K. It seems that the M23C6 carbide nucleated on the austenite-ferrite interphase boundary, and had the cubic to cubic orientation relationship with the austenitic grain and grew into the ferritic grain without any orientation relationship. The dual-phase nature of the Fe-Mn-Al-C alloy makes the clarification of the formation mechanism of the M23C6

carbide more difficult in the quaternary Fe-Mn-Al-C alloy.

The high manganese steel is a single phase of austenite at 1373 K. The austenitic grains are the parent phase even in the as-quenched condition. During the isothermal aging process of the high manganese steel at 873 K, the appearance of the M₂₃C₆ carbide belongs to the precipitation transformation. The sequence of the precipitation transformation of the M23C6 carbide is as follows: ’ + M23C6. The

’is a meta-stable supersaturated solid solution at 873 K and it has decomposed into the more stable solid solution and stable M23C6 carbide. The phase has the same crystal structure as’phase, but has a composition closer to the equilibrium state.

The M23C6carbide is a constituent phase in the binary Fe-C alloy system, and it was found once in a quaternary Fe-Mn-Al-C alloy [18]. However, the M23C6

carbide has never been found so far in ternary Fe-Mn-C alloy systems. Thus, the discovery of the M23C6 carbide in the ternary Fe-Mn-C alloy completes the missing portion of the M23C6carbide in the high manganese steels.

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Conclusions

In conclusions, we have investigated that M23C6carbide precipitates along the austenitic grain boundary of the high manganese steel after the solution heat treatment at 1373 K and aging process at 873 K. So far M23C6 carbide was found in the binary Mn-C and quaternary Fe-Mn-Al-C systems, and no literature indicates that M23C6

carbide exists in the ternary Fe-Mn-C system. Thus, the discovery of the M23C6

carbide fills the gap for the absence of the M23C6 carbide in the ternary Fe-Mn-C system. Thus, it is the first experimental evidence to show the M23C6carbide existing in the ternary Fe-Mn-C steels. The M23C6carbide has a complex FCC crystal structure with a lattice parameter of a=1.057 nm. As the M₂₃C₆ carbide precipitates on the austenitic grain boundary, it exhibits the cube to cube orientation relationship with one of its neighboring austenitic grains. The orientation relationship is as follows:

[001]_C//[001]γand (200)_C//(200)γ. The metal constituents of the M₂₃C₆ carbide and austenitic matrix without carbon in a 100% basis are Fe(65.5)-Mn(34.6) and Fe(78.2)-Mn(21.8), respectively. The content of manganese of M₂₃C₆carbide is higher than that of the austenitic matrix.

Acknowledgements

The authors are pleased to acknowledge the financial support of the paper by National Science Counsel, Taiwan, under Grand No. NSC-97-2221-E-011-009.

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Fig 1. (a) OM and (b) XRD analyses of the high manganese steel after being heated at 1373 K and quenched into room temperature water. The Miller indexes of the FCC matrix are underlined.

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Fig 2. (a) OM and (b) XRD analyses of the steel after being quenched from 1373 K and aged at 873 K for 100 h. (The arrows show the precipitates nucleated and grew along the grain boundaries)

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Fig. 3. (a) TEM bright-field image (γ: austenite; C: M23C6; GB: grain boundary), and (b) The selected area diffraction pattern taken from (a) in the region covering the upper-middle M23C6 carbide and the left-side austenitic grain. The directions of the zone axes of the FCC matrix and M23C6 carbide are all along [001]. The Miller indexes of the FCC matrix are underlined.