Materials and Design 55 (2014) 798–804

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Materials and Design

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A novel high manganese austenitic steel with higher work hardeningcapacity and much lower impact deformation than Hadfield manganesesteel

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.09.057

⇑ Corresponding author at: College of Manufacturing Science and Engineering,Sichuan University, 610065 Chengdu, PR China. Tel.: +86 28 85405320; fax: +86 2885460940.

E-mail address: wenyh@scu.edu.cn (Y.H. Wen).

Y.H. Wen a,b,⇑, H.B. Peng a, H.T. Si a, R.L. Xiong a, D. Raabe b

a College of Manufacturing Science and Engineering, Sichuan University, 610065 Chengdu, PR Chinab Max-Planck Institut für Eisenforschung, Max-Planck Str. 1, 40237 Düsseldorf, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 April 2013Accepted 25 September 2013Available online 8 October 2013

Keywords:Hadfield manganese steelWork hardeningTwinningInterstitialsMartensitic phase transformation

To tackle the problem of poor work hardening capacity and high initial deformation under low load inHadfield manganese steel, the deformation behavior and microstructures under tensile and impact wereinvestigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that thisnew steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Itsimpact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapidincrease of the amount of stress-induced e martensitic transformation account for this unique propertiesin Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twinleads to the anomalous work hardening in Hadfield manganese steel.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hadfield manganese steel, here represented by the nominalcomposition Fe–12Mn–1.2C (wt.% here and throughout), combineshigh toughness and ductility with high work hardening capacityand good resistance to wear [1]. These unique properties make itwidely used in the fields of steelmaking, mining, railroading, andin the manufacture of cement and clay products. However, its highwork hardening properties can be obtained only under heavystress or high load impact. Under low load impact, its work hard-ening properties is poor. In addition, its yield strength is low. Con-sequently, its initial deformation is high in service, and thus it isnot well suited for parts that must resist plastic deformation in ser-vice [1]. Solution strengthening and precipitation strengtheningwere used to improve the yield strength of Hadfield manganesesteel by additions of Cr, Mo, Ti and V elements [1]. However, theadditions of these elements raised cost a lot. Metastable middle-manganese steels, whose Mn contents were reduced to 6–8 wt.%,were developed to address the low work hardening properties un-der low load impact [1]. Under low load impact metastable austen-ite (c) transforms into alpha prime martensite (a0). The formationof a0 thus enhances work-hardening properties [2]. Unfortunately,

the toughness of the middle-manganese steels is much lower thanthat of Hadfield manganese steel [1]. Currently, it is still of signif-icance to improve the work hardening capacity of Hadfield manga-nese steel and to decrease its initial deformation under low loadimpact.

There are two dominating opinions responsible for the excep-tionally high work hardening in Hadfield manganese steel [3–5].One is dynamic strain ageing, and the other is deformation twinsand their interactions. The dynamic strain ageing cannot explainhigher work hardening at low temperature, where the dynamicstrain ageing cannot take place [4]. Recent studies by Karamanand co-workers further argued that the deformation twins andtheir interactions accounted for high work hardening in Hadfieldsteel [6]. However, only the formation of deformation twin is notenough to explain the anomalous hardening of Hadfield steel. ACo–33Ni alloy shows a lower work hardening capacity than Had-field manganese steel although it exhibits the same twinningkinetics as Hadfield manganese steel [4]. Adler and co-workerspostulated that after the formation of deformation twins, largeoctahedral interstitial sites originally occupied by carbon are con-verted to small tetrahedral ones. Unless carbon atoms undergo dif-fusive motions during the lattice shear process, they will betrapped in tetrahedral sites [4]. A high carbon occupation of thesmaller tetrahedral sites must produce a lattice distortion. There-fore, in Hadfield manganese steel the twined region is seriouslydistorted. The recent studies by Bayraktar and co-workers furtherconfirmed that the hardness of twined regions was higher than

a

0.00 0.05 0.10 0.15 0.20 0.25 0.300

200

400

600

800

1000

1200T

rue-

stre

ss (

MP

a)

True-strain

HadfieldFeMnSiC

Y.H. Wen et al. / Materials and Design 55 (2014) 798–804 799

that of untwined regions in Hadfield manganese steel [7]. On thecontrary, the distortion of the twined region in Co–33Ni alloy isvery small because it contains few carbon atoms. As a result, thedeformation twins in Hadfield manganese steel has a higher hard-ness than those in Co–33Ni alloy [4]. The above analyses show thatthe formation of deformation twins and the distortions in them in-duced by carbon may be responsible for the anomalous hardeningin Hadfield manganese steel.

Crystallographically, the {111}h11 �2i twinning process may beregarded as the passage of 1

6 h11 �2i Shockley partial dislocationsalong every {111}c plane and the c ? hcp martensite (e) transfor-mation along every other {111}c plane. Accordingly, the formationof e also converts the octahedral interstitial sites to the tetrahedralones. If enough carbon atoms exist, the concomitant carbon-in-duced distortions will also be produced in e. When the stackingfaults energy (SFE) of c is below about 15 mJ/m2, the e can be in-duced by external stress [8]. In high manganese steels, the additionof silicon element significantly decreases the SFE, but that of car-bon element increases it [9–11]. Accordingly, increasing the siliconcontent and decreasing the carbon content can lower their SFE,leading to the occurrence of stress-induced e transformation. Infact, the studies by Tsuzaki and co-workers clearly revealed thatthe stress-induced e transformation takes place very easily inFe17Mn6Si0.3C shape memory alloy [12]. However, so far few pa-pers reported the deformation behavior of FeMnSiC steels withlower carbon content and much higher silicon content comparedto conventional Hadfield steel although lots of papers reportedthe deformation behavior of twip/trip FeMnC steels [13–21]. Be-cause the start temperature of c ? e transformation of theFe17Mn6Si0.3C alloy is 323 K, some thermal e phase will exist atroom temperature. To avoid the effect of pre-existing thermal ephase on deformation behavior, the carbon content was raised to0.35. In addition, to guarantee a good ductility, the manganese con-tent was increased to 18, and the silicon content was decreased to5. Consequently, a new Fe18Mn5Si0.35C steel was designed, andits deformation behavior under tensile and impact were studied.The results showed that its work hardening rate was much higherthan that of Hadfield manganese steel, and its deformation underlow load impact was much lower than that of Hadfield manganesesteel.

0.00 0.05 0.10 0.15 0.20 0.252000

3000

4000

5000

6000

7000

Wor

k ha

rden

ing

rate

(M

Pa)

True strain

FeMnSiC

Hadfield

b

Fig. 1. (a) True stress–true strain curves and (b) corresponding work hardeningrate–true strain curves of Fe18Mn5Si0.35C and Hadfiled steels.

2. Experimental procedures

The experimental FeMnSiC and Hadfield steels were preparedby induction melting under atmospheric environment using pureiron, electrolytic manganese and graphite. The ingots were forgedinto bars with 15 mm diameter. The bars were solution treatedat 1373 K for 40 min, followed by water quenching. The chemicalcompositions of the tested steels are listed in Table 1. The as-quenched bars were machined into different specimens to deter-mine their mechanical properties, impact ductility and work hard-ening properties. The tensile tests were conducted according toASTM: E-8M. Two specimens of each steel were cylindrical andbutton headed, whose gauge diameter and length were 10 mmand 50 mm, respectively. The tensile tests were performed at astrain rate of 6.67 � 10�4 s�1. Two specimens of each steel forthe impact ductility were Charpy-U shape, whose dimensions were

Table 1Chemical compositions and mechanical properties of experimental alloys.

Alloy Element (wt.%)

C Mn Si Fe

Hadfield steel 1.0 13.30 0.35 Bal.Fe18Mn5Si0.35C steel 0.35 17.80 5.21 Bal.

10 � 10 � 55 mm3. To determine the work hardening propertiesunder impact, the specimens with 10 mm diameter and 12 mmheight were impacted under different energy (50 J/100 J) for differ-ent times. Each test point was determined by three samples.

The impact deformation amount g was calculated by the fol-lowing formula:

g ¼ h0 � h1

h0� 100% ð1Þ

where h0 was the height before impact and h1 the height after impact.The microhardness before and after impact were measured by FM-700 (FUTURE-TECH) with 100 g load and 10 s holding time. The con-stituent phases and their volume fraction were identified by X’PertPro XRD apparatus with a speed of 0.04 degree per second and a CuKa ray. The (200)c, ð10 �11Þe, and (110)a peaks were used to deter-mine the volume fraction of c, e and a0 phases. The optical color etch-ing method was used to distinguish the phases in theFe18Mn5Si0.35C steel using OLYMPUS CK40M optical microscope.The color etching solution comprised 1.2% K2S2O5 and 0.5% NH4HF2

in water. In the color optical micrographs c appears brown, a0 appearsas dark, and e as white except that thin e plate appear as dark lines

Mechanical properties

r0.2 (MPa) rb (MPa) d (%) ak (J/cm2)

352 ± 5 869 ± 10 28.8 ± 3 >300246 ± 8 895 ± 6 19 ± 3 176 ± 13

0100200300400500600

Inte

nsity

(cou

nt) solution treated FeMnSiC steel

0100200300400500600

Inte

nsity

(cou

nt) after rupture

111 γ

1011 ε

200 γ

'110 α

101 ε0 101 ε2

1011 ε

40 42 44 46 48 50 52 54 56 58 60 62 64

40 42 44 46 48 50 52 54 56 58 60 62 640

100200300400500600700

Inte

nsity

(cou

nt)

2θ (°)

solution treated hadfiled steel

0100200300400500600

Inte

nsity

(cou

nt) after rupture

111 γ

200 γ

2θ (°)

a

b

Fig. 2. XRD patterns of solution treated (a) Fe18Mn5Si0.35C and (b) Hadfield steelsbefore and after tensile to rupture.

0.00 0.05 0.10 0.15 0.20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Volu

me

fract

ion

of p

hase

Tensile deformation

ε martensite

α′ martensite

Fig. 4. Volume fraction of e and a0 phases as a function of tensile deformation inFe18Mn5Si0.35C steel.

800 Y.H. Wen et al. / Materials and Design 55 (2014) 798–804

[22]. The deformation twins were examined by the optical micro-scope after the samples were electropolished in a solution of 50 g Na2-

CrO4 and 105 ml glacial acetic acid.

100 μm

Fig. 3. Color optical micrographs of Fe18Mn

3. Results

3.1. Mechanical properties and microstructures after tensile rupture

The mechanical properties of Fe18Mn5Si0.35C steel and Had-field manganese steel at room temperature are also listed in Ta-ble 1. Fe18Mn5Si0.35C steel shows much lower 0.2% proof stressr0.2 and lower impact ductility than Hadfield manganese steel.Note that the impact ductility of Fe18Mn5Si0.35C steel reached176 J/cm2. This value is high enough for engineering applicationsalthough it is much lower than that of Hadfield manganese steel.

The true stress–true strain curves of Fe18Mn5Si0.35C and Had-field manganese steels are shown in Fig. 1a. Based on the data inFig. 1a, their corresponding work hardening rate versus true straincurves were calculated, as shown in Fig. 1b (both the elastic sec-tions and the sections close to rupture are remove). The work hard-ening rate of Fe18Mn5Si0.35C steel first rapidly decreased withincreasing the strain up to 0.10, and then it almost remained un-changed with further increasing the strain up to 0.13. Over 0.15,it rapidly increased as increasing the strain. The work hardeningrate of Hadfield steel gradually decreased with the strain up to12%, and then it almost remained unchanged with further increas-ing the strain up to 0.16. Over 0.17, it increased with the further in-crease of strain. Note that the work hardening rate of

50 μm

α′

ε

5Si0.35C steel after tensile to rupture.

b

150

200

250

300

350

400

450

500

550

600

Surfa

ce h

ardn

ess

(HV)

Impact deformation amount (%)

Hadfield FeMnSiC

0 2 4 6 8 10 12 14 16 18 20

0 20 40 60 80 100180

200

220

240

260

280

300

320

340

Impact times

Surfa

ce h

ardn

ess

(HV)

FeMnSiC

Hadfield

Hadfield

FeMnSiC

0

2

4

6

8

10

12

14

Impa

ct d

efor

mat

ion

amou

nt (%

)a

Fig. 5. (a) Effects of impact times on surface hardness and impact deformationamount under energy of 50 J and (b) the relationship between impact deformationamount and surface hardness in Fe18Mn5Si0.35C and Hadfiled manganese steels.

150

300

450

600

0

150

300

450

600

1.87%

3.22%

coun

t)

'110 α

b '110 α

36 40 44 48 52 56 60 64 68 72 76 800

150

300

450

600

750

Inte

nsity

(cou

nt)

before impact

0

150

300

450

600

750

900

Inte

nsity

(cou

nt)

after 100 times impact

a 111 γ

200 γ

311 γ

2θ (°)

Y.H. Wen et al. / Materials and Design 55 (2014) 798–804 801

Fe18Mn5Si0.35C was much higher than that of Hadfield steel atlower strain or higher strain.

XRD patters show that a few e presented in solution treatedFe18Mn5Si0.35C steel (Fig. 2a). After tensile to rupture, lots of e to-gether with some a0 were produced. However, no obvious e or a0

peaks appeared in the Hadfield steel after tensile to rupture(Fig. 2b). Color optical micrographs clearly shows that some a0

were introduced inside e bands (Fig. 3).

0 1 2 3 4

350

400

450

500

550

600

Har

dnes

s (H

V)

Distance from surface (mm)

FeMnSIC hadfield

Fig. 6. Evolutions of hardness with distance from surface in Fe18Mn6Si0.35C alloyand Hadfield manganese steel after 100 times impact under energy of 100 J.

Fig. 4 shows the volume fraction of phases as a function of ten-sile deformation in Fe18Mn5Si0.35C steel determined by XRDanalysis. The volume fraction of e increased rapidly with increasingthe deformation and reached a maximum of about 0.7 at 0.05, butthat of a0 was negligible. When the deformation is over 0.05, thevolume fraction of e remained almost unchanged, while that of a0

began to increase with the increase of deformation.

3.2. Impact properties and microstructures after impact

Fig. 5a shows evolutions of the impact deformation amount gand the surface hardness with the impact times under energy of

0

150

300

450

600

7500

150

300

450

600

0

1.10%

0.74%

Inte

nsity

(

200 γ

101 ε1

111 γ

101 ε0 101 ε2

38 40 42 44 46 48 50 52 54 56 58 60 62 642θ (°)

Fig. 7. XRD patterns before and after different impact deformation amount. (a)Hadfield manganese steel before and after 100 times impact under 100 J and (b)Fe18Mn5Si0.35C steel after different impact deformation amount.

802 Y.H. Wen et al. / Materials and Design 55 (2014) 798–804

50 J. The g of Hadfield manganese steel rapidly increased with im-pact times, while the g of Fe18Mn5Si0.35C steel slowly increasedand was much lower than that of Hadfield manganese steel. Thesurface hardness of both Fe18Mn5Si0.35C and Hadfield manganesesteels remarkably increased with impact times.

Fig. 5b shows a relationship between the surface hardness andthe g of Fe18Mn5Si0.35C and Hadfield manganese steels, whichwere obtained by different combinations of energy and times ofimpact. At the same g, the surface hardness of Fe18Mn5Si0.35Csteel was much higher than that of Hadfield manganese steel,and their difference increased with increasing the g. This resultclearly shows that the Fe18Mn5Si0.35C steel has higher workhardening rate and much lower deformation than Hadfield manga-nese steel under impact.

Fig. 6 shows the variations in the surface hardness of Fe18Mn5-Si0.35C and Hadfield manganese steels as a function of distancefrom surface after 100 times impact under energy of 100 J. Duringthe depth of 4 mm, the hardness of Fe18Mn5Si0.35C steel was al-ways higher than that of Hadfield manganese steel.

Fig. 7a shows the XRD patterns of Hadfield manganese steelbefore and after 100 times impact under energy of 100 J. Afterimpact, no evidence showed the formation of either a0 or e in Had-field steel, but a obvious broadening of diffraction peaks occurred.However, the stress-induced e was produced after impact inFe18Mn5Si0.35C steel, and its amount increased with the increaseof impact deformation amount. In addition, some a0 were produced

100 μm

a

100 μm

b

Fig. 8. Color optical micrographs of impacted surface of (a) Fe18Mn5Si0.35C

after 1.87% deformation. Color optical micrograhps clearly showsthat some a0 were introduced inside e bands (Fig. 8a), while onlydeformation twins were produced in Hadfield steel (Fig. 8b).

4. Discussion

As above stated, the addition of silicon element significantly de-creases the SFE in high manganese steels, but the addition of car-bon element remarkabled increases it [9–11]. BecauseFe17Mn6Si0.35C steel contains much more silicon and much lesscarbon than conventional Hadfield steel, its SFE is much lower thanthat of conventional Hadfield steel. As the result of much lowerSFE, the stress-induced c ? e transformation easily takes place be-fore the occurrence of dislocations glide. Accordingly, the r0.2 ofFe18Mn5Si0.35C steel is the critical stress for stress-inducedc ? e transformation, rather than the strength of austenite againstslip deformation at room temperature. This is the reason why ther0.2 of Fe18Mn5Si0.35C steel is much lower than that of conven-tional Hadfield steel.

Adler and co-workers postulated that at low strain (below0.05%), Hadfield manganese steel deforms primarily by slip. Athigher strain (from 0.06 to 0.22), it deforms by twining, but theoccurrence of twining first results in dynamic softening effect, asthe occurrence of stress-induced c ? a0 transformation does inmetastable austenite stainless steels [4]. The recent in situ andex situ strain field measurements by Efstathiou revealed that

50 μμm

α′

ε

steel and (b) Hadfield steel after 100 times impact under energy of 100 J.

Y.H. Wen et al. / Materials and Design 55 (2014) 798–804 803

primary and secondary twins may nucleate simultaneously, butthe primary twin-system remains predominantly with increaseddeformation. However, twin–twin intersections and second-ordertwin generation increase with increased deformation [23]. As a re-sult, Hadfield manganese steel exhibits high work hardeningcapacity only under heavy stress or high load. Our recent resultalso confirmed this [24]. On the other hand, the intrusion of lotsof twins must subdivide the grains. As the result of subdivision,the grains are remarkably refined. This may be the reason of peaksbroadening after impact (Fig. 7a).

For the Fe18Mn5Si0.35C steel, the stress-induced c ? e mar-tensitic transformation takes place at the start of plastic deforma-tion, and its amount rapidly increases with the strain and reached0.7 only at 0.05 (Fig. 4). However, the amount of twin in Hadfieldsteel gradually increased to 0.25 as increasing the strain from0.05 to 0.3 [4]. The studies by Koyama and co-workers showed thatthe work hardening rate induced by stress-induced e transforma-tion is greater than that by deformation twin [18]. Accordingly,the rapid increase of the amount of stress-induced c ? e and car-bon-induced its distorsion may be the reason why Fe18Mn5-Si0.35C steel shows a much higher work hardening rate thanHadfield manganese steel at low strain (below 0.05%). At higherstrain (above 0.13), because lots of stress-induced a0 were pro-duced, its work hardening rate rapidly increases. This result alsoindirectly confirms that the formation of distorted deformationtwin induced by the conversion of interstitial sites leads to theanomalous work hardening in Hadfield manganese steel.

During the process of impact, the occurrence of stress-inducedc ? e transformation, on the one hand, can absorb the impact en-ergy [25]. On the other hand, the product of e, as a hard secondphase, can strength austenite matrix. This explains why theFe18Mn5Si0.35C alloy exhibits much lower deformation underlow load impact than Hadfield manganese steel.

On the other hand, Idrissi and co-workers investigated the rela-tionship between the twin internal structure and the work-harden-ing rate of Fe–20Mn–1.2C and Fe–28Mn–3.5Si–2.8Al steels [14,15].Their observations showed that the twins formed in the Fe–Mn–Csteel are thinner and contain a much larger density of sessile de-fects than in Fe–Mn–Si–Al steel. Because the thinner twins full ofsessile dislocations are undoubtedly stronger obstacles against dis-locations movement, they though that the large difference in thework-hardening rate of common Fe–Mn–C and Fe–Mn–Si–Al TWIPsteels can be explained primarily by considering the structure ofthe created mechanical twins [14,15]. However, they did not givethe reason why the thickness of deformation twin was much thin-ner, and more sessile dislocations existed in the Fe–20Mn–1.2Csteel. We argue that the distortion in deformation twin is muchgreater in the Fe–20Mn–1.2C steel than in the Fe–28Mn–3.5Si–2.8Al steel because it contains much more carbon than the Fe–28Mn–3.5Si–2.8Al steel. Consequently, it is very difficult for thedeformation twins in the Fe–20Mn–1.2C steel to thicken. At thesame time, dislocations are introduced to relax the large distortioninside the deformation twin in the Fe–20Mn–1.2C. In other words,the large difference in the work-hardening rate between the Fe–20Mn–1.2C and Fe–28Mn–3.5Si–2.8Al steels results from the largedifference of carbon content, not from the different structure ofdeformation twin. The different structure of deformation twin isthe result of different carbon content.

5. Conclusions

To tackle the problem of poor work hardening capacity and highinitial deformation under low load in Hadfield manganese steel,the carbon-induced distortion in deformation twin resulting fromconversion of octahedral interstitial sites to tetrahedral ones had

been first put forward to explain its anomalous work hardeningcapacity. Based on the fact that the formation of e also convertsthe octahedral interstitial sites to the tetrahedral ones, a novel highmanganese austenitic steel Fe18Mn5Si0.35C was designed withthe aim of promoting stress-induced e transformation under lowload by remarkably decreasing its stacking fault energy. The tensileand impact deformation behavior as well as microstructures wereinvestigated, the following conclusions are obtained:

(1) Fe18Mn5Si0.35C steel shows much higher work hardeningrate at low and high strains than Hadfield steel under tensiledeformation. The easy occurrence and rapid increase of theamount of stress-induced e martensite at low strain areresponsible for this difference. Under impact, it shows notonly much higher work hardening rate but also much lowerdeformation than Hadfield manganese steel. The reason isthat the occurrence of stress-induced e not only can absorbthe impact energy but also can strengthen austenite matrix.

(2) The results indirectly confirms that the formation of dis-torted deformation twin induced by the conversion of inter-stitial sites can account for the anomalous work hardeningin Hadfield manganese steel.

Acknowledgements

The work was supported by the National Natural Science Foun-dation of China (Nos. 50971095, 51171123 and 51271128) and theNatural Science Foundation for Young Scientists of Sichuan Prov-ince in China (No. 2010A01-436). Thanks are also due to the Alex-ander von Humboldt foundation for its support of the stay of Dr.Y.H. Wen at Max-Planck-Institut für Eisenforschung GmbH.

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