deformation-induced martensite formation during cyclic deformation of metastable austenitic steel:...

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Materials Science and Engineering A 481–482 (2008) 713–717 Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content U. Krupp a,, C. West b , H.-J. Christ b a Faculty of Engineering and Computer Sciences, FH Osnabr¨ uck, University of Appplied Sciences, 49009 Osnabr¨ uck, Germany b Institut f ¨ ur Werkstofftechnik, Universit¨ at Siegen, 57068 Siegen, Germany Received 31 May 2006; received in revised form 18 December 2006; accepted 18 December 2006 Abstract To study the influence of the parameters carbon content, temperature and total strain amplitude on the deformation-induced martensite formation in metastable 301 austenitic steel, hollow cylindrical fatigue specimens were carburized and decarburized in methane–hydrogen gas mixtures. Fatigue experiments were carried out in a temperature range between RT and T = 100 C while monitoring the fraction of deformation-induced martensite versus the number of cycles by means of a magneto-inductive ferrite sensor. The results show that deformation-induced martensite formation leads to pronounced cyclic hardening. A certain amount of accumulated plastic strain is necessary and a threshold value of the plastic strain amplitude must be exceeded to trigger martensitic transformation. The effect of the carbon content and/or the temperature on the formation of martensite is very strong in such a way that high carbon concentrations and elevated temperatures stabilize the austenite phase. © 2007 Elsevier B.V. All rights reserved. Keywords: Deformation-induced martensite; Austenitic steel; Fatigue 1. Introduction and technical background 1.1. Mechanism of plasticity-induced austenite-to-martensite transformation As a result of monotonic or cyclic deformation, metastable austenitic steels show a phase transformation from fcc austen- ite to metastable martensite and, eventually, martensite [1,2]. This kind of deformation-induced phase transformation has been used to produce high-strength stainless steel prod- ucts, e.g., for the food industry, in gas-supply devices, or for structural purposes. It also provides low-alloy steels with an enormous formability and strength potential by transformation of retained austenite, i.e., the so-called transformation-induced plasticity (TRIP) effect. TRIP steels are used, e.g., for passenger- protecting elements in automotive industry. The stability regime of austenite at room temperature is strongly affected by the alloying elements nickel and chromium, or the respective Ni and Cr equivalents, and can be obtained by means of the Schaeffler diagram ([3], see Fig. 1a). While the Corresponding author. Tel.: +49 541 9692188; fax: +49 541 9693719. E-mail address: [email protected] (U. Krupp). Schaeffler diagram is valid only for spontaneous phase changes at room temperature without mechanical load being applied, the transformation from austenite to ferrite can be promoted by plastic-strain energy. From a thermodynamic point of view, the critical free energy change G for spontaneous transfor- mation at the temperature M s can be reached already at higher temperatures by the deformation energy contribution G mech adding to G thermal . This is schematically shown in Fig. 1b; the respective martensite start temperature M s and the temperature M d30 , referring to 30% deformation that leads to the formation of 50% martensite, can be calculated according to the following empirical relationships [4]: M S = 1305 61.1Ni 41.7Cr 33.3Mn 27.8Si 1667(C + N), (1) M d30 = 413 9.5Ni 13.7Cr 8.1Mn 9.2Si 18.5Mo 462(C + N), (2) where the concentrations of the respective alloying elements have to be inserted in wt.%. One of the most prominent and simple models for deformation-induced martensite formation was developed by 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.211

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Page 1: Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content

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Materials Science and Engineering A 481–482 (2008) 713–717

Deformation-induced martensite formation during cyclic deformation ofmetastable austenitic steel: Influence of temperature and carbon content

U. Krupp a,∗, C. West b, H.-J. Christ b

a Faculty of Engineering and Computer Sciences, FH Osnabruck, University of Appplied Sciences, 49009 Osnabruck, Germanyb Institut fur Werkstofftechnik, Universitat Siegen, 57068 Siegen, Germany

Received 31 May 2006; received in revised form 18 December 2006; accepted 18 December 2006

bstract

To study the influence of the parameters carbon content, temperature and total strain amplitude on the deformation-induced martensite formationn metastable 301 austenitic steel, hollow cylindrical fatigue specimens were carburized and decarburized in methane–hydrogen gas mixtures.atigue experiments were carried out in a temperature range between RT and T = −100 ◦C while monitoring the fraction of deformation-inducedartensite versus the number of cycles by means of a magneto-inductive ferrite sensor. The results show that deformation-induced martensite

ormation leads to pronounced cyclic hardening. A certain amount of accumulated plastic strain is necessary and a threshold value of the plastictrain amplitude must be exceeded to trigger martensitic transformation. The effect of the carbon content and/or the temperature on the formationf �′ martensite is very strong in such a way that high carbon concentrations and elevated temperatures stabilize the austenite phase. 2007 Elsevier B.V. All rights reserved.

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eywords: Deformation-induced martensite; Austenitic steel; Fatigue

. Introduction and technical background

.1. Mechanism of plasticity-inducedustenite-to-martensite transformation

As a result of monotonic or cyclic deformation, metastableustenitic steels show a phase transformation from fcc austen-te to metastable � martensite and, eventually, �′ martensite1,2]. This kind of deformation-induced phase transformationas been used to produce high-strength stainless steel prod-cts, e.g., for the food industry, in gas-supply devices, or fortructural purposes. It also provides low-alloy steels with annormous formability and strength potential by transformationf retained austenite, i.e., the so-called transformation-inducedlasticity (TRIP) effect. TRIP steels are used, e.g., for passenger-rotecting elements in automotive industry.

The stability regime of � austenite at room temperature is

trongly affected by the alloying elements nickel and chromium,r the respective Ni and Cr equivalents, and can be obtained byeans of the Schaeffler diagram ([3], see Fig. 1a). While the

∗ Corresponding author. Tel.: +49 541 9692188; fax: +49 541 9693719.E-mail address: [email protected] (U. Krupp).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.12.211

chaeffler diagram is valid only for spontaneous phase changest room temperature without mechanical load being applied, theransformation from � austenite to � ferrite can be promotedy plastic-strain energy. From a thermodynamic point of view,he critical free energy change �G for spontaneous transfor-

ation at the temperature Ms can be reached already at higheremperatures by the deformation energy contribution �Gmechdding to �Gthermal. This is schematically shown in Fig. 1b; theespective martensite start temperature Ms and the temperature

d30, referring to 30% deformation that leads to the formation of0% �′ martensite, can be calculated according to the followingmpirical relationships [4]:

S = 1305 − 61.1Ni − 41.7Cr − 33.3Mn

−27.8Si − 1667(C + N), (1)

d30 = 413 − 9.5Ni − 13.7Cr − 8.1Mn − 9.2Si

−18.5Mo − 462(C + N), (2)

here the concentrations of the respective alloying elementsave to be inserted in wt.%.

One of the most prominent and simple models foreformation-induced martensite formation was developed by

Page 2: Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content

714 U. Krupp et al. / Materials Science and Engineering A 481–482 (2008) 713–717

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ig. 1. Schematic representation of (a) the Schaeffler diagram and (b) the con-ribution of the mechanical deformation energy �Gmech to the driving force forustenite-to-martensite transformation.

lsen and Cohen [5]. It correlates the plastic deformation εplith the formation of a certain volume fraction of shear-bands

SB as follows:

SB = 1 − exp(−αεpl), (3)

here α is a temperature-dependent constant. Since stackingaults in the {1 1 1} slip planes and shear-band intersectionoints can be considered as preferred �′-martensite nucleationites [6,7], the volume-fraction f�′ of �′ martensite formed as aesult of plastic deformation can be given as

�′ = 1 − exp(−β(1 − exp(−αεpl))n), (4)

ith n being a material-depending exponent and β accountingor the probability that martensite embryos are formed at shear-and intersections.

.2. Deformation-induced martensite formation duringatigue loading

Beside material design and processing, the phenomenon ofeformation-induced martensite formation needs to be takennto account for a proper fatigue-life prediction of stainless-steel

roducts. Martensite formation is connected with a pronouncedolume increase by 2.57% [2,8,9]. This is the reason why thisind of deformation-induced phase transformation is stronglynisotropic and depends on the local stress state: under hydro-

mtCv

ig. 2. (a) �′ martensite in the vicinity of a fatigue crack (�εpl/2 = 0.4) [12] andb) schematic representation of transformation-induced fatigue-crack closure asconsequence of the volume increase during martensite formation.

tatic compression it is suppressed completely, it is moderateor pure shear and reaches a maximum under tension [10]. Cer-ainly, the anisotropy of transformation has implications forhe fatigue damage process of cyclically loaded structures of

etastable austenitic steels. Beside the development of meantresses, the reduction of the fatigue-crack driving force due toransformation-induced crack closure was reported to stronglynfluence the fatigue-crack-propagation rate. This was shown by

ei and Morris [11] for two different stress ratios R = 0.05 and.5, respectively, and supported by microstructural examinationf fatigue crack tips by Stolarz et al. [12] (cf. Fig. 2).

While constant-strain fatigue life in the LCF regime iseduced by pronounced deformation-induced martensite for-ation, the transformation effect is beneficial for the fatigue

trength for HCF conditions. This can probably be attributedo the formation of very fine martensite particles in the areaf local plasticity, which effectively block dislocation motion.ccording to Chanani and Antolovich [13] deformation-induced

artensite formation sets in only when a threshold value in

he plastic-strain amplitude �εpl/2 is exceeded. Mughrabi andhrist [14] and Bayerlein et al. [15] report such a thresholdalue of �εpl/2 = 0.3. Up to the threshold value, deformation is

Page 3: Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content

U. Krupp et al. / Materials Science and Engineering A 481–482 (2008) 713–717 715

Table 1Chemical composition of the AISI301 stainless steel material used in this study(wt.%)

C 0.11Si 0.98Mn 1.34Mo 0.32Cu 0.09Al 0.033Cr 16.8NF

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ppmtshpwsfAISI301 coupons from an initial thickness of 2 mm to an endthickness of 0.8 mm. The reduction in thickness was subdividedin 2, 4, 6, and 12 steps. It was shown that due to the tem-perature effect the martensite volume fraction was by a factor

i 7.41e Balance

overned by slip-band formation and � austenite to � marten-ite transformation, the mechanisms of which depend on theicrostructure of the material considered, such as the grain size

r the chemical composition. Furthermore the temperature has atrong effect. Section 3 of the present paper gives a few examplesf ongoing work to quantify these mechanisms.

. Experimental details

Deformation-induced martensite formation was studiedsing AISI301 austenitic stainless steel of the nominal composi-ion given in Table 1 as reference alloy. Since carbon is known toave a strong effect on the austenite stability (cf. Fig. 1 and Eqs.1) and (2)) decarburization and carburization treatments werepplied to the specimens using methane-hydrogen mixtures at= 1050 ◦C (1323 K) resulting in a carbon activity of

C = K(T ) · pCH4

(pH2 )2 , with log K(T ) = 4649K

T− 5.672.

(5)

To optimize the carburization/decarburization process withespect to homogeneity, reproducibility and the amount ofarbide precipitation, these treatments were accompanied byhermodynamic calculations using the commercial softwareactSageTM in combination with a tailor-made database of theystem Ni–Cr–C–O, as well as by means of chemical analysessing a LECO carbon/sulfur analyzer (cf. [16]). The alloy carbononcentration was varied from cC = 0.01 to 0.12%, maintainingn average grain size of d = 100 �m.

Fatigue experiments were carried out on cylindrical hollowpecimens of 1.5 mm wall thickness, which allow (i) internalooling and (ii) establishment of homogeneous carbon concen-ration profiles by the carburization/decarburization treatmentscf. Fig. 6). MTS 810 and Schenck S56 servohydraulic testingystems were used for fatigue tests under total-strain controlnd fully reversed (R = −1) triangular wave shape (ε = const.)arying the temperature between T = −100 and 22 ◦C. The low-emperature tests were carried out in an Airtemp temperaturehamber, which is operated by liquid nitrogen. During the room-emperature experiments the �′-martensite volume fraction was

ontinuously monitored by means of a ferrite probe (see Fig. 3).t reduced temperatures, the ferrite probe was discontinuouslysed during interruptions of the fatigue tests. The ferrite probeerritscopeTM is based on the magneto-inductive determination

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ig. 3. Experimental set-up for fatigue testing of hollow specimens of metastableustenitic steels.

f the ferromagnetic phase fraction. This allows the separationetween paramagnetic austenite and ferromagnetic �′ marten-ite. However, detection of the metastable � martensite requiredpplication of X-ray diffraction.

. Results and discussion

From the engineering point of view, the deformation tem-erature and the deformation degree are the most importantarameters determining the process of deformation-inducedartensite formation in metastable austenitic steels. To obtain

he desired microstructure, these parameters have to be cho-en carefully. A high degree of deformation leads on the oneand to a high fraction of �′ martensite, on the other handlasticity gives rise to a significant increase in the temperature,hich again stabilizes the fcc austenite phase. This relation-

hip is illustrated by Fig. 4, showing the martensite volumeraction and the temperature increase during cold rolling of

ig. 4. Development of the temperature and the martensite volume fractionuring cold-rolling of AISI301 austenitic stainless steel.

Page 4: Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content

716 U. Krupp et al. / Materials Science and Engineering A 481–482 (2008) 713–717

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Fig. 6. (a) Thermodynamic calculation (FactSageTM) of the concentration ofcarbides in equilibrium with dissolved carbon in AISI 301 stainless steel vs.tpD

pt

f

τ = A exp −B2

+ C exp −D2

. (6)

It should be noted that not only a critical threshold valueof the plastic-strain amplitude but also a certain amount of

ig. 5. Cyclic deformation curves for fully reversed fatigue of AISI301ustenitic stainless steel at three different temperatures.

f almost two higher for multi-step rolling than for two-stepolling.

Certainly, the temperature increase due to plastic strain needso be taken into account when studying the mechanisms ofeformation-induced martensite formation during fatigue load-ng. To avoid even slight temperature changes, the specimensor room-temperature testing were internally cooled.

While elevated temperatures stabilize the fcc austenite phase,ow temperatures promote the martensite formation. This effectan be used for strengthening purposes by cryo-forming [17].ig. 5 shows cyclic deformation curves for three different tem-eratures between T = −100 and room temperature. In the casef the relatively high carbon concentration of cC = 0.09%, roomemperature fatigue at �ε/2 = 0.5 does not cause any pronouncedtrengthening. This is completely different at sub-zero temper-tures: During the initial stage, strong cyclic strengthening inombination with massive deformation-induced martensite for-ation was observed, followed by a saturation stage. Since the

igher martensite volume fraction coincides with a completehange in the stress/stain response, the plastic strain amplitudeithin the saturation stage is small and falls below the critical

hreshold value (cf. Section 1).The decarburization treatments in hydrogen atmosphere were

ound to be most effective in adjusting the carbon concentra-ion. Carburization leads to the formation of internal carbidesf type M23C6 and type M7C3. Fig. 6a shows the results of aactSageTM calculation of the dissolved C concentration besides

he fraction of carbides versus the carbon activity. Fig. 6b illus-rates the development of carbon profiles for decarburization at= 1050 ◦C assuming a diffusion-controlled kinetics.The effect of decarburization on the deformation-induced

artensite formation during cyclic loading is shown in Fig. 7.he cyclic deformation curves are represented in combinationith the course of the martensite volume fraction for three dif-

erent carbon concentrations for tests performed at a total-strainmplitude of �ε/2 = 0.5. After a certain incubation time, cyclictrengthening and martensite formation sets in for the two low-arbon specimens, while the stress response for the specimenith a slightly higher carbon concentration remains almost con-

tant.

The cyclic martensite formation curves for materials show-

ng deformation-induced phase transformation are generally ofigmoidal shape (see Fig. 7). The martensite volume fraction f�′

as been tried to correlate with the number of cycles N and theFa

he carbon activity at T = 1050 ◦C and (b) calculation of the carbon diffusionrofiles during decarburization at T = 1050 ◦C (carbon diffusion coefficient:

C = 2.6472 × 10−11 m2/s) [16].

lastic-strain amplitude �εpl/2 in earlier work [18,19] resultingo the following equation:

�′ = 1 − exp

(−

(N

τ

)n)with

(�εpl

) (�εpl

)

ig. 7. Cyclic deformation curves for fully reversed fatigue of AISI301ustenitic stainless steel with three different carbon concentrations.

Page 5: Deformation-induced martensite formation during cyclic deformation of metastable austenitic steel: Influence of temperature and carbon content

U. Krupp et al. / Materials Science and En

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ig. 8. Deformation-induced martensite (f�′ = 40%) in AISI301 austenitic stain-ess steel after fully reversed fatigue at �ε/2 = 0.6% and T = −40 ◦C.

ccumulated plastic strain needs to be exceeded to initiate theeformation-induced martensite formation. This is manifestedy the incubation time, which can be observed in general duringyclic deformation of metastable austenitic steels (see Fig. 7). Inddition, also an upper limit of the martensite volume fractioneems to exist, but here it needs to be checked, if in total-train control this upper limit is not only due to the shift inhe stress/strain response and could be eliminated by increasinghe plastic strain even more.

As outlined in Section 1 the degree in deformation-inducedartensite formation depends strongly on the local stress state,hich is, even in the case of uni-axially loaded specimens, notomogeneous, but a function of the microstructural parame-ers, i.e., grain size and crystallographic orientations, materialefects and fatigue damage in the form of microcracks. Theicrograph in Fig. 8 shows the martensitic phase formed during

yclic deformation of an AISI301 specimen. The alignment ofhe martensite plates along crystallographic slip bands is clearlyisible. The center grain was obviously oriented in an elasti-ally hard direction, where the plastic strain was not sufficientor phase transformation. The quantitative correlation of the �′hase formed during cyclic loading with the local crystallo-raphic orientation relationships as well as with fatigue cracknitiation and early growth is subject of ongoing research.

. Summary

The phenomenon of deformation-induced martensite forma-ion in metastable austenitic steels is strongly dependent on theoading conditions and the chemical composition. It was shownhat

[

[

gineering A 481–482 (2008) 713–717 717

(i) decreasing the carbon concentration by high-temperaturedecarburization promotes the transformation to martensiteand leads to pronounced cyclic strengthening during strain-controlled fully reversed fatigue loading, and that

ii) fatigue at sub-zero temperatures also destabilizes theaustenitic phase for the benefit of �′-martensite formation.

threshold value of the plastic strain amplitude needs to bexceeded to initiate the phase transformation process. Sinceyclic plasticity generates heat, the temperature effect can onlye quantified, when the specimens are cooled and the tempera-ure is kept constant. First microstructural examinations revealedhat the volume fraction f�′ of deformation-induced �′ marten-ite is not homogeneously distributed within the gauge length.ather, the extent of transformation depends on the local stress

tate and the local microstructural parameters, as well as onamage concentration in form of cracks, which form later in theatigue life.

cknowledgement

The financial support of Deutsche ForschungsgemeinschaftDFG) is gratefully acknowledged.

eferences

[1] P. Marshall, Austenitic Stainless Steels, Microstructure and MechanicalProperties, Elsevier, New York, 1984.

[2] A.F. Padilha, P.R. Rios, ISIJ Int. 42 (2002) 325.[3] A.L. Schaeffler, Met. Prog. Databook 6 (1973) 207.[4] B. Cina, J. Iron Steel Inst. (1954) 406.[5] G.B. Olson, M. Cohen, Metall. Trans. A 6 (1975) 791.[6] K. Spencer, J.D. Embury, K.T. Conlon, M. Veron, Y. Brechet, Mater. Sci.

Eng. A 387–389 (2004) 873.[7] G. Ghosh, G.B. Olson, Acta Metall. Mater. 42 (1994) 3371.[8] P.L. Mongonen Jr., G. Thomas, Metall. Trans. A 1 (1970) 1577.[9] P.L. Mongonen Jr., G. Thomas, Metall. Trans. A 1 (1970) 1587.10] R.W. Neu, H. Sehitoglu, Acta Metall. Mater. 40 (1992) 2257.11] Z. Mei, J.W. Morris Jr., Enging Fracture Mech. 39 (1991) 569.12] J. Stolarz, N. Baffle, T. Magnin, Mater. Sci. Eng. A 319–321 (2001) 521.13] G.R. Chanani, S.D. Antolovich, Metall. Trans. 5 (1974) 217.14] H. Mughrabi, H.-J. Christ, ISIJ Int. 37 (1997) 1154.15] M. Bayerlein, H.-J. Christ, H. Mughrabi, Mater. Sci. Eng. A 114 (1992)

L11.16] C. West, V. Braz Trindade, U. Krupp, H.-J. Christ, Mater. Res. 8 (2005)

469.17] H.J. Maier, B. Donath, M. Bayerlein, H. Mughrabi, B. Meier, M. Kesten,

Z. Metallkd. 84 (1993) 12.18] K. Baldus, unpublished research (diploma thesis), University of Siegen,

1995.19] U. Krupp, H.-J. Christ, P. Lezuo, H.-J. Maier, R.G. Teteruk, Mater. Sci.

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