Anomalous stabilization of austenitic stainless steels at cryogenic temperatures

Author’s Accepted Manuscript Anomalous stabilization of austenitic stainless steels at cryogenic temperatures Michael. Hauser, Marco Wendler, Fabrichnaya, Olena Volkova, Javad Mola

Olga

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To appear in: Materials Science & Engineering A Received date: 6 July 2016 Revised date: 15 August 2016 Accepted date: 19 August 2016 Cite this article as: Michael. Hauser, Marco Wendler, Olga Fabrichnaya, Olena Volkova and Javad Mola, Anomalous stabilization of austenitic stainless steels at cryogenic temperatures, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.08.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Anomalous stabilization of austenitic stainless steels at cryogenic temperatures Michael Hauser1*, Marco Wendler1, Olga Fabrichnaya2, Olena Volkova1, Javad Mola1 1 Institute of Iron and Steel Technology, TU Bergakademie Freiberg, 09599 Freiberg/Saxony, Germany 2 Institute of Material Science, TU Bergakademie Freiberg, 09599 Freiberg/Saxony, Germany * corresponding author: e-mail: [email protected] Abstract The deformation-induced formation of α martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between -196 °C and 400 °C. The steel did not exhibit spontaneous α martensite formation at temperatures as low as -196 °C. Therefore, the critical driving force for the formation of α (-2780 J/mol) was obtained by determining the complementary mechanical energy necessary to trigger the deformationinduced α martensite at 0 °C. Driving forces for the γ→α transformation at other tensile test temperatures associated with the deformation-induced α formation were then obtained by subtracting the mechanical energies applied to trigger the martensitic transformation from the critical driving force. The triggering mechanical energies were obtained from in-situ magnetic measurements which enabled to mark the onset of the γ→α transformation. The driving forces for the γ→α transformation calculated using the preceding method indicated an increase in the stability of austenite which was attributed to changes in the mechanical and physical properties of austenite in the vicinity of the Néel temperature. The method can be used to calculate modified driving forces for the occurrence of the γ→α phase transformation.

Keywords: TRIP steel, Gibbs energy, Néel temperature, mechanical stability, austenitic steel, martensitic transformation

1. Introduction Thermodynamic modeling is a powerful tool in the design of engineering materials. It enables to calculate the Gibbs energies of phases in a given system and provides important information such as phase transformation temperatures [1–7]. For austenitic steels, the T0 temperature at which the Gibbs energies of austenite and ferrite are equal could be obtained from thermodynamic calculations [3,8,9]. Furthermore, with the knowledge of the minimum thermodynamic driving force for the spontaneous martensite (α) formation, the martensite start (Ms) temperature too can be predicted [3,8,9]. 1

Depending on the chemical composition and deformation temperature, deformation mechanisms such as dislocations cell formation, mechanical twinning, and martensitic transformation may occur in the austenite phase of high-alloy steels [10–20]. The martensite commonly forms at intersections of glide bands in the austenite. At low temperatures where martensite formation is enabled, glide bands may consist of stacking fault bundles, εmartensite, and mechanical twins [21,22]. In austenitic steels exhibiting the transformationinduced plasticity (TRIP) effect, the knowledge of the minimum driving force necessary for the martensite formation is of primary importance. The concept of defining a critical driving force for the martensite nucleation was first put forward by Cohen and coworkers [23,24] and has been extended by Ghosh and Olson to include the effect of alloying elements [1]. The ′

critical driving force (

) is equal to the Gibbs energy of the martensitic transformation ′

at the Ms temperature (

′

), namely

′

.

As long as the Ms temperature is known, the critical driving force can be determined from the knowledge of the temperature dependence of the Gibbs energy. The accuracy of such calculations will then depend on the reliability of the thermodynamic data. The thermodynamic database is particularly trustworthy in the region where it has been supported by experimental data [3,4]. Especially at low temperatures, reliability of the thermodynamic data decreases because it is no longer possible to reach equilibrium within a reasonable time. Another factor contributing to the low reliability of thermodynamic databases at low temperatures is the antiferromagnetic-paramagnetic transition of austenite at Néel temperature [25–29]. In this paper, modified Gibbs energies for the →α transformation were obtained by tensile testing of an austenitic TRIP steel between -196 °C and 400 °C and the determination of the triggering stress for the martensitic transformation by in-situ magnetic measurements. The approach used in this work is applicable to other austenitic TRIP steels and may be used to enhance the existing thermodynamic databases.

2. Materials and Methods

The investigated steel was melted in a vacuum induction furnace under a nitrogen partial pressure of 45 kPa before being cast into a water-cooled copper mould with a dimension of 230  35  95 mm3. The chemical composition of the cast steel is given in Table 1. To avoid pore formation in ingots, the nitrogen partial pressure was raised to 150 kPa in the subsequent casting step. 2

Table 1: Chemical composition of the investigated cast steel in mass-%

Alloy Cr19NC17.15

C 0.154

N 0.167

Cr 18.70

Mn 2.94

Ni 4.22

Si 0.52

Fe bal.

The steel was machined to round tensile test specimens with a gauge diameter of 6 mm. To ensure the absence of machining-induced martensite near the surface of tensile specimens, the solution heat treatment was performed after machining. The solution heat treatment aimed at the dissolution of carbides and nitrides likely present in the as-cast microstructure. It also led to the partial homogenization of the steel in the austenite phase field. The solution heat treatment consisted of holding the steel at 1150 °C for 30 minutes under an argon atmosphere. Quasi-static uniaxial tensile tests were performed using a Zwick 1476-type universal testing machine. The initial strain rate was 4  10-4 s-1. With the aid of a thermal chamber which surrounded the tensile specimen and its constraints, different temperatures in the range of -196 °C to 400 °C could be adjusted. For the ex-situ quantification of the ferromagnetic phase content in tensile specimens, a Metis MSAT-type magnetic saturation device equipped with a Lakeshore 480 fluxmeter was used. The equipment enabled the measurement of magnetic flux density after saturation magnetization of specimens cut from the gauge section of tensile specimens. The microstructure was studied by means of electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) techniques in a Zeiss LEO-1530 GEMINI-type field emission scanning electron microscope (FESEM). An in-situ magnetic measurement system was devised to determine the triggering stress for the martensite formation during tensile tests. The magnetic measurement system consisted of two coils. The first coil served to generate an electromagnetic field which magnetized the martensite phase as it formed during tensile loading. The magnetization of martensite phase in tensile specimens induced an electrical potential difference (voltage) in the second coil which was recorded [30]. To calculate Gibbs energies for the austenite (fcc) and martensite (bcc) phases and the Nèel temperature of austenite, the thermodynamic database developed by Franke et al. [31] was used. Calculations were done using the Thermo-Calc software [32].

3. Results and Discussion

Figure 1 shows the stress-strain curves for the tensile test specimens deformed until fracture at temperatures between -196 °C and 400 °C. The highest total elongation of 73% was reached 3

at 60 °C which is almost equal to the Md temperature, namely the highest temperature associated with the deformation-induced martensite formation. The formation of martensite at lower temperatures resulted in a pronounced strengthening and a steady decrease in elongation. Reduction of tensile elongation at temperatures below the Md temperature is a common occurrence in austenitic stainless steels [13,20,33,34]. The highest engineering stress level of 1500 MPa was reached at -80 °C. At deformation temperatures below -80 °C, tensile specimens failed at significantly lower stress levels. This could be related to the occurrence of surface decarburization/denitriding processes during the solution annealing. These processes enable the formation of a continuous martensite layer on the surface of tensile specimens thereby facilitating the nucleation of surface cracks [35,36].

Figure 1: Engineering stress-strain curves of tensile specimens in the temperature range of -196 °C to 400 °C

To specify the deformation mechanisms activated during tensile tests, SEM and EBSD investigations were carried out on tensile specimens deformed until fracture at -196 °C and 70 °C (figure 2). The prior tensile loading axis is horizontal in the micrograph. The ECCI micrograph of figure 2(a) shows the presence of a high density of nearly straight deformation bands in the microstructure of the specimen deformed at -196 °C. This indicates the dominance of planar glide due to the high activity of Shockley partial dislocations at -196 °C [19]. According to the EBSD phase map of the specimen tested at -196 °C (figure 2(c)), many of the deformation bands have transformed to martensite. Due to the coexistence of a small fraction of ε-martensite in the glide bands, the martensitic transformation has likely occurred according to the sequence →→α. This sequence has been similarly observed in the Fe16Cr-6Mn-6Ni (values in mass-%) stainless steel [13,37]. It is well established that the type 4

of deformation-induced processes in austenitic steels depends on the stacking fault energy; as the stacking fault energy decreases, the deformation mechanism changes from perfect dislocations glide to deformation twinning, -martensite formation, and α-martensite formation in that sequence [38]. The dominance of α- and -martensite at the expense of twinning after deformation at -196 °C (figure 2(c)) is in agreement with the reduction in the stacking fault energy at lower temperatures [39]. The ECCI micrograph of the specimen tensile tested at 70 °C (figure 2(b)) shows two austenite grains with different microstructural characteristics. Whereas the microstructure of the austenite grain to the left resembles the microstructure shown in figure 2(a), diffuse contrast changes in the austenite grain to the right imply the dominance of wavy glide [17,18,40]. Abrupt transition in the glide mode across a grain boundary is more consistent with the differences in the crystallographic orientation of the neighboring grains than with the possible segregation of alloying elements in the cast steel. After all, the forces exerted on the leading and trailing partial dislocations of the primary slip system depend on the crystal orientation [19,41]. These forces govern the separation distance of partials, the ease of the cross slip, and eventually the extent of glide planarity. According to the EBSD phase and twin boundary map of the specimen deformed at 70 °C (figure 2(d)), a high density of twin boundaries exist in the microstructure. The formation of deformation twins and -martensite are mechanistically very similar and differ only on the overlapping distance of stacking faults on {111} close-packed planes; whereas stacking faults on successive {111} planes generate deformation twins, their overlap on every second {111} plane results in the formation of -martensite [13]. Accordingly, these two byproducts of stacking faults have been observed to coexist in the microstructure of deformed austenitic steels [42]. The coexistence of twins and -martensite may also be inferred from the EBSD map of figure 2(d). Both of these products can evolve to α-martensite by plastic deformation [43–45]. This can explain the presence of α-martensite in some of the deformation bands in figure 2(d). The occurrence of a high density of deformation twins and the associated twinning-induced plasticity (TWIP) effect justify the high tensile elongations at temperatures in the vicinity of 70 °C. It must be noted that some of the more equiaxed bcc regions in the phase maps of figures 2(b,d) mark the delta ferrite formed during the primary ferritic solidification [46].

5

(a)

(b)

T= -196°C 𝜺= 12%

0

T= 70°C 𝜺= 63%

0

4μm

(c)

T= -196°C 𝜺= 12%

0

4μm

4μm

4μm

(d)

25μm

T= 70°C 𝜺= 63%

0

Figure 2: Microstructures obtained after tensile deformation at different temperatures: ECCI micrographs of specimens deformed at -196 °C (a) and 70 °C (b); EBSD phase map of specimens deformed at -196 °C (c) and 70 °C (d). In EBSD phase maps, red, yellow, and blue denote phases with fcc, hcp, and bcc crystal structures, respectively. 3 twin boundaries in the austenite are demarcated by white lines. The phase fraction of martensite after tensile deformation until fracture at different temperatures is shown in figure 3(a). The delta ferrite content of ~3% has been subtracted from the results. The steel does not form as-quenched martensite at temperatures as low as -196 °C. Therefore, the martensite contents in figure 3(a) represent the deformationinduced martensite only. The highest martensite content of about 78 vol.% was formed at -80 °C. At lower temperatures, the martensite content decreased which might imply a reduction in the driving force for the martensitic transformation. Based on the martensite contents alone, however, it is difficult to confirm the latter statement since the reduction in the martensite content at temperatures below -80 °C was associated with a decrease in both maximum stress and strain levels. Therefore, the stress level required to trigger the deformation-induced martensite formation was used to estimate the driving force for the martensitic transformation at each temperature. Figure 3(b) shows the stress levels required for the formation of 1, 5, and 10 vol.% martensite by tensile deformation at temperatures below the Md temperature. The values are based on in-situ magnetic measurements. For all threshold martensite fractions, the triggering stress initially decreases as the temperature decreases below the Md temperature. Reduction in the external stress required to trigger the 6

20μm

martensitic transformation is in agreement with the increased thermodynamic driving force at lower temperatures [47]. Below 0 °C, however, the triggering stress for the formation of 1 vol.% martensite increases from 530 MPa at 0 °C to 750 MPa at -196 °C. For higher threshold martensite fractions, the transition to the regime of increasing triggering stress began at lower temperatures. An increase in the mechanical stress required to supply the critical driving force for the martensitic transformation indicates a reduction in the chemical driving force. In other words, higher stress levels are needed to compensate for the decreasing chemical driving force for the martensitic transformation. Nevertheless, stabilization of austenite at lower temperature is not predicted by the thermodynamic data indicating that the latter needs to be modified.

(b)

(a)

Figure 3: (a) Temperature dependence of deformation-induced martensite fraction after tensile tests until fracture; (b) engineering stress levels required to trigger the indicated deformationinduced martensite fractions. An alternative approach towards the evaluation of austenite stability is based on the strain dependence of the deformation-induced martensite formation. As long as this is available, the austenite stability can be calculated with the following equation proposed by Sugimoto et al. [48] which has also been used elsewhere [49–51]:

′

In equation (1),

(1)

is the austenite stability parameter,

the deformation-induced martensite fraction, and 7

is the initial austenite fraction,

′

is

is the true strain applied by tensile

deformation. The lower the

value, the higher is the mechanical stability of austenite.

Figure 4 indicates that the austenite stability decreases below the Md temperature and reaches a minimum at -120 °C. At lower temperatures, however, the austenite stability increases which is in agreement with the increased triggering stress for the martensitic transformation.

Figure 4: Austenite stability parameter were calculated using equation (1).

as a function of tensile test temperatures. Values

The increased stability of austenite at low temperatures is often interpreted in terms of the transition in the magnetic state of austenite from paramagnetic above the Néel temperature (TN) to antiferromagnetic below it [52–54]. The transition in the magnetic state of austenite influences physical and mechanical properties such as shear modulus, Young’s modulus, thermal conductivity, thermal expansion, and Poisson’s ratio [25–29,55]. The property changes associated with the magnetic transition of austenite in turn influence the temperature dependence of the Gibbs energy of austenite [26,38,54]. Consequently, the driving force for the martensite formation decreases below TN and the spontaneous or deformation-induced martensite formation may come to a standstill. The Néel temperature of the steel was calculated by Thermo-Calc to be around -126 °C. As shown in Figure 5, abrupt changes occur in the calculated heat capacity of austenite in the vicinity of the Néel temperature.

8

Figure 5: Temperature dependent heat capacity of austenite as calculated by Thermo-Calc

For the thermodynamic description of the martensite formation, a critical driving force has be determined [1]. In austenitic steels with spontaneous martensite formation and a known Ms ′

temperature, the critical driving force is the chemical driving force (

) at the Ms

temperature [9].

′

′

(2)

In austenitic steels with a higher stability and no measurable Ms temperature, the critical driving force can be obtained by calculating the mechanical energy per mole needed to trigger ′

the deformation-induced martensite formation (

) and adding it to the available

′

chemical driving force (

′

′

):

′

(3)

′

The term

may be obtained by integrating the stress-strain curve until trig, namely

the triggering strain for the martensitic transformation: ′

∫

(4)

where v denotes the molar volume. The term trig can be obtained from in-situ magnetic measurements. 9

By applying the preceding method to the tensile test results at 0 °C and using equation (3), the critical driving force for martensitic transformation in the experimental steel was estimated to be about -2780 J/mol. Based on the assumption that the critical driving force for martensite formation is temperature independent and equals the value at 0 °C, the temperature dependence of the Gibbs energy change at lower temperatures can be calculated by subtracting the mechanical energy term at each temperature. The mechanical energies and the Gibbs energy changes obtained in this manner are summarized in Table 2. The Gibbs energy changes using the proposed method are expected to be better approximations of the Gibbs energy changes than the values calculated using the thermodynamic database [31].

Table 2: Mechanical energies to trigger deformation-induced martensite and recalculated Gibbs energy changes at various temperatures Temperature

′

[°C] 0 -40 -80 -120 -150 -196

′

[J/mol]

[J/mol]

-305 -191 -127 -108 -101 -113

-2476 -2589 -2654 -2672 -2679 -2668

In Figure 6, Gibbs energy changes for martensite formation based on the thermodynamic database (dashed line) [31] are compared with the values recalculated using the preceding method which relies on experimental results (solid line fitted to symbols). Whereas the dashed line predicts a continuous increase in the chemical driving force at lower temperatures (down to -196 °C), the chemical driving force based on the revised values remains almost constant below -80 °C which is a better reflection of the austenite stabilities deduced from the triggering stresses shown in Figure 3. The observed deviation is most likely caused by changes in the mechanical and physical properties of austenite near the Néel temperature.

10

Figure 6: Gibbs energy changes for γ→α phase transformations calculated by Thermo-Calc and recalculated with aid of tensile tests with in-situ magnetic measurements

4. Conclusions The deformation-induced formation of α martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between -196 °C and 400 °C. The steel did not exhibit spontaneous α martensite formation at temperatures as low as -196 °C. Therefore, the critical driving force for the formation of α could not be obtained by the calculation of the chemical driving force at the α martensite start temperature. The critical driving force for the formation of α (-2780 J/mol) was therefore obtained by determining the mechanical energy necessary to trigger the deformation-induced α martensite at 0 °C. Driving forces for the formation of α martensite at other tensile test temperatures were then obtained by subtracting the mechanical energies applied to trigger the martensitic transformation from the critical driving force. The triggering mechanical energies for the deformation-induced α martensite formation were determined by in-situ magnetic measurements during tensile tests. Just below the Md temperature, the triggering stress for the formation of α martensite decreased. This trend was however reversed at lower temperatures which implied an increase in the stability of austenite. The raised stability of austenite was attributed to changes in the mechanical and physical properties of austenite in the vicinity of the Néel temperature. The

11

results enable to calculate modified driving forces for the occurrence of the γ→α phase transformation.

Acknowledgements The financial support of the German Research Foundation (DFG) in the framework of Collaborative Research Center 799 is gratefully acknowledged. Thanks are also due to Mr. Benedikt Reichel from the Institute of Material Science for his assistance in SEM and EBSD characterizations.

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