Influence of surface morphology on fatigue behavior of metastable austenitic stainless steel AISI 347 at ambient temperature and 300°C

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Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, 2 International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal Madeira, Portugal

Influence of surface morphology on fatigue behavior of metastable Influence ofConference surface on morphology on fatigue behavior of metastable XV Portuguese Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal austenitic stainless steel AISI 347 at ambient temperature and 300°C austenitic stainless steel AISI 347 at ambient temperature and 300°C Thermo-mechanical modeling of a high pressure turbine blade of an Marek Smagaa* , Robert Skorupskiaa, Patrick Mayerbb, Benjamin Kirschbb, Jan C. Aurichbb, a* c c , Patrick d Marek Smaga , Robert Skorupski Mayer , Benjamin C.a Aurich , airplane engine Indek Raid , Jörg Seewig , Jiřígas Manturbine , Dietmar Eifleraa, Kirsch Tilmann, Jan Beck c c d a Indek Raid , Jörg Seewig , Jiří Man , Dietmar Eifler , Tilmann Beck P. Brandão , V. Infante , A.M. Deus *

Institute of Materials Science and Engineering, University of Kaiserslautern, P.O. Box 3049,c67653 Kaiserslautern, Germany a b b InstituteaInstitute for Manufacturing Technology andEngineering, Production Systems,University of Kaiserslautern, Box 3049,Kaiserslautern, 67653 Kaiserslautern, of Materials Science and University of Kaiserslautern, P.O. BoxP.O. 3049, 67653 GermanyGermany c b Institute for Measurement and Sensor-Technology,University of Kaiserslautern, P.O. Box 3049, Germany Institute Technology and Production Systems,University of Kaiserslautern, P.O. Box67653 3049, Kaiserslautern, 67653 Kaiserslautern, Germany a for Manufacturing Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, d c Department of Mechanical Institute of Physics of Materials ASCR,ofŽižkova 22, 616 62 Brno, Czech Republic Institute for Measurement and Sensor-Technology,University Kaiserslautern, P.O. Box 3049, 67653 Kaiserslautern, Germany Portugal d b Institute of Physics of Materials ASCR, Žižkova 22,Universidade 616 62 Brno,de Czech Republic IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, a

Portugal CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Portugal c

Abstract

The effect of surface modification by cryogenic turning on fatigue behavior of metastable austenitic stainless steel Abstract The347 effect surface modification by cryogenic turning on at fatigue behavior of metastable austenitic AISI wasofinvestigated in stress-controlled fatigue tests ambient temperature (AT) and 300 °Cstainless in air. steel Five AISI 347surface was investigated in were stress-controlled fatigue at ambient temperature andand 300 °C incryogenic air. Five different morphologies manufactured by thetests variation of turning parameters(AT) – with without During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, different surface morphologies were manufactured byconditions the variation turning parameters –different with and without cryogenic cooling feed turbine velocity as well as by the application ofofpolishing reference surfaces with a very small CO 2 snow especially the highand pressure (HPT) blades. Such cause these partsfor to undergo types of time-dependent cooling well as bythethe application of polishing reference surfaces verytosmall CO 2 snow surface roughness. For afeed comprehensive characterization of the surface and nearfor surface morphology, X-ray diffraction degradation, one ofand which is velocity creep. A as model using finite element method (FEM) was developed, in orderwith to beaable predict surface roughness. characterization of´-martensite the surface andand near surfaceprovided morphology, X-rayindiffraction investigations wereFor performed. ThreeFlight phases (-austenite, -martensite) were the aviation nearthe creep behaviour ofa comprehensive HPT blades. data records (FDR) for a specific aircraft, bydetected a commercial company, were usedperformed. toafter obtain thermal and mechanical data turning for threewithout different flight cycles. In were order to create thethe 3Dnearmodel investigations were Three phases (-austenite, ´-martensite and -martensite) in surface microstructure cryogenic turning while after cryogenic cooling the detected only microstructural neededmicrostructure forwas the γ-austenite. FEM analysis, a HPT blade scrap wasafter scanned, andwithout its chemical and properties were surface after cryogenic turning while turning cryogenic cooling thematerial only microstructural constituent Moreover, residual stress state, micro hardness andcomposition surface roughness play an important obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified constituent wasmorphology. γ-austenite. The Moreover, residual stress state, micro hardness and surface playresponse an important role in surface experimental data on the cyclic deformation behavior androughness stress-strain of all3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The role in surface morphology. The experimental data on the cyclic deformation behavior and stress-strain response all surface are in reported. Reference specimens with purely austenitic surface microstructure showofsuch the overall morphologies expected behaviour terms of displacement was observed, in particular at the trailing edge of the blade. Therefore a surface morphologies are reported. Reference specimens with purely austenitic surface microstructure show the highest plastic strain amplitude during cyclic loading at both AT and 300°C. At elevated temperature these specimens model can be useful in the goal of predicting turbine blade life, given a set of FDR data. highest strain amplitude during cyclic loading both AT and 300°C. At elevated temperature specimens achievedplastic the shortest fatigue life. Martensitic surfaceatlayers induced by cryogenic turning result in these the reduction of achieved the amplitude shortest Published fatigue Martensitic surface layers induced cryogenic resulttemperatures. in the reduction of © 2016 The Authors. by Elsevier B.V.and plastic strain duringlife. cyclic loading significantly enhancebyfatigue life atturning both tested Peer-review under responsibility of the loading Scientificand Committee of PCFenhance 2016. fatigue life at both tested temperatures. plastic strain amplitude during cyclic significantly

© 2017 The Authors. Published by Elsevier B.V. © 2017 The Authors. Published by Elsevier B.V. Finite Element Method; 3D Model; Simulation. Keywords: High Pressure Turbineby Blade; © 2017 Theunder Authors. Published B.V. Peer-review under responsibility of Elsevier the Creep; Scientific Committee ICSI 2017. Peer-review responsibility of the Scientific Committee of ICSIof2017 Peer-review under responsibility of the Scientific Committee of ICSI 2017.

Keywords: metastable austenitic stainless steels; martensite; surface morphology; cryogenic turning; fatigue Keywords: metastable austenitic stainless steels; martensite; surface morphology; cryogenic turning; fatigue * Corresponding author. Tel.: +49-631-205-2762; fax: +49-631-205-2137. E-mail address: [email protected] * Corresponding author. Tel.: +49-631-205-2762; fax: +49-631-205-2137. E-mail address: [email protected] 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review underThe responsibility of theby Scientific Committee of ICSI 2017. 2452-3216 © 2017 Authors. Published Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.150

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Nomenclature ASLp ASLt MSLp MSLt f α´   a,p t a n RS 

austenitic surface layer after mechanical and electrolytic polishing austenitic surface layer turned without CO2 snow cooling martensitic surface layer after mechanical and electrolytic polishing turned with CO 2 snow cooling martensitic surface layer turned with CO2 snow cooling cutting feed [rev/min] b.c.c. (body centered cubic) ´-martensite f.c.c. (face centered cubic) -austenite h.c.p. hexagonal close packed) -martensite plastic strain amplitude [-] total-strain [-] stress amplitude [MPa] nominal stress [MPa] residual stresses [MPa] fraction of magnetic phase, i.e. ferromagnetic ´-martensite measured with FeritscopeTM [FE-%]

1. Introduction According to their excellent mechanical and technological properties as well as their corrosion resistivity, austenitic stainless steels are widely used for components in nuclear power and chemical plants as well as in a great variety of industrial, architectonic and biological applications – for a review see e.g. Lo and Shek (2009). After quenching from solution annealing temperature a large number of technically relevant chromium-nickel stainless steels exhibit austenite in a metastable state. Due to plastic deformation, local phase transformations from paramagnetic austenite into ferromagnetic martensite occur in these alloys Smaga et al. (2008) which can affect the mechanical properties of the material in a positive manner Marshall (1984). Besides typical surface hardening mechanisms like increase of dislocation density or introduction of compressive residual stresses, it is also possible to modify surface morphology of metastable austenitic structure by the deformation induced martensite formation – see Altenberger et al. (1999). In this context, it should be noted that the surface morphology of metastable austenite includes both topographic and microstructural features, i.e. volume fraction of phases, thickness of affected surface layer and distribution of martensite as well as residual stress state and surface roughness (Fig. 1a). (a) (b)

Fig. 1. (a) Schematic representation of surface morphology; (b) fatigue testing specimen during cryogenic turning.

Since fatigue damage generally initiates at the surface Murghrabi (2009) a great effort is permanently exerted to the improvement of surface treatment and finishing processes which can considerably increase fatigue life of metals. Numerous diverse technologies of surface modification, e.g. cryogenic laser shot peening Ye et al. (2012) or abrasive ball blasting and cryogenic deep rolling Meyer (2012) are investigated at present. A possible variation in the morphology at the specimen surface can be realized also by a turning process utilizing carbon dioxide snow as cooling medium to achieve low temperatures in the cutting zone Aurich et al. (2014). In the case of metastable austenitic



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stainless steels this technology allows the net shape machining with simultaneous formation of martensitic layers at the specimen surface (Fig. 1b). The present paper is focused on the characterization of cryogenic turning-induced surface morphologies produced under different turning conditions in metastable austenitic stainless steel AISI 347. Experimental data on the influence of various surface morphologies on the cyclic stress-strain behavior and fatigue life are reported. 2. Experimental setup and investigated material 2.1. Cryogenic turning For cryogenic turning, a CO2 snow cooling system including two nozzles with an exit diameter of 10 mm each was applied. Due to its chemical and physical properties, the CO2 reached the workpiece as a solid-gas mixture with a temperature of -78 °C. One nozzle was applied for a pre-cooling the workpiece in front of the cutting zone and one nozzle was oriented behind the tool to cool the workpiece behind the cutting zone. To assure the required deformations, the passive force, being the crucial factor influencing the mechanical load in the workpiece surface layer, had to be enlarged. Therefore, a chamfered cutting edge (CNMA120416T02020), a negative tool orthogonal rake angle of -6 ° and tool cutting edge inclination of -6 ° were applied. The cemented carbide tool was equipped with a multilayer coating (TiN/TiCN/Al2O3). A comparably low cutting speed of vc = 30 m/min was applied to keep the process energy and, therewith, the generated heat and resulting temperature at low levels. As the cooling penetration of the workpiece decreases with increasing distance from the workpiece surface, a relatively low cutting depth of ap = 0.2 mm was chosen, as higher depths of cut would remove a larger previously cooled workpiece volume. More details about the cryogenic turning process were published elsewhere, see e.g. Aurich et al. (2014), Mayer et al. (2014). To vary the surface morphology generated in cryogenic turning process, two different feed f = 0.15 mm/rev and f = 0.35 mm/rev were selected to manufacture fatigue specimens with Martensitic Surface Layer after turning using the above mentioned CO2 snow-cooling system: MSLt, f=0.15, and MSLt, f=0.35. Futhermore, specimens of the type MSLt, f=0.15 were additionally polished mechanically and electrolytically (MSLp) to eliminate turning induced surface roughness. During polishing the layer with a thickness of 25–35 µm was removed. The diameter of each specimen was thus carefully measured using an optical system with a resolution of 1 µm before fatigue testing. Specimens with an Austenitic Surface Layer after turning (ASLt) with the same parameters as MSLt, f=0.15 specimens but without the C02 snow-cooling were manufactured as a reference. In addition, specimens of the ASLt type were polished mechanically and electrolytically (ASLp). In summary, specimens with five different surface morphologies were obtained, namely (i) MSLt, f=0.15, (ii) MSLt, f=0.35, (iii) MSLp, (iv) ASLt, f=0.15 and (v) ASLp. 2.2. Mechanical testing and analytical methods Tensile tests were performed in a Zwick electro-mechanical testing system with a maximum load capability of 250 kN. Cylindrical specimens for tensile and cyclic tests were machined from the central part of bars. Specimens with a diameter of 6 mm were used for monotonic tensile tests in agreement with the geometry requirements of the German Institute for Standardization (DIN) standards DIN 50125. Stress-controlled fatigue tests were carried out using a servohydraulic testing system MTS 810 with the load ratio of R = –1 and frequency of 5 Hz at ambient temperature (AT) and 300 °C, respectively, in air. For characterization of cyclic deformation behavior by means of stress-strain hysteresis, two extensometers were used. In fatigue tests at ambient temperature an extensometer with a gauge length of 8 mm and in fatigue tests at elevated temperature an extensometer with ceramics clips and a gauge length of 12 mm. For fatigue tests, specimens with a gauge diameter of 7.6 mm and different surface morphologies were manufactured applying the turning parameters specified above with and without CO2 snow cooling (see Fig. 1b). Optical micrographs before mechanical loading were taken with Zeiss Axio Imager Vario Z2 and Leica DM 6000M microscopes. A scanning electron microscope FEI Quanta 600 FEG equipped with EBSD (electron backscattering diffraction) technique was used for detailed characterization of microstructure and orientation of grains. The topography of the investigated specimens was measured using a confocal microscope (CM), Nanofocus, µSurf Explore. Ferromagnetic ´-martensite fraction was determined by a Feritscope™ magnetic sensor. Even though there

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is a linear correlation between the ferrite and ´-martensite content reported by Talonen et al. (2004), in the present work the indicated magnetic fraction is given in the volume percent of ferrite [FE-%]. For phase analysis and determination of residual stresses on the specimen surface and in the near surface regime, Xray diffraction measurements with Cu-Kα-radiation operating at 40 kV and 40 mA at a scan speed of 0.0011 °/s and a spot size of 1.5 mm × 1.5 mm were performed. The penetration depth of Cu-Kα-radiation in austenitic stainless steels is in the range of 4-10 µm. The quantitative analysis of the X-ray diffraction pattern was carried out by the Rietveld method Bish et al. (1988). To minimize the influence of texture, the diffraction profiles were measured at five different tilt angles in the range 0°    40°. Relative errors in the quantification of phases was about 1–3 % for each phase. The layer removal method for this kind of measurements was electrolytic polishing. The residual stress was determined by means of the sin2 method from diffraction peak at the (220) austenite lattice plane. Additionally, measurements of the micro hardness were performed with a computer-controlled Fischerscope H100C system using a Vickers indenter with a force of 100 mN and a load time of 10 s for each indentation. The micro hardness HV0.01 of all surface morphologies was measured at the 40 µm distance below the specimen surface. 2.3. Chemical composition and microstructure The investigated material was metastable austenitic stainless steel AISI 347 (X6CrNiNb1810, 1.4550) delivered as rolled bars with a diameter of 25 mm, stripped in solution annealed state from one single batch. To obtain a fully austenitic microstructure, an additional solution annealing heat treatment at 1050°C for 35 min and quenching in helium atmosphere in an industrial heat treatment furnace was performed. The chemical composition is given in Table 1 including the characteristic parameters of metastabilty, i.e. temperatures M s and Md30 calculated according to the empirical equations given by Eichelman and Hull (1953) and Angel (1954). Table 1. Chemical composition of AISI 347 in weight-% and Ms, Md30 temperatures C

N2

Cr

Ni

Nb

Si

Mn

Mo

Cu

P

S

Co

Fe

MS

Md30

0.024

0.019

17.29

9.25

0.41

0.63

1.55

0.19

0.21

0.023

0.008

0.5

bal.

-81°C

46°C

The fully austenitic microstructure after the additional solution annealing is shown in optical and scanning electron micrographs of a longitudinal section in Fig. 2. The rolling direction of the bars is oriented vertically. Figure 2a shows blue and brown band structure, grain boundaries and annealing twins as typical for austenitic steels. This optical micrograph was made after color etching using Bloch & Weld etching agent, revealing a band structure caused by slight inhomogeneities of the Cr and Ni content, which could not be removed during solution annealing. The blue band correlates with a lower Ni and higher Cr content, while the brown bands indicate higher Ni and lower Cr content Man et al. (2016). Note that this chemically induced band structure could not be observed in optical micrographs after etching with typical etching agents for stainless steels like V2A etchant (Fig. 2b), or in scanning electron micrographs using EBSD technique (Fig. 2c). The EBSD images show a homogeneous crystallographic microstructure with a grain size of 17 µm and very low defect density (Fig. 2c and Fig. 2d). (a)

(b)

(c)

(d)

Fig. 2. Microstructure of AISI 347 steel in the initial state after additional solution annealing. (a) Optical micrograph after color etching with Bloech & Wedl, (b) optical micrograph after etching with V2A, (c) EBSD grain orientation map and (d) EBSD misorientation map.



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3. Results and discussion 3.1. Monotonic properties at ambient and elevated temperature T = 300° At ambient temperature, an ultimate tensile strength of 621 MPa and an elongation after specimen fracture of 51 % were measured (Table 2). The ´-martensite formation was measured (only at AT) insitu during monotonic loading by a FeritscopeTM magnetic sensor (Fig. 3). After tensile testing at 300 °C no ´-martensite was detected due to higher austenite stability at elevated temperature. The achieved tensile strength is 428 MPa and the elongation after specimen fracture was 37 %. Fig. 3. Stress-strain response and ´-martensite formation in AISI 347 steel during tensile tests at AT and T = 300 °C.

Temperature in °C AT 300

Table 2. Mechanical properties and ´-martensite fraction at specimen failure

Rp0.2 in MPa 220 155

UTS in MPa 621 428

A in % 51 37

 in FE-% 33 0.0

3.2. Surface and near surface morphology Figure 4a shows an example of X-ray diffractograms obtained at the surface and at defined distances from the surface up to 285 µm after electrochemical removing of thin material layers from specimens of the MSLt, f=0.15 type. Three phases were detected in this type of surface morphology, namely f.c.c. -austenite, b.c.c. ´-martensite and h.c.p. -martensite.

Fig. 4. (a) X-ray diffractograms at different distances from the surface of specimen with MSLt, f=0.15 surface morphology. Distribution of ´-martensite (b), -martensite (c) residual stresses (d) and micro hardness (e) as a function of distance from the surface for three surface morphologies ASLt, f=0.15, MSLt, f=0.15 and MSLt, f=0.35.

Figure 4b and 4c show the phase distribution of ASLt, f=0.15, MSLt, f=0.15 and MSLt, f=0.35 specimens. Fully austenitic microstructure was confirmed by X-ray measurements for ASLt, f=0.15 surface morphology where no martensite fraction was detected both at the surface and in the near surface layer. A comparison of both MSLt specimens showed that the feed significantly influenced the distribution of ´-martensite (see Fig. 4b). A continuous decrease of the α´-martensite fraction from the maximum value of 18 vol.-% at the surface to 0 vol.-% at 150 µm was found for the small feed of 0.15 mm/rev. At higher feed an increase of α´-martensite fraction from 4 vol.-% at the surface to 32 vol.-% at 37 µm

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from the surface was followed by a continuous decrease up to zero detected at the depth of around 200 µm. For both feeds only γ-austenite and α´-martensite were detected at the surface while ε-martensite was detected only below the surface. The increase of feed led to an increase of the degree of deformation and an increase of the thermal load during the turning process. On one hand, the higher degree of deformation results in higher martensite fraction. On the other hand, with increasing thermal load the tendency to martensite formation is lowered Angel (1954), Hahnenberger et al. (2014). The shift of maximum of α´-martensite fraction for feed of 0.35 mm/rev to a greater depth is thus a consequence of the higher thermal loads at higher feed. Figure 4d shows the residual stress distribution in the MSLt, f=0.15, MSLt, f=0.35, and ASLt specimens. Generally, relatively high tensile residual stresses in the range of 800MPa were detected in the surface of all specimens, independent of the turning parameters and processes. The drop to zero residual stresses occurred within a surface distance of about 50 µm. For higher distances from the surface, residual stresses close to zero were measured. This distribution of macroscopic residual stresses is, generally, typical for turning processes and does not correlate with the austenite-to-martensite phase transformation. Figure 4e shows the development of micro hardness near the surface. The maximum micro hardness was measured for all three surface layers close to the surface. With increasing surface distance the micro hardness approaches to the core hardness of 231 HV0.01. The surface and near surface hardening can be attributed both to the formation of - and/or ’-martensite but also to an increase in dislocation density. Nevertheless, comparing Figs 4b and 4e a clear correlation between the micro hardness and ´-martensite content can be found within a group of various surface morphologies up to the depth of about 150 µm,. The MSLt, f=0.35 specimen shows in this depth range the highest values of micro hardness, the ASLt specimen the smallest values of micro hardness and micro hardness distribution in the MSLt, f=0.15 specimen is lying between above surface morphologies.

Fig. 5: 3D micrographs of different surface morphologies as documented by CM for fatigue testing specimens (martensitic surface layer after mechanical and electrolytical polishing turned with CO2 snow-cooling (MSLp), austenitic surface layer after mechanical and electrolytical polishing (ASLp), martensitic surface layer after turning with the CO2 snow-cooling (MSLt, f=0.15), austenitic surface layer after turning without CO2 snow-cooling (ASLt, f=0.15) and martensitic surface layer after turning with the CO2 snow-cooling (MSLt, f=0.35)).

Besides phase distribution, residual stresses and micro hardness, among important factors influencing fatigue life belongs the surface topography. The surface topographies of all investigated specimen variants measured by confocal microscope are displyed in three dimensions in Fig. 5. The polished reference specimens with (MSLp) and without martensite (ASLp) possess a similar, very flat surface while a considerable roughness is charateristic for the as-turned samples. Topography profiles measured in a surface area of 4.8 mm × 0.8 mm were used to determine the roughness parameter Rz (maximum height of profile) according to ISO 4287:1997. Comparing micrographs obtained for ASLt, f=0.15 and MSLt, f=0.15 samples the effect of CO2 snow cooling on the surface topography can be clearly seen. In case of CO2 snow cooling, the same turning parameters result in smaller surface roughness. During turning without CO2 snow cooling, chip adhesion takes place on the cutting edge and the process is generally not stable. An increase of feed leads to an increase of the roughness parameter Rz. 3.3. Fatigue life Fatigue life of specimens cyclically loaded at ambient temperature with the constant stress amplitude a = 270 MPa is presented for all five investigated surface morphologies in Fig. 6a. Results of fatigue tests performed for all surface morphologies at 300°C with the constant stress amplitude of 180 MPa are shown in Fig. 6b. The stress amplitudes were chosen from S-N curves of conventionally turned (without the CO 2 snow cooling) and mechanically / electrolytically polished specimens published in Skorupski et al. (2014) where at AT and T = 300°C, stress amplitudes of a = 270 MPa and 180 MPa, respectively, led to fatigue failure at numbers of cycles in the range of Nf > 104, i.e. at



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the beginning of the high cycle fatigue regime. The benefit of a martensitic surface layer on fatigue life can clearly be seen at both test temperatures. 1,E+07 107

(a)

a = 270 MPa

1,E+07 107  = 180 MPa a

(b)

MSLp

Nf

ASLp MSLt, f=0.15

MSLt, f=0.35 ASLt, f=0.15

1,E+05 105

Nf

106 1,E+06

106 1,E+06

1,E+05 105

MSLt, f=0.15

MSLp

MSLt, f=0.35 ASLt, f=0.15

ASLp 104 1,E+04

104

1,E+04

1 0.7

2 0.8

32

43

54 Rz in µm

65

76

87

98

1 0.7

2 0.8

32

43

54 Rz in µm

65

76

87

98

Fig. 6. Number of cycles to failure for specimens with different surface morphologies fatigued at (a) AT and (b) 300°C.

At AT, the MSLp specimen achieved even the ultimate number of cycles NU = 2×106 without failure, while the ASLp specimen with comparable surface roughness (see Fig. 5) but turned without CO 2 snow cooling, i.e. purely austenitic microstructure, failed at N = 2×105. Furthermore, an increase of the roughness parameter Rz by a factor of 3 and 10 for the MSLt, f=0.15 and MSLt, f=0.35 specimens, respectively, had no significant influence on fatigue life and both specimens achieved Nf > 105, i.e. in the range of conventionally turned and polished specimens. This result clearly indicates that the well known detrimental effect of surface roughness on fatigue life can be suppressed by martensitic surface layers. Accordingly, specimens without martensitic surface layer and higher roughness, e.g. ASLt, f=0.15, reach smaller number of cycles to failure (see. Fig. 6a). At 300 °C all specimens with martensitic surface layer obtained higher number of cycles to failure compared to the specimens with purely austenitic microstructure independent of surface roughness. The highest number of cycles to failure was achieved for the specimen MSLt,f=0.15 with Rz = 1.8 µm. Interestingly, at 300°C, the specimen with martensitic surface layer after mechanical and electrolytic polishing achieved a smaller number of cycles to failure compared to the specimen with as-turned surface morphology resulting from the same turning parameters. 3.4. Cyclic deformation behavior

Fig. 7. Development of plastic strain amplitude versus number of cycles during fatigue tests at (a) AT and (b) 300°C.

To analyse the influence of surface morphology on cyclic deformation behavior of metastable austenitic stainless steel, the plastic strain amplitudes are plotted versus the number of cycles for all investigated morphologies in Fig. 7. It can be clearly seen that the ASLp specimen achieved the highest plastic strain amplitude during the whole fatigue process at both testing temperatures. At ambient temperature, the initial cyclic softening was followed by significant cyclic hardening due to the gradual formation of deformation induced ´-martensite, as it was proved by in-situ magnetic sensor measurements Smaga et al. (2017a). At 300°C, the longer period of initial cyclic softening was followed by a saturation state up to end of fatigue life only for the ASLp specimen. All other specimens show slight cyclic hardening – see Fig. 7b. The possible reason for this behaviour could be a local formation of deformation

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induced -martensite and very localized ´-martensite formation which was recently observed in metastable austenitic stainless steel AISI 347 cyclically loaded at 300°C in both HCF regime Sorich et al (2014) and VHCF regime Smaga et al. (2017b). Furthermore, very recent detailed scanning electron microscopy investigations of surface after focused ion beam preparation have shown that during turning with appropriately chosen parameters nano-crystalline surface layers were formed. More details were published elsewhere Smaga et al (2017a). Conclusion Cryogenic turning with CO2 snow cooling allows in metastable austenitic stainless steel AISI 347 the production of net-shape geometry combined with the formation of martensitic surface layers due to deformation induced transformation processes. The morphologies of cryogenically machined and conventionally turned specimen surfaces were characterized by the roughness-, phase-, micro hardness- and residual stress measurements. X-ray diffraction measurements showed in cryogenically turned samples the existence of three phases: f.c.c. -austenite, b.c.c. ’martensite and h.c.p. -martensite in the surface layer of thickness up to 300 µm. In stress-controlled fatigue tests at ambient temperature and T = 300°C, the martensitic surface layer resulted in (i) a clear improvement of fatigue life compared to specimens with fully austenitic structure on the surface, even in case of higher surface roughness, and (ii) in a reduction of plastic strain amplitude. Acknowledgements The authors thank the German Research Foundation (DFG) for the financial support within the CRC 926 ‘‘Microscale Morphology of Component Surfaces’’. References Altenberger, I., Scholtes, B., Martin, U., Oettel, H., 1999. Cyclic deformation behavior and near surface microstructure of shot peened or deep rolled austenitic stainless steel AISI 304. Mater. Sci. Eng. A 264, 1–16. Angel, T., 1954. 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