Thermal fatigue behavior and dislocation substructures of 316-type austenitic stainless steels

Thermal fatigue behavior and dislocation substructures of 316-type austenitic stainless steels

jouM.lot Journal of Nuclear Matenals 191-194 (1992) 672-675 North-Holland nilmeal' mterials Thermal fatigue behavior and dislocation substructures...

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Journal of Nuclear Matenals 191-194 (1992) 672-675 North-Holland

nilmeal'

mterials

Thermal fatigue behavior and dislocation substructures of 316-type austenitic stainless steels A.F. Armas a, I. Alvarez-Armas a and C. Petersen b a CONICET, UNR, lnstimm de F[s~ca Rosario, Bt" 27 de Febrero 210 Bis. 2000 Rosario, Argentina h Kemforschungszentmm Karlsruhe, IMF II, Postfach 3640, D-7500 Karlsruhe 1, Germany

Results of a study on cychc behavmr and dislocation structures of type AISI 3161.. austemhc stainless steel arising from cychc thermal stresses are reported. The test method conmsts m an ohmic heating device to allow a thin tubular test specimen to serve as its own heater, converting any longitudinal thermal deformation of the specimen into elasnc or inelasttc deformatmn The effects of thermal stress cycling on the cychc hardening and/or softening behavior and the accompanying mlcrostructural ,.hanges m the specimen were evaluated" A ojchc hardening is observed for all temperature chahges Th~s hardening is followed by a continuous softemng that occupies the major part of total fatigue life Only for the temperature amphtude of 473-823 K an extended saturation plateau was observed. The dislocation structure formed after failure depends on the temperature amphtude. These structures will evolve from a veins and walls structure for the lowest temperature amphtude to a completely equiaxed cell structure for the h~ghest temperature amphtude.

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1. Introduction The radiation heating arising from the plasma burn phase will produce severe thermal stresses in the first wall of fusion reactors. If one is concerned wtth the effect of thermal stresses in fusion reactors the magmtude of thermal stress is not necessarily the real problem, but the repeated thermal cycles occurring m the material as product of the pulsed operational mode. Repeated thermal cycling will generate a high strata fatigue process which may be sufficiently severe to cause damage in the material. Numerous studies on the cychc behavtor of austenitic stainless steels under isothermal strata controlled conditions have been reported in the hterature [1,2]. However no ~tudy was found on the thermal fatigue behavmr and their corresponding dislocation substructure. In order to contribute to the understanding of the mechanisms of microstructural changes during thermal cycling we report here the results of a thermal fatigue study on type 316L stainless steel.

2. Experimental procedures The material used m this study was from a 30 mm thick plate of austenitic stamless steel type AISI 316L, ISPRA heat no. 12247. The chemical composition is the following: C, 0.022; Cr, 17.40; Ni, 12.34; Mo, 2.30; Mn, 1.82; N, 0.060; Si, 0.46; Cu, 0.20; Co, 0.17; S, 0.001; P, 0.027 wt%. From this material were fabricated - perpendicular to the roUmg direction and in the centerline of the sheet - hollow hourglass shaped samples known as H G R I M specimens (fig. 1).

Fig. 1. Geometry of the HGRIM specimen.

The essentml feature of the measurement method, a modification of the Coffin [3] method, is to convert any longitudinal thermal deformation of the specimen into elasuc or inelastic deformation. A brief description of the test procedure is given in another contribution to this conference [4]. After the specimens had been tested they were examined by transmission electron m~croscopy. Special care was taken to ensure that the observed region would have been in the same distance to the main crack for all faded samples. The observations were done with a 200 kV transmission electron microscope.

3. Experimental results Fig. 2 shows the curves giving the evolution of the tensile peak stress tr T, the compressive peak stress trc,

0022-3115/92/$05.00 © 1992 - Elsevier Science Pubhshers B.V. All nghts reserved

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the mean stress ff = ( t r T / 2 ) + (~rc/2) and the stress range Act = o"T --orC as a function of the number of cycles for a test wRh a temperature variation of 473-933 K. A cyclic hardening followed by a period of softening was observed. Furthermore it is also evident that, in spite of the wide range of temperature variation of the test, the mean stress remains stable, small and positive• This small positive stress could be attributed to the net shortening of the sample as a consequence of the strong plastic strain produced during the first compressive phase which is not completely compensated by the reversed stress. Fig. 3 shows the cyclic hardening-softening curves obtained by plotting only few experimental points of the continuously recorded stress r~.nge A~r for specimens tested under different thermal cyclic conditions of A T - - T H - T L with a constant T L = 4 7 3 K. The selected values of T 8 were: 823, 893, 933 and 973 K. The real curves are similar to the one for Ao~ shown in

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Fig• 4• Cyclic hardening-softening observed in a thermal fatigue test (AISI 316L) and two isothermal strain controlled fatigue tests (from ref. [1]) camed out on AISI 316H.

fig. 2• In fig. 3 we can see that the cyclic behavior of A I S I 316L stainless steel under thermal fatigue is characterized by a pronounced hardening. This short periud of hardening is followed by a period of softening that is much more extended than the previous hardening stage and always culminates with the material failure, except for the lowest temperature amplitude of 473-823 K. For this case the softening period is followed by a saturation plateau which dominates the major part of the total fatigue life•

4. Discussion

The mechamcal behavior observed in this thermal fattgue study is similar to that reported for AISI 316H [1] and AISI 316L [2] cycled under isothermal strain controlled conditions• In fig. 4 the cychc behavior observed in a thermal cyclic test with temperature amplitude of 473-823 K and two selected curves obtained [1] from strain controlled tests performed at 523 and 823 K are represented. The isothermal results correspond to low cycle fatigue tests carried out on AISI 316H with AEt=0.01 and d t = 2 × 1 0 -3 s - I . It is evident that, despite of the more pronounced cyclic hardening rate and the smaller saturation stress observed in the isothermal curves, the behavior looks simdar for both types of tests. In fact, cyclic hardening followed by a short softening leading to an emended saturation period is the feature of the thermal test. Corresponding to this similar mechanical behavior, the dislocation structures that AISI 316L can form under thermal fatigue, according to the applied temperature amplitude, are also similar to that obtained in isothermal strain controlled tests. The typical dislocation structures reported in the hterature for low cycle fatigued austenitic stainless steels as veins and walls,

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A.F. Armas et al / Thermal fatigue behat'tor of AIS! 3161_,

Fig. 5. Well developed veins and walls structure observed for AT = 473-823 K

labyrinth, PSBs, layers and cells, were all observed in the present work. Fig. 5 shows the well developed veins and walls structure observed on samples cycled up to failure in a test with temperature amplitude of 473-823

K. Only planar arrangements of dislocations generated in austenit~c steels when they are cycled m isothermal tests with low strain amplitudes [2] or at low temperatures [1] were never observed under thermal fatigue.

Fig 6. Dwect transformation from a wall structure into equiaxed cells observed in a sample thermal fatigued up to fadure with 473-933 K.

A F. Armas et al / Thermalfangue behat'lorofAISl316L

The smaller cychc hardening rate observed in hg. 4 for the thermal fatigue test, the higher saturation stress range reached and the lack of a dislocation structure characteristic for low strain fatigue arc evidences that thermal fatigue tests belong to the high strain fatigue category. Observations made on several thin foils and on grams with different orientations permit to assert that, similar to results described in the literature [2] for isothermal tests carried out on AIS! 316L, a marked evolution of the dislocation arrangements can be seen with cycling Depending on thc cychc temperature amphtude, different types o! dlslocaUon structures have been observed m the specimens cycled up to failure. Increasing the temperature amphtude the final predominant structure wdl evolve from a veins and walls structure (fig. 5) to a cell structure. It ts proposed in this work that the dislocation structure evolution for AISI 316L thermal fatigued will be analogous to that observed under isothermal fatigue in this material [2]. That is, the evolution operates, indirectly, by using a labyrinth structure or directly from a wall structure into eqmaxed cells. Fig. 6 is a striking picture showing this direct transformation m a sample thermal fatigued up to failure with AT = 473-933 K.

5. Conclusions

The mechanical behavior and the corresponding dislocation structure observed m a type AISI 316L austeniUc stainless steel cycled under thermal fatigue are similar to those reported for this material cycled under isothermal strain controlled conditions. The results may be summarized as follows" 1) A pronounced cychc hardening is observed for all temperature variations.

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2) After cyclic hardening a continuous softening that occupies the major part of the total fatigue life is observed under all test conditions, except for the lowest temperature amplitude of 473-823 K. For this case a continuous softening is followed by a saturation plateau similar to that observed in isothermal fatigue tests. 3) The dislocation structures observed after failure depend on the temperature amplitude of the tests and evolve, on increasing this parameter, from a veins and walls structure to an equiaxed cell structure.

Acknowledgements

The authors would like to thank to Mr. D. Rodrian for performing the experiments. The work was performed withm the Special Intergovernmental Agreement between Germany and Argentina, sponsored by Kernforschungszentrom Karisruhe (KfK), Germany; C O N I C E T and the University of Rosario, Argentina and in :he framework of the Nuclear Fusion Project of the KfK supported by the European Communities within the European Fusion Technology Program.

References

[1] A F. Armas, OR. Bettm, I Alvarez-Armas and G.H. Rublolo, J. Nucl Mate1. 155-157 (lO88) 646. [2] M. Gerland, J. Mendez, P. Vlolan and B. Pat Saadi, Mater SOL Eng. A l l 8 (1989) 83. [3] L F Coffin, Jr and R P Wesley, Trans ASME 76 (1954) 923. [4] !. Aivarez-Armas, A.F. Armas and C Petersen, m these Proceedings (ICFRM-5) J. Nucl Mater. 191-194 (1992) 841

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