RESPONSE OF AISI TYPE 316 STAINLESS STEEL TO INTERRUPTED QUASI-STATIC TO IMPACT TENSION AT ELEVATED TEMPERATURES

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1 RESPONSE OF AISI TYPE 316 STAINLESS STEEL TO INTERRUPTED QUASI-STATIC TO IMPACT TENSION AT ELEVATED TEMPERATURES A. Eleiche, C. Albertini, M. Montagnani To cite this version: A. Eleiche, C. Albertini, M. Montagnani. RESPONSE OF AISI TYPE 316 STAINLESS STEEL TO INTERRUPTED QUASI-STATIC TO IMPACT TENSION AT ELEVATED TEMPERATURES. Journal de Physique Colloques, 1985, 46 (C5), pp.c5-495-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C5, supplirnent au nos, Tome 46, aoot 1985 page C5-495 RESPONSE OF AISI TYPE 316 STAINLESS STEEL TO INTERRUPTED QUASI-STATIC TO IMPACT TENSION AT ELEVATED TEMPERATURES A.M. ~leiche*, C. Albertini and M. Montagnani Applied Mechanics Division, Joint Research Center, Commission of the European Comnities, Ispra Establishment, Ispra (Va), Italy Resume - Le comportement d'un acier inoxydable de type 316 H sournis S une traction interrompue avec un changement de vitesse de deformation, ainsi que la validite d'une equation mecanique d'etat, ont et6 determines par des essais experimentaux d 300, 400, 550 et 650 C. Les effets de vieillissement dynamique aux plus hautes temperatures sont aussi examines. Abstract - The behaviour of a stainless steel type AISI 316 H under interrupted strain-rate change tensile tests, as well as the validity of a mechanical equation of state for this material, are determined experimentally at 300, 400, 550 and 650'~. The effects of dynamic strain aging at the highest temperatures are also examined. I - INTRODUCTION In two recent studies, the behaviour of AISI type 316 H stainless steel when subjected, at the ambient temperature of 20 c, to a strain-rate history consisting of interrupted tensile jumps/l/ and incremental torsional shear jumps/2/ from quasistatic to impact, was investigated. The insensitivity of the flow stress to strainrate change at various values of quasi-static prestrain led to the conclusion that a mechanical equation of state relating stress, plastic strain and strain rate could be used to describe the material behaviour at room temperature. The objective of the present work is to examine the validity of this conclusion at more elevated temperatures for the same material and tensile loading history. This is particularly important from the practical standpoint, because most stainless steel components currently used in fast breeder reactors operate in a temperature range lying between 200 and 700 c, where material data bases for various thermomechanical histories are presently insufficient for adequate analysis and design. I1 - EXPERIMENTAL METHODS Details of the interrupted rate-change tests as used at the ambient temperature of 20 C have been presented previously/l/. The technique followed here was basi,cally the same. Each test consisted of the quasi-static loading in a Hounsfield tensometer of a tensile specimen, havin 5 mm active length and 3 mm diameter, at the nominal strain rate of about 4 X 1Od S-' and a given temperature, well into the plastic range, then unloading. The same specimen was subsequently stretched to fracture, also at the same temperature but at an impact strain rate of about 500 S-' using a modified split Hopkinson tension bar. Tests were conducted for various quasi-static prestrains, at the elevated temperatures of 300, 400, 550 and 650'~. Specimen heating was always achieved by means of a small powerful furnace surrounding its gauge length. Temperature was measured by means of a thermocouple attached to the specimen, and separately controlled by means of a Variac to f 5OC. Thermal gradients along the bars surrounding the heated specimen were found to be small, and their effects on the propagating waves neglected. Further details regarding the instrumentation involved, the testing procedure and the reduction of data can be found elsewhere/l,3/. * Dept. of Engineering, The American University in Cairo, on leave from Dept. of Mechanical Design & Production, Faculty of Engineering, Cairo University, Egypt. Article published online by EDP Sciences and available at

3 C5-496 JOURNAL DE PHYSIQUE The test material was the same as that tested previously at 20 c, namely AISI type 316 (nuclear grade H) stainless steel, of the following chemical composition (in wt%): 16.9 Cr, 12.4 Ni, 1.65 Mn, 2.45 MO, 0.05 C, 0.35 Si, S, P, CO, B, N and (balance) Fe. Specimens were tested without any post-machining heat treatment. Typical quasi-static as well as impact test records are presented elsewhere (Eleiche, Albertini and Montagnani, to be published). Engineering stress-strain curves for the impact part of each test were determined, within the validity of some common simplifying assumptions, using simple formulae/4/. Each impact true stress-true strain curve was then plotted as a continuation of the quasi-static true stress-true strain curve of prestraining, to obtain the overall material response to the xmposed strain-rate history. I11 - RESULTS AND DISCUSSION The monotonic true stress-true strain curves obtained at various temperatures for two constant quasi-static and impact strain rates are plotted together on a common strain scale in Fig. 1; each curve representing the average of at least three test results. TRUE STRAIN, et Fig. 1 Comparison of quasi-static and dynamic tensile true stress-true strain curves of AISI 316 H SS for various testing temperatures. The material has considerable strength, ductility and strain hardening even at the highest temperatures. With increasing temperature, the material strength is much more reduced, at a given strain rate, than ductility. Positive strain-rate sensitivity is also exhibited at all temperatures, except at 650 c, where negative sensitivity dominates at large deformations. This is not due to softening because of adiabatic heating at the impact strain rate, but rather due to dynamic strain aging (DSA) taking place during quasi-static deformation. DSA manifested itself also by serrations in the load-extension quasi-static records at 550 C and to a greater extent at 650 c, and also by abnormal strengthening and ductility plateaux taking place in this same '~ temperature range (Eleiche, Albertini and Montagnani, to be published). These phenomena fall in line to what has been observed previously associated with DSA in austeniticstainlesso steels of fcc structures in general within the temperature range of 200 to 700 C/5-81. True stress-strain curves resulting at 300, 400, 550 and 6 50~~ from interrupted testing with a strain-rate change from quasi static to impact at different prestrains are shown in Fig. 2, each jump curve being the result of a single test for each value of prestrain. Two types of behaviour are distinguished, at all testing temperatures. For prestrains less than about 0.10, the dynamic reloading curve is characterized by a well-defined yield point whose level is higher than the maximum reached in quasistatic prestraining, but which is very close to the flow stress level reached at the same strain in a test conducted exclusively at the impact rate (curve B in Fig. 2). On the other hand, for prestrains larger than 0.1, the yield stress levels reached on dynamic reloading are even higher than the corresponding levels at the same strains

4 I Restrain =(S-]) AlS l 316Y Test Temp 300 t P $(S-$ AlSl 316H Test Temp LOO 'C TRUE STRAIN. Et 0 0. I TRUE STRAIN, Et Prestrotn &(s-l) AIS1 316H Test T e r n ~. ~ ~ ; r TRUE STRAIN TRUE STRAIN. Et Fig. 2 Tensile true stress-true strain curves of AISI 316 H SS for monotonic loading to fracture at a quasi-static (curve A) and impact (curve B) strain rate, and for quasi-static prestraining followed by impact reloading to fracture, at the testing temperatures of 300, 400, 550 and 650'~. G -F \D.l

5 C5-498 JOURNAL DE PHYSIQUE on curves B. The amounts of deviation at 300 agd 400'~ are of the same order of magnitude as those observed previously/l/ at 20 C, also at large prestrains, for the same material, and hence can be accounted for by the lowering of the purely dynamic curves due to softening by adiabatic deformation. At 550 and 650 c, the situation is much more complicated due to the occurrence of DSA at large quasi-static deformations, and to accompanying changes in the dislocation cell structures and subgrain formation 161. This difference is also reflected in the work-hardening characteristics of the plastic deformation following yielding in the dynamic reloading curves, at all temperatures. For large quasi-static prestrains at 550 and 650 c, rapid softening rather than work-hardening occurs, indicative of structural instabilities upon the change in strain rate. In conclusion, it is clear that for the AISI 316 H SS used in this work, t p major effects of strain-rate history occurs only at large strains in the C DSA temperature range. At lower temperatures, a mechanical equation of state can be approximately used to describe the material behaviour, ACKNOWLEDGEMENTS We are pleased to acknowledge the assistance of Mr. M, Forlani and Mr. A. Pachera in the experimentation. The first author, A.M.E., extends his appreciation to the Division of Applied Mechanics for providing facilities during his visit to JRC, when the present work was initiated. REFERENCES /I/ Eleiche, A.M., Albertini, C. and Montagnani, M., Proc. 8th Int. Conf. on High Energy Rate Fabrication, San Antonio,Texas (1984) 213. /2/ Eleiche, A.M., Proc. SMiRT 8, Brussels (1985) Paper L8/3. /3/ Albertini, C. and Montagnani, M., Tech. Rept. No. EUR 5787 EN, JRC, Ispra, Italy (1977). /4/ Lindholm, U.S., in Techniques of Metal Research, Vol V, Part 1, Interscience Publishers, N.Y. (1971) 199. /5/ Jenkins, C.F. and Smith, G.V., Trans. AIME 245 (1969) /6/ Michel, D.J., Moteff, J. and Lovell, A.J., Acta Met. 21 (1973) /7/ Almeida, L.H. and Monteiro, S.N., Proc. 2nd Int. Conf. on Mech. Beh. of Mats., Boston (1976) /8/ Albertini, C., Del Grande, A., Montagnani, M. and Pachera, A,, Proc. of Stainless Steels '84, Gateborg (1984).