Austenite stability of low-temperature reversion-treated microstructures of an AISI 301LN stainless steel under monotonic and dynamic loading

Transkriptio

1 Thesis based on compiled publications on Austenite stability of low-temperature reversion-treated microstructures of an AISI 301LN stainless steel under monotonic and dynamic loading Antti Järvenpää / FMT-ryhmä CASR seminaari Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

2 Background Steel development (3 G) - Higher strength without impairing the ductility - Better steel without increasing the production costs Grain size refinement - Effective grain refinement enhances the strength and ductility may stay high by TRIP effect - Reversion treatment - Strain-induced martensite transforms back to austenite in austenitic steel - An effective route for grain size refinement increasing strength - Long research history in University of Oulu (Prof. Pentti Karjalainen) 2 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

3 Thesis - Complex structures obtained at C; stability XRD Gleeble / Induction C TEM EBSD OM Cold rolling Electropolishing Metastable austenite Chemical composition % deformation induced martensite XRD Fine-grained reversed austenite, retained martensite (and retained austenite) Sample preparation Strengthening rolling Cold rolling Mechanical testing Stress and strain controlled fatigue tests Tensile tests Hardness tests Feritscope 3 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

4 AISI 301LN Chemical composition Metastable austenite Feritscope XRD OM Sample preparation % deformation induced martensite Cold rolling Annealing C Gleeble / Induction Cold rolling Fine-grained reversed austenite, retained martensite (and retained austenite) Electropolishing Strengthening rolling Sample preparation Mechanical testing XRD Hardness tests TEM Feritscope Tensile tests EBSD 4 Stress and strain controlled fatigue tests OM

5 Austenite stability - What is known: - The stability of austenite against martensitic transformation depends on numerous factors such as temperature, chemical composition, crystallographic orientation, defect density, surrounding phases, and grain size (GS) /e.g. 1,2/ - It is commonly reported that the highest austenite stability exists with the average grain size (GS) of 1 2 µm whereas the stability decreases with both increasing and decreasing average GS /3-6/ - The focus of the study - The influence of GS and its distribution on the austenite stability What explains the inverse trend? 5 1) K. Nohara, Y. Ono, N. Ohashi, Composition and grain size dependencies of strain-induced martensitic transformation in metastable austenitic stainless steels, J. Iron Steel Inst. Jpn 2) S.K. Varma, J. Kalyanam, L.E.Murr, V. Srinivas, Effect of grain size on deformation-induced martensite formation in 304 and 316 stainless steels during room temperature tensile testing, J. Mater. Sci. Lett. 3) D. Maréchal, Linkage between mechanical properties and phase transformation in a 301LN austenitic stainless steel. PhD Thesis. 4) M.C. Somani, P. Juntunen, L.P. Karjalainen, R.D.K. Misra, A. Kyröläinen, Enhanced mechanical properties through reversion in metastable austenitic stainless steels, Metall. Mater. Trans. 5) A. Kisko, R.D.K. Misra, J. Talonen, L.P. Karjalainen, The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr 9Mn Ni Cu stainless steel, Mater. Sci. Eng. 6) P. Behjati, A. Kermanpur, A. Najafizadeh, H. Samaei Baghbadorani, Effect of annealing temperature on nano/ultrafine grain of Ni-free austenitic stainless steel, Mater. Sci. Eng. Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

6 Results: Austenite stability Inverse trend in austenite stability- average GS The order of the stability of the reversed structures vary similarly under monotonic and cyclic loading! 6 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

7 EBSD study of the details of the structure Heterogeneous microstructures 7 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

8 EBSD study of the details of the structure Average GS is not a good characteristic of low-t reversed structure FGA = 900 ºC / 1 s UFGA = 800 ºC / 1 s PRev = 750 ºC / 0.1 s WHY? Texture no Low-T reversion structures are unstable 8 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

9 Why low-t structures are unstable? The effect of precipitation! - Fine-grained stable austenitic structure, FGA obtained at 900 C - FGA was subsequently heated up 750, 850 and 900 C PRISMA modeling 9 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

10 STEM Nano-size Cr2N precipitation occurs during annealing at C The effect of precipitation! 10 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

11 Stability of reversed structures The effect of precipitation! The degree of stability drop follows the predicted kinetics of the precipitation! 11 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

12 CR and complex structures The effect cold rolling reduction On complex structure and its stability The presence of deformed austenite and larger austenite grain size decreases slightly monotonic austenite stability of structures rolled with low cold rolling reduction! No practical differences under cyclic loading! 12 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

13 Fraction of low-deformed martensite 13 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

14 The fraction of DA Some DA grains have the proper orientations to form martensite, but most of the DA grains are stable Prev-32CR: After cycles at 0.4% total strain amplitude Prev-32CR: After 7% tensile straining 14 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

15 CR and complex structures Equal strength can be achieved with cold rolling reductions from 32 to 63%, both under monotonic and dynamic loading 15 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

16 Cold strengthening Lower stability in low-t reverted structures can be utilized in strengthening by temper rolling 16 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

17 Conclusions Novel observations Austenite stability - The ultrafine grain size is not the reason for highly reduced austenite stability in tension and cyclic loading - Low-T microstructure consists of submicron, medium size and DA grains - The precipitation is the factor decreasing the mechanical stability of reversion-treated austenitic structures while formed at C. Especially the medium-size grains are unstable. - Precipitation at C binds the nitrogen from the austenite matrix and reduces the stability The role of the retained austenite (DA) - Equal mechanical strength can be achieved with partially reverted 32 63% cold rolled structures - 63CR: High strength is achieved due grain refinement - 32CR: Lower reduction impairs grain refinement, but the presence of cold-rolled retained austenite strengthens Other - Low stability can be utilized in - Cold strengthening (temper rolling) - Cyclic loading by stabilizing the structure - Enhancing fatigue life 17 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

18 Conclusions Courses: OK! 5th paper under preparation 6th paper and the actual thesis will be finished in 2017 Study on reversion structures continues A lot of papers have been published - 31 reviewed papers (corresponding author in 15 papers) since year In addition to the thesis, 7 publications on reversion structures have been published Presented. A. Järvenpää, P. Karjalainen, M. Jaskari, Effect of grain size on fatigue behavior of type 301LN stainless steel, XVI International Colloquium "Mechanical Fatigue of Metals" Conf. Sept , 2012, Brno, Czech Republic Published. A. Järvenpää, M. Jaskari, L. P. Karjalainen, M. Hietala, Enhancing mechanical properties and formability of AISI 301LN stainless steel sheet by local laser heat treatment, Key Engineering Materials, The Current State-of-the-Art on Material Forming (2013) ISBN- 13: Published. A. Järvenpää, M. Jaskari, M. Hietala, K. Mäntyjärvi, Local reversion of cold formed AISI 301LN, Physics Procedia Vol. 78, 2015, P Published. A.S. Hamada, A. Järvenpää, E.Ahmed, P. Sahu, A.I.Z. Farahat, Enhancement in grain-structure and mechanical properties of laser reversion treated metastable austenitic stainless steel, Materials and Design, Vol 94, 2016, P Presented. A. Järvenpää, M. Jaskari, L. P. Karjalainen, Grain size and austenite stability in fatigue of a reversion-treated 301LN type stainless steel, Extended abstract, NT2F16-conference, Published. J. Man, I. Kuběna, M. Smaga, O. Man, A. Järvenpää, A. Weidner, Z. Chlup, J. Polák, Microstructural changes during deformation of AISI 300 grade austenitic stainless steels: Impact of chemical heterogeneity, 21st European Conference on Fracture, ECF21, June 2016, Catania, Italy, Structural Integrity Procedia 2, 2016, Submitted. J. Man, A. Chlupová, I. Kuběna, T. Kruml, O. Man, A. Järvenpää, L.P. Karjalainen, J. Polák, LCF BEHAVIOUR OF 301LN STEEL: COARSE-GRAINED VS. UFG-BIMODAL STRUCTURE, LCF8 conference 18 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

19 Thesis Publications Paper 1: Effect of Grain Size on Fatigue Behaviour of Type 301ln Stainless Steel Paper 2: Austenite stability in reversion-treated structures of a 301LN steel under tensile loading Paper 3: Stability of grain-refined reversed structures in a 301ln austenitic stainless steel under cyclic loading Paper 4: Demonstrating the effect of precipitation on mechanical stability of austenite in a reversiontreated 301LN stainless steel Paper 5: REVERSED MICROSTRUCTURES AND PROPERTIES AFTER LOW COLD ROLLING REDUCTIONS IN AISI 301LN STEEL Paper 6: Effect of initial microstructure on mechanical behaviour of cold rolled AISI 301LN 19 Kerttu Saalasti instituutti Tulevaisuuden tuotantoteknologiat (FMT)

20 Science With Arctic Attitude!