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Deformation and Martensitic Phase Transformation in Stainless Steels

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Deformation and Martensitic Phase Transformation in Stainless Steels 2007:67 DOCTORAL T H E SIS Deformation and Martensitic Phase Transformation in Stainless Steels Peter Hedström Luleå University of Technology Department of Applied Physics and Mechanical Engineering Division of Engineering Materials 2007:67|: 02-544|: - -- 07⁄67 -- Doctoral thesis Deformation and Martensitic Phase Transformation in Stainless Steels Peter Hedström Division of Engineering Materials Department of Applied Physics and Mechanical Engineering Luleå University of Technology SE-971 87 Luleå, Sweden November 2007 Doctoral thesis Deformation and Martensitic Phase Transformation in Stainless Steels Peter Hedström © Peter Hedström, 2007 2007:67 ISSN: 1402-1544 ISRN: LTU-DT - 07/67 - SE http://epubl.ltu.se/1402-1544/2007/67 Division of Engineering Materials Department of Applied Physics and Mechanical Engineering Luleå University of Technology SE-97187 Luleå, Sweden Universitetstryckeriet Luleå, Sweden 2007 “A poet or prose narrator usually looks back on what he has achieved against a backdrop of the years that have passed, generally finding that some of these achievements are acceptable, while others are less so” Abstract High-energy x-ray diffraction has enabled three-dimensional structural characterization of the meso-scale, such as individual grains and dislocation structures, in polycrystalline materials. It is the first technique with sufficient spatial resolution and penetration power to probe the local structure embedded deep within the materials. This, together with good time resolution makes it suitable for investigations of e.g. phase transformations kinetics, stress-strain behavior, and texture evolution in stainless steels. The micromechanical response of a metastable austenitic stainless steel AISI 301 and a duplex stainless steel SAF 2304 during loading has been investigated by a series of high-energy x-ray diffraction experiments at the Advanced Photon Source (APS) in Argonne, IL, USA. Different measurement scales of the steels are tested, ranging from the behavior of individual grains up to the macroscopic material behavior. The experimental data is used as input to material models to validate and improve existing models for strain-induced martensite and mechanical properties. The x-ray investigations have revealed that autocatalytic α’-martensite transformation is triggered by strains induced by the transformation itself in 301 and this was evidenced as bursts of α’-martensite transformation during tensile loading. This behavior is of significant importance for the mechanical properties of the metastable austenitic stainless steels, since it provides strong local hardening and increases the time to neck formation. The behavior of 301 during tensile loading at different strain rates was also investigated and it was concluded that even moderate strain rates produce adiabatic heating sufficient to suppress the martensite formation. The strain- induced martensitic transformation and the stress-strain behavior was predicted by an extended Olson-Cohen strain-induced martensite model, finite element simulations for the temperature evolution and a radial return algorithm for the stress-strain behavior. The measured and modeled results were in fair agreement. In addition, the phase specific stresses were measured during the experiments and these were in good agreement with the predicted results from the finite element model. Thus, it was concluded that v the employed iso-work principle was a good assumption for the stress distribution between the phases. One way of tailoring the metastable austenitic stainless steels’ microstructure with different phase fractions and deformation structures is by the reverse transformation from martensite to austenite. This was investigated for the cold rolled 301 steel, and the reverse transformation was observed to occur via two different mechanisms, one diffusion controlled and the other a diffusionless transformation. The onset of the diffusion reversion was about 450°C and the shear reversion became active at higher temperatures. The microstructure of shear reversed austenite consists of highly faulted austenite with an inherited lath like structure. The stress response of 15 individual austenite and ferrite grains deeply embedded in the bulk of a 2304 duplex stainless steel was measured during tensile loading. These results showed large intergranular stresses acting between grains and these stresses can have a significant effect on the two- phase behavior during loading. The İ-martensite formation was investigated for 45 individual austenite bulk grains in 301 and the resolved shear stress was determined. Out of the 45 austenite grains probed one was observed to form İ-martensite. The grain that formed İ-martensite had the highest Schmid factor for the active slip system during fcc to hcp transformation. Finally, the use of a conical slit have allowed for investigations of 301 material response during tensile loading to high strains (15%). The rotation of seven austenite grains could be followed and α’-martensite transformation was coupled to two of the individual austenite grains. Thus, it was demonstrated that it is possible to investigate the three-dimensional structure evolution and deformation-induced phase transformations during tensile deformation to high strains. Preface The work presented in this doctoral thesis has been carried out during 2004-2007 at the Division of Engineering Materials at Luleå University of Technology (LTU), Sweden. The synchrotron experiments were conducted at the Advanced Photon Source, Argonne National Laboratory, IL, USA. The thesis consists of an introductory part followed by these six papers. I. Stepwise transformation behavior of the strain induced martensitic transformation in a metastable stainless steel Peter Hedström, Ulrich Lienert, Jon Almer and Magnus Odén Scripta Materialia, 55 (2007) 213-216. II. Load partitioning and strain-induced martensite formation during tensile loading of a metastable austenitic stainless steel Peter Hedström, Lars-Erik Lindgren, Jon Almer, Ulrich Lienert, Joel Bernier, Mark Terner and Magnus Odén Submitted for publication III. Reverse martensitic transformation and resulting microstructure in a cold rolled metastable austenitic stainless steel Axel Knutsson, Peter Hedström and Magnus Odén Submitted for publication IV. Load partitioning between 15 individual austenite and ferrite grains embedded in the bulk of a duplex stainless steel during tensile loading Peter Hedström, Tong-Seok Han, Ulrich Lienert, Jon Almer and Magnus Odén, Manuscript V. Elastic strain evolution and İ-martensite formation in individual austenite grains during in situ loading of a metastable stainless steel Peter Hedström, Ulrich Lienert, Jon Almer and Magnus Odén Materials Letters (2007) in press. VI. α’-martensite formation in individual bulk austenite grains during tensile loading of a metastable austenitic stainless steel Peter Hedström, Ulrich Lienert, Jon Almer, Mark Terner and Magnus Odén, Manuscript vii My contribution to the appended papers I. Planned and carried out most of the experimental work and analysis. Wrote the paper. II. Planned and carried out most of the experimental work and analysis. Took part in the modeling, but the modeling was done by Lars-Erik Lindgren. Wrote the major part of the paper. III. Planned the work and participated in the experiments and analysis, but the major experimental work was done by Axel Knutsson. Wrote a major part of the paper. IV. Planned and carried out most of the experimental work and analysis. Took part in the initiation of the modeling, but the modeling was done by Tong-Seok Han. Wrote the paper. V. Planned and carried out most of the experimental work and analysis. Wrote the paper. VI. Planned and carried out most of the experimental work and analysis. Wrote the paper. Other related reports not included in the thesis The use of SAXS/WAXS for structural characterization of stainless steels Peter Hedström and Magnus Odén Proceedings of Stainless Steel World 2005, Maastricht, Netherlands, 2005 Evolution of Residual Strains in Metastable Austenitic Stainless Steels and the Accompanying Strain Induced Martensitic Transformation. Peter Hedström, Jon Almer, Ulrich Lienert, Magnus Odén Materials Science Forum 524-525 (2006), 821-826 Residual stress evolution during decomposition of Ti(1-x)Al(x)N coatings using high-energy x-rays. Mark Terner, Peter Hedström, Jon Almer, Jan Illavsky and Magnus Odén Materials Science Forum 524-525 (2006), 619-624 Stress state and strain rate dependence of strain-induced martensitic transformation in a metastable austenitic stainless steel Peter Hedström, Lars-Erik Lindgren and Magnus Odén Presented at IS3, Kyoto, Japan, 2007 Contents CHAPTER 1. INTRODUCTION................................................................................... 1 1.1. SCOPE OF THIS WORK .......................................................................................... 2 CHAPTER 2. STAINLESS STEELS............................................................................. 3 2.1. HISTORICAL PERSPECTIVE ................................................................................... 4 2.2. PRODUCTION PROCESS ........................................................................................ 5 2.3. PHASE AND CONSTITUTION DIAGRAMS ................................................................ 6 2.4. CORROSION RESISTANCE ..................................................................................... 9 2.5. AUSTENITIC STAINLESS STEELS ........................................................................
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