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Microstresses and anisotropic mechanical behaviour of duplex stainless steels
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2001 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The evolution of deformation during monotonic and cyclic loading of a two-phase material like duplex stainless steel is more complex than in a single-phase material. One reason for this is the microstresses formed due to differences in thermal and mechanical properties between the two phases. Another factor contributing to a complex load partitioning between the two phases is that hot and cold rolled duplex stainless steel exhibits anisotropic material properties. The aim of this thesis has therefore been to investigate the influence of microstresses and an isotropy on the mechanical properties of duplex stainless steels.

The effect of microstresses was clearly revealed when X-ray diffraction was used to study the evolution of microstresses during cyclic loading. Even if the hardness and yield strength were found to be higher in the austenitic phase compared to the ferritic phase more plastic deformation occurs in austenite during cyclic tensile loading. This was also confirmed by transmission electron microscopy investigations of the dislocation structure in both phases. The main reason for the higher degree of plastic deformation in the austenitic phase is that the microstresses are tensile in this phase and compressive in the ferritic phase.

Measurements of the crystallographic texture were used as input to theoretical predictions of both elastic and plastic anisotropy. The predicted anisotropic material properties were then used in finite element simulations to study the flow behavior and the load partitioning between phases during deformation in different loading directions. The experiments and the simulations show that the microstresses and the anisotropy make the load partitioning between the two phases dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45°-direction more plastic deformation occurs in the ferritic phase.

The anisotropic flow behaviour of the as-received material can be predicted from the crystallographic texture. However, it was found that prestraining introduces a transient work hardening behaviour during the second stage deformation, whjch causes an anisotropic flow behaviour immediately after yielding that cannot be described by the crystallographic texture. Instead the an isotropy can be associated with the rearrangement of the dislocation structure that occurs during changes in the loading path. Prestraining also alters the microstresses from being higher in the transverse direction to being higher in the rolling direction. At the same time the fatigue limit is changed from being higher in the rolling direction to being higher in the transverse direction. This study shows that microstresses have a significant influence on fatigue crack initiation and the fatigue limit of duplex stainless steels.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2001. , 52 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 699
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-30059Local ID: 15519ISBN: 91-737-3043-2 (print)OAI: oai:DiVA.org:liu-30059DiVA: diva2:250880
Public defence
2001-06-11, Sal Schrödinger, Fysikhuset, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-02-14
List of papers
1. Evolution of the residual stress state in a duplex stainless steel during loading
Open this publication in new window or tab >>Evolution of the residual stress state in a duplex stainless steel during loading
1999 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 47, no 9, 2669-2684 p.Article in journal (Refereed) Published
Abstract [en]

The evolution of micro- and macrostresses in a duplex stainless steel during loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold-rolled sheet of alloy SAF 2304 solution treated at 1050°C was studied. Owing to differences in the coefficient of thermal expansion between the two phases, compressive residual microstresses were found in the ferritic phase and balancing tensile microstresses in the austenitic phase. The initial microstresses were almost two times higher in the transverse direction compared to the rolling direction. During loading the microstresses increase in the macroscopic elastic regime but start to decrease slightly with increasing load in the macroscopic plastic regime. For instance, the microstresses along the rolling direction in the austenite increase from 60 MPa, at zero applied load, to 110 MPa, at an applied load of 530 MPa. At the applied load of 620 MPa a decrease of the microstress to 90 MPa was observed. During unloading from the plastic regime the microstresses increase by approximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstresses originates from the initial thermal stresses. Finite element simulations show stress variations within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.

Place, publisher, year, edition, pages
Elsevier, 1999
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30076 (URN)10.1016/S1359-6454(99)00149-4 (DOI)15539 (Local ID)15539 (Archive number)15539 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
2. Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading
Open this publication in new window or tab >>Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading
2000 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 31, no 6, 1557-1570 p.Article in journal (Refereed) Published
Abstract [en]

The load sharing between phases and the evolution of micro- and macrostresses during cyclic loading has been investigated in a 1.5-mm cold-rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction (XRD) stress analysis and transmission electron microscopy (TEM) show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual microstresses present in the material. The origin of the microstresses is the difference in coefficients of thermal expansion between the two phases, which leads to tensile microstresses in the austenite and compressive microstresses in the ferrite. The microstresses were also found to increase from 50 to 140 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was, thus, mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from −40 to 50 MPa during the first 103 cycles. After the initial increase of microstresses and macrostresses, no fading of residual stresses was found to occur for the following cycles. A good correlation was found between the internal stress state and the microstructure evolution. The change in texture during cyclic fatigue showed a sharpening of the deformation texture in the ferritic phase, while no significant changes were found in the austenitic phase.

Place, publisher, year, edition, pages
SpringerLink, 2000
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30134 (URN)10.1007/s11661-000-0166-3 (DOI)15614 (Local ID)15614 (Archive number)15614 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
3. Influence of elastic and plastic anisotropy on the flow behavior in a duplex stainless steel
Open this publication in new window or tab >>Influence of elastic and plastic anisotropy on the flow behavior in a duplex stainless steel
2002 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 33, no 1, 57-71 p.Article in journal (Refereed) Published
Abstract [en]

The load partitioning between two phases in a cold-rolled duplex stainless steel has been experimentally studied in situ by X-ray diffraction, for different loading directions. It was found that the load partitioning between the two phases is dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45-deg direction, more plastic deformation occurs in the ferritic phase. The strong crystallographic texture in the ferritic phase makes the material anisotropic, with a higher stiffness and yield strength in the transverse direction compared to the rolling direction. The measured texture was used as input to theoretical predictions of both elastic and plastic anisotropy. The plastic anisotropy was predicted by assuming intragranular slip as the main deformation mechanism. The predicted anisotropic material properties were then used in finite-element simulations to study the flow behavior of the material in different directions. The predicted flow behavior was found to be in good agreement with the experimentally observed load partitioning between the phases for loading in the rolling and transverse directions. However, the yield strength of the ferritic phase during loading in the 45-deg direction was found to be lower than what was predicted. The reason for this is the difference in slip characteristics in different sample directions, because of the morphological texture.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30019 (URN)10.1007/s11661-002-0005-9 (DOI)15470 (Local ID)15470 (Archive number)15470 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
4. Deformation behaviour of a prestrained duplex stainless steel
Open this publication in new window or tab >>Deformation behaviour of a prestrained duplex stainless steel
2002 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 337, no 1-2, 25-38 p.Article in journal (Refereed) Published
Abstract [en]

The anisotropic flow behaviour due to a change in strain path during uniaxial deformation of a duplex stainless steel has been investigated. The anisotropic flow behaviour of the as-received material could be predicted from the crystallographic texture. However, it was found that prestraining introduces a transient work hardening behaviour during the second stage deformation, which causes an anisotropic flow behaviour immediately after yielding that cannot be described by the crystallographic texture. Samples subjected to a coaxial second stage deformation show a small increase in flow stress and similar work hardening rate as in the monotonic loading. Samples subjected to noncoaxial second stage deformation show a pronounced transient stage characterised by early yielding and a high work hardening rate in the first part of the transient stage. Beyond the earliest stage of deformation a higher flow stress and a lower work hardening rate compared to monotonic loading is observed for the noncoaxial samples. The transient phenomena are discussed based on the measured internal stresses and the differences in dislocation structures in the two different phases.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30008 (URN)10.1016/S0921-5093(02)00022-9 (DOI)15451 (Local ID)15451 (Archive number)15451 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13
5. Anisotropic high cycle fatigue behaviour of duplex stainless steels: influence of microstresses
Open this publication in new window or tab >>Anisotropic high cycle fatigue behaviour of duplex stainless steels: influence of microstresses
2002 (English)In: International Journal of Materials Research - Zeitschrift für Metallkunde, ISSN 1862-5282, E-ISSN 2195-8556, Vol. 93, no 1, 7-11 p.Article in journal (Refereed) Published
Abstract [en]

The anisotropic high cycle fatigue behaviour has been investigated for a duplex stainless steel in as-received and prestrained condition. It was found that the anisotropy of the fatigue limit is different from the anisotropy of yield strength. The main reason for this is the influence of microstresses on fatigue crack initiation. Fatigue cracks are seen to nucleate preferably in the austenite phase, which has tensile microstresses. Prestraining alters the microstresses from being higher in the transverse direction to being higher in the rolling direction. At the same time, the fatigue limit is changed from being higher in the rolling direction to being higher in the transverse direction.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30022 (URN)15473 (Local ID)15473 (Archive number)15473 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2017-12-13

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