Microstructural Design Of Duplex Stainless Steels For Enhanced Mechanical Properties

Duplex stainless steels,where the microstructure consists of a mixture of ferrite and austenite phases,are widely used in the petrochemical,marine and construction industries due to the good combination of corrosion resistance and mechanical properties that can be achieved in such steels.A significant effort has been made to further improve the mechanical properties of duplex stainless steels by structural refinement.However,as high strength and good ductility are often mutually exclusive when the grain size is refined down to sub-micrometer scale,achieving such a goal is very challenging.In this thesis the ability to achieve both high strength and good ductility in such steels by the development of microstructures is demonstrated.The approaches are illustrated for two types of duplex stainless steel,respectively – one where the austenite phase is mechanically stable at room temperature(RT),and one where it is meta-stable and transforms under RT deformation to martensite.For both steels,samples at different processing stages have been characterized by tensile tests and by microstructural analysis using optical microscopy,transmission electron microscopy,electron back-scatter diffraction and X-ray diffraction.The main results are summarized as follows.Firstly,a series of heterogeneous lamella(HL)structures were produced in a mechanically-stable Fe-25Cr-6Ni-3Mo-0.3N duplex stainless steel by cold rolling and subsequent annealing,with the processing conditions chosen to achieve a wide grain size distribution from nanometer scale to micrometer scale in both the austenite and ferrite phases.In these samples this unique microstructure allows both strength,from ultrafine-grains,and ductility,from coarse grains,to be achieved.In particular,annealing at 900?C for 1 min of samples cold-rolled to a reduction of 90% results in enhanced yield strength and tensile strengths of 805 MPa and 1002 MPa respectively,compared to just 488 MPa and 704 MPa in conventional equiaxed grain-size samples,with only a small decrease in tensile elongation from 38% to 33%.The resulting Fe-25Cr-6Ni-3Mo-0.3N duplex stainless steel,with HL microstructure,shows an excellent combination of strength and ductility,superior to that of commercial 2507 duplex stainless steels with similar alloying composition.Secondly,a nano-duplex structure,consisting of recovered ferrite and fully recrystallized austenite,was produced in a meta-stable Fe-23Cr-8.5Ni duplex stainless steel by 90% cold rolling following annealing at 700?C for 30 min.In this condition the average grain size in the ferrite phase is 350 nm and 400 nm in the austenite phase.This nano-structured Fe-23Cr-8.5Ni duplex stainless steel shows a good combination of strength and ductility,with an enhancement in both the yield strength and tensile strength from 395 MPa and 588 MPa to 738 MPa and 818 MPa,respectively,compared to equiaxed-grain counterpart material.The occurrence of transformation-induced plasticity in this meta-stable alloy ensures a good tensile elongation of 29%,which is only slightly lower than the value of 35 found in equiaxed-grained counterpart.Finally,the strain hardening behaviour of both the HL Fe-25Cr-6Ni-3Mo-0.3N and nano-structured Fe-23Cr-8.5Ni duplex stainless steels have been systematically investigated.Both samples show a heterogeneous plastic response resulting from the difference in mechanical properties between the individual microstructural components.This heterogeneous mechanical response leads to the evolution of long-range back stresses,and results in a strong Bauschinger effect.It is concluded that the back-stress hardening contributes to the total strain hardening during tensile deformation,and that this is responsible for the observed high ductility in both samples.In summary,the results presented in the thesis underline the benefical effects of the multi-scale grain size distribution and the transformation-induced plasticity for managing the strength-ductility trade-off in duplex stainless steels.As such it can be concluded that the proposed microstructural design offers a new scope for achieving enhanced combinations of both high strength and good ductility.