The Mechanical Behavior And Relevant Mechanisms Of High Nitrogen Nickel-free Austenitic Stainless Steel

As an important new class of structural materials,owing to its high strength and duc-tility,high work hardening ability,excellent corrosion resistance and low magnetic suscep-tibility,etc,high nitrogen austenitic stainless steel has become one of the focus in the cur-rent material research and has been obtained widely development in different areas gradu-ally.Nowadays,the influence of microstructures and experimental conditions on the me-chanical properties of high nitrogen austenitic stainless steel has been intensively studied.However,few attention has been paid to the influence of solid solution,cold rolling and hot rolling on these mechanical properties systematically.Thus,the research should be made systematically on these aspects.In addition,most of the studies focused on the mechanical behavior of high nitrogen austenitic stainless steels are only under specific loading condi-tions,and few studies on the mechanical behavior under wide loading strain rate are re-ported.In order to have a comprehensive and in-depth understanding of the deformation behavior and relevant mechanism of high nitrogen austenitic stainless steel,it is very far-reaching to study the influence of these treatments and loading strain rate on the me-chanical properties of high nitrogen austenitic stainless steel,which also can lay a good theoretical foundation for the rational use in its practical application.In this paper,a method with solution heat treatment under 1150? and 8 h followed by water quenching was used to obtain the solution treated high nitrogen nickel-free austenitic stainless steel?HNASS?.A method with rolling treatment at room temperature and 1140?under different total reduction of area?RA?was used to obtain the cold rolled HNASS and hot rolled HNASS with different microstructures respectively.A series of tensile tests and nanoindentation experiments were performed by using the MTS-810 material testing sys-tem and Agilent-G200 nanoindenter to study the mechanical behavior of the present HNASS.The systematic characterization of the present HNASS was carried out by using X-ray diffraction,scanning electron microscopy and transmission electron microscopy.Based on these methods,the relevant deformation mechanism of the present HNASS was revealed and discussed.The main results of this paper are as follows:1.The solution treated HNASS with a single face-centered cubic structure was obtained by solution heat treatment.The cold rolled HNASS with RA of 10%,30%and 50%was obtained by cold rolling at room temperature.The hot rolled HNASS with RA of 30%and 70%was obtained by hot rolling at 1140?.The uniaxial tensile tests with a wide loading strain rate range?10-4s-1-1s-1?at room temperature were studied on these HNASS.For the HNASS with same RA,it is found that the increased yield strength with increasing loading strain rate arises from the enhanced interactions caused by high storage rate or high density of dislocations,the slightly increased ultimate tensile strength arises mainly from the lower strain hardening ability?rapidly reduced strain hardening rate?after higher yielding,the decreased uniform strain and fracture strain is attributed to the lower strain hardening ability?rapidly reduced strain hardening rate?.For the HNASS with same loading strain rate,it is found that the increased yield strength with in creasing RA arises from the reduced grain interior space,the increased ultimate tensile strength arises mainly from the higher yielding before lower strain hardening ability?rapidly reduced strain hardening rate?,the decreased uniform strain and fracture strain is attributed to the lower strain hardening ability?rapidly reduced strain hardening rate?.2.The examination and analysis of the plastic flow behavior by performing uniaxial ten-sile tests at room temperature and at different loading strain rates shown that the pre-sent HNASS exhibits a significant two-stage strain hardening behavior,which can be described by using the Ludwigson equation.Based on the Ludwigson equation and Considere instability criterion,two equations of the maximum uniform plastic strain and the stress ratio?true ultimate tensile strength/true yield strength?to the strain hardening exponents n1 and n2 were derived:?_U= n₁ ?_U = ??_0.2?n₁n₂/??^n₁ Based on these two equations,the dependence of the strength and ductility on the strain hardening exponents is analyzed and discussed.3.The cold rolled HNASS with RA of 70%was obtained by cold rolling at room temper-ature.The creep deformation behavior of the solution treated HNASS and cold rolled HNASS with RA of 70%was carried out by nanoindentation with a wide loading rate range?0.5mNs-1-500mNs-1?at room temperature.It is found that the cold rolled HNASS exhibits an obvious creep deformation behavior as compared with that of the solution treated HNASS during the holding regime.The significant creep plastic de-formation of the cold rolled HNASS arises mainly from the rapid relaxation and ab-sorption of the dislocation structures,which is generated during the loading regime and is highly unstable during the holding regime.For the solution treated HNASS,disloca-tion structure would become highly stability due to the dislocation interaction during the loading regime,which make the rapid relaxation and absorption of dislocations become difficult during the holding regime.Thus there is no significant creep defor-mation behavior in the solution treated HNASS.4.The indentation size effect on the hardness of the solution treated HNASS and cold rolled HNASS with RA of 70%was investigated by the nanoindentation at room tem-perature with loading strain rate ranges from 1×10-3s-1 to 4×10-1s-1 and maximum indentation displacement ranges from 50 nm to 2000 nm without holding time.The experimental results show that at shallow indentation depth ranging from 50 nm to 200 nm,the measured hardness is obviously smaller than the values predicted by the Nix-Gao model in the solution treated HNASS,which is in contrast to the tendency in the cold rolled HNASS.At indentation depth less than 800 nm,obvious indentation size effect can be observed in the both solution treated HNASS and cold rolled HNASS.When the indentation depth is larger than 800 nm,the observed indentation size depth in the solution treated HNASS would gradually disappear in the cold rolled HNASS.Based on the deformation mechanism at different indentation depth,the den-sity of geometrically necessary dislocations and statistically stored dislocations con-structed by the Nix-Gao model are used to explain this phenomena.