Materials Technology Cold Work Hardening of High-Strength Austenitic Steels V.G. Gavriljuk 1 ), A.I. Tyshchenko 1 ), V.V. Bllznuk", I.L. Vakovleva 2 ), S. RiednerJ), H. Berns 3 ) 1) G.V. Kurdyumov Institute for Metal Physics, 03142 Kiev, Ukraine; 21 Institute for Metal Physics, 620041 Ekaterinburg, Russia; 3) Lehrstuhl Werkstofftechnik, Huhr-Universitat Bochum, 0-44780 Bochum, Germany Mechanisms of cold work hardening in three austenitic steels containing (mass%) 12Mn and 1.2C (Hadfield steel denoted as C1.2); 21Cr, 23Mn, 2Ni and 0.9N (Bohler steel P-560 denoted as NO.9); 18Cr, 18Mn, 0.345C, 0.615N (CARNIT steel denoted as CNO.96) were studied using mechanical tension tests and TEM studies of substructure formed in the course of plastic deformation. Hadfield steel C1.2 reveals the smallest yield and ultimate stresses and elongation but the highest cold work hardening. Similar yield and ultimate stresses were obtained for steels NO.9 and CNO.96 with a higher elongation and cold work hardening for the latter. The analysis of TEM results leads to the following conclusions: Cold work hardening of the carbon steel C1.2 is mainly due to intensive twinning with rather thick twins. Localized planar slip is a feature of the substructure in the nitrogen steel NO.9 and carbon-nitroqsn steel CNO.96 at strains up to 10 %, whereas twinning is involved in deformation at strains in the range of 10 to 50%. The strain-induced E martensite is rarely observed in both of these steels at strains above 30 %. The substructure and cold work hardening are discussed in terms of stacking fault energy, short-range atomic order and binding between interstitial atoms and dislocations. Keywords: Austenitic steel; Carbon; Nitrogen; Cold work hardening 001: 10.2374/SRI08SP051-79-2008-413; submitted on 15 February 2008, accepted on 8 March 2008 Introduction Interstitials in austenitic steels enhance their cold work hardening. The most persuasive example for that is the Hadfield type steel containing (mass %) carbon, 1.0-104, and manganese, 10-14 [I]. Heavy cold work hardening was also found in nitrogen austenitic steels (e.g. [2,3]) and, recently, in steels alloyed with carbon+nitrogen [4]. The mechanism according to which the interstitial elements increase the cold work hardening of fcc iron-based solid solutions has not been clear so far. It remains a controversial issue even in regard to the Hadfield type steels the studies of which have a rather long history. In early publications, the extremely heavy cold work hardening of Hadfield steel was related to strain-induced [5-7] or martensitic transformations [7,8]. These observations were later explained also as a result of segregation [9], precipitation [10] or decarburization [11]. Perhaps for the first time, the idea of twinning as a reason for the increase in the cold work hardening of Hadfield steel was expressed in [12]. Some other mechanism based on the dynamical strain ageing, namely the break-up of pinned dislocations from the carbon clouds and the restored pinning due to the accelerated pipe diffusion of carbon atoms along the dislocations was proposed in [13] as a cause of the enhanced cold work hardening as well as of the serrated flow. It is worth noting in this relation that, in comparison with bulk diffusion, pipe diffusion proceeds faster only in case of substitutional atoms because the migration of vacancies is accelerated along the dislocation pipes, which is not true for interstitials. In contrast, in comparison with the bulk, the interstitial atoms lose their mobility in the vicinity of dislocation cores (see, e.g., experimental data [14] about the retarded carbon diffusion in preliminary cold worked iron and other metals). In further studies of Hadfield type steel research int. 79 (2008) NO.6 steels the formation of twins, not of s-rnartensite, was confirmed and the decisive role of dynamical strain ageing refuted [16]. In studies of high nitrogen austenitic steels, the cold work hardening was attributed from the very beginning to the nitrogen-caused planar slip at lower strains and deformation twinning at higher strains (see e.g. [3,17]). At the same time, the increase in the dislocation density during cold working was considered as a factor preventing brittle fracture [18]: the dislocations shield stresses created by intersecting twins where the cracks are thought to be nucleated. Alloying with nickel assists the dislocation mode of plastic flow and shifts twinning to higher strains. With decreasing deformation temperature or increase in the nitrogen and manganese contents, the onset of twinning is shifted to lower strains, whereas the dislocation density decreases in between the twins. A particular role in cold work hardening of nitrogen steels was ascribed to second order twinning, i.e. the formation of twins within the space between the primary twins [19]. The multiple twin system is analysed in terms of an increase in the efficiency of twins as hardening obstacles for gliding dislocations and' formation of new twins. At the same time, no remarkable twining was found in other studies of cold worked nitrogen austenitic steels. For example Kubota et al. [20] examined the structure of SUS316L steels with 0.02 and 0.56 mass% nitrogen and Crl8Mn18 type steels with 0.51 and 0.84 mass% nitrogen after tension tests and observed an increase in work hardening with increasing nitrogen content and no deformation twinning but planar dislocation arrays forming Lomer-Cottrell barriers at their intersections. In a similar way, Saller et al. [21] demonstrated microstructures of strained CrMn austenitic steels alloyed with 0.3 or 0.9 mass% nitrogen, where planar slip was the 413