Austenitic stainless steels are probably the most important class of corrosion resistant metallic materials. In order to attain their good corrosion properties they rely essentially on two factors: a high chromium content that is responsible for the protective oxide film layer and a high nickel content that is responsible for the steel to remain austenitic. Thus the base composition is normally a Fe-Cr-Ni alloy. In practice the situation is much more complex with several other elements being present, such as, Mo, Mn, C, N among others. In such a complex situation one almost never has a single austenite phase but other phases invariably form. Those phases are, with few exceptions, undesirable and they can be detrimental to the corrosion and mechanical properties. It is therefore of considerable importance to study the formation of such phases. In this work the decomposition of austenite in austenitic stainless steels is reviewed in detail. First the binary equilibrium diagrams relevant to the system Fe-Cr-Ni are briefly presented as well as other diagrams, such as the Schaeffler diagram, that traditionally have been used to predict the phases present in these steels as a function of composition. Secondly the precipitation of carbides and intermetallic phases is presented in detail including nucleation sites and orientation relationships and the influence of several factors such as composition, previous deformation and solution annealing temperature. Next, the occurrence of other constituents such as nitrides, sulfides and borides is discussed. TTT diagrams are also briefly presented. Finally the formation of martensite in these steels is discussed.KEY WORDS: austenitic stainless steel; precipitation behaviour; carbides; intermetallic compounds; TTT diagram; strain induced martensite. Phase DiagramsPhase diagrams are important to predict the phases present in the austenitic stainless steels. Nevertheless they have limitations due to the complexity of multicomponent thermodynamic calculations and also due to the transformation kinetics that may prevent the attainment of the equilibrium phases. Regarding the first limitation, the number of relevant components is often more than five and published diagrams are rarely found to contain more than four components. As to the second limitation, the diffusion of alloying elements in austenite can be very slow and the precipitation of certain intermetallic compounds can take thousands of hours. Equilibrium DiagramsIn this section the most relevant features of the binary diagrams 3,4) Fe-Cr, Fe-Ni, Cr-Ni, Fe-Mo, Fe-Ti, Ni-Ti, FeNb, Fe-Mn and Fe-Si are considered. Next the Fe-Cr-Ni and Fe-Cr-Mo ternary diagrams and the quaternary FeCr-Ni-Mo diagram are discussed. The individual characteristics of each phase will be presented later in this review.The two main features of the Fe-Cr diagram, shown in Fig. 1, which are relevant to austenitic stainless steels, are the ferrite stabilizing character of Cr and the presence of sigma (s) phase.The Fe-Ni diagram clearly shows the strong ...

The evolution in the understanding of the recrystallization phenomena is summarized in this paper. Initially the main developments concerning recrystallization are presented from a historical perspective. Definitions and concepts involving recrystallization are presented regarding it as a solid-state reaction that occurs by nucleation and growth. The recrystallization nucleation mechanisms are subsequently discussed. Finally, the growth step is highlighted, emphasizing boundary and sub-boundary mobilities and the forces acting on the high angle grain boundaries that sweep the microstructure during recrystallization.

This article reviews the phenomena involved during the annealing of cold worked austenitic stainless steels. Initially the cold worked state is discussed, with special emphasis on the formation of deformation induced martensites. Following, the phenomena of martensite reversion, recovery, recrystallization and grain growth are discussed. The interactions between primary recrystallization and precipitation and between precipitate dissolution and secondary recrystallization are dealt with in detail. Finally, the textures resulting from hot and cold working, and from primary and secondary recrystallization, are presented.KEY WORDS: austenitic stainless steels; work hardening; recovery; recrystallization; grain growth; texture.cold working can be correlated to a lower SFE. 39) Deformation twinning is also dependent on SFE and grain size. Low SFE and large grain size favor deformation twinning. Recent work 40) has shown that deformation twin initiation requires a critical deformation (critical dislocation density). The SFE has an indirect effect by increasing dislocation density and strain hardening in low-SFE fcc metals and alloys. 40) Another aspect that must be remembered is that SFE of the ASSs increases with deformation temperature. 41) In summary, depending on the value of the SFE, two different microstructures may be observed in the cold worked ASSs: for a high SFE, a cellular dislocation distribution without DIM and, for a low SFE, a planar dislocation distribution containing DIM. It is worthy of note that deformation bands can be found in both.In terms of recrystallization, the lower is the SFE, keeping all others variables constant, the larger will be the stored energy during deformation and the corresponding driving force for recrystallization. This subject will be discussed later on in greater detail.ASSs are commonly considered to be materials having a low SFE. Actually the SFE of this class of steels are within quite a large range. [42][43][44][45][46][47] Fig. 2(b)). Several interesting features can observed in this figure. Microstructural analysis using optical microscopy, as shown in Fig. 2(a) for a high SFE ASS, does not allow differentiation between high and low SFE austenitic stainless steels after cold working. Furthermore, at the transmission electron microscopy scale deformation bands may be seen eventually (compare Figs. 2(b) and 2(c)). Monteiro and Kestenbach 49) using transmission electron microscopy observed that dislocation substructure is orientation sensitive, i.e., individual grains develop different substructures, according to their specific orientation and the orientation of their neighbours.As previously mentioned, two types of martensite may occur in the ASSs, namely: aЈ-(bcc, ferromagnetic) and e-(hcp, paramagnetic). The typical lattice parameters are: a aЈ ϭ0.2872 nm and a e ϭ0.2532 nm; c e ϭ0.4114 nm Assuming 50) a g ϭ0.3585 nm, we may conclude that the g→aЈ transformation leads to volume increase of about ISIJ International, Vol. 43 (2003) where the contents are in wt%. Since ...

Wrought austenitic stainless steels are widely used in high temperature applications. This short review discusses initially the processing of this class of steels, with emphasis on solidification and hot working behavior. Following, a brief summary is made on the precipitation behavior and the numerous phases that may appear in their microstructures. Creep and oxidation resistance are, then, briefly discussed, and finalizing their performance is compared with other high temperature metallic materials

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The production of American Petroleum Institute (API) class steels using the traditional controlled rolling route rather than the process involving accelerated cooling necessitates a careful adjustment of steel composition associated with the optimization of the rolling schedule for the deformation and phase transformation characteristics of these modified alloys. The current work presents a study of two, NbCr and NbCrMo, steel systems. The microstructure obtained is correlated not only with the resulting mechanical properties, but also with the weldability and resistance to damage in the aggressive environments to which the materials are exposed. The evaluation of the steels was undertaken at two stages along the production route, sampling the material as plate and as tubular product, according to the API 5L 2000 standard. Tensile testing, Charpy-V impact testing, and hardness measurements were used to determine the mechanical properties, and microstructural characterization was performed by optical and scanning electron microscopy. The results showed that it was possible to obtain good impact properties, for both steels, in plate and tube formats. The Charpy-V impact energy, measured at Ϫ20 °C from 100 to 250 J corresponds to a toughness level above that required by the API 5L 2000 standard, which specifies 68 to 101 J at 0 °C. The yield strength (YS) to ultimate tensile strength (UTS) ratio was determined to be 0.8, the API standard establishing a maximum limit of 0.93. Both of the alloys investigated exhibited a bainitic microstructure and were successfully processed to fabricate tubular products by the "UOE" (bending in "U", closing in "O," and expanding "E") route. With regard to weldability, the two experimental steels exhibited a heat-affected zone (HAZ) for which toughness levels (using the temperature associated with a 100 J impact energy as a base for comparison) were higher than those for both the base metal (BM) and the weld metal (WM) itself. In order to perform the evaluation of the behavior of the steels in an aggressive environment, more specifically their resistance to the deleterious effects of H 2 S, slow strain rate tests (SSRTs) were carried out, immersing the samples in a sodium thiosulfate solution during the tests. Though no secondary cracking was observed in the test samples, the ductility levels measured were lower than those for the same materials tested in air. Constant load tests were also conducted according to the standard NACE conditions. Despite the more aggressive nature of the test solution in these cases, no samples of either steel suffered failure.

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