Decomposition of Austenite in Austenitic Stainless Steels

Abstract

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 ...

Precipitation within localised chromium-enriched regions in a Type 316H austenitic stainless steel

“…G phase precipitates form within the ferrite regions of duplex [21,22] and nominally austenitic stainless steels (e.g. Type 300 series austenitic steels) [13,15,16,19] during ageing within the temperature range of 250-500°C [12,14,22]. This phase typically nucleates and grows at austenite grain boundaries [21,24], and/or at austenite-a-ferrite phase boundaries but in the latter case it has a cube-on-cube orientation relationship with the ferrite [19,21].…”

“…G phase evolution in austenite is associated with exposure to higher temperatures, within a nominal range of 500-800°C [13], where the kinetics have been purported to be controlled by the rate of Si diffusion [21]. Lower ageing temperatures in austenitic stainless steels favour austenite grain boundary precipitation of G phase [12,13], whilst intragranular precipitation is observed at higher temperatures [12]. Steels with high volume fractions of G phase have been found to be embrittled [19,25] when subjected to room temperature fracture, and as such the phase promotes intergranular fracture.…”

“…Depending on the relative concentrations of Mo and Ti, the composition of the chi phase ranges from (Fe/Ni) 36 [12,24,[26][27][28], and there is potential for substitution of other elements [28]. The composition of chi phase is frequently likened to that of sigma phase, with the principle differences being the solubility of carbon [12,26] and higher concentrations of molybdenum [26,29,30].…”

“…Chi phase is a BCC intermetallic phase (a = 0.881-0.895 nm [12]) the precipitation of which only occurs in Mo-or Ti-containing stainless steels [12,26]. Depending on the relative concentrations of Mo and Ti, the composition of the chi phase ranges from (Fe/Ni) 36 [12,24,[26][27][28], and there is potential for substitution of other elements [28].…”

“…One of the less encountered precipitates is G phase, a silicon containing FCC intermetallic phase (a = 1.115-1.120 nm [12]) with a nominal composition observed in austenitic stainless steels [12][13][14][15][16][17][18][19][20] of (Ni/Fe/Cr) 16 (Nb/ Ti) 6 Si 6 [12]. By comparison, in duplex stainless steels [12,21,22] the composition is (Fe/Ni) 16 (Mn/Cr) 6 Si 7 [19]. Other studies have found that substitution of Mo for Ni is possible [14,23].…”