Austenite stabilizers

Alloying elements either enlarge the austenite field or reduce it. The former include manganese, nickel, cobalt, copper, carbon, and nitrogen and are referred to as austenite stabilizers. 

N Austenitic stabilizer, economical substitute for nickel Enhances localized corrosion resistance, thermal stability and mechanical properties... 

Nickel (Ni). Nickel is the most important austenite stabilizer. The type 304 austenitic stainless steel, for example, has 8% nickel for a chromium content of 18%. Thus, depending upon the chromium content, the nickel content has to be adjusted so that the required austenitic structure could be obtained. Austenitic steels have good weldability. They exhibit good toughness at lower temperatures. Nickel content beyond 8% adds to the corrosion resistance of steel. Higher nickel content is, however, not desirable for sour service. 

Hai] Haidemenopoulos, G.N., Grajicic, M., Olson, G.B., Cohen, M., Thermodynamies Based Alloy Design Criteria for Austenite Stabilization and Transformation Toughening in the Fe-Ni-Co System , J. Alloys Compd., 220, 142-147 (1995) (Calculation, Experimental, Phase Relations, 15)... 

Provides increased resistance to pitting and crevice corrosion, especially in the presence of molybdenum. Nitrogen is a strong austenite-stabilizing element. 

One of the most widely used classifications of stainless steels is based on the Schaeffler diagram. Fig. 1-6 (Schaeffler, 1949). The figure shows the dependence of the structure of high-alloy steels on the chrome- and nickel-equivalent. It is obvious that the chrome-equivalent includes all the ferrite-stabilizing elements and the nickel-equivalent all the austenite-stabilizing elements. For example a high-alloy stainless steel with 25% Ni and 20% Cr has a fully austenitic structure whereas a stainless steel with 18% Cr has a mainly ferritic structure. 

If austenite stabilizers such as nickel or manganese are added to Fe-Cr alloys the structure changes to the face-centered-cubic austenitic structure. Austenitic stainless steels constitute the highest proportion of stainless steel products - ca. 70%. 

Today different high-quality grades are assured by improved metallurgy. Table 1-6. Corrosion resistance can be improved by addition of Mo, Cu, W and N. Nitrogen as an austenite stabilizer reduces precipitation of the a-phase. Overall, the precipitates in Duplex steels can be very complex. Beside the high temperature phases (a-phase, T-phase, Cr2N-nitride, M23C6) a 475 °C embrittlement can occur (Schlapfer and Weber, 1986 ... 

The passive film formed on austenitic stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit of hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers—nickel, manganese, and nitrogen—all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is foimd in greater abundance in the passive film than iron, which is the major element in the alloy. 

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

Nitrogen. The effect of nitrogen depends upon the amount contained in a steel. Nitrogen is an austenite stabilizer. 

Nickel 0.3-0.5 Austenitic stabilized, increases toughness, increases corrosion resistance... 

In liquid sodium, dissolution kinetics of austenitic steels is also heterogeneous. Indeed, selective dissolution of chromium, nickel, and manganese occurs. After reaching steady-state corrosion, the concentrations of austenite stabilizing components are decreased to such values that the stmcture is changed to the ferritic one. The thickness of the corrosion layer remains more or less constant, since dissolution at the surface removes the ferrite, while at the interface with the austenitic matrix, diffusion removes nickel, chromium, and manganese so that ferrite becomes stable [27]. 

The RAFM steels have been developed for fusion reactor applications, in particular as structural materials for the blankets. The current reference RAFM steel grades, such as EUROFER, exhibit creep resistance comparable to that of T91 [137]. There is an on-going effort by the fusion community to further improve the performance of RAFM steels by alloy chemistry optimization and TMT [136]. However, the low activation criteria impose severe constraints for instance austenite stabilizing elements Co and Cu cannot be used. 

Manganese. An alternative austenite stabilizer is sometimes present in the form of manganese, which in combination with lower amounts of nickel than otherwise required will perform many of the same functions of nickel in solution. The effects of manganese on corrosion are not well documented. Manganese is known to combine with sulfur to form sulfides. The morphology and composition of these sulfides can have substantial effects on the corrosion resistance of stainless steels, especially their resistance to pitting corrosion. 

Although the mechanical behavior of many commercial metals has been well tabulated for temperatures down as low as 20 K and in some cases 4 K, relatively few data are available on experimental alloys in this range. Just what properties may be obtained is somewhat uncertain. This paper deals with an investigation into the tensile and impact properties and austenite stability of several austenitic manganese steels. 

Since there was no simple way to measure C, the measured results are given in terms of the measured transformation. This in no way invalidates the austenite stability conclusions. 

The relationship of tensile properties, impact properties, and austenite stability to compositional variations is very complex. The generally desirable features of high tensile and yield strength, adequate ductility, and high impact strength at low temperatures cannot be simply related to the effect of compositional variations on the austenite stability. A detailed discussion of the effect of compositional changes follows. 

Although magnetic results are not available for the original 17 manganese alloys, it is felt safe to assume that the austenite stability should be somewhere between that exhibited by the 15 and 19 manganese alloys. On this basis we can discuss the effects of some of the other alloying elements. 

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