Austenitic Stainless Steel Family

This family of stainless accoimts for the widest usage of all the stainless steels. These materials are nonmagnetic, have face-centered cubic structures, and possess mechanical properties similar to those of the mild steels, but with better formability. The AISI designation system identifies the most common of these alloys with numbers beginning with 300 and resulted in the term 300 series stainless. 

Examination of the Schaeffler diagram offers insight into the reason for the composition of t) e 304, the cornerstone of the austenitic alloy family. After the corrosion resistance plateau of 18% chromium is reached, the addition of about 8% nickel is required to cause a transition from ferritic to austenitic. The primary benefit of this alloy addition is to achieve the austenitic structure that relative to the ferritics, is very tough, formable, and weldable. The added benefit, of course, is the improved corrosion resistance to mild corrodents. This includes adequate resistance to most foods, a wide range of organic chemicals, mild inorganic chemicals, and most natural environmental corrosion. 

The next major step in alloying additions comes from molybdenum. This element also provides excellent corrosion resistance in oxidizing 

Austenitic alloys also make use of the concept of stabilization. Stainless types 321 and 347 are versions of type 304 stabilized with titanium and niobium, respectively. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. 

Effects of environment and alloy content on anodic polarization behavior. 

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The design of thermal power plants and new-generation nuclear reactors has been the reason for carrying out many smdies on the behavior of tempered martensitic steels and austenitic stainless steels subjected to fatigue and/or creep at high temperature (450—650°C). This chapter reviews firstly the numerous recent experimental and simulation works concerning tempered martensite-ferritic steels. Then, creep and fatigue properties of the two steel families are compared on both micro- and macroscales. Finally, recommended further works are mentioned. 

Many austenitic stainless steel candidates for Generation IV systems belong to the same families as those successfully used in the past or at the present time. From the previous projects it can be learned that high quality and traceabUity in material supply and component fabrication are needed to guarantee safe and reliable behavior during... 

While the yield strength decreases with temperature, ductility increases, as shown in Fig. 3.82. Opposite behaviour may have FCC metals like austenitic stainless steels, aluminum and its alloys or copper, silver etc. in which flow stress has only the athermal component The characteristic stress-strain relationship for this family of metals is shown in Fig. 3.83. Now, temperature has little or no influence at all on yield strength but ductility may even increase, which gives these metals a formidable capability to be used at cryogenic temperatures. Figure 3.84 is an example of true stress-true strain curves obtained with silver specimens of 17 pm grain size [84]. 

Stainless steels can be divided into two main families austenitic and ferritic. The different structure, y for the austenite and a for the ferrite, seems to have an appreciable effect on FCGR properties of steels. This can be seen in Fig. 10.30 that collects 383 data obtained at RT by different researchers [29, 30, 49-53] on four austenitic stainless steels of the series 300, type 304, 316, 321 and 350, the last two with columbium-tantalum and titanium, respectively for high temperatures use (430-820 °C), an austenitic-ferritic (14 % S ferrite) steel type 351 cast [54, 55], a ferritic stainless steels, type 18Cr-Nb [56] and experimental data relative to 403 type martensitic stainless steel [57] (Fig. 10.28). 

Although the duplex stainless steels (austenitic-ferritic steels) have been known since 1940, they did not find wide application until 1975. This is somewhat surprising because this alloy family has a number of interesting properties. The microstracture of duplex stainless steels consists of the two phases austenite and ferrite (50% of each). This microstructure combines good corrosion behavior with interesting strength properties. 

The duplex stainless steels are not as ductile as the austenitic family of stainless steels. Welding requires more care than with the austenitic alloys due to a greater tendency towards compositional segregation and sensitivity to weld heat input. 

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