Austenitic Family

The third group is named after the austenite phase, which for pure iron exists as a stable structure between 1670 and 2552°F (910 and 1400°C). It is the major or only phase of stainless steel at room temperature, existing as a stable or metastable structure by virtue of its austenite-forming alloy additions, notably nickel and manganese. These stainless steels have face-centered austenite structure from below 32°F (0°C) up to near melting temperatures. 

This family of stainless steel accounts for the widest usage of all the stainless steels. These materials are nonmagnetic, are not hardenable by heat treatment. They can, however, be strain hardened by cold work, have face-centered cubic structures, and possess mechanical properties similar to those of mild steels, but with better formability. The strain hardening from cold work induces a small amoimt of ferromagnetism. 

It has been established that certain elements, specifically chromium, mol)d)denum, and silicon, are ferrite formers. Aluminmn and niobium may also act as ferrite formers depending upon the alloy system. Other elements, such as nickel, manganese, carbon, and nitrogen, tend to promote the formation of austenite. 

After the corrosion resistance plateau of 18% chromimn is reached, the addition of approximately 8% nickel is required to cause a transition from ferritic to austenitic. This alloy is added primarily to form the austenitic structure that is very tough, formable, and weldable. An additional benefit is the increased corrosion resistance to mild corrodents. This includes adequate resistance to most foods, a wide range of organic chemicals, mild inorganic acids, and most natural environmental corrosion. 

The corrosion resistance of the austenitic stainless steel is further improved by the addition of molybdenmn, titanimn, and other elements. 

---

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. These elements will preferentially combine with carbon that comes out of solid solution during weld solidification. Rather than a loss of corrosion resistance associated with formation of harmful chromium carbides, the carbides of titanium and niobium are not detrimental to corrosion resistance. The austenitic family of stainless also prompted another approach to avoiding the effects of chromium carbide precipitation. Because the amount of chromium that precipitated was proportional to the carbon content, lowering the carbon could prevent sensitization. Maintaining the carbon content to below about 0.035% vs. 

Although more formable than the ferritic alloys, they are not as ductile as the austenitic family of alloys. Welding requires more care than with the austenitic alloys because of a greater tendency toward compositional segregation and sensitivity to weld heat input. Improper fabrication techniques can result in equipment that falls short of expectations for corrosion resistance and mechanical properties. 

Corrosion resistance properties of undivided members of the austenitic family are discussed in Chapter 10. 

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. 

Stainless alloys that contain roughly equal amounts of austenite and ferrite are termed duplex stainless. This family of alloys grew out of one basic material originally identified as type 329. They are balanced to contain relatively high chromium contents, with only enough nickel and austenitizers to develop about 50% austenite. 

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 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. 

It seems that, beyond the progresses realized from the first 300-series steels to the present reference materials (15-15Ti and D9 derivatives), it would be possible to find an ultimate upgrade in the family of irradiation-resistant austenitic steels using a CW 12-15/15-25 Ti + Nb stabilized and P-doped matrix, but further work has yet to be done to specify the content of other alloying elements (Mo, Mn, C, Si, N, B) and to adjust the fabrication route to optimize the in-pile behavior of such advanced austenitic material for high-dose applications. 

---