01. Austenitic stainless steels - from kitchen sinks to fuel cell cars

This column is the first in a series of eight columns on the topic of the seven families of stainless steels; their characteristics, complementary properties and the astonishingly wide variety of applications ranging from tiny stents in the human body to large scale constructions in the process industry. It is natural to start with the family of austenitic stainless steels because of its large impact on our society.

In 1912 the first two patents on stainless steels were submitted by Krupp Stahl in Germany. One represented an austenitic stainless steel (ASS), which was a forerunner of what is today known as type 304. Today, this steel together with type 316 dominates the stainless steel market with a total share of about 70%. It is quite remarkable that one family of stainless steels has earned such a dominating position.

There is no other type of stainless steel used in such a wide temperature interval as the austenitic grades. They retain their toughness well below room temperature and retain a substantial part of their mechanical strength at elevated temperature, where creep deformation is a concern in many applications. In addition, they show good resistance to wet corrosion and high temperature corrosion. They are, therefore, suitable in applications ranging from cryogenic temperatures, where the fracture toughness is insufficient in other steels, to temperatures well above 1000°C. Moreover, they are easily formed and readily welded.

Figure 1. {111}-type glide plane in fcc. There are 4 such planes and 3 slip directions in each. Thus, there are altogether 12 glide systems in fcc.

Several features of ASS are intimately related to the face centered cubic crystal structure. The dense packing of atoms leads to low diffusion rates and creep resistance at elevated temperature. Dislocation slip occurs on the four {111}-type slip planes (Figure 1). The resulting 12 dislocation slip systems in combination with a low stacking fault energy leading to pronounced work hardening explains the exceptional drawability and stretchability required in forming operations.

Figure 2. Light optical micrograph of type 304 austenitic stainless steel etched to produce contrast from grain boundaries (curved) and twin boundaries (straight).Moreover, this explains the unique range of yield strength attainable by cold work (250 up to almost 2000 MPa). Austenitic stainless steels are easily identified from the presence of straight twin boundaries visible in micrographs of the kind shown in Figure 2. An abundance of twin boundaries reflect a low stacking fault energy, which is a distinctive feature of ASS and separates them from all other stainless steels.

Figure 3. An example of a heat exchanger under construction viewed from the tube sheet into which the tubes are inserted.To people in general, ASS are most well-known through the wide variety of kitchen utensils and domestic appliances, such as kitchen sinks, dish-washers, silver ware and washing machines. The importance of this in the improvement of hygiene and our quality of life can hardly be exaggerated. However, the large tonnage is represented by applications in the process industry such as heat exchangers (see Figure 3), pipe lines, tubes for a wide variety of purposes and super heaters. A list, which is far from complete, is shown in Table 1.

Figure 4. A view under the bonnet of a hydrogen fuel cell car. The fuel cell stack includes about 400 individual fuel cells. Photo: The author.It is tempting to consider future and novel applications of ASS. Fuel cell cars driven by hydrogen will need large quantities of a corrosion resistant stainless steel (see Figure 4). Each cell is separated from the others by a thin surface treated steel sheet of 0.1 mm thickness. The total quantity in each stack is about 100 m2 and type 316 surface modified ASS is preferred by many producers. An Achilles heel of ASS is the sensitivity to stress corrosion cracking. Fortunately, there are other steels which are almost immune to this type of corrosion. Ferritic stainless steels can provide an efficient and cost-effective remedy as we will see in my next column.

Table 1. Characteristic properties and typical applications of a selection of austenitic stainless steels
Alloy designation Characteristic properties Applications
AISI 304 The leanest type of Cr-Ni austenitics (301 and 302 types excluded) Heat exchangers, pipe lines, cooling and heating coils
AISI 316 Suitable in acidic environments where 304 is insufficient Heat exchangers, pipe lines, cooling and heating coils
AISI 201 High strength Ni-poor alternative to Cr-Ni austenitics
AISI 205 High strength Ni-poor alternative to Cr-Ni austenitics
AISI 321 Good resistance to corrosion in H2S gas. Good resistance to HT creep. Stabilized, resistant to intergranular corrosion. Piping in cracking furnaces (ethylene and vinyl chloride), heat exchangers in chemical and petrochemical plants
AISI 347 Good resistance to H2S gas corrosion. Good resistance to HT creep. Stabilized, resistant to intergranular corrosion. Good resistance to nitrogen absorption. Superheater tubes in steam power plants, cooling tubes in ammonia converters, furnace tubes in refineries
253MA Good creep strength combined with resistance to oxidation and carbon uptake. Recuperator tubes, radiation tubes in furnaces, thermocouple protection tubes, furnace rollers.

This article was first published in Stainless Steel World Magazine in March 2017.

Jan-Olof Nilsson

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