Austenitic stainless steels represent the major tonnage alloys of the high alloy corrosion resistant steels. Their primary matrix structure is austenite which has substantially different characteristics than other grades of carbon and low alloy steels. The structure also usually contain amounts of ferrite which enhance the strength and corrosion resistance of the alloys. The presence of small amounts of ferrite increases the castability of the alloys in this group by avoidance of hot cracking. In many applications the ferrite also improves corrosion resistance performance. The presence of ferrite in the microstructure is in contrast to wrought materials which are usually wholly austenitic to improve hot forgeability. The alloy group starts at approximately 18% Cr and 8% Ni and goes up in both elements from that point along with additions of Mo and other elements.
Balancing of the full chemistry of these alloys is critical to achieve optimum mechanical and corrosion properties. Although most material specifications allow for wide variations of chemistries this can be misleading. Normal foundry processing does not require special attention to gas pickup problems. The alloys do require a high temperature heat treatment to impart full corrosion resistance. A normal practice is also to acid passivate castings as a final finishing department process. As a general condition the castability of this alloy group is excellent.
When chemistry balancing is done to give a fully austenitic structure the parts can then be used in cryogenic service temperatures. This same balancing yields parts which have the highest magnetic permeability. Normal balancing yields parts which exhibit some magnetism even though austenitic stainless steels are considered to be non-magnetic.
The machinability of this group of alloys is generally considered to be poor. The alloys have a strong tendency to work harden and shallow cuts cannot be made. Very rigid machine tools are required to take the cuts and feeds necessary. A usual customer complaint is “hard castings” when in fact it is microstructurally impossible to harden these alloys. The true case is that shops unaccustomed to these materials work harden machined surfaces.
Applications for this group of alloys are diverse. Normal designing starts with alloys in this group when corrosion is considered to be a factor and then proceeds to lower grades of stainless for cost considereations or to higher groups for performance considerations.