Selection of the Proper Grade of Stainless Steel & Nickel Alloy

The first and most important step toward successful use of stainless steel or nickel alloy is selection of the proper type for the application. Stainless steels and nickel alloys include a large number of standard types, but these types differ greatly from one another in composition, corrosion resistance, physical properties, and mechanical properties, and selection of the optimum type for a specific application is the key to satisfactory performance at a minimum total cost. Below is a suggested checklist of properties to be considered in the selection of the proper type for a specific application. This includes not only the obvious properties but also some less frequently required, but occasionally overlooked, properties.

Property Checklist for Type Selection
Corrosion resistance
Oxidation and sulfidation resistance
Strength and ductility at service and ambient temperatures
Suitability for intended fabrication techniques
Suitability for intended cleaning procedures
Property stability in service
Toughness
Abrasion and erosion resistance
Galling and seizing resistance
Reflectivity
Magnetic properties
Thermal conductivity
Thermal expansion
Electrical resistivity
Sharpness (retention of cutting edge)
Rigidity
Dimensional stability

Corrosion resistance is frequently the most important characteristic of a stainless or heat resistant steel but is often also the most difficult to assess for a specific application. General corrosion resistance to natural conditions and to pure chemical solutions is comparatively easy to determine. However, general corrosion is often much less serious than localized forms such as stress corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, intergranular attack in "sensitized" material such as in weld heat affected zones, etc. Such localized corrosion can cause unexpected and sometimes catastrophic failures while most of a structure is unaffected and must, therefore, be considered carefully in design and in steel selection. Corrosive attack can also be dramatically increased by seemingly minor impurities in the medium, which may be difficult to anticipate but can have major effects even in parts-per-million concentrations. At elevated temperatures, an attack on the metal can be significantly accelerated by seemingly minor changes in atmosphere which affect scaling rate, sulfidation, or carburization.

Despite these complications, a suitable steel can be selected for most applications on the basis of experience, perhaps assisted by suggestions from the steel producer. However, it must be recognized that laboratory corrosion data can be misleading in predicting service performance of a particular type. Even service data have limitations because similar corrosive media may differ substantially due to slight variations in some of the above corrosion factors. For difficult applications, extensive study of comparative data may be necessary, sometimes followed by pilot or service testing.

Mechanical properties at service temperature are an obvious consideration, but sometimes overlooked is the necessity for satisfactory properties at other temperatures which are likely to be experienced. Thus, a product for arctic service must have suitable properties at subzero temperatures even though the steady-state operating temperature may be much higher, and post-service room-temperature properties can be important for a structure which may be intermittently shut down after operating at an elevated temperature.

Selection must consider not only performance requirements but also fabrication and cleaning requirements. Frequently, a particular type is chosen for a fabrication characteristic such as formability or weldability over other types, which would perform adequately, but cost more to fabricate. Even a required or preferred cleaning procedure may dictate steel selection. Sometimes, it is overlooked that a welded fabrication, which is to be cleaned in a medium which attacks sensitized stainless steel, such as nitric-hydrofluoric acid, should be produced from stabilized or low-carbon types even though sensitization may be unimportant under service conditions. Other properties listed in the checklist are of vital importance for some specialized applications but are of little concern for many other applications. Surface finish is important for many applications, and stainless steels are sometimes used because of the variety of attractive finishes available. Selection among these finishes may be made on the basis of characteristics such as appearance, slideability, or cleanability. Effect of finish on the cleanability is not as simple as sometimes thought, and tests of available finishes may be advisable. Selection of finish may in turn influence selection of type because of differences in availability of the various finishes with grade or differences in finish durability. A more corrosion resistant type, for example, will maintain a bright finish in a corrosive environment, which would dull a lower alloy steel.