The first and most important step toward successful use of a stainless steel is selection of the proper type for the application. Stainless steels 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.
Property checklist for type selection
The list as shown below is a suggested checklist of properties to be considered in the selection of the proper type for a specific application. This list includes not only the obvious properties but also some less frequently required, but occasionally overlooked, properties.
• 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
• Abrasion and erosion resistance
• Galling and seizing resistance
• Magnetic properties
• Thermal conductivity
• Thermal expansion
• Electrical resistively
• Sharpness (retention of cutting edge)
• 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 localised forms such as stress corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, intergranular attack in “sensitised” material such as in weld heat affected zones etc. Such localised 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 carburisation.
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.
Fabrication and cleaning
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 sensitised stainless steel, such as nitric-hydrofluoric acid, should be produced from stabilised or low-carbon types even though sensitisation may be unimportant under service conditions. Other properties listed in the checklist are of vital importance for some specialised 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.