Stainless Steel

Stainless steel is a family of iron-base alloy steel containing 10.5% or more chromium, and other alloys such as nickel, manganese, molybdenum, sulphur, etc. The chromium takes care of corrosion and heat resistance. Corrosion resistance is due to the presence of a naturally occurring chromium-rich oxide film on the steel’s surface. This inert film adheres tightly to the metal and is rapidly self repairing in the presence of oxygen or surface damage.

The other alloys in stainless steel also enhance corrosion resistance and impart certain characteristics with respect to strength and fabricability.

Stainless steel as used in fastener manufacture is normally identified by the following three terms on the basis of their metallurgical structure. Under each classification the material tends to have similar characteristics with regards corrosion resistance, hardenability and fabricability.

  • Austenitic - These stainless steels are chromium-nickel and chromium-nickel-manganese compositions. They are hardenable by cold working and are non-magnetic in the annealed condition. Typical of this group is Type 304 or A2 which contains at least 18% chromium and 8% nickel, hence its 18-8 name.
  • Ferritic – These stainless steels are straight chromium steels that are not hardenable by heat treatment and slightly hardenable by cold working. All this type is magnetic. Type 430 is a typical example.
  • Martensitic - These stainless steels are straight chromium that can be hardened by heat treatment only. All this type is magnetic and Type 410 is a typical example.

Stainless Steel - The Right Choice

To aid the designer to choose the correct grade of stainless steel in a particular application, the following should be considered.

  • All the austenitic stainless steels have excellent corrosion resistance. Type 304 (A2) is the most widely used and is virtually immune to foodstuffs, sterilising solutions, most organic chemicals and many inorganic chemicals.
  • If an application demands higher corrosion resistance, Type 316 (A4) is the next choice. This type resists corrosion in environments containing chlorides or sulphurous acid compounds. It can also be used in phosphoric and acetic acids that can cause corrosion to Type 304.
  • The relative resistance to corrosion among the common stainless steel fastener materials is: Austenitic (316) superior, (304) excellent, Ferritic (430) good, Martensitic (410) fair.

When these types have been considered on the basis of their corrosion-resistant properties the next concern will be their tensile strength or its capacity to withstand load. See the following chart.

Stainless Steel Fasteners - Installation Procedures

For a fastener to be effective it will be obvious to the designer the fastener must be installed correctly. Incorrect installation can lead to difficulty such as seizing and galling or alternatively the fastener may work loose with vibration. Reasonable care should be taken in handling fasteners to ensure threads are kept clean and free of dirt. It is also critical that the fastener is installed to the correct torque. Trouble occurs in this area when fastener installation is attempted without correct torque measuring tools.