The advantages of stainless steel for fasteners

Ideal for a wide range of applications, from construction to plumbing, stainless steel fasteners are the most popular choice for many industries due to their strength, durability and corrosion resistance. Here Specialinsert® Srl takes a look at the main features and advantages of this widely used material.

Stainless steel is a metal alloy consisting mainly of iron, carbon and over 10% of chromium. The presence of chromium produces a thin, dense, uniform oxide layer that is very adherent to the surface and not very reactive. This ‘passive layer’ protects the surface from corrosion by preventing corrosive agents from coming into contact with the iron in the alloy.

In addition to varying amounts of carbon and chromium, stainless steel alloys may also contain varying amounts of other elements, such as nickel, molybdenum, manganese, silicon and titanium. The variation in the percentages of these elements defines the type of stainless steel; its structural and mechanical properties; and its degree of corrosion resistance.

In mechanical applications, the use of stainless steel presents many advantages. First, stainless steel fasteners provide a greater tightness than fasteners made of brass, aluminum or galvanised steel and are, therefore, usually employed in applications that require higher mechanical strength or where they will be subject to corrosion.

With their corrosion resistance properties, stainless steel fasteners will also have a longer service life even in highly alkaline and humid environments. Some types of stainless steel are also resistant to chemicals, such as oils, solvents, and acids, so they are ideally suited for use in highly corrosive environments.

Types of stainless steel

The main stainless steels can be divided into three categories: Ferritic: With a body-centered, cubic crystal structure – ferrite – these are magnetic and have good mechanical properties, but low corrosion resistance.

Austenitic: With face-centered, cubic crystal structure –austenite – these stainless steels are non-magnetic and have good mechanical properties and high corrosion resistance. Martensitic: With body-centered, cubic crystalline structure –martensite – these stainless steels are magnetic and display remarkable mechanical properties and acquire excellent corrosion resistance – only after annealing, quenching and tempering.

Classifications of stainless steel

Stainless steels can be classified according to their resistance to localised corrosion and, in particular, pitting. This classification is based on their chemical composition and the calculation of the Pitting Resistance Equivalent Number (PREN) index. The higher this value, the greater the resistance to pitting.

Lean stainless steels: Do not contain molybdenum and the typical value of their PREN is around 25.

Standard stainless steels: Have a PREN value between 25 and 40, e.g AISI 316.

Super stainless steels: Have a PREN index ≥ 40.

Hyper stainless steels: Contain high percentages of chromium and their PREN value can be up to 50.

It is also very important to consider the grades of stainless steel, which indicate different ratios of chromium, nickel, molybdenum and carbon. The grade determines resistance to corrosion, durability, resistance to high temperatures, and other characteristics of the material. For this reason, it is essential to know which grade is appropriate for an intended fastener manufacture application. The 200 series (chromium-nickel-manganese) and the 300 series