Stainless steel fasteners: an in-depth look

Stainless steel fasteners are ideal for a wide range of applications, from construction to plumbing. Due to their strength, durability, and corrosion resistance, they are the most popular choice for many industries.

Leveraging its 50 year-long experience, Specialinstert is going to guide us through the main features of this material.

Stainless steel is a metal alloy consisting mainly of iron, carbon, and more than 10% 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, titanium, etc. The variation in the percentage (%) of these elements defines the type of stainless steel, its structural and mechanical properties, and its degree of corrosion resistance.

The use of stainless steel in mechanical applications offers many advantages. Firstly, stainless steel fasteners provide a greater tightness than fasteners made of brass, aluminum, or galvanized steel. Therefore, they are usually employed in applications that require higher mechanical strength and are subject to corrosion.

Stainless steel fasteners, thanks to their corrosion resistance, 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 perfect for use in highly corrosive environments.

The main stainless steels can be divided into three macro-categories:

FERRITIC = (body-centered cubic crystal structure – ferrite) – Magnetic. They have good mechanical properties but low corrosion resistance.

AUSTENITIC = (face-centered cubic crystal structure – austenite) – Nonmagnetic. They have good mechanical properties and high corrosion resistance.

MARTENSITIC = (body-centered cubic crystalline structure – Martensite) – Magnetic. They have remarkable mechanical properties and acquire excellent corrosion resistance only after annealing, quenching, and tempering.

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

• Lean stainless steels do not contain molybdenum, and the typical value of their P.R.E.N. is around 25;
• Standard stainless steels have a P.R.E.N. value between 25 and 40 (e.g., AISI 316);
• Super stainless steels have a P.R.E.N. index ≥ 40;
• Hyper stainless steels contain high percentages of chromium, and their P.R.E.N. 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 the intended use of the fastener.

Are you looking for the right stainless steel fasteners for your projects? Contact Specialinsert’s sales support to choose the best product based on the features you require.

With almost half a century of experience and an annual production of more than 100 MLN parts, Specialinsert is a leading company in the fasteners manufacture, offering an extensive selection of products, including special and standard inserts, with a strong focus on the Automotive sector. Innovation, R&D, and export are the cornerstones of thise Italy-based company, which meets customers' needs in more than 80 countries worldwide.