Stainless steel fasteners are classified according to the alloying composition, shape, and finish. Generally, the fasteners used in Europe, the USA, and other regions must meet specifications in terms of mechanical, chemical, and physical characteristics under specified conditions. Further, they must conform to standards in terms of performance, composition, toxicity, corrosion, resistance, and more.
Unlike other commonly used metals, the stainless steels are superior engineering materials used for the fabrication of corrosion-resistant, high-quality fasteners for a wide range of applications. The alloy, of low carbon and chromium, provides good corrosion resistance, high mechanical strength, and can be customized to provide the desired characteristics.
In addition to the low carbon and chromium, the other alloying elements which can be added to improve the characteristics include aluminum, copper, molybdenum, nickel, niobium, nitrogen, selenium, silicon, sulfur, and titanium. Additional hardening processes, surface treatments such as coatings and plating helps to improve the fastener’s properties such as corrosion resistance and appearance to suit specific applications.
Steel Stainless fasteners for the USA and Europe
The standards in the USA are set by the American Society for Testing Materials (ASTM), while those in Europe are set by the International Standards Organization (ISO). However, most of the physical and chemical properties of the stainless steel fasteners are similar for the corresponding equivalent grades.
Most stainless steel fasteners for the USA and Europe are made from the ASTM 304 (18-8) or DIN/ISO A2, and ASTM 316 or DIN/ISO A4. The 304/A2 grade provides good corrosion resistant protection and is used for most general applications. The acid-resistant, 316/A4, is used in more corrosive environments such as marine and chlorine environments where the 304 would be susceptible to corrosion.
In Europe, the grade A2 and A4 are used, with either a property class of 70 or 80, the A refers to the Austenitic Stainless steel while the property class indicates the mechanical properties. The Grade A2 is equivalent to the American standard grade 1.4301 (304), while the A4 is equivalent to 1.4401 (316).
The other less commonly used stainless steel grades for fasteners include 303, 309, 317,321,330, 347, 410, 416, 422, 430, 431, 440, and 501.
Stainless steel type 304 (A2)
The alloy contains conventional carbon, between 18 and 20 percent chromium, and 8-12 percent nickel. It is easy to manufacture and clean. The 304 grade is commonly referred to as 18-8 and is used in wide range environments free of chemicals.
The 304 is used for all kinds of fasteners from small screws to large bolts and nuts.
Stainless steel type 306 (A4)
The 316 alloy has 16-18 percent chromium, 10-14 percent Nickel and 2-3 percent molybdenum. The higher nickel concentration and the addition of molybdenum improve the protection against the crevice or pitting corrosion.
Stainless steel 316 has higher corrosion resistance and suited for salty water, marine and chlorine, moist and wet environments, and other applications requiring a higher degree of corrosion resistance. It is, however, more expensive than the 304.
Chemical composition for fasteners made of stainless steel
The chemical compositions of the various fastener grades under different standards are shown below.
Table 1 Chemical composition for stainless steel fasteners
|Grade||Chemical Composition (% maxima unless stated)||Alternative names|
|A1||0.12||1||6.5||0.2||0.15/0.35||16-19||0.7||5-10||1.75-2.25||303S31, 303S42, 1.4305|
|A2||0.1||1||2||0.05||0.03||15-20||–5||8-19||4||304, 394S17 (BS 3111), 1.4301, 1.4567|
|A3||0.08||1||2||0.045||0.03||17-19||–5||9-12||1||321, 1.4541, 347, 1.4550|
|A4||0.08||1||2||0.045||0.03||16-18.5||2-3||10-15||4||316, 1.4401, 1.4578|
|A5||0.08||1||2||0.045||0.03||16-18.5||2-3||10.5-14||1||316Ti, 1.4571, 316Cb, 1.4580|
|F1||0.12||1||1||0.04||0.03||15-18||–10||1||430, 1.4016, 430Ti, 1.4520 430Cb, 1.4511|
Table 2: The equivalent European and USA stainless steel fastener grades
|European standards||USA standards|
|Steel name||Steel number||AISI||UNS|
NB! Entries in parentheses are common names, not true AISI designations
The standards for the common types of stainless steel fasteners in Europe and the USA are similar in terms of composition and treatment; however, their naming is different as indicated in table 2 above.