What is the difference between a2 and a4 stainless steel fasteners?

Difference between a2 and a4 stainless steel fasteners

Fasteners in grades A2 and A4, tend to be amongst the most popular ones across global markets. Grade A2, or as commonly known as stainless steel 304 is highly valued for its economical cost and delivering a good performance. Likewise, grade A4, or as known as grade 316 is referred to as marine grade stainless steel. Due to its resistance to seawater and brine solutions, A4 screws work well in marine engineering applications. These fasteners are also resistant to salt spray, which is common in the shipping industry. Although in terms of popularity and availability, it is grade A2 that ranks first, the corrosion resistance and the mechanical strength of A4 supersedes the former, making it a valuable resource in times when high strength and resistance to corrosion is desired at elevated temperatures.

Although the base chemistry of both these stainless steels remains the same, A4 alloy is a molybdenum bearing grade. The addition of molybdenum in A4 improves its resistance to pitting. Generally, grade A4 stainless screws tend to be easier to fabricate, clean, finish, and weld. This is because the chemistry of A4 differs from A2. Moreover, the high alloying content makes it more expensive than grade 304 stainless steel. However, in consideration of the larger picture, these screws are an investment. And so many buyers find them as a more cost-effective solution in comparison to grade 304, over a longer period of time, simply because they lack requirements that would otherwise be necessary, as like in grade A2 fasteners such as maintenance or replacement, especially when they are used outside. The use of A4 material is considered to be vital in environments where resistance to pitting or even environments that contain caustic solutions, because of the resistance of the material to it. The higher content of chromium enables the stainless steel alloys to build a protective, self passivating oxide layer on their surface, which protects them from corrosion like oxidation corrosion or even pitting. The resistance to creep, creep strength properties as well as corrosion resistance properties of stainless steel grade A4 is what permits these screws to deliver a good performance at elevated temperatures without the occurrence of failure.

How to Identify a2 and a4 screws?

Identification of screws is made possible by the markings done on the head. Some specifications call for the use of marking on the heads, simply because it is easier to track the material. The markings are either in the form of a Code Or Reference. Usually, the code or the reference is to be either forged, stamped, or could even be engraved on the head of the screws. It is also possible to identify these stainless steel grades simply by the performance of a material test. A material test report or as known as a mill test report, detail the chemistry of an alloy. This test may also offer the buyer additional information such as the mechanical strength of the alloy as well as its physical properties.

A2 vs A4 stainless steel strength

Although the tensile and yield strength values for grade A2 or Alloy 304 tend to be at significantly higher levels in comparison to conventional low carbon steel alloys, they are weaker in comparison to alloy steel grades such as 8.8 or even 10.9. Whereas grade A4 is stronger than grade A2, and its tensile properties are almost similar to alloy steel grade 8.8.

Types of Grade A4 / A2 Stainless Steel fasteners

IS 1367 grade a2-70 Nuts and Bolts

  • A2-70 stainless steel Eye Nuts
  • A4-80 stainless steel Hex Nuts
  • A2 stainless steel Heavy Hex Nuts
  • IS 1367 grade a2-70 Lock Nuts
  • A4 Stainless Steel Jam Nuts
  • A2-80 stainless steel Square Nuts
  • Stainless steel grade a2-80 Panel Nuts

A4 Stainless Steel Bolts

  • A2-70 stainless steel Hex Bolts
  • A4-80 stainless steel Hex Head Bolts
  • A2 stainless steel Flange Bolts
  • IS 1367 grade a2-70 Heavy hex Bolts
  • A4 Stainless Steel Eye Bolts
  • A2-80 stainless steel U Bolts
  • A2-70 stainless steel Countersunk Bolts

A2-80 stainless steel Studs

  • A2-70 stainless steel Threaded Rod
  • A4-80 stainless steel Stud Bolt
  • A2 stainless steel Double Ended Studs
  • IS 1367 grade a2-70 Tie Rod
  • A4 Stainless Steel Unc Stud Bolt
  • A2-80 stainless steel Fully Threaded Studs
  • Stainless steel grade a2-80 Weld Studs

Stainless steel grade a2-80 Screws

  • A2-70 stainless steel hex cap screw
  • A4-80 stainless steel socket cap screw
  • A2 stainless steel countersunk slotted screw
  • IS 1367 grade a2-70 machine screw
  • A4 Stainless Steel allen cap screw
  • A2-80 stainless steel Set screws
  • Stainless steel grade a2-80 Button Head Socket Screw
  • A2-70 stainless steel self tapping screws
  • A4-80 stainless steel wood screws
  • A2 stainless steel coach screws

Grade A4 Stainless Steel Washer

  • A2-70 stainless steel Flat Washer
  • A4-80 stainless steel Punched Washer
  • A2 stainless steel Machined Washer
  • IS 1367 grade a2-70 Spring Washer
  • A4 Stainless Steel Countersunk Washer
  • A2-80 stainless steel Belleville Washer
  • Stainless steel grade a2-80 Spherical Washer

A2/ A4 Stainless Steel Threaded Rod

  • A2-70 stainless steel Acme Threaded Rod
  • M10 A4-80 stainless steel Threaded Rod
  • A2 stainless steel Fully Threaded Studs
  • IS 1367 grade a2-70 Threaded Studs
  • A4 Stainless Steel Double End Threaded Studs

304 A2 Stainless Steel Fasteners Chemical Composition

A2-70 Min 15.0
Max 0.1 2.0 0.05 0.03 1.0 20.0 19.0 4.0
304 Min 18.0 8.0
Max 0.08 2.0 0.045 0.03 0.75 20.0 10.5
304L Min 18.0 8.0
Max 0.03 2.0 0.045 0.03 0.75 20.0 12.0
304H Min 0.04 18.0 8.0
Max 0.10 2.0 0.045 0.03 0.75 20.0 10.5

Please Note:

  1. If the chromium (cr) content is below 17%, the min nickel (ni) content should be 12%
  2. If the carbon content is a max of 0.03%, nitrogen may be present to a max of 0.2%

A4 Stainless Steel Fasteners Chemical Composition (SS grade 316)

A4-70 Minimum 16.0 2.0 10.5
ISO Maximum 0.08 1.00 2.00 0.045 0.03 18.5 3.0 14.0 1.0
A4-70 Minimum 16.0 2.0 11.5 0.12
ISO² Maximum 0.03 1.00 2.00 0.045 0.03 18.5 3.0 14.0 1.0 0.22
316 Minimum 0 16.0 2.00 10.0
Maximum 0.08 0.75 2.0 0.045 0.03 18.0 3.00 14.0 0.10
316L Minimum 16.0 2.00 10.0
Maximum 0.03 0.75 2.0 0.045 0.03 18.0 3.00 14.0 0.10
316H Minimum 0.04 0 0.04 16.0 2.00 10.0
Maximum 0.10 0.75 0.10 0.045 0.03 18.0 3.00 14.0

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