A2-70 Stainless Steel vs 304: Fastener Property Class Explained

What Does A2-70 Mean?

A2-70 is the designation of an austenitic stainless steel fastener defined in ISO 3506-1, the international standard for the mechanical properties of corrosion-resistant stainless steel fasteners. The designation carries two pieces of information. A2 is the material group: it denotes an austenitic stainless steel corresponding to Type 304 (UNS S30400, EN 1.4301). The number 70 is the property class: the fastener has a minimum tensile strength of 700 MPa. Together, A2-70 identifies a cold-worked 304-type fastener with a guaranteed tensile strength of at least 700 MPa, used in general engineering, construction and automotive applications.

Chemical Composition

ISO 3506-1 defines the A2 group as austenitic stainless steel whose composition corresponds to Type 304. The typical limits for Type 304 per ASTM A240 and EN 10088 are given below.

Element (wt %)	A2 / Type 304
Carbon, max	0.08
Manganese, max	2.0
Silicon, max	0.75
Phosphorus, max	0.045
Sulfur, max	0.030
Chromium	18.0-20.0
Nickel	8.0-10.5

The A4 group corresponds to Type 316 (UNS S31600) with 2.0-3.0% molybdenum for improved chloride resistance. Within each group, the property class number defines the strength: class 70 for A2-70 and A4-70, class 80 for A2-80 and A4-80.

Mechanical Properties of Fastener Property Classes

Minimum mechanical requirements per ISO 3506-1 for austenitic fastener property classes are listed below.

Property	Class 70	Class 80
Tensile strength Rm, min (MPa)	700	800
0.2% proof stress Rp0.2, min (MPa)	450	600
Elongation, min	0.4 d	0.3 d

The elongation requirement applies to the fastener under test, expressed as a multiple of the nominal diameter d for diameters up to M20. The strength of A2-70 fasteners comes from cold working the austenitic material, which raises the tensile and proof strength of the base 304, about 515 MPa tensile in the annealed condition, to the guaranteed 700 MPa of the fastener.

A2 vs A4: Choosing the Right Group

A2 (304-type) provides good corrosion resistance in dry, indoor and mildly corrosive environments: kitchen equipment, interior structures, general machinery and building fittings. A4 (316-type) adds molybdenum and resists chlorides and acids better, making it the choice for marine, coastal, chemical and food-processing applications where washing chemicals and salt are present. A4 costs more than A2, so the material group should be matched to the actual environment rather than upgraded by default.

Applications and Standards for A2-70 Fasteners

A2-70 bolts, screws and nuts are used in wind tower assembly, rail infrastructure, automotive components, construction and structural steel connections, electronics enclosures, and general industrial equipment. The dimensional requirements of the fasteners are governed by the applicable product standards such as ISO 4014 and ISO 4017 for hex head bolts and screws, DIN 931 and DIN 933, and the Chinese equivalents GB/T 5782 and GB/T 5783; the mechanical properties are verified per ISO 3506-1, including proof load, tensile testing and hardness. A2-70 is suitable for most indoor and sheltered applications; for outdoor coastal or chloride-exposed connections, A4-70 or A4-80 is the safer specification.

Frequently Asked Questions

Q1. Is A2-70 the same as 304?
A2-70 uses 304-type base material, but it is a fastener property class with a guaranteed minimum tensile strength of 700 MPa, well above annealed 304 plate values.

Q2. What does the 70 in A2-70 mean?
It is the property class: minimum tensile strength of 700 MPa per ISO 3506-1.

Q3. What is the difference between A2 and A4?
A2 corresponds to 304 with no molybdenum; A4 corresponds to 316 with 2-3% molybdenum and better chloride resistance.

Q4. Can A2-70 bolts be used in marine environments?
For coastal or saltwater exposure, A4-70 or A4-80 should be specified instead because A2 will pit in chloride environments.

Q5. What standards apply to A2-70 bolts?
ISO 3506-1 for mechanical properties, and ISO 4014, ISO 4017, DIN 931, DIN 933 or GB/T 5782, GB/T 5783 for dimensions.

Q6. How strong is A2-70 compared with a carbon steel 8.8 bolt?
Tensile strength is close, 700 versus 800 MPa, but the proof stress of A2-70 is lower at 450 MPa; stainless fasteners also resist corrosion without any coating.