A2-70

DIN ES ISO 3506-1: 1997 specifies the mechanical properties of bolts, screws and nuts made of austenitic (amongst others) stainless steels, when tested over an ambient temperature range of 15ºC to 25ºC.

Trafitec UK, supplies a range of material that enable fastener manufacturers and others to produce products that meet or exceed the properties required in the above standard. Material to meet the properties of A2-70 is one element of our product range.

The designation A4-80 derives from “A” meaning the material is a cold worked austenitic stainless, divided into 5 grades (1-5) according to material characteristics (see Annex B of DIN ES ISO 3506-1: 1997 for full description of each grade). The “70” denotes the minimum tensile strength to be met by the material (actually the bolt, screw or nut after manufacture).

Therefore, A2-70 is an austenitic, acid proof stainless grade (usually 304 material – 304 or 304L) which has been cold worked to provide a minimum tensile strength of 700N/mm² (MPa).

We stock round bar from 10 mm to 60 mm diameter, and hexagons from 10 mm to 37 mm AF.

We can also produce flats and squares not covered by the standard, on request, to reduce machining costs (up to 101.6 mm (“4”) square and flats to 100 mm x 50 mm (4” x 2”]). Please enquire as to properties required.

Background

Grade 304 is the standard “18/8” stainless; it is the most versatile and most widely used stainless steel. It has excellent forming and welding characteristics. The balanced austenitic structure of Grade 304 enables it to be severely deep drawn without intermediate annealing; Grade 304 also has outstanding welding characteristics.

Grade 304L, the low carbon version of 304, does not require post-weld annealing and so is extensively used in heavy gauge components (over about 6 mm). Grade 304H with its higher carbon content finds application at elevated temperatures. The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Good oxidation resistance in temperatures up to 870ºC. Continuous use of 304 in the 425-860ºC range is not recommended if subsequent aqueous corrosion resistance is important. Grade 304L is more resistant to carbide precipitation and can be heated into the above temperature range. 304H will become sensitized in the temperature range of 425-860ºC; this is not a problem for high temperature applications, but will result in reduced aqueous corrosion resistance.

Grade 304 is excellent in a wide range of atmospheric environments and many corrosive media, although it is subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60ºC. Grade 304 is considered resistant to potable water with up to about 200mg/L chlorides at ambient temperatures, reducing to about 150mg/L at 60ºC. The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures. Heat Treatment is by solution annealing – heated to 1010-1120ºC and cooled rapidly. These grades cannot be hardened by thermal treatment. Cold working (normally drawing) procedures enhanced levels of tensile strength. All austenitic stainless steel fasteners are normally non-magnetic in the annealed condition.

It is common for 304 grades to be stocked in “Dual Certified” form. These items have chemical and mechanical properties complying with both 304 and 204L specifications.

Chemical Composition

See table below for A2-70 material chemical composition (standard chemical compositions for a range of 304 stainless steels is also given for information).

Grade C Mn P S Si Cr Ni Cu
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

Notes:

  1. If the chromium content is below 17%, the minimum nickel content should be 12%
  2. If the carbon content is a maximum of 0.03%, nitrogen may be present to a maximum of 0.2%

Mechanical Properties

The mechanical properties of material supplied for conversion into bolts, screws and studs conforms to the requirement of the standard below. For comparison, the “normal” mechanical properties of 304 stainless are given for information.

Grade Tensile Strength (MPa) min Yield Strength
0.2% Proof
(MPa) min
Elong
(% in 4D min)
Red of Area (% min) Hardness (HRC max)
A4-70 700 450 40 Not required Not required
304 515 205 40 55 15
304L 485 170 40 55 15
304H 515 205 40 55 15

Note: the standard allows that, for diameters over 1.1/2” (38.1 mm), centre (core) properties may be lower than indicated by test results which are based on values determined at ½ radius.

Physical Properties

Typical physical properties of 304 grade stainless steel in the annealed condition.

Grade Density
(kg/m³)
Elastic Modulus (GPa) Mean Co-eff of Thermal Expansion (μm/m/°C)
Thermal Conductivity (W/m.K)
Specific Heat
0-100 °C (J/kg.K)
Elec Resistivity (nΩ.m)
0-100 °C 0-315 °C 0-538 °C
At 100 °C At 500 °C
304/L/H 8000 193
17.2 17.8 18.4
16.2 21.5
500 720