Q: I have difficulties getting A4-70 A4-80 and A4-100 screws in small sizes (M4, M5 and M6). Can you suggest suitable material alternatives?
A: A4 (A4 (1.4401 / 1.4404) is an excellent material for high strength bolts. A4-70 and higher strength screws in small sizes (up to M8) are not standard products. The reason is that A4 (AISI 316) stainless steel is a austenitic material. Therefore, the only way gain high
strength fasteners (with a tensile strength of 700 MPA (A4-70), 800 Mpa (A4-80) or higher is by hardening of the material. The hardening for larger sizes (M10 and up) is obtained during the cold heading of the screws. For smaller screws however, only a tensile strength in the range of 500 MPA (equal to A4-50) or strength class 5.x is typically obtained during production.
It is possible to manufacture A4-70, A4-80 or A4-100 in small sizes by machining those from full hard material (i.e machining a hardened bar) . This is a difficult process as the pre-hardened material can be fairly brittle – which can make small A4 bolts in these sizes quite expensive.
The alternative may be to go for Duplex screws. Machined Duplex screws will have a tensile strength of min (113 ksi / 780 MPA) if machined and can replace A4-70. For higher strength requirements, you should consider titanium grade 5 screws which have a tensile strength of Min 138 ksi / 950 MPA corresponding to A4-80 or A4-90. Replacement of A4-100 is best done by a precipitation hardened material like 17-4PH or Inconel 718 bolts.
The corrosion resistance of Duplex stainless steel, titanium grade 5 and Inconel 718 are
all better than A4. The A4 has PREN numbers in the range 23-28, whereas Duplex is above 30. The 17-4PH material contains slight lower amount of chromium compared to A4 so the pitting resistance may also be lower.