The question is never “which is stronger”. It is “which one is still there in five years”. Grade 8.8 carries more load; A2-70 never rusts. Here is how to choose between them without over-specifying or under-specifying.
What the numbers actually say
| Grade | Material | Tensile Rm | 0.2% proof Rp0.2 |
|---|---|---|---|
| A2-50 | 304 stainless, soft | 500 MPa | 210 MPa |
| A2-70 | 304 stainless, cold worked | 700 MPa | 450 MPa |
| A4-70 | 316 stainless, cold worked | 700 MPa | 450 MPa |
| 4.8 | Low-carbon steel | 400 MPa | 320 MPa |
| 8.8 | Medium-carbon, quenched and tempered | 800 MPa | 640 MPa |
| 10.9 | Alloy steel, quenched and tempered | 1040 MPa | 940 MPa |
| 12.9 | Alloy steel, quenched and tempered | 1220 MPa | 1100 MPa |
Read the proof-stress column, not the tensile column: that is the load the fastener can carry without taking a permanent set. Grade 8.8 carries roughly 40% more than A2-70, and grade 12.9 about 2.4 times as much. That gap is the whole reason stainless is not simply “the better option”.
Where stainless wins outright
Outdoors, near the coast, in wash-down areas, in food and pharmaceutical plant, and anywhere a coating would be scratched during assembly. Stainless needs no plating, so there is nothing to chip, and it does not depend on a coating that a fitter can damage. 316 (A4) is the grade for chlorides — marine air, salt spray, swimming pools, chemical plant. 304 (A2) is the general-purpose choice for inland, sheltered and indoor work.
Where steel wins
Where the load is high and the environment is dry: machine frames, structural steelwork, press tooling, engine and drivetrain work. Grade 8.8 and above is also more predictable under repeated load, and cheaper per unit of strength than stainless by a wide margin.
The two traps
Hydrogen embrittlement
High-strength steel is sensitive to hydrogen absorbed during acid pickling and electroplating. A grade 10.9 or 12.9 bolt that has been electroplated with zinc and then baked inadequately can crack under load days or weeks after installation, with no warning and no visible corrosion. This is why 10.9 and 12.9 parts should be supplied black oxide or Dacromet rather than bright zinc — both avoid the acid step. If you are buying 12.9 with clear zinc plating, ask how it was baked.
Galling
Stainless on stainless will cold-weld under pressure and seize, particularly at larger diameters and with power tools. It is a mechanical problem with a mechanical fix, covered in why stainless fasteners seize.
Galvanic corrosion: the pairing matters as much as the grade
Two different metals in contact in the presence of moisture form a cell. The less noble one corrodes, and the ratio of areas decides how fast. A small stainless screw in a large aluminium plate is the worst case: the aluminium around it can be eaten away quickly. The practical rules are to keep the pair close together in the series, to insulate with a nylon or plastic washer, and to avoid stainless fasteners into large aluminium structures without isolation.
A short decision table
| Environment | Sensible choice |
|---|---|
| Dry, indoor, controlled | Zinc-plated grade 4.8 or 8.8 |
| Sheltered outdoor, inland | Zinc-plated 8.8, or 304 stainless if the finish matters |
| Coastal or marine air, chlorides | 316 / A4-70 or A4-80 |
| Food, pharmaceutical, wash-down | 316 / A4, passivated, brushed or plain |
| Structural steel, high load | 8.8 or 10.9, hot-dip galvanised or Dacromet |
| High-stress tooling and machinery | 12.9, black oxide or Dacromet |
| Aluminium structures | Aluminium fasteners, or stainless with isolation washers |
| Dissimilar metals in damp air | Isolate the joint; do not rely on the coating |
Where a design is load-bearing, have the grade confirmed against the actual load case by a competent engineer — the table above is a starting point, not a calculation.
Buying them
We stock 304, 316 and carbon steel lines across the common grades, sold by the piece with no minimum order, and in the finishes named above — black oxide, Dacromet, zinc and plain.