Property classes are the most misread two digits in the fastener world. They tell you how strong the steel is — nothing about corrosion resistance, hardness-vs-brittleness, or whether the screw suits your joint. This guide decodes the markings and the sourcing mistakes around them.

Carbon steel: the property class numbers

ISO 898-1 defines classes for carbon and alloy steel screws, marked on the head. Each class is two digits, read as:

  • First digit × 100 = nominal tensile strength in MPa.
  • First digit × second digit × 10 = yield strength in MPa.
Class Tensile ≈ Yield ≈ Where you meet it
4.8 400 MPa 320 MPa Ordinary hardware, low-strength machine screws
5.8 500 MPa 400 MPa General machinery
6.8 600 MPa 480 MPa Sheet-metal and structural light joints
8.8 800 MPa 640 MPa The workhorse: machinery, automotive chassis
9.8 / 10.9 / 12.9 900 / 1000 / 1200 MPa ~0.9× tensile High-strength bolts, socket cap screws, tooling

Higher is not automatically better. A 12.9 socket screw in an undersized, soft or corrosion-prone application is a classic failure setup — very hard steels are more sensitive to notch effects and hydrogen embrittlement (a plating-side risk more on that in Nickel vs Zinc Plating). Design the joint to a strength, then buy the class that matches.

The SAE system (Grade 2 / 5 / 8) used in North America is a different table, not a different spelling of the same one; roughly, Grade 5 ≈ mid-8.8 territory and Grade 8 ≈ 9.8–10.9, but match by measured requirements when it matters, not by approximation.

Stainless: A2/A4, not 8.8

Stainless screws are classed by alloy group plus strength code (ISO 3506):

  • A2-70 — the common 304-type austenitic stainless. "70" is the tensile class (≈700 MPa). Lower strength options exist: A2-50 (cold-formed, more ductile) and A2-80.
  • A4-70 / A4-80 — 316-type with added molybdenum for marine/chloride environments.
  • Stainless gains strength by work-hardening, not by quench-and-temper like carbon steel — which is why you rarely see stainless above class 80.

So "SS304 grade 8.8" written on an RFQ is a contradiction; a supplier who quotes it without question should raise your pulse. For stainless, the honest pairing is material group + strength class (A2-70), and corrosion behavior is covered in 304 vs Carbon Steel.

What to confirm when ordering by grade

  1. Class marking on the head — 8.8/12.9 socket screws should carry the stamp; small sizes may be exempt by standard, then packaging labels do the work.
  2. The material behind the class — 12.9 from proper alloy steel with quench and temper behaves differently from cheap over-hardened parts that pass a hardness test and fail a sideways one.
  3. Documents, if the joint matters — material/test documentation availability depends on the product and the supplying factory, so state what you need in the RFQ and require the answer in the quotation (see How We Quote).
  4. Hydrogen-embrittlement exposure — high-strength classes plus electroplating is the risky combination. Ask about baking if you specify plated 10.9/12.9.

How grade language works on this site

Our product pages show the specifications confirmed at catalog level; where a property class has not been verified with the supplying factory for a given line, the field stays empty rather than optimistic, and your quotation is where grade gets pinned down — in writing, per line, before sourcing starts. If your application is structural or safety-relevant, say so in the RFQ: it changes what we ask the factory to document, not just what we offer.

This page explains publicly defined property-class systems (ISO 898-1, ISO 3506, SAE J429) for buyers' own reference. It is not a certification of any listed product — strengths and classes per order line are confirmed in the quotation against your stated basis.