Bolt Strength Grade Chart (4.6–12.9 + Stainless A2/A4)

The two numbers encode strength. The first is the nominal tensile strength Rm divided by 100 — 8.8 gives about 800 MPa — and the second is the yield-to-tensile ratio times 10 (0.8 for 8.8). The matrix above uses the ISO 898-1:2013 Table 1 minima: 8.8 → Rp0.2 640 MPa / Rm 800 MPa; 10.9 → Rp0.2 940 MPa / Rm 1040 MPa; 12.9 → Rp0.2 1100 MPa / Rm 1220 MPa. Note that for 10.9 and 12.9 the standard minimum is slightly higher than the class arithmetic.

Rp0.2 is the 0.2% offset proof strength — the stress at which the bolt starts to yield and no longer returns to its original length; Rm is the strength at break. For class 8.8 the ISO 898-1 minima are Rp0.2 640 MPa and Rm 800 MPa. Tightening torque is estimated from Rp0.2 (target preload = 70% of Rp0.2 × thread stress area), so the yield column drives torque, not the breaking load. Class 4.8 is a special case: Table 1 has no Rp0.2 column, so this chart uses Rpf,min 340 MPa.

Hardness is the fastest shop-floor check behind a class: ISO 898-1:2013 Table 4 gives HV ranges of 120–220 for 4.6, 130–220 for 4.8, 250–320 for 8.8, 320–380 for 10.9 and 385–435 for 12.9. The 8.8 range is diameter-dependent: 250–320 HV up to 16 mm nominal diameter, 255–335 HV above. Austenitic A2-70 and A4-80 have no hardness requirement in ISO 3506-1:2009 / GB/T 3098.6-2014 Table 2, which is why the matrix shows no HV value for those two classes.

Match the nut class to the bolt class: class 8 nuts (ISO 4032) are the standard pairing for 8.8 bolts, class 10 for 10.9 and class 12 for 12.9. The rule behind it is that the nut must be able to develop the bolt's minimum tensile load — an under-strength nut strips first and turns a ductile bolt failure into a sudden one. Stainless A2-70/A4-80 are normally paired with A2/A4 nuts of the same grade family. Nut height and thread tolerance depend on the nut type: check ISO 4032 (style 1) or ISO 4033 (style 2) for what you order.

Follow the View torque link at the end of any row: it opens the M12 bolt torque chart with that class highlighted. The torque family covers 6 classes (4.8, 8.8, 10.9, 12.9, A2-70, A4-80), giving a min–typical–max range for each friction state, and its figures come from the same source as this matrix — K-factor method T = K·D·F with target preload 70% of Rp0.2. Class 4.6 is not on the torque axis (low-strength, rarely tightened to a spec).