Grades & Materials | Class Selection and Properties
8 material and class families covering ambient to 550°C, marine atmospheres and CIP washdown — from food grade to heavy industry.
| Class | Material & standard | Service temp. | Service pressure | Size range | Tensile Rm |
|---|---|---|---|---|---|
| 8.8 | Medium-carbon steel (35#/45#), GB/T 3098.1 | — | — | — | Rm ≥ 800 MPa |
| 10.9 | Medium-carbon alloy steel (35CrMo), quenched & tempered | — | — | — | Rm ≥ 1000 MPa |
| 12.9 | Alloy steel (42CrMo etc.), quenched & tempered | — | — | — | Rm ≥ 1200 MPa |
| A2-70 | Austenitic stainless 304 / 304L (ISO 3506) | — | — | — | Rm ≥ 700 MPa |
| A4-80 | Austenitic stainless 316 / 316L (ISO 3506) | — | — | — | Rm ≥ 800 MPa |
Figures shown are typical for class 8.8 (GB/T 3098.1 / ISO 898-1). Values vary with class, coating and heat treatment - full data on each class page.
Class & finish selection by duty
Typical class and finish guidance - send your parameters for engineering confirmation.
How is tightening torque calculated?
Use the simplified formula T = F x (0.16P + 0.58d2mu + 0.775dmu): preload F = 0.7 x yield strength x tensile stress area As (d2 = d - 0.6495P, d3 = d - 1.2269P, As = pi/4 x ((d2+d3)/2)^2), mu = overall friction coefficient. For quick field checks use T = 0.2 x F x d.
Grade 8.8 or 10.9 - which to choose?
8.8 (800 MPa tensile, 640 MPa yield) covers ordinary static structures. Choose 10.9 (1000/940 MPa) under alternating loads, vibration or high reliability demands - it allows higher preload and fights micro-movement fatigue. 10.9 typically costs 20-40% more.
Which friction coefficient should I use?
Three steps: well lubricated (oil / phosphated + soaped) mu about 0.10; lightly oiled (normal as-shipped) mu about 0.14; dry assembly mu about 0.20. Dropping mu from 0.20 to 0.10 nearly doubles the preload from the same torque - so standardize lubrication and use torque wrenches in production.
Why must plated 10.9+ bolts be de-embrittled?
Acid pickling during plating drives hydrogen into the steel, and high-strength quenched-and-tempered parts are sensitive to hydrogen embrittlement. Skipping the 190-220°C bake plants a delayed-fracture risk - sudden failure hours to days after assembly. Confirm the bake when ordering plated high-strength fasteners.
Can A2-70 stainless replace grade 8.8?
Not in load-bearing service. A2-70 provides 700 MPa tensile and 450 MPa yield, below 8.8 (800/640 MPa), with no 8.8-level yield guarantee; its strength advantage is corrosion resistance. For high strength plus corrosion resistance, use alloy steel Q&T with a hydrogen-free coating such as Dacromet.
Fastener Property Classes Explained | 8.8 / 10.9 / 12.9 and A2-70 / A4-80
Fastener class numbers read straight off the mechanical properties: in 8.8 the leading 8 means 800 MPa nominal tensile strength and the .8 means a yield ratio of 0.8, i.e. 640 MPa yield. Class 10.9 is 1000 MPa tensile with 940 MPa minimum yield. Values follow GB/T 3098.1 (ISO 898-1); stainless classes follow ISO 3506. Every class is supplied as a matched bolt-and-nut set with a mill test certificate and tensile / hardness reports.
Carbon Steel Class 8.8 Bolts & Nuts | 800 MPa Tensile · 640 MPa Yield · 22–32 HRC
Class 8.8 is the broadest-reaching fastener grade, made from 35# / 45# medium-carbon steel with 800 MPa nominal tensile strength, 640 MPa yield and 22–32 HRC hardness. Its fracture toughness is good and quenching and tempering is not mandatory. Priced as the baseline (1.0), it is the default class for machinery, steel structures and automotive assembly. Our 8.8 bolts and nuts come off the same batch, M6–M24 are held in stock, and each shipment carries a mill certificate plus tensile and hardness reports.
Alloy Steel Class 10.9 High-Strength Bolts | 1000 MPa Tensile · 940 MPa Yield · Quenched & Tempered 35CrMo
Class 10.9 is made from medium-carbon alloy steel 35CrMo and is quenched and tempered to 1000 MPa nominal tensile strength, 940 MPa minimum yield and 32–39 HRC. It is the workhorse class for high load, vibration and alternating-load duty. Because it accepts a higher preload than 8.8, the joint faces are clamped harder and micro-movement fatigue resistance improves markedly. Price runs about 1.2–1.4× class 8.8. Note that plated parts at 10.9 and above must be hydrogen baked at 190–220°C, otherwise pickling-induced hydrogen leaves a delayed-fracture risk.
Alloy Steel Class 12.9 Socket Screws | 1200 MPa Tensile · 39–44 HRC
Class 12.9 is the highest commonly supplied metric property class: quenched-and-tempered 42CrMo-type alloy steel at 1200 MPa nominal tensile, 1100 MPa minimum yield and 39–44 HRC. It serves moulds, hydraulics and power heads where envelope is tight and high preload is required, and ships in black oxide as standard. The trade-off is toughness: 12.9 is brittle, dislikes impact and is the most sensitive class to delayed fracture, so hydrogen baking after plating is mandatory. In corrosive service switch to A4-80 stainless or a lower class with a Delta-Tone coating.
Low-Carbon Steel Class 4.8 Fasteners | The Economical Choice for Furniture and Light Assembly
Class 4.8 is the entry grade of the carbon-steel fastener system, intended for light-duty work such as furniture, general brackets and trim, priced below class 8.8 and usually supplied in blue-white or yellow zinc. It looks almost identical to an 8.8 part, which is why substituting 4.8 for 8.8 is one of the most common misuses — the 4.8 head mark is easy to read, so verify marks part by part on receipt. For any load-bearing, vibrating or alternating-load joint, design at class 8.8 or above rather than saving here.
Stainless Steel A2-70 (304) Fasteners | 700 MPa Tensile · PREN 18–20 Corrosion Data
A2-70 is the most widely used stainless fastener class, made from 304 / 304L (18Cr-8Ni, no molybdenum) at 700 MPa nominal tensile strength. It relies on a passive oxide film for corrosion resistance and suits indoor dry, inland outdoor and food-equipment environments at a mid-range cost. Two points matter: its strength sits below class 8.8 and must not be treated as an equal-strength substitute, and because 304 carries no molybdenum it will pit — and can even stress-corrosion crack — in chloride service such as seawater, salt spray and de-icing salt. Coastal projects must move to 316. Stainless is not rust-proof in an absolute sense; the deciding question is whether chlorides are present.
Stainless Steel A4-80 (316) Fasteners | PREN 24–26 for Coastal and Chemical Service
A4-80 is made from austenitic stainless 316 / 316L (16Cr-10Ni-2–3Mo), and the molybdenum gives it clearly better pitting resistance than 304: PREN rises from 18–20 to 24–26, about 35% higher, which in practice moves the corrosion safety envelope by a whole environment class. Coastal, salt-spray, de-icing-salt, chemical and marine projects should go straight to A4-80 — this is why the legacy material table assigns 316 to marine and high-corrosion service. The price is a higher raw-material cost than 304, and the strength remains mid-range; 316 cannot reach class 10.9.
Delta-Tone Coated Class 10.9 Bolts | 500–1000 h+ Salt Spray · 300°C · No Hydrogen Embrittlement
When you need class 10.9 strength together with salt-spray life, the answer is not stainless but Delta-Tone, a zinc-flake coating. The process is non-electrolytic, so no hydrogen is introduced and no baking step is needed. Film thickness is only 4–10 µm, which lets the parts mate on the standard thread — unlike hot-dip galvanising, where the nut has to be opened up by 0.2–0.4 mm. Neutral salt spray reaches 500–1000 h+ and temperature capability about 300°C. Our class 10.9 hex bolts, socket screws and studs can all ship Delta-Tone coated; for demanding export markets we default-recommend Delta-Tone on anything at class 8.8 and above.
Brass and Aluminium Fasteners | For Electrical, Light-Duty and Decorative Joints
In the legacy material table, brass and aluminium occupy the 'electrical and light-duty decorative' row — they sit outside the 8.8 / 10.9 / 12.9 property-class system, so selection is driven by conductivity, density, corrosion behaviour and appearance rather than tensile strength. Brass serves terminals, instruments and decorative hardware; aluminium serves weight-critical structures and panel parts. Two cautions: aluminium or brass against steel creates a galvanic couple, and these non-classed parts carry no metric property mark, so the order must state material and standard, not just the thread size.
Standard, size and length, load, media and service environment.
We size the class, coating and torque against load and corrosion.
Samples with test reports, so performance is proven before volume.
Not sure which class or finish? Let our engineers help
Submit your joint conditions — temperature, media, load, size — and get class selection plus a quotation within 24 hours.