How to Choose Stainless Fastener Grades: A2-70 and A4-80 Explained per GB/T 3098.6
Executive Summary
The "A2-70" and "A4-80" stamped on stainless bolt heads are a completely different language from the "8.8" on carbon-steel bolts. Per ISO 3506 and GB/T 3098.6, it is a two-part code of "group + property class": A denotes an austenitic stainless group (A2≈304, A4≈316), and the figure after the dash is one-tenth of nominal tensile strength. This article explains group differences, class meaning, corrosion-resistance comparison, and the engineer's biggest headache—galling.
Table of Contents
How to Read the Code: Two-Part Group + Class Meaning
Per GB/T 3098.6 (corresponding to ISO 3506-1) Chapter 5 "Designation of stainless-steel groups and property classes", the property-class code for corrosion-resistant stainless fasteners has two parts: the first part is the stainless-steel group code (a letter plus a digit), and the second part is the property class (the figure after the dash). In "A2-70" for example, "A2" denotes austenitic group 2, and "70" denotes one-tenth of nominal tensile strength, i.e. Rm,nom = 70×10 = 700 MPa.
| Group Code | Microstructure | Rough Equivalent Grade | Typical Traits |
|---|---|---|---|
| A1, A2, A3, A4, A5 | Austenitic | A2≈304 (18-10), A4≈316 (Mo-bearing) | Non-magnetic, corrosion-resistant, strengthened by cold work; by far the most used |
| C1, C3, C4 | Martensitic | C1≈410, C3≈420 series | Can be quenched and tempered; high strength, moderate corrosion resistance |
| F1 | Ferritic | ≈430 series | Magnetic, moderate corrosion resistance; cannot be hardened by quenching |
Of these, the A-series austenitic stainless is the source of over 90% of "stainless bolts" in engineering. The common classes after the dash are 50, 70 and 80: A2-50 = Rm≥500 MPa, A2-70 = Rm≥700 MPa, A2-80 = Rm≥800 MPa. The larger the figure, the higher the cold-work hardening, the higher the strength and the slightly lower the ductility.
Austenitic Groups and Corrosion Resistance (A2/A4/A5)
Within the "A" series, corrosion resistance differs mainly by alloy content, centered on chromium (Cr), nickel (Ni) and molybdenum (Mo). A2 corresponds to the classic 18-10 chromium-nickel steel (roughly US-grade 304) and is the most versatile; A4 adds about 2% molybdenum on top of A2, markedly raising pitting and chloride-corrosion resistance (roughly 316).
| Group | Composition Trait | Corrosion-Resistance Positioning | Suitable Medium |
|---|---|---|---|
| A1 | Free-machining austenitic (raised sulfur) | General corrosion resistance | Standard parts, self-tappers, assembly with low corrosion demands |
| A2 | 18Cr-10Ni (≈304) | Atmosphere, fresh water, food, general chemicals | Indoor equipment, appliances, construction, general environments |
| A3 | Ti/Niobium-stabilized austenitic (≈321) | Better intergranular-corrosion resistance | Welded or higher-temperature service |
| A4 | Mo-bearing austenitic (≈316) | Pitting and chloride resistance | Coastal, seawater, chemical, medical, damp environments |
| A5 | Mo-bearing stabilized austenitic (≈316Ti) | Pitting + intergranular corrosion resistance | Welded structural parts, higher-spec corrosion-resistant joints |
Strength Figures of A2-70 and A4-80 Compared
Below are the typical mechanical-property figures for austenitic stainless bolts, screws and studs (per ISO 3506-1 / GB/T 3098.6 tabulated values). The biggest difference from carbon steel is that austenitic stainless cannot be strengthened by quenching; its strength comes from cold-drawn/cold-headed cold-work hardening, so "50, 70, 80" correspond to different degrees of cold deformation.
| Class | Tensile Rm,min | 0.2% Proof Rp0.2,min | Condition |
|---|---|---|---|
| A2-50 / A4-50 | 500 MPa | 210 MPa | Soft / lightly cold-worked; good ductility |
| A2-70 / A4-70 | 700 MPa | 450 MPa | Cold-work hardened; the most common stock class |
| A2-80 / A4-80 | 800 MPa | 600 MPa | Higher cold work; high strength, slightly lower ductility |
| C1-110 (martensitic) | 1100 MPa | — | Quenched and tempered; reaches near carbon-steel 8.8 strength |
Note: even when A4-80 reaches 800 MPa tensile, its yield is lower than carbon steel and its Young's modulus is only about 193 GPa (steel about 206 GPa); during assembly you cannot simply apply carbon-steel torque values, and because austenitic is not hard, repeated tightening easily damages the threads by adhesion.
This modulus difference matters in practice: under the same clamp force, a stainless bolt stretches slightly more than a steel bolt of the same diameter, which changes how much preload you actually achieve for a given torque turn. Combined with the much higher and more scattered friction coefficient of dry stainless, the rule "use the carbon-steel torque table" is exactly how under-tightened or over-tightened stainless joints get built. The proper path is always a measured K value and a torque-clamp-force test on the actual coated/lubricated pair, then tightening to that—not to a number copied from a carbon-steel chart.
Why Stainless Bolts Gall, and How to Prevent It
Galling (cold welding / thread seizure) is the number-one headache of stainless assembly: when two stainless threads rotate relative to each other under high pressure, the contacting surfaces directly adhere and tear, eventually seizing bolt to nut or even twisting the bolt off. The cause is that austenitic stainless is soft with a strong self-fusing tendency, and frictional heat locally welds the surfaces.
- Lubrication is the first countermeasure: coat threads and bearing surfaces with molybdenum disulfide (MoS₂), nickel-based anti-seize compound or dedicated stainless thread grease to markedly reduce friction and prevent adhesion.
- Control speed and torque: use slow tools; avoid high-speed spinning that generates heat; apply torque per a measured K value, not by "feel".
- Avoid same-material mating: an A2 bolt with an A2 nut gall easiest under high pressure; pair different groups or add lubrication.
- Thread precision and cleanliness: ensure 6g threads are free of burrs and iron chips, bearing faces flat, reducing local high-pressure points.
Selection: When You Must Use A4
Combining group and class into one selection decision table:
- Indoor, atmosphere, fresh water, food equipment: A2-70 is enough; universal, fully stocked, low cost.
- Coastal, salt spray, seawater, damp chemical, medical environment: choose A4-70 or A4-80 directly; molybdenum resists chloride pitting.
- Welded or higher-temperature service: choose A3/A5 stabilized groups to lower intergranular-corrosion risk.
- Need higher strength with moderate corrosion demands: consider the martensitic C series (hardenable) rather than blindly raising austenitic cold-work class.
- High-pressure, galling-prone service: thread lubrication and slow assembly are mandatory; pair different-group materials if needed.
Conclusion: How to Remember A2 vs A4
Stainless fasteners are designated per GB/T 3098.6 (ISO 3506-1) as "group + class": A2≈304, A4≈316 (Mo-bearing), and the figure after the dash is one-tenth of nominal tensile strength—A2-70 means Rm≥700 MPa, Rp0.2≥450 MPa. Use A2-70 indoors; upgrade to A4-70/80 in coastal/chloride environments, and during assembly be sure to lubricate, control speed and prevent galling.
YF Zhichengjia, founded in Shenzhen in 2005, is a fastener manufacturer and can provide selection and supply for A2-70, A4-70, A4-80 and other stainless bolts, screws and custom parts. For more thread-joining technical material, see the technical documentation library, or contact our technical engineers for a service-condition and material-matching evaluation.
