How to Choose Stainless Fasteners: The Applicability Boundaries of GB/T 3098.6 and A2/A4
Executive Summary
Stainless fasteners do not "never rust," and the difference between A2 and A4 is not just price. This article, based on GB/T 3098.6-2023, explains the property class system for stainless fasteners, the composition and applicability boundaries of A2/A4, why their strength is lower than carbon steel, and the typical failures from choosing the wrong grade in chloride environments.
Table of Contents
- "Stainless" Does Not Mean "Stain-Free"
- The Property Class System of GB/T 3098.6
- Why Stainless Strength Is Lower Than Carbon Steel
- Composition and Boundaries of A2 and A4
- Three Engineering Considerations for Stainless Fasteners
- Quick Selection Reference for Stainless Fasteners
- Conclusion: Breaking "Stainless" Down into Verifiable Conditions
"Stainless" Does Not Mean "Stain-Free"
A common purchasing misconception is treating stainless fasteners as maintenance-free, never-corroding parts. In actual failure cases, the proportion of stainless bolts exhibiting pitting, crevice corrosion, and stress corrosion cracking is not low, and it often occurs in coastal or outdoor environments where problems seem least expected. The root cause is: the corrosion resistance of stainless steel comes from an extremely thin passivation film on the surface (mainly Cr2O3, about 1–3 nm thick). This film can self-repair in oxidizing environments, but in three types of environments—chloride ions, reducing acids, and oxygen-depleted crevices—it is damaged and difficult to rebuild. Once this is understood, it becomes clear why A2 and A4 have explicit applicability boundaries.
The Property Class System of GB/T 3098.6
GB/T 3098.6-2023 (Mechanical properties of fasteners—Stainless steel bolts, screws, and studs) uses a class notation completely different from carbon steel, marked as a combination of "steel group + property class," such as A2-70 and A4-80. A2 denotes austenitic stainless steel (mainly Cr, Ni); A4 denotes austenitic stainless steel containing Mo; the trailing number 70/80/100 denotes one-tenth of the tensile strength (MPa).
| Property Class | Tensile Strength Rm (MPa) | Rpf (MPa) | Typical Application |
|---|---|---|---|
| A2-50 / A4-50 | 500–700 | 210 | Soft condition, for forming |
| A2-70 / A4-70 | 700–850 | 450 | Cold-worked, most common |
| A2-80 / A4-80 | 800–1000 | 600 | Heavily cold-worked |
| A4-100 | 1000–1200 | 900 | Special strengthened, size-limited |
Why Stainless Strength Is Lower Than Carbon Steel
The difference between GB/T 3098.6 and the carbon steel system is essentially a difference in strengthening mechanism: carbon steel relies on martensitic structure formed by quenching and tempering and can achieve tensile strength above 1000 MPa; austenitic stainless steel has no martensitic transformation and can only rely on cold work hardening to raise strength, with a theoretical limit far below that of quench & tempered steel.
| Item | Grade 8.8 Carbon Steel | A2-70 Stainless |
|---|---|---|
| Tensile strength (MPa) | 800–965 | 700–850 |
| Rpf (MPa) | 640 | 450 |
| Elongation | ≥12% | about 40% or more |
As shown, the yield strength of A2-70 is only about 70% that of grade 8.8. This means: replacing a carbon steel bolt with a stainless bolt of the same size will markedly lower the upper preload limit—if the torque is not recalculated, the joint may loosen due to insufficient preload. This is the most commonly overlooked effect of substituting stainless.
Composition and Boundaries of A2 and A4
| Item | A2 (304 Series) | A4 (316 Series) |
|---|---|---|
| Main composition | Cr 17%–19%, Ni 8%–11% | Cr 16%–18.5%, Ni 10%–14%, Mo 2%–3% |
| Pitting resistance | Average | Significantly improved |
| Chloride resistance | Weak, prone to pitting | Stronger, tolerates a certain chloride concentration |
| Applicable environment | Indoor, dry atmosphere, fresh water | Coastal, chloride media, chemical, food/pharma |
| Relative cost | Baseline | about 20%–40% higher |
A2 should be used cautiously or prohibited in three types of scenarios: coastal high-salt-spray environments (high chloride concentration; A2 pits very readily, usually showing rust spots within 500 h of neutral salt spray); long-term damp structures with crevices (oxygen-depleted crevices under washers and at thread engagements are the starting points of pitting and crevice corrosion); and environments with chlorine-containing cleaners or disinfectants (hypochlorite, common in food, pharma, and pool equipment, rapidly destroys the passivation film).
The improved corrosion resistance of A4 comes from molybdenum, but it is not universal. Against stress corrosion cracking from the combination of high-concentration chloride + tensile stress + rising temperature, it still lacks sufficient resistance; when the medium temperature exceeds 60°C, higher-grade alloys (such as 904L) or titanium should be considered.
Three Engineering Considerations for Stainless Fasteners
Consideration 1: Galling (Cold Welding)
After the passivation film on austenitic stainless steel is broken by friction, fresh metal surfaces contact directly and, under pressure and frictional heat, cold weld, causing thread galling. Countermeasures include: using anti-galling lubricating coatings containing MoS2 or PTFE; lowering the tightening speed to reduce frictional heat accumulation; and wherever possible using dissimilar grades for mating internal and external threads.
Consideration 2: Galvanic Corrosion
The potential of stainless steel is higher than that of carbon steel and aluminum. When a stainless fastener joins aluminum, the aluminum becomes the anode and corrodes faster; when it joins carbon steel, the carbon steel corrodes faster. The countermeasure is to add an insulating barrier at the contact surface or select compatible materials.
Consideration 3: Cannot Be Strengthened by Heat Treatment
Austenitic stainless fasteners cannot be strengthened by quenching; if higher strength is needed, one can only choose martensitic stainless steel (group C in GB/T 3098.6) or precipitation-hardening stainless steel—but the former has lower corrosion resistance than A2, and the latter is more costly.
Quick Selection Reference for Stainless Fasteners
| Environment | Recommended Grade and Class | Notes |
|---|---|---|
| Indoor dry, general assembly | A2-70 | Best cost-performance |
| Outdoor industrial atmosphere, fresh water | A2-70 / A4-70 | Upgrade to A4 if needed |
| Coastal, salt-spray environment | A4-70 / A4-80 | A2 not applicable |
| Food/pharma, requires washing | A4-70 with passivation | Watch for chloride in cleaners |
| Low temperature (down to -196°C) | A2-70, must use low-carbon grade (L type) | Austenitic stainless has excellent low-temperature toughness |
A guiding principle: when choosing stainless, first define the medium and environment, then the grade, and only finally the property class. Working in reverse—fixing strength first and then looking for a material—very easily leads to a dead end, because the strength ceiling of austenitic stainless steel is set by the material system, not something process can break through.
Conclusion: Breaking "Stainless" Down into Verifiable Conditions
The difficulty in selecting stainless fasteners is not in knowing that A4 is better than A2, but in knowing under what conditions A2 is already sufficient and under what conditions even A4 is not enough. The class system of GB/T 3098.6-2023, combined with an understanding of the passivation film mechanism, turns this from choosing the expensive option into choosing the right one.
