Choosing between A2-70 stainless steel bolts and Grade 8.8 carbon steel bolts is one of the most common fastener specification questions we field. Both are workhorse grades, but they solve different problems. At Jade Alloys, we manufacture and stock both, so this comparison is written from the position of someone who has to get the grade call right on both sides, not sell whichever one is closer to hand.
Grade 8.8 wins on raw strength, with higher tensile and noticeably higher yield strength. A2-70 wins on everything that matters once the bolt is installed outdoors, in a washdown area, or anywhere near moisture, since no coating is required to survive. Neither is simply “better.” Match the grade to the job.
A2-70 vs Grade 8.8 at a GlanceÂ
| Property | A2-70 Stainless Steel | Grade 8.8 Carbon Steel |
|---|---|---|
| Base material | Austenitic stainless, 18/8 (AISI 304 / EN 1.4301) | Medium carbon or carbon-boron steel |
| How it gets its strength | Cold work during forging and thread rolling | Quenched and tempered after forming |
| Minimum tensile strength | 700 MPa | 800 MPa (830 MPa typical mill result) |
| Minimum yield strength | 450 MPa | 640 MPa |
| Corrosion resistance | Good in atmosphere, fresh water, mild chemicals | None without coating |
| Typical finish supplied | Plain, passivated | Zinc plated, hot-dip galvanised, or black oxide |
| Relative cost | Roughly 3–5x a plated 8.8 bolt of the same size | Baseline, lowest cost per piece |
| Best suited to | Outdoor, washdown, food contact, mild chemical exposure | Indoor structural and machinery loads where coating is maintained |
What Sets A2-70 and Grade 8.8 Apart
A2-70 marking: A2 identifies an austenitic stainless alloy carrying 17.50–19.50% chromium and 8.00–10.50% nickel under ISO 3506-1. The “70” is one-tenth of the guaranteed minimum tensile strength in N/mm², so class 70 guarantees 700 MPa minimum.
Grade 8.8 marking: Under ISO 898-1, the first “8” is one-tenth of minimum tensile strength in kgf/mm². The second “8” is the yield-to-tensile ratio as a fraction of ten, so yield sits at roughly 80% of tensile. Multiply the two: 8 × 8 × 10 = 640 MPa minimum yield against an 800 MPa minimum tensile.
The bigger difference is how each grade gets there. A2-70 reaches its strength purely through cold work; there is no heat treatment step, and the head is cold-forged and the threads are rolled, which work-hardens the material as it’s shaped. Grade 8.8 reaches its strength through quenching and tempering. Austenitic stainless cannot go through that same cycle without destroying the microstructure that makes it corrosion resistant, which is exactly why a genuinely stainless bolt is never available in a heat-treated high-strength class the way carbon steel is.
This explains two things people notice on site:
- A brand-new A2-70 bolt sometimes pulls faintly on a magnet, because the worked areas around the head and threads convert some austenite grain structure to magnetic martensite, while the bulk stays non-magnetic austenite.
- An 8.8 bolt left outdoors without its coating rusts within weeks, because the strength mechanism in carbon steel provides no corrosion protection of its own.
Chemical Composition
| Element | A2-70 (wt %) | Grade 8.8 (wt %) |
|---|---|---|
| Carbon (C) | 0.07% max | 0.25%–0.55% typical, grade dependent |
| Chromium (Cr) | 17.50%-19.50% | Not specified; no corrosion protection from alloy content |
| Nickel (Ni) | 8.00%–10.50% | Not specified |
| Manganese (Mn) | 2.00% max | 0.60%–1.40% typical |
| Boron (B) | Not present | Present in some 8.8 boron steels, 0.0008%–0.005%, for hardenability |
| Silicon (Si) | 1.00% max | 0.15%–0.40% typical |
| Phosphorus (P) | 0.045% max | 0.025% max typical |
| Sulphur (S) | 0.030% max | 0.025% max typical |
ISO 3506-1 fixes A2-70 composition tightly, because corrosion performance depends on chromium and nickel sitting in a defined band. ISO 898-1, governing Grade 8.8, does not fix a single composition at all. It specifies mechanical performance only and leaves the steelmaker free to choose the carbon- or boron-treated chemistry that hits those figures. If a drawing needs a specific chemistry, call it out separately from the 8.8 marking.
Mechanical Properties Side by Side
| Property | A2-70 | Grade 8.8 |
|---|---|---|
| Tensile strength | 700 MPa min (101,500 psi) | 800 MPa min (116,000 psi) |
| Yield strength (0.2% proof) | 450 MPa min (65,250 psi) | 640 MPa min (92,800 psi) |
| Elongation at break | 0.4d min (multiple of diameter, ISO 3506-1) | 12% min (ISO 898-1) |
| Hardness | 250 HV max / 238 HB max / 100 HRB max | 255 HV max / 245 HB max / 99 HRB min |
| Behaviour at low temperature | Stays ductile, no brittle transition | Can suffer brittle fracture below roughly -20°C without a cold-rated grade |
Elongation is expressed differently between the two standards. ISO 3506-1 states A2-70 elongation as a multiple of nominal diameter, not a flat percentage, so the two columns aren’t directly comparable numbers even though both describe ductility before fracture. For cold-climate or refrigerated service, the low-temperature line alone can decide the grade.
Corrosion Resistance in Real Conditions
| Environment | A2-70 | Grade 8.8 |
|---|---|---|
| Dry indoor, climate-controlled | No advantage, unnecessary cost | Performs well, standard choice |
| Outdoor, rain and condensation | Performs well unprotected | Needs zinc plating or hot-dip galvanising; coating wears over years |
| Coastal or salt spray | Can pit over time; A4-80 is the better upgrade, not A2-70 | Corrodes quickly even coated; avoid |
| Food and beverage washdown | Standard choice, cleans without rust staining | Not suitable; coating fails under repeated washdown |
| Buried or embedded in concrete | Rarely needed; cost usually not justified | Standard choice with adequate cover and coating |
| High chloride chemical exposure | Not adequate; step up to 316 or duplex | Not suitable at all |
A2-70 is not the universal stainless answer even in corrosive service. Chlorides, whether from seawater, de-icing salt, or pool chemistry, will pit A2-70 over time because it lacks the molybdenum content that gives 316-grade and A4-80 their chloride resistance. Specifying A2-70 for a coastal railing because “it’s stainless” is a common, avoidable mistake. The correct upgrade is A4-80, not more A2-70.
Can You Substitute One for the Other on an Existing Drawing?
- Direct swap (8.8 to A2-70). A straight tensile swap rarely works cleanly. Substituting A2-70 for Grade 8.8 at the same nominal size drops available tensile capacity by roughly 12% and yield capacity by close to 30%. Any joint designed close to the 8.8 limit needs a size increase in A2-70, not a one-for-one swap.
- Worked example (M12). A Grade 8.8 M12 bolt carries a proof load of roughly 58.8 kN. The same nominal size in A2-70 carries roughly 51.7 kN. If the original design has a thin margin, stepping up to M14 in A2-70 restores an equivalent safety margin.
- Reverse swap (A2-70 to 8.8). More common and usually better justified, since most structural and machinery joints run with margin well below the ultimate tensile figure. Even so, confirm the original design was never pushed close to the top of the 8.8 tensile capacity before changing.
- Match the nut to the bolt. An A2-70 stud paired with a Grade 8.8 rated nut, or the reverse, defeats the point of the substitution and can gall on the stainless side or overload the softer nut on the carbon steel side.
Working through a substitution on your own drawing? Our stainless steel fastener range and carbon steel fastener range both list full size and grade options.
Torque and Clamp Load Comparison
| Size | A2-70 Lubricated (Nm) | Grade 8.8 Lubricated (Nm) | Grade 8.8 Dry (Nm) |
|---|---|---|---|
| M6 | 3.7 | 9.9 | 13.2 |
| M8 | 8.9 | 24.0 | 32.0 |
| M10 | 17.7 | 47.0 | 63.0 |
| M12 | 31.0 | 83.0 | 110.0 |
| M16 | 77.3 | 205.0 | 273.0 |
| M20 | 150.0 | 400.0 | 533.0 |
Lubricated figures assume a friction coefficient K = 0.15 on clean, greased threads. Dry figures for Grade 8.8 assume K = 0.20. Preload basis: roughly 45% of proof stress. (Pending QA confirmation against controlled ISO 3506-1 / ISO 898-1 copies; see open items below.)
Preventing Galling on A2-70 Threads
Stainless threads are prone to galling under dry, fast assembly. Running a stainless bolt down with an impact wrench rather than a controlled torque wrench is the single most common cause of a seized stainless fastener on site.
1. Always Lubricate Before Assembly
Use an anti-seize or a lubricant rated for stainless-to-stainless contact on every A2-70 joint, even where corrosion isn’t a concern for the lubricant itself. The K = 0.15 torque figures above assume it.
2. Assemble by Controlled Torque, Not Impact Wrench
Run A2-70 fasteners down with a calibrated torque wrench. The heat generated by fast, high-speed impact assembly is what causes the thread surfaces to gall and seize.
3. Avoid Repeated Assembly and Disassembly Without Relubrication
Each removal and reinstallation increases gall risk. Re-apply lubricant before reassembly, and inspect threads for early gall marks before reuse.
Cost Comparison and When the Premium Is Worth It
A2-70 typically costs three to five times a plated Grade 8.8 bolt of the same size. That gap narrows for larger diameters bought in bulk and widens for small fasteners, where base material cost is a smaller share of total price.
The comparison that actually matters is total cost over the service life of the joint, not the purchase-order price. A coated Grade 8.8 bolt in a mild outdoor environment can still be cheaper across a ten-year horizon, provided the coating is inspected and refreshed on schedule. In washdown, coastal, or chemical exposure, the same coated bolt is usually more expensive over that horizon once you count replacement labour, downtime, and a failed fastener discovered too late, against specifying stainless once and not thinking about it again.
Need piece weights across a full bill of materials? Our weight calculator does the sums for either grade.
Decision Chart: Which Grade for Which Job
| Application | Recommended Grade | Why |
|---|---|---|
| Structural steel framing, indoors | Grade 8.8 | Higher load capacity; coating risk is manageable indoors |
| Outdoor railing and architectural fixings | A2-70 | No coating maintenance; handles weather without rust staining |
| Food processing equipment | A2-70 | Washdown chemistry and hygiene requirements rule out coated carbon steel |
| Machinery mounting, indoor, high load | Grade 8.8 | Load capacity is the driver; corrosion risk is low |
| Coastal handrails and fixings | A4-80, not A2-70 | Chlorides pit A2-70 over time; upgrade to 316 grade |
| Water treatment plant, fresh water | A2-70 | Good resistance, no chloride exposure to worry about |
| Automotive underbody fixings | Grade 8.8, coated | Load-driven; coating is standard practice in this industry |
If your application doesn’t sit cleanly on this list: will the joint see moisture, and does the load calculation genuinely need the extra strength Grade 8.8 offers? Answer those honestly and the grade usually picks itself.
Frequently Asked Questions
1. Is A2-70 stronger than Grade 8.8?
2. Can Grade 8.8 bolts be used outdoors?
3. Does A2-70 stainless steel rust?
4. What is the equivalent of Grade 8.8 in stainless steel?
5. Why is Grade 8.8 not made from stainless steel?
6. Which is cheaper, A2-70 or Grade 8.8?
7. Can I mix A2-70 bolts with Grade 8.8 nuts?
7. What thread standards do A2-70 and Grade 8.8 bolts share?
Jade Alloys has manufactured and exported industrial fasteners from Mumbai since 1985. We are ISO 9001:2015 certified; every shipment carries mill test certificates to EN 10204 3.1, third-party inspection is available through SGS, TUV and Bureau Veritas, and we supply more than forty countries across the Gulf, Southeast Asia, Europe, Africa and the Americas.
We manufacture and stock both A2-70 stainless steel bolts and Grade 8.8 carbon steel bolts, along with the full range of nuts, washers and stud bolts to match either grade. If you’re still weighing the two for a specific joint, our standard information pages and stud bolts range cover the dimensional side, and our technical team is happy to talk through the application.
