⚡ Key takeaways
1. The property class is a code, not decoration. On a carbon or alloy steel bolt, 8.8, 10.9 and 12.9 mean: the figure before the point is one hundredth of tensile strength (8.8 = 800 MPa) and the figure after is the yield ratio (0.8 = 80 %). On a stainless bolt, A2-70 means steel group A2 (304 family) with 700 MPa tensile.
2. Stainless is weaker than alloy steel but far more corrosion resistant. In M10, A4-80 gives 800 MPa against 1,220 MPa for class 12.9 — 50 % less. But stainless lasts twenty years outdoors or on the coast where a zinc-plated alloy bolt is rusted through in five.
3. Stainless is not for high temperature. Between 425 and 860 °C, 304 and 316 precipitate chromium carbides and become sensitive to intergranular corrosion. Continuous limits are about 425 °C for 304 and 450 °C for 316; above that you need Inconel or alloy steel.
4. Stainless galls against stainless. A stainless bolt in a stainless nut, hot, dry and heavily torqued, can seize halfway and have to be cut off. It is the most common stainless fastener failure on the shop floor.
5. Cost differs by three to ten times. For an M8×30 socket head: class 8.8 zinc plated = 1×, A2-70 ≈ 3–4×, class 12.9 black oxide ≈ 5–6×, A4-80 ≈ 10×. Buy for the duty, not for the highest number.
6. Do not use 12.9 as insurance. Class 12.9 is more brittle, more hydrogen sensitive and actually worse in fatigue than 10.9. Aerospace practice says take 10.9 unless 12.9 is unavoidable.
7. Three questions settle it: is the environment corrosive, what is the temperature, and how much strength is needed?
1. Reading a property class
2. Carbon and alloy steel versus stainless
3. Mechanical properties compared
4. Choosing A2, A4 or A5
5. Choosing 8.8, 10.9 or 12.9
6. Preventing galling
7. Cost comparison (2026)
8. Decision tree
9. Common misconceptions
10. FAQ
1. Reading a property class
The numbers stamped on a bolt head are defined by ISO 898-1 for carbon steel and ISO 3506-1 for stainless. Understanding both codes is half of the selection job.
1.1 Carbon and alloy steel: 8.8, 10.9, 12.9
For 8.8:
- The 8 before the point: tensile strength Rm = 8 × 100 = 800 MPa
- The 0.8 after: yield ratio Re/Rm = 0.8, so Re = 800 × 0.8 = 640 MPa
| Class | Tensile Rm (MPa) | Yield Re (MPa) | Typical material |
|---|---|---|---|
| 4.8 | 400 | 320 | Low-carbon steel Q235 |
| 5.8 | 500 | 400 | Low-carbon steel |
| 6.8 | 600 | 480 | Medium-carbon steel |
| 8.8 | 800 | 640 | Quenched and tempered medium carbon (ML35, SWRCH35K) |
| 10.9 | 1040 | 940 | Quenched and tempered alloy (42CrMo, SCM440) |
| 12.9 | 1220 | 1100 | Quenched and tempered alloy (SCM440, 35CrMo) |
1.2 Stainless: A2-70, A4-80
Format A2-70:
- A2: steel group. A1 free-machining, A2 = 304 family (18 % Cr, 8 % Ni), A3 = 321, A4 = 316 family with molybdenum, A5 = 316L.
- 70: tensile strength = 70 × 10 = 700 MPa.
| Class | Alloy | Tensile (MPa) | Typical use |
|---|---|---|---|
| A2-50 | 304, half hard | 500 | General structure, trim |
| A2-70 | 304, cold headed | 700 | The default for 80 % of stainless work |
| A2-80 | 304, work hardened | 800 | Where 304 needs more strength |
| A4-70 | 316, cold headed | 700 | Marine, coastal, chloride |
| A4-80 | 316, work hardened | 800 | Marine, high strength |
2. Carbon and alloy steel versus stainless
2.1 Chemistry
| Family | Typical grade | C | Cr | Ni | Mo |
|---|---|---|---|---|---|
| Low-carbon | Q235 / SWRCH18A | 0.18 % | — | — | — |
| Q&T alloy | 42CrMo / SCM440 | 0.42 % | 1.0 % | — | 0.2 % |
| 304 | A2 / 06Cr19Ni10 | ≤ 0.08 % | 18 % | 8 % | — |
| 316 | A4 / 06Cr17Ni12Mo2 | ≤ 0.08 % | 17 % | 12 % | 2.2 % |
2.2 Different strengthening mechanisms
Carbon and alloy steel are strengthened by heat treatment. Quench and temper turns martensite into tempered sorbite and lifts strength sharply. Classes 8.8, 10.9 and 12.9 are all quenched and tempered.
Stainless is strengthened by cold work. Austenitic stainless cannot be hardened by quenching — heating softens it — so strength comes from grain deformation during cold heading and drawing. A2-70 is stronger than A2-50 because it has seen more cold reduction.
2.3 Where corrosion resistance comes from
Carbon and alloy steel need a coating:
- Zinc plating: 8–12 μm, 72–100 hours of salt spray.
- Geomet (zinc flake): 500–1,000 hours.
- Zinc-nickel: 720–1,000 hours; the automotive chassis standard.
Stainless contains 18 % chromium and forms a self-healing Cr₂O₃ passive film, so it needs no coating to survive atmosphere, fresh water and mild acids. The molybdenum in 316 adds chloride resistance for seawater and coastal air.
3. Mechanical properties compared
M10×50 socket head cap screw, same size, different materials:
| Class | Tensile (MPa) | Yield (MPa) | Recommended preload (kN) | Max temperature | Corrosion |
|---|---|---|---|---|---|
| 4.8 carbon steel | 400 | 320 | 12 | 300 °C | Poor (needs plating) |
| 8.8 carbon steel | 800 | 640 | 25 | 400 °C | Moderate (zinc / Geomet) |
| 10.9 alloy steel | 1040 | 940 | 36 | 425 °C | Moderate (Geomet / Zn-Ni) |
| 12.9 alloy steel | 1220 | 1100 | 43 | 425 °C | Moderate (Geomet / Zn-Ni) |
| A2-70 (304) | 700 | 450 | 18 | 425 °C continuous | Good (atmosphere, fresh water) |
| A4-70 (316) | 700 | 450 | 18 | 450 °C | Very good (coastal, mild acid) |
| A4-80 (316) | 800 | 600 | 24 | 450 °C | Very good |
| Inconel 718 | 1200 | 1000 | 40 | 650 °C | Excellent (hot corrosion) |
4. Choosing A2, A4 or A5
4.1 A2-70 (304): the 80 % default
A2-70 is simply cold-headed 304. It resists:
- General indoor environments (equipment housings, cabinets, electronics)
- Fresh water (cooling towers, pumps)
- Weak acids and alkalis (food processing, pharmaceutical equipment)
- Non-coastal outdoor use (street furniture inland)
Do not use A2-70 for:
- Within 5 km of the coast: chloride breaks down the 304 passive film and causes pitting. Use A4-70 (316).
- Pools, chemicals, bleach: chloride is too high even for 316; use duplex or Inconel.
- Food contact with brine or soy sauce: A4-70 or A4-80.
4.2 A4-70 / A4-80 (316): marine and aggressive
316 carries 2.2 % molybdenum, giving about three times the pitting resistance of 304. Typical A4 applications:
- Ships, ports, offshore platforms
- Coastal buildings and bridge railings
- Food machinery with brine, soy or vinegar
- Medical equipment exposed to disinfectants and body fluids
- Swimming pool plant
A4-70 is cold headed, A4-80 is work hardened: 14 % stronger with slightly less ductility. A4-70 covers most work; take A4-80 only when you need the extra preload.
4.3 Above that: A5-80 (316L) and duplex
316L is the low-carbon version (C ≤ 0.03 %) that resists intergranular corrosion after welding, so welded stainless structures should specify A5. Duplex 2205 has twice the corrosion resistance of 316 at five times the cost, and is confined to chemical plant and offshore wind.
5. Choosing 8.8, 10.9 or 12.9
5.1 Class 8.8: the workhorse
Class 8.8 is the industrial default: strong enough, cheap, tough and not brittle. Eighty percent of steel structures, machine assembly and general machinery run on it. Material is ML35 medium carbon, cold headed then quenched and tempered, finished in blue-white zinc, yellow zinc or Geomet.
5.2 Class 10.9: high strength
When a smaller bolt must carry more load — wheel hubs, con-rods, robot joints — take 10.9 in 42CrMo or SCM440. It is the sweet spot: 30 % stronger than 8.8 with better toughness and fatigue life than 12.9. About 90 % of high-strength automotive bolts are 10.9.
5.3 Class 12.9: handle with care
12.9 is the highest commercial class at 1,220 MPa, with three problems:
- Hydrogen embrittlement. The higher the strength, the greater the risk. A 12.9 bolt plated without adequate baking can fracture hours to days after assembly. ASTM F1941 requires baking at 230 °C for four hours after plating.
- Brittleness. It fails with almost no plastic warning, which is a safety issue.
- Worse fatigue life. Higher strength means sharper stress concentration at the thread root, so the fatigue limit is actually below 10.9.
The only genuine case for 12.9 is a space-limited joint where the diameter cannot grow but the load exceeds 10.9 capacity. Otherwise 10.9 is the safer specification.
6. Preventing galling
This is the most painful stainless problem. Two stainless surfaces sliding under pressure, heat and no lubrication micro-weld and then tear — the nut seizes halfway down the bolt and has to be cut off.
6.1 Why stainless galls
The passive film is extremely thin. Once it ruptures, fresh metal contacts fresh metal under high pressure and welds. Carbon steel carries oxide and lubricant films that limit the effect.
6.2 Prevention
- Use different grades for bolt and nut: A2 bolt with A4 nut, or 304 with 316. Dissimilar chemistry welds less readily.
- Apply anti-seize: MoS₂, copper or nickel-based compounds. This is the single most effective measure.
- Slow the driver down: low speed on the power tool, not full speed.
- Use a looser thread fit: 6g/6H rather than an interference fit.
- Never assemble dry: a little oil or thread grease on the thread before assembly.
7. Cost comparison (2026)
M8×30 socket head cap screw, South China 2026 prices including tax:
| Class | Material / finish | Unit price (USD) | Relative |
|---|---|---|---|
| 4.8 carbon steel | Blue-white zinc | 0.011 | 1× |
| 8.8 carbon steel | Blue-white zinc | 0.021 | 2× |
| 10.9 alloy steel | Geomet | 0.063 | 5.6× |
| 12.9 alloy steel | Black oxide / Geomet | 0.111 | 10× |
| A2-70 (304) | Plain | 0.083 | 7.5× |
| A4-70 (316) | Plain | 0.208 | 19× |
| Inconel 718 | Plain | 4.86 | 440× |
8. Decision tree
- Above 425 °C?
- Yes: Inconel 718 or a high-temperature alloy steel.
- No: continue.
- Is the environment corrosive?
- Inland, indoor, dry: class 8.8 zinc plated.
- Outdoor, damp: A2-70.
- Coastal, chloride, food: A4-70 (316).
- Chemical, aggressive: A4-80 or duplex.
- How much load?
- Normal: 8.8 carbon steel.
- High load, limited space: 10.9 alloy steel.
- Extreme load at minimum size: 12.9, with caution.
- Impact or low temperature?
- Yes: prefer 10.9 over 12.9; stainless should be A4-80.
- No: as above.
9. Common misconceptions
No. A2-70 is 700 MPa, class 8.8 is 800 MPa and 10.9 is 1,040 MPa. Stainless buys corrosion resistance, not strength.
No. 12.9 is hydrogen sensitive, brittle and worse in fatigue. Aerospace guidance is explicit: 10.9 unless 12.9 is unavoidable.
No. 304 pits on the coast, sensitises between 425 and 860 °C and galls under dry friction. Stainless is not universally rust-proof.
That is the highest galling risk combination. Use an A2 bolt with an A4 nut, or apply anti-seize.
No. Heavy electroplating on class 12.9 introduces hydrogen and increases risk. For high-strength parts use Geomet or zinc-nickel rather than a thicker zinc layer.
10. FAQ
Q1. Can I tell 304 from 316 by eye?
No. A molybdate test liquid turns red on 201/304 and stays clear on 316. Optical emission spectroscopy is definitive.
Q2. Why do class 12.9 bolts break by themselves?
Hydrogen embrittlement. Hydrogen from the plating bath diffuses into the steel and collects at the thread root, causing delayed fracture. Bake at 230 °C for four hours after plating.
Q3. Can stainless bolts be used on engines?
Generally not. Exhaust temperatures exceed 500 °C, where 304 and 316 lose strength sharply. Exhaust fasteners use Inconel 718 or high-temperature alloy steel.
Q4. Which is stronger, A2-70 or 8.8?
8.8 at 800 MPa against 700 MPa, so 8.8 is 14 % stronger. Note the yield ratios differ (0.64 for A2-70, 0.80 for 8.8), so actual capacity depends on the joint.
Q5. What torque for a class 12.9 M10?
About 65–75 N·m at 90 % of yield with μ = 0.14. Check the ISO 898-2 tables rather than relying on habit.
Q6. Can zinc-plated bolts be used outdoors?
Blue-white zinc rusts within one to two years outside. Geomet gives five to ten. For coastal outdoor work specify A4-70 or zinc-nickel.
Q7. How many times can a stainless bolt be reused?
Five to ten. Every assembly wears the passive film and raises the galling risk. Inspect the thread on critical joints each time.
Q8. How do I know a bolt really is 12.9?
Genuine parts carry "12.9" and the maker's mark. Missing, blurred or suspiciously soft parts are often 8.8 relabelled. Critical applications need spectrometry and hardness sampling.
Q9. What class is a titanium bolt?
Ti-6Al-4V runs about 900 MPa at 60 % of the density of steel, giving the highest specific strength. It sits outside ISO 898 and is specified by material standard, for aerospace, motorsport and robotics where every gram counts.
Q10. What happens with stainless bolts in aluminium parts?
Galvanic corrosion. Aluminium is anodic and corrodes faster in damp conditions. Insulating grease, an insulating washer or an aluminium fastener solves it.
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