Why fasteners are made from wire rod
Almost all bolts, screws, nuts and rivets up to about M24 are cold formed from wire rod: round steel, 5.5 to about 40 mm thick, supplied in coils. Cold-heading wire rod is made to tighter limits than ordinary steel: low sulphur and phosphorus, a clean surface and controlled decarburisation, because any flaw opens up into a crack when the head is formed. Larger bolts and studs are made from straight bar.
What ISO 898-1 allows for each property class
ISO 898-1 does not name a specific steel. It sets limits on chemistry and heat treatment, and the bolt must then meet the strength and hardness tests. The main limits (cast analysis, %) are:
| Class | Steel type allowed | Carbon | P / S max | Heat treatment |
|---|---|---|---|---|
| 4.6, 5.6 | Carbon steel, with or without additives | up to 0.55 (5.6: 0.13–0.55) | 0.050 / 0.060 | Not required |
| 4.8, 5.8, 6.8 | Carbon steel, with or without additives | up to 0.55 | 0.050 / 0.060 | Not required (strength from cold work) |
| 8.8 | Carbon steel with B, Mn or Cr; carbon steel; or alloy steel | 0.15–0.40 / 0.25–0.55 / 0.20–0.55 | 0.025 / 0.025 | Quench and temper, min 425 °C |
| 10.9 | Carbon steel with B, Mn or Cr; carbon steel; or alloy steel | 0.20–0.55 / 0.25–0.55 / 0.20–0.55 | 0.025 / 0.025 | Quench and temper, min 425 °C |
| 12.9 | Alloy steel | 0.30–0.50 | 0.025 / 0.025 | Quench and temper, min 425 °C |
Boron is limited to 0.003% (0.005% if extra boron is tied up by titanium or aluminium). "Alloy steel" must contain at least 0.30% Cr, 0.30% Ni, 0.20% Mo or 0.10% V. From 8.8 upward the steel must also be hardenable enough to reach about 90% martensite in the core of the thread before tempering.
Typical steels used in practice
Each maker picks its own grade within those limits. These are common choices; the exact grade varies with size, supplier and region.
| Use | Typical wire rod grades | Why |
|---|---|---|
| 4.6 / 4.8 bolts, rivets, soft nuts | Low-carbon cold-heading steel: C4C–C15C (EN 10263-2), SWRCH10A–18A (JIS), 1008–1018 (SAE) | Very ductile, forms easily, no heat treatment |
| 8.8 bolts, smaller sizes | Boron steels: 20MnB4, 23MnB4 (EN 10263-4), 10B21 (SAE) | A small boron addition makes low-carbon steel hardenable, so it forms easily and still reaches 8.8 |
| 8.8 bolts, larger sizes | Medium-carbon steel: C35, C45, SWRCH35K/45K, 1035 | More carbon for strength in thicker sections |
| 10.9 bolts | Boron or Cr-boron steels: 32CrB4, 35B2; Cr-Mo for larger sizes | Higher hardenability for full hardening through the section |
| 12.9 socket screws, high-strength studs | Cr-Mo alloy steels: 34CrMo4, 42CrMo4 (SCM435 / SCM440, 4140) | Hardens through thick sections and stays tough after tempering |
Chemistry of the common fastener steels
Nominal chemical composition ranges (%) of the steels most used for high-tensile bolts, from the steel standards. Phosphorus and sulphur are typically held at 0.025–0.030% maximum for cold-heading quality.
| Steel | Standard | C | Si | Mn | Cr | Mo | B | Typical use |
|---|---|---|---|---|---|---|---|---|
| SAE 1010 / C10C / SWRCH10A | SAE J403 / EN 10263-2 / JIS G 3507 | 0.08–0.13 | ≤0.10 | 0.30–0.60 | – | – | – | 4.6, 4.8, rivets, soft nuts |
| SAE 1018 / C17C / SWRCH18A | SAE J403 / EN 10263-2 / JIS | 0.15–0.20 | ≤0.10 | 0.60–0.90 | – | – | – | 4.8, 5.8, case-hardened screws |
| SAE 10B21 | SAE J403 (boron) | 0.18–0.23 | 0.15–0.30 | 0.70–1.00 | – | – | 0.0005–0.003 | 8.8 up to about M16 |
| 20MnB4 | EN 10263-4 | 0.18–0.23 | ≤0.30 | 0.80–1.10 | ≤0.30 | – | 0.0008–0.005 | 8.8 |
| SAE 1035 / C35 / SWRCH35K | SAE J403 / EN 10263-4 / JIS | 0.32–0.38 | 0.10–0.35 | 0.60–0.90 | – | – | – | 8.8 larger sizes, nuts |
| SAE 1045 / C45 | SAE J403 / EN 10083-2 | 0.43–0.50 | 0.10–0.40 | 0.60–0.90 | – | – | – | 8.8 bolts and studs, class 8 nuts |
| 32CrB4 | EN 10263-4 | 0.30–0.34 | ≤0.30 | 0.60–0.90 | 0.90–1.20 | – | 0.0008–0.005 | 10.9 |
| SCM435 | JIS G 4053 | 0.33–0.38 | 0.15–0.35 | 0.60–0.90 | 0.90–1.20 | 0.15–0.30 | – | 10.9, 12.9 socket screws |
| 42CrMo4 / EN19 | EN 10083-3 / BS 970 | 0.38–0.45 | ≤0.40 | 0.60–0.90 | 0.90–1.20 | 0.15–0.30 | – | 12.9, high-strength studs |
| AISI 4140 | SAE J404 | 0.38–0.43 | 0.15–0.35 | 0.75–1.00 | 0.80–1.10 | 0.15–0.25 | – | A193 B7 studs, 12.9 |
Ranges are the usual heat-analysis limits of each steel standard; check the mill certificate for the actual values of your lot.
Why boron matters
A few thousandths of a percent of boron greatly increases how deeply steel hardens when it is quenched. That lets fastener makers use a softer, lower-carbon steel that is easy to cold form and kind to dies, and still reach 8.8 or 10.9 after heat treatment. This is why boron steels are so widely used for high-tensile bolts.
Stainless steel: A2 and A4
Stainless fasteners follow ISO 3506. The letter and number give the steel group: A2 is the 304 family (18% chromium, 8% nickel) and A4 adds molybdenum, as in 316, for better resistance to chlorides, salt water and many chemicals. The number after the dash gives strength: 50, 70 or 80 means 500, 700 or 800 MPa minimum tensile strength.
Austenitic stainless steel cannot be hardened by heat treatment. Its strength comes from cold working during drawing and heading: class 50 is lightly worked or machined, class 70 is the normal cold-formed product, and class 80 is heavily cold worked. That is why A2-70 and A4-70 are the usual stock grades, and A4-80 is a stronger, special-order option.
Other materials
- Brass is used for electrical parts and decorative fixings. It conducts well and does not spark, but is much weaker than steel.
- Aluminium is used for blind rivets and light fixings. It matches aluminium sheet and avoids galvanic corrosion.
- Titanium (Grade 2, Grade 5 / Ti-6Al-4V) is used for aerospace, marine and medical parts. It is very strong for its weight and highly corrosion resistant.
- Nickel alloys such as Alloy 718 and C-276 are used for very high temperatures and aggressive chemicals.
- ASTM grades such as B7 (Cr-Mo) studs with 2H nuts are standard for flanges and pressure service.
Raw materials in the Indian market
- Cold-heading steel to IS 11169 (Part 1). This is the Indian standard for carbon and low-alloy steels for cold heading and cold extrusion. These steels are covered by the Ministry of Steel's quality control order, so the wire rod itself must be BIS certified.
- Wire rod supply. Low-carbon and medium-carbon wire rod (SAE 1008–1018, 1035–1045 types) is rolled by Indian steel plants. Many makers buy boron grades (10B21, 15B25, 20MnB4 types) and Cr-Mo grades (SCM435, 42CrMo4) from Indian mills or import them, depending on size and quantity.
- EN grades for studs and turned parts. Indian buyers often order studs and machined bolts in British EN names: EN8 (080M40, a 0.4% carbon steel), EN19 (709M40, Cr-Mo, similar to 42CrMo4 / AISI 4140) and EN24 (817M40, Ni-Cr-Mo, similar to AISI 4340). EN19 and EN24 are hardened and tempered to the strength required; EN8 is used for lower-strength studs.
- Free-cutting steel such as EN1A is popular for turned parts because it machines fast. ISO 898-1 only allows free-cutting steel for low classes such as 4.6 and 5.8, so it is not suitable for 8.8 and above.
- Stainless. SS 304 and 316 wire and bar for A2 and A4 fasteners are produced in India, with large stainless makers in Maharashtra and Gujarat, and also imported.
- Brass fasteners and inserts come largely from the Jamnagar cluster in Gujarat, India's main brass-parts centre.
When you order, name the property class (for example 8.8 or 10.9) rather than just "EN8" or "EN19". A steel name says nothing about how the bolt was heat treated, while the class guarantees the tested strength.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.