What Is 20MnMoNi4-5 (1.6311) Steel?
20MnMoNi4-5 (material number 1.6311) is a quenched-and-tempered, low-alloy manganese–molybdenum–nickel steel used for thick-walled pressure equipment. It pairs clean weldability with high toughness from −10 °C up to about 375 °C, offers 470 MPa minimum yield and 590–750 MPa tensile strength in thin sections, and is standardised as plate to EN 10028-2 and as open-die forgings to EN 10250-3 for boilers and pressure vessels.
What is 20MnMoNi4-5 (1.6311) steel?
20MnMoNi4-5, designated material number 1.6311 in the European steel-numbering system, is a low-alloy special steel built around a manganese–molybdenum–nickel chemistry. The grade name reads like a recipe: roughly 0.20% carbon, with manganese and the Mo/Ni pairing balancing the strength–toughness equation.
Unlike plain carbon steels that lose toughness as section thickness grows, 1.6311 was developed so that thick walls still harden through and stay tough. Molybdenum suppresses temper embrittlement and lifts hot strength; nickel keeps the impact transition temperature low; manganese drives hardenability. The result is a steel that is calm to weld yet dependable under cyclic pressure and heat.
It is supplied as heavy plate to EN 10028-2 and, importantly for our work, as open-die forgings to EN 10250-3 — shells, tubesheets, nozzles, flanges and vessel components where forging refines the grain and seals internal soundness.
What is the chemical composition of 1.6311?
The composition below follows EN 10028-2 (weight %). Carbon is deliberately moderate to protect weldability, while the molybdenum band of 0.45–0.60% is the signature that separates this grade from leaner Mn-Ni steels.
| Element | Min | Max |
|---|---|---|
| Carbon (C) | 0.15 | 0.23 |
| Silicon (Si) | — | 0.40 |
| Manganese (Mn) | 1.00 | 1.50 |
| Nickel (Ni) | 0.40 | 0.80 |
| Molybdenum (Mo) | 0.45 | 0.60 |
| Chromium (Cr) | — | 0.20 |
| Copper (Cu) | — | 0.20 |
| Vanadium (V) | — | 0.02 |
| Nitrogen (N) | — | 0.012 |
| Phosphorus (P) | — | 0.020 |
| Sulphur (S) | — | 0.010 |
Balance Fe. Cr, Cu, V are residual ceilings, not intentional additions.
What are the mechanical properties of 20MnMoNi4-5?
Properties are quoted in the quenched-and-tempered (+QT) condition. The notable feature is how gracefully strength tapers with section size — even a 250 mm wall still guarantees 400 MPa yield, which is why the grade dominates heavy forged shells.
| Section thickness | Tensile Rm (MPa) | Yield ReH (MPa) | Elong. A (%) |
|---|---|---|---|
| ≤ 40 mm | 590–750 | 470 min | 18 |
| 40–60 mm | 590–730 | 460 min | 18 |
| 60–100 mm | 570–710 | 450 min | 18 |
| 100–150 mm | 570–710 | 440 min | 18 |
| 150–250 mm | 560–700 | 400 min | 17 |
Notch toughness
The Mo–Ni balance keeps Charpy V-notch energy high even as temperature drops — decisive for vessels that may be hydro-tested or operated in cold ambient conditions.
| Test temperature | Impact energy KV |
|---|---|
| +20 °C | 50 J |
| 0 °C | 40 J |
| −20 °C | 27 J |
How is 20MnMoNi4-5 heat treated?
The published strengths exist only after a disciplined quench-and-temper cycle. For forgings, this is performed after rough machining so the whole cross-section transforms uniformly.
Austenitise & quench
870–940 °C soak until the full section is at temperature, then water quench to lock in a fine, hardenable structure.
Temper
610–690 °C, hold ~30 min once through-heated, then cool in still air. This trades a little hardness for the toughness and ductility the grade is prized for.
Stress relieve (post-weld)
580–620 °C. Keep total time ≤150 min; if cycles repeat or exceed 90 min, work at the lower end of the range to preserve properties.
Can 20MnMoNi4-5 be welded and formed?
Yes — 1.6311 can be hot or cold formed, welded and flame cut with standard procedures. The low, controlled carbon level keeps the hardened heat-affected zone manageable, while clean sulphur/phosphorus limits reduce cracking risk.
Welding generally follows EN 1011 guidance together with the relevant material data sheet. Where pressure equipment is built under frameworks such as PED / AD 2000, qualified matching-strength consumables and a post-weld stress-relief cycle on thick joints are the usual practice.
• Use qualified Mn-Mo-Ni filler metals
• Control heat input per EN 1011
• Apply PWHT / stress relief on heavy welds
• Re-test impact properties after stress relief
What is 20MnMoNi4-5 used for?
Wherever a wall must contain pressure at elevated temperature and stay tough when cold, this grade is a default choice — typically rated from −10 °C up to about 375 °C.
Boiler Installations
Drums and headers built to rules such as TRD 101 / EN 12952 for steam-raising plant up to ~375 °C.
Pressure Vessels
Reactors, separators and heat-exchanger shells — forged tubesheets, nozzles and flanges for petrochemical and power plant.
Rings, Shafts & Blocks
Large rings, shafts, end-caps and custom forged blocks where through-thickness toughness is essential.
We produce these to order as 20MnMoNi4-5 (1.6311) forged parts. The grade is also a recognised material for reactor and containment components in the wider industry; the codes named above describe where the steel grade is typically used and are not a statement that any one supplier is accredited under those schemes.
Which standards cover 1.6311 steel?
The grade lives in two principal European product standards. The material itself conforms to these specifications — this is a property of the steel grade, not a company accreditation.
| Item | Reference |
|---|---|
| Grade | 20MnMoNi4-5 |
| EN number | 1.6311 |
| Class | Alloy special steel |
| Plate std. | EN 10028-2 |
| Forging std. | EN 10250-3 |
| Mill cert. | EN 10204 / 3.1 (on request) |
Frequently asked questions
Is 20MnMoNi4-5 the same as 1.6311?
Yes. “20MnMoNi4-5” is the descriptive EN name and “1.6311” is its EN material number — two labels for the same low-alloy Mn-Mo-Ni pressure-vessel steel.
What is the chemical composition of 20MnMoNi4-5 steel?
Per EN 10028-2 (weight %): carbon 0.15–0.23, silicon up to 0.40, manganese 1.00–1.50, nickel 0.40–0.80, molybdenum 0.45–0.60, chromium up to 0.20, with phosphorus held to 0.020 max and sulphur to 0.010 max. The balance is iron.
What are the mechanical properties of 1.6311 steel?
In the quenched-and-tempered condition, sections up to 40 mm offer a tensile strength of 590–750 MPa, a minimum yield of 470 MPa and 18% elongation. Yield decreases with thickness, reaching 400 MPa minimum at 150–250 mm. Charpy impact energy (transverse) is 50 J at +20 °C, 40 J at 0 °C and 27 J at −20 °C.
How is 20MnMoNi4-5 heat treated?
Austenitise at 870–940 °C and water quench, then temper at 610–690 °C with about 30 minutes hold and cool in still air. Post-weld stress relief is performed at 580–620 °C, with total time generally kept under 150 minutes.
What is the maximum service temperature of 20MnMoNi4-5 steel?
It is commonly approved for service up to roughly 375 °C in boiler and pressure-vessel codes, with a lower bound near −10 °C. The governing design code and certificate always take precedence.
Can 20MnMoNi4-5 steel be welded?
Yes. It welds with standard processes under EN 1011 guidance. On heavy sections, preheat, matching-strength Mn-Mo-Ni consumables and a post-weld stress-relief cycle are recommended to keep the heat-affected zone tough.
Why choose a forging over plate in 1.6311 steel?
Forging refines grain flow, closes internal porosity and improves through-thickness properties — advantages that matter most in pressure-boundary components where reliability is critical.
Key takeaways
20MnMoNi4-5 (1.6311) in brief
- Identity: low-alloy Mn-Mo-Ni pressure-vessel steel; EN name 20MnMoNi4-5, EN number 1.6311.
- Composition: ~0.20% C, 1.00–1.50% Mn, 0.40–0.80% Ni, 0.45–0.60% Mo (EN 10028-2).
- Strength: 590–750 MPa tensile, 470 MPa min yield (≤40 mm), through-hardening in heavy sections.
- Toughness: 27 J Charpy at −20 °C; reliable to about 375 °C service.
- Processing: austenitise 870–940 °C / water quench; temper 610–690 °C; weldable per EN 1011.
- Best form: forged components for boilers and pressure vessels.
Reference standards
- EN 10028-2 — Flat products of steels for pressure purposes (elevated-temperature properties).
- EN 10250-3 — Open steel die forgings for general engineering purposes: alloy special steels.
- EN 10204 — Metallic products: types of inspection documents (3.1 certificate).
- EN 1011 — Welding recommendations for metallic materials.
Need 20MnMoNi4-5 forged to spec?
We forge and heat-treat 1.6311 components — rings, shafts, tubesheets, nozzles and custom blocks — and can supply EN 10204 type 3.1 mill test certificates with your order on request. Send a drawing or your operating conditions and we’ll quote the optimal forged solution.