What Is 1.3912 / NI36 Steel?
1.3912 / NI36 is the DIN designation for Invar 36, a 36% nickel-iron alloy (also designated UNS K93600 / Alloy 36) with an ultra-low coefficient of thermal expansion (CTE) of ≤ 1.5 × 10⁻⁶/°C at room temperature — about one-tenth that of carbon steel. Its low CTE arises from the magnetovolume effect below the Curie temperature (~230°C). Above 230°C, the Invar property is completely lost.
1.3912 is the DIN Werkstoffnummer (German material number) for a binary nickel-iron alloy containing approximately 36% nickel by weight. Across different national standards and commercial suppliers, this exact same material carries multiple designations — all referring to the identical alloy:
| Standard / System | Designation | Region / Usage |
|---|---|---|
| DIN / EN (Germany / Europe) | 1.3912 / NI36 | Werkstoffnummer — primary European drawing reference |
| UNS (United States) | K93600 | Most common in North American procurement |
| ASTM (USA) | F1684 / A658 | Strip and forging product specifications |
| AMS (Aerospace, USA) | AMS 1444 | US aerospace material specification |
| Trade Name — Carpenter Technology | Invar 36® | Most widely recognized global trade name |
| Trade Name — Special Metals / Inco | Nilo 36® | Alternate commercial designation |
| Generic Industry Name | Alloy 36 / Invar | Non-proprietary shorthand used internationally |
| JIS (Japan) | JIS G 3214 / SUS F36 | Japanese industrial standard equivalent |
The defining characteristic of 1.3912 / NI36 is its ultra-low coefficient of thermal expansion (CTE) of ≤ 1.5 × 10⁻⁶/°C at room temperature — roughly one-tenth that of carbon steel (11–13 × 10⁻⁶/°C) and one-eighth that of austenitic stainless steel (16–17 × 10⁻⁶/°C). This makes it the preferred material wherever dimensional stability under changing temperatures is essential.
When ordering from non-European manufacturers, specifying "1.3912" alone may cause confusion. Always cross-reference with at least one additional designation — UNS K93600, Invar 36, or Alloy 36 — to ensure unambiguous material identification on Mill Test Certificates and inspection documentation.
Need Custom 1.3912 / NI36 Forged Parts?
Jiangsu Liangyi — ISO 9001:2015 China manufacturer since 1997. Open die forgings and rolled rings 30 kg–30,000 kg. Free DFM review, 24-hr quote, 20–45 day lead time.
Why Does 1.3912 / NI36 Have Ultra-Low Thermal Expansion?
The low CTE of 1.3912 / NI36 is caused by the magnetovolume effect: below its Curie temperature (~230°C), a magnetically-driven volume expansion in the iron-nickel lattice almost exactly cancels the normal positive thermal expansion. At exactly 36% Ni, these two effects cancel most completely across the 20°C–200°C range, yielding CTE ~1.2 × 10⁻⁶/°C. Above 230°C, the magnetic effect collapses and CTE rises to ~12–14 × 10⁻⁶/°C.
The Magnetovolume Effect: The True Mechanism
The low CTE of NI36 is not a compositional accident — it is caused by a quantum mechanical phenomenon called the magnetovolume effect (spontaneous volume magnetostriction). The mechanism works in three linked stages:
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1Normal positive thermal expansion
Like all metals, the Fe-Ni crystal lattice expands when heated due to increased atomic vibration amplitude. Without any compensation, this would produce a CTE of ~12–14 × 10⁻⁶/°C — identical to ordinary carbon steel.
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2Spontaneous volume magnetostriction (negative compensating component)
In the ferromagnetic state (below ~230°C Curie temperature), magnetic exchange interactions between neighboring Fe atoms cause a spontaneous volume expansion of the crystal lattice. As temperature rises, spontaneous magnetization weakens, so this magnetically-driven volume expansion also decreases — producing an effective negative CTE contribution that opposes the positive thermal expansion.
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3Near-perfect cancellation at exactly 36% Ni
At the specific composition of ~36% nickel, the two opposing effects cancel almost exactly across the 20°C–200°C temperature range, yielding a net CTE of only ~1.2 × 10⁻⁶/°C. Charles Édouard Guillaume discovered this in 1897 and was awarded the Nobel Prize in Physics in 1920.
Why 36% Ni Is the Critical Composition
The magnetovolume compensation is extremely sensitive to nickel content. Below ~34% Ni, the Curie temperature drops below room temperature, eliminating the compensation effect entirely. Above ~40% Ni, the magnetic ordering changes in ways that reduce the compensation magnitude. The 35.5%–36.5% Ni range is the narrow sweet spot where both effects cancel most completely — which is why NI36 composition must be controlled so tightly in production.
Even a 0.3% deviation in nickel content can shift room-temperature CTE by 0.1–0.2 × 10⁻⁶/°C. For aerospace and precision instrument applications, verify that your forging supplier controls Ni to ±0.5% internal tolerance (35.5%–36.5%), not just the ASTM F1684 standard limit of ±1% (35%–37%).
CTE of 1.3912 / NI36 Across the Full Temperature Range
At 20°C–100°C: ≤ 1.5 × 10⁻⁶/°C (typically ~1.2). At cryogenic −196°C to 20°C: ~1.0–1.8 × 10⁻⁶/°C. At 100°C–200°C: CTE rises from ~1.5 to ~4.0. Above 230°C (Curie temperature): CTE jumps to 12–14 × 10⁻⁶/°C — same as carbon steel, Invar effect completely lost.
The CTE of 1.3912 / NI36 is not constant — it varies significantly with temperature and magnetic state. Using room-temperature CTE data for elevated-temperature calculations is one of the most common engineering errors with this alloy:
Above ~230°C, 1.3912 / NI36 becomes paramagnetic and CTE rises to 12–14 × 10⁻⁶/°C — identical to carbon steel. The Invar advantage disappears completely. Maximum recommended service temperature for low-CTE applications: 200°C. This also means: (1) Stress relief must be performed at 150°C–180°C, not at standard nickel alloy temperatures of 450°C–600°C. (2) Post-weld heat treatment must use solution annealing at 850°C–900°C — NOT standard steel PWHT at 600°C+, which is above the Curie point.
Chemical Composition of 1.3912 / NI36 (UNS K93600)
Per ASTM F1684: Ni 35.0%–37.0%, C ≤ 0.15%, Mn ≤ 0.60%, Si ≤ 0.40%, P ≤ 0.006%, S ≤ 0.004%, Cr ≤ 0.25%, Co ≤ 0.50%, Fe balance. For aerospace and precision applications, specialist manufacturers maintain tighter internal controls — notably Ni at 35.5%–36.5% — to guarantee consistent CTE ≤ 1.5 × 10⁻⁶/°C every batch.
| Element | ASTM F1684 Limit | Precision Internal Control | Engineering Reason |
|---|---|---|---|
| Nickel (Ni) | 35.0%–37.0% | 35.5%–36.5% | Most critical CTE-controlling element — ±0.5% ensures CTE ≤ 1.5 × 10⁻⁶/°C every batch |
| Carbon (C) | 0.15% max | ≤ 0.08% | Lower C prevents grain boundary carbide precipitation — improves cryogenic toughness and weldability |
| Manganese (Mn) | 0.60% max | 0.30%–0.50% | Optimizes strength without promoting MnS inclusion stringers in large-section forgings |
| Silicon (Si) | 0.40% max | ≤ 0.25% | Lower Si reduces oxide inclusions during ESR remelting; improves ultrasonic testing cleanliness rating |
| Phosphorus (P) | 0.006% max | ≤ 0.004% | P at grain boundaries causes hot-shortness during forging and embrittlement at cryogenic temperatures |
| Sulfur (S) | 0.004% max | ≤ 0.002% | Ultra-low S eliminates MnS stringer formation — critical for isotropic properties in heavy forgings |
| Chromium (Cr) | 0.25% max | ≤ 0.10% | Cr above 0.1% shifts the Curie temperature, causing unpredictable CTE in the 180°C–230°C range |
| Cobalt (Co) | 0.50% max | ≤ 0.30% | Co raises Curie temperature — controlled for predictable CTE transition behavior |
| Iron (Fe) | Balance | Balance | — |
Mechanical Properties of 1.3912 / NI36
In solution annealed condition: Tensile Strength ≥ 490 MPa (forged: 510–550 MPa); Proof Strength ≥ 240 MPa (forged: 270–310 MPa); Elongation ≥ 42%; Charpy at −196°C ≥ 70 J (forged: 80–120 J); Hardness 140–180 HB; Density 8.05 g/cm³; Elastic Modulus ~141 GPa. NI36 is not precipitation-hardenable; these are the optimal properties of the base alloy.
| Property | Standard Requirement | Forging Typical (Achieved) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 490 MPa | 510–550 MPa | ASTM E8 / ISO 6892-1 |
| 0.2% Proof Strength (Rp0.2) | ≥ 240 MPa | 270–310 MPa | ASTM E8 / ISO 6892-1 |
| Elongation at Fracture (A%) | ≥ 42% | 45–50% | ASTM E8 / ISO 6892-1 |
| Reduction of Area (Z%) | ≥ 60% | 65–75% | ASTM E8 / ISO 6892-1 |
| Hardness | 140–180 HB | 145–165 HB | ASTM E10 / ISO 6506 |
| Charpy Impact (+20°C) | ≥ 120 J | 140–180 J | ASTM E23 / ISO 148-1 |
| Charpy Impact (−100°C) | ≥ 100 J | 110–150 J | ASTM E23 / ISO 148-1 |
| Charpy Impact (−196°C) | ≥ 70 J | 80–120 J | ASTM E23 / ISO 148-1 |
| ASTM Grain Size (Annealed) | — | 5–8 (fine) | ASTM E112 |
Physical Properties Reference
1.3912 / NI36 vs. Alternative Low-CTE Materials
For most structural forging applications needing CTE < 2 × 10⁻⁶/°C, 1.3912 / NI36 is the best overall choice — balancing CTE performance, strength, weldability, forgeability, and cost. Super Invar has ~0.5 × 10⁻⁶/°C CTE but costs 5× more with limited forgeability. Kovar and Invar 42 match specific glass/ceramic CTEs for sealing. CFRP achieves near-zero CTE but cannot be welded or used in large forged structures.
Why Forged 1.3912 / NI36 Outperforms Cast Material
Yes, significantly. Forged 1.3912 / NI36 delivers 20–25% higher tensile strength, 60–170% better cryogenic Charpy impact at −196°C, and 8× better CTE uniformity across the section (±0.05 vs ±0.4 × 10⁻⁶/°C) compared to cast NI36. Cast material suffers from dendritic Ni segregation causing local CTE variations up to 0.8 × 10⁻⁶/°C — unacceptable for LNG, aerospace, or precision applications.
| Property | Forged NI36 | Cast NI36 (Typical) | Advantage |
|---|---|---|---|
| Tensile Strength (Rm) | 510–550 MPa | 420–460 MPa | +20–25% |
| Proof Strength (Rp0.2) | 270–310 MPa | 200–230 MPa | +30–35% |
| Elongation (A%) | 45–50% | 25–35% | +30–50% |
| Charpy Impact (−196°C) | ≥ 80 J | 30–50 J | +60–170% |
| CTE Uniformity (cross-section) | ±0.05 × 10⁻⁶/°C | ±0.4 × 10⁻⁶/°C | 8× better |
| ASTM Grain Size | 5–8 (fine, uniform) | 2–4 (coarse, dendritic) | Finer = stronger |
| Internal Porosity | Virtually zero (SEP 1921 C/C) | ~3–4% shrinkage typical | Near-zero defects |
| Ni Segregation Across Section | <0.5% variation | 3–6% dendritic variation | 6–12× more uniform |
In cast NI36, dendritic solidification causes Ni segregation where dendrite cores contain ~33–34% Ni while interdendritic zones reach 37–39% Ni. This 3–6% chemical inhomogeneity produces local CTE variations up to 0.8 × 10⁻⁶/°C within the same casting. In precision applications such as satellite structures or OLED frames, the actual dimensional behavior can deviate significantly from design calculations — causing assembly errors that are nearly impossible to correct after installation. For application-specific guidance on specifying the correct forging grade, see our 1.3912 / NI36 forging parts procurement guide.
Industrial Applications of 1.3912 / NI36 Forged Parts
The main applications of 1.3912 / NI36 forged parts are: LNG cryogenic membrane fittings and valve seats (−163°C service); aerospace CFRP autoclave mold frames; satellite structural brackets (−200°C to +120°C orbital cycles); OLED shadow mask tension frames; industrial laser optical bench components; oil and gas cryogenic valve bodies (API 6A / NORSOK); and precision metrology length bars and CMM reference frames.
Membrane tank fittings, valve seats, pipe connectors at −163°C. CTE stability prevents seal failure through thousands of thermal cycles. Supplied to PED 2014/68/EU and API 6A standards with EN 10204 3.2 third-party inspection.
Autoclave mold frames and bases for CFRP wing skins and fuselage panels. CTE matched to carbon fiber layup. Flatness <0.3 mm over 3 m length after full heat treatment and dimensional stabilization.
Bus structural brackets, optical bench frames and thermal compensation rods surviving −200°C to +120°C orbital thermal cycles within ±0.05 mm dimensional tolerance. VIM+VAR melting route available via our partner network upon request.
Gen 6/8/10.5 display production lines. Tension frames and fine metal mask support structures: <50 µm flatness over 1,500 mm, residual magnetism <5 Gauss, Ra ≤ 0.4 µm surface finish.
Optical bench frames, mirror mount housings, Fabry-Pérot interferometer spacers for high-power industrial laser systems (Nd:YAG, CO₂, fiber laser). Maintains micron-level beam alignment under ambient thermal changes.
Valve bodies, bonnet forgings, stems and seats for LNG processing trains, FLNG vessels, and deepwater subsea systems. manufactured to meet API 6A PSL and NORSOK MDS D46 requirements, with full traceability documentation.
CMM reference frames, length standard bars, and geodetic measurement rods. The historical application of Invar — Guillaume's Nobel Prize work in 1897 was based on NI36's use in precision geodetic surveying standards.
Pendulum rods, balance wheels, bi-metal thermostat elements, barometers, aneroid capsules. Also used in radar cavity resonators, high-voltage circuit breaker assemblies, and semiconductor process tooling.
Machining, Welding & Surface Treatment of 1.3912 / NI36
Machining Guidelines for 1.3912 / NI36
- Tool material: Carbide K05–K15 grade (uncoated or TiN coated); positive rake angle 8°–15° essential to minimize work hardening
- Cutting speeds: Turning 60–100 m/min; Milling 40–80 m/min; Feed: 0.1–0.2 mm/rev roughing, 0.05–0.08 mm/rev finishing
- Coolant: Flood cooling mandatory — never dry machine. Low thermal conductivity of NI36 concentrates heat at the cutting edge and induces thermal residual stresses in dry cutting
- Tool dwell rule: Never dwell or rub the tool on the surface — even brief contact causes immediate surface work-hardening that degrades all subsequent passes
- Precision tolerances (<±0.05 mm): Perform intermediate stress relief at 150°C–180°C for 4–8 hr per 25 mm section after rough machining, before final finishing
- Post-grinding: Demagnetize to <5 Gauss after magnetic chuck grinding for sensitive optical or OLED applications
- Final inspection: Allow 24 hours thermal stabilization at room temperature before CMM measurement of critical tolerances
Welding 1.3912 / NI36: Key Requirements
- Filler material: Invar 36 matching filler wire only (AWS ERNi-1, ER36 grade) — never use austenitic stainless steel or generic nickel alloy fillers; they create CTE mismatch at the weld joint
- Preheat (<10 mm section): Not required
- Preheat (10–50 mm section): 80°C–120°C; maximum interpass temperature 150°C
- Preheat (>50 mm section): 120°C–150°C throughout the full welding sequence
- PWHT: Solution anneal at 850°C–900°C, 1 hr per 25 mm section, then rapid controlled cooling — restores CTE ≤ 1.5 × 10⁻⁶/°C throughout the HAZ
- Shielding gas: 100% argon or Ar/He mixture only — no CO₂-containing gases (carbon pickup increases HAZ hardness and reduces ductility)
Specifying PWHT at standard carbon steel or nickel alloy temperatures (450°C–650°C) is the single most frequent error by fabricators new to NI36. These temperatures are all above the 230°C Curie point. During PWHT, the HAZ behaves as ordinary steel (CTE 12–14 × 10⁻⁶/°C), causing macroscopic dimensional changes. The result is a welded assembly with localized CTE anomalies of 3–8 × 10⁻⁶/°C in the HAZ region — permanent dimensional drift that cannot be corrected after fabrication.
Procurement Checklist: How to Specify 1.3912 / NI36 Forged Parts
- Material designation: Specify at least one: 1.3912 / NI36 / Invar 36 / UNS K93600. For high-purity grade, specify melting route: ESR (industrial), VIM+ESR (aerospace), or VIM+VAR (satellite/space)
- Dimensions and tolerances: Provide 2D drawing (PDF/DXF) or 3D model (STEP/IGES) with all tolerances and surface roughness (Ra) for machined surfaces marked
- Heat treatment condition: Standard is solution annealed (+AT). State explicitly if different hardness or strength required
- Service conditions: Min/max service temperature; working pressure if pressure-containing; exposure environment (LNG, seawater, vacuum, etc.)
- CTE verification: Not in default EN 10204 3.1 scope — specify if dilatometer CTE curve (−50°C to 250°C) is required in the MTC
- Mechanical tests: Tensile at room temperature; impact temperature(s) (−100°C, −196°C for LNG/cryo); hardness method
- NDT requirements: UT class (EN 10228-3 Level 3 or 4), PT/MT class and applicable standard
- MTC level: EN 10204 3.1 (standard) or 3.2 (third-party). Name inspection body: buyer's nominated agency (TÜV, SGS, BV, DNV, and others accepted)
- Applicable codes: ASME Section II / API 6A PSL 1–3 / NORSOK MDS D46 / AMS 1444 / JIS G 3214 / PED 2014/68/EU (specify which apply — products manufactured to meet these upon customer specification) — list all applicable codes and revision levels
(1) "Standard grade" without specifying melting route for aerospace — always specify VIM+ESR or VIM+VAR. (2) Not requesting CTE dilatometer testing — never included by default. (3) Stress relief above 200°C — permanently destroys NI36's CTE property. (4) Standard steel PWHT at 600°C+ for welded assemblies — above the Curie point. (5) No dimensioned drawing provided — verbal descriptions cannot be accurately quoted or produced.
Source Custom 1.3912 / NI36 Forgings from Jiangsu Liangyi
Jiangsu Liangyi Co., Limited (established 1997, Jiangyin City, Jiangsu Province, China) is an ISO 9001:2015 certified manufacturer of 1.3912 / NI36 forging parts with 25+ years of specialized nickel alloy forging experience and more than 2,000 completed NI36 projects. Manufacturing capabilities:
- Production range: 30 kg to 30,000 kg per piece; annual capacity 120,000 tons
- Forging equipment: 2,000T–6,300T hydraulic presses; 1M and 5M seamless ring rolling machines
- Full in-house vertical integration: steel melting → open die forging → heat treatment → CNC machining → NDT → CMM inspection, no outsourcing
- Proprietary 3-stage CTE verification: spectrometric Ni control (35.5%–36.5%) → dilatometer CTE test (−50°C to 250°C) → section CTE uniformity check for forgings over 5 tons
- Certification held: ISO 9001:2015 QMS. Products manufactured to meet ASTM, AMS, DIN, EN, JIS, ASME, API 6A, NORSOK MDS D46, and PED 2014/68/EU requirements upon customer specification
- Third-party inspection by buyer's nominated agency fully accommodated (TÜV, SGS, BV, DNV, Lloyd's, and others)
- Export to 50+ countries; standard lead time 20–45 days; free DFM review with every enquiry
To request a custom quote or review the full product range — including forged bars, seamless rings, hollow forgings, and custom shapes with 20–45 day lead time — contact our engineering team directly at www.jnmtforgedparts.com/Contact-us.html.
Frequently Asked Questions About 1.3912 / NI36 Steel
The following questions and answers are structured to directly answer the most common engineering and procurement queries about 1.3912 / NI36 (Invar 36 / UNS K93600).