Alloy 751 (UNS N07751, EN 2.4694) — commercially known as Inconel® 751 — is a precipitation-hardenable nickel-chromium superalloy containing ≥70% Ni, 14–17% Cr, 2.0–2.6% Ti, and 0.9–1.5% Al. It is a modified version of Alloy X-750 with higher aluminum for superior stress-rupture life at 871 °C (1600 °F). Primary application: heavy-duty diesel engine exhaust valves. Governing standards: ASTM B637 / AMS 5706 / AMS 5710 / EN 2.4694.
This guide is authored by Jiangsu Liangyi Co., Limited — an ISO 9001:2015 certified open-die forging manufacturer founded in 1997, located in Jiangyin, Jiangsu Province, China. With over 25 years of experience producing nickel superalloy forgings and an annual capacity of 120,000 tonnes, our metallurgical engineering team has compiled this reference from ASTM B637, AMS 5706, Special Metals technical bulletins, and ASM Handbook data. All technical data in this article is sourced from published international standards and the open literature — not from proprietary internal testing. EN 10204 3.1 Material Test Reports are supplied with every order.
What Is Alloy 751 (UNS N07751)?
Alloy 751 — commercially known as Inconel® 751, a registered trademark of Special Metals Corporation (now part of Precision Castparts Corp.) — is a member of the γ′-strengthened nickel-based superalloy family. These alloys derive high-temperature strength from a coherent precipitate phase (gamma-prime, Ni₃(Al,Ti)) dispersed within the face-centered cubic nickel-chromium matrix.
The alloy was engineered as a direct modification of Alloy X-750 (UNS N07750). By increasing the aluminum content to 0.90–1.50 wt% and adding a controlled niobium contribution of 0.70–1.20 wt%, the composition was optimized to achieve superior stress-rupture life at 871 °C (1600 °F) — the critical service temperature for diesel and gas engine exhaust valves. In all other respects, Alloy 751 and Alloy X-750 are essentially equivalent: same oxidation resistance, same corrosion behavior, same physical properties, same weldability.
"INCONEL alloy 751 is a high-strength, high-temperature nickel-chromium alloy. It is essentially the same as INCONEL alloy X-750, except that its slightly modified composition imparts increased rupture properties at 1600 °F (871 °C). It is used for diesel engine exhaust valves."— ASM Alloy Digest, Filing Code Ni-116, Huntington Alloy Products Division (now Special Metals / PCC)
"Inconel® 751" is a trade name. The material standard designation is UNS N07751 (USA), EN 2.4694 (Europe), or Alloy 751 (generic). When issuing purchase orders, always specify the UNS number or EN designation alongside the applicable standard (ASTM B637 / AMS 5706) — not the trade name alone — to avoid dependency on any single brand. Other trade names for the same composition include CarTech 751 (Carpenter Technology) and Nicrofer 7016 TiAl (VDM Metals).
Chemical Composition
The nominal chemical composition per ASTM B637 and AMS 5706 is listed below. Key differentiators from Alloy X-750 (UNS N07750) are the higher aluminum range (0.90–1.50 vs. 0.40–1.00 wt%) and the controlled niobium addition (0.70–1.20 wt%).
| Element | Symbol | Min (wt%) | Max (wt%) | Metallurgical Role |
|---|---|---|---|---|
| Nickel | Ni | 70.0 min | balance | Matrix; base for oxidation and corrosion resistance |
| Chromium | Cr | 14.0 | 17.0 | Forms Cr₂O₃ passive layer; sulfidation and oxidation resistance |
| Iron | Fe | 5.0 | 9.0 | Matrix stabilizer; cost-balancing element |
| Titanium | Ti | 2.00 | 2.60 | Primary γ′ former (Ni₃Ti); key high-temperature strengthener |
| Aluminum | Al | 0.90 | 1.50 | γ′ former (Ni₃Al); elevated vs Alloy X-750 → superior rupture life |
| Niobium | Nb | 0.70 | 1.20 | γ″ former and carbide stabilizer; controlled addition |
| Molybdenum | Mo | 0.85 | 1.10 | Solid-solution strengthener; pitting corrosion resistance |
| Manganese | Mn | — | 1.00 | Deoxidizer; sulfur scavenger during melting |
| Silicon | Si | — | 0.50 | Minor deoxidizer |
| Copper | Cu | — | 0.50 | Controlled residual impurity |
| Carbon | C | — | 0.10 | Carbide former; grain boundary pinning at low levels |
| Sulfur | S | — | 0.010 | Tramp element; controlled low for hot workability |
| Phosphorus | P | — | 0.020 | Tramp element; controlled low |
Titanium and aluminum are highly reactive with oxygen and nitrogen. UNS N07751 must be melted under vacuum using VIM + ESR (Vacuum Induction Melting + Electroslag Remelting) or VIM + VAR (Vacuum Arc Remelting). These processes eliminate oxide inclusions and achieve the compositional homogeneity required for aerospace-grade and safety-critical engineering applications. Material not produced by one of these routes should not be accepted for critical service without additional verification.
Physical Properties
Physical constants for Alloy 751 (UNS N07751) closely mirror those of Alloy X-750 (UNS N07750), as expected from the nearly identical chemistry. All values are at room temperature unless otherwise stated. Data sourced from published ASTM / ASM reference literature.
| Property | Value | Units | Notes |
|---|---|---|---|
| Density | 8.25 | g/cm³ (0.298 lb/in³) | Room temperature |
| Melting Range | 1393–1427 | °C (2540–2600 °F) | Solidus to liquidus |
| Elastic Modulus (E) | 214 | GPa (31.0 × 10⁶ psi) | Decreases significantly at elevated temperature |
| Poisson's Ratio | 0.29 | — | Typical for Ni-Cr alloys |
| Electrical Resistivity | 122 | μΩ·cm | Room temperature |
| Magnetic Permeability | ~1.0002 | — | Non-magnetic (paramagnetic) |
| Hardness — annealed | ≤ 302 | HBW | Solution-annealed condition |
| Hardness — aged | 320–380 | HBW | Precipitation-hardened condition |
Mechanical Properties
Mechanical properties of UNS N07751 depend on heat treatment condition and test temperature. Values below represent typical data for wrought bar and forging product forms in the precipitation-hardened (aged) condition at room temperature, per ASTM B637 and AMS 5706.
| Property | Typical (RT) | Minimum (RT) | Units |
|---|---|---|---|
| Ultimate Tensile Strength (UTS) | 1241 | 1138 | MPa (180 / 165 ksi) |
| 0.2% Proof Stress (Yield Strength) | 1034 | 862 | MPa (150 / 125 ksi) |
| Elongation at Break | 20 | 15 | % |
| Reduction of Area | 20 | 15 | % |
| Charpy Impact (V-notch, RT) | ~100 | — | J |
| Stress Rupture @ 871 °C / 100 h | ~138 | — | MPa (20 ksi) — key advantage vs Alloy X-750 |
At 760 °C (1400 °F), UNS N07751 retains tensile strength in the range of 900–1000 MPa. At 871 °C (1600 °F), it holds approximately 700–800 MPa — sufficient for exhaust valve service with appropriate design margins. Its high fatigue limit enables it to withstand the thermal cycling and mechanical vibration typical of heavy-duty diesel engine service (1–2 million cycles at operating temperature).
Thermal Properties
Low thermal conductivity is a defining characteristic of UNS N07751 — it acts as a thermal insulation benefit in some designs but creates heat build-up at the tool tip during machining that must be managed with appropriate coolant strategies.
| Property | At 21 °C (70 °F) | At 538 °C (1000 °F) | At 871 °C (1600 °F) |
|---|---|---|---|
| Thermal Conductivity (W/m·K) | 11.5 | 16.4 | 22.2 |
| Specific Heat Capacity (J/kg·K) | 431 | 544 | 586 |
| Mean CTE (μm/m·°C from 21 °C) | — | 13.0 | 14.4 |
The mean CTE of UNS N07751 (~13–14 μm/m·°C) differs from cast iron (~11 μm/m·°C) and carbon steel (~12 μm/m·°C). Always calculate differential expansion across the full thermal cycle when designing press-fitted valve inserts or shrink-fit assemblies, to prevent fretting, galling, or interference loss at operating temperature.
Heat Treatment of Alloy 751 (UNS N07751)
The precipitation-hardening sequence for UNS N07751 consists of two stages — simpler than the two-step aging required for UNS N07718 (Alloy 718). The complete cycle per AMS 5706 and ASTM B637 is as follows:
Heat to 1177 °C (2150 °F) and hold for a minimum of 1 hour. Quench by rapid air cooling or water quench. This dissolves all γ′ precipitates and carbides, producing a supersaturated solid solution with a uniform, equiaxed grain structure. Do not exceed 1200 °C — grain coarsening above this threshold degrades fatigue performance.
Age at 843 °C (1550 °F) for 24 hours, then air cool. This nucleates and grows γ′ (Ni₃(Al,Ti)) precipitates coherent with the nickel matrix, producing peak tensile strength and stress-rupture life. For exhaust valve applications specifically, an alternative cycle of 760–790 °C for 16 hours may be specified to achieve peak γ′ precipitation with improved dimensional stability at operating temperature. Always confirm the cycle with the applicable specification (AMS 5706 or AMS 5710).
After aging: verify hardness (target 320–380 HBW); conduct tensile testing per ASTM E8/E8M to confirm UTS ≥ 1138 MPa and YS ≥ 862 MPa; perform microstructural examination if required by the project ITP. All Jiangsu Liangyi deliveries include EN 10204 3.1 MTR with heat-treatment time-temperature records.
Forging & Hot Working
Open die forging is the preferred manufacturing route for UNS N07751 bars, discs, rings, and custom components. The forging process breaks down the cast dendritic structure of VIM/ESR ingots and produces controlled grain flow — essential for fatigue and stress-rupture performance in critical applications.
| Process Stage | Temperature Range | Notes |
|---|---|---|
| Pre-forge furnace soak | 1177–1205 °C (2150–2200 °F) | Max 2 hours; minimize soak time to limit grain coarsening |
| Forging window | 1038–1205 °C (1900–2200 °F) | Maintain uniform temperature across full section thickness |
| Minimum working temperature | 983 °C (1800 °F) | Reheat before temperature drops below this threshold |
| Ring rolling / open die drawing | 983–1205 °C (1800–2200 °F) | Apply progressive, uniform reductions; avoid duplex grain structures |
| Cold working | Room temperature | Standard tooling; intermediate anneals required for reductions exceeding 20% |
A minimum total reduction ratio of 4:1 from ingot to finished product is required to fully eliminate the cast grain structure. Hydraulic presses (2000–6300 tonne) are preferred over hammer forging for large cross-sections, providing superior control of strain rate and temperature uniformity through heavy sections.
We manufacture UNS N07751 (Alloy 751) forged bars up to 2000 mm diameter and 12 m length, seamless rolled rings up to 6000 mm OD, and custom forgings up to 30 metric tons per piece — from VIM+ESR stock billets, using 2000T / 4000T / 6300T hydraulic presses. EN 10204 3.1 MTR with forging process records is supplied with every order. Third-party inspection by SGS, Bureau Veritas, or TÜV Rheinland can be arranged upon request. custom UNS N07751 forgings from Jiangsu Liangyi
Machining UNS N07751 (Alloy 751)
Alloy 751 / UNS N07751 can be machined using conventional equipment, but requires disciplined technique. Its high strength, rapid work-hardening tendency, and low thermal conductivity (11.5 W/m·K at RT) place it in the "difficult-to-machine" category alongside other γ′-strengthened nickel superalloys.
Three Primary Machining Challenges
- Rapid work hardening: The surface work-hardens within the first pass, requiring consistent chip loads and avoidance of tool dwell or repeated light passes over the same surface.
- Heat concentration at the tool tip: Low thermal conductivity prevents heat from dissipating into the workpiece, causing rapid tool wear and potential surface damage if coolant flow is inadequate.
- Built-up edge (BUE): Titanium and nickel content promotes adhesion of workpiece material to cutting tool edges, particularly at low cutting speeds, leading to sudden tool failure and poor surface finish.
| Parameter | Recommended Practice |
|---|---|
| Preferred machining condition | Rough machine in solution-annealed state; finish-machine after aging for best chip control and surface finish |
| Cutting speed — turning | 20–40 m/min with carbide tooling; 5–8 m/min with HSS |
| Feed rate | 0.1–0.25 mm/rev — avoid very fine feeds that cause rubbing and BUE |
| Depth of cut | Maximize DOC to stay below the work-hardened surface layer |
| Tooling | Coated carbide inserts (TiAlN or AlTiN coating); sharp cutting edges; positive rake geometry |
| Coolant — turning / milling | Flood water-based coolant at high flow rate |
| Coolant — drilling / tapping | Neat cutting oil with internal delivery preferred |
| Grinding | Mandatory flood coolant — never dry-grind UNS N07751 |
Welding Alloy 751 (UNS N07751)
UNS N07751 is weldable using standard nickel alloy processes. Its precipitation-hardening nature means the heat-affected zone (HAZ) experiences γ′ over-aging or dissolution during welding, which softens the HAZ and must be corrected by post-weld heat treatment for structural applications.
Qualified Welding Processes
- GTAW (TIG) — preferred for thin sections and root passes; best control, lowest risk of contamination or porosity
- GMAW (MIG) — suitable for production welding of sections above 6 mm
- SMAW (Stick) — acceptable for site repairs; low-hydrogen nickel-alloy electrodes required
- SAW (Submerged Arc) — suitable for high-volume production welds with approved flux/wire combinations
Recommended Sequence for Structural Weldments
- Solution anneal at 1177 °C / 1 h before welding to establish a uniform, ductile starting condition
- Weld in the annealed condition; maintain interpass temperature below 150 °C (300 °F)
- Post-weld age at 843 °C (1550 °F) for 24 hours, air cool, to restore γ′ strengthening across the weldment
- Inspect per AWS D1.6 or project NDE procedure — PT and/or UT as required by design class
Corrosion Resistance
Alloy 751 / UNS N07751's corrosion performance derives from two mechanisms: a self-healing chromium oxide (Cr₂O₃) passive layer that reforms rapidly after mechanical damage, and the inherent electrochemical nobility of the high-nickel (≥70 wt%) matrix.
Lead Oxide Screening Test — Exhaust Valve Qualification Data
Nickel-chromium alloys were evaluated for exhaust valve applications using a standardized lead oxide screening test at 913 °C (1675 °F). UNS N07751 consistently achieved corrosion rates averaging 4.31 g/dm²/h — a result that has established it as the benchmark alloy for heavy-duty diesel exhaust valves operating on leaded and heavy fuel oil. The alloy also resists attack from sulfur, bromine, and chlorine compounds found in diesel combustion gases. (Source: Special Metals technical bulletin for INCONEL® alloy 751.)
| Environment | Resistance Level | Notes |
|---|---|---|
| Oxidation in air to 980 °C | Excellent | Stable Cr₂O₃ + NiO protective scale; self-healing |
| Lead oxide at 913 °C | Excellent | 4.31 g/dm²/h — primary exhaust valve qualification benchmark |
| Hot sulfur-bearing exhaust gases | Good | Superior to austenitic stainless steels; limited sulfidation above 900 °C |
| Seawater (splash / spray zones) | Good | Low corrosion rate; suitable for marine splash environments |
| Aqueous chloride solutions | Moderate | Susceptible to stress-corrosion cracking under tensile stress above ~100 °C |
| Concentrated mineral acids | Limited | Conduct site-specific corrosion rate testing above 30% concentration |
Alloy 751 (N07751) vs. Alloy X-750 (N07750)
Because Alloy 751 is a direct derivative of Alloy X-750, engineers frequently face this specification decision. The critical variable is operating temperature and the primary design-limiting failure mode.
Decision rule: Specify Alloy 751 (UNS N07751) when operating temperature exceeds 816 °C, or when stress-rupture is the design-limiting failure mode. Specify Alloy X-750 (UNS N07750) for spring, fastener, and seal applications below 816 °C where high-cycle fatigue resistance is the governing performance criterion.
Industrial Applications of UNS N07751
Heavy-Duty Diesel Exhaust Valves
The primary engineered application for UNS N07751. Specified for trucks, locomotives, marine engines, and stationary diesel generators — particularly on leaded and heavy fuel oil where lead oxide corrosion resistance at 871 °C is the qualifying criterion.
Aerospace Turbine Components
Turbine blades, nozzle guide vanes, combustion chamber hardware, and high-pressure compressor fasteners. Service temperatures and stress levels exceed the capability of standard austenitic stainless steels.
Power Generation
Gas turbine hot-section hardware, steam turbine high-temperature fasteners, spring elements in valve actuation systems, and waste-to-energy incineration components exposed to sulfur-rich combustion gases.
Oil & Gas / Downhole
High-strength wellhead fasteners, valve stems, and spring elements in subsea completion equipment. Products manufactured to API 6A requirements and qualified to ISO 15156-3 (NACE MR0175) for sour service.
Nuclear Energy
Control rod mechanisms, reactor core hardware, and spring elements in nuclear reactor auxiliary systems where radiation resistance and long-term dimensional stability are design requirements.
High-Temperature Fasteners & Springs
Bolting and spring elements for pressure vessels, heat exchangers, and industrial furnaces above 600 °C — replacing 316L stainless steel where creep at temperature causes progressive bolt load relaxation.
Standards, Specifications & Grade Equivalents
Alloy 751 / UNS N07751 forgings and bars must be ordered to the specific standard mandated by the application. The main international specifications are listed below. Note that standards compliance (e.g., products manufactured to API 6A dimensional and testing requirements) is different from company-level certification (e.g., ISO 9001:2015 management system certification held by Jiangsu Liangyi).
| Standard Body | Spec. No. | Product Form | Scope |
|---|---|---|---|
| ASTM | B637 | Bars, forgings, rings | Primary procurement standard for precipitation-hardening Ni alloy bars and forgings |
| AMS (SAE) | 5706 | Bars & forgings | Aerospace-grade UNS N07751; more stringent cleanliness and testing than ASTM B637 |
| AMS (SAE) | 5710 | Bars & forgings | Alternative aging heat treatment cycle; different minimum tensile requirements |
| EN / DIN | 2.4694 | All wrought forms | European designation; required for EU and German defense/aerospace procurement |
| ISO / NACE | 15156-3 | Material qualification | Sour service qualification for oil & gas; equivalent to NACE MR0175 for UNS N07751 |
| UNS | N07751 | Chemical identity only | Unified Numbering System — defines chemistry only; not a procurement specification |
Trade Name Cross-Reference
| Trade Name | Owner / Manufacturer | UNS / EN Equivalent |
|---|---|---|
| Inconel® 751 | Special Metals Corporation / PCC (registered trademark) | UNS N07751 |
| CarTech 751 | Carpenter Technology Corporation | UNS N07751 |
| Nicrofer 7016 TiAl | VDM Metals GmbH | EN 2.4694 |
| Alloy 751 | Generic / non-branded designation | UNS N07751 / EN 2.4694 |
Frequently Asked Questions — Alloy 751 / UNS N07751
Alloy 751 (UNS N07751, EN 2.4694) — commercially marketed as Inconel® 751 by Special Metals Corporation — is a precipitation-hardenable nickel-chromium superalloy containing ≥70% Ni, 14–17% Cr, 5–9% Fe, 2.0–2.6% Ti, and 0.9–1.5% Al. It is a modification of Alloy X-750 with increased aluminum for superior stress-rupture life at 871 °C vs X-750's 816 °C limit. X-750 is preferred for springs, fasteners, and seals below 816 °C. Both alloys are equivalent in oxidation resistance, corrosion performance, and weldability.
Alloy 751 / UNS N07751 should be forged at 1038–1205 °C (1900–2200 °F). Reheat whenever temperature drops below 983 °C (1800 °F). Minimum reduction ratio 4:1 from ingot to final product. Hydraulic presses (2000–6300 tonne) are preferred for large cross-sections.
Primary standards: ASTM B637 (general procurement), AMS 5706 (aerospace-grade bars and forgings), AMS 5710 (alternative heat treatment condition), EN 2.4694 (European designation), ISO 15156-3 (sour service qualification for oil and gas). Required documentation: EN 10204 3.1 Material Test Reports. Melting practice: VIM+ESR or VIM+VAR.
Primary application: exhaust valves for heavy-duty diesel engines (trucks, locomotives, marine engines, stationary generators). Additional applications include aerospace turbine blades and vanes, power generation hot-section hardware, oil and gas wellhead fasteners and valve stems, nuclear reactor components, and high-temperature springs and fasteners for service above 600 °C.
UNS N07751 must be melted under vacuum because titanium and aluminum react readily with oxygen and nitrogen. Standard practice is VIM+ESR (Vacuum Induction Melting + Electroslag Remelting) or VIM+VAR (Vacuum Induction Melting + Vacuum Arc Remelting), which minimize oxide inclusions and achieve the compositional homogeneity required for critical applications.
Density: 8.25 g/cm³ (0.298 lb/in³). Maximum recommended service temperature for stress-bearing components: 871 °C (1600 °F). Melting range: 1393–1427 °C (2540–2600 °F). Oxidation resistance in air: good to approximately 980 °C (1796 °F).
Engineering Summary — Alloy 751 / UNS N07751
Key Facts for Engineering Specification
- Precipitation-hardenable Ni-Cr-Fe superalloy (UNS N07751 / EN 2.4694); modified Alloy X-750 with higher Al for superior stress-rupture at 871 °C
- Composition: ≥70% Ni · 14–17% Cr · 5–9% Fe · 2.0–2.6% Ti · 0.9–1.5% Al · 0.7–1.2% Nb (all wt%, per ASTM B637)
- Reactive elements (Ti, Al) require VIM+ESR or VIM+VAR vacuum melting — no air-melt material for critical service
- Standard heat treatment: 1177 °C / 1 h solution anneal → 843 °C / 24 h age → air cool (per AMS 5706)
- Forging range: 1038–1205 °C; minimum working temperature 983 °C; 4:1 minimum reduction from ingot
- Aged RT properties: UTS ~1241 MPa · YS ~1034 MPa · Elongation ≥20% · Hardness 320–380 HBW
- Stress rupture at 871 °C / 100 h: ~138 MPa — key differentiator vs Alloy X-750
- Lead oxide corrosion rate at 913 °C: 4.31 g/dm²/h — validated for diesel exhaust valve service
- Specify Alloy 751 over X-750 when service temperature exceeds 816 °C or stress-rupture governs design
- Procurement standards: ASTM B637 · AMS 5706 · AMS 5710 · EN 2.4694 · ISO 15156-3 depending on application