UNS N07718 is the Unified Numbering System designation for a
precipitation-hardenable nickel-chromium superalloy, sold under the trade name
Inconel 718. It contains 50 to 55 % nickel plus cobalt,
17 to 21 % chromium, 4.75 to 5.50 % niobium plus tantalum and
2.80 to 3.30 % molybdenum, with iron as the balance. Double ageing precipitates
γ″ and γ′, giving 1276 MPa (185 ksi) tensile strength.
The alloy stays tough from −253 °C to about 700 °C.
Jiangyin Jiangnan Metal Co., Ltd. forges UNS N07718 to ASTM B637,
ASTM B564 and AMS 5662/5663 as seamless rolled rings, forged rings, discs, shafts,
flanges, round bars, sleeves, bushings, tube sheets and valve components. Forgings are
ultrasonically tested to EN 10228-3, SEP 1921 or ASTM A388 and certified to
EN 10204 3.1 or 3.2. The plant is at No.1 Chengxiqiao Road, Zhouzhuang Town,
Jiangyin City, Jiangsu Province, China. Telephone
+86-189-2135-9659, email
sales@steelforgepieces.com.
What UNS N07718 is
UNS N07718 is a nickel-chromium superalloy hardened by precipitation, not by cold
work or carbon content. Niobium is the important addition. During ageing it forms a
body-centred-tetragonal γ″ (Ni3Nb) phase, which strengthens the matrix much
more than γ′ (Ni3(Al,Ti)) does on its own.
This has two practical effects for buyers.
Good weldability. The γ″ reaction is slow, so a weld and its
heat-affected zone can cool through the ageing range without strain-age cracking. Welded turbine
casings and subsea structures are made in UNS N07718 for this reason, in applications where
Waspaloy or René 41 would crack.
A service limit near 700 °C. Above about 650 °C the
metastable γ″ starts converting to the stable, coarse δ phase and strength drops.
Hotter gas turbine sections use Waspaloy, Alloy 720 or single-crystal alloys instead. There is
no equivalent limit at the cold end. The alloy stays ductile at −253 °C, so it is
used in liquid-hydrogen rocket hardware and LNG service.
δ phase also pins grain boundaries during hot working. Forging temperature control
therefore sets the final grain size, and with it the fatigue life. See
forging and heat treatment below.
Equivalent designations and governing standards
UNS N07718 is listed under a different name in most purchasing systems. All of the designations
below cover the same chemistry. A trade name identifies the producer, not a different alloy.
Table 2. Governing specifications for UNS N07718 forgings
Specification
Scope
Typical use
ASTM B637 / ASME SB-637
Precipitation-hardenable nickel alloy bars, forgings and forging stock for high-temperature service
The default industrial specification
ASTM B564 / ASME SB-564
Nickel alloy forgings
Flanges, fittings, pressure parts
AMS 5662
Bars, forgings and rings, solution heat treated
Aerospace, supplied for later ageing
AMS 5663
Bars, forgings and rings, solution treated and precipitation hardened
Aerospace, ready to machine
AMS 5664
Forgings, solution treated and precipitation hardened, higher solution temperature
Where creep and rupture govern
API 6A718
Alloy 718 for oil and gas wellhead and Christmas tree equipment
Controlled lower strength, higher toughness
NACE MR0175 / ISO 15156-3
Materials for H2S-containing environments
Sour service qualification
EN 10204
Types of inspection document
3.1 works certificate, 3.2 third-party witnessed
Specifications are revised periodically. Name the revision on your purchase order,
for example ASTM B637-24, and we will confirm it on the order acknowledgement.
Chemical composition of UNS N07718
Composition limits below follow ASTM B637 and AMS 5662 for UNS N07718, in weight percent.
Every heat is verified by optical emission spectrometry. Finished parts are checked by positive
material identification before shipment.
Table 3. UNS N07718 chemical composition, wt%
Element
Min
Max
Role in the alloy
Nickel + Cobalt (Ni+Co)
50.00
55.00
Austenitic matrix
Chromium (Cr)
17.00
21.00
Oxidation and corrosion resistance
Iron (Fe)
Balance
Roughly 17 to 20 %, keeps cost down
Niobium + Tantalum (Nb+Ta)
4.75
5.50
Forms γ″, the main strengthener
Molybdenum (Mo)
2.80
3.30
Solid-solution strengthening, pitting resistance
Titanium (Ti)
0.65
1.15
Forms γ′
Aluminium (Al)
0.20
0.80
Forms γ′
Cobalt (Co)
–
1.00
Residual
Carbon (C)
–
0.08
Carbides at grain boundaries
Manganese (Mn)
–
0.35
Residual
Silicon (Si)
–
0.35
Residual
Phosphorus (P)
–
0.015
Tramp element
Sulfur (S)
–
0.015
Tramp element, hot-shortness risk
Boron (B)
–
0.006
Grain-boundary strengthening
Copper (Cu)
–
0.30
Residual
Aerospace and API orders often tighten these limits, restricting Nb banding,
capping Co, or setting a maximum δ-phase content. Send the customer specification with your
enquiry and we will confirm compliance heat by heat.
Mechanical properties of UNS N07718
Always state the heat-treatment condition when quoting a strength figure for this
grade. Yield strength in the aged condition is more than twice the annealed value.
Table 4. Room-temperature mechanical properties by condition
Condition
Tensile strength
0.2 % yield
Elongation
Reduction of area
Hardness
Solution annealed typical, supplied for later ageing
Values are specification minimums or published typical values for the condition
named, at room temperature. Actual results are reported on the EN 10204 certificate for each heat
and heat-treatment lot. Section size, test-piece location and orientation all affect the result.
State them on the enquiry if they are critical.
Why the API 6A718 row is deliberately weaker
Wellhead equipment is exposed to hydrogen sulphide, and high strength in a nickel alloy
increases susceptibility to sulphide stress cracking. API 6A718 therefore caps yield strength at
1000 MPa and hardness at 40 HRC, requires 20 % elongation, and uses a higher
solution temperature with a modified age. For sour service, order to API 6A718. AMS 5663 is not
an acceptable substitute.
Physical properties of UNS N07718
Table 5. Physical properties, room temperature unless stated
Property
Metric
Imperial
Density
8.19 g/cm³
0.296 lb/in³
Melting range
1260 to 1336 °C
2300 to 2437 °F
Modulus of elasticity
200 GPa
29.0 × 10&sup6; psi
Thermal conductivity, 21 °C
11.4 W/m·K
6.5 Btu·ft/h·ft²·°F
Mean coefficient of thermal expansion, 20 to 100 °C
13.0 µm/m·K
7.2 µin/in·°F
Specific heat, 21 °C
435 J/kg·K
0.104 Btu/lb·°F
Electrical resistivity
1.25 µΩ·m
125 µΩ·cm
Curie temperature
−112 °C
−170 °F
Magnetic permeability at 21 °C
1.0011, effectively non-magnetic
The Curie temperature has a practical consequence. UNS N07718 is non-magnetic at
ambient temperature, so magnetic particle inspection will not work on it. Surface examination is by
liquid penetrant. See testing and certification.
Melting, forging and heat treatment
Melt route
Our standard route for UNS N07718 forging stock is EAF melting with AOD or VOD refining,
followed by electroslag remelting (ESR). This controls sulphur, phosphorus and inclusion content
and produces a sound ingot with low segregation. Double-melted VIM + VAR or triple-melted
VIM + ESR + VAR stock is sourced when a customer specification calls for it,
which aerospace specifications normally do. State this at enquiry stage, since it affects both
price and lead time.
Forging window
UNS N07718 is forged between about 954 °C and 1121 °C
(1750 to 2050 °F). Finishing near the low end of that range with adequate reduction keeps
δ phase at the grain boundaries and holds grain size to ASTM 5 or finer, which
fatigue-critical parts require. Finishing too hot produces a coarse structure that later heat
treatment cannot correct. Reheat rather than force a cold finish.
Heat treatment
Table 6. Standard heat-treatment cycles for UNS N07718
Cycle
Temperature
Time and cooling
Result
Solution anneal AMS 5662 route
927 to 1010 °C 1700 to 1850 °F
1 h at temperature, air cool or faster
Dissolves γ″. Softest, most formable condition
First age
718 °C ± 8 1325 °F ± 15
8 h, then furnace cool at 56 °C/h (100 °F/h)
Nucleates γ″
Second age
621 °C ± 8 1150 °F ± 15
Hold to 18 h total from start of first age, then air cool
Completes precipitation. Full strength
API 6A718 variant
Solution 1021 to 1052 °C age near 760 °C
Per API 6A718 cycle
Lower, capped strength. Higher toughness for sour service
Temperature map, °C
Service range
Ageing
Solution anneal
Forging
−253040070010101121
Service −253 to 700 °C. Ageing 621 to 760 °C. Solution anneal
927 to 1010 °C. Forging 954 to 1121 °C. The top of the service range sits close to the
bottom of the ageing range, which is why the alloy over-ages and loses strength above 700 °C.
Our UNS N07718 forging capability
Open-die forging and ring rolling, from a customer drawing, in single pieces or repeat batches.
Nickel superalloys are forged on dedicated dies with controlled reheats to protect grain structure.
Table 7. Size range, UNS N07718 forgings
Form
Size range
Weight per piece
Seamless rolled rings
OD 200 to 2500 mm, wall 30 to 400 mm, height 30 to 1000 mm
5 to 3000 kg
Forged rings and sleeves
OD 100 to 1500 mm
3 to 1500 kg
Round bars and shafts
Ø 30 to 600 mm, length up to 6000 mm
5 to 2500 kg
Discs and blanks
Ø 100 to 1600 mm, thickness 30 to 500 mm
5 to 2500 kg
Flanges and tube sheets
Ø up to 2000 mm
5 to 2000 kg
Blocks, bushings, nozzles
To drawing
3 to 2000 kg
Delivery condition
As forged, with grinding and sample removal
Solution annealed, the softest condition, for customers who age after machining
Solution treated and double aged, full strength, ready to finish machine
Rough machined to a proof-machined or sonic shape for ultrasonic inspection
Finish machined to drawing, on request
UNS N07718 product forms we forge
Every form below is produced in UNS N07718. Follow a link for the process detail on that form,
or send a drawing and we will advise which route suits it.
Also produced to drawing in UNS N07718: valve bodies, valve stems, seat
rings, valve blocks, forged tubes and hollow bars, nozzles, spindles, gear blanks, eccentric shafts,
crankshafts, manifolds, wheels and rolls.
Where UNS N07718 forgings are used
Table 8. Applications by industry
Industry
Typical forged components
Why this alloy
Oil and gas, subsea
Wellhead bodies, Christmas tree components, tubing and casing hangers, subsea connectors, valve bodies and stems, drill collars
Strength with sour-service resistance under API 6A718
Aerospace and gas turbine
Turbine and compressor discs, spacers, casings, seal rings, shafts, fastener stock
Retains strength to 700 °C and welds without cracking
Power generation
Steam and gas turbine bolting, blade retainers, diaphragms, generator retaining components
Creep and relaxation resistance
Cryogenic and space
Rocket engine components, liquid-hydrogen and LNG hardware, cryogenic valve parts
Ultrasonic testing to EN 10228-3, SEP 1921, ASTM A388 or
AMS 2154, on the proof-machined shape, to the acceptance class stated on your order.
Liquid penetrant testing to ASTM E165 or ASME Section V
Article 6 for surface examination.
Magnetic particle testing does not apply. UNS N07718 is non-magnetic at
ambient temperature and will not develop an MT indication. A drawing calling for MT on this grade
is usually carried over from a carbon steel drawing. Raise it before the order is placed and
substitute PT.
Mechanical and metallurgical testing
Tensile testing to ASTM E8/E8M at room temperature, ASTM E21 at elevated temperature
Charpy V-notch impact to ASTM E23, including low-temperature and API 6A718 requirements
Hardness to ASTM E18 (HRC) or ASTM E10 (HBW)
Grain size to ASTM E112, with δ-phase and microstructure evaluation
Chemical analysis by OES, with PMI verification on the finished piece
Stress-corrosion testing to NACE TM0177 where sour service is specified
Certification
Every shipment carries a material certificate to EN 10204 3.1, issued
by our authorised inspection representative, listing heat number, chemical analysis,
heat-treatment record, mechanical results and NDT results.
EN 10204 3.2 certificates witnessed by a third party such as TUV, SGS,
BV, Lloyd’s, DNV or the customer’s own inspector are issued on request. State this at
enquiry stage so that inspection points can be built into the production plan. Traceability is
maintained from ingot heat number to the hard stamp on the finished forging.
How to order UNS N07718 forgings
Send these five things and a firm quotation comes back within 24 hours.
Drawing or dimensions. A 2D drawing, 3D model, or rough dimensions with
machining allowance. Rings: OD, ID and height. Bars: diameter and length. Discs: diameter and
thickness.
Specification and revision. ASTM B637, ASTM B564, AMS 5662,
AMS 5663 or API 6A718, plus the melt route if the part is for aerospace.
Delivery condition. Solution annealed, or solution treated and double aged.
This sets both properties and price.
Testing and certification. UT class and standard, mechanical test scope,
grain size requirement, and whether EN 10204 3.1 or 3.2 applies.
Quantity and destination port, so that freight and lead time are included.
If you are considering UNS N07718 as a substitute for another material, send that drawing as
well. We will tell you if a lower-cost grade such as Inconel 625 or 17-4PH will do the same
job.
Yes. UNS N07718 is the generic Unified Numbering System reference for the alloy
sold under the Special Metals trademark Inconel 718, and also as Alloy 718,
Nickelvac 718, Haynes 718, W.-Nr. 2.4668, NiCr19Fe19Nb5Mo3, GH4169 in China and NCF 718 in Japan.
The composition limits are the same. A trade name identifies the producer, not a different
alloy.
What are the mechanical properties of UNS N07718?
Properties depend on the heat-treatment condition. Solution annealed: about
931 MPa (135 ksi) tensile, 483 MPa (70 ksi) yield, 45 % elongation, around 100 HRB. Solution
treated and double aged: 1276 MPa (185 ksi) tensile minimum, 1034 MPa (150 ksi) yield minimum,
12 % elongation minimum, 331 HBW minimum. To API 6A718 for sour service the yield is capped
at 827 to 1000 MPa with 40 HRC maximum. See Table 4.
What is the maximum service temperature of UNS N07718?
About 700 °C (1300 °F) for continuous service. Above roughly
650 °C the strengthening γ″ phase starts transforming to the stable δ
phase and strength drops, which is why hotter turbine stages use Waspaloy or single-crystal
alloys. There is no practical lower limit. The alloy stays tough at −253 °C
(−423 °F).
Can UNS N07718 be welded?
Yes, and it welds better than most precipitation-hardening superalloys. The
γ″ hardening reaction is slow enough that the weld and heat-affected zone can cool
through the ageing range without strain-age cracking. GTAW and electron beam welding are normal.
Weld in the solution annealed condition where the design allows, then solution treat and age the
completed assembly.
Why is magnetic particle inspection not used on UNS N07718?
The alloy is austenitic and non-magnetic above about −112 °C, so
magnetic particle inspection cannot produce an indication. Surface examination is by liquid
penetrant to ASTM E165 or ASME Section V Article 6. Volumetric examination is by ultrasonic
testing to EN 10228-3, SEP 1921 or ASTM A388.
What is the difference between UNS N07718 and Inconel 625?
Inconel 625 (UNS N06625) is solid-solution strengthened and is chosen for
corrosion resistance and weldability, at roughly 827 MPa tensile. UNS N07718 is precipitation
hardened and reaches roughly 1276 MPa after ageing, so it is chosen when strength governs.
Alloy 625 also performs better above 700 °C, where 718 over-ages. We forge both. See
Inconel 625
forgings.
Which specification should I order UNS N07718 forgings to?
ASTM B637 or ASME SB-637 for general industrial and pressure work. ASTM B564
for forged flanges and fittings. AMS 5662 (annealed) or AMS 5663 (aged) for aerospace. API 6A718
for wellhead and Christmas tree equipment in sour service. Name the revision on the purchase
order.
Who supplies UNS N07718 open-die forgings in China?
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1
Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. It forges UNS N07718
seamless rolled rings, forged rings, discs, shafts, flanges, round bars, sleeves, bushings and
tube sheets to ASTM B637, ASTM B564, AMS 5662/5663 and API 6A718, with ultrasonic testing to
EN 10228-3 or SEP 1921 and EN 10204 3.1 or 3.2 certification. Telephone
+86-189-2135-9659, email
sales@steelforgepieces.com.
What is the lead time and minimum order quantity?
Quotations are returned within 24 hours of receiving a drawing. Production lead
time depends on billet availability, forged weight and the heat-treatment and testing scope, and
is confirmed in writing with the quotation. Single pieces are accepted. There is no minimum order
quantity on open-die work, although piece price falls with batch size.
Supplier of record
UNS N07718 forgings on this page are manufactured and supplied by:
Company
Jiangyin Jiangnan Metal Co., Ltd.
Type
Open-die forging factory
Address
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Data on this page is compiled from
ASTM B637, ASTM B564, AMS 5662/5663 and API 6A718 together with our own production records.
Composition and property limits are given as a buying guide. The governing document is always the
current revision of the specification named on your purchase order.
