Inconel 718 Socket Weld Fittings | UNS N07718 | Tailan Metal

Inconel 718 socket weld fittings are the age-hardened member of the nickel alloy fitting family, and they behave differently from every other grade on this site. UNS N07718 is a precipitation-hardening nickel-chromium-iron alloy with niobium, titanium and aluminium additions; it is not forged and left soft, but solution annealed and then aged through a controlled cycle, and the result is a minimum tensile strength of 185 ksi (1275 MPa) at a minimum yield of 150 ksi (1034 MPa) with 12 % elongation. That combination of aerospace-level strength with resistance to oxidation, chloride pitting and sour service is not available from any other nickel alloy in the range.

Supplied to ASME B16.11 in the socket weld form, these fittings are used on small-bore lines where a full-penetration weld is wanted but the pipe cannot easily be bevelled, and where the joint has to be inspectable and the component has to carry load. Note at the outset that ASME SB-366 does not list UNS N07718 among its grades, so the material route for these fittings is ASME SB-834 for the pressure consolidated powder metallurgy product, or ASME SB-637 forging stock machined to the B16.11 form. This page covers the socket weld form; the butt weld form is on our Inconel 718 butt weld fittings page and the full nickel alloy range is on our nickel alloy socket weld fittings overview.

Specification

Chemical Composition

Limits as published for UNS N07718 in ASME SB-834 Table 1, which covers pipe flanges, fittings, valves and parts in this alloy. ASME SB-637 Table 1 publishes the same grade for bars, forgings and forging stock, and ASME SB-834 Table 2 sets the product analysis tolerances. The niobium and tantalum figure is given as a combined limit, which is why the certificate reports niobium with tantalum.

Mechanical Properties

From ASME SB-834 Table 3 for UNS N07718. ASME SB-637 Table 4 gives the same tensile and yield minima with 12 % elongation in 2 in. (50 mm) and 15 % reduction of area, and adds a minimum hardness of 331 HB, equivalent to 36 HRC, in the aged condition. Because that hardness is a requirement of the specification rather than a characteristic of the melt, it is the figure to check against any maximum hardness limit the project applies, particularly in sour service, before the grade is confirmed.

Size Range and Dimensional Standard

Manufacturing Route and Heat Treatment

The material is delivered in the solution annealed plus precipitation hardened condition, and that condition is reached in two steps that must both be completed. Solution annealing at 1700 to 1850 °F (924 to 1010 °C) dissolves the carbides and the gamma prime phase into a homogeneous matrix. Precipitation hardening then holds the material at 1325 ± 25 °F (718 ± 14 °C) for 8 hours, furnace cools it to 1150 ± 25 °F (621 ± 14 °C) and holds it there until the total precipitation treatment time reaches 18 hours before air cooling. The strength of the finished fitting is set by the second step, so an under-aged body is not a slightly weaker fitting, it is a different material from the one that was ordered. Forming and machining are completed before the final heat treatment, and the socket is then machined and dimensionally checked.

Inspection, Testing and Certification

• Material certificate: EN 10204 3.1 is supplied as standard and
  3.2 is available on request; the certificate is issued against the heat number
  stamped on the fitting.
• Chemical analysis: one test per heat lot, reported against the
  certificate, with the actual values rather than the specification limits.
• Mechanical testing: one tension test per lot, in the delivered
  condition, on test specimens taken from the lot.
• Dimensional inspection: against the governing dimensional
  standard, with the socket or bore, wall, length and end condition recorded.
• Supplementary testing: hardness, intergranular corrosion testing,
  charpy impact testing, ultrasonic or eddy current examination, positive material
  identification and liquid penetrant examination are supplied when ordered, and
  are named on the purchase order rather than assumed.
• Third-party inspection: witnessed by Lloyd’s Register, BV, DNV,
  SGS or TUV on instruction, arranged at enquiry stage.

Packing and Delivery

Fittings are supplied solution annealed and pickled, with a machined surface
finish. Each fitting carries the grade, the governing material specification, the
dimensional standard, the nominal size and the heat number, linked to the
EN 10204 certificate issued with the shipment. Export packing is by wooden
crate, steel pallet or case, with plastic end caps and, where required, VCI
protection inside the case. Beveled or plain ends are confirmed with the
order. Delivery schedule, quantity, minimum order quantity and price are
confirmed only against a specific enquiry.

Applications

• Aerospace, gas turbine and rocket engine fluid and fuel systems
• High-pressure hydraulic and instrumentation lines on offshore installations
• Sour gas, oil and gas wellhead and downhole control equipment
• Chemical injection lines in refinery and petrochemical plant
• Nuclear reactor primary and secondary circuits
• High-temperature process lines where creep and stress rupture govern
• Oil and gas gathering lines where strength and chloride resistance are both required

Technical Notes

The weld joint does not inherit the aged properties. This is the single most important thing to understand about 718. The strength of the parent metal comes from a fine dispersion of a nickel-niobium precipitate formed during ageing, and the heat-affected zone of a welded joint is heated well above the ageing temperature and then cools through the precipitation range without ever being held at it. The result is a joint that is stronger than the unaged material but weaker than the aged parent, and no filler selection can change that, because filler chemistry controls strength in the weld metal, not in the heat-affected zone. Where full aged strength has to be carried through the joint, the answer is a machined component that is not welded, or a post-weld solution and ageing cycle that the fabricator and the code allow.

718 is weldable, and it is crack resistant when welded properly. The alloy is a deliberate choice for welded structures precisely because it resists strain age cracking and hot cracking in the heat-affected zone far better than the precipitation hardening grades that came before it. But that resistance depends on controlling heat input and interpass temperature, and on keeping the heat-affected zone clean. Contamination of the nickel-rich surface with copper-bearing material, or with sulphur, is the classic way to manufacture a cracking problem in a nickel alloy that is supposed to be crack resistant.

The hardness requirement cuts both ways. ASME SB-637 makes a minimum hardness of 331 HB, or 36 HRC, part of the aged condition, because that is the hardness that comes with the precipitate. The same hardness is what a sour-service specification typically limits, so an aged 718 fitting and a maximum-hardness sour requirement can be in direct conflict. Resolve this on the enquiry, in writing, against the actual governing document, rather than discovering it at the manufacturing stage.

Temperature capability is not the same as creep capability. 718 is often described simply as a high-temperature alloy, and it is qualified for service to about 700 °C. The distinction that matters in design is between the alloy being able to sit at a temperature and the component being able to hold a stress at that temperature for a long time. The strength of 718 at 700 °C is well below its room temperature strength, and it is the long-term rupture behaviour, not the short-term tensile behaviour, that governs a pressure boundary design.

Related Products

• Inconel 718 butt weld fittings
• Nickel alloy socket weld fittings — every form and grade
• Nickel alloy flanges
• Nickel alloy pipe and tube

Frequently Asked Questions

Which material specification governs Inconel 718 socket weld fittings?

ASME SB-834 covers UNS N07718 pipe flanges, fittings, valves and parts in the pressure consolidated powder metallurgy form, and ASME SB-637 covers the precipitation hardening bar, forgings and forging stock from which fittings are machined. ASME SB-366 does not list UNS N07718 among its grades.

What condition are the fittings supplied in?

Solution annealed plus precipitation hardened. Solution annealing at 1700 to 1850 °F, then 1325 ± 25 °F for 8 hours with a furnace cool to 1150 ± 25 °F held to a total of 18 hours, then air cooling.

What is the strength of the aged material?

A minimum tensile strength of 185 ksi (1275 MPa) and a minimum yield strength of 150 ksi (1034 MPa) with 12 % elongation, per ASME SB-834 Table 3. ASME SB-637 adds a minimum hardness of 331 HB (36 HRC) in the aged condition.

Does the weld joint keep the aged strength?

No. The heat-affected zone of a welded joint does not receive the ageing treatment and does not reach the strength of the aged parent metal. Where that matters, the design should avoid a welded joint or allow a post-weld treatment cycle.

Can these fittings be used in sour service?

The alloy resists chloride pitting and H2S attack well, but the aged condition carries a specified minimum hardness. Confirm that the hardness requirement of the governing sour-service document is compatible before the grade is confirmed.

Which pressure classes are available?

ASME B16.11 designates socket weld end fittings as Class 3000, Class 6000 and Class 9000. The class must match the pipe wall, since the class describes the pairing rather than a pressure in isolation.

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