UNS N07718 Capillary Tube: Composition, Standards and Selection Guide

What UNS N07718 Capillary Tube Is

UNS N07718 is a precipitation-hardening nickel-based alloy in which nickel is the base element at roughly 50 to 55 percent, with chromium, molybdenum, niobium and titanium added to build strength. After a solution anneal followed by a two-step ageing treatment, niobium and titanium form finely dispersed gamma double prime and gamma prime phases that give the alloy a very high yield strength while it remains ductile and oxidation resistant.

Capillary tube is the small-bore form of the material. It is produced by cold drawing to outside diameters of roughly 0.5 to 6 mm with wall thicknesses of about 0.05 to 1.0 mm, and is supplied in coils or straight lengths with controlled concentricity, surface finish and internal cleanliness. That combination of strength, pressure capability and fine bore makes the grade a standard choice for instrumentation lines, sensor sheaths, sampling systems and compact heat exchanger elements.

Composition and Physical Properties

The table below gives the usual composition limits for the grade together with the physical values that matter when a tube is being designed.

In the solution-treated and aged condition, the minimum room-temperature properties required by ASTM B637 for bar, forgings and rings are a tensile strength of 1240 MPa, a yield strength of 1034 MPa and an elongation of 12 percent. Reported values for capillary tube follow the same order of magnitude, but the actual figures depend on reduction, grain size and the exact ageing cycle, so the mill certificate should always be checked against the specification that governs the order.

Two frequently quoted figures for this grade are incorrect and should not be used in design work: a density of 8.44 g/cm3 and a melting range of 1290 to 1310 degrees Celsius. The correct values are approximately 8.19 g/cm3 and 1260 to 1336 degrees Celsius.

Standards and Heat Treatment

Product specifications are organised by form. Bar, forgings and rings are covered by ASTM B637; plate, sheet and strip by ASTM B670; aerospace sheet, strip and plate by AMS 5596; and aerospace bar, forgings and rings by AMS 5662. Seamless capillary and small-bore tube are ordered against the applicable seamless tube specification together with the general requirements documents for nickel alloy pipe and tube, and the purchase order should state both the dimensional tolerances and the acceptance test schedule.

The standard heat treatment has two stages:

Solution anneal: hold at 954-982 degrees Celsius, typically for one hour, then cool rapidly. A high-temperature solution anneal at 1038-1065 degrees Celsius is used when a coarser grain size is wanted for creep duty.

Ageing: hold at 720 degrees Celsius for eight hours, cool in the furnace at about 55 degrees Celsius per hour down to 620 degrees Celsius, hold for a further eight hours, then air cool.

A solution treatment near 1150 degrees Celsius is not the normal practice for this grade. It coarsens the grain structure, can dissolve the phases that carry the strength, and usually leaves the material below specification after ageing.

Selection and Fabrication Notes

Three points decide whether this alloy is the right choice for a capillary application.

Service environment: the grade is intended for high temperature, high pressure and corrosive duty. For a room-temperature instrumentation line carrying a benign fluid, an austenitic stainless steel such as grade 304 or 316 is usually more economical.

Grade confusion: UNS N07718 differs from UNS N06625 and UNS N06600 in both composition and strengthening mechanism. N07718 is precipitation hardened, while the other two are solid-solution grades, so they cannot be substituted without re-checking pressure ratings and heat treatment.

Processing capability: the alloy work-hardens rapidly and is difficult to machine and draw. Cold drawing must be interleaved with annealing, and a final solution plus ageing treatment is required after forming. Suppliers should be able to show the drawing schedule, the intermediate anneals and the finished heat treatment.

For capillary tube, cleanness and dimensional control matter as much as composition. Specify the maximum bore contamination, the concentricity and the surface roughness, and request a pressure test or eddy current test where the tube will carry high-pressure gas or hydraulic fluid.

Frequently Asked Questions

Q: What is UNS N07718 capillary tube?
It is precipitation-hardening nickel-based alloy, containing about 50 to 55 percent nickel with chromium, molybdenum, niobium and titanium, drawn into small-bore seamless tube with outside diameters of roughly 0.5 to 6 mm.

Q: How is UNS N07718 strengthened?
By a solution anneal followed by a two-step ageing treatment that precipitates gamma double prime and gamma prime phases, giving a minimum tensile strength of 1240 MPa and yield strength of 1034 MPa in the aged condition.

Q: What is the standard heat treatment for this alloy?
Solution anneal at 954-982 degrees Celsius for about one hour and rapid cool, then age at 720 degrees Celsius for eight hours, furnace cool to 620 degrees Celsius, hold eight hours and air cool.

Q: What is the density of UNS N07718?
About 8.19 g/cm3, with a melting range of roughly 1260 to 1336 degrees Celsius. Values of 8.44 g/cm3 or a 1290 to 1310 degrees Celsius melting range do not belong to this grade.

Q: Where is UNS N07718 capillary tube used?
Instrumentation and impulse lines, sensor and thermocouple sheaths, sampling systems, hydraulic and pneumatic control lines, and compact heat exchanger or cooling elements in aerospace, energy and chemical plants.

Q: Can UNS N07718 be replaced by UNS N06600 or UNS N06625?
Not directly. The other two grades are solid-solution alloys, so a substitution changes strength level, heat treatment requirement and pressure rating and must be re-qualified.