Overview of Alloy X-750 Pipe and Tube
Alloy X-750 (UNS N07750) is a nickel-chromium alloy that is made precipitation hardenable by additions of titanium and aluminium. The result is a material that combines good corrosion and oxidation resistance with high tensile and creep-rupture strength at elevated temperature, and it keeps that strength far longer than most austenitic grades. Pipe and tube made from this grade are specified where a component must carry pressure or load at high temperature without relaxing, for example in gas turbine and nuclear service.
Unlike a solid-solution strengthened alloy, X-750 is strengthened by a controlled ageing treatment applied after forming. That means the mill condition, the amount of cold work and the heat treatment cycle all influence the final properties, so the procurement specification must be stated precisely.
Specifications and Grade Equivalency
| Product form | Specification | Remarks |
|---|---|---|
| Bar, forgings and forging stock | ASTM B637 | Precipitation-hardening nickel alloy bars and forgings for high-temperature service |
| Solution treated and precipitation hardened bar | AMS 5667 | Defines the conditioning and ageing treatment plus minimum room-temperature properties |
| Seamless and welded tubing | AMS 5582 | Solution heat treated tubing for high-temperature pressure and structural use |
| Wire | AMS 5670 | Cold drawn wire for springs and fasteners |
| Material designation | UNS N07750 | Nickel-chromium alloy with titanium, aluminium and niobium additions |
Chemical Composition
Limits in weight percent for the wrought grade:
| Element | Nickel | Chromium | Iron | Titanium | Aluminium | Niobium plus tantalum | Manganese | Silicon | Sulfur | Copper | Carbon |
|---|---|---|---|---|---|---|---|---|---|---|---|
| wt % | 70.0 min | 14.0 - 17.0 | 5.0 - 9.0 | 2.25 - 2.75 | 0.40 - 1.00 | 0.70 - 1.20 | 1.00 max | 0.50 max | 0.010 max | 0.50 max | 0.08 max |
Mechanical and Creep Properties
Typical properties for precipitation hardened material at room temperature:
| Property | Typical range | Comment |
|---|---|---|
| Ultimate tensile strength | 1100 - 1300 MPa | Age-hardened condition |
| Yield strength, 0.2% offset | 750 - 950 MPa | Age-hardened condition |
| Elongation at break | 10 - 20% | Falls as strength rises |
| Hardness | 30 - 40 HRC | Depends on heat treatment and cold work |
| Creep and stress rupture | usable at 650 °C | Sustains significant load for long periods before failure |
Above about 700 °C the strengthening phases begin to coarsen and long-term strength falls, so continuous service is normally limited to roughly 700 °C while short-term excursions to higher temperature can be tolerated. At 650 °C the alloy still carries meaningful stress for thousands of hours, which is what makes it attractive for turbine and reactor internals.
Heat Treatment
Three practical conditions are supplied. Solution treated material is heated and rapidly cooled to put the strengthening elements into solution, and is suitable for forming before ageing. Solution treated and aged material receives a precipitation treatment that develops full strength and is supplied when little further forming is required. The third, often called the equalised and aged condition, applies an additional high-temperature step to improve creep resistance at the cost of some room-temperature strength. Ageing temperatures, soak times and cooling rates must follow the governing AMS or ASTM specification rather than shop practice, because the balance between tensile strength, ductility and stress-rupture life is set by that cycle.
Welding and Fabrication
Welding: gas tungsten arc welding with matching filler metal is standard; the joint area must be clean, and ageing after welding restores strength in the weld zone.
Cold forming: the alloy work hardens rapidly and should be formed in the solution treated condition with generous bend radii, followed by the specified ageing treatment.
Machining: age-hardened material machines to a good finish with carbide tooling and low cutting speeds; heavy interrupted cuts are best taken before ageing.
Oxidation: a protective surface oxide forms at high temperature and limits further degradation, so the alloy performs well in air and combustion atmospheres.
Typical Applications
Gas turbine blades, discs and casings; steam turbine components; aircraft engine fasteners, springs and seals; nuclear reactor internals and control rod hardware; pressure vessels and high-temperature tooling; and petrochemical reactors and heat exchangers that combine corrosive media with elevated temperature and pressure. The ability to resist relaxation at temperature also makes the alloy a common choice for bolting on hot flanges.
Frequently Asked Questions
Q: What grade does alloy X-750 correspond to in ASTM terms?
The wrought precipitation-hardening grade is designated UNS N07750 and appears in ASTM B637 for bar and forgings, with tubing and wire covered by the relevant AMS specifications.
Q: Why is the alloy supplied in more than one heat treatment condition?
Because the ageing cycle trades room-temperature strength against stress-rupture life. Components that must resist creep choose the equalised and aged condition; parts that need maximum short-term strength use the standard aged condition.
Q: Can X-750 pipe be used above 700 °C?
Short excursions are acceptable, but continuous service above roughly 700 °C causes the strengthening phases to coarsen and strength falls away, so other grades are usually selected for sustained higher duty.
Q: How does cold work affect the finished pipe?
Cold work raises strength but reduces ductility and can change the ageing response, so the amount of reduction is controlled and recorded, and the ageing treatment is matched to it.
Q: Is the material corrosion resistant in chloride service?
It has useful resistance to many oxidizing and reducing media and to high-temperature oxidation, but it is not a substitute for the molybdenum-bearing nickel alloys in severe wet chloride environments.
Q: What certification accompanies pipe and tube orders?
Each order is supplied with an EN 10204 3.1 mill certificate covering chemistry, tensile and hardness results, plus dimensional and non-destructive test records where the specification requires them.
