Alloy X-750, 2.4669, UNS N07750, Inconel® X-750

Alloy X-750, 2.4669, UNS N07750, Inconel® X-750 according to ASTM B637 and EN 10302

Alloy X-750, Inconel® X-750, N07750 – description

Alloy X-750, widely known as Inconel® X-750 is one of the nickel-chromium alloys. It was specifically developed to achieve high creep-resistance - to resist deformation at elevated temperatures.

The titanium and aluminium content make welding diffucult, but since the Al+Ti level is below 6%, welding do not pose outright strain-age cracking danger. Proper technique and heat treatment are required. More information in the "welding" section.

Enrichment of the X-750 alloy with elements such as cobalt, molybdenum, boron, zirconium, tungsten and/or niobium would result in superalloys capable of working as gas turbine components.

Applications

Typical applications are gas turbine components (both aero and industrial turbines), high temperature fasteners, springs and rocket engines. After slightly different heat treatment, alloy X-750 is also used for nuclear reactor components, typically pressurised water reactors (PWR's) and boiling water reactors (BWR's). It can also be used for applications in cryogenic temperatures.

Alloy X-750, Inconel® X-750, N07750 - physical and mechanical properties

Room temperature physical properties:

• Density: 8.28 g/cm³
• First melting temperature: 1290 °C
• Precipitation temperature: 955 °C

Room temperature mechanical properties of Inconel® X-750 bar:

• Tensile strength: 1200 MPa
• Yield strength: 815 MPa
• Elongation: 27%

Elevated temperature (540°C) mechanical properties of Inconel® X-750 bar:

• Tensile strength: 1050 MPa
• Yield strength: 725 MPa
• Elongation: 26%

Rupture strength of Inconel X750 bar and temperature:

• 650°C: 470 MPa
• 870°C: 50 MPa

Corrosion behaviour

As a super alloy, X-750 exhibits high corrosion resistance in moderate environments. The results of resistance tests to some challenging environments are discussed below.

Molten carbonates - corrosion data generated in molten carbonate salt at 900°C for 504 h show adequate resistance to molten carbonates of Inconel® X-750. In the test, total depth of corrosion attack was 0.27mm.

High-temperature water - Inconel® X-750 is susceptible to stress-corosion cracking in deaerated high-purity water at 300-350°C and intergranular SCC in aerated high-temperature water application. Unoptimal heat treatment further weakens this alloy against high-temperature water (more info in section "heat treatment").

Halids - X-750 is not resistant to hydrogen fluoride HF and hexafluorosilicic acid H₂SiF₆ at temperature above 50°C. Contact with these agents is the cause of SCC.

X-750 is not resistant to hydrogen fluoride HF, which causes stress-corrosion cracking.

Weldability

The alloy belongs to group 43 according to ISO 15608.

Selection of welding consumables for Inconel® X-750:

• Filler metals: ERNiFeCr-2

Best practices for welding alloys containing Al and Ti.

• Use appropriate joint design, welding process and filler metal to minimize residual and thermally induced stresses
• Minimize heat input during welding
• For heavy-section weldments use multiple stress-relieve anneals in between welding sessions.

Heat treatment, working and machining

Metallic particles of aluminium or copper on the surface of Inconel® X-750 will rapidly alloy at elevated temperatures, degrading corrosion resistance and mechanical properties. All contamimants should be removed from the piece before heat-treatment.

Hot-forming pressure for Alloy X-750:

• 870°C: 335 MPa
• 1040°C: 299 MPa
• 1095°C: 265 MPa
• 1150°C: 230 MPa

Forging - Below 980 °C, the alloy is stiff and hard to move, and attempts to work it may cause splitting. All heavy forging should be done at about 1040 °C, and the metal should be reheated whenever it cools to below that temperature. Forgings can be finished with some light reduction in the range between 980 and 1040 °C.

• Forging: 980-1205 °C
• Heavy forging: 1040-1205 °C
• Air cooling

Variable crosssections are especially susceptible to thermal cracking during cooling. In very large crosssections, furnace cooling may be necessary.

Machining - alloy X-750 belongs to group D-2, meaning it's very hard to machine. It should be rough machined in solution annealed condition and then finish machined after aging. A size contraction up to about 0,07% takes place upon aging, which must be allowed for in rough machining.

Power spinning is preferred over manual spinning for nickel-base alloys. Practical limits on blank thickness for manual spinning of Alloy X-750 is 0,94mm. The maximum blank hardness is 94 HRB.

Soft annealing should be done in 955-1155°C for 30-60 min (continous annealing) or 60-180 min (batch annealing). Air cooling.

Solutioning prior to age hardening - nickel alloys normally do not require solution treating prior to age hardening. However in case of alloy X-750 it may be used to enhance maximum creer, relaxation and rupture strength at temperatures above 600°C, which is essential for high-temperature springs and turbine blades. To achieve this, alloy X-750 may be solution treated and air cooled prior to a double (high and low temperature) aging cycle. AMS 5668 and AMS 5671 give different temperatures.

Two-stage solutioning does not give optimal corrosion protection in very hot deaerated water (an typical environment for BRW's, boiling water reactors). For such applications the literature recommends single-stage solution treatment (note, it may reduce corrosion resistance in aerated water):

Electropolishing

Electropolishing techniques for X-750 alloy:

Electrolyte composition - 25ml H₃PO₄, 25 ml HNO₃, 50ml H₂O. Platinum cathode. 17,8 A/cm² for 5-10 sec.

We deliver this steel alloy as:

• Forgings
• Rods
• Tubes
• Sheets

Replacements, equivalents and other designations:

alloy X-750, alloy x750, N07750, AISI 688, 72Ni15.5Cr-0.95Cb,2-5Ti,0.70Al-7.0Fe, AMS5698, ASTM B637 Grade 688, Pyromet Alloy X-750, NAS 750, AMS 5747, AMS 7246, AMS 5542, AMS 5583, AMS 5667, AMS 5671, AMS 5779, AMS 7246B, AMS AS7245, Inconel® X-750, Unitemp® 750, Nickelvac® X-750, J467, SAE J467, 2.4669, NW7750, NiCr15Fe7TiAl, NC750, EP601, Sanicro® 75XT, Sanicro® 75X1T