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Reactor material Inconel 718

By Thomas Huang December 11th, 2023 79 views
Contents

I, What is Inconel718

Inconel 718 is a precipitation-hardening nickel-chromium alloy containing considerable amounts of iron, niobium, and molybdenum, as well as small amounts of aluminum and titanium. 
Microsstructure of Inconel 718:
Inconel718 alloy is a nickel-based high-temperature alloy strengthened by body-centered tetragonal γ" and face-centered cubic γ′ phase precipitation.
Temperature property:
It has good comprehensive performance in the temperature range of -253~700℃. It combines corrosion resistance and high strength with outstanding welding properties, including resistance to post-weld rupture. 
Below 650 ℃, Inconel718 has the best yield strength of the deformation among high-temperature alloys. It also has good resistance against fatigue, radiation, oxidation, and corrosion. It has good machining properties, welding properties, and long-term organizational stability, able to manufacture a variety of complex shapes of parts.

II, Inconel 718 Microstructure

Inconel718 alloy is an age-hardening Fe-Cr-Ni-based deformable high-temperature alloy. Its structure consists of the γ matrix, NbC, Y', and Y" δ phase. The Y" phase is the main strengthening phase in the alloy, and it is easy to turn into δ phase at high temperatures and a long time.  The number of the Y' phase is a little less than the γ" phase.  Y' phase also plays a certain role in strengthening.
picture of Inconel 718 Microstructure
The Figure above shows the microstructure of Inconel 718 alloy in the direct aging state of different processes.
ᆞAccording to the grain size metallographic rating standard, under the state of 750 ℃ × 8h + 650 ℃ × 8h its grain size is about grade 11, see 1a. 
ᆞUnder the state of 720 ℃ × 8h + 620 ℃ × 8h, its grain size is about grade 13, see 1b; 
The grain size is about grade 12 in the state of 690 ℃ × 8h + 620 ℃ × 8h. 
So, the alloy organization is particularly sensitive to thermal processing technology. We can make a variety of parts to meet the different requirements on strength levels and the usages, after mastering the laws of alloy phase Precipitation and Dissolution, as well as the interrelationships organization and process, performance.

III, Inconel 718 composition

The chemical composition of this alloy is divided into three categories: standard composition, high-quality composition, and high-purity composition, see Table below:
Table of Inconel 718 composition grade 1

Table of Inconel 718 composition grade 2
Compared with Standard Composition, high-quality composition has less carbon, but more niobium, to reduce the niobium carbide. It makes the High-quality composition have less proportion of fatigue source and more reinforcing phases, to improve fatigue resistance.  The content of harmful impurities and gases is reduced at the same time.
High-purity Composition further reduces the content of sulfur and harmful impurities on the basis of High-quality composition to improve the purity and comprehensive performance.
Inconel718A
Inconel718 alloy for nuclear energy applications, needs to control the boron content (other elements remain unchanged). When ω (B) ≤ 0.002%, the alloy grade is named as Inconel718A.

IV. Inconel 718 material properties

4.1 Inconel 718 Physical Properties:

Density: =8.24g/cm3
Melting temperature scale: 1260 ~ 1320 ℃
machining property

4.2 Easy processing.

Hard special nickel 718 alloy has good plasticity, ductility, fatigue properties, creep properties, and tensile strength, so it is widely used in various fields. The typical applications are liquid rockets, columns, protective sleeves, and a variety of aircraft on the molding sheet metal parts, gas turbine engines on the ground, cryogenic storage tanks, as well as fasteners and instrument parts. 
Inconel 718 easy to process in many shapes

4.3 It has high tensile strength, fatigue strength, creep strength, fracture strength, excellent corrosion resistance, and easy processability at 700℃.

4.4 High oxidation resistance at 1000℃.

4.5 Stable chemical properties at low temperatures.

4.6 Good welding performance.

V, Inconel718 Processing and Heat Treatment

  Inconel 718 alloy is easy to process in a variety of industrial processes. 

5.1 Inconel 718 preheating

  Workpieces must be surface cleaned before and during heating. If the heating environment contains sulfur, phosphorus, lead, or other low melting point metals, Inconel 718 alloy will become brittle. Impurities come from marking paints, chalk, lubricants, water, fuels, and so on. 
The sulfur content of fuels should be low, for example, it should be less than 0.1% in liquefied petroleum gas and natural gas, less than 0.25g/m3 in city gas, and less than 0.5% in petroleum gas. 
 The electric furnace should have the function of precise temperature control. The furnace gas must be neutral or weakly alkaline. The fluctuation between oxidizing and reducing of the furnace gas composition is not allowed. 

5.2 Hot working on Inconel 718

The hot working temperature of Inconel 718 alloy is 1120-900 ℃. Cooling can be water quenching or other rapid cooling methods, annealing needs to be done promptly after hot working to ensure the best performance. 
The material should be heated to the upper limit of the processing temperature during hot working, to ensure plasticity during processing. The final processing temperature should not be lower than 960℃ when the deformation reaches 20%. 

5.3 Inconel 718 Cold working

  Cold working should be carried out after solution treatment. The work hardening rate of Inconel 718 is greater than austenitic stainless steel, so the processing equipment should be adjusted accordingly, and there should be an intermediate annealing process during the cold working. 
Cold working machine

5.4 Inconel 718 Heat Treatment

Different solution treatment and aging processes result in different material properties. Due to the low diffusion rate of the γ" phase, the best mechanical properties of Inconel 718 alloy can be obtained by prolonged aging. 

5.5 Inconel 718 Sanding

It requires fine abrasive belt sanding because oxides around welds of Inconel 718 workpieces are more difficult to remove than in stainless steel. 
The removal of oxides with sandpaper or salt bath pretreatment is also needed before pickling in a mixture of nitric and hydrofluoric acids. 

5.6 Inconel 718 Machining

Machining of Inconel 718 needs to be carried out after solution treatment. Inconel 718 is suitable for low cutting speeds because of its work-hardening properties of the material.
ᆞTurning
Inconel 718 turning processing  
The main processing difficulties of nickel-based alloys are large plastic deformation, high cutting force, high cutting temperature, severe hardening, and severe tool wear, making it difficult to ensure surface quality and accuracy. 
So, people usually use new high-performance cutting tool materials, suitable cutting fluid, and appropriate heat treatment processes to improve the cutting processability of the material. It also requires suitable cutting parameters to optimize cutting, to achieve a high machining speed of Inconel 718. 
Burrs may appear on the machined surface during dry machining Inconel 718. Adjusting the feed amount to change the direction of chip flow and the shape of the chip, can reduce the generation of burrs.

5.7 Inconel 718 Welding

Precipitation-hardened Inconel 718 alloy is well suited for welding. There is no tendency to crack after welding.  
Inconel718 is suitable for arc welding, plasma welding, and so on. 
arc welding for inconel 718
ᆞNotice:
Before welding, the surface of the material should be clean, with no oil, and no chalk marks, and the weld should be polished to reveal bright metal within a range of 25mm around the weld. 
Inconel 718 is recommended welding material:  
ᆞGTAW/GMAW 
ᆞNicrofer S 5219 
ᆞW.-Nr. 2.4667 
ᆞSG-NiCr19NbMoTi 
ᆞAWS A 5.14 ERNiFeCr-2 
ᆞBS 2901 Part 5: NA 51 

VI, Similar grades

Inconel 718 similar grades:
ᆞGH4169, GH169 (China),
ᆞNC19FeNb (France),
ᆞNiCr19Fe19Nb5, Mo3 (Germany),
ᆞNA 51 (UK)
ᆞInconel718, UNS NO7718 (USA)
ᆞNiCr19Nb5Mo3 (ISO)

VII. Applications:

Inconel718 application areas: due to the high-temperature strength and excellent corrosion resistance at 700 ℃, easy processing, can be widely used in a variety of high-demanding occasions.
ᆞSteam turbine.
ᆞLiquid fuel rockets.
ᆞLow temperature engineering.
ᆞAcidic environment.
ᆞNuclear engineering.

7.1Aerospace

Inconel 718 is one of the most commonly used nickel-based alloys in the aerospace industry because it excels in high-temperature and high-pressure environments. 
It is used in the manufacture of various static and rotating parts in aviation and aero-engines, jet propellers, jet engines, and turbine components, such as disks, rings, magazines, shafts, blades, fasteners, elastomeric elements, gas ducts, sealing elements, etc. and welded structural parts. 
In addition, it is also used in the manufacture of other key components in aerospace equipment, such as gas turbine blades, braking systems, gas turbine generators, rocket engine parts, and so on.

7.2 Energy Sector

Inconel 718 is also one of the most commonly used high-temperature alloys in the energy sector because of its ability to maintain strength and corrosion resistance under extreme operating conditions. 
It is widely used in the manufacture of energy equipment, such as the manufacture of various elastomeric components and lattices used in nuclear energy industry applications, nuclear reactors, gas turbines, steam turbines, heat exchangers, and gas burners.

7.3 Chemical Industry

Inconel 718 is also widely used in petroleum extraction, refining, the chemical industry, natural gas processing, and other fields, such as oil well casing, oil well drilling bit, chemical reactors, high-temperature heaters, pipes, and valves.
high temperature chemical equipment made by inconel718

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