Inconel Alloy 718 for oil and gas industry
Inconel alloy 718 was discovered in 1980s for downhole apparatus. The need for meeting high pressure gas wells demands a high strength alloy with excellent corrosion resistance to gas containing high carbon dioxide and hydrogen sulphide for completion apparatus. The superalloys like Hastelloy C-276 were available and provided outstanding corrosion resistance to the anticipated fluids. Although, it was available as high strength tubing and didn’t offer wall thickness and uniform strength around the wall thickness. The technique for cold processing, high strength bar was not found during that period. An analysis for different heat treatments for under and over aging was conducted to obtain required strength and toughness. The heat processing and mechanical properties were determined on the base of stress corrosion cracking outcomes, the heat processing and maximum hardness were monitored for sour service operations.
The controlled strength can be received in Inconel 718. The application of Inconel 718 was based on sour service at medium temperatures and in absence of free sulphur.
Inconel 718 has been employed in downhole measurement equipments, particularly in measurement while drilling. Here the main cause is nonmagnetic alloy with high strength as the magnetic analysis is followed to determine the well orientation. Simultaneously, the material should prevent corrosion in drilling mud that may be enriched with salt.
Inconel 718 has been used for drill string parts, where it provides a suitable blend of high fatigue strength and excellent corrosion resistance while being non-magnetic.
In 718 is chosen among other nickel based superalloys for providing highs strength, excellent corrosion resistance and it is easily available in the variety of sizes. In the beginning, it was commonly used in safety valves. The alloy was heat treated to meet the needs of aerospace applications in a high strength condition. The application service temperature varies from low temperatures below 100oF to a manufacturing temperature up to 300oF. The high strength condition was not required for the maximum resistance to stress corrosion cracking.
Heat processing of In 718
The standard heat processing of In 718 is conducted for aerospace applications to obtain higher creeping resistance and cracking strength for the excellent transverse ductility. This heat processing creates 150 ksi minimum yield strength. Annealing temperature about 1850 to 1925oF was selected because a low annealing temperature approximate 1700oF to 1750of was not adequate to homogenize the microstructure from earlier processing procedures. Annealing at 1925of was selected as maximum annealing temperature to prevent extensive grain growth. For specification purposes, a temperature limit of 1850oF to 1900of was chosen.
The modern applications of In 718 in downhole apparatus are noticed in wells in severe corrosive conditions. The general corrosion resistance of alloy 718 in the various sour gas media is described in the following media:
| Alloy | H2S pressure | Corrosion rate-300oF | Corrosion rate at 400of |
| 718 | 10 psi | 3 mpy or 0.076 mm/y | 0.3 mpy or 0.008 mm/y |
| 718 | 50 psi | 0.7 mpy or 0.018 mm/y | 2.3 mpy or 0.058 mm/y |
| 718 | 100 psi | 0.1 mpy or 0.003 mm/y | 1.2 mpy or 0.030 mm/y |
| 925 | 0 psi | 0.1 mpy or 0.003 mm/y | 0.1 mpy or 0.003 mm/y |
| 925 | 50 psi | 0.4 mpy or 0.010 mm/y | 0.5 mpy or 0.058 mm/y |
| 925 | 100 psi | 0.1 mpy or 0.003 mm/y | 0.4 mpy or 0.010 mm/y |
The lowest corrosion rates are found in warm sour brines up to 400oF. The localized corrosion and stress corrosion cracking becomes a concern in availability of sulfur. The materials were analyzed for 14 days in 15% sodium chloride + distilled water with gas pressure about 1000 psi plus 500 psi carbon dioxide plus nitrogen and variable magnitudes of hydrogen sulphide.
Effect of heat processing on corrosion resistance
The general corrosion and pitting resistance are prone to heat processing. The drop in corrosion resistance can be featured to the precipitation of carbides and segregation of alloy composition while heat processing. The nominal drop of resistance found is caused by chromium loss in the localized areas because of heat processing. For downhole completion apparatus applications, the heat processing has been done to offer strength, toughness and corrosion resistance. The heat work for several applications includes a solution annealing at 1850oF to 1900oF for 60 minutes at least, for maximum two hours. The aging processing is conducted at 1450 oF for six to eight hours.
Applications for Inconel alloys
Inconel alloy 718, Inconel 625 and Incoloy 925 are popularly employed in oil and gas industry for the different applications. The severe corrosive conditions in wells are resulted by large amounts of H2S, carbon dioxide, chlorides and high temperatures at the holes. The major use is for downhole tools like subsurface security valves, packers, flow control devices and other apparatus. The Inconel alloys are employed for well head components and valve bodies. Cast Inconel 718 is also employed for the sophisticated shapes.
As compared to other stainless steels utilized, these alloys also receive nominal reduction in strength at the application temperatures. These alloys are made in the different sizes of bars. They can be easily forged and welded which make them a popular material for use in the production of oil well tools. Inconel 718 has been utilized in the production of side pocket mandrels utilized for artificial lifting of oil through injection gas. These are completely annealed and aged subsequent to welding to obtain higher strength and restore corrosion resistance properties.
Drilling industry applications
The extensive application of directional drilling has given rise to extreme applications of Inconel 718. These applications need a nonmagnetic material with significant corrosion resistance that are offered by titanium alloys and Inconel 718 that are commercially introduced to meet the strength criteria. Alloy 718 even meets the hardenability needs for large sizes.
The drilling media is severe in other modes. The drilling mud can be highly attacking as it contains high chlorides, in the vigorous drilling conditions; hydrochloric acid is pumped through the drill. The standard nickel superalloys and stainless steels and copper alloys can be employed in the conditions. Inconel 718 is nominally attacked in this media and it is not attacked by stress corrosion cracking from sulphur. The benefit of using In 718 is its higher fatigue strength and freedom from corrosion.
Heanjia Super-Metals completely understands the severity of oil and gas well conditions, therefore superalloys like Inconel 718 and Inconel 625 are manufactured following the stringent needs of this industry. In order to get any kind of information about our stock of corrosion resistance alloys, contact us at 12068907337 or send an email at sales2@alloywirestrip.com