Effect of heat treatment temperature on pitting resistance of G3 alloy

With the continuous exploration and development of high sulfuric acid oil and gas fields, the severity of downhole environment is increasing. Nickel base corrosion resistant alloy is widely used in the development of acid oil and gas wells because of its excellent corrosion resistance and good mechanical and processing properties. The results show that the excellent corrosion resistance of nickel base alloy is due to the passivation film formed on its surface, which isolates the corrosion medium from the alloy and improves the corrosion resistance of the alloy. The corrosion resistance of nickel base alloy mainly depends on its chemical composition and microstructure.

Nickel base alloys used in acid environment belong to solution strengthened cold working corrosion resistant alloys, which will form certain residual stress in the cold working production process. A series of heat treatment is needed to eliminate the residual stress and structural defects. However, in the process of heat treatment, the intermetallic compounds and carbides which damage the corrosion resistance of nickel base alloy are easy to precipitate. Under the actual working condition, there are many cases that corrosive medium causes severe local corrosion to the workpiece with sensitized precipitation structure, and pitting is one of the most destructive and hidden corrosion forms. Heat treatment temperature can further affect the pitting resistance of nickel base alloy by affecting the diffusion rate of elements and precipitation and dissolution of precipitates.

In the past, the heat treatment process or corrosion resistance of the alloy was studied, but there were few reports about the effect of heat treatment process on the corrosion resistance of the alloy. By analyzing the influence of different heat treatment processes on the microstructure of G3 alloy, the researchers conducted corrosion simulation experiments under laboratory conditions to study the influence of heat treatment process and corresponding microstructure characteristics on the pitting resistance of G3 alloy, in order to establish the relationship between heat treatment process, microstructure characteristics and pitting resistance.

The experimental material is G3 nickel base alloy. The trial production process is VIM + ESR + forging + extrusion + cold rolling + 650 ℃ annealing + 1100 ℃ solution treatment. Its chemical composition (mass fraction,%) is Cr22, mo7, Cu2, Fe20, Ni residual. In order to compare the effect of different heat treatment temperature on the pitting corrosion resistance of G3 alloy, the annealing treatment was carried out at 500 ℃, 700 ℃ and 900 ℃ for 2h and then air cooling.

SEM, EDS, TEM and other methods were used to study the effect of different heat treatment temperature on the microstructure and grain boundary precipitates of G3 alloy. The test results show that:

(1) The pitting resistance of G3 alloy increased after low temperature annealing, and decreased with the increase of annealing temperature.

(2) Grain boundary precipitation plays a key role in the pitting resistance of G3 alloy. After annealing at 500 ℃ for 2 h, the precipitates are very few and the homogeneity of microstructure is increased, so the pitting resistance is the best. A large number of precipitates were formed on the grain boundaries of the samples annealed at 900 ℃ for 2 h. The formation of precipitates increased the inhomogeneity of the microstructure, resulting in the poor stability of the passive film in the Mo poor area, easy to occur local activation dissolution, and the pitting sensitivity increased significantly.

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