What is the effect of heat treatment temperature on the pitting resistance of G3 alloy?
With the continuous exploration and development of high sulfur-bearing sour oil and gas fields, the harshness of the downhole environment has increased. Nickel-based corrosion resistant alloys are widely used in the development of sour oil and gas wells due to their superior corrosion resistance and good mechanical and processing properties. Studies have shown that the excellent corrosion resistance of nickel-based alloys is due to the passivation film formed on their surfaces, which isolates the corrosive media from the alloy and thus improves the corrosion resistance of the alloy. The corrosion resistance of nickel-based alloys depends mainly on their chemical composition and microstructure.
Nickel-based alloys used in acidic environments are all solid solution strengthened cold-working state corrosion resistant alloys, which will form certain residual stresses during cold-working production and require a series of heat treatments to eliminate residual stresses and tissue defects. However, during heat treatment, nickel-based alloys tend to precipitate intermetallic compounds and carbides that impair their corrosion resistance. Under actual working conditions, there are many cases of corrosive media causing severe localized corrosion of workpieces with sensitized precipitation tissues, and pitting is one of the most destructive and potentially dangerous corrosion forms. Heat treatment temperature can further affect the pitting resistance of nickel-based alloys by influencing the diffusion rate of elements and the precipitation and dissolution of precipitated phases.
Previous studies have mostly focused on the heat treatment process or corrosion resistance of the alloy, while relatively few reports have addressed the effect of heat treatment process on the corrosion resistance of the alloy. By analyzing the effects of different heat treatment processes on the tissue characteristics of G3 alloy and then conducting corrosion simulation experiments under laboratory conditions, the researchers investigated the effects of heat treatment processes and the corresponding tissue characteristics on the pitting resistance of G3 alloy in order to establish the relationship between heat treatment processes-tissue characteristics-pitting resistance.
The experimental material is the trial G3 nickel-based alloy, the trial process is: VIM + ESR + forging + extrusion + cold rolling + 650 ℃ annealing + 1100 ℃ solution treatment, its chemical composition (mass fraction, %) is Cr22, Mo7, Cu2, Fe20, Ni balance. To compare the effect of different heat treatment temperatures on the pitting resistance of G3 alloy, the annealing treatments performed were 500, 700 and 900°C for 2h followed by air cooling.
The effects of different heat treatment temperatures on the morphology of G3 alloy and grain boundary precipitation phases were investigated by SEM, EDS, and TEM; then, immersion simulation experiments and electrochemical experiments were used to analyze the effects of tissue changes and grain boundary precipitation phases on the pitting resistance of G3 alloy. The experimental results show that.
(1) the pitting resistance of G3 alloy increased after low-temperature annealing, and the pitting resistance decreased significantly with the increase of annealing temperature.
(2) The influence of grain boundary precipitation phase on the pitting resistance of G3 alloy plays a key role. 500℃ annealing 2h specimen has very little precipitation phase, the organization uniformity increases, thus the best pitting resistance. 900℃ annealing 2h specimen generates a large number of precipitation phase at the grain boundary, the large number of precipitation phase generation makes the organization inhomogeneity increases, resulting in poor stability of the Mo-poor region passivation film, easy to occur local activation dissolution The susceptibility to pitting corrosion increases significantly.