Effect of heat processing on Inconel alloy 718 properties

Single step aging treatments are used to develop the mechanical properties in oil and gas applications. The annealing temperatures are often in the range 1850oF – 1950oF or 1210oC – 1265oC followed by one step age at 1200oF to 1450oF or 648oC to 988oC. It offers yield strengths of 130,000 – 140,000 psi. The strength can be receive by underaging near 1275oF or overaging near 1400oF. Considering the needs any of these heat treatments are used for a specific yield strength. The hardness reduces with increase in aging temperature however the yield strength reaches a peak value between 1200oF to 1400oF. Hence for excellent hardness, underaging may be suitable. For instance, grain boundary precipitation may affect hardness, as is shown by either the ductile or intergranular nature of fracture surfaces received in toughness tests.

Some applications demand the highest yields strength about HRC 40 hardness. Inconel 718 is supplied and configured in oil field applications for corrosive media. Using these extreme strength levels is unlike to the justification for the recent amendments made in NACE specification MR0175. User should understand that most of Inconel alloy 718s used in market are designed for high temperature services. They are heat processed to the standard double aging treatments. Reheat treatment of these alloys may not offer the necessary properties for oil and gas applications as the heat treatment history has an effect on the microstructure. For instance, grain boundary precipitates or second phases can retain in the grain boundaries to subsequently coarsen in future heat processing. So the oil and gas industry should consider the heat treatment history.

Inconel alloy 718 is used for oil and gas applications because of its excellent corrosion resistance. As downhole conditions are fully deaerated the chance of general corrosion or pitting attack in production environments is nominal. Inconel alloy 718 is useful in offering excellent resistance to carbon dioxide, hydrogen sulfide and acetic acid based conditions. Corrosion rates of below 1 mil per year occur in high temperatures. Just with acid, oxygen or elemental sulfur significantly corrodes Inconel alloy 718. So the pitting corrosion resistance is considered in the extremely severe conditions occurred in the industry. Various corrosion resistant alloys are used in most production conditions. Inconel 718 offers pitting corrosion resistance similar to duplex stainless steels.

Inconel 718 applications are not only limited to corrosion resistance. Both general corrosion resistance and pitting corrosion resistance properties of Inconel 718 wire are affected by heat processing. Increase in pitting temperatures show enhanced corrosion resistance. Heat processing up to 1350oF do not have a damaging effect on the pitting temperature in the Yellow Death Solution. Although heat processing at 1450oF affects the pitting corrosion resistance. The loss of pitting resistance can be added to precipitation of carbides and to the segregation of alloying composition during the precipitation hardening processing.