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Schlumberger started using alloy 718 in the early 1980s for downhole equipment. A need for completing high-pressure gas wells required a high strength alloy with good corrosion resistance to gas containing high COz and H2S for completion equipment. Alloys like C-276 were available and offered excellent corrosion resistance to anticipated fluids. However, it was available only as high-strength tubing and did not provide wall thickness and uniformity of strength across the wall thickness. The technology for cold-worked, high-strength bar was not developed at that that time. A study for various heat treatments for underand over-aging was carried out to achieve desirable strength and toughness. The heat treatment and mechanical properties were finalized based on stress corrosion cracking results. The heat treatment and maximum hardness were governed for sour service application. This limited the maximum strength that can be obtained in 718. A welding procedure was developed for artificial gas lift applications. The use of 718 was limited to sour service at moderate temperatures and in the absence of free sulfur. For severe corrosive applications at higher temperatures, alloy 725 with higher Molybdenum was introduced. This alloy provided a better cracking resistance in severe applications based on laboratory testing. Later the heat treatment was modified to offer higher strength. The use of higher strength 725 allows equipment to be manufactured with the maximum ID and minimum OD to increase oil and gas production at a lower completion cost. 718 has also been used in downhole measurement tools, especially in measurement while drilling (MWD). Here, the primary reason is the need for a nonmagnetic alloy with high strength, because magnetic surveying is used to measure the well orientation. At the same time, the material must resist corrosion in drilling mud that may be saturated with salt. Most other nonmagnetic materials available at the same strength level rely on cold work to some extent and are, therefore. not readily available in large diameters. Finally. 718 has been used directly for drill string components, where i t applies a favorable combination of high-fatigue strength and high-corrosion resistance with non-magnetism. In order to maximize fatigue resistance, a modified heat treatment has been developed to produce fine grain size in large diameter forgings. Superuilo! s 718. 6 3 . 706 and Various Derivatives Edited by E.A. Lorin TMS (The Alinernls. hletnls S: Materials Society). 2001

Use of Alloy 718 and 725 in Oil and Gas Industry