Stress corrosion cracking problems with non-magnetic drillcollar (NMD) have occurred frequently in recent years. To prevent these problems, a steel having the chemical composition of 35%Ni-20%Cr-2%Ti-0.5%AI was developed as NMD material with high resistance to stress corrosion cracking by making use of nickel and precipitation hardening. This steel bar was obtained by hot-rolling and successive heat treatment. It displays a mechanical performance similar to conventional Mn-Cr steels, which have been used for NMD, under the stress condition of drilling. As for stress corrosion cracking and corrosion fatigue, this steel has much stronger resistance than Mn-Cr steels. Moreover, it contains little residual stress, because it was strengthened by precipitation, not by warm working, and its proof stress does not decrease up to 300° C, while conventional Mn-Cr steels reduce their proof stress at lOO°C. These properties are also beneficial to prevent stress corrosion cracking.
Non-magnetic drillcollars (NMD) are placed in a string during directional drilling to help determine the actual location of the string and bit. These collars are mainly made of Mn-Cr austenitic steels strengthened by solid solution hardening and warm working. The environment in which NMD must serve is drilling muds. These muds contain water, clay, and chloride-containing salt, such as NaCI and KCI. High levels of chloride in the muds, coupled with tensile stress, either service stress or residual stress, can lead to chloride stress corrosion cracking. These NMD stress corrosion cracking problems have occurred frequently in recent years. Conventional NMD, made of Mn-Cr steels strengthened by solid solution hardening and warm working, are susceptible to stress corrosion cracking for the following reasons. Mn-Cr steels have little resistance to stress corrosion cracking, and they have relatively high residual stress produced by warm working and following rapid cooling processes. Therefore, reducing susceptibility to stress corrosion cracking and removing residual stress are both necessary to develop an NMD with high resistance to stress corrosion cracking. This paper reports the development process of new NMD with high resistance to stress corrosion cracking, its physical and mechanical properties, and its corrosion resistance.
35%Ni-20%Cr was designed as a matrix chemical comf,0sition for the following reasons. Nickel is an effective element) in improving resistance to stress corrosion cracking in a chloride environment. Chromium is the most effective element to general corrosion. The equilibrium phase of this matrix steel is a complete austenite and excludes the possibility of the formation of magnetic phases even after heavy cold working. There are several means of strengthening this matrix, such as solid solution hardening, precipitation hardening, and warm (cold) working. Most conventional NMD were strengthened by solid solution hardening and warm working. But the problems associated with these strengthening methods were the occurrence of residual stress and inhomogeneity of properties. Considering these problems, precipitation hardening was chosen as a strengthening method. We selected precipitation hardening by intermetallic compounds, not by carbides, because carbon has a strong affinity to chromium, and Cr-carbide precipitates during precipitation hardening treatment, which reduces the corrosion resistance. As the nickel content is high, we tried to make use of the precipitation of gamma-prime (Ni3Til).
