Abstract
High-strength oil country tubular goods (OCTG) like C110, according to standard API 5CT (yield strength at least 758 MPa), are subject to requirements in terms of mechanical and corrosion properties. In this work, we studied the microstructure of seamless casing tubes made of class C110 high-strength steel with a 194.5 mm diameter and 19.7 mm wall thickness, and its influence on sulfide stress corrosion cracking (SSC). Casing tubes were obtained from continuous billets by screw piercing with preliminary quenching and tempering. It was shown that cracking during the tests always begins from the inner surface of the tube. Rough segregation bands were found on the inner tube surface, which occupies about a third of the thickness of the wall. To increase the resistance of 0.3C-Cr-Mn-Mo + 0.15(V + Nb + Ti) steel to SSC, primary recommendations for adjusting the chemical composition, production technology and heat treatment were developed.
Highlights
During the development of deep oil and gas fields, casing is influenced by the aggressive environment of the well (H2S and CO2), and by high pressures and temperatures [1,2,3,4]
It is shown that the main reason for the unsatisfactory resistance of Casing Tube (CT) samples to stress corrosion cracking (SSC) is the presence of segregation bands and niobium carbonitrides, from which corrosion cracking begins
Coarse segregations were found in the center of the CCB, in which the content of all alloying elements were increased, and coarse niobium nitrides and manganese sulfides were found
Summary
During the development of deep oil and gas fields, casing is influenced by the aggressive environment of the well (H2S and CO2), and by high pressures and temperatures [1,2,3,4]. Extreme operating conditions require high quality of steels and determine the requirements for the level of alloying and purity for detrimental impurities, as well as for the chemical and structural homogeneity of the finished casing tubes. To produce these tubes, it is important to choose the optimal chemical composition of steel and consider that the formation of the final structure is influenced by all processes: melting and casting [8], rolling [9] and final heat treatment [10]. The main idea o3f otfh1e4 calculation is the assumption that the diffusion of impurity elements in the solid phase is completely suppressed, and completely proceeds in the liquid phase
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