High-strength OCTG tubes like C110, according to API 5CT (yield strength at least 758 MPa), are subject to requirements in terms of mechanical and corrosion properties. Steels of that group must combine high strength characteristics and resistance to a process of general corrosion and sulfide corrosion cracking (SSC). Extreme operating conditions require the high quality of the 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 fabricated tubes. The production of these tubes is complicated because of influence on final structure by all parts of manufacturing from smelting and casting, to rolling and final heat treatment.
In this work, we studied the influence of seamless tubes microstructure with a 177.8 mm diameter and 10.36 mm wall thickness of high-strength steel (yield strength more than 758 MPa) to sulfide stress corrosion cracking (SSC) and sulfide stress corrosion cracking with low strain rates (SSRT). Tubes were obtained from continuous billets by screw piercing with preliminary quenching and tempering.
It was established 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. The segregation bands consist of bainite varying degrees of tempering with a hardness from 280 (light band) to 320 HV (dark band). In the dark band, in comparison with the soft light ones, the content of chromium, molybdenum and niobium is increased. In addition to dispersed niobium, molybdenum, and chromium carbides formed during rolling and heat treatment, coarse niobium and titanium carbonitrides formed in the solidifying metal were found in the band. Thus, it has been shown that zonal segregations formed in continuous casting are not eliminated during rolling but are transformed in the region of banding of the inner pipe surface, which cannot be eliminated by heat treatment. Recommendations for the ladle treatment and casting of these steels are proposed, as well as modes of heat treatment to minimize the harmful effect of the segregated banding found on the pipe properties.
SSRT method helps more accurately and faster study the performance of steels in the development of technology for their production. But to verify of SSRT methods and the development of criteria for evaluating materials, a large statistics of two parallel tests is needed - standard tests according NACE TM 0177 (SSC) and SSRT methods.
