engleski

Critical Stress of Low Alloy Mn-V and Cr-Mo Steel Tubing

During exploatation of oil and natural gas, the built-in equipment is considerably exposed to different corrosion processes. One of the most frequent types of corrosion is sulfide stress cracking (SSCC) which is especially emphasized in steels of higher mechanical properties. This paper presents the investigation in the Iron and Steel Works Sisak, based on hot rolled tubing, made of medium carbon low alloy Mn-V and Cr-Mo (microalloyed with Nb) steels of high purity (<0.010% S). After numerous heat treatment variants (quenching+tempering, quenching+double tempering) carried out on the pipes in labaratory conditions, the SSCC resistance of Mn-V steel, API grades C-90 and L-80, as well as Cr-Mo steel, grade C-95, was tested, in order to determine the critical stress (S-value) by the Shell method. The testing results show that tubing in as-rolled condition has a poor SSCC resistance (low critical stresses) while, by quenching and tempering at temperatures 700-720oC, high SSCC resistance microstructure with spheroidised in oil, because of the tempered martensite microstructure with spheroidised carbides in the ferrite matrix. The acquired cognitions on the critical stress values for particular conditions of pipes are confirmed by the fractured surface analysis, as well as X- ray diffraction phase analysis.

critical stress, tubings, corrosion, heat treatment

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