Due to their low ratio of cost versus performances, quenched and tempered (Q&T) high strength low alloyed steels are widely used as tubular goods. For several decades, steel suppliers and academics are investigating the mechanisms of sour corrosion of these materials. Several publications highlighted the different parameters influencing the KISSC of Q&T high strength low alloyed steels like the prior austenitic grain size, the dislocation density or the precipitation state. Niobium is known to have an impact on these 3 parameters, but authors often looked at its impact separately. Additionally, complete data relating the Niobium content and the KISSC are extremely rare in the open literature.
The present study focuses on the impact of Nb on the Prior Austenite Grain Size (PAGS) refinement for several quenching and tempering cycles and additionally on the role of Nb on the critical stress intensity factor for sulfide stress cracking (KISSC). A common Cr-Mo high strength OCTG steel alloyed with three different Nb contents ranging from 300 to 1000 ppm is used. It was found that Nb addition allows the use of higher strength material without compromising the resistance to crack propagation (KISSC). The results are discussed in terms of microstructure, hydrogen pick-up and precipitation state.
The use of Oil Country Tubular Goods (OCTG) material in harsher environments (deeper wells, higher H2S concentrations and/or higher pressure and temperature) is pushing suppliers to design new steel grades with higher strength and better SSC resistance. This is extremely challenging since it is well known that the SSC resistance of a given material decreases with increasing its strength. Since the late 80's, people have been studying the metallurgical factors of low alloyed steels playing a role on the SSC resistance [1]–[3]. Quenched and tempered (Q&T) steels progressively became the reference since this processing allows to meet a high yield to tensile ratio answering the hardness limit imposed by successive API standards.
