Structure and Properties of Microalloyed HSLA Steels Resistant to Sour Gas

• Published in: Key engineering materials 1993-01, Vol.84-85, p.472-489
• Main Authors: Datta, R., Mishra, Sanak, Ravi, K.
• Format: Article
• Language: English
• ISSN: 1013-9826; 1662-9795; 1662-9795
• DOI: 10.4028/www.scientific.net/KEM.84-85.472

This investigation dealt with the effects of composition, finish rolling temperature (FRT), and cooling rate on the microstructure, strength, toughness, and corrosion resistance of steels evaluated for linepipe applications in transportation of oil and natural gas contaminated with hydrogen sulphide. Six steels (Fe--0.17C--0.88Mn--0.4Cr--0.38Cu--0.05Nb--0.04Al--0.04V, Fe--0.15C--0.86Mn--0.34Cu--0.3Cr--0.05Al--0.05V--0.04Ti, Fe--0.14C--0.9Mn--0.35Cr--0.35Cu--0.06Nb, Fe--0.14C--0.9Mn--0.34Cu--0.31Cr--0.05V--0.04Al--0.03Nb, Fe--0.1C--1Mn--0.33Cu--0.32Cr--0.08Al--0.05V--0.03Ti, and Fe--0.11C--0.8Mn--0.35Cr--0.35Cu--0.07Nb) microalloyed with niobium, Nb--V, and Ti--V and containing different levels of sulphur (0.0005-0.026) were studied. Finish rolling the steels at 800 and 850 deg C followed by air cooling resulted in ferrite--pearlite microstructure with moderate strength level (YS: 350-390 Mpa, UTS: 450-500 MPa), high ductility ( epsilon sub T sub : 35-40%), and excellent impact toughness (KCV: 100-200 J). A change in the cooling rate from air cooling to spray water cooling led to the formation of bainitic microstructures with higher strength (YS: 350-500 mPa, UTS: 550-800 MPa), but lower ductility ( epsilon sub T : 15-30%) and impact toughness (KCC sub -40 : 20-90 J). The sulphide stress corrosion cracking (SSCC) and hydrogen induced cracking (HIC) resistance of the steels subjected to different processing conditions were also evaluated. Steels containing lower S ( approx 0.01%) level were found to exhibit superior SSCC and HIC properties, as expected, compared to the same steels containing a higher S ( approx 0-0.2%) level. The Nb-steel containing approx 0.01% S showed the best SSCC resistance, resulting in no failures after 720 h of testing at a stress level 0.95-1.0 of yield stress, irrespective of the microstructure. Overall, the steels exhibiting ferrite--pearlite microstructure resulted in comparatively superior SSCC resistance to that of steels having bainitic microstructure. However, no such trend was observed in the case of HIC resistance. The CLR ranged between 4-9% for 0.01% S steels and 9-12.5% for 0.02% steels.