Effect of tempering on hydrogen induced cracking in accelerated cooled pipeline steels

Abstract

Hydrogen induced cracking (HIC) occurs in pipeline steels used in oil and gas applications rich in hydrogen sulfide gas. The presence of high amounts of H2S gas, also known as sour service, allows for ingress of hydrogen and internal cracking in the absence of an externally applied stress. It is generally believed that HIC susceptibility increases with increasing strength, which limits the use of high strength steel in sour service. Pipeline steels are low carbon microalloyed steels produced using controlled rolling. Higher cooling rates induce a microstructure that often consists of a mixture of non-equiaxed ferrite with smaller islands of solute rich secondary microconstituents. This multiphase microstructure is called granular bainite and is the primary microstructure of interest in this work. Two different alloys, X65 and X70, with granular bainitic microstructures were used for this study. Two separate steel ingots with the X65 chemistry were cast, reheated, and thermomechanically processed (TMP) with finish rolling into the intercritical regime followed by air cooling for one ingot and accelerated cooling to 540 C followed by air cooling to room temperature for the other ingot. The intercritical finish rolling step can be referred to as warm rolling, and was conducted in order to understand the effect of increased dislocation density in proeutectoid ferrite. The air cooled X65 steel produced a mixture of quasi-polygonal ferrite and pearlite. Upon HIC testing the air-cooled steel exhibited no cracking while the accelerated cooled steel exhibited considerable cracking. In contrast, the X70 steel had additions of Mo and Si and was processed with the intent of initiating accelerated cooling in the single-phase austenite regime. The X65 steel was susceptible to HIC in the as-received (AR) condition, while the X70 steel was not. The lack of cracking in the X70 AR steel was hypothesized to be due to the lower area fraction, homogeneous distribution, and equiaxed shape of secondary microconstituents in the X70 as opposed to the X65 steel. In order to understand the effect of changes in microstructure on HIC, both steels were tempered at 300, 400, 500, and 600 C for 40 minutes, and the X70 steel had an additional tempering temperature of 700 C. After tempering, hardness and yield strength were maintained or increased in both steels at all temperatures. Upon tempering, the accelerated cooled X65 steel exhibited austenite decomposition and cementite formation similar to that observed in fully martensitic steels, while hardness and strength were nominally maintained if not slightly increased. In contrast, additions of Si in the X70 steel retarded the decomposition of austenite and formation of cementite to higher tempering temperatures. Tensile strength in the X65 steel decreased upon tempering at 300 C and was maintained at higher temperatures; tensile strength was maintained in the X70 steel with the exception of a decrease at 700 C. HIC testing revealed that tempering improved HIC resistance in all X65 conditions except 400 C. The reduced HIC resistance of the AR and 400 C conditions was attributed to elongated secondary microconstituents (SM) that resulted from warm rolling and elongated grain boundary cementite that formed from elongated SM in the 400 C condition, observations that are often associated with tempered martensite embrittlement (TME) in fully martensitic steels. Charpy impact test results at room temperature in the X65 steel, interpreted to be in the upper shelf regime, revealed that the same conditions that were HIC susceptible, the AR and 400 C conditions, also exhibited the lowest room temperature toughness indicating that toughness might be a better predictor of HIC resistance than hardness or strength. The X70 condition exhibited HIC susceptibility in the 300 C tempered condition in which phosphorous was also observed within cracks. Observations of phosphorous in cracks (X65 AR and X70 300 C) and elongated grain boundary cementite (X65 400 C) indicates that a phenomenon like TME occurs in these accelerated cooled steels. As long as TME regimes are taken into consideration, tempering appears to be a promising option to increase HIC resistance of linepipe steels.