Phase Transformation Behaviours, Microstructural Formation And Properties Of High-Strength Low-Alloy Pipeline Steels

Aimed to acquire optimum comprehensive properties for the oil and gas pipeline steels, heat treatment should be controlled to achieve ideal microstructures. However, the influence of thermal treatment on the microstructural transformation of pipeline steels still remains indeterminate. The acquired microstructures of low-carbon- micro-alloying steels during continuous cooling process are very complex, and the transformation kinetics awaits being clarified. Although acicular ferrite has been widely considered one of the most attractive candidate microstructures for pipeline steels, agreed definition on its characteristics has not been made yet, nor the transformation mechanism. Moreover, difficulties still exist in massive production of large diameter thick wall seamless pipeline steel for commercial purposes and the cost needs to be controlled.With a view to questions mentioned above, possible influence of several crucial heat-treatment processes, such as austenization, tempering, and continuous cooling transformation, on the pipeline steels properties were systematically studied by means of high-resolution differential dilatometry. Conclusions were achieved as follow: (i) Prior austenite grains grew as austenitizing temperature increased. NbC precipitates effectively hindered the coarsening of austenite grains. (ii) Increasing austenitizing temperature promoted martensite formation. (iii) In cooling rates ranging from 5oC/min to 3000oC/min, the transformed microstructures were observed to be a combination of polygonal ferrite, pearlite, acicular ferrite and bainitic ferrite in present investigated high strength low alloy steel. The result showed that, the increasing of cooling rate accelerated acicular ferrite formation and refined the matrix. It was also found that acicular ferrite formed in a wide range of cooling rates, which was beneficial to obtain a uniform microstructure especially for thick wall pipelines. (iv) With increasing of austenitizing temperatures, austenite grain size increased, whereas critical transformation temperatures decreased. Correspondingly, the microstructures of the matrix changed from ferrite to acicular ferrite, and then to bainite as temperature went up further. In other words, austenitizing temperature, apart from cooling rate, was another important factor in promoting the transformation of acicular ferrite.