Hydrogen induced cracking（HIC） behaviors of a high strength pipeline steel with three different microstructures, granular bainite ＆ lath bainite（GB ＋ LB）, granular bainite ＆ acicular ferrite（GB ＋ AF）, and quasi-polygonal ferrite（QF）, were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide（H_2S）-saturated solution. The experimental results show that the steel with GB ＋ AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB ＋ GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF ＋ GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.
The carbon dioxide corrosion behavior of low alloy pipeline steel was investigated by immersion experiment. Optical microscopy （OM）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and X-ray diffraction （XRD） were used to reflect the microstructure of the tested material and the corrosion morphology characterization. Results show that precipitate particles may accelerate the iron cabonate crystal formation process of the nucleation growth and promote the formation of compact layer. The major corrosion phases are FeCO3 and complexity compound containing Fe and Cr. The corrosion behavior consists of three stages. At the first stage, a thin inner layer and an inhomogeneous outer layer have appeared. At the second stage, the outer layer becomes homogeneous and compact, which prevents corroding the steel substrate further. At the third stage, iron carbonate crystal tends to nucleate and grow locally. The corrosion rate obtained by weight loss method increases abruptly first and then decreases quickly with increasing corrosion time. The mutual relation among microstructure, corrosion surface morphology, corrosion phases and corrosion kinetics is discussed.
For refining grain and obtaining excellent properties, the experiments of asymmetric and symmetric monotonic hot rolling were carried out to investigate the role of shear strain on the microstructures and mechanical properties of V-microalloyed steel. The study demonstrates that the gradient ferrite distribution and dispersive pearlite through the sheet thickness are observed in asymmetric rolled specimen, and the homogeneous microstructure with ferrite and large pearlite is found in symmetric rolled specimen. The average grain size in asymmetric rolled specimen is smaller than the one in symmetric rolled specimen. The styles of precipitate morphology in asymmetric rolled specimen are random precipitate and obvious interphase precipitate, while the ones in symmetric rolled specimen are random precipitate and unobvious interphase precipitate. The additional shear strain results in the microstructure difference between asymmetric rolled specimen and symmetric rolled specimen. The impact energy of asymmetric rolled specimen, 58 J, is more than the one of symmetric rolled specimen, 48 J. Both deflection-energy curve and fracture morphology show that the fracture style of asymmetric rolled specimen is ductile, and the ones of symmetric rolled specimen are brittle and ductile.
