Abstract
The aim of this research is to study hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) properties of the pressure vessel steel (ASTM A516) having three different kinds of microstructure obtained by different cooling rates in normalizing heat treatment. HIC test was performed in reference to NACE TM0284 standard method. For the effective analysis on SSC, 2 kinds of test method were utilized: the constant load test (CLT: dead weight device) and 4-point bent beam test in reference to NACE TM0177 and ASTM G39, respectively. In addition, fractography analysis by using field emission scanning electron microscope (FE-SEM) was performed to clarify the crack initiation sites and propagation paths with respect to the different microstructures ranging from ferrite/pearlite to bainite. The results indicated that the HIC resistance was increased in the order of water quenched (WQ) specimen, air cooled (AC) specimen and oil cooled (OC) specimen whereas, the SSC resistance was increased in the order of WQ specimen, OC specimen and AC specimen, with AC specimen being the most resistant to SSC. It suggests that the microstructure with high resistance to HIC does not directly assure high resistance to SSC. For OC and WQ specimens, however, tempering heat treatment decreases the crack length ratio (CLR) determined by ultrasonic detector (UT) and increases significantly the threshold stress (σth) determined by the CLT, resulting in the best performance with respect to both HIC and SSC susceptibility. Also, the fractography analysis revealed that the failure mode of AC specimen under external stress was type I SSC (SOHIC), while the OC and WQ specimens showed a mixture of type I and type II SSC failure mode.
