Hydrogen Cracking and Stress Corrosion of Pressure Vessel Steel ASTM A543.

摘要

The purpose of conducting this research is to develop fundamental understanding of the weldability of the modern Quenched and Tempered High Strength Low Alloy (Q&T HSLA) steel, regarding the cracking behavior and susceptibility to environmental cracking in the base metal and in the heat affected zone (HAZ) when welded. A number of leaking cracks developed in the girth welds of the pressure vessel after a short time of upgrading the material from plain carbon steel to Q&T HSLA steel. The new vessels were constructed to increase the production of the plant and also to save weight for the larger pressure vessel.;The results of this research study will be used to identify safe welding procedure and design more weldable material. A standardized weldability test known as implant test was constructed and used to study the susceptibility of the Q&T HSLA steel to hydrogen cracking. The charged hydrogen content for each weld was recorded against the applied load during weldability testing. The lack of understanding in detail of the interaction between hydrogen and each HAZ subzone in implant testing led to the need of developing the test to obtain more data about the weldability. The HAZ subzones were produced using two techniques: standard furnace and GleebleRTM machine. These produced subzones were pre-charged with hydrogen to different levels of concentration. The hydrogen charging on the samples simulates prior exposure of the material to high humidity environment during welding process. Fractographical and microstructural characterization of the HAZ subzones were conducted using techniques such as SEM (Scanning Electron Microscopy). A modified implant test using the mechanical tensile machine was also used to observe the effects of the hydrogen on the cracking behavior of each HAZ subzone.;All the experimental weldability works were simulated and validated using a commercial computational software, SYSWELD. The computational simulation of implant testing of Q&T HSLA with the previously used plain carbon steel and other currently used pressure vessel steels was successfully completed. The experimental and computational results of the Q&T HSLA steel agreed well with each other. The susceptibility of the Q&T A543 steel to stress corrosion cracking was investigated using the slow strain rate testing under different environments and conditions. Also, advanced corrosion study using the electrochemical impedance spectroscopy was done at different conditions. The corrosion study revealed that this A543 steel is prone to form pits in most of the conditions. The model results in the corrosion study were validated with the Gamry Echem Analyst software that A543 steel tends to form pits in the tested environment.