Stress corrosion cracking in a high strength steel
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
This thesis falls into four fields of study. The first is a survey of relevant literature concerning the many theories of stress corrosion cracking and hydrogen embritt1ement. This includes descriptions of the mechanisms of stress corrosion cracking (SCC) and outlines electromechanical processes and stress - sorption theory. Four widely accepted mechanisms for environment assisted cracking are also outlined. They are, 1) Embritt1ement resulting from accumulated hydrogen at embritt1ement sites, 2) Lowering of surface energy by adsorption of hydrogen, 3) Hydrogen interaction with dislocations, and 4) Lowering of the binding energy by interaction of hydrogen. The literature survey is a significant part of this thesis. The overall objective of the survey is to review a series of current SCC tests on high strength steels. The principal findings from these previous studies are summarized, they provide concrete evidence for the conclusion that SCC of high strength steels is due to hydrogen embrittlement. The second part of the project deals with the development of a stress corrosion loading clevis suitable for testing compact tension specimens. Three existing constant load rigs were developed, and equipment was designed for the successful operation of the rigs. Corrosive environment was applied to the standard compact tension specimen using a novel circulation system based on a magnetic plate stirrer. Corrosive solution (3.5% NaCl) was stirred by the magnetic plate, and the vortex created by the magnetic stirrer was used to create a pumping head. The third area of work dealt with the testing of compact tension specimens of ULTIMO 200 steel using the developed apparatus. The experimental procedures used are based on the application of linear elastic fracture mechanics to stress corrosion cracking. The fourth area of work carried out was to perform slot length calibration experiments on CT specimens by using strain gauges. The results indicated that the specimens pre-cracked in air with a higher dynamic load gave higher threshold stress intensities (KIscc ) than those pre-cracked in air with a lower dynamic load. An electron microscope study indicated evidence of a largely inter granular fatigue crack having occurred in the specimens pre-cracked with a high dynamic load.