Study of hydrogen assisted cracking in ship building grade steel

Abstract

High strength, high toughness and good weldability are the major demanding factors for engineering structural materials uses in naval construction. HSLA-80 steel is essentially low carbon micro alloyed and copper strengthened high strength steel typically used for ship building application. The hydrogen presence in high strength steels causes untimely brittle fracture under static load condition. Hydrogen enters into metal and reduces the cohesive strength of metallic atom, creates void and enhance the brittlement. In the present study, the loss in ductility and susceptibility to fracture of HSLA80 grade steel was studied by using fracture toughness parameters (J integral and CTOD) in hydrogen and air environment. The tests were carried out with very slow displacement rate through cyclic loading. The hydrogen environment was generated with NaOH aqueous solution. Single Edge Notched Beam (SENB) samples with fatigue precrack were used for the tests. J-R and Ä-R curve were developed by ASTM standards. It is observed that fracture toughness of air environment tested sample was higher than hydrogen environment tested sample and fracture occurs after stable tearing. The fracture surface of the failed samples was examined by Scanning Electron Microscope (SEM) and it was observed that the failure occurred by cleavage, quasi cleavage and transgranular fracture. From the present study it was clear that in hydrogen environment the brittleness is more and the related fracture toughness values confirm the same. The hydrogen enhanced local plasticity is more applicable mechanism for hydrogen embrittlement. Mechanical property of the as received steel was studied by tensile testing and hardness test. Microstructural observation revealed presence of lath martensite and acicular ferrite in the steel. The experimentalresult obtained from the present study was found closeto the models proposed earlier.