INFLUENCE OF HYDROGEN ON DELAYED FRACTURE IN HIGH MANGANESE TWINNING INDUCED PLASTICITY (TWIP) STEELS AND OTHER ADVANCED HIGH STRENGTH STEELS (AHSS)

Abstract

Delayed fracture (DF) behaviour by presence of hydrogen is a non–negligible obstacle for fully application of excellent formability of advanced high strength steel (AHSS) such as high manganese Twinning Induced Plasticity (TWIP) steel, Transformation Induced Plasticity (TRIP) steel and Dual Phase (DP) steel. TWIP steels have austenitic structure and are a type of specific AHSS which show higher strength and ductility than that of other AHSS of ferritic and martensitic structure (i.e. DP and TRIP). The current work investigated the influence of hydrogen on delayed fracture behaviour in high manganese TWIP steels and other advanced high strength steel (AHSS) by carrying out slow strain rate test (SSRT) and deep drawing test (DDT). Slow strain rate test (SSRT) were employed on both tensile and notched tensile specimens of investigated materials to evaluate the influence of diffusible hydrogen on the hydrogen embrittlement effect (by tensile specimens) and influence of complex stress state (by notched tensile specimens) of the high manganese TWIP steel and other AHSS. Deep drawing test (DDT) was conducted on round blanks of investigated materials to investigate the effect of diffusible hydrogen on the delayed fracture behaviour of high manganese TWIP steel and other AHSS. By addition of Aluminium (Al) as an alloying element in high manganese TWIP steel such as X30MnAl22-1 showed postponed initiation of mechanical twinning upon deformation than X60Mn22. Since Al increases the stacking fault energy of austenite, the tendency for mechanical twinning is decreased, and the formation of deformation-induced martensite eliminated. It explored benefit of homogeneously dispersed fine Aluminium Nitrides as hydrogen traps that acts as an obstacle for diffusion of hydrogen in steel. Hydrogen–dislocation interaction is mostly increased at low strain rate (in SSRT) and high tri-axial stressed condition (in DDT).The fracture mechanisms in the investigated materials were identified by observing the fracture surfaces in scanning electron microscopy (SEM).