Abstract
Tensile and impact properties have been examined on isothermally transformed plain 0.2%C steel and 0.2%C-0.5%Mo steel with particular attention to a characteristic structure formed in a molybdenum steel.
In a plain carbon steel, ferrite-lamellar pearlite structure is formed over transformation temperatures of 450-680°C, while in a molybdenum steel, divorced pearlite structure with blocky carbides is formed at 600-680°C and bainite structure at 450-550°C.
Lamellar pearlite structures show increasing tensile strengths and decreasing FATT (fracture appearance transition temperature) with decreasing the lamellar spacing, but ductility in tensile tests and shelf energy in impact tests are remained unaffected. With appearance of divorced pearlite in a molybdenum steel, both tensile and impact properties are markedly impaired; tensile strength, ductility, and shelf energy are lowered and FATT is raised.
Detailed observations of ductile fracture processes show that void initiation occurs at smaller strains in the divorced pearlite compared to the lamellar pearlite or bainite structure.
Detrimental effects of the divorced pearlite structure on both impact and tensile properties can be attributed to the large, irregular-shaped carbides in the structure.
