Vol.10, No3, 2010, pp.173-178
UDC  620.178.74:669.15

A V-Nb HSLA steel was heat treated to martensite and lower bainite with different grain size, reheated for 3 s at 750°C with conduction heating and air cooled. Charpy notch tests were performed from –100°C to 60°C and notched tensile tests at –115°C.

For as delivered steel and lower bainite, the upper shelf toughness was above 200 J and the transition temperature low, for martensite the upper shelf toughness threshold above 60°C. After reheating, notch toughness has slightly decreased for martensite, while for lower bainite it decreased about ten times at 0°C and the upper shelf temperature increased above 40°C. In conclusion, independently on grain size, lower bainite was more prone than martensite to form in heat affected zone local brittle zones.

Charpy notch toughness, cleavage fracture and transition temperature were found to be related to the cleavage strength at –115°C. EBSD examinations of tensile fracture surface have shown that for bainite and martensite the cleavage occurred in the plane (001).

Keywords: HSLA steel, martensite, lower bainite, reheating, Charpy notch toughness, transition temperature, cleavage strength