Charpy Notch Toughness and Hardness of Reheated Martensite and Lower Bainite

The resistance to hydrogen embrittlement is of essential importance for steels for vessels for storage of hydrocarbons and depends on the effect of absorbed hydrogen on steel ductility. API tests have shown for the 490 MPa yield stress HSLA steels with the microstructure of ferrite and cementite particles a much greater reduction of area than for the 350 MPa steel with the microstructure of polygonal ferrite and pearlite 1,2 . By routine tests during the construction of a 60.000 m vessel, it was found that by the equal welding procedure, Charpy toughness was lower for 15 mm plates than for 25 mm plates of the same HSLA steel. It was assumed that the differences were related to the propensity of constituents of heat affected zone (HAZ) to embrittlemenst by short reheating in the two phase (ferrite+austenite) range for the steel that produces local brittle zones in the HAZ of welds 3–15 . Earlier simulation tests have shown 3 that the sensibility to embrittlement after reheating was different for a microalloyed than a conventional steel. As shown in Figures 1 and 2, after cooling from 1300 C Charpy notch toughness was slightly lower for the steel ^.0562 with yield stress of 350 MPa than for the microalloyed steel Niomol 490 K with yield stress of 490 MPa, while, after reheating toughness was for this steel sligthly lover. The different behaviour of both steels was confirmed with Charpy tests on HAZ of welds of 20 mm plates 3 . The aim of this work was to check these findinds with more systematical tests and to verify if the embrittlement propensity was related to the difference of steels in chemical composition or to the constituents of microstructure obtained with cooling from high temperature. Martensite and lower bainite in the microalloyed steel Niomol 490 K were investigated because assumed to be most sensible to reheat embrittlement than other constituents of HAZ of welds. For comparison, the steel with the as delivered microstructure was used.