Comparison of impact and fracture toughness of ductile iron and ferritic steel

Namen magistrske naloge je določitev lomne in udarne žilavosti sive litine s kroglastim grafitom kvalitete EN-GJS-400-18LT pri različnih temperaturah ter primerjava z valjanim konstrukcijskim jeklom S235JR+AR. Ulitih je bilo več t. i. Y-prob iz sive litine, iz katerih smo izdelali preizkušance za preizkus lomne in udarne žilavosti ter vzorce za določitev mikrostrukturnih lastnosti. Iz vroče valjane jeklene plošče, debeline 25 mm smo prav tako izdelali preizkušance za preizkus lomne in udarne žilavosti ter vzorce za analizo mikrostrukture. Kemijsko sestavo smo določili s pomočjo optične emisijske spektroskopije, karakterizacija mikrostrukture je potekala s pomočjo svetlobne mikroskopije in programa Analysis 5.0. Določili smo udarno žilavost obeh materialov po Charpyu pri sobni temperaturi, 0 °C, -20 °C in -40 °C. Pri enakih temperaturah smo določili še kritične vrednosti J-integrala, odpiranje vrha razpoke oziroma CTOD (crack tip opening displacement) ter lomno žilavost z uporabo tri-točkovnega upogibnega preizkusa. Mikrofraktografija je bila narejena z vrstičnim elektronskim mikroskopom. Mikrostruktura litine EN-GJS-400-18LT je sestavljena iz feritno-perlitne matrice in ogljika, strjenega v obliki grafitnih krogel. Mikrostruktura valjanega jekla S235JR+AR je feritno-perlitna, pri čemer lahko opazimo, da so kristalna zrna nekoliko podolgovata in usmerjena v smeri valjanja. Izkazalo se je, da je udarna žilavost jekla pri sobni temperaturi bistveno večja kot udarna žilavost sive litine, a z nižanjem temperature drastično pade in se pri -40 °C celo izenači z udarno žilavostjo sive litine. Lomna žilavost sive litine je pri vseh temperaturah večja od lomne žilavosti jekla in s padcem temperature celo nekoliko naraste.The purpose of this master thesis is to determine the fracture and impact toughness of EN-GJS-400-18LT ductile iron at different temperatures and compare it to S235JR+AR hot-rolled structural steel. Several Y-block ductile iron castings were made in order to make specimens for determining the fracture and impact toughness and samples for metallographic examination. We made specimens for impact and fracture toughness as well as metallographic samples out of 25 mm thick hot rolled steel plate. Microstructure examination was performed by light microscopy, while chemical analysis was performed by optical emission spectroscopy. Impact toughness was determined with the use of the Charpy method at room temperature, 0 °C, -20 °C and -40°C. At the same temperatures, J-integral values as well as crack tip opening displacement – CTOD and fracture toughness were determined by three-point bending test. Microfractography was performed by using scanning electron microscope. EN-GJS-400-18LT has a ferritic – perlitic microstructure with carbon in form of graphite nodules embedded in the matrix. Hot rolled S235JR+AR steel has a ferritic – perlitic microstructure with crystal grains extended and orientated in the direction of rolling. It turned out that the impact toughness of steel at room temperature is greatly higher than that of ductile iron, but it decreases sharply with decreasing temperature. At -40 °C the impact toughness of both materials is practically the same. Fracture toughness of ductile iron is higher at all temperatures tested than fracture toughness of steel and it actually rises a bit with decreasing temperature