On the hot crack formation during solidification of iron-base alloys

Sammanfattning: Hot cracking during solidification is a severe problem inmany casting processes, for example in continuous casting.Combinations of low strength and low ductility of the materialat high temperatures and applied tensile stress can cause hotcracking. In order to investigate the hot crack susceptibilityof several iron-base alloys, the transition temperature betweenbrittle and ductile fracture, TDB, was determined by high temperature tensiletesting of 'in situ' solidified samples using cooling rates inthe range of 10-1900ºC/min.Fe-Ni alloys, including Invar steel, with a narrowsolidification interval, and Fe-Cu alloys, with a broadsolidification interval, were investigated. The compositionswere selected to study both solidification to austenite and toferrite. The ferritic solidifying Fe-Cu and Fe-Ni alloys werenot sensitive to hot cracking, as the brittle ductiletransition temperature occurred near the solidus temperatureand was not affected by various cooling rates. The austeniticsolidifying Fe-Cu alloy was very sensitive for hot crackingasall samples fractured in the brittle mode in the testedtemperature range. The austenitic solidifying Fe-Ni alloys werealso very susceptible for hot cracking, as the ductile/brittletransition occurred more than hundred degrees below the solidustemperature and with decreased cooling rate, TDBwas decreased. The results found in the Fe-Nialloys could not be explained by the presence of liquid films.In order to explain the observations of the Fe-Ni alloys a newtheory was proposed and analysed. It suggested thatsupersaturation of vacancies formed during the solidificationprocess will initiate a crack and favour the growth of thecrack by vacancy diffusion.The hot cracking susceptibility of commercial alloys wasalso determined, such as rock tool steel, free cutting steeland stainless steel. It was shown that the primary austeniticsolidifying stainless steels were much more sensitive for hotcracking and various cooling rates compared to the primaryferritic solidifying stainless steels.Copper is notorious for causing hot cracks in steels. Inorder to study hot cracking in presence of liquid films,experiments of liquid copper penetration into iron grainboundaries were investigated. A new theory was proposed for thepenetration process and included the Kirkendall effect, i.e. anuneven diffusion of Cu an Fe will generate vacancies which willdiffuse and condense at grain boundaries. High temperaturetensile tests were also performed on an Fe-10%Ni alloy duringliquid copper penetration in order to study how the penetrationprocess was affected by applied tensile stress. Microprobemeasurements across a crack gave a diffusion constant, whichwas at least 1000 times the value found in literature. Itshowed that there was an enhanced diffusion process caused bystraining the lattice.Keywords:hot cracking, hot tearing, cracking(fracturing), tearing, ductility, brittleness, embrittlement,fractures, ductile brittle transition, solidification, iron,copper, nickel, Invar, steels, free machining steels, stainlesssteels, penetration, crack initiation, crack propagation,lattice vacancies, diffusion.