Directional solidification of Ni base superalloy IN738LC to improve creep properties

The high Cr, Ni base superalloy IN738LC has been directionally solidified o n both laboratory and industrial scales using Bridgman and liquid metal coo ling (LMC) methods respectively. In the Bridgman experiments, cylindrical r ods were grown using a graphite chill with temperature gradient G = 5.0 K m m(-1) and a water cooled copper chill with G = 8(.)5 K mm(-1) and a wide ra nge of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h(-1). In the LMC rigs, several turbine blades were grown using a wide range of withdraw al rates of R = 120, 225, 330, 420, and 630 mm h(-1). Grain and dendritic s tructures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions, Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 6 50 degrees C and creep tests at 152 MPa and 982 degrees C, 340 MPa and 850 degrees C, and 586 MPa and 760 degrees C. It was shown that at R = 600 mm h (-1) with water cooled copper chill, directionally solidified rods with a w ell orientated dendritic structure and better segregation pattern gives hig her tensile properties at 25 degrees C and creep properties at 340 MPa and 850 degrees C. Tension and creep tests of turbine blades showed that althou gh the yield and tensile strength of directionally solidified specimens are in the range of conventionally cast ones, the creep properties of the blad es have been significantly improved using the LMC process.