Solidification microstructure formation in HK40 and HH40 alloys

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatPro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L -> L + gamma -> L + gamma + M7C3 -> gamma + M7C3 -> gamma + M7C3 + M23C6 -> gamma + M23C6 and L -> L + delta -> L + delta + gamma -> L + delta + gamma + M23C6 delta + gamma + M23C6, respectively. The solidification mode was determined to be the austenitic mode (A mode) in HK40 alloy and the ferritic-austenitic solidification mode (FA mode) in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary gamma dendrites grow at the 60A degrees angle to each other. On the other hand, in HH40 alloy, residual delta forms because of the incomplete transformation from delta to gamma. Cr23C6 carbide is produced in solid delta ferrite delta but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.