Effect of solidification rate on microstructure and primary carbides of AISI DC 53 cold work tool steel

The solidification behavior of AISI DC 53 cold work tool steel was investigated by means of a cooling curve and its first derivative. Copper and sand wedge-shaped molds were used to obtain various solidification rates. To reveal the cooling rate degree during solidification,the secondary dendrite arm spacing of the steel alloy was examined by scanning electron microscopy(SEM). The solidification rates of each section for both wedge steel samples were calculated by means of the secondary dendrite arm spacing using a research-based empirical relation from the literature. Experiment results revealed that at the tip region of the cast specimen in the copper wedgeshaped mold,the carbide size was 7–8 μm,where the solidification rate was approximately 4,830 °C·s-1. The greatest carbide size obtained in the upper region of the sand cast wedge-shaped specimen was 250–270 μm.

Microstructural evolution of directionally solidified DZ125 superalloy castings with different solidification methods

The properties of Ni-base superalloy castings microstructure, and different solidification methods have are closely related to the uniformity of their as-cast serious effect on microstructural uniformity. In this paper, the influences of high rate solidification （HRS） process （with or without superheating） and liquid metal cooling （LMC） process on the microstructure of DZ125 superalloy were investigated. Blade-shape castings were solidified at rates of 40 pm.s-1 to 110 tJm.s1 using HRS process and a comparative experiment was carried out at a rate of 70 IJm.s1 by LMC process. The optical microscope （OM）, scanning electron microscope （SEM） were used to observe the microstructure and the grain size was analyzed using electron back scattered diffraction （EBSD） technique. Results show that for the castings by either HRS or LMC process, the primary dendrite arm spacing and size of 7＇ precipitates decrease with increasing the withdrawal rate; the dendrites and 7＇ precipitates at the upper section of the blade are coarser than those in the middle, especially for the HRS castings without high superheating technique. When the withdrawal rate is 70 iJm.s1, the castings by HRS with high superheating technique have the smallest PDAS with fine 7＇ precipitates; while the size distribution of 7＇ precipitates is more homogenous in LMC castings, and the number of larger grains in LMC castings is smaller than that in the HRS castings. Moreover, high superheating technique yields smaller grains in the castings. Both the LMC method and HRS with high superheating technique can be used to prepare castings with reduced maximum grain size.