Effect of Heat Treatment on the Structure and Morphology of Amorphous Steel Produced Using Spray Metal Forming
In order to investigate the effect of heat treatment on the structure and morphology of spray formed structurally amorphous steel, several small samples were cut from one spray formed amorphous steel ingot (DAR – 35). The samples were heat treated at 700oC in a tube furnace under a constant flow of 99.9% argon for up to 120 hours. The samples were furnace cooled to 300oC. Later they were quenched in water. The structure was investigated using x-ray diffraction and the morphology of the polished and lightly etched samples was examined under both optical and scanning electron microscopes. The results indicate that the spray formed steels were amorphous. However, their microstructure is not homogeneous. The microstructure is composed of three distinctively different phase contrasts (viz. light gray, dark gray and bright white contrasts). The bright white phase contained maximum amount of tungsten and molybdenum, it had the least amount of iron. Both light and dark gray phases contained nearly the compositionally expected amount of iron, and more chromium than compositionally expected. The effect of heat treatment at 700oC had no effect on the three different phase contrasts. However, the heat treatment at 700oC showed some effect on the crystallization of the structure. The heat treatment for up to 24 hours has very little effect on the structure and morphology of the amorphous steel samples. However, if the samples were heat treated at 700oC for more than 24 and below 72 hours, it appears that a partial crystallization of the steel occurs. Significant crystallization of the spray formed amorphous steel occurs due to prolonged heat treatment above 72 hours.
T. Chandra, K. Tsuzaki, M. Militzer , C. Ravindran
A. S. Rao et al., "Effect of Heat Treatment on the Structure and Morphology of Amorphous Steel Produced Using Spray Metal Forming", Materials Science Forum, Vols. 539-543, pp. 4185-4190, 2007