THE EFFECT OF QUENCH RATE ON THE MICROSTRUCTURE, MECHANICAL-PROPERTIES, AND CORROSION BEHAVIOR OF U-6 WT PCT NB

The effect of cooling rate on microstructure, mechanical behavior, corrosion resistance, and subsequent age hardenability is discussed. Cooling rates in excess of 20 Ks** minus **1 cause the parent gamma -phase to transform martensitically to a niobium supersaturated variant of the alpha -phase. This phase exhibits low hardness and strength, high ductility, good corrosion resistance, and age hardenability. As cooling rate decreases from 10 Ks** minus **1 to 0. 2 Ks** minus **1, microstructural changes (consistent with spinodal decomposition) occur to an increasing extent. These changes produce increases in hardness and strength and decreases in ductility, corrosion resistance, and age hardenability. At cooling rates less than 0. 2 Ks** minus **1 the parent phase undergoes cellular decomposition to a coarse two-phase lamellar microstructure which exhibits intermediate strength and ductility, reduced corrosion resistance, and no age hardenability. An analysis of the cooling rates indicates that fully martensitic microstructures can be obtained in plates as thick as 50 mm.