An Ultrasonic Technique for the Identification of Crack Susceptible Microstructures in Nickel-Copper Alloy K-500 Forgings

Abstract

Centerline cracking has recently been observed in nickel-copper alloy K-500 large forged bars and components. These cracks were identified to be intergranular and completely internal, with varying degrees of continuity. The cracks appeared as transverse or longitudinal in orientation and were flat and tightly closed. They initiated at the center of the forging and arrested just short of the outer diameter. The extensive nature of the cracks, and quantity of cracks observed indicated that there was a problem inherent in this material that needed to be resolved to decrease the expense of using this alloy. The cause of the intergranular cracking has been attributed to a grain boundary graphite content greater than 15%. It is currently believed that these graphite precipitates weaken the grain boundaries such that the stresses produced during the forging and heat treating process are sufficient to cause intergranular cracking. Identification of material susceptibility to cracking prior to finish machining of K-500 components can reduce the cost of using this alloy.