Nondestructive detection of embrittlement by copper in steel

Abstract

Copper can be used as an alloying element to increase the strength and hardness of steel. However, it can also act as an embrittling agent under certain circumstances. In particular, the decrease in fracture toughness of reactor pressure vessels following long term exposure to neutrons has been blamed on the formation of very small copper rich precipitates (CRPs) in the iron lattice. In order to develop a nondestructive technique for detecting these CRPs and for predicting the onset of embrittlement, several ultrasonic and magnetic properties have been measured on a high strength, low alloy steel containing 1.1 mass % copper in which the CRPs could be formed by proper heat treatments. The particular properties measured were the longitudinal and shear wave velocities and their associated internal frictions as well as the acoustic nonlinearity parameter, β. The magnetic properties were the coercive force, the saturation magnetization, a dynamic permeability, με, and the magnetostrictive coefficient, λ. Small angle neutron scattering studies confirmed that CRPs were formed during the heat treatments used and that their presence could be correlated with observed changes in hardness. These hardness changes could then be correlated with some of the physical properties that were measured.