Copper precipitates in 15 NiCuMoNb 5 (WB 36) steel: material properties and microstructure, atomistic simulation, and micromagnetic NDE techniques

The material investigations presented confirm the results of earlier MPA investigations that the service-induced hardening and decrease in toughness in WE 36 materials are caused by the precipitation of copper. In the initial state of the material, generally only a part of the alloyed copper is precipitated. The other part is still in solution and can be precipitated during long-term operation at temperatures above 320-350 degreesC. The copper precipitation leads to a distortion of the crystal lattice surrounding the copper precipitates and yields internal micro-stresses. If the number and size of the copper precipitates change during operation of a component, a change of the residual-stress level occurs. Formation and growth of copper precipitates was simulated using atomistic calculations. In addition, it was possible to mathematically follow the movement of dislocations and their attachment to precipitates. In this way the nano-simulation was established as a scientific method for the numerically based understanding of precipitation hardening. The results obtained from load stress-related Barkhausen noise measurements demonstrated that these micro-magnetic procedures are generally suitable for the verification of copper precipitation. The goal of current research is to establish these findings statistically through further experimental measurements. In addition, the influence of different deformation states, macro residual stress, and thermal-induced residual stress have to be researched. This is important for future developments of tron-destructive inspection techniques applied to inservice components. (C) 2001 Elsevier Science B.V. All rights reserved.