Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles

The effects of welding and post-weld heat treatment (PWHT) on nanoscale co-precipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes grain coarsening in the fusion zone, resulting in a soft and ductile weld without any cracks in the as-welded condition. A 550 degrees C PWHT induces fine-scale re precipitation of NiAl and Cu co-precipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of duttility with intergranular failure, whereas a 600 degrees C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in a great recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation microstructure of nanoparticles, together with grain size and reversion of austenite, while the ductility recovery is sensitive to the grain-boundary structure. APT reveals that the grain-boundary segregation of Mn and P may be the main reason for the 550 degrees C embrittlement, and the enhanced ductility at 600 degrees C is ascribed to a possible reduction of the segregation and reversion of austenite. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.