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

Abstract

The effects of welding and post-weld heat treatment (PWHT) on nanoscale coprecipitation, 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 aswelded condition. A 550 °C PWHT induces fine-scale re-precipitation of NiAl and Cu coprecipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of ductility with intergranular failure, whereas a 600 °C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in greater recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation

Atom-probe study of Cu and NiAl nanoscale precipitation and interfacial segregation in a nanoparticle-strengthened steel

“…It should be pointed out that the matrix of most Cu and NiAl nanoparticle-strengthened steels is primarily martensite, but contains a small amount of austenite, the amount of which depends on the alloy composition and heat-treatment histories. While most of the previous studies were focused mainly on the understanding of precipitation behavior of Cu and NiAl nanoparticles in the martensite and ferrite phases [15][16][17][18][19][20][21][22], limited detailed information is available on the precipitation behavior and related mechanism in the austenite phase.…”

“…Recently, in a welding study of a newly developed Cu/NiAl co-precipitation strengthened steel [22], we observed an interesting microstructure containing both martensite and austenite phases together with complex nanoscale precipitation behavior in the heat-affected zone. Although the thermal history of the heat-affected zone is complicated in nature, while for an understanding of the welding microstructure, it is interesting and important to study the precipitation behavior and interfacial segregation in the heat-affected zone.…”

“…It was found that there are two types of pathways for the formation of the NiAl/Cu co-precipitates in the martensite phase (i.e. 'solid solution → Cu particles → Cu particles + NiAl particles' and 'solid solution → NiAl particles → NiAl particles + Cu particles'), depending upon the Cu, Ni, and Al concentrations [21,22]. In addition, the welding study revealed that the NiAl/Cu coprecipitates which are dissolved in the matrix during welding can be re-precipitated upon proper post-weld heat-treatments, leading to a recovery of the strength [22].…”

“…'solid solution → Cu particles → Cu particles + NiAl particles' and 'solid solution → NiAl particles → NiAl particles + Cu particles'), depending upon the Cu, Ni, and Al concentrations [21,22]. In addition, the welding study revealed that the NiAl/Cu coprecipitates which are dissolved in the matrix during welding can be re-precipitated upon proper post-weld heat-treatments, leading to a recovery of the strength [22]. It should be pointed out that the matrix of most Cu and NiAl nanoparticle-strengthened steels is primarily martensite, but contains a small amount of austenite, the amount of which depends on the alloy composition and heat-treatment histories.…”

“…Practically, for many high-strength steels, the matrix is tailored to have a mixture of two or more phases, rather than simply a single phase, to obtain an optimum combination of high strength and high ductility [11][12][13][14]. Among them, Cu and NiAl nanoparticle-strengthened steels represent an important class of advanced highstrength steels, which have attracted significant attention in recent years due to their good combination of high strength, good ductility, and good weldability [15][16][17][18][19][20][21][22]. It was found that there are two types of pathways for the formation of the NiAl/Cu co-precipitates in the martensite phase (i.e.…”