Plasticity of Coarse Martensite Laths Within the Heat-Affected Zone of 9Ni Steel Girth-Welded Pipes

Presenting a low Ductile-Brittle transition temperature compared to other body-centered cubic structural steels, lath martensitic steels, such as 9Ni steels, are the most prominent high-strength steels proposed for low-temperature and cryogenic services. In the past couple of decades, broad access to tools and instrumentation required to map complex crystallographic relations allowed authors to draw several relations between the lath martensite microstructure and the mechanical properties of those steels. Recently, morphological heterogeneity of lath martensite microstructure has been pointed out as a microstructural feature responsible for the scattering in local yield strength within the microstructure, with coarse martensite laths acting as soft zones with enhanced deformation ability. The present work conducts an investigation covering macro- to micro-plasticity of 9Ni steel-welded joints, focusing on the heat-affected zone (HAZ) and the contribution of these coarse laths to the strain mechanisms of the microstructure. Tensile tests and Electron Backscatter Diffraction (EBSD) analyses were conducted over a selected area in tensile specimens at progressive strain levels. Local strain measurements revealed that the coarse-grained HAZ (CGHAZ) is the only HAZ region that experiences significant strain, with the Subcritically reheated CGHAZ (SC-CGHAZ) presenting the highest strain levels. Regarding the coarse lath contribution, EBSD analyses revealed that, up to 7 pct strain, these coarse constituents present a progressive increase in geometrically necessary dislocations density increasing rate, indicating that they may carry plasticity for even further stages of the plastic regime. However, at the beginning of plastic deformation, dislocation activity is mainly observed at thin lath areas due to the deformation-driven martensitic transformation of interlath austenite films.