Phase transformations in low-alloy steel laser deposits

We examined the microstructure evolution in medium-carbon low-alloy steel upon laser engineering net shape (LENS) (LENS is a trademark of Sandia National Laboratories and the US Department of Energy, Albuquerque, NM). Involved was the deposition of 14 superimposed fine layers. Several characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoindentation and electron back scattered diffraction (EBSD) were used in conjunction with three-dimensional finite element thermal modeling to rationalize the transformation mechanisms. Delta ferrite was the primary phase to solidify from the melt. Solid-state austenitisation led to allotriomorphic hexagonal prisms that grew following a unique direction depending upon the parent delta-grain crystallographic orientation. A supersaturated lower bainitic plate was the main phase to have transformed from austenite, except for the first two deposited layers where martensite predominated. The supersaturated plate underwent a sudden-tempering reaction at the 10th layer but was confined at the plate boundaries. This tempering reaction became more transgranular and increasingly affected retained austenite and microphases for the underlying layers. Coalescence of carbon-depleted ferrite plates gave rise to the steady-stage microstructure at the fourth layer. This complicated microstructural evolution corroborated the microhardness fluctuations through all deposited layers.