Materials Science and Engineering A 494 (2008) 10–20 Phase transformations in low-alloy steel laser deposits Haitham El Kadiri a,∗ , Liang Wang a , Mark F. Horstemeyer a , Reza S. Yassar b , John T. Berry c , Sergio Felicelli c , Paul T. Wang a a Center for Advanced Vehicular Systems, Mississippi State University, Mississippi State, MS 39762, United States b Mechanical Eng-Eng Mechanics Department, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States c Mechanical Engineering Department, Mississippi State University, Mississippi State, MS 39762, United States Received 28 May 2007; received in revised form 3 December 2007; accepted 5 December 2007 Abstract 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. Published by Elsevier B.V. Keywords: Laser deposition; Low-alloy steel; Bainite; Martensite; SEM; TEM 1. Introduction A large class of metal alloys has been used to fabricate parts with the laser engineered net shaping (LENS) process [1]. These materials include low-alloy steels [2], stainless steels [3,4], nickel-based alloys [5,6], and titanium alloys [7,8]. Recent advances in system capabilities, thermal modeling, measure- ment, and processing parameters have succeeded to a certain extent in overcoming the inherent constraints related to net shaping with LENS [9–11]. However, most structural compo- nents are multi-phase-based alloys. This makes them highly sensitive to cooling rates. Because of the very high cooling rates associated with laser deposition, the processing won- dow for multi-phase generation is substantially constricted. For steels, the finely generated austenite grain size leads to a high ∗ Corresponding author. Tel.: +1 662 325 5568; fax: +1 662 325 5433. E-mail address: elkadiri@cavs.msstate.edu (H. El Kadiri). density of allotriomorphic ferrite strings, and the mostly diffu- sionless transformations generate phases containing high solid solution concentrations. The high fractions of allotriomorphic ferrite strings and solid solution elements lead to very brittle microstructures. To recover some acceptable properties, mod- ern designers resorted to post-mortem heat-treatment processes. These post-tempering processes induced tempering embrittle- ment problems, increased the cost and labor in the overall manufacturing cost, and eliminated the possibility of obtaining gradients of microstructures. Because of the complicated relationship between the process parameters and the resulting microstructure, phase and com- position modulations in LENS-processed steels were largely neglected. This lack of understanding is a substantial barrier against the development of novel methodologies to better control the microstructure and design of multifunctional alloys. In this paper, we attempt to thoroughly explain the microstructural and property gradients observed along a single- walled deposit (SWD) from a pre-alloyed AISI 4140 graded steel 0921-5093/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.msea.2007.12.011