Laser-directed energy deposition additive manufacturing of a lean hot work tool steel: Tempering behavior and impact toughness

Abstract

HWTS 50 is a Cr, Mo, V is a new lean hot work tool steel with ∼0.2 wt% carbon, designed with chemical composition modifications to achieve comparable properties and temper resistance to those of medium carbon hot work tool steels such as AISI H13 (∼0.4 % C in wt.), while offering improved processability in laser additive manufacturing (LAM) processes. This paper reports on the processing and properties of this tool steel by laser-directed energy deposition (L-DED). Results suggest achievement of near-fully dense and crack-free martensitic microstructure with up to 6 vol% retained austenite (RA), which is substantially lower than that typically found in laser AM-processed AISI H13 (i.e., up to 20 vol%). As-built (AB) material exhibits a hardness of ∼47 HRC and Charpy V-notch impact energy of ∼20 J. Hardness of 48–50 HRC can be achieved by tempering slightly above the secondary hardness peak of 575 °C, either through quenching and tempering or direct double tempering from AB condition. Direct tempering improves temper resistance due to higher dislocation density and higher matrix supersaturation in elements carbon, nitrogen, and vanadium in AB condition, leading to a higher number density of fine and stable secondary carbides through over-tempering. In the above hardness range, the impact toughness of quenched and tempered steel was substantially higher than that of directly tempered one (i.e., ∼18 J vs. ∼12 J). Increased impact energy by prior quenching could be ascribed to microstructural homogenization, removal of inter-dendritic micro-segregation, and columnar prior austenite grain boundaries, which act as preferential sites for chains of alloy carbides precipitation, serving as low energy preferential crack initiation and propagation path. The new steel grade showed enhanced tempering resistance compared to AISI H13, particularly at elevated temperatures (i.e., >600 °C). Enhanced AM processability, optimum balance of hardness-, impact toughness-, and tempering resistance suggest it can be used for the manufacturing and repair of hot work tool steels in laser AM processes.