On the Microstructure Development under Cyclic Temperature Conditions during WAAM of Microalloyed Steels

This paper shed light on the kinetics of transformation and the developed microstructure during wire arc additive manufacturing (WAAM). Three microalloyed alloys, two of them are high strength low alloyed steel (HSLA) grades and the third is a Ni-Cr-Mo steel, from which the welding wires are being produced, were investigated. Repeated cycles around varied temperatures from a reheating temperature of 1350 degrees C and down to a temperature 35 degrees C below the A(e1) are applied using dilatometer on samples from the steels. After applying the cycles, the dilatometric-samples were investigated metallographically and their macro- and microhardness values were measured. It is shown that the WAAM using HSLA steels produce softer structure than the steel of the welding wire. Combined microalloying with Ti and Nb can present a useful strategy for producing finer structure in the WAAM components due to the effect of Ti in inhibiting the prior austenite grain-growth and that of Nb in refining the final structure. Additionally, repeated heating near A(e3) refines the prior austenite grains and produced fine ferrite-pearlite structure in case of HSLA steels and a microstructure predominated by the granular bainite in case of welding wire alloy. The former microstructure was the softest one for the case of HSLA steels, whereas the softest structure in case of the welding wire alloy was the tempered martensite structure developed by reheating below A(e1). Idealized temperature curves were chosen for the heat treatment, which could be characterized in a well-defined manner. In future work such idealized curves together with temperature histories obtained in WAAM-process will be used to set up a database to train an AI-model for predicting structure and material properties.

Automated summary

This paper investigates the kinetics of transformation and microstructure development in wire arc additive manufacturing (WAAM). The study shows that the use of HSLA steels in WAAM produces a softer structure compared to the welding wire steel. Microalloying with Ti and Nb can be an effective strategy for producing finer structures in WAAM components. Repeated heating near A(e3) refines the microstructure and produces different structures in HSLA steels and welding wire alloy. Idealized temperature curves are chosen for heat treatment, and future work aims to use these curves along with temperature histories to predict structure and material properties using an AI-model.