Laser based additive manufacturing for hot rolling contacts: Alloy development, laser processing and high temperature tribology

Work rolls are used for hot rolling of steel in early- and rear-stands of the finishing mill. Due to the aggressive working environment, the work rolls suffer from various degradation mechanisms such as thermo-mechanical fatigue and tribo-oxidative wear, and require frequent replacement. In this work, laser-based additive manufacturing, also referred to as laser-based Directed Energy Deposition (DED), is studied as a possible technique for manufacturing novel compound work roll systems. Application specific alloys of materials being deposited are designed with an emphasis on enhanced resistance to thermo-mechanical fatigue and wear damage. The new product development process
comprised of alloy development, optimizing laser-material processing, as well as analysis of samples including comprehensive microstructural characterization, mechanical testing, high temperature tribological testing and non-destructive evaluation.

Additive manufacturing was conducted in different processing configurations to avoid excessive oxidation and in-situ monitoring was employed to analyse the dynamics of the melt pool during manufacturing. With optimized processing parameters, additive manufacturing of multilayer samples to a thickness up to 20 mm was successfully demonstrated. Softening of the matrix of the newly developed high speed steel alloys under complex thermal cycling was investigated and addressed by modifying the alloying elements. High resolution transmission electron microscopy identified presence of nanometer-sized complex precipitates in the additively manufactured alloys. Strain scanning by Neutron diffraction of additively manufactured samples was inconclusive, indicating the possible existence of a strong crystallographic texture in the bulk.

Among the newly developed alloys, additively manufactured High Speed Steel (HSS) alloy LC3 (Febal-C-Cr-V-Mo-W) for early-stands work rolls, showed an enhanced resistance to the thermo-mechanical fatigue damage compared to that of reference roll grade HSS alloy produced by casting and was recommended for pilot scale production. Furthermore, additively manufactured HSS alloy R1 (Febal-C-Cr-V-Mo-Si) for rear-stands work rolls, showed an improved resistance to high temperature tribo-oxidative wear compared to that of reference roll grade alloy indefinite-chilled-double-poured iron; however the anti-sticking characteristics of this new alloy, to avoid adhesion and micro-welding, requires further development.