Effect of thermal cycles on formation of pearlitic heat-resistant steel structure under wire arc additive manufacturing

The authors investigated the microstructure and mechanical properties of a model wall manufactured by arc wire 3D printing. 3D printing was performed using heat-resistant pearlitic steel wire in coldArc reduced heat input mode. Stationary thermal imager was employed to analyze the thermal cycles during layer deposition. Compressed air cooling to 200 °C was applied before each layer deposition to reduce heat accumulation. The high temperature gradients between the molten metal and the cooled layer resulted in areas with non-uniform structure, typical of welded joints after arc welding. Such areas with non-uniform structure were formed during the printing of each new layer and repeated throughout the wall height. It was observed that each solidified layer undergoes cyclic thermal effects during the deposition of subsequent ten layers. Intensive heating from deposition of two to three new layers leads to partial structural-phase transformations in the underlying layer. Deposition of the next 7 – 8 layers leads to heating similar to the “tempering” thermal operation. Microstructure analysis across different areas of the wall revealed acicular bainite with a small proportion of lath ferrite, bainitic ferrite, and martensitic-austenitic constituents. A slight increase in the width dimensions of acicular structure laths was observed with increasing wall height compared to the lower layers. The highest microhardness values were observed at the wall and substrate fusion zone (320 ± 7 kgf/mm 2 ) due to rapid heat conduction and high cooling rates during the initial stages of printing. In the wall bulk, microhardness values ranged from 260 to 300 kgf/mm 2 . The scatter of values and the periodic nature of the microhardness curve are associated with the formation of areas with non-uniform structure within each deposited layer of the wall. The wall material exhibits high strength characteristics (up to 800 MPa) and relative elongation (9 – 12 %).