Fatigue crack growth of untreated and heat-treated WAAM ER70S-6 carbon steel

• FCG and crack closure measurements are explored for WAAM ER70S-6. • The effect of stress ratio, loading direction and heat treatment is studied. • The stress ratio significantly influenced the PICC mechanism. • Heat treatment and orientation did not substantially affect FCG mechanisms. • Results are compared with prior studies on similar steels and design standards. The paper addresses the fatigue crack growth behaviour of untreated and heat-treated WAAM ER70S-6 carbon steel. Specimens were extracted from the printed wall along different directions (vertical and horizontal) and tested under mode-I loading at two stress ratios ( R = 0.05 and R = 0.25 ). Crack closure was measured using Digital Image Correlation (DIC). The microstructure of the untreated material mainly consisted of polygonal ferrite and intergranular lamellar pearlite. After heat treatment, pearlite decomposed, allowing ferrite to grow and reducing hardness. The load ratio influenced fatigue crack growth rates due to variations in crack closure levels. However, the loading direction relative to the print layer orientation did not significantly affect the crack growth rate. Fracture surfaces were examined by scanning electron microscopy to identify the main fatigue crack growth mechanisms associated with the different loading orientations and material conditions. Fractographic analysis revealed a mixed fracture mechanism, characterised by cleavage in the harder pearlite-rich regions and fatigue crack propagation striations in the softer ferrite-dominant areas. Minor manufacturing defects, such as inclusions and porosity, were also observed. The tested WAAM carbon steel exhibited slightly lower performance than conventional steels of a similar grade, aligning closely with the existing literature for WAAM ER70S-6 carbon steel.