The Effect of Stress Relieving Temperature on Mechanical Properties, Residual Stresses and Lattice Parameter Evolution of L-PBF High-Carbon Tool Steel D2

Laser powder bed fusion of tool steel D2 was done with 360 °C platform preheating. Industrially relevant sized (d9 X L51 mm cylinders) samples were printed without cracking and heat-treated at 550, 600 and 650 °C for two hours. Hardness and residual stress profiles along the sample length, tensile properties and fracture surfaces are presented. A thorough x-ray diffraction analysis revealed M7C3 carbides emerging during heat treatments. The carbides were concluded to be at least partially coherent with the BCC phase based on the lattice parameter changes in the BCC phase. Maximum hardness of 625 HV1, yield strength of 1441 MPa and tensile strength of 1813 MPa were achieved after 2 h stress relieving at 600 °C. Fracture surfaces revealed tensile cracks propagating through distinct areas in the melt pool. Residual stress measurements showed almost 800 MPa tensile stresses in the as-manufactured sample. Residual stresses were measured for a dual-phase sample annealed for 2 h at 550 °C showing over 600 MPa residual tensile stress in FCC and 140 MPa residual tensile stress on the BCC phase. The results show high-carbon tool steel being manufacturable with L-PBF and that the extremely high residual stresses can be relieved with a relatively low-temperature tempering.