Effects of yttrium on the microstructures, internal fraction and martensitic transformation in H13 die steel

H13 die steels with varied yttrium (Y) content were prepared by vacuum induction melting, multiple forging, annealing and quenching treatment with stepwise heating. The effects of Y on the microstructures, internal fraction and martensitic transformation of H13 die steel were investigated using electron backscattering diffraction, transmission electron microscopy and a multifunctional internal friction meter. The results showed that the martensite start temperature first decreased but then increased with the increasing Y content, reaching a minimum in the 0.013Y-H13 steel. The refinement of the prior austenite grain size afforded more nucleation sites in Y-modified H13 die steels. The Snoek–Köster–Kê peak indicated that the solid solution of Y atoms provided additional martensitic transformation dynamics to increase the martensitic transformation rate and promote the formation of V1−V2 (Σ3) variants during the initial stage of transformation. The transformation rate decreased in Y-modified H13 steels during the late stage of transformation (70% completed). Therefore, the addition of Y elements was beneficial for refining the size of the martensite and promoted the formation of twin-type martensite in H13 steel.