Phase evolution and carbon redistribution during continuous tempering of martensite studied with high resolution techniques

Abstract

Using high resolution techniques, such as, 3D atom probe tomography, transmission electron microscopy and synchrotron radiation, the continuous tempering process of a 50CrMo4 steel is analyzed. Differences of continuous heating compared to isothermal holding at certain tempering temperatures are discussed. Focus is laid on in-depth characterization of phase development and carbon redistribution effects, such as, segregation and clustering. The results of this work suggest that segregation is the dominant effect at low temperatures (< 150 °C), while cluster formation within these segregated areas is prominent at intermediate temperatures (150–250 °C). Epsilon carbide is formed during continuous heating with off-stoichiometric chemical composition (270–350 °C). It transforms finally into cementite (T > 350 °C).