Effect of annealing on microstructure, phase composition and mechanical properties of thixo-cast 100Cr6 steel

Steel is a particularly challenging material to semi-solid process because of the high temperatures involved and the potential for surface oxidation. Hot forged and annealed 100Cr6 steel was used as feedstock for the thixoforming process. Samples were heated in an argon atmosphere up to 1425 °C which corresponded to about 25% of liquid fraction. Specimens after heating were transported into a modified shot sleeve of a high pressure die casting machine and pressed into a die preheated up to 150 °C. After thixoforming the microstructure of thixo-casts consisted of large globular grains (average grain size about 300 μm). Martensitic plates in the form of globules were found inside austenitic matrix. Among large globules, a fine grain structure was identified to appear from the liquid phase (average size 20 μm). A mixture of ferrite and carbides was identified within the region of fine grains. Average hardness of thixoformed samples was 759 HV5, while the compression strength attained 3850 MPa. Dilatometric and calorimetric studies carried out directly after thixoforming allowed the determination of characteristic temperatures of retained austenite decomposition and carbide formation. They were used to choose temperatures of heat treatment conditions: 150 °C, 180 °C, 250 °C, 320 °C and 400 °C. After tempering at 150 °C, precipitates of ε carbide in martensite needles were found and at the same time the retained austenite transformed into martensite. X-ray phase analysis showed that thixo-cast contained: 78.7% – α′-Fe, 15.3% – γ-Fe and 6% – ε carbides. Hardness increased after annealing at 150 °C up to 808 HV5, while after tempering at 180 °C and 250 °C the hardness decreased to 693 HV5 and 665 HV5, respectively. This effect was connected with the decomposition of austenite into ferrite and transformation of martensite into ferrite. After treatment at 320 °C the hardness decreased to 556 HV5, which, according to quantitative phase analysis was due to the presence of α-Fe and Fe3C. After the treatment at 400 °C for 2 h the microstructure consisted of ferrite and coarse cementite. Its average hardness was 508 HV5. The increase of annealing temperature caused a gradual decrease of compression strength to 2420 MPa at 400 °C with the increase of plastic contraction strain to 22%.