Abstract: Taking homogenizing annealed G115 steel casting as research subject, the effects of different (normalizing+tempering) treatment process on microstructure and mechanical properties of the steel casting were studied. The normalizing temperatures were respectively 1070 ℃ and 1100 ℃, both holding for 1 h then air cooling (AC). The tempering processes were single tempering (780 ℃×3 h, AC), and double tempering (780 ℃×3 h, AC then 750 ℃×3 h, AC), respectively. The results show that with the increase of normalizing temperature, both the room temperature strength and the 650 ℃ high temperature strength of the G115 steel casting increase somewhat, while the toughness decreases somewhat, and the plasticity does not change obviously. With the increase of tempering times, both the room temperature strength and the 650 ℃ high temperature strength decrease somewhat, while the toughness and plasticity do not change obviously. Laves phase, M₂₃C₆ and MX (NbC, VN) phases are the main precipitated phases in the G115 steel casting after normalizing and tempering, and the impact fracture is characterized by cleavage or quasi-cleavage fracture. With the increase of normalizing temperature, the width of martensite blocks increases and the arrangement is relatively neat. The prior austenite grain size is the effective grain size for grain boundary strengthening in the contribution value of the room temperature strength of G115 steel casting. In summary, the recommended heat treatment process is normalized at 1100 ℃ for 1 h, AC, then tempered at 780 ℃ for 3 h, AC.

Key words: heat treatment process, G115 steel casting, mechanical properties, precipitated phase, EBSD, TEM