The effects of normalizing and tempering temperatures on the microstructures and mechanical properties of China Low Activation Martensitic (CLAM) steel with and without yttrium were studied. Based on the optimized traditional heat treatment, two special intermediate heat treatments were studied to improve the mechanical properties of the steels. The A_C3 temperature of the CLAM steel was increased by adding yttrium. The optimized traditional heat treatments were 1000 °C × 30 min + 755 °C × 90 min and 1050 °C × 30 min + 755 °C × 90 min for the C1 and C2 alloys, respectively. The intermediate heat treatment results indicated that the precipitation behavior of carbides was effectively controlled. The size of the M₂₃C₆ carbides was refined during the two intermediate heat treatments due to the priority precipitation of MX particles. However, coarsening of grains and martensite laths occurred during heat treatment with furnace cooling. The twice-quenched tempering samples had a smaller grain size and martensite lath width than the other samples. The ductile–brittle transition temperature was − 69 and − 103 °C for the C1 and C2 alloys with twice quenching, and the yield strengths were 745 and 760 MPa, respectively. An excellent balance of strength and impact toughness was obtained with the twice-quenching and once tempering heat treatment process.