Effect of tempering temperature on the toughness of 9Cr–3W–3Co martensitic heat resistant steel

Effect of tempering temperature on the toughness of 9Cr–3W–3Co martensitic heat resistant steel was studied on the basis of the microstructures after normalized at 1100 °C for 1 h and then tempered at 740–780 °C for 3 h. With increasing tempering temperature from 740 °C to 780 °C, the absorbed energy of the 9Cr–3W–3Co steel increased greatly from 26 J to 115 J. The change of the toughness with increasing tempering temperature was attributed to the softening of the base metal and the increase of the crack propagation path. The softening of the base metal was caused by the decrease of the dislocation density and the supersaturation of the interstitial atoms. The reason for the increase of the crack propagation path was that the length of the large angle boundaries increased and then the propagation direction of the cleavage crack was deflected more frequently.