Dispersing heating improves impact toughness of high Si F/M steel by optimizing phase interface structure

High Si ferritic/martensitic (F/M) steel is a candidate structure material for the advanced nuclear reactors, while the formation of δ ferrite can be promoted by high Si content which damages the toughness. In this paper, to toughen high Si F/M steel, three concentrated heating (CH) processes with different parameters and dispersing heating (DH) process were designed for self-developed high Si F/M steel. CH samples share similar microstructure and mechanical property. While the impact energy of DH sample (about 30 J) is twice that of CH samples (about 15 J) and the strength is similar (about 730 MPa at room temperature, about 250 MPa at 650 °C). The toughness of DH sample is improved because the nucleation of cracks at the phase interface is effectively inhibited by reduced M 23 C 6 precipitating and martensite/δ ferrite strength disparity (strength of martensite near phase interface is decreased through DH process while δ ferrite strength is increased). Reduction of M 23 C 6 precipitating and martensite-strength is attributed to more uniform distribution of C and Cr around phase interface. Increase of δ ferrite-strength is caused by more residual VN clusters in δ ferrite of DH samples, because there is less dislocation for clusters to transform to precipitates and clusters have better strengthening effect. CH samples show similar microstructure and mechanical properties because steady state deformation appears after the strain amount exceeds 50 % according to flow stress curve, which indicates structure characteristics such as dislocation density do not change continuously in forging procedure.