The effect of parameters on internal crack healing in 30Cr2Ni4MoV steel for 600-ton ultra-super ingots was systematically investigated. The results show that the degree of crack healing increases with increasing healing temperature, holding time, reduction ratio, and hot pressure, and with decreasing strain rate. Internal crack healing in 30Cr2Ni4MoV steel should be conducted at less than 1200 °C to avoid grain coarsening. Hot pressure, in the high-temperature elastic zone and perpendicular to the crack faces, not only promotes the rapid crack healing, but also prevents grain coarsening of the matrix.

Internal cracks could be healed under the process of hot plastic deformation. In this study, mechanical properties recovery after crack healing in SA 508–3 steel were investigated. Microstructures of the crack healing zones were observed using an optical microscope (OM) and electron back scattered diffraction (EBSD) technology, and the recovery degrees of mechanical properties in the crack healing zones with the healing temperature and a reduction ratio were tested systematically. The results showed that the internal cracks in SA 508–3 steel disappeared and were replaced by newly formed grains, achieved by recrystallization and abnormal grain growth. The tensile properties of crack healing zones could be fully restored, while their impact and low cycle fatigue properties could only be partially achieved. The recovery degrees of mechanical properties in crack healing zones increased with increasing the healing temperature and reduction ratio in the temperature range of 950–1050 °C. When the temperature was above 1150 °C, the impact properties began to deteriorate because of grain coarsening and larger MA (martensite–austenite) constituents. The microstructural evolution of the crack zone in the SA 508–3 steel was sketched.

After thermal deformation and heat treatment, it can be observed that there are small, newly formed grains in the crack healing zone, which indicates that the internal crack is fully healed. However, in our previous study, the impact properties of the internal healing zone could only be partially healed. In this study, the process of homogenizing treatment was adopted to achieve grain size homogeneity. The effects of homogenizing treatment and quenching and tempering treatment on crack healing were also systematically analyzed. With the same heat treatment method, SA508-3 samples, which were subjected to multi-pass deformation, had a higher percentage recovery than those that underwent uniaxial compression. The percentage recovery of the crack healing zone was significantly improved after the process of homogenizing treatment. The impact property of the crack healing zone could be fully restored after homogenizing treatment followed by quenching and tempering treatment. However, after several episodes of heating, the grain-boundary strength decreased, and the impact value was relatively low.

The forging–healing of the internal porosity defects affects the tensile, impact and fatigue properties of heavy forgings. In the present work, the effect of deformation process on the microstructure in the joint area as well as the tensile strength, impact toughness and fatigue strength was studied experimentally. It is shown that the tensile strength is restored once the porosity defects were healed, and the impact toughness is recovered when the flat grain band is eliminated. The fatigue strength can be restored if a uniform grain structure can be achieved in both the joint area and the matrix, whereafter precipitate become the key factor affecting the fatigue strength. A complete healing of the porosity defects, a uniform grain structure in the joint area and the matrix, and a fully controlled precipitate are essential to guarantee the mechanical properties and in-service performance of the heavy forgings.

In this study, the microstructural evolution and mechanical properties of B50A789G steels with different Nb/C ratios (7, 9, 11, and 13) after tempering at 495 °C were investigated through mechanical property testing, X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM). The role of Nb in B50A789G steel was also explored. The results indicate that, at the same tempering time, the strength and hardness of the steel increase with increasing the Nb/C ratio. The maximum tensile strength exceeded 1240 MPa when the Nb/C ratio reached 13. With prolonged tempering time, the tensile strength of steels with low Nb/C ratios (7 and 9) gradually decreases, whereas steels with high Nb/C ratios (11 and 13) exhibit a decline in tensile strength only after 6 h of tempering. In contrast, the impact toughness shows an opposite trend to the strength. As the Nb/C ratio increases, both coarse primary NbC and nanoscale NbC precipitates in the steel gradually increase. The primary roles of Nb in B50A789G steel are grain refinement strengthening and precipitation strengthening. For steels with Nb/C ≤ 11, the improvement in strength is attributed to the combined effects of grain refinement strengthening and precipitation strengthening provided by Nb. However, for steels with Nb/C > 11, the increase in strength is primarily driven by the precipitation-strengthening effect of the nanoscale NbC phase.