The evolutions of microstructure and mechanical properties during tempering at 700 °C, of normalized and oil-quenched 2.25Cr-1Mo-0.25V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated. It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure. After 0.5 h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite. However, when the tempering time is increased from 0.5 to 128 h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample. This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample.

•Pre-deformation changes the morphology of the oxide scale locally.•Internal oxidation zone disappears locally in pre-deformed sample.•Inner oxide scale thickness decreases locally in pre-deformed sample.•Changes in morphology are related to the enhanced Cr-rich belt in inner oxide layer.•Pre-deformation deteriorates the oxidation resistance of the material.The oxidation resistance of a high Si ferritic/martensitic steel with and without pre-deformation in oxygen-saturated stagnant lead-bismuth eutectic at 550 °C has been investigated. It is found that pre-deformation changes the morphology of the oxide scale locally. In some positions, the internal oxidation zone beneath the inner oxide layer disappears and/or the thickness of the inner oxide layer decreases. These changes are related to the formation of an enhanced Cr-rich belt locally in the inner oxide layer. Additionally, the local enhanced Cr-rich belt induces many voids at the inner/outer oxide layer interface, which deteriorates the oxidation resistance of the material.

The compatibility of T91 steels having different preparation processes with oxygen-saturated stagnant lead-bismuth eutectic have been investigated at 450 °C and 550 °C. It is found that cold rolling decreases the thickness of the oxide scale of T91 steel by forming a continuous enhanced Cr-rich belt in the inner oxide layer next to the internal oxidation zone, which is attributed to the rapid diffusion of Cr induced by numerous non-equilibrium grain boundaries and migrating dislocations.

Deformation-induced phase transformation (DIPT) of metastable austenite in Fe–13% Cr–4% Ni steel occurs during the macroscopic elastic stage and is accompanied by the yielding of metastable austenite at microscopic level. The DIPT rate is accelerated by yielding the martensite matrix, which increases the available nucleation sites for DIPT by enhancing the plastic deformation in austenite.

In order to investigate the effect of N on the microstructure and room temperature mechanical properties of new-type high silicon martensitic heat-resistant steels, three steels containing the same total content of C and N but different N contents have been designed and prepared according to the thermo-calc calculation. The thermodynamic calculation and experiments indicate that the replacing of C by N changes the kind and volume fraction of precipitates of the high Si martensitic steel significantly. Along with the N content increasing, the precipitates in the samples after 750 °C tempering change from (Cr23C6 + VN + TaC) to (Cr23C6 + VN + TaC + TaN) and finally to (Cr23C6 + VN + Cr2N) according to both experimental results and thermodynamic calculations. The room temperature mechanical tests show that the strength of the steel decreases as the N content increases. However, the Charpy impact toughness increases with N content increasing. According to the calculation and SEM observation, it is inferred that the decrease of amount and size of precipitates accounts for the changes of the mechanical properties.

The mechanical stability of reversed austenite at room temperature in two 13%Cr–4%Ni low carbon martensitic stainless steel samples after different heat treatments has been investigated. The uniaxial tensile tests indicate that the reversed austenite resulting from the one-stage and two-stage intercritical tempering heat treatment have different mechanical stability, which induces distinct strength-ductility balance of the material. Experiments and crystal plasticity finite method simulations reveal that the special grain orientation relationship between the reversed austenite and martensite matrix, besides chemical composition, plays important roles on the mechanical stability of the reversed austenite. It is found that in both samples the Nishiyama–Wassermann or Kurdjumov–Sachs relationship between the reversed austenite and the martensite matrix provides an easy way for the active slip systems in austenite to penetrate the phase boundary to the adjacent martensite. This results in a high mechanical stability of the reversed austenite. In addition, the larger austenite grains in the second-stage tempering sample have higher mechanical stability in the martensitic steel due to the favored austenite stabilizing elements distribution behavior, and the favored stress distribution originating from the intrinsic strengths of the austenite and martensite matrix.

The phase transformation from martensite to austenite during intercritical tempering with high heating rate in a low carbon martensitic stainless steel Fe-13%Cr-4%Ni-Mo has been investigated to clarify the microstructure evolution in some regions of the weld joint heat affected zone (HAZ). The experimental results indicate that the start and finish temperatures of the martensite to austenite transformation keep constant when the heating rate is higher than 10 K/s, and the transformation is much faster than nickel diffusion. The mechanism of the martensite to austenite transformation changes from diffusion to diffusionless during the intercritical tempering when the heating rate is higher than 10 K/s. The diffusionless transformation and higher As temperature render it difficult for any austenite to remain at room temperature during the intercritical tempering with high heating rate that occurs in the HAZ. Adding a proper intercritical tempering with low heating rate can induce some reversed austenite in the rapid heated sample.