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High strength and toughness are usually hard to obtain simultaneously because of the trade-off. In this research, cyclic intercritical tempering (IT) was applied to a low-carbon Cu-bearing 7Ni steel to pursue a better strength-toughness balance than what conventional single intercritical tempering can achieve. The mechanical properties and microstructure of cyclic IT and single IT were studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in addition to dilatometry. It was found that cyclic IT can significantly improve the strength without much sacrifice of toughness. The additional strength comes from dislocation and precipitation strengthening. The mechanism of reverse transformation was studied, and it was found that the mechanism changes from diffusional at single IT or first-cycle IT to a combination of interface-dominated and diffusional at the following cyclic IT. It was suggested that enrichment of Ni after the first cyclic IT is responsible for the mechanism change by thermodynamic calculation. Furthermore, although the Ni content is higher in fresh martensite (FM) after following cyclic IT, no distinct decrease of Ms was found, which is related to the inhomogeneous elemental distribution of FM. [ABSTRACT FROM AUTHOR]

In this paper, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray stress meter (XRSA), atom probe tomography (APT), hardness, and tensile tests were used to study the effect of tempering temperature on the microstructure and properties of Fe-9Ni-2Cu steel. The results show that after the quenched samples were tempered at 460 °C for 2 h, the hardness values increased from 373 to 397 HV, and elongation also increased from 13% to 16%. With the tempering temperature increasing from 460 to 660 °C, the hardness firstly decreases from 397 to 353 HV and then increases to 377 HV, while the elongation increases to 17% and then decreases to 11%. The variation of the mechanical properties greatly depends on the evolution of the Cu-rich phase and carbides. The precipitation strengthening of the Cu-rich phase and carbides leads to the increase of hardness, but when the precipitate is coarsened, the precipitation strengthening weakens, and then, the hardness increases. When the tempering temperature is 560 °C, a large amount of stable reverse transformation austenite was formed with a content of 7.1%, while the tensile strength reached the lowest value of 1022 MPa and the elongation reached the maximum value of 17%. [ABSTRACT FROM AUTHOR]

A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500°C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well. • Ultra-high strength steel without carbide-forming elements is designed. • Two-stage hardening behavioris discovered. • Strengthening by nanoscale co-precipitation of Cu and β-NiAl precipitates. • The effect of Co addition in precipitation reaction is discussed. [ABSTRACT FROM AUTHOR]

High toughness, especially cryogenic toughness, is usually hard to be obtained in precipitation strengthening steels. In this report, intercritical tempering plus tempering were applied on a low carbon Cu-contained 7Ni steel. Results reveal that high strength (819MPa, 25 °C) combined with high cryogenic toughness (138 J, −196 °C) can be realized simultaneously by two steps tempering. The microstructural evolution during intercritical tempering and tempering was investigated systematically by scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was found that both Ni-rich regions and Ni-depleted regions exist after first step intercritical tempering and no stable austenite was found, whereas nano-sized austenite (with average thickness of 20 nm) was produced on the Ni-rich regions after following second tempering process. Thermo-dynamic calculation and DICTRA simulation was applied to systematically study formation of austenite and partition of Ni during two steps tempering process. Simulation results coincides well with experiments. Meanwhile, it is suggested that high cryogenic toughness is ascribed to the stable austenite because of its nanometer thickness and high Ni content. Precipitation of Cu-rich phases at second tempering process maintained high strength during final tempering. [ABSTRACT FROM AUTHOR]

The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on the mechanical properties during tempering at 650 °C were studied to reveal the time-dependent toughness and temper embrittlement. The results show that the toughening of the material should be attributed to the decomposition of the martensite/austenite constituents and uniform distribution of carbides. When the tempering duration was 5 h, the strength of the investigated steel decreased to strike a balance with the material impact toughness that reached a plateau. As the tempering duration was further increased, the material strength was slightly reduced but the material impact toughness deteriorated drastically. This time-dependent temper embrittlement is different from traditional temper embrittlement, and it can be partly attributed to the softening of the matrix and the broadening of the ferrite laths. Moreover, the dimensions and distribution of the grain carbides are the most important factors of the impact toughness. [ABSTRACT FROM AUTHOR]