Qingdong Liu

On the Role of Ni in Cu Precipitation in Multicomponent Steels

The nature of Cu precipitation in quench-tempered multicomponent high-strength low-alloy steels is characterized by atom probe tomography. The detected nanometer sized Ni-rich clusters act as preferential nucleation sites for Cu-rich clusters, and Ni segregation at the Cu-rich precipitate/matrix heterophase interface contribute to fast growth of Cu precipitates. Molecular dynamics simulation indicates that local Ni clustering at atomic scale significantly quickens the solute diffusion. The initial Ni composition has a profound effect on the nature of Cu precipitation.

Direct observation of Cu interphase precipitation in continuous cooling transformation by atom probe tomography

Atom probe tomography (APT) combined with electron back scatter diffraction and transmission electronic microscopy (TEM) is utilized to characterize the nature of copper precipitation during austenite–ferrite transformation in a continuous cooling high-strength low-alloy steel. The copper precipitation manners in association with the austenite decomposition kinetics are studied. The prevailing microstructure of the continuous cooling steel consists of acicular ferrite (AF), which is formed at an intermediate cooling rate of 10 °C/s. Besides, a limited volume of polygonal ferrite (PF) because of fast cooling rate and a trace of retained austenite are detected. Numerous copper-rich phase is found by TEM observation both in highly dislocated AF and dislocation-free PF. Generally, the copper-rich precipitates have comparatively large sizes and are considered to be formed by interphase precipitation during austenite–ferrite transformation. A high number density of nanometre sized copper-rich clusters that are lack of diffraction contrast in conventional TEM observation are detected by APT. These smaller copper-rich clusters, which are usually located between the linear-arranged copper-rich precipitates, are considered to be formed from supersaturated solid solution after the cessation of austenite–ferrite transformation. That means an ageing reaction for Cu precipitation occurs during continuous cooling transformation. The copper-rich precipitates and clusters are both rich in nickel, manganese and iron.

Fine structure characterization of martensite/austenite constituent in low‐carbon low‐alloy steel by transmission electron forward scatter diffraction

Transmission electron forward scatter diffraction and other characterization techniques were used to investigate the fine structure and the variant relationship of the martensite/austenite (M/A) constituent of the granular bainite in low‐carbon low‐alloy steel. The results demonstrated that the M/A constituents were distributed in clusters throughout the bainitic ferrite. Lath martensite was the main component of the M/A constituent, where the relationship between the martensite variants was consistent with the Nishiyama–Wassermann orientation relationship and only three variants were found in the M/A constituent, suggesting that the variants had formed in the M/A constituent according to a specific mechanism. Furthermore, the Σ3 boundaries in the M/A constituent were much longer than their counterparts in the bainitic ferrite region. The results indicate that transmission electron forward scatter diffraction is an effective method of crystallographic analysis for nanolaths in M/A constituents.

Microstructure Evolution of a Reactor Pressure Vessel Steel During High-Temperature Tempering

The electron microscopy techniques were employed to analyze the microstructure evolution during high-temperature tempering of a reactor pressure vessel steel. The results show that carbon enriched martensite/austenite (M/A) constituents decomposed into ferrite laths and accumulated carbides during initial stage of tempering. Simultaneously, the carbon atoms in the constituents diffused into the matrix continuously. With further prolonging of tempering, Mo2C carbides were found to be distributed uniformly in bainitic ferrite. In case of longer tempering, bainitic ferrite would combine and broaden, and grain boundary carbides grew up sequentially and coarsened. The newly formed austenite was detected during tempering at 660 °C for 5 h, and at 650 °C for 100 h,which shown that the Ac1 is time-related. This phenomenon may be depend on the component fluctuation of M/A constituents and segregation of carbides.

Decomposition of Supercooled Austenite in Continuous Cooling Transformation Process of a Mn-Mo-Ni Low Alloy Steel

Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni low alloy steel was evaluated by dilatometric measurements, light microscopy, electron backscatter diffraction, and microhardness testing and other methods. The results show that at the cooling rates of 1°C/s or below, ferrite initially formed and continuously rejecting C into the untransformed austenite, which transforms to C-rich lower bainite at lower temperature, resulting in ferrite-bainite dual-phase microstructures. At the cooling rates between 1 °C/s and 5 °C/s, the successive transformation products are bainite ferrite, upper bainite and lower bainite, and islands of carbon-enriched austenite transform to martensite (plus retained austenite) at low temperatures. The upper bainite and martensite dual-phase microstructures are formed at the range of 5 °C/s to 50 °C/s with a lower Ms. When cooling rates greater than approximately 50 °C/s, the microstructure are martensite and retained austenite at room temperature. With the increase of cooling speed, the residual austenite content increased before decreased at the cooling speed of 2 °C/s, which is probably associated with the incompleteness of bainite transformation.