Effect of welding heat input on microstructural evolution, precipitation behavior and resultant properties of the simulated CGHAZ in high-N V-alloyed steel

Effect of initial martensite and tempered carbide characteristics on mechanical properties of 3Cr2MnNiMo mold steel were investigated. The results indicated that the initial martensite microstructure can be effectively adjusted by changing the quenching temperature. With the increase of quenching temperature from 760 °C to 1010 °C, the initial microstructure gradually changed from acicular ferrite + plate martensite + M7C3 carbides to lath martensite, and the grain size increased monotonically. The coarsening of the initial martensite led to the increase of tempered carbide size, and the morphology of tempered carbide gradually changed from spherical to long strip-shaped. The large-size and long strip-shaped carbides promoted the formation of micro-voids during the tensile deformation and significantly decreased the elongation. In addition, with the coarsening of the martensite microstructure, both the decrease of high-angle grain boundaries (HAGB) density and the appearance of long strip-shaped tempered carbides significantly reduced the impact absorbed energy. The fracture mechanism transformed from ductile fracture caused by micro-voids coalescence to intergranular cleavage fracture. Moreover, when the initial microstructure completely transformed into lath martensite, the yield and tensile strength remained approximately unchanged. The precipitation strengthening compensated the adverse effect of martensite coarsening.

Effect of initial martensite and tempered carbide on mechanical properties of 3Cr2MnNiMo mold steel

In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND//<111> preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} <110> orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α 12 GND tensor average value (Ave. α 12 ) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior. • The deformation behavior of ferrite and bainite dual phase was studied by in situ EBSD. • Orientation evolution revealed the grains heterogeneous deformation and texture evolution behavior. • The factors of affecting slip transfer were proposed. • α 12 GND tensor average value was used to describe the coordinated deformation. 

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND// preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.

Effect of cooling rate on microstructure and mechanical properties of a low-carbon low-alloy steel

Microstructure and mechanical properties of the Fe-1.2Mn-0.3Cr-1.4Ni-0.4Mo-C tempered steel plate in three cooling processes (direct quenching, accelerated cooling + air cooling and air cooling) after hot rolling were investigated in this paper. The lath-martensite is obtained by direct quenching, whereas granular bainite, lath bainite, retained austenite and M/A constituents are observed in other two cooling processes. Meanwhile, the lath-martensite with higher cooling rate after hot-rolling could enhance the intensity of {332} texture, which is beneficial for the strength and toughness of the steel. After tempering at 680°C, the difference of yield strength is mainly caused by grain boundary strengthening and precipitation strengthening. The higher proportion of high misorientation boundary, stronger intensity of {332} texture, weaker intensity of {001} texture and fewer of M/A constituents (or retained austenite) will improve the impact toughness. All in all, the steel with optimum combination of strength and toughness is obtained by using direct quenching to room temperature after hot rolling.

Strength and toughness of Fe-1.2Mn-0.3Cr-1.4Ni-0.4Mo-C tempered steel plate in three cooling processes

The work-hardening characteristics and deformation behavior of high deformability pipeline steel with a polygonal ferrite and bainite microstructure were studied in this paper. The high work-hardening ability of polygonal ferrite with 35% bainite (PF+35%B) dual phase sample contributed to its high tensile properties. Electron backscattered diffraction was conducted to analyze the mechanism of the micro-damage behavior. The results of kernel average misorientation and grain reference orientation deviation of the deformation microstructure revealed that the main deformation mechanism of the polygonal ferrite grain was slip with a number of slip systems. Dislocation rearrangement and a local orientation gradient caused by crystal rotation during sliding to cause the formation of micro-voids. The strain dispersion ability of the PF+35%B sample was better than that of the polygonal ferrite with 27% bainite (PF+27%B) dual phase sample. The risk of micro-voids nucleation and crack propagation were reduced to an improved plastic deformation ability for PF+35%B sample. The average Taylor factor (M) of the PF+35%B sample was higher than that of the PF+27%B sample, which indicated the grains of the PF+27%B sample was more liable to slip during deformation and yield under low stress. The evolution characterization of the micro-texture showed that the γ-fiber was more stable compared with the α-fiber, and the stronger γ-fiber may contribute to the improved plasticity.

Xingyang Tu

Papers

Strength and toughness of Fe-1.2Mn-0.3Cr-1.4Ni-0.4Mo-C tempered steel plate in three cooling processes

Tensile deformation damage behavior of a high deformability pipeline steel with a ferrite and bainite microstructure

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

Effect of finish rolling temperature on microstructures and mechanical properties of 1000 MPa grade tempered steel plate for hydropower station

Microstructure and Properties of 1.0C–1.5Cr Bearing Steel in Processes of Hot Rolling, Spheroidization, Quenching, and Tempering