V-alloyed Steel Research Articles

Significant Influence of Welding Heat Input on the Microstructural Characteristics and Mechanical Properties of the Simulated CGHAZ in High Nitrogen V-Alloyed Steel

AbstractThe microstructural characteristics and mechanical properties of the simulated coarse grained heat affected zone (CGHAZ) in high N V-alloyed steel have been conducted under different welding heat input, characterized by the cooling time taken from 800°C to 500°C (t8/5). The experimental results show that the microstructure is dominantly composed of lath bainite (LB) and granular bainite (GB) at t8/5 30 s– 90 s. The content of LB decreases with t8/5 increasing, and that of GB increases. When t8/5 further increases to 120 s and 180 s, the microstructure mainly consists of intragranular polygonal ferrite (IPF) and acicular ferrite (IAF). The higher t8/5 leads to the increased content of intragranular ferrite (IGF). Meanwhile, the prior austenite grain size (PAGS) progressively increases from 56 ± 6.0 μm to 148 ± 9.9 μm as t8/5 increases from 30 s to 180 s. Besides, EBSD analysis indicates that the fraction of high angle grain boundaries (HAGBs) is 0.570, 0.427 and 0.624, respectively, corresponding to t8/5 30, 90 and 180 s. Moreover, the impact toughness decreases as t8/5 increases from 30 s to 90 s caused by the increased PAGS and GB content, and then sharply increases with t8/5 exceeding 90 s due to the increased formation of IGF, especially IAF. Furthermore, the high nitrogen content accelerates V(C,N) precipitation, which not only inhibits the coarsening of prior austenite grains, but promotes the formation of IGF, resulting in the increased number of HAGBs and raising impact toughness.

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

The effect of welding heat input characterized by the cooling time taken from 800 °C to 500 °C (t8/5) on the microstructural evolution, V(C,N) precipitation and resultant mechanical properties of the simulated CGHAZ in high-N V-alloyed steel was comparatively investigated using a Gleeble-1500D thermomechanical simulator. Metallographic analysis indicated that the dominant microstructure transformed from lath bainite to granular bainite when t8/5 increased from 30 s to 90 s and then changed to intragranular ferrite at t8/5 of 180 s. The nanoscale V(C,N) precipitates were coarsened as t8/5 increased, coupled with a slightly increased number density. Furthermore, the orientation relationship [001]α//11¯0V(C,N) reduced the interfacial structural energy (MC/α) and enhanced the V(C,N) precipitation in the ferrite matrix. Moreover, the microhardness progressively decreased due to the combined effect of microconstituents, precipitates and grain boundaries. The impact toughness first decreased and then increased, and the optimal value was obtained at t8/5 of 180 s. Furthermore, the formation of intragranular ferrite, especially acicular ferrite, and the increased fraction of high angle grain boundaries could completely remedy the detrimental effect caused by the increased content of grain boundary ferrite and the coarsened effective grain size and precipitates. In addition, the high nitrogen content of 240 ppm accelerated V(C,N) precipitation in the austenite region, and the submicron V(C,N) precipitates could promote the formation of intragranular ferrite by providing heterogeneous nucleation sites, in combination with the larger prior austenite grain size caused by higher welding heat input.

Effect of Precipitation Control on Strength and Impact Toughness in Advanced Pressure Vessel Steel

This paper reports the effect of increase in vanadium (V) content on changes in precipitation reaction and on mechanical properties in SA1017 grade 23 steel (2.25Cr-1.5W-VNbTi) for application to pressure vessels in advanced nuclear reactors. Increase in V content prohibits formation of coarse M23C6 carbide and allows only fine MX carbo-nitrides precipitation in this steel during tempering. Calculation based on Ashby–Orowan equation suggests that increase in yield strength is mainly attributed to precipitation hardening of fine MX carbo-nitrides and it shows a good agreement with experimental results. Meanwhile, step-cooling heat treatment causing temper embrittlement decreases Charpy impact toughness of steels at sub-zero temperatures, but higher V-alloyed steel shows higher toughness. Because interfaces between MX carbo-nitrides and the matrix act as strong trap sites of segregating P, its segregation toward prior austenite grain boundaries is decreased. Therefore, alloy design of V content for SA1017 grade 23 improves strength without degradation of impact toughness and temper embrittlement.

The effect of age-hardening mechanism on hydrogen embrittlement in high-nitrogen steels

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr2(N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr2(N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.

Super-high-strength and formable medium Mn steel manufactured by warm rolling process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy could produce not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby–Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in the ultrafine RA grains. The best combination of strength and ductility included 1.5 GPa ultimate tensile strength and 28% total elongation. This was achieved due to the sustainable TRIP and TWIP effects in the later straining stage resulting from the ultrafine austenite grains, the latter were reversely transformed from the recrystallized ferrite grains.

Super-High-Strength and Formable Medium Mn Steel Manufactured by Warm Rolling Process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy produced not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing (IA) of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby-Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening during successive deformation due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in fine RA grains. The best achieved combination of tensile properties included 1.8 GPa ultimate tensile strength, 1.0 GPa yield strength and 18% total elongation.

State of Hydrogen in V-alloyed Steel

State of Hydrogen in V-alloyed Steel

Effect of Vanadium on the Hot Deformation Behavior of Vanadium-Microalloyed Steel for Thin Slab Direct Rolling

The effects of V on hot deformation properties of low-carbon steel were investigated in the temperature range of 1173 K to 1473 K (900 °C to 1200 °C) and for strain rates from 0.1 to 5 s−1 for compositions with a V content in the range of 0 to 0.125 wt pct. The critical stress and strain for dynamic recrystallization (DRX) initiation were obtained from the stress dependence of the strain hardening rate. The hot deformation properties of V-alloyed steel were studied in function of the temperature-corrected strain rate. The experimental results were used to construct a kinetic model of DRX. V was found to have no influence on the hot deformation properties for V contents less than 0.125 pct, indicating the absence of both solute drag and precipitation effects at low V content. When the V content reached 0.125 wt pct, the activation energy for DRX increased and DRX was suppressed in high strain rate condition.

Effect of Vanadium on Microstructure and Mechanical Properties of Air Cooled Medium Carbon High Silicon Bainite Steels

The effect of vanadium on the bainite transformation of medium carbon high silicon steel during air cooling was studied by using Optical Microscopy (OM) and Transmission Electron Microscopy (TEM). The mechanical properties of the test steels subjected to heat treatment were measured by tensile, hardness and impact tests. The results showed that, through the same heat treatment process, the microstructure of the V-alloyed steel was comprised of Carbide-Free Bainite (CFB, bainite + retained austenite) and martensite while the microstructure of the V-free steel was composed of ferrite/pearlite, which made the V-alloyed steels exhibit superior combination of strength, hardness and toughness to the V-free steel, but the elongation of the V-alloyed steel was worse than that of the V-free steel somewhat. Vanadium was helpful for the transformation of bainite in the tested medium carbon high silicon steel under air cooling condition. The carbon-enriched retained austenite films in the CFB enhanced the toughness of the V-alloyed steel.