Influence of Vanadium Content on Bainitic Transformation of a Low-Carbon Boron Steel During Continuous Cooling

The phase transformation behaviors during continuous cooling of low-carbon boron steels with different vanadium contents were studied by means of dilatometric measurement and microstructure observation. The bainite transformation behavior is not noticeably altered when the vanadium content is 0.042 and 0.086 wt%, and these steels exhibit full bainitic microstructure even at a cooling rate of 5 ℃/s. When vanadium content is increased to 0.18 wt%, ferrite is still present in the microstructure even at a cooling rate of 40 ℃/s. Vickers hardness of the steels with 0.042 and 0.086 wt% V is remarkably higher than that of the steel with 0.18 wt% V at a cooling rate higher than 10 ℃/s, and the difference is increased with the increase in cooling rate. Moreover, the amount of coarse vanadium precipitates formed in austenite is increased with the increase in vanadium content. The optimum content of vanadium to obtain bainitic microstructure is 0.086 wt% in this experimental low-carbon boron steels.

On hot deformation behavior and workability characteristic of 42CrMo4 steel based on microstructure and processing map

In order to determine the safe region of 42CrMo4 steel during hot working and obtain excellent workability,the hot deformation behavior at the temperatures of 850-1150℃and the strain rates of 0.01-10 s^(-1)was investigated through single-pass compression test of thermo-simulation.Through observing and analyzing the true stress-strain curves,the conclusion may be drawn that the flow stress value increases with the decrease in deformation temperature and the increase in strain rate.Raising temperature and reducing strain rate are conductive to dynamic recrystallization(DRX)nucleating and growing,but adiabatic heating caused by higher strain rate can also promote it.Since the Zener-Hollomon(Z)value and dynamic recrystallized grain size(D_(DRX))have completely opposite trends with deformation condition parameters,the expression of Z value and DDRX can be determined as:D_(DRx)=15,567.645Z^(-02174).The processing map and instability map constructed at a strain of 0.9 show that the suitable window for hot working with a true strain of 0.9 is in the temperature range of 970-1150℃and strain rate range of 0.01-0.25 s^(-1),as well as at the temperature of 1150℃ and strain rate range of 0.25-10 s^(-1).The instability phenomenon appears in the process interval of 850-1096℃ and 0.22-10 s^(-1).

Corrosion Behavior of High-Strength Steel for Flexible Riser Exposed to CO_2-Saturated Saline Solution and CO_2-Saturated Vapor Environments

The corrosion behavior of high-strength steel used for flexible riser exposed to CO_(2-)saturated saline solution and CO_(2-) saturated vapor environments was studied through immersion experiment and electrochemical corrosion experiment. The corrosion behavior and mechanism of the tested steel were analyzed on the basis of corrosion kinetics, nature of corrosion products, corrosion product morphology, elemental distribution and polarization curves. The experimental results showed that the microstructure of the tested steel was bainitic microstructure. The corrosive activity of the tested steel exposed to CO_(2-) saturated vapor environment was significantly lower than that exposed to CO_(2-) saturated saline solution environment.On prolonging the exposure time, the corrosion rate gradually decreased, the corrosion heterogeneity increased, and the dimensions of FeCO_3 crystals gradually became small. At later stages of corrosion, the corrosion current density decreased significantly and the anodic Tafel slope increased, indicating that the corrosion process was strongly inhibited. The corrosion mechanism of low-alloy steel with bainitic microstructure was proposed based on experimental results.

Erosion corrosion of low-alloy wear-resistant steels in alkaline slurry

Erosion corrosion causes significant problems in various industrial environments through a synergistic effect which results in much greater weight loss than the sum of the weight losses in the individual processes.The erosion-corrosion behavior of three low-alloy steels was investigated in a simulated concrete slurry using the rotation method.The key influencing factors and mechanism of material degradation were analyzed.The experimental results indicate that the weight loss increases with the linear velocity according to a nearly exponential relationship（W =KV^n）,where nis 1.40-2.14.This weight loss is mainly caused by erosion in the alkaline slurry,and steels with higher tensile strengths show higher erosion-corrosion resistance.The formation of many platelets and ring cracks and their removal from the sample surface during erosion corrosion in the slurry are thought to constitute the mechanism responsible for this weight loss.These platelets and ring cracks are formed by solid particles striking the sample surface.Craters are initially produced and subsequently disappear as they grow and come in contact with each other.Fewer craters were observed on the surfaces of samples that exhibited higher weight loss.The surface of the material became work-hardened because of the effect of the particles striking and scratching,and a deformed layer was produced on the surface for steels of lower strengths,leading to deeper and more abundant gouges.

Hydrogen trapping and electrochemical corrosion behavior of V–N microalloyed X80 pipeline steels consisting of acicular ferrite and polygonal ferrite

Hydrogen trapping behavior of V–N microalloyed X80 pipeline steels was studied by means of hydrogen permeation and hydrogen induced cracking(HIC)tests.In addition,the electrochemical performance of the steels in 3.5 wt.%NaCl solution was investigated.Results indicated that the microstructure of experimental steels mainly consisted of acicular ferrite and polygonal ferrite(PF).When the fraction of PF was 9.1%and 30.4%,hydrogen effective diffusion coefficient was 1.624×10^(−6) and 3.121×10^(−6) cm^(2)/s,respectively.The pipeline steels were not susceptible to HIC.Numerous potential hydrogen traps distributed in homogeneous dispersion were conducive to high HIC resistance.With increasing the fraction of PF from 9.1%to 30.4%,the corrosion current density increased from 5.39×10^(−6) to 9.49×10^(−6) A cm^(−2),the corrosion potential decreased from−0.48 to−0.57 V,and the charge transfer resistance decreased from 2301 to 2068Ωcm^(2),respectively.Increased fraction of PF was disadvantageous for corrosion resistance because of galvanic corrosion.

Structure-property relationships in heat-affected zone of gas-shielded arc-welded V-N microalloyed steel

The structure-property relationship in heat-affected zone (HAZ)of a low-carbon steel bearing V-N subjected to gasshielded arc welding was explored.The microstructural characteristics of base metal (BM),coarse-grained HAZ (CGHAZ),fine-grained HAZ,and intercritical HAZ were significantly different.The effect of grain-refinement strengthening and transformation hardening on HAZ contributed to equivalent hardness of 260.8-278.5 HV in comparison with BM hardness of 272.0 HV.Moreover,excellent impact toughness at -20 ℃ was obtained because of high resistance to crack propagation by high-misorientation boundaries,leading to impact fracture consisting of dimples.In CGHAZ,free N was partly fixed by V(C,N)precipitates,such that the deterioration effect of N on toughness was considered to be nearly eliminated.In comparison with CGHAZ,weld metal contained higher fraction of acicular ferrite with fine plates,while the impact toughness was inferior because of the detrimental influence of coarse inclusions from the welding wire.The nanoscale V(C,N)precipitates in CGHAZ had weak effect on toughness because of small size.

Influence of Vanadium Content on Hot Deformation Behavior of Low-Carbon Boron Microalloyed Steel

Single-pass compression tests were performed to investigate the hot deformation behavior of low-carbon boron microalloyed steel containing three various vanadium contents at 900-1100℃and strain rate of 0.01-10 s-1 using the MMS-300 thermal mechanical simulator.The flow stress curves of investigated steels were obtained under the different deformation conditions,and the effects of the deformation temperature and strain rate on the flow stress were discussed.The characteristic points of flow stress were obtained from the stress dependence of strain hardening rate;the activation energy of investigated steels was determined by the regression analysis;the flow stress constitutive equations were developed;the effect of vanadium content on the flow stress and dynamic recrystallization(DRX)was investigated.The result showed that the flow stress and activation energy(3-6.5 kJ mol-1)of the steel containing 0.18 wt%V were significantly higher than those of the steels with0.042 wt%and 0.086 wt%V,which was related to the increase in solute drag and precipitation effects for higher vanadium content.DRX analysis showed that the addition of vanadium can delay the initiation and the rate of DRX.

Corrosion Behavior of Low-Alloy Pipeline Steel with 1% Cr Under CO_2 Condition

Carbon dioxide corrosion behavior of low-alloy pipeline steel with 1% Cr exposed to CO2-saturated solution was investigated by immersion experiment. SEM, EDX, TEM, EPMA and XRD were utilized to investigate the microstructure, corrosion morphologies, corrosion phases and elements distribution of corrosion scale. The results demon strate that the microstructure of tested steel consists of ferrite and carbides. During the corrosion process, ferrite dissolves preferentially, leaving carbide particles behind. The residual carbide particles may promote the nucleation of FeCO3 crystal. The phase comprising of the inner layer is Cr compound, and the one of the outer layer is FeCO3. The formation process of corrosion scale can be illustrated as follows： Firstly, a thin scale consisting of thin inner layer and outer layer is formed, which represents poor corrosion resistance; then, the inner layer changes little, once it has been formed, and the outer layer becomes thick and compact, which demonstrates that a fine corrosion resistance is obtained. The chemical elements of chromium and molybdenum accumulate in the inner layer of corrosion scale. The corrosion behavior of low- alloy steel based on microstructure and morphology characterization is also discussed.