Hydrogen-assisted fracture features of a high strength ferrite-pearlite steel

Up to now, the exact reason of hydrogen-induced fracture for ferrite-pearlite(FP) steel is still not fully understood. This study presents detail observations of the feature beneath the fracture surface with the aim to reveal the hydrogen-induced cracking initiation and propagation processes. Slow strain rate tensile(SSRT) testing shows that the FP steel is sensitive to hydrogen embrittlement(HE). Focused ion beam(FIB)was used to prepare samples for TEM observations after HE fracture. The corresponding fractographic morphologies of hydrogen charged specimen exhibit intergranular(IG) and quasi-cleavage(QC) fracture feature. Pearlite colony, ferrite/pearlite(F/P) boundary and the adjacent ferrite matrix are found to be responsible for the initial HE fracture and the subsequent propagation. With increasing of the stress intensity factor, fracture mode is found to change from mixed IG and QC to entire QC feature which only occurs at the ferrite matrix. No crack is observed at the ferrite/cementite(F/C) interface. This may be mainly due to the limited pearlite lamella size and relatively low interface energy.

The synergy between cementite spheroidization and Cu alloying on the corrosion resistance of ferrite-pearlite steel in acidic chloride solution

In this work,the corrosion behavior of medium-carbon steels(45,45 Cu and 45 Cuq steels)in acidic chloride environment was investigated.The results indicated that the micro-galvanic effect between the anodic ferrite matrix phase and the cathodic cementite secondary phase notably affected the corrosion resistance of the three steels.For 45 steel,serious pitting corrosion happened in and around the pearlite regions,and a large number of lamellar cementite was fixed in the corrosion pits.Meanwhile,the continuously increasing superficial area of cathodic cementite enhanced the micro-galvanic corrosion,resulting in a rapidly increase in corrosion rate with time.While for 45 Cu and 45 Cuq steels,macroscopic uniform corrosion occurred,and the cementite accumulation was markedly reduced as compared with 45 steel,thus the micro-galvanic effect was weakened and the corrosion rate was decreased accordingly.Among these,45 Cuq steel showed the most stable and excellent corrosion resistance during long-term corrosion,indicating the occurrence of a synergistic effect between cementite spheroidization and Cu alloying,thereby significantly improving the corrosion resistance of 45 steel.

Influence of cementite spheroidization on relieving the micro-galvanic effect of ferrite-pearlite steel in acidic chloride environment

The corrosion behavior of the as-received steel and the spheroidized steel in acidic chloride environment was investigated. The results indicate the corrosion mode and corrosion rate of two steels are diverse due to their difference in microstructure. For as-received steel with ferrite-pearlite microstructure, severe localized corrosion happens on the pearlite regions, and plenty of cathodic cementite remains in the pits, further strengthening the micro-galvanic effect and accelerating the corrosion rate. While for spheroidized steel with tempered martensite microstructure, the nanosized cementite particles evenly distributed on the ferrite substrate are easy to fall off, which can significantly reduce the cementite accumulation on the steel surface, relieving the acceleration effect of micro-galvanic corrosion.

Prediction of mechanical behavior of ferrite-pearlite steel

A new approach describing the flow stress of ferrite-pearlite steel has been proposed,which divided the deformation process into three stages based on whether ferrite or pearlite yielded. Iso-work increment assumption was applied to describe the transfer of load between the components. The physically based model to describe ferrite was approximated with Swift＇s equation in order to obtain the analytic solution.The tensile strength of ferrite-pearlite had a linear relation with pearlite volume fraction,square root reciprocal of ferrite grain size and reciprocal of pearlite interlamellar spacing. Moreover,a model to calculate the tensile strength of ferrite-pearlite steel was proposed. The predicted values of tensile strength were in good agreement with experimental results when the pearlite volume fraction was less than 20%.Considering the plastic relaxation mechanisms,the internal stress was modified with pearlite volume fraction,total strain,yield stress of ferrite and pearlite when the pearlite volume fraction was more than 20%.

Microstructure and strength in ultrastrong cold-drawn medium carbon steel

Via traditional wire drawing,the medium carbon ferrite-pearlite(MCFP)steel wires can achieve the ultrahigh strength beyond 4 GPa normally for high-carbon pearlitic steel wires,but have a 30-60%lower production cost.The microstructural evolution and mechanical properties of medium carbon ferrite-pearlite steel wires have been investigated by means of scanning electron microscopy,transmission electron microscopy and tensile testing.The tensile strength of medium carbon ferrite-pearlite steel wires increases from 750 MPa up to 4120 MPa when the drawing strain increases up toε=6.4,which represents the highest strength reported so far-to our knowledge for a carbon steel with such low carbon content.At low and medium strains(ε≤1.95),the proeutectoid ferrite forms dense dislocation walls(DDWs)via dislocation activities,including sliding,accumulation,interaction,and tangling.With the drawing strain increase,the reorientation of DDWs to the drawing direction forms the coarse proeutectoid ferrite lamellae.Finally,the proeutectoid ferrite deformed to high strains is characterized by a lamellar morphology and the average lamellar spacing of proeutectoid ferrite is about 55 nm atε=6.4.The interlamellar spacing of pearlite and thickness of cementite decreases with the drawing strain increases.The dislocation density in ferrite lamellae increases with the drawing strain increases,and the dislocation density in ferrite lamellae is 7.8×10^(15)m^(2)atε=4.19.A higher dislocation density of 3.1×10^(16)m^(2)can be obtained atε=6.4 by means of extrapolation and TEM investigations.The stress contributions of proeutectoid ferrite and pearlite to the flow stress are estimated based on quantified structural parameters.Based on the assumption that the stress contributions from different strengthening mechanisms are linearly additive and the general rule of mixtures,a good agreement between the measured and estimated flow stresses has been found in a large range of flow stress.The good application of the general rule of mixture to the medium carbon ferrite-pearlite steel wires indicates the importance of quantitative characterization of microstructural evolution and parameters with the strain.

Effects of metallic microstructures on fatigue fracture of Q345 steel

Effects of high-frequency cyclic loading on the banded ferrite-pearlite steel were analyzed through crack initiation and propagation. Interfaces of ferrite and pearlite colony with a small angle deviation from the loading axis were verified to be the most potential sites to fabricate the microcracks caused by the high strain gradient. The initial crack extension inside ferrite grain was driven by shear stress in model II along the direction with a 45° angle to the loading axis. Banded pearlite colony and the high-angle grain boundaries were considered as the dominant factors that promote the fatigue resistance of the material through arousing crack deflection in short crack propagation range and crack branching in long crack propagation range to reduce the crack propagation driving force in the crack tip. P-S-N curves were used to quantify the dispersion of fatigue lifetimes and evaluate the effect of elevated volume content of pearlite colony on the fatigue performance of the material.