Pearlitic Steel Wires Research Articles

Hydrogen embrittlement of high-strength steel submitted to slow strain rate testing studied by nuclear resonance reaction analysis and neutron diffraction

Embrittlement of high-strength steel is known to be caused by hydrogen penetration into the network of the metal. However, the penetration mechanism of hydrogen into the steel is still under discussion, as it is not so easy to detect and measure the hydrogen in steels. In this paper, relatively large commercial cold drawn pearlitic steel wires have been submitted to stress corrosion cracking under different environments. After these tests, Nuclear Resonance Reaction Analysis, NRRA, has been used to detect the presence of the hydrogen and its quantity, which has confirmed the presence of H in the case of brittle fracture. In addition, once the presence of hydrogen has been confirmed, the penetration of the hydrogen into the unit cell of the ferrite has been deduced from the analysis of the steels by neutron diffraction, from the shift of the d-spacing after the experiments. The application of NRRA and Neutron Diffraction techniques have been shown as very promising techniques to study the mechanisms of the embrittlement of the steel to the hydrogen present in the metallic lattice.

Effect of Annealing Time on Microstructural Evolution and Corresponding Mechanical Properties of Cold Drawn Pearlitic Steel Wires

The effect of annealing time on microstructural evolution and corresponding mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. The increase of annealing time at 400 °C caused the spheroidization of lamellar cementite in the pearlite. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of lamellar cementite and the occurrence of recovery of lamellar ferrite in the pearlite. Mechanical properties of annealed steel wires showed the good combination of elongation and tensile strength.

Fatigue crack propagation in cold drawn steel

This paper deals with the fatigue crack growth in pearlitic steel wires in the form of hot rolled bar and cold drawn wire. The progress of crack front was analysed by means of the evolution of the aspect ratio with the relative crack depth, and this latter was evaluated during the tests by a compliance method. Results show that cold drawing is beneficial from the point of view of crack growth rate, i.e., cracking is slower in the cold drawn wire (final commercial product) than in the hot rolled bar (base material). In spite of the oriented pearlitic microstructure of the cold drawn steel, fatigue crack propagation develops in mode I, i.e., cracking takes place by maintaining its original plane. A materials science reasoning is proposed to explain this behaviour on the basis of the pearlitic microstructure of the steel and the large geometry changes in the vicinity of the crack tip.

The prediction of the occurrence of the delamination in cold drawn hyper-eutectoid steel wires

Abstract The effects of mechanical properties and microstructural features such as interlamellar spacing on the occurrence of the delamination in cold drawn hyper-eutectoid steel wires were investigated. Since the cementite dissolution occurred during wire drawing would become one of the main reasons to cause the occurrence of the delamination, the dependence of the occurrence of the delamination on mechanical properties was examined using tensile test and scanning electron microscopy. The results showed that tensile strength would become a proper parameter to predict the occurrence of the delamination in hyper-eutectoid pearlitic carbon steels and Cr-added steels. It was found that while the initial interlamellar spacing showed little influence on the occurrence of the delamination, the Cr addition would improve tensile strength of cold drawn pearlitic steel wires without the occurrence of the delamination.

３次元アトムプローブによる球状セメンタイトの元素定量解析

To examine the quantitative performance of three-dimensional atom probe (3D-AP) in steel materials, composition analysis of spherical cementite (Fe3C) formed in an annealed pearlitic steel wire was conducted. It was visualized in three dimensions that manganese and chromium atoms were concentrated in cementite, while silicon atoms were concentrated in ferrite. Phosphor atoms were segregated at the cementite-ferrite interface. By taking all carbon molecular ions into consideration, the carbon concentration in cementite was in the range of 25-27 at%, which approximately coincided with the stoichiometric composition of 25 at%. On the other hand, the iron concentration in cementite was in the range of 70-73 at%, which was a few atomic % lower than 75 at%. The reduction of the iron concentration is caused by the substitution of manganese, chromium and silicon atoms for iron atoms in cementite. It was shown that the 3D-AP enables the quantitative composition analysis with high accuracy. Inhomogeneous distribution of manganese and chromium in cementite was observed, and it was explained by the change in the partition coefficient depending on the interface mobility of spherical cementite growth.

Effect of Annealing Temperature on Mechanical Properties of Cold Drawn Pearlitic Steel Wires

The effects of annealing temperature and annealing time on mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. Annealing treatment was performed on cold drawn steel wires for the temperature range of 200°C to 450°C with the different annealing time of 30sec, 1min, 15min and 1hr. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of cementite plates and the occurrence of recovery of the lamellar ferrite in the pearlite.

고탄소강 펄라이트 조직의 인발 공정 시 전단응력의 해석

This paper presents a study on defects in pearlite lamella structure of high carbon steel by means of finite-element method(FEM) simulation. High-carbon pearlite steel wire is characterized by its nano-sized microstructure feature of alternation ferrite and cementite. The likely fatigue crack is located on interface of the lamella structure where the maximum amplitude of the longitudinal shear stress and transverse shear stress was calculated during cyclic loading. The FEM is proposed for maximum shear stress from loading of lamella structure, and a method is predicted to analyze the likely fatigue crack generation. It is possible to obtain the important basic data which can be guaranteed in the ductility of high carbon steel wire by using FEM simulation.

신선가공조건에 따른 고탄소강 선재 Pearlite 층상구조의 유한요소해석

This paper presents a study on defects in pearlite lamella structure of high carbon steel by means of finite-element method(FEM) simulation. High carbon pearlite steel wire is characterized by its nano-sized microstructure feature of alternation ferrite and cementite. FEM simulation was performed based on a suitable FE model describing the boundary conditions and the exact material behavior. Due to the lamella structure in high carbon pearlite steel wire, material plastic behavior was taken into account on deformation of ferrite and cementite. The effects of many important parameters(reduction in area, semi-die angle, lamella spacing, cementite thickness) on wire drawing process can be predicted by DEFORM-2D. It is possible to obtain the important basic data which can be guaranteed in the ductility of high carbon steel wire by using FEM simulation.

超高強度パーライト線の引張応力下における格子面ひずみの飛行時間法中性子回折その場測定(オーガナイズドセッション,材料と組織)

Work hardening of perlitic steel is governed by the stress partitioning between ferrite(α) and cementite (θ) during deformation. However by heavily drawn processing, cementite dissolves to disappear. Hence, the strength of heavily drawn processed pearlitic steel originates in a matrix. In this paper, a pearlitic steel wire with ultra-high strength was studied by in situ time-of-flight diffraction during tensile deformation. Lattice plane strains for various (hkl) planes were measured in the tensile and the perpendicular directions simultaneously.

Strength Anisotropy and Residual Stress in Drawn Pearlite Steel Wire

Strength anisotropy is found by tensile and compressive tests for a drawn pearlite steel wire, where the compressive tests are performed for prepared specimens along directions of 0, 45 and 90 degrees with respect to the drawing direction. The influence of annealing on such anisotropic strength is also examined. To make clear the origin of the strength anisotropy, texture and residual strain are measured using neutron diffraction. It is revealed that residual stresses in the ferrite and cementite phases are the cause of strength anisotropy.

Residual Stresses in Wires: Influence of Wire Length

Residual stresses are one of the causes of failures in structural components. These stresses may arise in the fabrication process from many causes. They cannot be easily accounted for because they are both difficult to predict and to measure. X-ray diffraction (XRD) is nowadays a widespread technique for measuring surface residual stresses in crystalline materials. Very small specimens are often used for this purpose due to geometrical restrictions of either the diffractometer sample holder or the component to be inspected. However, the cutting process itself may affect the residual stress state in these specimens, so measured stresses could be misleading. In this work, the influence of specimen length on residual stresses was investigated in cold-drawn ferritic and pearlitic steel wires by XRD measurements and finite element simulations. In the ferritic wires, numerical simulations coincide with experimentally measured stresses. However, in the pearlitic wires the effect of the stresses in cementite (which could not be measured by XRD) has to be taken into account to explain the observed behavior. The results obtained have shown that in both materials the cutting process affects residual stresses, so it is recommended that specimens larger than five times the wire diameter be used.

410 Residual Stress in a Drawn Pearlitic Steel Wire

410 Residual Stress in a Drawn Pearlitic Steel Wire

A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel

A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of cold-drawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C H). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain (ɛ eff f ) and C H, obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe3C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe3C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.

Drawing Behavior of Pearlitic Steel Wire Rods Controlled by Boron Addition in Medium Carbon Steel

One of the most effective methods to increase the productivity and cost down in wire manufacturing is eliminating the patenting heat treatment during drawing process. For that purpose, the ductility increase of wire rods is essential to draw wire without breakage. Dilute pearlite, i.e., pearlitic structure with low cementite volume fraction, would be optimum microstructure for increasing the drawing strain, because ferrite-pearlite interface and cementite, which act as void initiation sites, are reduced. To obtain such microstructure in production line, addition of boron was examined. As results, mechanical properties of eutectoid steel are little affected by boron addition. But, in boron added medium carbon steel, ductility is enhanced, especially large elongation at sever deformed wire after bluing. It would be due to increase the strain hardening rate by reducing of proeutectoid ferrite and cementite stabilization.

In situ neutron diffraction study of drawn pearlitic steel wires upon tensile deformation

Abstract The strengthening mechanism of heavily cold-drawn pearlitic steel wires was studied using the highresolution neutron diffraction experiment performed in situ upon tensile deformation. Microstructural parameters of lattice strain, microstrain and coherent block size, the evolution of which evidences the deformation mechanism of this ultrahigh strength steel, were extracted from the position, width and shape of the in situ recorded neutron diffraction profiles, respectively, using a newly proposed method of profile shape analysis. It was found that the deformation mechanism of cold-drawn pearlitic steel wires resembles the behavior of nanocrystalline materials.

Comment on “Effect of interlamellar spacing on cementite dissolution during wire drawing of pearlitic steel wires”

Relevant mechanisms of cementite dissolution due to cold work of pearlitic steel are discussed based on experimental data obtained using Mössbauer spectroscopy and internal friction. It is concluded that a strong interaction between carbon atoms and dislocations at the interface is the most probable reason for decomposition of the cementite lamellae.

Microstructure evolutions in pearlitic steels and Cu/Nb wires resulting from severe plastic deformation during drawing

The aim of this paper is to present recent microstructural investigations concerning two kinds of materials produced by industrial processes through high level of plastic deformations. To help in further understanding in the strengthening mechanism of such nanocomposite wires, the evolution of the microstructure was investigated using field ion microscopy (FIM), Transmission Electron Microscopy (TEM) and Three Dimensional Atom Probe (3D-AP). Niobium fibres and copper channels a few nanometers width with smooth interfaces were exhibited in the Cu/Nb wire. A deep interdiffusion of Cu and Nb was pointed out. This mechanical alloying induced by drawing is attributed to the strong increase of the interfacial energy of the system. Shear bands were also observed perpendicular to the wire axis. They were attributed to the tracks of moving dislocations in the (111) Cu plane, along the direction. Cold drawn pearlitic steel wires were also studied. Because of the plastic deformation. cementite lamellae aligned themselves along the wire axis and are thinned down to a few nanometres. 3D-AP data clearly reveal partly dissolved cementite lamellae. As in the Cu/Nb system, the dissolution of cementite is discussed on the basis of thermodynamical arguments.

Cementite decomposition in heavily drawn pearlite steel wire

Heavily drawn pearlitic wires have been investigated extensively over the years for its unusual strainhardening behavior as well as for the high strength with acceptable level of ductility. They are widely used for tire cord, springs, wire rope, and suspension bridge cable. They are typically produced by drawing wire of approximately eutectoid composition to an intermediate diameter, patenting to produce a fine pearlitic microstructure, and then cold drawing to strains between 1.5 and 5.0. Strength increases as a function of strains, and tensile strength exceeding 5 GPa was recently reported in a fine steel cord developed in a laboratory [1]. Previous transmission electron microscopy (TEM) observations have revealed a number of unique microstructural features associated with the wire drawing process [2-4]. The pearlitic lamellar distance decreases with drawing strain, and strengths are related with the lamellar distance with Hall-Petch like relationship [2]. Despite the limited ductility of monolithic cementite crystals, the cementite lamella in pearlitic wire co-deform with ferrite [5-7], but they appear to fragment into planar arrays of small particles. Recent high resolution electron microscopy study by Languillaume et al. [8] suggested that these fragmented cementite eventually dissolves into ferrite to reduce the surface energy associated with the nanoscale fragmented particles. Recent atom probe field ion microscopy (APFIM) [9-11] and three dimensional atom probe (3DAP) studies [12,13] indicate that substantial amount of carbon is dissolved in the ferritic phase in heavily deformed pearlite. Internal friction [14] and Mossbauer[15,16] experiments also suggest a substantial proportion of the cementite (from 20 to 50 volume percent) dissolves during deformation at room temperature. This phenomenon is interesting since cementite is stable at room temperature and the solubility of carbon in ferrite is quite low. In our previous APFIM and TEM studies [11,13], we reported the microstructures of pearlitic steel wires of maximum strain of 4.2 with tensile strength of 3,930 MPa. In this specimen, cementite was present as fragmented nanoscale particles, and evidence for partial dissolution of cementite was also observed. It is interesting to see how these microstructure change in a pearlitic steel wire with a higher strain. Hence, in the present study, we have characterized the microstructure of a pearlitic steel wire with a higher strain as much as 5.1 with tensile strength of 5,170 MPa.

Strain aging of pearlitic steel wire during post­drawing heat treatments

The thermal treatment involved in galvanising affects the mechanical properties of cold drawn pearlitic steels. There is a trough in the ductility at short heating times, which recovers after further heating, but only at the expense of strength. The objective of this work was to gain an understanding of the underlying processes that lead to these changes in properties. A 0.85 wt-%C, vanadium microalloyed, fully pearlitic steel was studied. Samples were heat treated for up to 90 s in a salt bath at 500°C to simulate the galvanising process. The effects of the post­drawing heat treatment on the microstructure and carbon atom distribution were investigated using differential scanning calorimetry, SEM, TEM, and atom probe field ion microscopy. Carbide fragmentation occurs during the heat treatment, representing the onset of the spheroidisation process. The carbon concentration in the ferrite matrix remains low in all heat treatment conditions. However, localised regions of ferrite containing approximately 2.5 at.-%C are observed, indicating the presence of Cottrell atmospheres around dislocation lines. Planar regions enriched in carbon are also seen, which demonstrate segregation to ferrite sub­boundaries. A simple model of the carbon redistribution process is proposed.

Specific dislocation multiplication mechanisms and mechanical properties in nanoscaled multilayers: The example of pearlite

Dislocation mechanisms responsible for mechanical properties of nanoscaled multilayers are studied using the example of heavily drawn pearlitic steel wires. Two specific types of dislocation source are observed during transmission electron microscopy in situ straining experiments: one dislocation source operates at wide interlamellar spacings by dislocation propagation in ferrite around cementite islands; the other consists of dislocations bulging in ferrite from interfaces, and is more likely to operate at narrow interlamellar spacings. The constitutive law of the material is derived from a balance between dislocation multiplication and annihilation rates, and predicts a linear dependence of the flow stress with the reciprocal interlamellar spacing, and a stress saturation for very small spacings.