High-strength Pearlitic Steel Research Articles

A fracture criterion for cold drawn pearlitic steel notched wires with circumferentially-shaped notches of different geometries: From Eduardo Torroja to José Antonio Torroja

This paper deals with the formulation and development of fracture criteria for high-strength structural members containing surface damage in the form of notches (i.e., blunt defects). The important role of the yield strength of the material and its strain hardening capacity (evaluated by means of the constitutive law or stress-strain curve) is analyzed in depth by considering the fracture performance of notched samples taken from high-strength pearlitic steels with different levels of cold drawing (the most heavily cold drawn pearlitic steel being commercial prestressing steel used in prestressed concrete). The final aim of the paper is to establish fracture-based design criteria for structural members made of steels with distinct yield strength and containing very different kinds of notch-shape surface damage.In Memoriam: The paper is dedicated to the memory of the Spanish civil engineers Eduardo Torroja and José Antonio Torroja.

Hydrogen embrittlement of pearlitic steel in the presence of notches: A kinematic fracture criterion based on the notch tip strain rate

This paper offers a kinematic fracture criterion for hydrogen embrittlement (or hydrogen assisted fracture) of notched samples of high strength pearlitic steel tested at very different loading/straining rates under cathodic electrochemical conditions promoting hydrogen entry and diffusion into the specimens. The local strain rate at the notch tip (notch tip strain rate NTSR) is shown to be the relevant variable governing the hydrogen embrittlement process. To this end, a fracture criterion is kinematically formulated over the hydrogen affected zone: fracture will take place when the equivalent stress reaches a critical value over a critical zone, both being a function of the local strain rate at the notch tip (or notch tip strain rate NTSR). Experimental results show that under certain conditions the results are universal and independent of the specimen geometry, which supports the validity of the present approach and the relevance of the role of local strain rate in the close vicinity of the notch tip.

Hydrogen embrittlement of pearlitic steel in the presence of cracks: A kinematic fracture criterion based on the crack tip strain rate

This paper offers a kinematic fracture criterion for hydrogen embrittlement (or hydrogen assisted cracking) of cracked samples of high strength pearlitic steel tested at very different loading/straining rates under cathodic electrochemical conditions promoting hydrogen entry and diffusion into the specimens. The local strain rate at the crack tip (crack tip strain rate CTSR) is shown to be the relevant variable governing the hydrogen embrittlement process. Calculation of the local strain rate is carried out on the basis of the elastic displacement distribution in the vicinity of a crack tip in conditions of plain strain. A local reference length is chosen next to the crack tip, at a distance estimated from the fractographic results of the tests that show a zone microscopically affected by hydrogen. The other relevant variable in hydrogen assisted cracking is the depth of the maximum hydrostatic stress point, which is obtained by using approximate stress distributions in the vicinity of the crack tip in the elastic-plastic regime, or calculated as the asymptotic depth (for quasi-static tests) of the hydrogen affected area.

A fracture mechanics approach to hydrogen assisted microdamage and failure analysis of high-strength pearlitic steel wires: Resembling Michelangelo Stone Sculpture Texture

A fracture mechanics approach to hydrogen-assisted microdamage and failure analysis of high-strength pearlitic steel wires is presented. Fractographic analysis revealed micromechanical effects of hydrogen in the form of tearing topography surface(TTS). The progress of this microdamage is modelled as a macroscopic crack that extends the original fatigue precrack and involves linear elastic fracture mechanics (LEFM) principles. In this case, the change from hydrogen-assisted microdamage (HAMD) in the form of TTS (subcritical mode) to cleavage-like topography (critical regime associated with failure) takes place when a critical stress intensity factor (KH) is reached, and this value depends on the amount of hydrogen which penetrated the vicinity of the actual crack tip (the fatigue precrack plus the TTS extension). It is seen that the crack growth by TTS corresponds to the horizontal part (plateau) in the crack growth kinetics (CGK) curve da/dt-K until reaching the critical level KH.

Coupling physics in machine learning to predict interlamellar spacing and mechanical properties of high carbon pearlitic steel

The present study proposed an improved composition-structure–property model which incorporates physical features in the machine learning(ML) process. Herein, the physical parameters, the volume fraction of ferrite, cementite and carbides and the transformation temperature of pearlite, were included to the dataset as extra input variables to guide the ML process. As a result, the dataset mixing physical features greatly improved the generalization ability and prediction accuracy of the generalized regression neural network(GRNN) model, which clearly demonstrates the practicability of the present physics coupled ML approach. Furthermore, several optimization algorithms are applied to improve the GRNN model and the fruit fly optimization algorithm(FOA) is demonstrated more effective than particle swarm optimization(PSO) algorithm. Thus then, a novel high-strength pearlitic steel was synthetized and experimentally validated with outperformed microstructural and mechanical characteristics.

On the cleavage hoop stress responsible for crack path deflection and notch-induced anisotropic fracture in cold drawn pearlitic steel: Revisiting John Ford’s Monument Valley

In this paper a study is presented of the fracture behaviour in the presence of notches of high-strength pearlitic steel with different cold drawing degree. To this end, axisymmetric notched samples with a circumferentially-shaped notch of very distinct geometries (sharp and blunt, deep and shallow) were taken from wires with different levels of drawing from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire: heavily cold drawn). Experimental results show that cold drawn steels exhibit a markedly anisotropic fracture behaviour in the form of fracture path deviation from the original transverse fracture path perpendicular to the wire axis or cold drawing direction. Present paper proposes a key variable governing the notch-induced anisotropic fracture process: the cleavage hoop stress as the relevant factor promoting this sort of fracture path deviation in the most heavily cold drawn pearlitic steel wires

Numerical modeling of hydrogen embrittlement of pearlitic steel in the presence of blunt notches

This paper offers a numerical modeling of hydrogen embrittlement and notch tensile strength of high-strength pearlitic steel (supplied in the form of hot rolled bars) in the presence of blunt notches, by using the finite element method in order to determine how the notch depth influences the concentration of hydrogen in the steady-state regime for different loading values. Numerical results show that the maximum hydrostatic stress location (towards which hydrogen is directed by a mechanism of stress-assisted diffusion) shifts from the notch tip to the inner points of the specimen under increasing load, even reaching the sample axis for the most elevated levels of externally applied stress.

Macro- and micro-approach to the notch-induced fracture process and notch tensile strength in eutectoid hot-rolled pearlitic steel: Weakest Link versus Process Zone Fracture Criterion

This paper presents a combined micro- and macro-approach to the fracture of high-strength pearlitic steel bars subjected to multiaxial stress states produced by notches of very different geometries. A fracture criterion based on the distortional part of the strain energy density (or, accordingly, the equivalent stress in the von Mises sense) averaged over a critical distance characteristic of the microstructure of the material is developed towards a more physically sound criterion on the basis of the process zone concept, i.e., the critical domain or fracture region is not constant, but depends on the stress triaxiality in the notched geometry under consideration. Although this combined approach is more difficult due to microstructural considerations, it may also be implemented in a computer, since the size of the afore-said process zone can also be predicted by numerical analysis.

Analysis of the Bauschinger Effect in Cold Drawn Pearlitic Steels

Prestressing steel wires usually undergo cyclic loading in service. Therefore, it is of interest to analyse certain features of their mechanical behaviour under this type of loading, such as the Bauschinger effect (BE) or the hardening rule, that fit the real mechanical behaviour appropriately. In this study, different samples of high strength pearlitic steel wires were subjected to cyclic tension-compression load exceeding the material yield strength, thus generating plastic strains. From the experimental results, various parameters were obtained revealing that analysed steels exhibited the so-called Masing type BE. In addition, the variation of the BE characteristics (of the effective and internal stresses) with the applied plastic pre-strain indicated that the studied materials followed a mixed strain hardening rule with the domination of the kinematic component.

Hydrogen embrittlement and notch tensile strength of pearlitic steel: a numerical approach

This paper offers a numerical approach to the problem of hydrogen embrittlement and notch tensile strength of sharply notched specimens of high-strength pearlitic steel supplied in the form of hot rolled bars, by using the finite element method in order to determine how the notch depth influences the concentration of hydrogen in the steady-state regime for different loading values. Numerical results show that the point of maximum hydrostatic stress (towards which hydrogen is transported by a mechanism of stress-assisted diffusion) shifts from the notch tip to the inner points of the specimen under increasing load, with numerical evidence of an elevated inwards gradient of hydrostatic stress “pumping” hydrogen inside the sample.

Recipe for improving the impact toughness of high-strength pearlitic steel by controlling the cleavage cracking mechanisms

Achieving superior impact toughness in high-strength pearlitic steels is extremely desirable and challenging due to the brittle character of the intervening cementite lamellae. Besides the ‘Hall-Petch’ type lamellar strengthening, a systematic understanding of various micro-toughening mechanisms through cleavage cracking resistance is essential to develop an integrated microstructure for toughness improvement. In this study, both thermal as well as thermo mechanical processing routes were adopted to correlate the processing and microstructural parameters with the mechanical properties. The prime contribution of pearlite nodule size refinement towards improving the impact toughness is discussed in terms of frequent cleavage crack deflection at the nodule boundaries. Other microstructural parameters controlling the toughness are the pearlite morphology (lamellar or spheroidized), interlamellar spacing and the cementite lamellae orientation with the crack path. It appears that both spheroidized and fine lamellar pearlite are more effective crack arresters than their coarse lamellar pearlite counterparts. Besides, the variation of crack growth resistance with the pearlite lamellae orientation and the ferrite-cementite interface (habit) plane is also presented, relating to the mechanism of interface decohesion.

Size Effects of High Strength Steel Wires

This study examines the effects of size on the strength of materials, especially on high strength pearlitic steel wires. These wires play a central role in many long span suspension bridges and their design, construction, and maintenance are important for global public safety. In particular, two relationships have been considered to represent strength variation with respect to length parameters: (i) the strength versus inverse square-root and (ii) inverse length equations. In this study, existing data for the strength of high strength pearlitic steel wires is evaluated for the coefficient of determination (R2 values). It is concluded that the data fits into two equations equally well. Thus, the choice between two groups of theories that predict respective relationships must rely on the merit of theoretical developments and assumptions made.

Influence of the Structural Parameters of High-Carbon Steel on the Impact Strength

For a coil of rolled C86D steel subjected to accelerated continuous cooling from the hot-deformation temperatures, change in austenite grain size by scores no greater than 1–2 according to State Standard GOST 5639–82 has no significant influence on the impact strength. For C86D steel, the impact strength is directly related to the extent of the ferrite–cementite phase boundaries: the impact strength increases with increase in the boundary length. These results are of practical interest in developing new alloys and conditions of deformation and heat treatment for high-strength pearlitic steels.

An efficient computational approach to evaluate the ratcheting performance of rail steels under cyclic rolling contact in service

A comprehensive study was carried out to numerically evaluate the ratcheting performance of three high strength pearlitic rail steels under different wheel–rail cyclic rolling contact conditions, i.e. free rolling, partial slip, and full slip conditions, different friction coefficients and different axle loads. The wheel–rail cyclic rolling contact was simulated by repeatedly passing a distributed contact pressure and a distributed tangential traction on the rail surface. This study combined the non-Hertzian contact pressure from finite element analysis with the longitudinal tangential traction from Carter’s theory to simulate the wheel–rail cyclic rolling contact problems. A cyclic plasticity material model considering the non-proportionally loading effect developed recently by the authors was applied to simulate the ratcheting behaviour of rail steels. The ratcheting performance of the rail steels was evaluated by the crack initiation life which was determined from the stabilized ratcheting strain rate and the ductility limit of the rail materials. The numerical results indicate that the crack initiation life decreases with the increase of the normalized tangential traction, the friction coefficient and the axle load for all three rail steels. Among the three rail steels, the hypereutectoid rail steel grade with a lower carbon content provides the best ratcheting performance under higher axle loads such as those used railway transport of mineral products in Australia. Furthermore, the numerical results obtained in this study are in reasonable agreement with the in-service performance of the three rail steels. This indicates that the developed approach has the capacity to evaluate the ratcheting performance of other rail steels under service loading conditions. The outcomes can provide useful information to the development and application of rail steels and the development of effective rail maintenance strategies in order to mitigate rail degradation.

비원형 신선을 이용한 고강도-고연성 펄라이트 강선의 제조

본 연구에서는 고강도-고연성 펄라이트 강선을 제조하기 위하여 비원형 신선 공정을 적용하였다. 다단 비원형 신선 공정을 A와 B로 정의한 2 종류의 가공경로를 이용하여 상온에서 12 패스까지 수행하였다. 비원형 신선 공정과의 비교를 위해서 기존의 원형 신선 공정을 수행하고 기계적 특성과 집합조직발달에 대해 비교를 수행하였다. 원형 신선 공정으로 제조된 강선은 10 패스에서부터 박리파괴가 관찰되었지만, 비원형 신선 가공경로 B의 경우는 12 패스에서도 박리파괴가 발생하지 않았으며, 이는 X-선회절로부터 측정된 집합조직 결과에서 원형집합조직의 발달이 적은 것과 연관된다. 따라서, 다단 비원형 신선 공정을 통하여 기존의 신선 공정보다 박리파괴의 발생 가능성을 저감시킴으로써 높은 비틀림 연성을 갖는 고강도 펄라이트 강선을 제조할 수 있음을 확인할 수 있었다.

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.

Multi-Scale Approach to the Fatigue Crack Propagation in High-Strength Pearlitic Steel Wires

Abstract This paper deals with the influence of the manufacturing process on the fatigue behavior of pearlitic steels with different degrees of cold drawing. The fatigue crack growth rate (da/dN) is related to the stress intensity range (ΔK) by means a compliance method to evaluate the crack depth a in the samples at any instant during the tests. The analysis is focused on the Region II (Paris) of the fatigue behavior in which da/dN=C(ΔK)m, measuring the constants (C and m) for the different degrees of drawing. From the engineering point of view, the manufacturing process by cold drawing improves the fatigue behavior of the steels, since the fatigue crack growth rate decreases as the strain hardening level in the material increases. In particular, the coefficient m (slope of the Paris Law) remains almost constant and independent of the drawing degree, whereas the constant C decreases as the drawing degree rises. The paper focuses on the relationship between the pearlitic microstructure of the steels (progressively oriented as a consequence of the manufacturing process by cold drawing) and the macroscopic fatigue behavior. To this end, a detailed metallographic analysis was performed on the fatigue crack propagation path after cutting and polishing on a plane perpendicular to the crack front. It is seen that the fatigue crack growth path presents certain roughness at the microscopic level, such a roughness being related to the pearlitic colony boundaries more than to the ferrite/cementite lamellae interfaces.

Micro- and Macro-Approach to the Fatigue Crack Propagation in High-Strength Pearlitic Steel Wires

This paper analyzes how the cold drawing process influences the fatigue behaviour of eutectoid steel, with special emphasis on the role of microstructural changes induced during such a manufacturing process. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. The net fatigue surface increases with cold drawing due to the higher angle of crack deflections.

Fractographic and numerical study of hydrogen–plasticity interactions near a crack tip

This paper offers a fractographic and numerical study of hydrogen–plasticity interactions in the vicinity of a crack tip in a high-strength pearlitic steel subjected to previous cyclic (fatigue) precracking and posterior hydrogen-assisted cracking (HAC) under rising (monotonic) loading conditions. Experiments demonstrate that heavier cyclic preloading improves the HAC behaviour of the steel. Fractographic analysis shows that the microdamage produced by hydrogen is detectable through a specific microscopic topography: tearing topography surface or TTS. A high resolution numerical modelling is performed to reveal the elastoplastic stress–strain field in the vicinity of the crack tip subjected to cyclic preloading and subsequent monotonic loading up to the fracture instant in the HAC tests, and the calculated plastic zone extent is compared with the hydrogen-assisted microdamage region (TTS). Results demonstrate that the TTS depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but rather to random-walk lattice diffusion. It is, however, stress-assisted diffusion in which the hydrostatic stress field plays a relevant role. The beneficial effect of crack-tip plastic straining on HAC behaviour might be produced by the delay of hydrogen entry caused by residual compressive stresses and by the enhanced trapping of hydrogen as a consequence of the increase of dislocation density after cyclic plastic straining.

Dependences of magnetic Barkhausen emission and magnetoacoustic emission on the microstructure of pearlitic steel

The relationships between microstructure, mechanical behaviour and magnetic properties of completely pearlitic steels have been investigated, with the objective of determining the applicability of magnetic measurements to non-destructive evaluation of the properties of high-strength pearlitic steels. High-carbon steels were heat treated to produce completely pearlitic structures with various interlamellar spacings, and their magnetic properties, including hysteresis loops, Barkhausen emission (BE) and magnetoacoustic emission (MAE) profiles, were measured. MAE profiles were found to have two peaks at the knee of hysteresis loop, while BE profiles show only a single peak at about the coercive field for all samples. The peak height ratio of the MAE profiles and the amplitude of the BE profiles increase monotonically with increasing pearlite spacing, whereas coercivity is inversely proportional to pearlite spacing. These results can be interpreted in the context of magnetic domain structures and magnetization reversal processes observed by Lorentz transmission electron microscopy, which revealed that, in specimens with smaller pearlite spacing, reverse domains nucleated and grew at higher reverse magnetic field, and domain wall jumps across cementite lamellae were smaller than in samples with coarser pearlites. The good correlation observed between the magnetic properties, mechanical strength and microstructures of these steels provides the basis for rapid and effective non-destructive assessment of their properties for industrial applications.