Pearlite Colonies Research Articles

Hydrogen-assisted cracking paths in oriented pearlitic microstructures: Resembling Donatello Wooden Sculpture Texture (DWST) & Mantegna’s Dead Christ Perspective (MDCP)

Progressive cold drawing in eutectoid steel produces a preferential orientation of pearlitic colonies and ferrite/cementite lamellae, thus inducing strength anisotropy in the steel and mixed mode propagation. While in the hot rolled steel (not cold drawn) the pearlitic microstructure is randomly oriented and the crack progresses in hydrogen by breaking the ferrite/cementite lamellae, in heavily drawn steels the pearlitic microstructure is fully oriented and the predominant mechanism of hydrogen assisted cracking is the delamination (or decohesion) at the ferrite/cementite interface.

Hierarchical microstructure evolution in cold drawn pearlitic steels: In the conceptual frame of Fray Luis de León, Miguel de Cervantes, Victor Vasarely, Maurits Cornelis Escher & Johann Sebastian Bach

Cold drawn pearlitic steels posses a hierarchical microstructure consisting of pearlitic colonies (first microstructural level) and pearlite (ferrite/Fe & cementite/Fe3C) lamellae (second microstructural level) that evolves during the manufacturing process by cold drawing towards a preferential orientation aligned in the drawing (wire axis) direction, so that these materials acquire microstructural anisotropy that influences their posterior fracture behaviour. The paper establishes an analogy with the literature of Spanish writers Fray Luis de León and Miguel de Cervantes (through the alternate distribution of ferrite/cementite lamellae), as well as with the painters Maurits Cornelis Escher and Victor Vasarely (through the multi-level organization of their paintings) and the composer Johann Sebastian Bach (through the hierarchical structure of his music).

Unconventional pearlitic pseudocolonies affecting macro-, micro- and nano-structural integrity of cold-drawn pearlitic steel wires: Resembling van Gogh, Bernini, Mantegna and Picasso

Prestressing steel wires are manufactured by cold drawing during which a preferential orientation is achieved in the matter of pearlitic colonies and lamellae. In addition to this general trend, special (unconventional) pearlitic pseudocolonies evolve during the heavy-drawing manufacture process affecting the posterior macro-, micro- and nano-structural integrity of the material. This paper discusses the important role of such a special microstructural unit (the pearlitic pseudocolony) in the fracture process in air (inert) environment in the presence of crack-like defects, as well as in the case of environmentally assisted cracking (stress corrosion cracking by localized anodic dissolution) or hydrogen embrittlement. Results clearly demonstrate the key role of pearlitic pseudocolonies in promoting crack deflection (and thus mixed-mode propagation) after a global mode I cracking, especially in the case of fracture in air and stress corrosion cracking.

Anisotropy of hydrogen embrittlement in cold drawn pearlitic steel: A tribute to Mantegna

Progressive cold drawing in eutectoid steel produces a preferential orientation of pearlitic colonies and ferrite/cementite lamellae, thus inducing strength anisotropy in the steel and mixed mode propagation. While in the hot rolled steel (not cold drawn) the pearlitic microstructure is randomly oriented and the crack progresses in hydrogen by breaking the ferrite/cementite lamellae, in heavily drawn steels the pearlitic microstructure is fully oriented and the predominant mechanism of hydrogen assisted cracking is the delamination (or decohesion) at the ferrite/cementite interface. A link is established between the oriented microstructure of the heavily cold drawn pearlitic steels wires and their anisotropy of hydrogen embrittlement behaviour associated with hydrogen assisted cracking path deflection with angle close to 90º, thereby resembling the Mantegna’s Dead Christ perspective.

Identification of a new microstructural unit in cold drawn pearlitic steel: The pearlitic pseudocolony

This paper provides a definition and identification of a new (non-conventional) microstructural unit in cold drawn pearlitic steel: the pearlitic pseudocolony, a special pearlitic colony in which the lamellae are not oriented along the wire axis or cold drawing direction (contrarily to what happens in the normal conventional colonies), thereby producing a clearly anomalous (very high) local interlamellar spacing, making the pseudocolonies weakest units or potential fracture initiation loci and promoting crack deflection associated with anisotropic fracture behavior and mixed-mode propagation

Fatigue & fracture crack paths generated by manufacturing-induced microstructural & strength anisotropy in cold drawn pearlitic steels: (a) In the conceptual framework of Maurits Cornelis Escher and Johann Sebastian Bach; (b) An Orteguian approach as well as a heartfelt tribute to Fray Luis de León’s “decíamos ayer”

This paper analyses both fatigue & fracture crack paths generated by manufacturing-induced microstructural & strength anisotropy in cold drawn pearlitic steels, on the basis of their microstructural evolution during manufacturing by multi-step cold drawing that produces slenderizing and orientation of the pearlitic colonies, together with densification and orientation of the Fe/Fe3C lamellae, reviewing previous research by the author and co-workers on fracture behaviour in inert and aggressive environments in the presence of crack and notches, with focus on hydrogen embrittlement. Results demonstrate the key role of manufacturing-induced microstructural anisotropy (orientation of the two microstructural levels of pearlitic colonies and ferrite/cementite lamellae as a consequence of the progressive/repetitive cold drawing) in the fatigue & fracture crack paths, thus producing crack path deflection/deviation/branching with mixed-mode crack growth and associated anisotropy of fatigue & fracture resistance and its linked anisotropic fatigue & fracture behaviour (strength anisotropy), allowing the definition of a directional toughness depending on the specific crack path with its propagation direction. From the point of view of the analysed material (progressively cold drawn pearlitic steel, a hierarchically structured material with colonies and lamellae), the present scientific work is related to the conceptual frame of Maurits Cornelis Escher and Johann Sebastian Bach. In addition, the paper represents an Orteguian approach (on the basis of the circumstance formulated by José Ortega y Gasset) to the problem of fatigue & fracture of progressively cold drawn pearlitic steel and a tribute to Fray Luis de León´s “decíamos ayer”.

Study on the Nucleation and Growth of Pearlite Colony and Impact Toughness of Eutectoid Steel

The relationship between microstructure parameters and mechanical properties was studied in this paper. The steel was heat-treated at different austenitizing temperatures to acquire varying microstructure. The results showed that austenite grain size increases with austenitizing temperature, while the pearlite colony size was relatively constant. The strength followed a Hall–Petch relationship with the austenite grain size, but the austenite grain size has nothing to do with the impact toughness. The control unit for determining the impact toughness of pearlitic steel is the pearlite colony size using a comparison method. Further studies have found that, in the hypoeutectoid steel and hypereutectoid steel, the pearlite colony size changes with the austenitizing temperature. However, when the eutectoid steel with a carbon content of 0.81% undergoes the isothermal transformation, the number of grain boundary precipitates is very few. There are many nucleation sites at the grain boundary. The pearlite colonies randomly nucleate at the grain boundaries and grow into the interior of the grains. Simultaneously, new pearlite colonies nucleate by the side of the existing pearlite colony. The intragranular pearlite colonies are also randomly nucleated. These nucleation sites increase the chance of the growing pearlite colonies colliding with each other, eventually resulting in a constant pearlite colony size.

Effect of cooling mode on the microstructure and mechanical properties of medium carbon steel after warm rolling

ABSTRACT The effects of cooling modes (air, furnace and water) on the variation of microstructure and mechanical properties of medium carbon steel were investigated. SEM, TEM and EBSD techniques were used to observe the microstructure. The observed results show that the effect of cooling mode on the grain size is insignificant. The average sizes of ferrite grain obtained under different cooling modes were little different. Moreover, in all the cases, most of the cementite lamellas were transformed into cementite particles, except for some of the residual cementite lamellas which were present on the original pearlite colonies. It has also been found that the ferrite substructure and precipitation of cementite particles are significantly affected by the cooling modes. Compared to other cooling modes, the furnace cooling is beneficial to the homogeneity of microstructure. The particles are distributed on the ferrite matrix more homogeneously after furnace cooling which results in a better combination of strength and ductility.

Effect of Soaking Time on Evolution of Microstructure and Hardness during Annealing of EN-47 Spring Steel

The study presents the results of the examination on the effect of soaking time on the Microstructure, Grain shape and hardness properties of annealed EN-47 Spring Steel. The EN-47 Spring Steel samples were heated at 860 °C for soaking times of 60, 120, 240, 480 minutes. Optical Microscopy and Rockwell Hardness were carried out to determine the morphology, grain shape and hardness of the material with increasing soaking time. EN-47 Spring Steel samples showed up a decreased value in hardness with the extent of soaking time. Pearlite colonies expand with the increase in soaking time. The linear relationship between the soaking time and other factors were also observed. Keywords: EN-47, Annealing, Microstructure, Rockwell Hardness.

Failure Analysis of Superheater Boiler Tube SA 213 T12

Superheater tubes SA213 T12 (OD = 57 mm, WT = 6 mm) has been in service for 11 years in a supercritical coal fired boiler (315 MW). Tubes operating temperature are 430°C and pressure is 11 MPa. Failure occurred on tube no. 20, row 2. Three tubes specimens from the same row (failed tube, tube no. 3, and no. 4), were drawn for laboratory examinations. Vickers microhardnesses of failed and no. 3 tubes are the same (140 VHN), while tube no. 4 hardness is 170 VHN. These hardness values are much lower compared to the new tube (192 VHN). The lower hardness value of failed tube is due to complete decomposition of pearlite colonies in inner and outer side of failed tube to ferrite and spheroidized cementite. Failure of tube is characterized by thin lip rupture which indicated that the tube circumferential deformation occur gradually following spheroidization process. Failure by intergranular crack occurred along grain bondaries when the stress on the tube wall (hoop stress) exceeding material strength at service temperature. Tube no. 3 and 4 are partially spheroidized, mostly on the innerside. The severity of microstructure damage is related to the flow direction of flue gas, from tube no. 1 to no.2, no.3 and so on. Tube no. 1 was survived because it was protected by a front cover that will limit heat transfer from the flue gas. The previous failure analysis of similar boiler tube revealed that the hardness of the bulging tube is about 140 VHN which is the same as the hardness of the failed tube (no. 2) and no.3. Tube no. 3 is in the early stage of damage and shall be replaced because the hardness is much lower than 170 VHN. The effect of flue gas flow direction not only affects the hardness and microstructure but also deposit thickness. Deposit thickness in tube no. 2, 3, and 4 are 149, 123, and 100 µm respectively. Thicker deposit in tube no. 2 is also contributed to premature failure of this tube compared to the others. The local temperature of tube wall will be higher because thick deposit will reduce heat transfer between flue gas and steam inside the tube.

Study of partition to non-partition transition of austenite growth along pearlite lamellae in near-eutectoid Fe-C-Mn alloy

The austenite formation from lamellar pearlite was studied in a near-eutectoid Fe-0.6mass%C-2mass%Mn alloy with focus on the transition between partitioned and non-partitioned growth of austenite. The austenite was nucleated at pearlite colony boundaries and grew along the lamellae of pearlite. The isothermal austenite transformation curves revealed that the transition from slow to fast growth of austenite occurred over a narrow temperature range (720–725 °C). The ghost pearlite which had formed above the transition temperature transformed to a very fine mixture of martensite (previously pearlitic ferrite) and austenite (pearlitic cementite) upon cooling to room temperature. According to thermodynamic analysis which takes into account carbon transport from θ/α and θ/γ interfaces toward γ/α interfaces the transition temperature appears to be closer to experiment and significantly (ca. 20 °C) lower than that previously proposed assuming that the austenite was nucleated at the θ/α interface. Phase field simulation of the carbon and Mn distributions around the tip of a cementite lamella also seems to indicate the change of diffusivity which plays a leading role in the growth of austenite.

Effect of increased nitrogen content on continuous cooling transition of γ→α in hot-deformed low-C Mo–V–Ti steels

The two-stage controlled rolling and cooling at 0.5–50 °C/s of low-carbon Mo–V–Ti steels with the increasing nitrogen content from 0.0032 to 0.0081 and 0.0123 wt.% were simulated through a Gleeble 3500 system. The continuous cooling transition (CCT) of γ→α in each steel was estimated via microstructure characterization and CCT diagram. The results indicated that CCT diagram for each steel was divided into three regions of γ→ferrite, γ→pearlite and γ→bainite, and the increasing N content elevated all the starting temperatures for γ→α. Consequently, the polygonal ferrite (PF) and pearlite formed in each steel cooled at 1 °C/s and, however, the increasing N content led to slightly coarser ferrite grain and pearlite colony. With the increased cooling rate to 10 and 30 °C/s, a mixed microstructure of acicular ferrite (AF), granular bainite (GB) and lath bainite (LB) formed in 32N steel and in contrast, the mixture of PF+AF+GB in 81N and 123N steels. The increasing N content promoted (Ti,V)(C,N) precipitation, enhanced the intragranular PF/AF nucleation, increased martensite/austenite constituent and depressed LB. In addition, the mechanisms dominating the effect of increasing N on this CCT of γ→α were discussed.

Corrosion Behavior of Annealed Steels with Different Carbon Contents (0.002, 0.17, 0.43 and 0.7% C) in Freely Aerated 3.5% NaCl Solution

Four annealed carbon steels with different carbon contents (0.002, 0.17, 0.43, and 0.7% C), consisting of ferrite, ferrite-pearlite and fully pearlite microstructures, were selected to understand the corrosion behavior of carbon steel as a function of carbon content in freely aerated 3.5% NaCl solution. Dynamic polarization, electrochemical impedance and linear polarization methods were used. The corrosion rate obtained from the different carbon steels was found to increase greatly from ultra-low carbon steel (0.002%) to low carbon steel (0.17%) due to the presence of pearlite in the low carbon steel. However, the increase in corrosion rate was marginal with the increase in carbon content from low carbon (0.17%) to medium (0.43% C) and high carbon steels (0.7% C). The mirostrucural evolution of the steels before and after polarization test without etching as observed by scanning electron microscopy could show that the corrosion behavior of the steels with the presence of pearlite was due to the combined effect from % pearlite, interlamellar spacing and cementite/ferrite area ratio in pearlite. Pearlite morphology also led to the differential corrosion within the pearlite colony in all the steels except the steel with 0.002% C. Catalytic activity of cementite on enhancing oxygen reduction reaction attributes to the higher corrosion rates in case of the steels with the presence of pearlite.

Cleavage fracture and microstructural effects on the toughness of a medium carbon pearlitic steel for high-speed railway wheel

The fracture behavior and microstructural effects on the toughness of a medium carbon wheel steel were systematically investigated by using instrumented impact tests. The microstructure of the steel consisted of lamellar pearlite and a small amount of proeutectoid ferrite. The results showed that with the refinement of prior austenite grain size, the pearlite block size decreased and the ferrite content increased, while the pearlite interlamellar spacing kept almost unchanged. The impact toughness was improved in the temperature range from −80 °C to room temperature. Observation of fracture surfaces revealed that the fracture occurred in a brittle manner, which was characterized by cleavage and a small proportion of ductile fracture. However, the toughness was positively correlated to the ductile fracture area. Cleavage cracks were found to be initiated at the sites of proeutectoid ferrite, pearlite colony or pearlite/ferrite interface. The values of macroscopic cleavage fracture stress measured were similar (~1833 MPa) for the specimens with different pearlite block size and ferrite content but the same pearlite interlamellar spacing. The toughness improvement by microstructural refinement in the transition temperature region was attributed to the increase in cleavage crack initiation energy. The cleavage crack propagation energy was, however, relatively low and mainly unchanged. With decreasing the test temperature, the difference in crack initiation energy was decreased, causing the difference in total impact absorbed energy to decrease. The results also suggested that in the lower shelf temperature region where complete cleavage fracture occurred, the toughness was insensitive to the microstructural parameters.

Fabrication of dissimilar Fe-Cu-C powder metallurgy compact/steel joint using the optimized resistance spot welding

The main parameters of resistance spot welding were optimized using Taguchi method for dissimilar joining of low carbon steel and Fe-Cu-C powder metallurgy compact. In order of importance, preheat current, preheat time, hold time, welding current and welding time affect the strength of weld. Different constituents such as lath martensite, ferrite, pearlite and carbide colonies were observed in the microstructure of welds. The maximum hardness and strength of joints was 735 HV and 5.75 kN, respectively. The presence of hard zone in the fusion zone improved strength values and encouraged the pullout failure mode. The width of hard zone was increased by application of high preheat current and low welding current.

Improvement of wear resistance in a pearlitic rail steel via quenching and partitioning processing

Improvement of wear resistance and mechanical performance of rails used in heavy-haul railway are essential to reduce railroad maintenance costs. A novel heat treatment based on quenching and partitioning (Q&P) processing was proposed to improve the wear resistance of a hypereutectoid pearlitic rail. 50% of austenite was transformed into martensite under an interrupted quenching from full austenitization temperature to 140 °C. A multiphase microstructure resulted from the quenching and partitioning process, consisting of tempered martensite, bainite, retained austenite, and pearlite colonies. The partitioning step was performed in the range of 350–650 °C. Microstructure characteristics were investigated using scanning electron microscopy, microhardness measurements, X-ray and electron backscattered diffraction. Uniaxial tensile and pin-on-disc tests were also performed to evaluate the mechanical properties and wear resistance. The best combination of wear resistance and mechanical performance was obtained in samples partitioned at 450 and 550 °C, which may be applied in the railway industries.

Effect of Microstructure and Microtexture on the Mechanical Properties of Small-Strain Cold-Drawn Pearlite Steel Wires

The effect of microstructure and microtexture on the mechanical properties of small-strain (e < 0.8) cold-drawn pearlitic steel wires was investigated by scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. A quantitative statistical analysis of the measured parameters of interlamellar spacing indicated a slight decrease during the testing and a relatively high deviation from the calculated values. Calculations on the dislocation density increased from approximately $$ 4.25\; \times \; 10^{14} \;{\text{m}}^{ - 2} $$ at e = 0 to approximately $$ 4.33 \times 10^{15} \;{\text{m}}^{ - 2} $$ and then to $$ 5.81 \times 10^{15} \;{\text{m}}^{ - 2} $$ with increasing cold-drawing strain. As calculated using the misorientation angle across the dislocation boundaries and the dislocation boundary area per unit volume, the dislocation density generally increased consistently. Under these conditions, a microstructure tends to form through the deflection of pearlitic colonies, consequently affecting the crack propagation path and improving the yield and tensile strengths.

Application of the Mixed-Mode Model for Numerical Simulation of Pearlitic Transformation

The multiscale model of pearlitic transformation, including the finite element solution of the second Fick’s equation with moving boundary in the microscale and Fourier equation in the macroscale, is presented in this paper. According to the mixed-mode approach, both the volume diffusion and the interface mobility were considered. Model describes sidewise and frontal growth of cementite and ferrite plates in a single grain of the austenite. Assessment of the possibility of the developed model application to determine the key parameters in terms of strength properties of pearlitic steel, i.e., pearlite grain size and colony size as well as interlamellar spacing for various cooling conditions, was the main aim of this work. The model was validated and verified on the basis of experimental tests performed for two eutectoid steels. In practice, developed model can support design process for a technology of high-strength rods and long products manufacturing. Rails were selected as a case study in this work, and therefore, numerical simulations of accelerated cooling of the rail head were performed and the relationship between the parameters of heat treatment, parameters of the structure and properties of the finished product was determined on the basis of obtained results.

Effectiveness of an intercritical heat-treatment on localized corrosion resistance at the microstructural boundaries of medium-carbon steels

The localized corrosion resistance at the microstructural boundaries of AISI 1045 carbon steels was accessed. Under KFM measurements, regions with low corrosion potentials were found at the pearlite colony boundary in the ferrite-pearlite structure and the prior austenite grain boundary in the martensite structure, and phosphorus was segregated in these regions. The localized corrosion resistance at these boundaries was improved via intercritical heat-treatment at 1023 K (the two-phase region of austenite and ferrite) after austenitizing, which may be attributable to the apparently homogenous dissolution of ferrite forming elements, such as phosphorus, in the ferrite phase during the treatment.

Analysis of Fractured Weldment of Hadfield Steel

This contribution is divided into two sections. The first one deals with identifying the cause of fracture in a ring, which fractured after weld surfacing. Optical and scanning electron microscopy and hardness testing were employed. Chemical composition was determined using the GDOES and EDX methods. Although weld surfacing has led to fracture in the base material, the actual problem was in the inadequate microstructure of the base material. Microscopic analysis revealed that it was heterogeneous. It contained distinctive segregation bands with bands of complex carbides and pearlite colonies, which are undesirable. The microstructure of this steel should be fully austenitic. In order to remove the heterogeneities, solution annealing was proposed and carried out successfully. The second section of this paper explores the options for revealing the microstructure of Hadfield steel. Etching reagents from the literature and authors' experience were put to test.