Nano-sized Cementite Research Articles

Comprehensive improvement of corrosion resistance and strength of ferrite-pearlite low alloy steel by cementite spheroidization treatment

In this work, the cementite spheroidization was realized by quenching and high-temperature tempering, which improved the anti-corrosion performance and strength of ferrite-pearlite low alloy steel. The spheroidized steel was composed of lath-shaped tempered ferrite matrix and nano-sized cementite particles, and the electron backscattered diffraction (EBSD) analysis revealed that grain refinement and dislocation strengthening of tempered ferrite were the reasons for its enhanced strength. The electrochemical and weight loss tests showed that the corrosion rate of the two steels increased with time due to the accumulation of cathodic cementite, which increased the cathodic-anode area ratio and enhanced the micro-galvanic effect. Compared to the as-received steel, the spheroidized steel demonstrated a slower corrosion rate during long-term immersion, which was attributed to the weaker micro-galvanic effect caused by reduced accumulation of cathodic cementite.

Transformation of structural-phase states in rail head at extremely long-term operation

Quantitative transformations of the structure at a depth of 0, 2, 5, 10 mm along the central and symmetry axis of the fillet of long differentially quenched rails head after extremely long-term operation (passed tonnage of 1770 mln. gross tons) were identified at macro-, micro- and nanoscale levels by methods of optical, scanning and transmission electron diffraction microscopy. At a macroscale level, the numerous shallow parallel cracks of contact fatigue are detected on the surface of working fillet, and on the surface of inoperative fillet there are only small spallings. The lateral wear of the rail was 2.5 mm and the vertical wear was 2 mm. Microstructure of the rail head metal corresponds to the requirements of standards and specifications of the Russian Railways. At microscale level, the transformation of cementite plates was established by cutting it with moving dislocations and dissolving with the escape of carbon to the dislocation lines, low- and high-angle boundaries. A decrease in microstructure dispersion is noted with a distance from the tread surface. At the nanoscale level, subgrain structure formed in the surface layers (subgrain size 110 – 200 μm) contains nanosized cementite particles (25 – 60 nm) localized at the joints and along the subgrain boundaries. It is supposed that this type of structure is formed as a result of dynamic recrystallization under megaplastic deformation in the process of extremely long-term operation of rails. The content of subgrain structure in the fillet layer is five-fold higher than that in surface layer of the tread surface. It was established that during operation, the transformation of lamellar perlite along the central axis of the head proceeds more slowly than along the symmetry axis of the fillet.

A novel approach of high-voltage low-current electric energy input to synthesise cost-effective ultra-strong ductile material

ABSTRACT Adopting a unique electrical circuit design, here we treat a significant low-cost engineering material (eutectoid steel not containing costly alloying elements) with a high-voltage (100 kV) low-current (150 mA) energy input (energy level exceeding cohesive energy). A distinctive structural evolution is ascertained with treatment duration of only 5 min as an outcome of lamellar fragmentation and matrix supersaturation. This envisages an origin of dispersed nano-sized hard cementite spheroids embedded in nano-thick martensite crystals of stratified-tile-morphology along with distributed α-ferrite regions containing sub-microscopic cementite particles of various shapes. Apart from the conjoint effect of nano-scale dispersion strengthening and martensitic strengthening overhauling the effect of conventional lamellar strengthening on a gross scale; high dislocation density and systematically arranged dislocations of similar sign at incoherent cementite particle-matrix interface provides a unique combination of ultra-high strength (UTS ∼ 1.5 GPa), significantly high specific strength (188 MPa/g cm−3) and large ductility (%Elongation = 20). Therefore, in terms of the adopted synthesis route, structural evolution and mechanical property achieved, a new dimension is hereby added to the next-generation material development so as to meet the ever increasing demand for low-cost structural application. In turn, we elucidate a fundamental conceptualization for the first time which exemplifies disproportionate atomic migration at highly incoherent nano-sized cementite particle-martensite matrix interface in steel under high-voltage low-current energy input, resulting in accumulation of dislocations of similar sign so as to significantly enhance strength along with retention of substantial ductility.

Regeneration of the initial structure in white alloy pig irons by their heat treatment

The regime parameters providing the improvements of initial cast structure in a white alloy quasi-eutectic pig iron are specified for the applied heat treatment which is based on the early defined principles of microstructure «regeneration» in hetero-phase materials with polymorphic matrixes. Extremely undesirable, typical original microstructure of the iron comprising big, plate-like, «ledeburite» cementite crystals along with coarse-grained colonies of pearlite-like structures was observed. Formation of dispersed, homogeneous in size and spacial distribution sole alloy cementite crystals instead of the «ledeburite» one, within fine grained matrix containing nano-sized cementite particles is shown as a result of the optimal «regenerative» heat treatment employment. Conserving the original iron hardness on the level 60-63 HRC was observed that confirms stability of its phase composition despite the obtained structure`s morphology changes. Unified features of the structure «regeneration» processes in hetero-phase metallic materials of various chemical composition are outlined that provides reliable obtaining high mechanical performance of the materials

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.

Design of high-strength and damage-resistant carbide-free fine bainitic steels for railway crossing applications

A novel high-strength steel design is proposed, with a fine bainitic microstructure free from inter-lath carbides, for railway crossings applications. The design is based on the phase transformation theory and avoids microstructural constituents like martensite, cementite and large blocky retained austenite islands in the microstructure which are considered to be responsible for strain partitioning and damage initiation. The designed steel consists of fine bainitic ferrite, thin film austenite and a minor fraction of blocky austenite which contribute to its high strength, appreciable toughness and damage resistance. Atom probe tomography and dilatometry results are used to study the deviation of carbon partitioning in retained austenite and bainitic ferrite fractions from the T0/T0ʹ predictions. A high carbon concentration of 7.9 at.% (1.8 wt%) was measured in thin film austenite, which governs its mechanical stability. Various strengthening mechanisms such as effect of grain size, nano-sized cementite precipitation and Cottrell atmosphere at dislocations within bainitic ferrite are discussed. Mechanical properties of the designed steel are found to be superior to those of conventional steels used in railway crossings. The designed steel also offers controlled crack growth under the impact fatigue, which is the main cause of failure in crossings. In-situ testing using micro digital image correlation is carried out to study the micromechanical response of the designed microstructure. The results show uniform strain distribution with low standard deviation of 1.5% from the mean local strain value of 7.7% at 8% global strain.

Development of new-generation low-carbon steel: Part II-wear behaviour

ABSTRACTA new-generation unalloyed low-carbon steel (containing 0.1 wt.% C) has been recently developed by the research group of the present corresponding author through incomplete austenitisation-based cyclic ice-brine quenching possessing an exceptionally high strength (UTS = 1.7 GPa) along with elimination of a yield point phenomenon. This is attributed to the evolution of a novel microstructure that consists of fine plate martensite crystals with a dispersion of nano-sized cementite particles and clusters. The present research work is conceived as the Part II of this investigation to establish this new-generation ultrahigh strength low-carbon steel as a unique wear-resistant steel substituting the conventional dual-phase steel along with the readily awaited in-depth correlation between wear mechanism and structural evolution. The wear behaviour of heat-treated steels is investigated against an alumina disc using a pin-on-disc tribometer. The steel subjected to incomplete austenitisation-based cyclic ice-brine quenching exhibits much better wear resistance than conventional dual-phase steel. Dominant microcutting and microploughing abrasion aggravate wear loss, especially at higher load, in dual-phase steel that inherently possesses lower matrix hardness. But, very high-surface hardness is attained in the incomplete austenitisation-based cyclic ice-brine quenched steel by virtue of a significant strain hardening of martensite matrix in between hard nano-sized cementite particles. Besides, the wear rate is not allowed to shoot up even at the highest load through the generation of hard abrasion-resistant tribo-oxide layer of Al2FeO4. This envisages an advent of novel wear-resistant steel as a better substitution for the dual-phase steel.

Attainment of an exceptionally high strength in low-carbon steel along with modest ductility through a novel heat treatment route

ABSTRACTOver the last few decades, the use of steel (the most significant structural engineering material) is facing a significant challenge due to its replacement by other materials (such as composites) possessing higher strength-to-weight ratio/specific strength. This necessitates further enhancement in the strength of steel. In particular, low-carbon steel, in the annealed condition, suffers from inherent problems of poor strength and discontinuous yielding. In this research work a novel heat treatment route of incomplete austenitisation-based cyclic ice-brine quenching has been adopted on low-carbon steel (AISI 1010 steel containing 0.1 wt.% C) without considering any costly alloying or thermo-mechanical treatment. Accordingly, exceptionally high strength (UTS = 1.7 GPa) and specific strength (226 MPa g−1cm3) are achieved after three cycles along with a modest ductility (% Elongation = 9). This is the highest strength reported so far for low-carbon steel containing 0.1 wt.% C. Yield point phenomenon is also eliminated. This is attributed to a novel microstructure consisting of highly sub-structured fine plate martensite crystals containing internal twin and dislocation tangles along with dispersion of nano-sized cementite particles and clusters of cementite particles.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

Effect of Combined Cyclic Heat Treatment on AISI 1080 Steel: Part II-Mechanical Property Evaluation

A typical plain carbon eutectoid steel (without addition of any significant alloying element) is developed possessing notably high strength (UTS = 2016 MPa) and reasonable ductility (%Elongation = 11) with an application of typical cyclic heat treatment that involves a combination of reconstructive and displacive transformations in successive two heat treatment cycles. Apart from fine clusters and spheroids (less than 1 μm in size), the dispersion of nano-sized cementite particles in a martensitic matrix contributes to significant strengthening of steel. The strength enhancement is much higher than the cyclic heat treatment processes involving only reconstructive transformation or displacive transformation. Accordingly, such a combined cyclic heat treatment paves a new way for high strength plain carbon steel development.

A transmission Kikuchi diffraction study of cementite in a quenched and tempered steel

This is the first transmission Kikuchi diffraction (TKD) study to report the indexing of nano-sized cementite as distinct structures and its orientation relationship with the body-centered cubic matrix in a quenched and tempered steel. Crystallographic analysis via TKD and selected area diffraction returned the well-known Bagaryatskii and Isaichev orientation relationships. However, the indexing of nano-sized cementite via TKD was sensitive to the thickness of the electron transparent region such that TEM remains the most precise method to characterise such precipitates.

Ultrafine-Grained HSLA Steel Processed Using MAF: Dry Sliding Wear and Corrosion Behaviour

As ultrafine grained (UFG) steels obtained by severe plastic deformation (SPD) are useful for structural purposes and micro devices, it is necessary to establish the relation of microstructure with wear and corrosion properties. Very few studies are available that correlate the microstructural changes and the mechanical properties of UFG steels to the degradation phenomena such as wear and corrosion. In present work a HSLA steel was severely deformed by warm (500°C) multiaxial forging (MAF) technique using up to nine strain steps. Submicron sized grain size was obtained after warm MAF. The hardness and strength improved significantly. The MAF processed UFG HSLA steel did not show any improvement in wear resistance. Low pull-off-work and fragmented nanosized pearlitic cementite particles were the two factors found responsible for it. The corrosion resistance of HSLA steel after warm MAF remained largely unaffected.

Strain-induced refinement in a steel with spheroidal cementite subjected to surface mechanical attrition treatment

Steel with spheroidal cementite particles dispersed in a ferrite matrix was subjected to surface mechanical attrition treatment. Strain-induced microstructure evolution, including grain refinement of ferrite and cementite as well as dissolution of cementite, was examined using transmission electron microscopy. Upon straining, dense dislocation walls were developed in ferrite grains, which evolved gradually into sub-boundaries and highly misoriented grain boundaries (GB) at increasing strains, leading to grain refinement of the ferrite. The ferrite refinement process is greatly facilitated by the presence of dispersed cementite particles, as the cementite/ferrite interfaces are effective nucleation sites for dislocations and also barriers for dislocation motions. When ferrite grains are refined to sizes smaller than that of cementite particles, plastic deformation occurs in cementite particles, characterized by edge dislocation gliding along the {110} and {100} planes. Dislocation gliding in cementite is initiated at intersections of the cementite/ferrite interfaces and ferrite GB. Accumulated multiple gliding progressively refines cementite into nano-sized cementite particles mixed within ferrite nano-grains in the top surface layer. A decreasing volume fraction of cementite is observed with decreasing depth in the top deformed layer, indicative of dissolution of cementite induced by increased plastic straining.

Grain growth and Hall–Petch relation in dual-sized ferrite/cementite steel with nano-sized cementite particles in a heterogeneous and dense distribution

Characteristic grain growth Occurred coincident with the Ostwald ripening of cementite particles upon annealing in a submicron-grained ferrite/cementite steel. The predictive capability of the Hall-Petch relation between the hardness and average ferrite size was demonstrated, independent of the distribution and size of cementite particles. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ultrafine grained steels processed by equal channel angular pressing

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

Formation of fine cementite precipitates in an ultra-fine grained low carbon steel

Low-carbon steel containing 0.06 wt.% vanadium was deformed by a severe plastic deformation technique and annealed in a temperature range from 420 to 600 °C. The treatment resulted in a formation of nano-sized cementite precipitates as well as refinement of ferrite grains to a submicrometer size. This phenomenon is discussed based on dislocation-cementite interactions.

WITHDRAWN: Effect of pressing temperature on microstructure and tensile behavior of low carbon steels processed by equal channel angular pressing

Abstract Two grades of low carbon steels, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing at a temperature range of 623–873 K. For steel without vanadium, the equal channel angular pressing at 623 K resulted in ultrafine (∼0.3 μm) ferrite grains with high angle boundaries. At higher pressing temperature, coarser grains with low angle boundaries were formed, which indicates that the recovery occurs at a significant rate during the pressing. For the steel containing vanadium, submicrometer order ferrite grains and high dislocation density were preserved up to pressing temperature of 873 K. The enhanced thermal stability of the steel containing vanadium was attributed to its peculiar microstructure consisted with fine ferrite grains with uniformly distributed nano-sized cementite particles. In addition, the tensile behaviors of the equal channel angular pressed steels were characterized and discussed based on the microstructure.

Effect of pressing temperature on microstructure and tensile behavior of low carbon steels processed by equal channel angular pressing

Two grades of low carbon steels, one containing vanadium and the other without vanadium, were subjected to equal channel angular pressing at a temperature range of 623–873 K. For steel without vanadium, the equal channel angular pressing at 623 K resulted in ultrafine (∼0.3 μm) ferrite grains with high angle boundaries. At higher pressing temperature, coarser grains with low angle boundaries were formed, which indicates that the recovery occurs at a significant rate during the pressing. For the steel containing vanadium, submicrometer order ferrite grains and high dislocation density were preserved up to pressing temperature of 873 K. The enhanced thermal stability of the steel containing vanadium was attributed to its peculiar microstructure consisted with fine ferrite grains with uniformly distributed nano-sized cementite particles. In addition, the tensile behaviors of the equal channel angular pressed steels were characterized and discussed based on the microstructure.

Ultrafine Grained Low Carbon Steels Fabricated by Equal Channel Angular Pressing: Microstructures and Tensile Properties.

Equal channel angular pressing (ECAP) was conducted on the two grades of low carbon steel, with or without vanadium, in order to produce an ultarfine grained structure. As a result, the ferrite grains were re-fined from 30 µm to 0.2–0.3 µm. The strength of the ECAPed steels increased remarkably, over twice of the strength of the steels before ECAP. A series of static annealing experiments showed that the increment of ECAP strain and the dilute addition of microalloying element such as vanadium were very effective on enhancing thermal stability of the ultrafine gained low carbon steels produced by ECAP in terms of microstructure and tensile properties. This enhanced thermal stability resulted from; (a) presence of excessive carbon content in the ferrite matrix by carbon dissolution from pearlitic cementite during ECAP; (b) preservation of high dislocation density due to addition of vanadium, providing the effective diffusion path for dissolved carbon atoms; (c) precipitation of excessive carbon as the form of nano-sized cementite particles during subsequent annealing and its effect on suppressing grain growth.