Showing papers on "Pearlite published in 2019"

Microstructural evolution and mechanical properties of a low-carbon low-alloy steel produced by wire arc additive manufacturing

Abstract: The emerging technology of wire arc additive manufacturing (WAAM) has been enthusiastically embraced in recent years mainly by the welding community to fabricate various grades of structural materials. In this study, ER70S-6 low-carbon low-alloy steel wall was manufactured by WAAM method, utilizing a gas metal arc welding (GMAW) torch translated by a six-axis robotic arm, and employing advanced surface tension transfer (STT) mode. The dominant microstructure of the fabricated part contained randomly oriented fine polygonal ferrite and a low-volume fraction of lamellar pearlite as the primary micro-constituents. Additionally, a small content of bainite and acicular ferrite were also detected along the melt-pool boundaries, where the material undergoes a faster cooling rate during solidification in comparison with the center of the melt pool. Mechanical properties of the part, studied at different orientations relative to the building direction, revealed a comparable tensile strength along the deposition (horizontal) direction and the building (vertical) direction of the fabricated part (~ 400 MPa and ~ 500 MPa for the yield and ultimate tensile strengths, respectively). However, the obtained plastic tensile strain at failure along the horizontal direction was nearly three times higher than that of the vertical direction, implying some extent of anisotropy in ductility. The reduced ductility of the part along the building direction was associated with the higher density of the interpass regions and the melt-pool boundaries in the vertical direction, containing heat-affected zones with coarser grain structure, brittle martensite–austenite constituent, and possibly a higher density of discontinuities.

Comparative Corrosion Behavior of Five Microstructures (Pearlite, Bainite, Spheroidized, Martensite, and Tempered Martensite) Made from a High Carbon Steel

Abstract: The present work discusses the comparative corrosion behavior of five microstructures of steels, namely, pearlite, bainite, spheroidized, martensite, and tempered martensite, which have been processed, respectively, by air cooling, isothermal transformation, spheroidizing, quenching, and tempering of a steel with composition 0.70C, 0.24Si, 1.12Mn, 0.026P, 0.021S, 0.013Nb, 0.0725Ta, and 97.7Fe (all are in wt pct). Dynamic polarization and alternating current (AC) impedance spectroscopic tests in freely aerated 3.5 pct NaCl solution show that the corrosion resistance of the steel specimens consisting of the preceding five microstructures decreases in the following sequence: pearlitic – bainitic – spheroidized – martensitic – tempered martensitic steels. The variation in the corrosion rate has been attributed to the shape, size, and distribution of the ferrite and cementite.

Microstructure and properties of NiCrBSi coating by plasma cladding on gray cast iron

Abstract: Plasma cladding has been widely applied in surface modification and repairing. In this study, the NiCrBSi coating was prepared on gray cast iron by plasma cladding. Scanning electron microscopy (SEM), energy dispersive microanalysis (EDS), X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to identify the microstructure and phase types of the cladded layer, the bonding zone and the heat affected zone. The microhardness, elastic modulus, tensile performance and wear performance of the cladded layer were analyzed by the microhardness tester, nanoindentation, tensile test and wear test. The results show that typical microstructure of the cladded layer is dendrites of γ-(Fe, Ni), with interdendritic phases rich in B(Fe, Si)3, Cr15.58Fe7.4C6 and Cr7C3. The bonding zone mainly consists of martensite, retained austenite, graphite, pearlite and a small amount of carbide network, while the heat affected zone is mainly a mixed microstructure of martensite, retained austenite and graphite. The Ni-based coating presents a significantly superior microhardness, elastic modulus and tensile strength and wear performance resistance than those of the gray cast iron. In addition, elastic modulus test and tensile test show that the interface between the cladded layer and the substrate has good metallurgical bonding characteristics.

Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing

Abstract: Metal Big Area Additive Manufacturing (MBAAM) is a novel wire-arc additive manufacturing method that uses a correction-based approach developed at the Oak Ridge National Laboratory (ORNL). This approach is an integrated software method that minimizes the dynamic nature of welding and compensates for build height. The MBAAM process is used to fabricate simple geometry thin walled specimens, using a C-Mn steel weld wire, to investigate the scatter in mechanical properties and correlate them to the underlying microstructure. The uni-axial tensile tests show isotropic tensile and yield properties with respect to building directions, although some scatter in elongation is observed. Large scatter is observed in the Charpy Impact tests. The microstructure characterization reveals mostly homogenous ferrite grains with some pearlite, except for some changes in morphology and grain size at the interface between the build and the base plate. The measured properties and microstructure are compared with the toughness and strength values reported in the literature, and a hypothesis is developed to rationalize the differences. Overall, the MBAAM process creates stable, isotropic, and weld-like mechanical properties in the deposit, while achieving a precise geometry obtained through a real-time feedback sensing, closed loop control system.

Fracture mechanisms and microstructure in a medium Mn quenching and partitioning steel exhibiting macrosegregation

Abstract: A medium-Mn steel, exhibiting manganese macrosegregation, was investigated. In order to study how the microstructure development influences the fracture mechanisms, the steel was quenching and partitioning processed using two different partitioning temperatures. At 400 °C partitioning temperature, the microstructure exhibits intergranular fracture at low plastic strain, following Mn-rich regions in which fresh martensite predominates. Elongated thin precipitates at prior austenite grain boundaries facilitate the initiation and progress of cracks at these locations. After partitioning at 500 °C, the redistribution of carbon triggers the formation of pearlite, the precipitation of carbides in the carbon-enriched austenite and the formation of spheroidal carbides at prior austenite grain boundaries. All these microstructural features result in an interlath fracture with more ductile character than after partitioning at 400 °C. In both cases, manganese macrosegregation triggers brittle fracture mechanisms by creating large hardness gradients.

A comparison of microstructure and mechanical properties of laser cladding and laser-induction hybrid cladding coatings on full-scale rail

Abstract: With the rapid development of high-speed and heavy-haul trains, the surface damages of rails are becoming more and more severe, and how to promote the surface strength of the rail and prolong its service life with high efficiency are becoming extremely important. Laser cladding (LC), with small heat affected zone (HAZ) and low dilution, is a promising novel way to hardface and repair the rail. However, there are two great barriers for the traditional LC to apply on full-scale rails: one is how to prevent the coating from cracking under the rapid heating and cooling cycle; the other is how to eliminate the martensite structure in HAZ, which may threaten the safety of railway transportation due to its high hardness and low fracture toughness and usually be forbidden in almost all the Railway Standards over the world. In this paper, laser-induction hybrid cladding (LIHC) was innovatively proposed to deposit Ni-based coatings on a full-scale rail. The cracking behaviors, microstructures and mechanical properties of the coatings and HAZs by LC, LIHC with induction pre-heating (pre-LIHC) and LIHC with induction post-heating (post-LIHC) were studied systemically. The results indicate that the cracking and martensite transformation occurred in the HAZ can only be prevented by post-LIHC, where fine pearlite with smaller pearlite block size and lower interlamellar spacing formed instead. Therefore, the abrupt change of microstructure and mechanical properties in the HAZ could be avoided by post-LIHC, and the hardness, strength and toughness of the rails can be improved significantly. The post-LIHC technology shows the potentiality to hardface and repair the full-scale rail.

Effects of cooling rate on microstructure, mechanical properties, and residual stress of Fe-2.1B (wt%) alloy

Abstract: The continuous cooling transformation (CCT) curve of an Fe-2.1B (wt%) alloy is obtained using a Gleeble 1500D thermomechanical simulator. The microstructure, mechanical properties, and residual stress of alloy specimens with various cooling rates are examined. The results reveal that the cooling rate has a great influence on the matrix microstructure of the Fe-2.1B (wt%) alloy. Pearlite is formed at the cooling rate of 0.1 K/s, pearlite and martensite are formed in the cooling rate range of 0.2–0.5 K/s, and only martensite remains in the matrix when the cooling rate exceeds 0.5 K/s. In addition, as the cooling rate increases, the dislocation density in the matrix increases, and this, in turn, leads to an increase in the volume fraction of the M23(B, C)6 phase. The precipitation of M23(B, C)6 causes the decrease in the (B + C) contents of the matrix, which, in turn, reduces the microhardness of the matrix to some extent. Meanwhile, the large residual compressive stress of the alloy increases with increasing cooling rate. The maximum residual compressive stresses induced by the cooling rates of 0.5 and 30 K/s are approximately 18% and 36%, respectively, higher than that induced by the cooling rate of 0.1 K/s. Moreover, when the cooling rate increases from 0.1 K/s to 0.5 K/s, the macrohardness and bending stress increase significantly owing to an increase in Vm/Vpr (where Vm represents volume fraction of martensite, Vpr is the volume fractions of pearlite and austenite). When the cooling rate exceeds 0.5 K/s, the macrohardness and bending stress decrease gradually because of the decrease in the (B + C) contents of the matrix and an increase in the residual stress. The critical cooling rate (0.5 K/s) may be the optimal cooling rate of Fe-B alloys.

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

Abstract: 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.

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

Abstract: 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.

Influence of the decomposition behavior of retained austenite during tempering on the mechanical properties of 2.25Cr-1Mo-0.25 V steel

Abstract: The as-quenched microstructure of 2.25Cr-1Mo-0.25 V steel heavy forgings is granular bainite, which is composed of bainitic ferrite and blocky islands of martensite and retained austenite (RA). In this study, the characteristics of RA decomposition and its effects on the mechanical properties of the steel are investigated. The results show that RA decomposes into a cluster of coarse M23C6 carbides and ferrite during standard tempering at 700 °C. These coarse carbides decorate the boundary of the cluster, thus deteriorating the impact toughness of the steel. Accordingly, the size and distribution of these carbides are tentatively modified by introducing pre-tempering at different temperatures ranging from 180° to 650°C before the standard tempering at 700 °C. This is because during pre-tempering, RA first decomposes into various transitional microstructures such as martensite, bainite or pearlite, which further transform into M23C6 carbide clusters during the subsequent 700 °C tempering. The experimental results show that 455 °C is the optimal pre-tempering temperature to improve the impact toughness of the steel after the 700 °C tempering. Microstructural observations reveal that during the 455 °C pre-tempering step, the RA completely decomposes into bainite consisting of fine bainitic packets and a high density of M3C carbides, which provide additional nucleation sites for M23C6 carbides inside the carbide clusters during the subsequent 700 °C tempering, and thus avoid the formation of coarse M23C6 distributed along carbide cluster boundaries.

High/very high cycle fatigue behaviors of medium carbon pearlitic wheel steels and the effects of microstructure and non-metallic inclusions

Abstract: The high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behaviors of medium carbon pearlitic wheel steels were investigated by the combination of conventional tension-compression fatigue test (up to 108 cycles, frequency of 150 Hz) and ultrasonic fatigue test (up to 109 cycles, frequency of 20 kHz). In the S-N curves, fatigue limit plateaus were found in the range of 107–108 cycles for the conventional fatigue test and in the range of 108–109 cycles for the ultrasonic fatigue test. No fatigue failures were found in the VHCF regime. The fatigue fractures were mainly originated from the surface matrix of specimens, and a small amount of fracture origins were the surface/subsurface inclusions or internal inclusions. Only oxide inclusions were found to cause fatigue fracture, while crack initiation was not found to be associated with sulfides in this work. The effects of pearlite block size and inclusion size on the fatigue strength were discussed based on the Murakami model. The fatigue limit was slightly improved by the grain size refinement, but was insensitive to inclusions. The main reason was that the sizes of most inclusions in the tested steels were smaller than the critical size, below which fatigue failure can hardly occur from inclusions. When most of the oxides were enveloped in sulfides, the fatigue properties, however, were not obviously improved. This is because the sizes of those oxides encapsulated by sulfides were substantially smaller than the critical inclusion size, while large-sized oxides were not found to be encapsulated by sulfides. Compared to the conventional high strength steels with the tensile strength beyond 1200 MPa, the lower sensitivity of fatigue behavior to inclusions was found in the studied steels, mainly due to the larger critical inclusion sizes.

Effect of Microstructure on Contact Angle and Corrosion of Ductile Iron: Iron-Graphite Composite.

Abstract: Ductile iron samples with similar compositions and varying microstructures were uniformly abraded, and the effects of phase fractions (ferrite, pearlite, and graphite) on the apparent contact angle...

Further improvement in ductility induced by the refined hierarchical structures of pearlite

Abstract: Significant improvement in ductility without strength loss is achieved by the hierarchical refinement of the pearlite in hypoeutectoid steels, through two approaches, i.e., the grain refinement and increasing the undercooling degree. The increased undercooling is achieved by a temporary undercooling treatment without austenite decomposition at bainitic temperature before the transformation of pearlite. Further refined colonies with increased misorientation are developed in the undercooled sample, whereas such colonies exhibit thicker ferrite plates and thinner cementite plates than fine-grained sample. The two aspects account for the higher ductility manifested by the undercooled sample, and the thickness ratio of cementite/ferrite plates can therefore be used to evaluate the ductility of lamellar pearlite. In addition, the uniformity of the colonies structure has positive effect on the enhanced ductility. This work provides a new understanding of high-performance fully pearlitic steels, which are expected to be applicable in drawing processes.

Towards an integrated experimental and computational framework for large-scale metal additive manufacturing

Abstract: Using the Metal Big Area Additive Manufacturing (MBAAM) system, a thin steel wall was manufactured from a low carbon steel wire. The wall was then characterized comprehensively by high-throughput high-energy X-ray diffraction (HEXRD), electron backscatter diffraction (EBSD), and in-situ HEXRD tensile tests. With the predicted temperature histories from the finite element-based additive manufacturing process simulations, the correlations between processing parameters, microstructure, and properties were established. The correlation between the final microstructure with the predicted temperature history is well explained with the material's continuous cooling transformation (CCT) diagram calculated based on the composition of the low carbon steel wire. The final microstructure is dependent on the cooling rate during austenite to ferrite/bainite transformation during initial cooling and the subsequent reheating cycles. Fast cooling rate resulted in small ferrite grain size and fine bainite structure at the location closest to the base plate. Slower cooling rate at the side wall location and repeated reheating cycles to the ferrite-pearlite regions resulted in all allotriomorphic (equiaxed) ferrite with medium grain size with small amount of pearlite. With no reheating cycles, the top location has the slowest cooling rate and a large grained allotriomorphic ferrite and bainitic structures. The measured mechanical strength is then related to the microstructural feature size (grain or lath size) observed in those locations. A good correlation is found between the mechanical properties, microstructure features and the temperature history at various locations of the printed wall.

Effects of niobium alloying on microstructure, toughness and wear resistance of austempered ductile iron

Abstract: Austempered ductile iron (ADI) is a heat-treated nodular iron variant that has for a given strength level much higher elongation than conventional iron with spheroidized graphite. This paper considers the effect of niobium addition on the graphite microstructure, bainite microstructure, bainite transformation process and properties such as hardness and impact toughness under given heat-treatment conditions. The resulting properties are significant with regard to the wear resistance of the material. The effects of niobium addition on the graphite morphology can be understood based on a detailed precipitation analysis of the niobium precipitation behavior in the liquid phase. Niobium influences the temperature and kinetics of pearlite as well as bainite transformation. Furthermore, niobium refines the bainite microstructure generated during austempering treatment. It was found that niobium addition in the range of 0.2–0.5 wt percent allows obtaining an optimum combination of hardness, impact toughness and wear resistance.