Showing papers on "Pearlite published in 2020"

Recent progress in third-generation low alloy steels developed under M 3 microstructure control

Abstract: During the past thirty years, two generations of low alloy steels (ferrite/pearlite followed by bainite/martensite) have been developed and widely used in structural applications. The third-generation of low alloy steels is expected to achieve high strength and improved ductility and toughness, while satisfying the new demands for weight reduction, greenness, and safety. This paper reviews recent progress in the development of third-generation low alloy steels with an M3 microstructure, namely, microstructures with multi-phase, meta-stable austenite, and multi-scale precipitates. The review summarizes the alloy designs and processing routes of microstructure control, and the mechanical properties of the alloys. The stabilization of retained austenite in low alloy steels is especially emphasized. Multi-scale nano-precipitates, including carbides of microal-loying elements and Cu-rich precipitates obtained in third-generation low alloy steels, are then introduced. The structure–property relationships of third-generation alloys are also discussed. Finally, the promises and challenges to future applications are explored.

Achieving a desirable combination of mechanical properties in HSLA steel through step quenching

Abstract: Step quenching and tempering (SQT) treatment was applied on HSLA steels to achieve a multi-phase microstructure with a superior deformation performance. Intercritical quenching and tempering (IQT) as well as direct quenching and tempering (DQT) were also processed for comparison. Moreover, effects of intercritical temperatures before quench for different treatment methods on microstructure evolution and mechanical property were also investigated. Compared with IQT and DQT, SQT treatment produces a fine microstructure composited of soft ferrite/pearlite and hard martensite, with precipitates distributed along boundaries to maintain a large quantity of dislocations on deformation. Experiments involving tension and impact tests show that SQT samples with such multi-phase microstructure exhibit a desirable combination of mechanical properties. With increasing the intercritical temperature, the phase fraction of martensite is increased, which improves the strength but weakens the impact toughness in regardless of heat treatment type. Meanwhile, the yield ratio (YR) is found to be increased by higher intercritical temperature in all samples, due to the larger phase fraction of martensite with a large dislocation density and a finer grain structure. Such scenario is elucidated by using a model proposed through Swift's equation. That indicates that the content of multi-phase microstructure can be efficiently controlled via adjusting the intercritical temperature using SQT, which enables the optimization of phase composition according to the requirements of actual productions in engineering.

Microstructural characterization and tensile behavior of Nd:YAG laser beam welded thin high strength low alloy steel sheets

Abstract: Laser beam welding (LBW) has many advantages to join high strength low alloy (HSLA) steels compared with conventional fusion welding processes. The present work is focused on joining HSLA plates of 2 mm thickness efficiently through the application of Nd:YAG laser source. Only one process variable i.e. the welding speed was operated between 70 mm/s to 120 mm/s while the rest of the variables were maintained constant. The microstructure evolution of the fusion zone was recorded through optical, scanning and transmission electron microscopy. The fusion zone was observed to be wider but no major defects were present. Fusion zone experienced a change in morphology of ferrite and pearlite depending upon the rate of cooling and exposure time. Fusion zone showcased various ferrites such as acicular ferrite, widmanstatten ferrite, and grain boundary ferrite. Advancement in the welding speed helped in the formation of acicular ferrite. EBSD results presented a descending tendency of prior austenite grain size against increasing welding speed. TEM micrographs affirmed the ferrite transformation and showed the presence of dense dislocations. The changes in hardness and tensile behavior under the experimental conditions were further reported.

Phase change materials as quenching media for heat treatment of 42CrMo4 steels

Abstract: In the present work, paraffin phase change material is used as quenchant for the heat treatment of 42CrMo4 alloy and compared with water, air, and CuO doped paraffin. The samples were prepared based on ASTM E 8M-98 standard for tensile test and then heated up to 830 °C, kept for 4 h in an electric resistance furnace and then quenched in the mentioned media. Elastic modulus, yield strength, ultimate tensile strength, elongation, and modulus of toughness were determined according to the obtained stress-strain curves. Moreover, the hardness and microstructural evolution were investigated after the heat treatment at different media. The samples quenched in paraffin and CuO-doped paraffin are higher in ultimate tensile strength (1439 and 1306 MPa, respectively) than those quenched in water (1190 MPa) and air (1010 MPa). The highest hardness, with a value of HV 552, belonged to the sample quenched in CuO-doped paraffin. The microstructural studies revealed that the non-tempered steel had a ferrite/pearlite microstructure, while by quenching in water, paraffin and CuO-doped paraffin, ferrite/martensite microstructures were achieved. It is also observed that using the air as quenchant resulted in a three-phase bainite/martensite/ferrite microstructure.

On the Post-Printing Heat Treatment of a Wire Arc Additively Manufactured ER70S Part.

Abstract: Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, ie, acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample; however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component

Repair of light rail track through restoration of the worn part of the railhead using submerged arc welding process

Abstract: A surface worn C-Mn rail is repaired by retrieving the lost part of the railhead using a commercial rutile flux-cored wire submerged arc welding (SAW) method. Optical microscopy (OM), scanning electron microscopy (SEM), electron-dispersive X-ray spectroscopy (EDS), and Rockwell B hardness test are employed to investigate the properties of the repaired rail specimen. After the first set of analysis, the as-repaired rail is heated up to 1100 °C and water-quenched to room temperature to increase hardness. Each specimen is analytically partitioned into three zones including weld zone (WZ), heat-affected zone (HAZ), and the unaffected rail substrate. The as-repaired rail WZ is primarily composed of pearlite, ferrite, and austenite with a low hardness of 80 HRB, whereas the austenite phase is gone in the as-quenched rail and a massive extent of carbides are precipitated which increased hardness to 95 HRB. The microstructure of the HAZ in the as-repaired sample is a uniform distribution of fine-grained ferrite, pearlite, and carbide with the hardness of 92 HRB, while the microstructure of the identical zone in the as-quenched specimen is mainly martensitic-pearlitic with the highest average hardness among all zones, 110 HRB. The results presented an immense potential for SAW in rail repair.

Improvement of strength, toughness and ductility in ultrafine-grained low-carbon steel processed by warm bi-axial rolling

Abstract: A 0.15%C-0.3%Si-1.5%Mn steel bar with ultrafine elongated grain structures of transverse grain size 1.2 μm was fabricated via multi-pass bi-axial rolling process at a warm working temperature. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and 0.15% low-carbon steel with a ferrite/pearlite structure were also prepared. The full-size Charpy V-notch impact and tensile tests were conducted at a temperature range of −196 °C to 200 °C, and the relationship between microstructures, yield stress, reduction in area and impact energy was studied. In the developed steel bar, the main orientation in the microstructure changed in the cross-sectional plane, and it was dominated by {001} cube orientation at the center, {111} at the quarter and rolling direction// at the surface. Crack branching started to occur with decreasing temperature in the Charpy test. The fracture surfaces are very complicated at temperatures below −100 °C and the specimen did not separate into two pieces even at a low temperature of −196 °C. The strength–toughness balance of the developed steel was significantly improved compared with conventional steels. This advantage in the developed steel was also seen in the strength–reduction in area balance. As a result, the steel fabricated by warm bi-axial rolling was best balance in correlation between strength and ductility and between strength and toughness at all temperatures.

The effect of niobium addition on the microstructure and properties of cast iron used in cylinder head

Abstract: Cylinder head is one of the most important parts of heated components in the engine, while cracking due to low strength and large residual stress at high temperature is the main failure mode of cylinder head. With the growing demand of good performance and high reliability, gray cast iron with trace alloying element and optimized annealing process are probably effective ways in obtaining suitable materials. The effect of niobium addition on the microstructure and properties of cast iron used in cylinder head is studied in this paper. The results show that, the tensile strength, toughness, fatigue properties and thermal fatigue properties of gray cast iron are improved by adding trace niobium elements. The specimen with 0.20% niobium shows better comprehensive mechanical properties than others. The addition of trace niobium elements can not only refine the graphite, eutectic cell, carbide and phosphorus eutectic, but also reduce the pearlite lamellar space, thus strengthening the matrix. The cylinder head is made by cast iron with 0.20% niobium addition, and the annealing process of thermal aging is optimized. The residual stress of cylinder head can be maintained at a low level by adopting annealing process with higher holding temperature, longer holding time, lower cooling rate and lower outlet temperature. The analysis of microstructure, mechanical properties and residual stress for niobium alloyed cast iron is helpful for solving the cracking problem of cylinder head, and providing scientific basis for improving engine reliability.

An EBSD Investigation on the Evolution of the Surface Microstructure of D2 Wheel Steel During Rolling Contact Fatigue

Abstract: In this work, the relationship between rolling contact fatigue (RCF) failure and the microstructure of D2 wheel steel was studied using a GPM-30 fatigue tester under oil lubrication conditions. The microstructural evolution during the RCF process can be divided into three stages: In the first stage, the misorientation of the proeutectoid ferrite is 2°–10°, the ferrite phase in pearlite is less than 2°, and the dislocation density is low. In the second stage, with the increase in cycles, the misorientation of the proeutectoid ferrite increases to more than 10°, and the ferrite phase in pearlite increases to 2°–10°. In the third stage, the misorientation of the ferrite phase in pearlite increases to more than 10°, the ferrite phase is divided into fine grains, and the dislocation density is high. RCF cracks are formed in the third stage. Crack initiation is ascribed to the refinement of the surface ferrite phase and proeutectoid ferrite and the increase in dislocation density. RCF cracks are initiated and propagate primarily at the interface of pearlite/proeutectoid ferrite and in proeutectoid ferrite.

Microstructure–Mechanical Property Relationships for Post-Fire Structural Steels

Abstract: The objectives of this study are (1) to investigate the microstructural changes in structural steels that are exposed to fire accidents; and (2) to understand their influence on post-fire m...

Influence of microstructure on the thermo-mechanical fatigue behavior and life of vermicular graphite cast irons

Abstract: Cylinder head is subjected to the combination changes of temperature and mechanical loads in service. In such harsh condition, thermo-mechanical fatigue (TMF) failure is a main problem. In this study, two kinds of vermicular graphite cast irons were selected to reveal the TMF properties and life prediction. Vermicular graphite casts with different microstructures have different ferrite cluster morphologies, which result in different fracture mechanisms. The hysteresis loops of both materials display obvious asymmetry. Hysteresis energy and mechanical strain amplitude were selected as damage parameters to determine the fatigue life. It is found that the TMF performance of vermicular graphite cast iron with higher pearlite content is better; meanwhile, the life of TMF at different constraint ratios was also studied. Hysteretic energy related to constrain ratio is in a parabolic form, and the life of TMF decreases with the absolute value of constraint ratio increasing. Finally, the quantitative relationship between constraint ratio and TMF life was also discussed.

Microstructure and mechanical properties of cold drawn pearlitic steel wires: Effects of drawing-induced heating

Abstract: To investigate the effects of drawing-generated heating on the microstructural and mechanical properties of cold drawn pearlitic steel wires, cryogenic and room temperature (RT) drawing were carried out. Microstructure evolution of the steel wires was examined by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Experimental results show that with the increase of cold drawing strain interlamellar spacing of pearlite was reduced, while the crystalline cementite was gradually transformed into amorphous state. Only slight difference in the microstructure and mechanical properties was identified between the RT and cryogenic drawings. Specifically, as the drawing strain increased, the cryogenic drawn wires have lower strength but better ductility. For example, the tensile strength of cryogenic drawn wires (e = 2.2) is about 2050 MPa, about 90 MPa lower than that of RT drawn wires. On the other hand, cryogenic drawing wires have produced a lower volume fraction of nano-crystalline cementite particles. As the nano-crystalline cementite particles could hinder the movement of dislocations, a lower fraction of nano-crystalline cementite in cryogenic drawn wires is believed to be responsible for observed lower strength and better ductility.

Investigation of Hydrogen Embrittlement Susceptibility and Fracture Toughness Drop after in situ Hydrogen Cathodic Charging for an X65 Pipeline Steel

Abstract: The present research focuses on the investigation of an in situ hydrogen charging effect during Crack Tip Opening Displacement testing (CTOD) on the fracture toughness properties of X65 pipeline steel. This grade of steel belongs to the broader category of High Strength Low Alloy Steels (HSLA), and its microstructure consists of equiaxed ferritic and bainitic grains with a low volume fraction of degenerated pearlite islands. The studied X65 steel specimens were extracted from pipes with 19.15 mm wall thickness. The fracture toughness parameters were determined after imposing the fatigue pre-cracked specimens on air, on a specific electrolytic cell under a slow strain rate bending loading (according to ASTM G147-98, BS7448, and ISO12135 standards). Concerning the results of this study, in the first phase the hydrogen cations’ penetration depth, the diffusion coefficient of molecular and atomic hydrogen, and the surficial density of blisters were determined. Next, the characteristic parameters related to fracture toughness (such as J, KQ, CTODel, CTODpl) were calculated by the aid of the Force-Crack Mouth Open Displacement curves and the relevant analytical equations.

Laminar plasma jet surface hardening of the U75V rail steel: Insight into the hardening mechanism and control scheme

Abstract: Insight into the mechanism of laminar plasma jet (LPJ) surface hardening is critical for the controllable preparation of ideal hardened layer to improve the wear and fatigue resistance of U75V rail steel. To reveal the surface hardening mechanism, a series of surface hardening experiments on the U75V rail steel surface were performed under different working parameters using a self-designed laminar plasma generator. Results showed that the geometrical dimension, microstructure and hardness distribution of the hardened layer can be adjusted by changing the arc current, the scanning velocity and the anode nozzle diameter. Further characterization and numerical simulation suggested that the geometrical dimension of the hardened layer is determined by the heat affected zone over the critical austenitizing temperature (730 °C), while its microstructure and hardness distribution depend on the local cooling rate below the critical austenitizing temperature. By changing the working parameters, the heat flux density applied on the workpiece surface and the heating time can be controlled to obtain the desired temperature field within the heat affected zone. Not only a full hardened layer but also a transition layer consisting of martensite, pearlite, ferrite and carbides can be achieved. This configuration of LPJ surface hardening process for U75V rail steel indicates that the LPJ surface hardening with the ability of producing hardness gradient is promising for reducing the risk of crack generation on the rail.

Bainite kinetic transformation of austempered AISI 6150 steel

Abstract: This research studies the kinetic transformation and austempering temperature effect on the morphologies of bainite for AISI 6150 steel. A minimum of 10 different holding times for each austempering temperature were utilized. All the AISI 6150 steel samples with original spheroidal pearlite were austenized at 855 °C for 20 min, followed by a lower temperature salt bath for austempering at various holding times. The hardness for each austempered temperature and holding time was measured, and the microstructures were observed by optical microscopy. The kinetic energy for needle like lower bainite and granular like upper bainite was analyzed by using kinetic transformation equations. The aim of this research was to study the transformation of bainite, in particular lower bainite, to obtain improved mechanical properties and higher ductility of AISI 6150 steel, and then apply AISI 6150 steel to more applications.