Showing papers on "Pearlite published in 2018"

Austempered Ductile Iron (ADI): Influence of Austempering Temperature on Microstructure, Mechanical and Wear Properties and Energy Consumption

Abstract: Alloyed Ductile iron, austenitized at 840 °C for 30 min in a special sealed austempering furnace, was austempered for 30 min in molten salt mixture at 4 trial temperatures of 300 °C, 320 °C, 340 °C and 360 °C. Tensile strength, yield strength, percentage elongation and impact energy were evaluated for the as-cast and austempered samples. Microstructures were investigated using microscopy, coupled with analyzing software and a scanning electron microscopy. The specific wear of samples was tested using pin-on-disc wear testing machine. X-ray diffraction was performed to calculate the amount of retained austenite present in the ausferrite matrix. As-cast microstructure consists of ferrite and pearlite, whereas austempered ductile iron (ADI) contains a mixture of acicular ferrite and carbon enriched austenite, called “ausferrite”. Hardness and strength decreased, whereas ductility and impact strength improved with an increase in the austempering temperature. XRD analysis revealed that the increase in austempering temperature increased the retained austenite content. A decrease in wear resistance with austempering temperature was observed. Modified Quality Index (MQI) values were envisaged, incorporating tensile strength, elongation and wear resistance. MQI for samples austempered at 340 °C and 360 °C showed a better combination of properties. About an 8% reduction in energy consumption was gained when the heat treatment parameters were optimized.

Microstructure evolution during laser cladding Fe-Cr alloy coatings on ductile cast iron

Abstract: Ductile cast iron illustrates well cast characteristics and abrasive resistance, excellent corrosion resistance and good mechanical properties. Fe-based alloys were always used to repair invalid ductile cast iron components owing to their cheapness in price. But it is difficult to eliminate defects like pores and chilled structure for the remanufacturing of ductile cast iron without preheating and post heat treatment. In this research work, a kind of Fe-based alloy powders were used as the cladding material for the laser cladding process on ductile cast iron, and different cladding procedures had been successfully completed. The microstructure and phase evolution in the partially melted zone (PMZ) and heat affected zone (HAZ) had been investigated. Mechanical properties also had been studied. Scanning electron microscopy (SEM), energy dispersive microanalysis (EDS), X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to determine the microstructure and phase composition of the cladding layers and transition zones. The result revealed that distinct phases were formed during a single pass and multi-layer cladding process. Different kinds of pearlite structures formed in the transition zone during cladding process. Element migration was not apparent near the interface. The wear resistance of the cladding layers was not higher than that of the substrate.

Contributions of Rare Earth Element (La,Ce) Addition to the Impact Toughness of Low Carbon Cast Niobium Microalloyed Steels

Abstract: In this research Rare Earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel to disclose their influence on the microstructure and impact toughness. It is suggested that RE are able to change the interaction between the inclusions and matrix during the solidification process (comprising peritectic transformation), which could affect the microstructural features and consequently the impact property; compared to the base steel a clear evolution was observed in nature and morphology of the inclusions present in the RE-added steel i.e. (1) they changed from MnS-based to (RE,Al)(S,O) and RE(S)-based; (2) they obtained an aspect ratio closer to 1 with a lower area fraction as well as a smaller average size. Besides, the microstructural examination of the matrix phases showed that a bimodal type of ferrite grain size distribution exists in both base and RE-added steels, while the mean ferrite grain size was reduced from 12 to 7 μm and the bimodality was redressed in the RE-added steel. It was found that pearlite nodule size decreases from 9 to 6 μm in the RE-added steel; however, microalloying with RE caused only a slight decrease in pearlite volume fraction. After detailed fractography analyses, it was found that, compared to the based steel, the significant enhancement of the impact toughness in RE-added steel (from 63 to 100 J) can be mainly attributed to the differences observed in the nature of the inclusions, the ferrite grain size distribution, and the pearlite nodule size. The presence of carbides (cementite) at ferrite grain boundaries and probable change in distribution of Nb-nanoprecipitation (promoted by RE addition) can be considered as other reasons affecting the impact toughness of steels under investigation.

Effect of Different Cooling Rates on the Corrosion Behavior of High-Carbon Pearlitic Steel

Abstract: The present work discusses the effect of pearlitic morphology on the corrosion behavior of high-carbon fully pearlitic steel in 3.5% NaCl solution. Four different types of pearlitic steels (furnace-cooled, as-received, air-cooled and forced-air-cooled) consisting of coarse, medium, fine and very fine microstructures, respectively, were tested. Electrochemical behavior of these steels was studied with the help of dynamic and linear polarization and AC impedance spectroscopic tests. The corrosion resistance improved with fineness of the microstructure in general. However, with further reduction in interlamellar spacing beyond a limit, the corrosion resistance reduced slightly. Formation of homogeneous distribution of microgalvanic cells between cementite and ferrite lamellae of fine pearlitic steel improved the corrosion resistance. However, entanglement of the lamellae of pearlite in very fine pearlitic structure as well as breaking of cementite lamellae due to finer pearlitic colonies was attributed to the higher corrosion of the forced-air-cooled steel as compared to the air-cooled steel.

Title Synergistic Effect of Al₂O₃ Inclusion and Pearlite on the Localized Corrosion Evolution Process of Carbon Steel in Marine Environment.

Abstract: The initiation and evolution of the localized corrosion in carbon steel were investigated in a simulated marine environment of Xisha Island in the South China Sea. In the initial stage, localized corrosion occurred in the form of corrosion spot. The localized corrosion morphology and electrochemical information during corrosion process were tracked by field emission scanning electron microscopy energy dispersive spectrometry (FE-SEM-EDS), scanning vibrating electrode technique (SVET) and scanning Kelvin probe force microscopy (SKPFM). Localized corrosion was induced by the microcrevices around Al₂O₃ inclusions. The occluded cells and oxygen concentration cell formed in the pits could accelerate the localized corrosion. Pearlite accelerated the dissolution of the inside and surrounding ferrite via the galvanic effect between Fe₃C and ferrite. Overall, the localized corrosion was initiated and evaluated under a synergistic effect of crevice corrosion, occluded cells, oxygen concentration cell and the galvanic couple between FeC₃ and ferrite.

Fatigue strength model based on microstructures and damage mechanism of compacted graphite iron

Abstract: The as-casted compacted graphite irons (CGIs) with different microstructures were opted to investigate the relation between tensile and fatigue strengths as well as the correspondingly influencing mechanisms. The results show that the fatigue strength increases first and then decreases with increasing the tensile strength. The fatigue strength is mainly determined by the area percentage ratio of ferrite to graphite and the strength of ferrite, while the tensile strength is basically dominated by the pearlite area percentage, resulting in the non-synchronous relation above. Finally, based on these investigations, a prediction model for the fatigue strength of CGI is proposed.

Micro and nanoscale characterization of complex multilayer-structured white etching layer in rails

Abstract: Micro- to nano-scale characterization of the microstructures in the white etching layer (WEL), observed in a Dutch R260 Mn grade rail steel, was performed via various techniques. Retained austenite in the WEL was identified via electron backscatter diffraction (EBSD), automatic crystallographic orientation mapping in transmission electron microscopy (ACOM-TEM), and X-ray diffraction (XRD). EBSD and ACOM-TEM methods were used to quantify grains (size range: 50 nm–4 μm) in the WEL. Transmission electron microscopy (TEM) was used to identify complex heterogeneous microstructural morphologies in the WEL: Nano-twinning substructure with high dislocation density in the WEL close to the rail surface and untransformed cementite and dislocations in the WEL close to the pearlite matrix. Furthermore, atom probe tomography (APT) revealed a heterogeneous through-thickness distribution of alloying elements in the WEL. Accordingly, the WEL is considered a multi-layered martensitic microstructure. These findings are supported by the temperature calculations from the shape analysis of the manganese profile from APT measurements, related to manganese diffusion. The deformation characteristics of the WEL and the pearlite beneath the WEL are discussed based on the EBSD measurements. The role of deformation in the martensitic phase transformation for WEL formation is discussed.

In situ study of cementite deformation and its fracture mechanism in pearlitic steels

Abstract: The deformation and fracture behaviors of cementite in pearlitic steels during tensile tests were investigated by in situ scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD). The results showed that the deformed structure had three different types of shear bands: shear deformation in the pearlite colonies, at the interfaces of the pearlite colonies and at the phase boundary between ferrite and cementite. The orientations of the shear bands were nearly anisotropic with respect to the tensile axis. The path of shear deformation developed at the dislocation walls and was propagated by the offset of the cementite along the dislocation walls. As the dislocations cut perpendicularly through the cementite layers, the single-crystalline cementite platelets were divided into many nanometer-sized subgrains by the effects of the shearing stress. This process hindered dislocation movement and led to the accumulation of complex dislocations at the adjacent ferrite lamellae. Thus, the plastic deformation of the cementite layers was characterized by bulging until they fractured. The pearlitic texture component was evenly distributed, and the crystallographic axes of the colonies were randomly oriented. Hence, three different shear models characterized the crack propagation process, which occurred in a linear fashion and could be considered the pearlite colonies as a displacement unit for forward expansion.

Partition and non-partition transition of austenite growth from a ferrite and cementite mixture in hypo- and hypereutectoid Fe-C-Mn alloys

Abstract: The growth of austenite from a ferrite and cementite mixture in low Mn steel of hypo- and hypereutectoid composition is investigated with focus upon the Mn partitioning between dissolving cementite (or ferrite) and austenite. Under the assumption that austenite is nucleated on cementite, two critical temperatures which characterize the transition between Mn-partitioned and non-partitioned growth of austenite are noticed; below the 1st and lower critical temperature the austenite grows with redistribution of Mn from the beginning, and above the 2nd and higher critical temperature, without Mn redistribution until completion. Between them the growth mode switches from carbon-diffusion to Mn-diffusion control during growth. The influence of carbon and/or Mn diffusion through the matrix becomes progressively more significant with time, but may not affect the growth mode transition temperatures. Above the 2nd critical temperature, which is at most ca. 50 °C higher than Acm or Ae3 in alloys studied, the distribution of Mn in as-transformed or spheroidized pearlite is preserved at the completion of austenitization irrespective of the last dissolving phase, leading to the formation of an ultrafine mixture of martensite and austenite upon quenching.

Enhancing Corrosion Resistance and Hardness Properties of Carbon Steel through Modification of Microstructure

Abstract: Steel has played a primary role as structural and fabricating materials in various industrial applications—including the construction sector. One of the most important properties of steel that required a constant improvement is corrosion resistance specifically in corrosive environment. For this purpose, various approaches have been conducted through different heat treatment parameters to compare its microstructural engineering on chemical and mechanical properties. In this paper, correlation of different microstructure on corrosion resistance and hardness properties have been investigated. Three different heat treatment cycle have been applied on carbon steel with same composition to prepare dual-structure (DS) steel that consisted of ferrite/pearlite and triple-structure (TS) with ferrite/pearlite/bainite and ferrite/bainite/martensite. Phase transformation during heat treatment process was analyzed through in-situ ultra-high temperature confocal microscopy. Effect of corrosion behavior on these steels was investigated by Tafel plot, Scanning Electron Microscopy (SEM), 3D laser scanning confocal microscopy (3DLSCM), and calculation of phase volume fraction by ImageJ. Mechanical test was conducted by Vickers hardness test. It has been found that TS steels that have improvement in corrosion resistance accounted around 5.31% and hardness value for up to 27.34% more than DS steel, because of tertiary phase—bainite/martensite. This corrosion rate was reduced due to decreased numbers of pit growth and lower level of boundary corrosion as bainite/martensite phases emerged.

Understanding the Interaction between a Steel Microstructure and Hydrogen.

Abstract: The present work provides an overview of the work on the interaction between hydrogen (H) and the steel’s microstructure. Different techniques are used to evaluate the H-induced damage phenomena. The impact of H charging on multiphase high-strength steels, i.e., high-strength low-alloy (HSLA), transformation-induced plasticity (TRIP) and dual phase (DP) is first studied. The highest hydrogen embrittlement resistance is obtained for HSLA steel due to the presence of Ti- and Nb-based precipitates. Generic Fe-C lab-cast alloys consisting of a single phase, i.e., ferrite, bainite, pearlite or martensite, and with carbon contents of approximately 0, 0.2 and 0.4 wt %, are further considered to simplify the microstructure. Finally, the addition of carbides is investigated in lab-cast Fe-C-X alloys by adding a ternary carbide forming element to the Fe-C alloys. To understand the H/material interaction, a comparison of the available H trapping sites, the H pick-up level and the H diffusivity with the H-induced mechanical degradation or H-induced cracking is correlated with a thorough microstructural analysis.

Hydrogen behavior in high strength steels during various stress applications corresponding to different hydrogen embrittlement testing methods

Abstract: Atomic scale phenomena during various types of hydrogen embrittlement testing were examined by investigating hydrogen desorption during the tests using tempered martensitic steel and cold-drawn pearlitic steel specimens. Hydrogen desorption increased in the elastic stage of constant stress/strain and cyclic stress testing, implying hydrogen transportation by dislocations. In contrast, hydrogen desorption increased in the elastic stage but turned downward near proof stress and finally deceased to less than that before stress application. This implies that hydrogen-enhanced strain-induced lattice defects such as dislocations and vacancies formed in addition to hydrogen accumulation by mobile dislocations. These results suggest that one of the reasons for the high hydrogen embrittlement susceptibility of high strength steels is hydrogen accumulation on the grain boundaries and cementite interfaces due to hydrogen transportation by mobile dislocations under elastic deformation.

Enhancing the strength of structural steel through severe plastic deformation based thermomechanical treatment

Abstract: Structural steel (hypoeutectoid steel) was subject to severe deformation through a novel method of thermo-mechanical processing coupled with equal channel angular pressing (ECAP). The ECAP experiment was conducted at a specific temperature for the hot ECAP (H-ECAP) process after which the sample gained a superior strength of 811 MPa with reasonable elongation. Whereas when the ECAP was conducted at room temperature (RT-ECAP), the sample strength was 659 MPa with an elongation of 16%. The electron backscattered diffraction (EBSD) analysis revealed grain refinement and grain misorientation as a result of the ECAP process. The H-ECAP process resulted in an increased pearlite volume fraction and uniform pearlite fragmentation due to the strain induced transformation generated by hot deformation. The internal structural modification of the pearlite grain was revealed in the TEM micrographs. Nanoindentation analysis further exposed carbon supersaturation in the ferrite matrix. The X-ray line profile analysis (XRDLPA) illustrated that the H-ECAP processed sample has a higher dislocation density than the RT-ECAP processed sample. Factors such as grain refinement, morphological changes in the grains and the dislocation density predominantly contribute to the increased strength of the H-ECAP processed sample. In the overall observation, H-ECAP processed sample exhibited superior mechanical properties than the RT-ECAP processed sample.