Carbon High Silicon Research Articles

Influence of microstructure on slurry erosion resistance of medium carbon high silicon steels

The present study compares slurry erosion resistance of austempered and tempered medium carbon high silicon steels. A 0.62C-1.91Si-0.85Mn-0.23Cr steel was subjected to austempering at 300, 350 and 400 °C for 10 min to develop a microstructure comprising of bainite and retained austenite. With the increase in the austempering temperature, the mechanical stability of retained austenite decreased, and the size of bainitic ferrite laths increased. As a result, strain hardening ability and toughness of the steels decreased with austempering temperature. Another set of steel with the same composition was quenched and tempered at 300, 450 and 550 °C for 60 min. An increase in the tempering temperature led to an increase in the size and fraction of carbides and a decrease in the mean free path and dislocation density. Strain hardening ability and toughness of tempered steels was substantially small than austempered steels. Both the austempered and tempered steels were subjected to slurry erosion. Austempered steels exhibited higher slurry erosion resistance as compared to tempered steels. Slurry erosion resistance of austempered steels has been noted to depend on the mechanical stability of retained austenite and the size of bainitic ferrite laths. Due to the difference in the extent of deformation-induced transformation of retained austenite into martensite in the steels austempered at different temperatures, number of mobile dislocations generated (to accommodate the dilatation due to transformation) in bainitic ferrite laths differed. A large number of mobile dislocations in the specimens austempered at 300 °C blunted the propagating crack and delayed the material loss during erosion. The erosion resistance of tempered steels was observed to decrease with an increase in tempering temperature. A large fraction of coarse carbides led to the formation of many decohesion sites in steels tempered at 450 and 550 °C. Further, a small number of mobile dislocations present per unit volume in the steels tempered at high temperatures could not resist the propagation of cracks nucleated from the decohesion sites leading to a high erosion rate in these steels.

Influence of austempering temperature on carbide free bainite development, a critical review

In this review paper, it is well established that high carbon high silicon steels on austempering result in carbide free bainite (CFB) which exhibits enhanced mechanical and tribological properties without adding any heavy alloying elements. Most of the research has been pointed out regarding the improvement of CFB steels that consists of the fine structure of bainitic ferrite laths and retained austenite (RA). In some cases, it was observed that the tensile strength of CFB steels was found similar to the commercial quenched and tempered steel alloys. Regarding other properties, it is estimated that out of the various failures that take place due to wear, 50% are caused by abrasive wear. To improve the lifetime of components, the selection of material and optimized processing techniques plays a pivotal role.

Effect of Post-austempering Cold Work on Thermal Stability of Retained Austenite and Mechanical Properties of Medium Carbon High Silicon Multiphase Steels

Effect of Post-austempering Cold Work on Thermal Stability of Retained Austenite and Mechanical Properties of Medium Carbon High Silicon Multiphase Steels

Effect of the ausforming deformation mode on bainitic transformation in a medium carbon high silicon steel

Ausforming treatments are thermomechanical treatments that consist in plastically deforming a fully austenitized steel, usually leading to either a bainitic or a martensitic microstructure. The effects of the mentioned previous deformation have been mainly studied under compression conditions. However, as widely known, many large-scale processes do not only require the application of deformation under a single mode. For that reason, it is important to assess whether different deformation modes could induce similar effects on the final microstructure. In this work, the effect of the deformation mode was analyzed in a medium carbon high silicon steel. Samples were deformed under compressive and tensile stresses at different temperatures and subsequently subjected to isothermal holding steps in the bainitic range for prolonged times. The final microstructures consisted of bainitic ferrite and retained austenite, where the bainitic ferrite was formed at different stages: a certain fraction was dynamically formed during the deformation step, while the remaining fraction was formed during the isothermal holding. The study of the changes in length observed during these treatments enabled to understand both phase transformations, while the analysis of the final microstructures by different characterization techniques, such as X-Ray Diffraction and Scanning Electron Microscopy, enabled to better understand the effect of the deformation mode at different levels. It was proved that tensile ausforming led to more rapid and intense strain induced transformations than compression ausforming. Moreover, results showed that ausforming led to anisotropic structures, regardless of the deformation mode, where bainitic ferrite plates were aligned with respect to the deformation direction. The effect of the deformation mode on the bainitic ferrite plate thickness, the volume fractions of the different phases, several crystallographic parameters and hardness is also discussed.

Study of one-step and two-step quench and partition heat treatments on a medium carbon high silicon alloy using dilatometry

This study evaluated the microstructural evolution in a medium carbon high silicon steel during one-step, and two-step quench and partition (Q&P) processes using dilatometry experiments. The two-step Q&P process was carried out using different quench temperatures ranging from 180 to 260 °C. In the one-step process, Q&P heat treatment samples were held isothermally for ten minutes after quenching at specified temperatures ranging between 200 and 4°50 C. The two-step Q&P process yielded a higher fraction of retained austenite than a one-step Q&P process. During the isothermal hold step, the volume expansion due to carbon partitioning and austenite decomposition behavior was interpreted by experimentally determined strain values. For the one-step Q&P process, the austenite decomposition kinetics above and below the Ms temperature differed, as evidenced by the JMAK parameters. The TTT diagram generated for the one-step Q & P process showed a “swing back” at a temperature of around 355 °C.

Effect of electrolysis parameters on corrosion resistance of extra-low carbon high silicon iron-based alloy

PurposeHigh silicon iron-based alloys possess excellent corrosion resistance in certain specific media, but the effects of electrolysis parameters on corrosion resistance remain unknown. This study aims to guide the development and application of an extra-low carbon high silicon iron-based alloy (ECHSIA) in electrode plates.Design/methodology/approachThe corrosion resistance of ECHSIA and a conventional high-silicon cast iron (CHSCI) was analyzed through experimental characterizations. The morphology was observed by scanning electron microscopy. The influence of electrolysis parameters on the corrosion resistance of ECHSIA was investigated through corrosion experiments. The relationship between the electrolysis parameters and the corrosion resistance of ECHSIA was statistically investigated using the grey correlation analysis method.FindingsThe corrosion resistance of the ECHSIA is better than that of the CHSCI. The corrosion rate showed an increasing tendency with the increase in the nitric acid concentration (CHNO3), electrolyte temperature and current density. The grey correlation analysis results showed that the CHNO3 was the main factor affecting the corrosion rate of the ECHSIA.Originality/valueAn ECHSIA with a single ferrite microstructure was prepared. This study provides a guideline for the future development and application of ECHSIAs as electrode plates.

Effect of Multi-Step Austempering Treatment on the Microstructure and Mechanical Properties of a High Silicon Carbide-Free Bainitic Steel with Bimodal Bainite Distribution

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350 °C and 370 °C, respectively), and three kinds of two-step routes (370 °C + 300 °C, 370 °C + 250 °C, and 350 °C + 250 °C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature.

Cold work induced stability of retained austenite at elevated temperature in a medium carbon high silicon steel

In the present work, a potential solution has been suggested for arresting the depletion of the retained austenite during prolonged holding at elevated temperature. A medium carbon high silicon steel specimen was austenitized and then air cooled for 20s followed by austempering at 350 °C for 10 min. 20% retained austenite was measured from X-Ray diffraction studies in the specimen and 23.4 ± 1.7% uniform elongation was achieved. On holding the similar austempered specimen at 350 °C for 120 min, all the RA austenite depleted from the microstructure and uniform elongation values dropped to 5.7 ± 0.6%. In further thermomechanical treatments, austempered specimens were cold worked by 4, 7 and 10% at room temperature. The cold worked specimens were further held at elevated temperature (350 °C) for prolonged duration (120 min). Retained austenite content was observed to increase from 14 to 18% with the amount of cold work and tensile elongation value of up to 14% was observed. The decomposition of the retained austenite from the microstructure of cold worked specimens was arrested due to the increase in the dislocation density during cold work applied before holding at the elevated temperature. First principle calculations indicate migration of large number of carbon atoms towards the dislocations at elevated temperature thereby inhibiting the carbide precipitation during decomposition of retained austenite. Application of cold work to the austempered specimens could arrest the depletion of retained austenite at elevated temperature.

Influence of Austempering of As-Cast Medium Carbon High-Silicon Steel on Wear Resistance.

The aim of this study was to evaluate the effect of austempering compared to quenching and low-temperature tempering on wear resistance of an as-cast medium carbon high-silicon steel intended for rock breaking. Austempering was done by isothermal holding at 270, 300 and 350 °C in molten salt baths, while quenching was done in water. The austempering treatments resulted in microstructural combinations of bainite and martensite. The isothermal holding at 270 °C resulted in bainite and self-tempered martensite, while isothermal holdings at 300 and 350 °C resulted in bainite and untempered martensite. The two quench and temper treatments resulted in tempered martensite. In general austempering resulted in lower hardness values when compared to quenching and tempering but higher impact toughness. The wear resistance was best for quenching and low temperature tempering, followed by austempering at 270 °C, but at slightly lower hardness and 25% higher impact toughness. The other two austempering treatments resulted in worse wear resistance.

The role of plastic strains on variant selection in ausformed bainitic microstructures studied by finite elements and crystal plasticity simulations

Abstract The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.

The study on corrosion behavior and corrosion resistance of ultralow carbon high silicon iron-based alloy

The ultralow carbon high silicon iron-based alloy (sample A) with a single ferrite matrix was prepared. The corrosion performance difference and mechanism between it and conventional high silicon cast iron (sample B) were systematically studied using optical microscope, scanning electron microscope, electric corrosion equipment, electrochemical corrosion workshop and atomic force microscope. The results show that the corrosion rate of ultralow carbon iron-based alloys was significantly lower than that of high-silicon cast iron. For example, the static corrosion rate after 310 h, the corrosion rate of sample A was 0.2399 g/(m2 · h), and the corrosion rate of sample B was 1.5159 g/(m2 · h). The potential difference of the substrate was significantly reduced. That is, the ultralow carbon iron-based alloy exhibits better corrosion resistance, which is mainly attributed to the denser passivation film formed on its surface. The results of microhardness show that the hardness of Sample A was higher than that of Sample B.

Effect of Compressive Deformations on the Final Microstructure of a Low Carbon High Silicon Bainitic Steel Thermomechanically Processed

Due to a combination of advantages, high-performance steel components, especially for automotive manufacturing applications, are generally forged parts. In the forging industry, bainitic steels are being increasingly used, because they can reduce the processing chain and energy consumption. In this case, the bainitic microstructure can be obtained immediately after forging, with controlled cooling, and without any subsequent heat treatment. In the present work, the effect of thermomechanical routes performed in the austenitic and bainitic fields on the final microstructure and final hardness of 18MnCrSiMo6-4 bainitic steel has been discussed. Thermomechanical processing routes were tested and evaluated in a Gleeble® 3800 testing machine with one and two-step deformation. In both cases, the sample had its height reduced by 40% and the strain rate used was 0.1s-1. It could be shown that the plastic deformation promoted in the bainite field induces the bainite transformation. The results also show a strong dependence of bainite morphology concerning the deformation temperature of the steel. Moreover, the knowledge of the hot and warm stress-strain curves is an important result because it allows estimating the necessary stress and the energy consumption per volume unit to deform the material.

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

The role of ausforming in the stability of retained austenite in a medium-C carbide-free bainitic steel

In this work, the influence of thermomechanical treatments, consisting of a combination of ausforming followed by isothermal holding and cooling to room temperature, on the stability of retained austenite of a carbide-free medium carbon-high silicon steel was investigated. The amount of retained austenite and its transformation to martensite for a range of ausforming treatments was determined by means of X-ray diffraction, dilatometry signal analysis, and metallographic investigation. Different amounts of deformations were applied at 600°C in the austenite region prior to fast cooling into the bainitic transformation region. Four isothermal temperatures (325, 350, 375, and 400°C) with a holding time of 1800s were selected to estimate the extent of the stability of the retained austenite after the completion of the bainitic transformation. The results show that the deformation-free austenite was more stable for the samples isothermally treated at temperatures between 325 and 350°C. However, the decomposition of retained austenite to martensite during cooling to room temperature increased for isothermal holding temperatures above 350°C. It was found that the stability of retained austenite was a function of the temperature and percent deformation, and the critical temperature and percent deformations were determined. It was also found that ausforming enhanced the formation of blocky-shaped retained austenite for isothermal holdings above 350°C, resulting in a decrease in retained austenite stability. The results were analyzed in terms of the influence of thermomechanical conditions on bainitic transformation and its impact on the transformation of retained austenite to martensite.

Effect of Tempering Temperature on Microstructure and Properties of Low Carbon High Silicon Alloy Steel Treated by Q-P-T Process

The mechanical properties and microstructure of low-carbon high-silicon alloy steel were examined under various tempering temperatures using the quenching, partitioning and tempering (Q–P–T) process. The performance changed with the variation in tempering temperature. The results show that the microstructure of low carbon high silicon alloy steel treated by Q-P-T process was mainly ferrite, martensite, carbide-free bainite and film-like retained austenite. This alloys exhibited good mechanical properties at tempering temperature of 300 °C. The product of strength and elongation were 33.7 GPa%. Specifically, the Ultimate tensile strength were 1508 MPa, the yield strength were 1048 MPa, and the elongation were 22.4%. At this temperature of 300 °C, the volume fraction of retained austenite reached 10.4%.

Effect of the Microsegregation on Martensitic and Bainitic Reactions in a High Carbon-High Silicon Cast Steel

Casting processes show some weaknesses. A particular problem is presented when the workpiece needs to be subjected to heat treatments to achieve a desired microstructure. This problem arises from the microsegregation phenomena typically present in cast parts. The effect of the microsegregation on the martensitic and bainitic transformations has been investigated in a high carbon-high silicon cast steel, with the approximate composition Fe-0.8C-2Si-1Mn-1Cr (in wt. %), which was poured into 25 mm keel block-shaped sand molds. The microsegregation maps of Cr, Si, and Mn characterized by electron probe microanalysis (EPMA) show that interdendritic regions are enriched while dendrites are impoverished in these elements, implying that their partition coefficients are lower that the unity (k < 1). As-quenched martensitic and austempered bainitic microstructures (at 230 °C) were obtained and analyzed after applying an austenitization heat treatment at 920 °C (holding for 60 min). The thermal etching method used to reveal the prior austenite grain size showed a bimodal grain size distribution, with larger grains in the dendritic regions (≈22.4 µm) than in the interdendritic ones (≈6.4 µm). This is likely due to both the microsegregation and the presence of small undissolved cementite precipitates. Electron Backscatter Diffraction (EBSD) analysis carried out on the martensitic microstructure do not unveil any differences in misorientation distribution frequency and block size between the dendritic and interdendritic zones related to the microsegregation and bimodality of the austenite grain size. On the contrary, the bainitic transformation starts earlier (incubation time of 80 min), proceeds faster and bainitic ferrite plates are longer in the dendritic zones, were prior austenite grains are larger and impoverish in solute. The presence of these microsegregation pattern leads to the non-uniform development of the bainitic reaction in cast parts, modifying its kinetics and the resulting microstructures, which would probably have a major impact on the mechanical properties.

Wear behaviour analysis of medium carbon high silicon alloy steel at different process parameter

ABSTRACT In railway wheel, track and mechanical component major issue losses of material due to rolling contact and sliding contact. A dual-wear type wear testing machine has been designed and developed, in which two discs rotate perpendicular to each other in the same direction with different velocity. In this machine bot type of wear, we can calculate rolling as well as sliding. As a result, the outer most peripheries of these discs are in symmetrical rolling contact. Discs were made up of medium carbon high silicon alloy steel with different treatments performed and were rubbed against each other rolling and sliding wear test performed in dissimilar metal also. All combinations of discs were tested including the same metal pairs. In similar metal combinations, the wear characteristics depended on the surface properties and microstructure, but not on the material hardness. Wear characteristics of material test performed on different surface treated sample by self-developed dual-wear testing machine Experimental data listed in this research paper. Characterisation of material/samples with field emission scanning electron microscopy and atomic force microscopy.

Influence of cast part size on macro- and microsegregation patterns in a high carbon high silicon steel

In this work, the macro and microsegregation of Cr, Si and Mn have been investigated in a high-carbon high-silicon cast steel using X-ray fluorescence (XRF) and electron probe microanalysis (EPMA), respectively. Two different keel block sizes with leg thicknesses of 12.5 and 75 mm have been compared. In each of the keel blocks, three different locations along the leg thickness have been analyzed: A) zone near surface (≈1 mm); C) the central zone of the leg thickness and B) an equidistant zone from A and C. After comparing the analysis performed by XRF in these three zones no macrosegregation of the Cr, Mn and Si has been observed in any of the two keel blocks. However, clear microsegregation patterns have been obtained by EPMA for these three elements; interdendritic zones are enriched while dendrites are impoverished in these elements implying that their partition coefficient is lower that the unity (k < 1). This coefficient has been estimated using the EPMA measurements and Thermo-Calc calculations, finding good agreement between both approaches for Si and Mn but not for Cr. Finally, for both keel block leg thicknesses, similar microsegregation intensity, measured in terms of alloying element concentration has been observed. This result suggests that the cast part size (or dimension) do not have a strong influence on the microsegregation profiles. This it attributed to the back diffusion phenomena involving redistribution of solute during the solidification process.

Constitutive Modelling of High Temperature Flow Behaviour for a Low Carbon High Silicon Bainitic Steel

It is well recognized the importance of the rheological characterization for the development of the steel in thermomechanical treatments, especially for the mechanical properties improvement of bainitic steels in subsequent hot forging optimization. Therefore, the plastic strain behaviour of a low carbon high silicon bainitic steel was studied through isothermal compression tests using a thermomechanical simulator at temperatures of 1123 K – 1423 K and strain rates of 0.1 – 5 s-1. Arrhenius equation was used to obtain the constitutive constants, which represents the material behaviour of flow stress in high temperature. Besides, work hardening, dynamic recovery, and the JMAK model in the dynamic recrystallization (DRX) of the steel parameters were determined. The second part of this research compared two proposed modified models from the literature, which showed the differences in modelled flow curves behaviour when they are applied for high strain levels. The flow curves were modelled in high strain levels for further implementation in numerical simulation, thus allowing an adjustment of parameters in hot forming processes for this bainitic steel. The proposed models presented an agreement with experimental values. However, only the Avrami equation to DRX showed the dynamic recovery mechanism in high strain levels, which has represented physical behaviour during the thermomechanical process.

Retained austenite stabilisation in low carbon high silicon steel during isothermal holding

ABSTRACTWe elucidate here the role of isothermal hold temperature of 300–500°C after intercritical annealing at 760°C on bainitic transformation and in governing the stabilisation of retained austenite in a 0.23C-1.35Si-1.82Mn steel. A critical analysis was attempted to explain the observations using displacive mechanism of bainite formation in the attempt to endeavour to understand the kinetics of bainitic transformation during isothermal holding. The model predicted that carbon enrichment in austenite was of particular significance in governing the stability of retained austenite. Thus, through the contribution of transformation induced plasticity effect of retained austenite, high tensile strength (964 MPa) and excellent ductility (uniform elongation of 24.5% and total elongation of 32%) was obtained on isothermal holding at 400°C.