Efforts are being made worldwide to transform road transport to minimise its contribution to greenhouse gas emissions, with a focus on reducing vehicle mass. Lightweight manganese–aluminium steels ...have gained popularity for this purpose due to their low density and combination of strength and plasticity. In this particular study, the solidification process of five lightweight manganese–aluminium steels with different silicon content was investigated. The steels were fabricated by inductive melting in a vacuum and remelted during further thermal analysis. A reference steel composition (Fe-14Mn-10Al-0.2Si-0.8C) was used, and the remaining four steels contained different amounts of silicon (0.5, 1.0, 2.0 and 4.0 mass%). The impact of silicon on solidification was analysed by thermodynamic calculations (CALPHAD method), differential thermal analysis and optical microscopy. The thermodynamic calculations gave good estimates for the liquidus and austenite liquidus temperatures, but higher solidus temperatures than the experimental results. The presence of silicon decreases the density and lowers the solidus and liquidus temperatures, resulting in a prolonged solidification interval. In addition, the spacing of the secondary dendrite arms decreases, except at a silicon concentration of 0.5 mass%, where an increase was observed compared to the reference steel.
51CrV4 spring steel is a martensitic steel grade that is heat treated by quenching and tempering. Therefore, austenitisation is an important step that influences steel properties. The main goal of ...austenitisation is to obtain a single-phase austenite structure that will transform into martensite. We studied the influence of austenitisation parameters on grain growth and martensite transformation temperatures. The samples were quenched from different austenitisation temperatures (800–1040 °C) and were held for 5, 10 and 30 min. The martensite start transformation temperatures (MS) were determined from dilatometric curves, and the hardness was measured using the Vickers method. The microstructure of the samples and the size of the prior austenite grains were characterised using optical microscopy. The increase in the size of the prior austenite crystal grains increases the MS temperature. However, this trend is visible up to 960 °C, where the results start to deviate. High temperatures, 960 °C and above, cause both grain growth and increased carbide dissolution along with chemical homogenization of the steel. The added influence of strong solute diffusion caused a big deviation in the results. The stability of carbides during austenitisation were evaluated with scanning electron microscopy (SEM) and thermodynamic calculations of equilibrium phases using the Thermo-Calc program. MC-type vanadium carbides are stable up to 956 °C under equilibrium conditions, but the SEM results show that they were present in the microstructure even after annealing at 1040 °C. This means that crystal growth is slowed down, which is positive, and that the austenite contains less carbon, so the hardness is lower.
Recently, low-density Fe–Mn–Al–C steels have attracted attention because of their low density with excellent combinations of strength and ductility. They can be divided into three categories: ...ferritic steels, duplex steels and austenitic steels, depending on the microstructure. The solidification of five duplex low-density steels was investigated. Five steels Fe–15Mn–10Al–0.8C, Fe–15Mn–10Al–1.7Ni–0.8C, Fe–15Mn–10Al–3.9Ni–0.8C, Fe–15Mn–10Al–5.6Ni–0.8C and Fe–15Mn–10Al–8.6Ni–0.8C were produced in a vacuum induction furnace under Ar atmosphere and remelted in a simultaneous thermal analyser (NETZSCH STA 449 C Jupiter). The influence of Ni on the solidification process was analysed using thermodynamic calculations and differential scanning calorimetry. The samples were analysed by light microscopy. The thermodynamic modelling was performed with commercial software Thermo-Calc using the CALPHAD method. The Thermo-Calc calculations show a better agreement for the liquidus temperatures at lower Ni additions, while the higher Ni additions show a better agreement for the solidus temperatures. The Ni additions slightly increase the density, reduce the distance of the secondary dendrite arms and lower both the solidus and liquidus temperatures, but do not influence the solidification interval.
The aim of this study was to investigate the surface deterioration of high-Cr roll steel caused by the spalling of larger particles during thermal fatigue. The mechanisms of surface deterioration due ...to spalling of larger particles are discussed. Using a laboratory thermal fatigue test that replicates hot rolling conditions, samples were tested cyclically (up to 4500 times) at maximum cycle temperatures of 500, 600 and 700 °C, followed by water cooling. Specimens with surface deterioration were selected for analysis, revealing important influencing parameters, i.e., the combination of test temperatures, chemical composition, thermal stress and microstructural properties, leading to oxidation-assisted crack growth in different directions and consequent surface deterioration due to early spalling of larger particles. Here, we describe the mechanisms of crack propagation, especially in the lateral direction, and their relation to the subsequent spalling of larger particles, which depend on the influence of the local chemical composition on the microstructural constituents, as well as their distribution and properties. The results obtained in this study can be used in the development of roll steel microstructures with improved resistance to the identified mechanisms of surface degradation.
The influence of austenitisation temperature and time on the martensitic and isothermal bainite transformation of 51CrV4 spring steel was analysed. Based on the analysis of dilatometric curves, the ...martensite start temperatures (MS) were determined at different austenitisation temperatures (800–960 °C) and austenitisation times (5–30 min). At a temperature of 800 °C, a partial austenitic transformation occurred, and undissolved chromium carbides were present in the matrix. At higher temperatures, the austenitic transformation was complete, and the temperature MS increased with the austenitisation temperature. The temperature of the isothermal phase transformation has a stronger effect on the bainite transformation, which has different effects on the stability of the austenite and the diffusion processes. The microstructure of isothermal bainite transformation samples at 330, 430 and 520 °C was characterised by optical microscopy and dilatometric curves. Lower bainite was formed at a bainitic transformation temperature of 330 °C, and a combination of upper and lower bainite was characterised at a transformation temperature of 430 °C. In the samples transformed at 520 °C, a smaller proportion of lower bainite formed in addition to the upper bainite and martensite. Some allotriomorphic ferrite formed along the boundaries of the austenitic grains.
The biodegradable WE43 magnesium alloy is an attractive biomedical material for orthopaedic implants due to its relatively high strength and corrosion resistance. Understanding the long-term ...corrosion behaviour in the human body plays a crucial role in the biomedical development and application of WE43 alloy for orthopaedic implants. In this work, the corrosion of an extruded WE43 magnesium alloy was investigated in a physiological environment using Dulbecco’s Modified Eagle Medium’s (DMEM) over a period of up to 10 weeks. To assess the in vitro corrosion process, we analysed the corrosion pits of the specimens’ cross sections and the composition of the corrosion layer by scanning electron microscopy. The experimental results indicated that the long-term corrosion process of WE43 magnesium alloy consists of three stages: (1) The rapid corrosion stage within the first 7 days, (2) the steady corrosion stage between 7 and 28 days, (3) the accelerated corrosion stage between 28 and 70 days. The microchemical analysis revealed a heterogeneous three-layer corrosion product with varying thicknesses of 10 to 130 µm on the surfaces of the samples for all corrosion times. It is composed of an inner layer of Mg-O, an intermediate layer of Mg-O-Ca-P, and an outer layer of Mg-O-Ca-P-C. The corrosion layers have many microcracks that allow limited contact between the liquid medium and the surface of the alloy. In addition, microgalvanic corrosion was observed to cause corrosion pits between the intermetallic rare earth element-rich phases and the Mg matrix.
Medium carbon high-silicon abrasion resistant (AR) steel was examined by performing dilatometry, light optical microscopy (LOM), scanning electron microscopy (SEM), and hardness measurements after ...isothermal bainitization and modified martempering and compared to direct quenching technology. A commercial thermodynamic tool was used for hardness prediction and compared to the measured one and revealed a rather good agreement for direct quenching, as was the case for isothermal holdings near to the martensite start (Ms). The predicted martensite start temperatures were in good agreement with the experimental data, the experimental value was 321 °C, while the predicted values were 324 and 296 °C. However, a higher discrepancy appeared for isothermal holding much above the martensite transition in the bainite region resulting in lower measured hardness compared to the predictions related to the actual kinetics and complexity of the formed final volume percentages of phase constituents such as bainite, martensite, and rest austenite, later as a part of unfinished bainite transformation at studied temperature. The predicted hardness values for quenching, isothermal holding at 280, 300 and 350 °C were 50.6, 50.6, 49.4 and 49.4 HRC, while the measured values were 53.3, 48.3, 48 and 43 HRC, respectively. A very good agreement between the thermodynamic prediction was achieved by comparing the measured Ms concerning prior austenite grain size as one of the crucial parameters for setting a proper heat treatment strategy of various isothermal quenchings making thermodynamic predictions for low alloyed steels a powerful tool for optimizing the heat-treating operations.
This article analyses the as-cast state of practically unknown Fe-P-based cast alloys with or without an addition of carbon and/or boron, cast into a grey cast iron mould. The melting intervals of ...the alloys were determined by DSC analysis, and the microstructure was characterized by optical and scanning electron microscopy with an EDXS detector. The hardness and microhardness of the alloys were also measured. Their hardness reached values between 52 and 65 HRC depending on chemical composition and microstructure, showing their high abrasion resistance. The high hardness is a consequence of the eutectic and primary intermetallic phases of Fe
P, Fe
C, Fe
B or mixed type. By increasing the concentration of metalloids and combining them, the hardness and brittleness of the alloys were increased. The alloys with predominantly eutectic microstructures were the least brittle. Depending on the chemical composition, the solidus and liquidus temperatures ranged from 954 °C to 1220 °C and were lower than those of the well-known wear-resistant white cast irons.
The aluminium alloy AA 6086 attains the highest room temperature strength among Al-Mg-Si alloys. This work studies the effect of Sc and Y on the formation of dispersoids in this alloy, especially L1
...-type ones, which can increase its high-temperature strength. A comprehensive investigation was carried out using light microscopy (LM), scanning (SEM), and transmission (TEM) electron microscopy, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and dilatometry to obtain the information regarding the mechanisms and kinetics of dispersoid formation, particularly during isothermal treatments. Sc and Y caused the formation of L1
dispersoids during heating to homogenization temperature and homogenization of the alloys, and during isothermal heat treatments of the as-cast alloys (T5 temper). The highest hardness of Sc and (Sc + Y) modified alloys was attained by heat-treating alloys in the as-cast state in the temperature range between 350 °C and 450 °C (via T5 temper).
The purpose of the present study was to evaluate the contribution of distinct regions of the simulated heat-affected zone (HAZ) to the overall creep behavior of welded joints in the X20 and P91 ...steels. The HAZ was simulated by means of dilatometry at four peak temperatures (900, 1000, 1200, and 1350 °C) with a consequent tempering at 650 °C. Microstructure features of the four simulated HAZ regions including precipitates, prior austenite grains, and subgrains were quantified by means of electron microscopy. The quantified parameters and the measured hardness were used in three physical models for evaluation of the stationary creep rate (ε˙ at 170 MPa and 580 °C. The resulting ε˙ values fall within the range 10−8–10−7 s−1, being in good agreement with the experimental data with a similar thermal history, but an order of magnitude lower than the measured values for the parent metal of the studied steels (10−7–10−6 s−1). Depending on the model utilized, their output can be linearly related to hardness, subgrain size, or interparticle spacing. The model relating ε˙ to hardness was the most consistent one in prediction, being always lower for higher peak temperatures.
PDF