Wt Pct Research Articles

Destabilization Treatment and Its Influence on Microstructure and Matrix Hardness of High-Cr Cast Iron

AbstractHigh-chromium cast irons are an essential class of wear-resistant materials commonly used for wear-resistant applications in the mining and steel industries. There is ongoing debate on the secondary carbide types and their formation sequences during heat treatment. This work examines the microstructural evolution during destabilization treatment of a hypoeutectic high-chromium cast iron containing 2.2 wt pct C and 16.5 wt pct Cr. Starting from an inhomogeneous as-cast microstructure consisting of ~ 28 pct M7C3 eutectic carbide and a mixed matrix of martensite and retained austenite, destabilization treatments resulted in the establishment of near homogeneous structure with a near equilibrium level of carbon concentration in the matrix, which fully transformed to martensite upon cooling. Homogeneously distributed M23C6 secondary carbides with a square-shaped morphology and 100 to 500 nm in size precipitated during heating up to the destabilization temperature. For higher destabilization temperatures (1000 °C), M7C3 secondary carbides formed together with M23C6 and were identifiable by a distinctly different morphology (elongated). It was found that the carbon content of the matrix, a function of the destabilization temperature and subsequent eutectic carbide dissolution, controls the martensite start temperature and has a dominating influence on bulk-hardness.

Effect of Solution Heat-Treatment on the Oxidation Resistance of Ni-Base Single-Crystal Superalloy

To clarify the effect of solution heat-treatment on the oxidation resistance of Ni-base single-crystal superalloy TMS-238, the evaluation of dendrite/inter-dendrite segregation of alloying elements in the as-cast and heat-treated samples, and its effect on cyclic oxidation resistance were investigated. Cyclic oxidation test results at 1100 °C for up to 150 cycles clearly showed that the as-cast samples with element segregations had lower oxidation resistance compared to heat-treated samples with homogeneous structure. Further, for the as-cast sample, rapid growth and spallation of oxide consisting of NiO, Cr2O3, and Al2O3 were observed around the dendrite core for 10 cycles of oxidation. Analysis of sub-surface on sample isothermally oxidized at 1100 °C for 10 minutes showed that rapid oxide growth is due to the formation of discontinuous Al2O3 layer at dendrite core with lower Al concentration. Furthermore, in this study, the threshold value of Al concentration and Gibbs energy for the formation of continuous Al2O3 layer were estimated and determined to be around 5.2 wt pct and − 556.6 ± 0.5 kJ/mol, respectively. This indicated that the solution heat-treatment for TMS-238 should be conducted above 1305 °C for exhibiting oxidation resistance at 1100 °C, to meet the threshold value within the whole region between dendrite and inter-dendrite.

Tolerance of Al–Mg–Si Wrought Alloys for High Fe Contents: The Role of Effective Si

Aluminum scrap is often contaminated with steel parts, leading to accumulation of Fe in recycled Al alloys. Consequently, low limits for Fe in Al wrought alloys are difficult to meet by recycling without dilution with primary Al. Wrought alloys with a higher tolerance for Fe could help overcome this problem and improve the sustainability of Al wrought products. Here we study the effects of increasing the Fe content in EN AW-6060, 6005A, and 6082 from 0.2 to 0.7 wt pct. The microstructure and mechanical properties of the alloys after extrusion and artificial ageing are compared to the standard alloys. We found that 6082 is more tolerant to above-standard Fe contents than 6005A, which in turn is more tolerant than 6060: the strength of the 6082-based alloy with increased Fe content is comparable to that of standard 6082 and the elongation at break is increased. In contrast, the artificial ageing potential of the 6060-based alloy with more Fe is drastically reduced compared to 6060. This data and literature values show that the effective Si content is a good overall predictor of alloy strength. Effective Si is not bound in AlFeSi-type phases and is available for precipitation hardening. Additional effects of increased Fe levels are discussed.

Quasi-In Situ Localized Corrosion of an Additively Manufactured FeCo Alloy in 5 Wt Pct NaCl Solution

FeCo alloys are important materials used in pumps and motors in the offshore oil and gas drilling industry. These alloys are subjected to marine environments with a high NaCl concentration, therefore, corrosion and catastrophic failure are anticipated. So, the surface dissolution of additively manufactured FeCo samples is investigated in a quasi-in situ manner, in particular, the pitting corrosion in 5.0 wt pct NaCl solution. The local dissolution of the same sample region is monitored after 24, 72, and 168 hours. Here, the formation of rectangular and circular pits of ultra-fine dimensions (less than 0.5 µm) is observed with increasing immersion time. In addition, the formation of a corrosion-inhibiting surface layer is detected on the sample surface. Surface dissolution leads to a change in the surface structure, however, no change in grain shape or grain size is noticed. The surface topography after local dissolution is correlated to the grain orientation. Quasi-in situ analysis shows the preferential dissolution of high-angle grain boundaries (HAGBs) leading to a change in the fraction of HAGBs and low-angle grain boundaries fraction (LAGBs). For the FeCo sample, a potentiodynamic polarisation test reveals a corrosion potential (Ecorr) of − 0.475 V referred to the standard hydrogen electrode (SHE) and a corrosion exchange current density (icorr) of 0.0848 A/m2. Furthermore, quasi-in situ experiments showed that grains oriented along certain crystallographic directions are corroding more compared to other grains leading to a significant decrease in the local surface height. Grains with a plane normal close to the langle {1}00rangle direction reveal lower surface dissolution and higher corrosion resistance, whereas planes normal close to the langle {11}0rangle direction and the langle {111}rangle direction exhibit a higher surface dissolution.

Kinetics and Mechanism of Hydrogen Reduction of Lead-Silicate Slag

A systematic study on the microstructure evolution and kinetics analysis of PbO–SiO2 slag reduction using hydrogen was conducted. The reduction was carried out on PbO–SiO2 pellets (70 wt pct PbO–30 wt pct SiO2; or 38.6 mol pct PbO and 61.4 mol pct SiO2) using 15 pctH2/85 pctN2 gas, with a flowrate of 500mL/min for various reaction time (30 minutes to 3 hours) isothermally at 300 °C to 700 °C. The kinetics and reaction mechanism were assessed by measuring the weight loss over reaction time and applying kinetics models on the data; supported by detailed samples characterizations. The results from microstructure observation show a viscous and blackish glass structure formed on the pellets when the reduction was carried out above softening point (glass transition temperature). This viscous structure appeared to reduce the overall reduction rate. The kinetics analysis shows that the reduction appeared to be a diffusion-controlled process. The activation energy, Ea, was calculated to be 70.7 kJ/mol at temperature range of 300 °C to 500 °C; while between 500 °C and 700 °C the kinetics were found to decrease with increasing temperature due to the formation of the viscous glass. These results suggest that for a complete reduction in industrial process, the formation of the viscous glassy state should be avoided.

Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV_{10}) of 320 J cm^{-2} compared to 66 J cm^{-2} in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

Origins of Strength, Strain Hardening, and Fracture in B2 Tailored Fe–0.8C–15Mn–10Al–5Ni Wt Pct Austenitic Low Density Steel

Origins of Strength, Strain Hardening, and Fracture in B2 Tailored Fe–0.8C–15Mn–10Al–5Ni Wt Pct Austenitic Low Density Steel

Investigation of Cracking Behavior of Fe–6.7 Wt Pct Si Manufactured Using Laser Powder Bed Fusion

Investigation of Cracking Behavior of Fe–6.7 Wt Pct Si Manufactured Using Laser Powder Bed Fusion

Phase Equilibrium studies in the system “FeO”–SiO2–Al2O3–CaO–MgO at 1200 °C and Po2 10-8atm

Iron oxides and silica are the major components of copper smelting slag. The oxides of aluminum, calcium and magnesium are also present in the slag that is introduced through copper concentrate, flux and refractories. Liquidus temperatures of the copper smelting slags are usually controlled by Fe/SiO2. The concentrations of Al2O3, CaO and MgO, and FeO/Fe2O3 in the slag can also affect the liquidus temperatures where FeO/Fe2O3 is a function of oxygen partial pressure. High temperature equilibration under controlled oxygen partial pressure followed by quenching and electron probe microanalysis were used to determine the compositions of the liquid and solid phases at 1200 °C and Po2 10-8 atm. The experimental results are presented in the forms of pseudo-ternary sections “FeO”-CaO-SiO2 at fixed 2, 4 and 6 wt pct MgO, and 2 + 2, 4 + 4 and 6 + 6 wt pct MgO + Al2O3. Spinel and tridymite are the major primary phases in the composition range investigated. In addition, CaSiO3, pyroxene, olivine, and melilite are also present. The isotherms in the spinel and tridymite primary phase fields move towards higher SiO2 concentration directions with increasing CaO, Al2O3, and MgO concentrations. The experimentally determined results are compared with the FactSage calculations.

Influence of the Sulfur Species on the Current Efficiency and Carbon Consumption in the Aluminum Electrolysis Process

The influence of sulfur species on the current efficiency and carbon consumption in the aluminum electrolysis was investigated. Prebaked and graphite anodes with varying levels of sulfur were used. It was found that increasing the sulfur content in the anodes decreases the current efficiency and increases the carbon consumption of both types of anodes. The current efficiency decreased by 1.3 pct and the carbon consumption rose by 6.1 pct per 1 wt pct S in the prebaked anodes. The addition of sodium sulfate to the electrolyte during electrolysis with graphite anodes led to a decrease in current efficiency from 0.9 to 1.0 pct per 1 wt pct S, and to an increase in carbon consumption from 3.1 to 7.0 pct per 1 wt pct S. Electrolyte analysis showed the presence of sulfates, sulfides and polysulfides. Some link between sulfur content in the anodes and sulfur content in the electrolyte and outgoing gases was found. A comparison of the polarized and non-polarized conditions showed that the electrolysis increases the sulfur depletion from the cell and promotes the formation of soluble sulfides. Reactions of sulfur participation in redox processes have also been proposed.

Nucleation and Growth Dynamics of Equiaxed Dendrites in Thin Metallic Al–Cu and Al–Ge Samples in Microgravity and on Earth

In this study, the nucleation and growth characteristics of equiaxed dendrites in near-isothermal solidification experiments, performed during two sounding rocket missions, MAPHEUS-6 and 7, are investigated. Two samples of the composition Al–15 wt pct Cu and one sample of the composition Al–46 wt pct Ge were processed in microgravity and several samples on ground. In situ X-radiography was performed to observe the nucleation dynamics and microstructure evolution during solidification of the 200 µm thin, disc-shaped samples. The measured dendritic growth rates and observed concentration distributions in the liquid indicate no difference between microgravity and on-ground horizontal experiments, in line with previous observations. On the contrary, a difference in the nucleation behavior was found. In all microgravity experiments bursts of nucleation were observed, which is different from comparable on-ground solidification experiments, where the nucleation occurs continuously. The combined analyses suggest that small variations of the concentration fields that probably originate from convective flow and buoyancy of the grains in the on-ground experiments, lead to a spatially variable nucleation undercooling distribution and hence to continuous nucleation. In contrast, the absence of convective flow in microgravity results in a more uniform nucleation undercooling distribution. The grains therefore nucleate in bursts at the surface walls, when the necessary nucleation undercooling is reached.

Effects of Initial Microstructure and Induction Heating Cycle on the Microstructure and Torsional Fatigue Performance of a Steel Containing 0.6 wt pct Carbon

Effects of Initial Microstructure and Induction Heating Cycle on the Microstructure and Torsional Fatigue Performance of a Steel Containing 0.6 wt pct Carbon

Influence of the Cooling Rate on Mechanical and Corrosion Properties of Ferritic Si–Mo Ductile Cast Iron

In this study, the effect of the cooling rate controlled by different castings thickness ranging from 3 to 25 mm on the mechanical and corrosion behavior of Si–Mo ductile iron (DI) castings was investigated. SEM-EBSD investigations demonstrated that the nodule counts, nodularity, and ferrite phase increased while the ferritic grain size decreased with decreasing in the casting wall thickness; this in turn led to improvements in the tensile properties of the DI. The corrosion behavior of Si–Mo DI castings was investigated after different immersion times in a 3.5 wt pct NaCl solution via electrochemical experiments. The electrochemical results showed that the corrosion resistance of Si–Mo DI decreased in the following order: 3 > 5 > 13 > 25 mm. The results indicated that all of the tested DI samples exhibited galvanic corrosion between the nobler carbide and ferritic matrix phases. SEM analysis showed that the dissolution of the ferrite phase with the pearlite phase exposed and accumulated on the DI’s surface, which led to intense corrosion acceleration. The higher corrosion rates that were observed for those samples with higher casting wall thicknesses could be associated with the higher contents of pearlite/carbide.

High-temperature Corrosion of ~ 30 Pct Porous FeCr Stainless Steels in Air: Long-Term Evaluation Up to Breakaway

In this work, a long-term (up to 6000 hours) corrosion evaluation of three porous (~ 30 pct of initial porosity) ferritic iron-chromium alloys with different Cr contents (20, 22, and 27 wt pct of Cr) was carried out at 600 °C, 700 °C, 800 °C, and 900 °C in air. Mass gain measurements and SEM analyses revealed that at temperatures above 600 °C, all alloys exhibit breakaway corrosion, whereas at 600 °C, none of the alloys were heavily oxidized even after 6000 hours. Based on the results, the diffusion character of the corrosion of porous chromia-forming alloys was identified. The microstructure changes at high temperatures in porous alloys containing 22 wt pct of Cr were determined in detail by transmission electron microscopy. The proposed prediction model indicated that the lifetimes of the Fe20Cr and Fe22Cr alloys were determined as 1250 hours (± 535 hours) and 1460 hours (± 640 hours), respectively. It is in agreement with the long-term oxidation experiment. For the Fe27Cr alloy, the deviation between predicted and observed lifetimes occurs. The proposed model allows for qualitative estimation of the porous alloys’ lifetime with experimentally validated accuracy.

An Improved Cellular Automata Solidification Model Considering Kinetic Undercooling

A cellular automata (CA) model has been developed for solidification simulation considering the kinetic undercooling at the interface. The state-of-the-art model incorporates a decentered growth algorithm to suppress the grid anisotropy and a generalized height function method to calculate the curvature accurately. To develop a CA model which is independent of the mesh size, a new diffusion term is proposed to handle the diffusion between the interface cells and liquid cells. The developed CA model is employed to simulate the single-dendritic solidification of an Al–3Cu (wt pct) alloy. The simulated tip velocities agree with the prediction of the Kurz–Giovanola–Trivedi (KGT) model. Further studies show that the developed CA model converges to an equilibrium model with increasing kinetic mobility values. Moreover, it is found that the virtual liquid cell assumption which is commonly used in existing CA models may lead to a deviation in the mass balance. The mass balance error has been resolved by redistributing solutes from neighboring liquid cells in each time step. The developed CA model could be potentially used in solidification simulations with a high undercooling, which is common in welding and additive manufacturing.

Evolution of Martensite Tetragonality in High-Carbon Steels Revealed by In Situ High-Energy X-Ray Diffraction

The martensitic transformation was studied by in situ and ex situ experiments in two high-carbon, 0.54 and 0.74 wt pct C, steels applying three different cooling rates, 15 °C/s, 5 °C/s, and 0.5 °C/s, in the temperature range around Ms, to improve the understanding of the evolution of martensite tetragonality c/a and phase fraction formed during the transformation. The combination of in situ high-energy X-ray diffraction during controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching was used to study the transformation behavior. The cooling rate and the different Ms for the steels had a clear impact on the martensitic transformation with a decrease in average tetragonality due to stronger autotempering for a decreasing cooling rate and higher Ms. A slower cooling rate also resulted in a lower fraction of martensite at room temperature, but with an increase in fraction of autotempered martensite. Additionally, a heterogeneous distribution of martensite tetragonality was observed for all cooling rates.

Hardening Due to Vanadium Carbides Formed During Short-Time Aging of Hadfield Steels

Precipitation hardening is a promising approach for strengthening of Hadfield steels. The present study examines the potential to achieve this by combining vanadium addition (up to 2 wt pct) with short-time aging (15 minutes) at 1173 K (900 °C). It was found that such a treatment is sufficient to generate a dispersion of nanoscale precipitates that provided a significant increase in hardness. Small-angle neutron scattering and transmission electron microscopy measurements were performed to quantify the particle dispersion, and Orowan precipitate hardening predictions made using the parameters thus obtained show good correspondence with the observed rates of age hardening, suggesting the precipitates are resistant to shearing. The present steels containing vanadium showed a small reduction in work-hardening capacity and this is believed to be due to carbon depletion from the matrix. It is concluded that the addition of vanadium and a short aging treatment at 1173 K (900 °C) provide a promising pathway to imparting hardness increases that provide gouge resistance during the running-in period of components made from Hadfield steel. For optimum performance, additional carbon should be added to maintain the solute carbon content of the matrix, and hence the matrix work-hardening rate.

Operation and Fundamentals of Direct Anode Copper Production From Matte

Copper matte converting is an essential process in copper pyrometallurgy. Replacing traditional batch converting in Peirce–Smith converters (PSCs), continuous converting is becoming a new trend. In this work, a thermodynamic modeling of the copper matte continuous converting process was carried out under actual production conditions. There were five phases in the process: copper matte, copper, Cu2O, slag, and gas. To directly produce anode copper, the continuous converting process was divided into an oxidation period and a reduction period. The multiphase transformation during converting was investigated. The thermodynamic equilibria of the Cu–Fe–S–O–Si system in the process were evaluated. The results showed that the maximum S and O content dissolved in copper was approximately 1 wt pct. The copper matte appeared when the P(S2) was higher than 1.58 × 10−6 atm at T = 1523 K. When P(O2) was higher than 9.55 × 10−5 atm, the Cu2O phase was produced at T = 1523 K. During the conversion of Cu2S to Cu2O dissolved in copper, P(O2) significantly increased, P(S2) rapidly decreased, and copper loss in the slag increased. Reducing the residual rate of oxidation slag and controlling the flow rate of natural gas could reduce the oxygen content in the anode copper and improve the grade of the anode copper.

Effect of Silicon on the Metastable Eutectic Temperature of Fe–C–Si Alloys

An attempt has been made to establish experimentally the temperature along the metastable eutectic in the Fe–C–Si system, down to the invariant three-phase eutectic involving austenite, cementite, and an iron silico-carbide. This was carried out by standard thermal analysis using cups with a coating rich in tellurium that is known to impede graphite growth. It turned out that evaluating the metastable temperature is possible up to a silicon contents of 3.8 wt pct, beyond which the driving force for graphite growth becomes so high that the development of stable eutectic cells can no more be hindered. This is quite unfortunate for melt control on the foundry floor but triggers further study for understanding and describing graphite shape in cast irons.

Effect of Accelerated Fe-Rich Precipitation Induced by Ag Addition on the Microstructure and Comprehensive Properties of Cu–10 Wt Pct Fe Alloys

Effect of Accelerated Fe-Rich Precipitation Induced by Ag Addition on the Microstructure and Comprehensive Properties of Cu–10 Wt Pct Fe Alloys