Time-Dependant Microstructural Evolution and Tribological Behaviour of a 26 wt% Cr White Cast Iron Subjected to a Destabilization Heat Treatment

By employing destabilization heat treatments (HT), it is possible to create microstructures possessing diferent fractions of carbides, martensite, and austenite, which lead to varying tribological responses in abrasion-resistant high-chromium white cast irons. In the current work, the destabilization temperature was kept constant at 980 °C, whereas the time was varied from 0 to 90 min. As a result, the microstructure of the 26 wt% Cr white cast iron had a mixture of M23C6 secondary carbides (SC), martensite, and a decrease in the amount of retained austenite (RA) with increasing destabilization holding time. The microstructures as well as their tribological characteristics were evaluated by combining confocal laser scanning microscopy, SEM, XRD, and EBSD, together with dry-sliding linear reciprocating wear tests. Results show that the volume fraction of SC were statistically comparable in samples destabilized for 0 and 90 min, although the average size was almost two-fold in the latter. This had direct implications on the wear properties where a decrease of up to 50% in the wear rate of destabilized samples compared to the non-treated material was observed. Furthermore, the sample with the lowest increase in the matrix hardness (~20% higher than non-treated), showed the highest wear resistance. This was attributed to a favourable distribution of the RA (~10%) and SC volume fraction (~5%), in combination with the harder martensitic matrix. Finally, the results obtained from this study shed light on the ability to alter the HT parameters to tune the microstructure depending upon the application prerequisite

A Comparative Study on the Influence of Chromium on the Phase Fraction and Elemental Distribution in As-Cast High Chromium Cast Irons: Simulation vs. Experimentation

The excellent abrasion resistance of high chromium cast irons (HCCIs) stems from the dispersion of the hard iron-chromium eutectic carbides. The surrounding matrix on the other hand, provides sufficient mechanical support, improving the resistance to cracking deformation and spalling. Prior knowledge of the microstructural characteristics is imperative to appropriately design subsequent heat treatments, and in this regard, employing computational tools is the current trend. In this work, computational and experimental results were correlated with the aim of validating the usage of MatCalc simulations to predict the eutectic carbide phase fraction and the elemental distribution in two HCCI alloys, in the as-cast condition. Microstructural observations were carried out using optical microscopy and SEM. The chemical composition and fraction of each phase was measured by electron probe microanalysis and image analysis, respectively. In all cases, the values predicted by the pseudo-equilibrium diagrams, computed with MatCalc, were in accordance with the experimentally determined values. Consequently, the results suggest that time and resource intensive experimental procedures can be replaced by simulation techniques to determine the phase fraction and especially, the individual phase compositions in the as-cast state

The Effect of Thermal Processing and Chemical Composition on Secondary Carbide Precipitation and Hardness in High-Chromium Cast Irons

The excellent abrasion resistance of high-chromium cast irons (HCCIs) is given by an optimal combination of hard eutectic and secondary carbides (SC) and a supporting matrix. The tailoring of the microstructure is performed by heat treatments (HTs), with the aim to adjust the final properties (such as hardness and abrasion resistance). In this work, the influence of chemical composition on the microstructure and hardness of HCCI_26%Cr is evaluated. An increase in the matrix hardness was detected after HTs resulting from combining precipitation of M23C6 SC during destabilization, and austenite/martensite transformation during quenching. Kinetic calculations of the destabilization process showed that M7C3 secondary carbides are the first to precipitate during heating, reaching a maximum at 850 °C. During subsequent heating up to 980 °C and holding at this temperature, they transformed completely to M23C6. According to the MatCalc simulations, further precipitation of M23C6 occurred during cooling, in the temperature range 980–750 °C. Both phenomena were related to experimental observations in samples quenched after 0-, 30-, 60- and 90-min destabilization, where M23C6 SC were detected together with very fine SC precipitated in areas close to eutectic carbides

Wear Induced Sub-surface Deformation Characteristics of a 26 Wt% Cr White Cast Iron Subjected to a Destabilization Heat Treatment

In the present work, the sub-surface microstructure of a heat treated and worn 26 wt% Cr white cast iron was investigated to gain better insight into the tribological behaviour of the material. The samples were destabilized at 980 °C for 0 (Q_0), 30 (Q_30) and 90 (Q_90) minutes followed by air cooling, and later subjected to dry-sliding linear reciprocating wear tests. The microstructural characterization of the area under the wear track was carried out using a combination of SEM, EDS and EBSD. Additionally, nanoindentation (NI) measurements were used to corroborate the mechanical behaviour with the microstructural observations. EBSD and NI measurements indicated that the matrix area underneath the wear track in Q_0 had undergone signifcant plastic deformation resulting in a drastic increase in hardness, whereas no such phenomena was observed in the Q_90. This was attributable to the relatively high amount of retained austenite in the former and a predominately martensitic matrix in the latter. Moreover, the large M7C3 eutectic carbides were less cracked in the destabilized samples compared to the as-cast sample owing to the presence of martensite and dispersed secondary carbides, leading to an increased matrix load-bearing capacity. These factors led to the destabilized samples showing a lower wear rate compared to the as-cast sample, and the Q_0 showing the best wear resistance amongst all the samples

Development of a Protective Coating for Evaluating the Sub-surface Microstructure of a Worn Material

In the current study, electrolytic deposition using two diferent electrodes, copper (Cu) and nickel (Ni) was investigated with the aim of protecting the worn surface during mechanical sectioning and polishing, for a posterior examination of the sub-surface microstructure. The efcacies of the two coatings were visually assessed based on its adhesivity and the ability to protect the wear tracks of an as-cast 26% Cr high chromium cast iron (HCCI) alloy. It was observed that electrodeposition using Cu as the electrode was inefective owing to a poor adhesivity of the coating on the HCCI surface. The coating had peeled of at several regions across the cross-section during the mechanical sectioning. On the other hand, Ni electroplating using Ni strike as the electrolyte was successfully able to protect the wear track, and the sub-surface characteristics of the wear track could be clearly visualized. A uniform coating thickness of about 8 µm was deposited after 30–40 min with the current density maintained between 1 and 5 A/dm2 . The presence of the Ni coating also acted as a protective barrier prevent ing the ejection of the broken carbide fragments underneath the wear track

Analysis of the carbide precipitation and microstructural evolution in HCCI as a function of the heating rate and destabilization temperature

Microstructural modifcation of high chromium cast irons (HCCI) through the precipitation of secondary carbides (SC) during destabilization treatments is essential for improving their tribological response. However, there is not a clear consensus about the frst stages of the SC precipitation and how both the heating rate (HR) and destabilization temperature can afect the nucleation and growth of SC. The present work shows the microstructural evolution, with a special focus on the SC precipitation, in a HCCI (26 wt% Cr) during heating up to 800, 900, and 980 °C. It was seen that the HR is the most dominant factor infuencing the SC precipitation as well as the matrix transformation in the studied experimental conditions. Finally, this work reports for frst time in a systematic manner, the precipitation of SC during heating of the HCCI, providing a further understanding on the early stages of the SC precipitation and the associated microstructural modifcations

Quantification of the Phase Transformation Kinetics in High Chromium Cast Irons Using Dilatometry and Metallographic Techniques

Further development of high chromium cast irons (HCCI) is based on tailoring the microstructure, necessitating an accurate control over the phase transformation and carbide precipitation temperatures and can be achieved by thermal treatments (TT). To understand the underlying mechanisms controlling the transformation kinetics during the different stages of the TT, it is imperative to adjust the TT parameters to have information of the transformations occurring during non-thermal and isothermal heating cycles, since proper selection of the TT parameters ensures the optimum use of the alloying elements. In this work, the boundaries of the phase transformations for a HCCI containing 26 wt pct Cr for different cooling rates (continuous cooling transformation, CCT, diagram) were established by applying dilatometric measurements. Based on the CCT diagram, a temperature-time-transformation (TTT) diagram was constructed by isothermally holding the samples until complete phase transformation. For determining the initiation and finishing of the transformation, the lever rule assisted by derivatives was applied. The phases present after transformation were determined by combining X-ray diffraction (XRD) and metallographic characterization using optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the data obtained from the dilatometer was experimentally verified by isothermally heat treating some samples using laboratory furnaces. The transformed phase fraction from OM and SEM images was then correlated to the fraction obtained from the TTT diagram

A Comparative Study on the Influence of Chromium on the Phase Fraction and Elemental Distribution in As-Cast High Chromium Cast Irons: Simulation vs. Experimentation

The excellent abrasion resistance of high chromium cast irons (HCCIs) stems from the dispersion of the hard iron-chromium eutectic carbides. The surrounding matrix on the other hand, provides sufficient mechanical support, improving the resistance to cracking deformation and spalling. Prior knowledge of the microstructural characteristics is imperative to appropriately design subsequent heat treatments, and in this regard, employing computational tools is the current trend. In this work, computational and experimental results were correlated with the aim of validating the usage of MatCalc simulations to predict the eutectic carbide phase fraction and the elemental distribution in two HCCI alloys, in the as-cast condition. Microstructural observations were carried out using optical microscopy and SEM. The chemical composition and fraction of each phase was measured by electron probe microanalysis and image analysis, respectively. In all cases, the values predicted by the pseudo-equilibrium diagrams, computed with MatCalc, were in accordance with the experimentally determined values. Consequently, the results suggest that time and resource intensive experimental procedures can be replaced by simulation techniques to determine the phase fraction and especially, the individual phase compositions in the as-cast state

Modifying the Characteristics of the Electrical Arc Generated during Hot Switching by Reinforcing Silver and Copper Matrices with Carbon Nanotubes

Switching elements are crucial components in electrical and electronic systems that undergo severe degradation due to the electrical arc that is generated during breaking. Understanding the behavior of the electrical arc and modifying its characteristics via proper electrode design can significantly improve durability while also promoting optimal performance, reliability, and safety in circuit breakers. This work evaluates the feasibility of carbon nanotube (CNT)-reinforced silver and copper metal matrix composites (MMCs) as switching electrodes and the influence of CNT concentration on the characteristics of the arcs generated. Accordingly, three different concentrations per MMC were manufactured via powder metallurgy. The MMCs and reference materials were subjected to a single break operation and the electrical arcs generated using 100 W and 200 W resistive loads were analyzed. The proposed MMCs displayed promising results for application in low-voltage switches. The addition of CNTs improved performance by maintaining the arc’s energy in the silver MMCs and reducing the arc’s energy in the copper MMCs. Moreover, a CNT concentration of at least 2 wt.% is required to prevent unstable arcs in both metallic matrices. Increased CNT content further promotes the splitting of the electrical arc due to a more complex phase distribution, thereby reducing the arc’s spatial energy density