Showing papers in "Metallography, Microstructure, and Analysis in 2015"
TL;DR: In this paper, the authors focus on the role of topology in metallurgical matters, but practitioners in other fields will notice close analogies with the structures and spatial relationships discussed.
Abstract: The structure of matter, on both an atomic and macroscopic scale, is a result of the interplay between the requirements of the physical forces operating between the individual parts and the mathematical requirements of space filling. Unlike the underlying forces, the shapes of objects and their components are immediately apparent, and for this reason the observation and cataloguing of shapes is usually the first stage in the development of any branch of science. As a science matures, attention is focused more on forces, and shape tends to be taken for granted. Day-to-day contact with the limitations of space filling makes us accept them without further thought. Nevertheless, it is desirable from time to time to examine the background and to attempt to deduce relationships of greater complexity and utility than those that are immediately obvious. The present paper is concerned mainly with the role of topology in metallurgical matters, but practitioners in other fields will notice close analogies with the structures and spatial relationships discussed. The microstructure of a piece of metal consisting of several grains must conform to space-filling requirements. It is the intent of the present paper to call attention to some elementary laws of topology and to show their possible application to metallurgical problems. For proofs of the fundamental relations used, reference should be made to mathematical works, though these rarely contain relations expressed in a form directly applicable to the present type of problem—or, it must be admitted, in terms immediately comprehensible to the average nonmathematician. Workers in other fields (particularly biologists interested in cell structure) have concerned themselves with related problems and have performed experimental statistical studies on the shapes of cells in unspecialized biological tissue (1), in soap froth (2), and the polyhedra formed by compression of lead shot in contact with each other in various modes of stacking (3). Much of this has been done empirically, although Lewis (1a-1f) invoked topological principles to good effect. D’Arcy Thompson, in his truly admirable book, Growth and Form (4), dealt in general with the relation between shape and function in plants and animals, and included a delightful discussion of topology, geometry, and surface tension as a basis to the understanding of cell shapes. Desch (5), in 1919, made a detailed study of the shape of metal grains and showed the relation of these to cells in a soap froth and to the ideal space-filling bodies of Archimedes and Kelvin, but he did not invoke any topological principles. Harker and Parker (6) first showed the importance of grain corner angles in relation to grain growth in a metal, and the present writer (7) has pointed out the significance of this in relation to space filling, and has extended the concept to three-dimensional grains. Scheil
62 citations
TL;DR: In this paper, the microstructural and mechanical properties of a transformation-induced plasticity-aided martensitic (TM) steel that is expected to serve as an advanced structural steel for automotive applications were examined.
Abstract: This paper deals with the microstructural and mechanical properties of a transformation-induced plasticity-aided martensitic (TM) steel that is expected to serve as an advanced structural steel for automotive applications. The microstructure consisted of a wide lath-martensite-structured matrix and a mixture of narrow lath-martensite and metastable retained austenite of 2–5 vol% (MA-like phase). When 1%Cr and 1%Cr–0.2%Mo were added into 0.2%C–1.5%Si–1.5%Mn steel to enhance its hardenability, the resultant TM steels achieved a superior cold formability, toughness, fatigue strength, and delayed fracture strength as compared to conventional structural steel such as SCM420. These enhanced mechanical properties were found to be mainly caused by (1) plastic relaxation of the stress concentration, which resulted from expansion strain on the strain-induced transformation of the metastable retained austenite, and (2) the presence of a large quantity of a finely dispersed MA-like phase, which suppressed crack initiation or void formation and subsequent void coalescence.
31 citations
TL;DR: In this article, the effects of molybdenum on the microstructure, wear resistance, and corrosion resistance of carbidic austempered ductile iron were studied and the results showed that the microstructures of all heat-treated samples mainly consist of nodular graphite, acicular ferrite, retained austenite, and a certain number of carbides.
Abstract: The effects of molybdenum on the microstructure, wear resistance, and corrosion resistance of carbidic austempered ductile iron were studied. The results show that the microstructures of as-cast samples are mainly composed of nodular graphite, pearlite and carbides, and the nodularization grade is 2–3 and graphite size is 6–7 magnitude in all samples. With the increase of Mo content, the amount of nodular graphite decreases, but the pearlite and carbides increase. The microstructures of all heat-treated samples mainly consist of nodular graphite, acicular ferrite, retained austenite, and a certain number of carbides. As the molybdenum content increases, the quantity of retained austenite and carbides increases, the acicular ferrite becomes finer, and the wear resistance and corrosion resistance increased. When the concentration of molybdenum reaches 0.570 wt%, the average wear weight loss decreased to 0.218 mg/m, the corrosion potential reached to −0.5626 V, and the corrosion current density decreased to 6.361 × 10−8 A/cm2.
28 citations
TL;DR: In this article, the mechanical and corrosion behavior of Al alloy 6082-T6 subjected to solution heat treating condition at temperatures varying from 400 to 600 °C and soaking times of 3-24h have been investigated.
Abstract: The mechanical and corrosion behavior of Al alloy 6082-T6 subjected to solution heat treating condition at temperatures varying from 400 to 600 °C and soaking times of 3–24 h have been investigated. Solution heat treating at 550 °C for 24 h led to the dissolution of the Mg2Si and AlSi6Mg3Fe precipitates into the matrix, and the Al12(FeMn)3Si2 phase transformed into Al85(Fe0.28Mn0.72)14Si phase. The solution-treated alloy showed equiaxed grain morphology with an average grain size of 85.7 µm. Increasing the solution heat treating temperature beyond 550 °C caused a reduction in corrosion resistance of the alloy. The pitting potential increased due to the presence of the anodic phase Mg2Si (dissolved at 550 °C) in Al matrix and it decreased with the formation of cathodic phases such as AlSi6Mg3Fe and Al12(FeMn)3Si2 (dissolved at 550 °C/24 h) in the alloy. General corrosion resistance of the alloy increased with decreasing Mg2Si concentration in the Al matrix. The optimum solution heat treating condition of 550 °C for 24 h resulted in an improvement in hardness (63 VHN), ultimate tensile strength (UTS—208 MPa), and pitting potential (−675 mV) and uniform corrosion potential (−1.255 mV) of 6082 Al alloy.
26 citations
TL;DR: In this article, a highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene polyethylene tubes.
Abstract: A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich η-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.
25 citations
TL;DR: In this paper, the effects of vanadium and austempering temperature on the microstructure and mechanical properties of carbidic austempered ductile iron were investigated, and the results showed that the micro-structure of as-cast samples mainly consisted of pearlite, nodular graphite, and carbides.
Abstract: The effects of vanadium and austempering temperature on the microstructure and mechanical properties of carbidic austempered ductile iron were researched. The results showed that the microstructure of as-cast samples mainly consisted of pearlite, nodular graphite, and carbides, and the amount of carbides increased gradually with the increase of vanadium content (0–0.71 wt%). After austenitizing at 900 °C for 1.5 h and austempering at different temperatures for 1.5 h, the microstructure was mainly composed of acicular ferrite, retained austenite, carbides, and nodular graphite. The x-ray diffraction analysis indicated that the carbides consisted of (Cr, Fe)3 C and VC. With the increase of V (0–0.71 wt%) at the same austempering temperature, the tensile strength and impact toughness increased initially and then decreased above a critical V concentration, the Rockwell hardness and wear resistance always increased. As the austempering temperature (220–320 °C) increased, the tensile strength increased initially and then decreased after a critical austempering temperature, but the impact toughness increased, the Rockwell hardness and wear resistance decreased.
20 citations
TL;DR: In this article, an X-ray diffraction line profile analysis technique was employed to investigate the dislocation density evolution during high temperature creep of Mg-3Al-1Zn alloy.
Abstract: X-ray diffraction line profile analysis technique was employed to investigate the dislocation density evolution during high temperature creep of Mg-3Al-1Zn alloy. The microstrains within the domain and dislocation density were calculated by the simplified Williamson–Hall and Williamson–Smallman methods. Further analysis on the possible dynamic recrystallization (DRX) and dynamic recovery (DRV) shows a relation between the number of dynamically recrystallized grains and the dislocation density. At constant temperature, higher stresses lead to more DRX and an enhancement on the dislocation density; whereas, at lower stresses the DRV is dominant leading to decrease in the dislocation density.
19 citations
TL;DR: In this paper, the effect of various parameters, in particular the welding speed and shoulder diameter to pin diameter ratio (D/d ratio), on microstructures and mechanical properties of the joints were addressed.
Abstract: The dissimilar AA2014-T6 and AA7075-T6 aluminum alloy butt joints were made by friction stir welding with different process parameters. Friction stir welding is an efficient technique to join dissimilar aluminum alloys when compared to other traditional techniques. This paper aims to address the effect of various parameters, in particular the welding speed and shoulder diameter to pin diameter ratio (D/d ratio) on microstructures and mechanical properties of the joints. Six joints were made using two different welding speeds and three different D/d ratios without altering the tool rotational speed and the axial load. The results showed that the D/d ratio greatly influences the mechanical properties of the joints. Plastic deformation and recrystallization during the weld process resulted in the formation of distinct zones which could be identified by their respective microstructures. X-ray diffraction analysis was carried out to predict the formation of the precipitates which influences the properties of the joints. Fractographic images collected with a scanning electron microscope for establishing D/d ratio of 3, for two different welding speeds, were discussed. The dissimilar joints fabricated with the tool rotational speed 1200 RPM, welding speed 20 mm/min, axial load 8 kN, and D/d ratio of 3 yielded the higher tensile strength and hardness.
18 citations
TL;DR: In this paper, the behavior of indentation properties was examined as a function of the indentation loads, and the results indicated that the CoB and Co2B coatings exhibited an apparent hardness of 20 and 17 GPa respectively, in which the fracture toughness of the cobalt boride coatings only varied slightly in the set of experimental conditions proposed in this work.
Abstract: Some mechanical properties for cobalt boride (CoB and Co2B) coatings were obtained using the Vickers depth-sensing indentation technique. The coatings were developed on the surface of a CoCrMo alloy using the powder-pack boriding process at temperatures between 1223 and 1273 K using different exposure times for each temperature. Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For the entire set of experimental conditions, the behavior of the indentation properties was examined as a function of the indentation loads. Universal expressions were used to determine the apparent or real hardness, the indentation Young’s modulus, and fracture toughness of the CoB and Co2B coatings. The results indicated that the CoB and Co2B coatings exhibited an apparent hardness of 20 and 17 GPa, respectively, in which the fracture toughness of the cobalt boride coatings only varied slightly in the set of experimental conditions proposed in this work.
17 citations
TL;DR: The shipwreck designated as the Akko Tower Wreck was discovered inside Akko harbor, Israel, in 1966 and the 105 nails that were retrieved were tentatively divided into two groups (A and B) according to their general shape and size as mentioned in this paper.
Abstract: The shipwreck designated as the Akko Tower Wreck was discovered inside Akko harbor, Israel, in 1966. It was surveyed in 1975 and 1981, and excavated in 2012–2013. Hull planks were connected to the frames by brass nails, some of which were in situ, emerging vertically from the planking, where frames had disappeared, and others were detached. The 105 nails that were retrieved were tentatively divided into two groups (A and B) according to their general shape and size. Nails sampled from each group were characterized by non-destructive and destructive metallurgical methods, including XRF, light microscopy, SEM–EDS, microindentation hardness measurements, and lead isotope analysis, in order to reveal their composition and microstructure, as well as to determine their manufacturing process and date the origin of the raw material. All nails were manufactured by casting and made of binary copper-zinc alloy: Type A with 35–36 wt% zinc, and Type B with 33–34 wt% zinc (XRF results). These compositions confirmed the preliminary typing of the nails. Their microstructure indicates that they were manufactured differently: Type A nails have Widmanstatten thin lamellae plates, and Type B nails have α-brass grains containing a dendritic microstructure. Based on their zinc and lead concentrations it is suggested that the nails were manufactured in the first half of the nineteenth century. The lead isotope analyses of the nails suggest that the raw material most probably originated in Great Britain. Hence, it is suggested that the Akko Tower Wreck is the remains of a European merchantman which sank in Akko harbor about the middle of the nineteenth century.
16 citations
TL;DR: In this paper, the effects of carton content and other alloying elements on the solidification behavior and associated microstructure of Stellite alloys are investigated and the phase transformation temperatures of these alloys during heating and cooling are determined under the DSC test.
Abstract: Stellite alloys are a family of superalloys, which can be categorized as CoCrW and CoCrMo systems. Ten selected Stellite alloys from these systems, with low carbon or high carbon content, are studied with respect to solidification and microstructure using differential scanning calorimetry (DSC), scanning electron microscopy with an energy dispersive x-ray spectroscopy, and x-ray diffraction. The objective of this work is to investigate the effects of carton content and other alloying elements on the solidification behavior and associated microstructure of Stellite alloys, to provide the materials industry with technical guidance in design, manufacturing, processing, and heat treatment of these alloys. The phase transformation temperatures of these alloys during heating and cooling are determined under the DSC test. The solidified microstructures and the phases present are analyzed and discussed with emphasis on the relation to chemical composition.
TL;DR: In this paper, the aging kinetics of the GTD-111 (used in gas turbine blades for thermoelectric energy generation) were analyzed using scanning electron microscopy and Vickers hardness measurements.
Abstract: Nickel-based superalloys are normally present in mechanical components requiring resistance to high temperatures and extreme mechanical stresses, e.g., gas turbine blades. The large amount of γ′-precipitates coherent with the γ-matrix in these superalloys ensures excellent properties at high operating temperatures. Nevertheless, after long service periods, the γ′-precipitates grow and lose coherence with the matrix. Intermediate thermal treatments are a potential solution to recover the microstructure, extending the component lifetime. Thermal cycles for γ′-dissolution and reprecipitation define the efficiency of such thermal treatments. The objectives of this work are to analyze the aging kinetics of the nickel-based superalloy GTD-111 (used in gas turbine blades for thermoelectric energy generation) and to study the effects of solution and preprecipitation treatment temperatures on the microstructure and hardness obtained by rejuvenation thermal treatment. For the experiments, samples were previously aged under laboratory conditions during 2,000 h at 1,000 °C. The subsequent rejuvenation thermal treatments consisted of four steps: solution (at 1,150, 1,175, and 1,190 °C), preprecipitation (at 1,055 and 1,120 °C), and reprecipitation (at 845 °C). The resulting microstructures were analyzed using scanning electron microscopy and Vickers hardness measurements. During the aging treatment, the average size of γ′-particles changed from 0.20 µm2, as found in virgin material, to 0.99 µm2 after 2,000 h of aging; the hardness changed from 417 to 320 HV, respectively. For solution treatment at 1,150 °C, total dissolution of γ′-particles did not occur and they presented a bimodal size distribution of coarse and fine particles. For solution treatment at 1,175 and 1,190 °C, the γ′-particles were fine and homogeneous, and the maximum hardness obtained after reprecipitation was 460 HV for 1,190 °C solution treatment and 1,120 °C preprecipitation.
TL;DR: In this paper, three nickel- and cobalt-based superalloys were exposed to Na2SO4−25%NaCl for 100 cycles and the resulting corrosion products were characterized using scanning electron microscopy and energy dispersive spectroscopy.
Abstract: Superalloys are widely used for high-temperature applications But these alloys also suffered from severe degradation where corrosion problem frequently encountered due to the presence of molten salt deposits To obviate this problem, three nickel- and cobalt-based superalloys had been exposed to (Na2SO4–25%NaCl) at 900 °C for 100 cycles The resulting corrosion products were characterized using scanning electron microscopy and energy dispersive spectroscopy It was observed that both Ni-based alloys exhibited improved corrosion resistance over the Co-based alloy in this aggressive environment In Ni-based alloys, the scale contained oxides such as NiO, Cr2O3, Fe2O3, and spinel NiCr2O4, whereas in case of Co-based alloy, the scale contained CoO and CoNiO2
TL;DR: In this article, the effects of manganese addition on the microstructure, tensile properties and hardness of the as-cast (AC) and heat-treated (HT) Mg-2Zn-0.5, 1.0 and 1.5-wt alloys were investigated for biomedical osteosynthesis applications.
Abstract: The effects of manganese (Mn) addition on the microstructure, tensile properties and hardness of the as-cast (AC) and heat-treated (HT) Mg-2Zn-0.5Ca-xMn (x = 0.5, 1.0 and 1.5 wt%) alloys were investigated for biomedical osteosynthesis applications. The transformation of the microstructure and precipitation of the AC and HT alloys with different Mn additions were analyzed by optical microscope, x-ray diffraction, scanning electron microscope and transmission electron microscope. Mechanical properties of the alloys were tested on an electronic universal testing machine and a micro-hardness tester. The results showed that the polygonal α-Mn precipitate was the existing form of Mn in the alloys, which was attached by Mg-Zn phases. With increasing the Mn addition, the grains in the AC Mg-2Zn-0.5Ca-xMn alloys became more refined, and changed from bulky and uneven dendrite crystal to uniform equiaxed crystal. Meanwhile, the MgZn2 and Mg2Zn3 phases increased in HT alloys with the increase in Mn addition. Moreover, the strength and elongation of the AC and HT alloys both decreased after reaching a peak with the increase in Mn addition. The HT Mg-2Zn-0.5Ca-1.0Mn alloy also possessed the optimal tensile strength of 205 MPa and the elongation of 15.7% among the alloys in this study.
TL;DR: In this article, the effect of filler metals on the weldability, microstructure, tensile strength, and impact toughness of bimetallic joints involving ferritic stainless steel (AISI 430) and super-austenitic stainless steel, were reported using continuous current and pulsed current gas tungsten arc-welding (GTAW) processes.
Abstract: This research article reports the effect of filler metals on the weldability, microstructure, tensile strength, and impact toughness of bimetallic joints involving ferritic stainless steel (AISI 430) and super-austenitic stainless steel (AISI 904L). These dissimilar combinations were obtained using continuous current (CC) and pulsed current (PC) gas tungsten arc-welding (GTAW) processes, employing duplex stainless steel (ER2553) and Inconel 625 (ERNiCrMo-3) fillers. Microstructure studies revealed the presence of grain boundary martensite and a slight grain coarsening effect in the HAZ of AISI 430 for all the cases. The study also inferred that both CCGTA and PCGTA welding employing these fillers resulted in better joint strength by observing failure at the parent metal of AISI 430 in all the trials. Impact toughness was found to be better for the PCGTA weldments employing ERNiCrMo-3 filler due to the refinement of grains. The impact studies showed that the root pass corroborated higher toughness values than the cap zone for all the cases. Based on the structure–property relationships, this study recommends the use of PCGTA employing ERNiCrMo-3 for joining AISI 430 and AISI 904L.
TL;DR: In this article, the authors used artificial neural network (ANN) to predict the microhardness of the center of the stir zone of mild steel plates and found that the number of passes, rotational speed, traverse speed, and nano-sized Al2O3 powder were considered as input parameters for the ANN model.
Abstract: Parameters of the friction stir processing of mild steel plates were correlated with microhardness of the stir zone using artificial neural network (ANN) modeling and experimental methods. For this purpose, the number of passes, rotational speed, traverse speed, and addition of nano-sized Al2O3 powder were considered as input parameters for the ANN model, while microhardness of the center of the stir zone was obtained as the output. To examine the accuracy and capability of the model in predicting the microhardness, the effects of all ANN input parameters on the microhardness were also examined experimentally with and without the addition of Al2O3 nanopowder. For the surface nanocomposites produced, increase in the number of passes and rotation speed led to increased microhardness values, whereas higher traverse speed resulted firstly in a rise in microhardness followed by a decreased microhardness. Using optical and scanning electron microscopy, the variations in microhardness were closely discussed based on the microstructural changes. Experimental results proved to show excellent conformity with the ANN model.
TL;DR: In this paper, the microstructures and crystallographic textures of single-pass and double-pass welds have been studied as a first approach to understand a multipass weld, and the highest hardness has been measured in the high-temperature heat-affected zone (HAZ) inside the base metal.
Abstract: 13Cr4Ni martensitic stainless steels are frequently used in hydroelectric industries. Considering the size and geometry of the turbine runners manufactured in hydroelectric industries, multipass welding procedures are common methods for fabrication and repair. In this research, the microstructures and crystallographic textures of single-pass and double-pass welds have been studied as a first approach to understand a multipass weld. The highest hardness has been measured in the high-temperature heat-affected zone (HAZ) inside the base metal. Similarly, it has been found that the heat of the second pass increases the hardness of the previous pass and produces a finer martensite microstructure. In areas of the HAZ, 3–6 mm from the fusion line, a tempering-like effect is reported; traces of austenite have also been found in these areas documenting the complexity of the microstructure found in the multipass welds.
TL;DR: In this article, the authors investigated the creep behavior of AZ31 magnesium alloy at three different temperatures (230, 270, and 350 °C) and stresses of 1-13 MPa.
Abstract: Investigation of creep behavior of AZ31 magnesium alloy at three different temperatures (230, 270, and 350 °C) and stresses of 1–13 MPa reveals that grain boundary sliding (GBS) is the dominant creep mechanism at elevated temperatures and low stresses. GBS and Mg17Al12 precipitates in Mg–Al alloys result in stress concentration sites for cavity formation during high-temperature low-strain rate deformation leading to premature failures. Analysis of fractured surfaces of samples deformed at 350 °C reveals that brittle-type fracture (inter-granular and trans-granular) is the dominant mechanism at low stresses (σ = 1–5 MPa) while at higher stresses (σ = 7–13 MPa) dimple ruptures are predominant. Grain growth, dynamic recovery, and a decrease in dislocation density are characteristics of low-stress deformation of AZ31 alloys in GBS region whereas increase in dislocation density and dynamic recrystallization is noted during deformation under higher stresses where dislocation creep was noted to be predominant.
TL;DR: In this paper, the role of microstructural analysis conducted on the burst tested propellant tank in locating the origin of crack initiation was highlighted, which is the material of choice for the design of propellant tanks for cryogenic engines of satellite launch vehicles.
Abstract: Owing to its excellent combination of weldability and strength at low temperatures (up to 4 K), aluminum alloy AA 2219 is the material of choice for the design of propellant tanks for cryogenic engines of satellite launch vehicles. In order to assess the design margins available, one propellant tank has been subjected to burst test. This paper highlights the role of microstructural analysis conducted on the burst tested propellant tank in locating the origin of crack initiation. Specimens collected from different locations of the burst tested tank have been subjected to optical microscopy and scanning electron microscopy to locate the origin of crack initiation.
TL;DR: In this paper, the effect of boron addition on the strength of steel Q690D plate was studied and the results showed that the microstructure became finer distinctly with an increase in the content of soluble borone in steel.
Abstract: The effect of boron addition on strengthening of steel Q690D was studied. The metallographic microstructures of Q690D plate samples after controlled rolling and cooling process and roughing rolling in bench-scale were observed by optical microscope and transmission electron microscope, and the mechanical properties were determined. The results showed that the microstructure of Q690D became finer distinctly with increase in the content of soluble boron in steel. The yield strength and tensile strength also increased with the increase of soluble boron. Most of boron in steel Q690D existed in solid solution state. The main precipitates in Q690D steel were the Ti(C, N) and NbC. The amount of BN was small. Among the strengthening performance of steel Q690D given by boron addition, refinement strengthening gave the greatest contribution on the strengthening effect.
TL;DR: In this article, an account of a study of the austenite: pearlite transformation in simple-carbon eutectoid steels in terms of the rate of formation of nuclei and the growth of these nuclei is given.
Abstract: This lecture is an account of a study of the austenite: pearlite transformation in simple-carbon eutectoid steels in terms of the rate of formation of nuclei and the rate of growth of these nuclei. The austenitizing reaction is considered from this point of view; the structure and the mode of formation of pearlite are described, and the results of the application of the electron microscope to the study of the structure of steel announced; and data are given on the rate of nucleation and rate of growth of pearlite correlated with austenitizing treatment and the degrees of austenite heterogeneity, which this determines.
TL;DR: In this paper, a Si-modified aluminide coating on the substrate of a polysilicon-modified Al-2Nb-2Cr (at.%) alloy was used as the substrate material.
Abstract: In this study, Ti-48Al-2Nb-2Cr (at.%) alloy was used as the substrate material. The coatings’ formation process was prepared by the slurry method. After the deposition of water-based slurry containing Al and Si powders on the substrate, the diffusion treatment has been carried out at 1,000 °C for 6 h in vacuum. The structure of the silicon-modified aluminide coating is as follows: (1) an outer layer consisting of a TiAl3 matrix and Ti7Al5Si12 grains dispersed within this matrix; (2) a middle layer composed of a TiAl3 matrix and columnar grains of Ti5Si3; and (3) an inner layer as TiAl2 phase. Mechanisms and reasons for the formation of such silicon-modified aluminide structure on the γ-TiAl alloy were discussed in this paper. The results of isothermal oxidation test of 1,000 °C/200 h showed that formation of Si-modified aluminide coating on γ-TiAl alloy by a slurry method can greatly improve its high temperature oxidation resistance.
TL;DR: In this article, the tensile and creep deformation behavior of HP40Nb micro-alloyed steel at temperatures ranging from 800 to 1200°C was analyzed and the Young's modulus, yield strength, and ultimate tensile strength of the steel decreased and the ductility increased with the increase in temperature up to 1000°C.
Abstract: Reformer tubes are designed for prolonged service conditions at high temperatures and pressures in petrochemical industries for the production of hydrogen-rich gas. Frequent premature failures of reformer tubes are encountered due to the microstructural degradation of the material. Analysis of the tensile and creep deformation behavior of HP40Nb micro-alloyed steel at temperatures ranging from 800 to 1200 °C is presented in this paper. The Young’s modulus, yield strength, and ultimate tensile strength of the micro-alloyed steel decreased, and the ductility increased with the increase in temperature up to 1000 °C. The stress–temperature-dependent steady state creep rate obeys Young’s modulus compensated power-law relationship. The stress exponent and activation energy for creep were determined to be 7.96 and 332.65 kJ/mol. The predicted minimum creep rates were compared with the experimental values. It is possible to predict the value of minimum creep rate within an error less than ±7%. Analysis of fractured surfaces reveals brittle fracture up to 800 °C, while at higher temperature, dimple fractures are predominant.
TL;DR: In this paper, the authors analyzed how a deep cryogenic process changes the microstructure and mechanical properties of a medium-carbon low-alloy steel and found that η-carbides grow in a Hirotsu and Nagakura orientation to the martensitic matrix that enables highly coherent interphase boundaries.
Abstract: This paper analyzes how a deep cryogenic process changes the microstructure and mechanical properties of a medium-carbon low-alloy steel. The light microscopy, microhardness testing, and transmission electron microscopy reveal that η-carbide and martensite are constituent phases of deep cryogenic treated (DCT) steel, while the microstructure of conventional heat-treated (CHT) steel consists of cementite, martensite, and retained austenite. Transmission electron microscopy also shows that the particles of η-carbide have the shape of ultra-fine globules in DCT martensite. The η-carbides grow in a Hirotsu and Nagakura orientation relationship to the martensitic matrix that enables highly coherent interphase boundaries. The results of the mechanical tests, including tensile and Charpy impact tests, show that the deep cryogenic process can improve toughness in terms of elongation (~12.81%), tensile fracture energy (~266 MPa), and the ductile–brittle transition temperature (~−17.2 °C). The results of fractography are also consistent with the improvement in toughness. It is also found that the strength and macrohardness values are increased. Unlike CHT steel, discontinuous yielding is observed in DCT steel. Moreover, there is no change in Young’s modulus due to the deep cryogenic treatment.
TL;DR: In this article, an attempt has been made to weld nickel-based superalloy, Inconel 718 and ferritic stainless steel, AISI 430, using a gas tungsten arc welding process employing ER2553 and ERNiCrMo-4 fillers.
Abstract: In this research work, an attempt has been made to weld nickel-based superalloy, Inconel 718 and ferritic stainless steel, AISI 430, using a gas tungsten arc welding process employing ER2553 and ERNiCrMo-4 fillers. Microstructure studies revealed the formation of an unmixed zone and the presence of tiny Nb-rich phases in the heat-affected zone (HAZ) of Inconel 718; also, grain coarsening was observed at the HAZ of AISI 430. Tensile studies corroborated that the failures occurred at the parent metal of AISI 430 in all the experimental trials. Bend test results revealed that no cracks occurred in either of the weldments reporting for ductility and soundness of the weld joints. This work also articulates the detailed structure–property relationships of these dissimilar weldments using the combined techniques of optical and scanning electron microscopy. The outcomes of the present study will be helpful to the aerospace and nuclear sectors.
TL;DR: In this article, the microstructure and low-temperature superplasticity of ZK60 magnesium alloy processed by equal-channel angular pressing (ECAP) were investigated.
Abstract: The microstructure and low-temperature superplasticity of ZK60 magnesium alloy processed by equal-channel angular pressing (ECAP) were investigated. Homogeneous and refined microstructure was achieved after four passes of ECAP at 200 °C. After ECAP, the ZK60 alloy showed excellent superplastic elongation of 628% at 250 °C under strain rate of 1.67 × 10−3 s−1. Low-temperature superplasticity was obtained after four passes of ECAP at 200 °C. A theoretical foundation for severe plastic deformation of magnesium alloy at low temperature is developed in this work.
TL;DR: A billet of hypoeutectic high-chromium white iron (2.5% C, 19% Cr) was spray formed using gas-to-metal ratios of 0.9, 1.0, and 1.1 as discussed by the authors.
Abstract: A billet of hypoeutectic high-chromium white iron (2.5% C, 19% Cr) was spray formed using gas-to-metal ratios of ~0.9, ~1.0, and ~1.1. The as-sprayed material was close to full density and contained fine (Fe,Cr)7C3 carbides (27–30 vol%) uniformly distributed in a matrix consisting of pearlite and ferrite. Length of the carbides rarely exceeded 30 μm. This was in stark contrast to a conventionally cast starting material, which contained coarse (Fe,Cr)7C3 carbides (~22 vol%) non-uniformly distributed in a matrix consisting of austenite and martensite. Length of the carbides in the cast material occasionally exceeded 100 μm. Varying gas-to-metal ratio between 0.9 and 1.1 did not result in any significant changes in carbide morphology, although slightly coarser carbide morphology was produced with the gas-to-metal ratio of 0.9. Regardless of the gas-to-metal ratio, the finest carbide morphology was found in the surface region of the spray-formed billet.
TL;DR: In this paper, an ultrafine-grained Al alloy was produced from its bulk alloy by cryoforging followed by cryorolling, and the results have shown that combined cry-oforging and aging led to remark- able improvement in strength and ductility.
Abstract: In the present investigation, ultrafine-grained Al alloy was produced from its bulk alloy by cryoforging followed by cryorolling. The bulk Al-Mg-Si alloy, with initial grain size 400 lm, was subjected to solid solution treatment (ST) followed by water quenching at room temperature. The ST treated alloy was subjected to aging at 100 C for 4 and 8 h prior to cryoforging. The cryoforged alloy was subjected to cryorolling up to 2.4 true strain for producing long sheets. Finally, the deformed alloy was subjected to low temperature aging at 120 C to improve the tensile properties of the alloys. Microstructure and mechanical properties were evaluated through Vickers hardness testing, tensile testing, and electron back scattered diffraction. The results have shown that combined cry- oforging ? cryorolling followed by aging led to remark- able improvement in strength (UTS-452 MPa) and ductility (8%). The average grain size of the alloy was found to be 240 nm, with increased fraction of high angle grain boundaries. Low temperature differential scanning calorimetry was used to study thermal behavior of bulk and severely deformed alloy.
TL;DR: Weldability of kinetically activated bainite steels with the process of activated tungsten inert gas welding was evaluated by means of mechanical testing and metallographic techniques as discussed by the authors.
Abstract: Weldability of kinetically activated bainite steels with the process of activated tungsten inert gas welding was evaluated by means of mechanical testing and metallographic techniques. The latter were used to identify which phases formed and to determine their volume fractions as well as formation mechanisms. Based on this information, correlations were established between the welding parameters and the observed metallurgical phenomena.
Particular emphasis was put on the morphology and the amount of retained austenite and the local formation of martensite. All tests were performed without preheating or post weld heat treatment of the high-carbon steel.
TL;DR: In this paper, a failure analysis of a tool steel (AISI A2) beater head of an impact crusher and development of suitable heat treatment process to improve its performance have been presented.
Abstract: Failure analysis of a tool steel (AISI A2) beater head of an impact crusher and development of suitable heat treatment process to improve its performance have been presented. The beater heads were failing prematurely by brittle fracture from its pin-hole locations. The investigation consisted of visual inspection, fractography, chemical analysis, characterization of microstructures using optical and scanning electron microscopes (SEM), EDS analysis, and determination of micro-hardness profile. Microstructural characterization using SEM and EDS analysis revealed significant amount of coarse continuous Cr-carbide networks in the martensite matrix. It increased hardness (64 HRC) as well as heterogeneity of the matrix as depicted by the micro-hardness profile, and decreased the toughness (3 J) since coarse carbide networks are very hard and brittle. The austenitizing temperature as well as tempering temperature of heat treatment was found lower at the manufacturer’s end. The new recommended heat treatment resulted in lower amount of discontinuous Cr-carbides along with significant amount of fine precipitates uniformly distributed throughout the matrix which led to an optimum combination of both hardness (59 HRC) and toughness (6.5 J) required for the application. The beater heads manufactured following the recommended heat treatment exhibited better performance (life increased by 4 times) compared to the earlier ones.