Ferrite Matrix Research Articles

Microstructure Evolution and Mechanical Behaviour of SAW Weldments of CSEF Steel on Preheating and PWHT

In this study, double-sided submerged-arc-welded CSEF steel joints were considered for microstructural and mechanical (hardness, ductility and strength) characterisations for four different cases of preheating and PWHT processes. The XRD and EDS analyses were also performed to investigate the phase transformation and chemical composition. Preheating was observed to be an improving factor for ductility and hardness of the welds. Similarly, PWHT enhanced the uniformity in the hardness distribution owing to reduced austenitic grain size in the HAZ as well as equiaxed fine-grained ferrite matrix in the FZ. It was observed that preheating and combined preheating and PWHT increased the ductility of the weld by 16.9% and 56.3%, respectively, from the as-welded case. Furthermore, it was also observed that preheating individually and preheating combined with PWHT reduced the maximum hardness values by 8.2% and 60.2%, respectively.

Local strain hardening behavior in a dissimilar metal welded joint with buttering layer of ultra-supercritical turbine rotor

Stratified tensile tests were performed to quantitatively investigate strain hardening behavior along a dissimilar metal welded joint (DMWJ) with buttering layer of ultra-supercritical turbine rotor. The results show weld region has lower tensile strength but better strain hardening capacity than base metals (BMs). The buttering layer (BL) exhibits higher strain hardening capacity than weld metal (WM). Two strain hardening exponents were found based on the fitted stress-strain curves using Hollomon equation. The lower exponent is related to the ferrite matrix, whereas the larger exponent is related to the precipitation phase, including martensite, bainite and sorbite. The gradient distribution of strain hardening exponent in heat affected zones (HAZs) is related to the gradient change of tempered martensite and grain size. The strain hardening mismatch in interface is related to the drastic changes in microstructure and amount of carbides. The dislocation cell structure and sub grain were observed after tensile tests, and carbides were elastic during tensile deformation. Kocks–Mecking (K–M) type plots of strain hardening rate versus true stress presented two hardening stages (stages III and IV) in welded joint. More ferrite and finer tempered martensite are conductive to extending the strain hardening period. Local strain hardening parameters play an important role in the accurate evaluation of crack behavior in HAZ and interface.

Influence of Different Soaking Times at 1050 °C on the UT Response Due to Microstructure Evolution of 2205 Duplex Stainless Steel

Under standard conditions, DSS (duplex stainless steel) features differing amounts of ferrite and austenite, essentially depending on the thermal treatment performed. This study is focused on the ultrasonic tests (UTs) response of DSS 2205, as a function of the microstructure, in terms of austenite volume fraction and austenitic grains evolution owing to different soaking times at 1050 °C. UTs were carried out on several samples. The samples underwent varying thermal treatments characterized by a constant maintenance temperature with different soaking times that allowed for microstructure evolution and modification of the structural constituents’ fraction. The UTs have highlighted an attenuation trend with the response mainly dependent upon the wave scattering and energy absorption caused by the grain features. In particular, the peak of sound attenuation was shown to correspond with the microstructure, which featured a major amount of austenite (in terms of volume fraction and the grain dimensions) and the disappearance of austenitic precipitates within the ferritic matrix. In order to obtain less UT attenuation, without affecting the mechanical and corrosion properties, the soaking should last as little time as possible.

Microstructure authentication on mechanical property of medium carbon Low alloy duplex steels

The present study emphasizes on the mechanical characterization of medium carbon AISI 1040 and 4140 duplex steels. AISI 1040 and 4140 steels are generally used as structural steels for varying applications, ranging from construction field material to heat resistant parts for moderate temperatures applications. The objective of the present work is to corroborate microstructure and mechanical properties by varying the intercritical temperature in duplex steels. The normalized specimens of as bought steel are heated to a different intercritical temperatures (750, 770, and 790 °C), followed by isothermal holding for 2 h and quenching to room temperature develops duplex phase structure. This treatment grows microstructure with a variable quantity of martensite embedded in a fine ferrite matrix. For the same carbon content in the duplex alloy steels, the addition of Cr, Mo, Si, and Mn, increases the tensile strength and hardness of the material but decreases the toughness and ductility. As the intercritical temperature varies, the amount of martensite and ferrite phases present in the duplex steel also varies, which intern alters the strength and hardness of the material. Duplex steels have higher strength and hardness than as bought, even though the impact strength and ductility are poor. Microhardness is used to identify the micro phases present in the material viz., ferrite and martensite. Microstructure shows the evidence of the formation of a duplex structure with a variation in the amount of aforesaid phases. The result obtained validates the influence of microstructure on mechanical properties of AISI 1040 and 4140 steels.

Investigation of electrical, magnetic, and optical properties of silver-substituted magnesium–manganese ferrite nanoparticles

In this work, a series of silver-doped Mg–Mn ferrite nanoparticles, Mg0.9Mn0.1AgxFe2−xO4 (x = 0.0, 0.1, 0.2), has been synthesized by solution combustion method and effects of Ag+ ions on the structural, electrical, magnetic as well as optical properties have been investigated. The phase recognition has been confirmed by utilizing X-ray diffraction (XRD) study. The particle size and strain has been estimated using the Williamsons–Hall plots, while Nelson–Riley plots have been utilized to predict the lattice parameter. An increase in the particle size (17.5–21.8 nm) and lattice parameter (8.37–8.39 A) has been viewed with the adding up of Ag+ ions. Incorporation of Ag+ ion has decreased the resistivity and has produced a normal dispersion curve in the dielectric properties. The M–H study has confirmed an increase in the saturation magnetization (16.9–22.7 emu/g), which has reflected the strengthening of magnetic exchange interactions with the addition of silver content in the Mg–Mn ferrite nanoparticles matrix. The bandgap estimated from the Tauc plots has been viewed to decrease (6.08–4.85 eV) by the rise in silver ions content. Raman spectroscopy study has predicted a slight shift in the Raman modes towards the lower frequency. The electric and dielectric properties have been explained in the light of Verwey’s hopping mechanism and Maxwell–Wagner model, respectively. The obtained magnetic parameters have suggested that present nanoferrites are quite suitable for electromagnet application.

Mechanism for H-shaped precipitate formation in 1.25Cr-0.5Mo steel

Needle-like M2C precipitates with a hexagonal close-packed (HCP) structure, which are referred to as Mo2C, are well known to be responsible for the precipitation strengthening of low Cr heat-resistant steel. This paper reports H-shaped precipitation of M2C on CrN in a 1.25Cr-0.5Mo ferritic steel aged at 550 °C for 5000 h and explains the precipitation mechanism. Mo diffuses into CrN from the ferrite matrix during the aging. Chemical elemental mapping in transmission electron microscopy (TEM) shows that Mo-enriched regions in CrN act as nucleation sites of M2C. Furthermore, the thermodynamic calculation indicates that the solubility of Mo in CrN is higher than that in the matrix at 550 °C. The precipitation of M2C on CrN seems to reduce the misfit strain energy between the ferrite matrix and CrN during long-term aging. High resolution transmission electron microscopy (HRTEM) reveals that M2C has a specific orientation relationship with the ferrite matrix as well as CrN. Only M2C precipitates meeting the orientation relationship with the matrix tend to grow dominantly in one direction, which seems to be responsible for the H-shaped precipitation.

Microstructural Adjustment of the Degenerated Graphite Layer in Ductile Iron for Targeted Evaluation on the Fatigue Properties

Graphite degeneration is the most prominent microstructural defect in the casting skin of ductile iron. Induced by either sulfur or oxygen contained in the molding material, its occurrence can be observed in a large range of iron castings, having substantial negative influences on the mechanical properties, especially fatigue resistance. Previous investigations predominately focused on the influence of the degenerated graphite layer, while accompanying changes of the iron matrix were neglected. The superposition of these effects hinders the evaluation of casting skin defects in state-of-the-art design of ductile iron components. The presented solution utilizes an experimental procedure, which enables the production of specimens with specific, individual microstructural configurations in the casting skin. This approach is based on the application of a modified sand core coating. By adding sulfurizing additives to the coating and adjusting its binder content and viscosity, a predominately homogeneous degenerated layer of 0.5 mm thickness was obtained. Using specific heat treatment steps assured a fully ferritic or pearlitic iron matrix, isolating the degenerated layer as the main microstructural defect. Fatigue testing of these specimens will further enable the numerical evaluation of the fatigue resistance in dependence of the casting skin microstructure.

METALS HEAT TREATMENT MODEL

This paper considers the problem of developing a model of thermal metal processing by multiprocessor computing systems. The obtained metal is used for high-strength fasteners manufactured by cold forging method without final heat treatment. The model is based on the heat treatment method of a billet from low- and medium-carbon steels intended for cold heading. The model aims at improving technological properties of a billet by ensuring high dispersion and uniformity of a billet structure across the entire plane of its cross-section.Implementation of the proposed model ensures the technical result of high dispersion and uniformity of the structure of the billet. The technological process of steel heat treatment is characterized by high performance, low power consumption, and improved performance characteristics. The apparatus for implementation of the spheroidization annealing regime determines the uniform distribution of cementite globules in the ferrite matrix, which means that it provides the necessary mechanical properties of the metal for its further cold deformation. The multiprocessor computing system software allows controlling the temperature conditions, both on the entire plane of the billet section, and across its length. Such temperature conditions are controlled in the center of the plane of the billet cross-section.Experimental studies of the heat treatment of metal products were conducted. In order to test the functions of the proposed model, several experiments were performed when a 20 mm diameter wire from 20G2G steel was subjected to heat treatment. Experimental studies have shown that metal has the necessary elasticity properties, saving the required hardness.

Evaluation of Pitting Corrosion in Duplex Stainless Steel Fe20Cr9ni for Nuclear Power Application

A spinodal decomposition is often carried out in the austenite-ferrite duplex stainless steel Fe20Cr9Ni when it is serviced at 280-500℃ for a definite time, resulting in a decrease of pitting corrosion resistance. Fe-rich α phase rather than G phase has been suggested as the arch-criminal in causing the declining corrosion resistance of thermally-aged steel. Here, we found that ~76.8% of the decline in pitting resistance for the duplex stainless steel Fe20Cr9Ni was attributed to G phase, and ~23.2% to Fe-rich α phase after the spinodal decomposition. In this study, a suitable thermal aging treatment was introduced to obtain a larger size of the G-phase and to study the role of the phase in the corrosion process. Through immersing thermally-aged TEM specimen treated at 475℃ for 3000 h in NaCl solution, the preferential position of corrosion pits formed in the ferrite was obtained. The composition changes and strain field distribution around the G phase were analyzed by TEM-EDS, 3DAPT and GPA techniques. We further found that, although the concentration difference of Cr element between α and α' phases was as high as 60 at.%, corrosion pits were initiated at the interface between the G phase and the ferrite matrix rather than in the Fe-rich α phase, indicating that the Cr-depleted theory could not explain the aforesaid phenomenon. The strain energy at the interface between the G phase and the ferrite matrix was found to be the largest. The atoms at the interface have higher energy than in the intracrystalline, and thus easily react with Cl- ions in the solution to form pits finally.

Role of tempering cooling rate on impact toughness of 2CrMoV weld metal

Abstract

Creep of the Matrix During Coalescence and Overgrowth of Graphite Precipitates in a High-Silicon Spheroidal Graphite Iron Submitted to Thermal Cycling in the Ferritic Domain

Thermal cycling of a high-silicon spheroidal graphite cast iron within the ferritic domain leads to coalescence of graphite particles with dendritic protuberances on the largest ones. This phenomenon occurs if there is some dissolution of graphite at the maximum temperature of the thermal cycle. Upon cooling, carbon redeposits in preferred locations, leading to the protuberances. This communication demonstrates that the ferritic matrix around the overgrowths can creep and remain in close contact with graphite.

Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy

The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth of γ′ iron nitride upon controlled gaseous nitriding was investigated. The nitrided specimens were characterized by XRD, light microscopy, TEM, EELS, EPMA and EBSD. The combination of difficult and thus delayed precipitation of (1) silicon nitride, because of a large misfit with the ferrite matrix, and (2) γ′ iron nitride, because of the negligible solubility of Si, was shown to lead to a series of unusual, nonequilibrium phenomena: high nitrogen supersaturation, development of Si-containing metastable γ′ phase of peculiar morphology, precipitation of amorphous silicon nitride in γ′ and development of ε nitride at thermodynamic conditions unlikely to allow its formation.

Micromechanism of Damage of the Graphite Spheroid in the Nodular Cast Iron During Static Tensile Test

This work was focused on two particular phenomena contributing to a damage process of nodular cast iron under tensile stress: Internal destruction of graphite nodule and debonding at graphite/matrix (G-M) interface. The G-M debonding was analyzed depending on the phase characteristics of the metal matrix and with the increase in the distance of the observation field from the main crack surface. Typical morphological effects of decohesion in the graphite-matrix microregions related to an internal structure of graphite nodule were revealed and classified. The obtained results of the microscopic observations suggest that the path of both types of internal cracks in the graphite nodule passed through areas of weakened cohesion. Detailed microscopic observations allowed revealing some additional phenomena associated with G-M debonding along the G/M interface. In the most ductile of the tested alloys, with ferritic and ausferritic matrix, the G-M debonding was preceded by the formation of a layer of shifted graphene plates in the external envelope of the spheroid. In the alloys of polyphase pearlitic and ausferritic matrix, the revealed morphology of the G-M interface suggests that G-M debonding might be delayed by the interaction with some phase components as cementite lamellae and austenite plates.

Strengthening mechanisms of creep-resistant 12%Cr–3%Co steel with low N and high B contents

The contributions from the martensitic laths, dislocations, secondary phase particles, and supersaturated solid solutions to the overall strength of the 12%Cr–3%Co–2.5%W creep-resistant steel with low N and high B contents were calculated after various heat treatments consisting of normalizing followed by medium-temperature tempering. An increase in the normalizing temperature from 1050 to 1150 °C led to an increase in the average size of the prior austenitic grains from 44 to 68 μm, but the δ-ferrite fraction did not significantly change. Medium-temperature tempering in the range of 750–800 °C ensured the formation of a tempered martensite lath structure with an average martensitic lath/subgrain size of 0.23–0.34 μm, along with a high dislocation density inside the laths/subgrains, fine secondary phase particles such as M23C6 carbides along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths/subgrains, and (Ta,Nb)X carbonitrides uniformly distributed inside the matrix. After medium-temperature tempering in the range of 750–800 °C, the ferritic matrix was supersaturated with substitutional elements such as Cr, W, Mo, and Cu. An increase in the tempering temperature from 750 to 800 °C led to decreases in the yield strength and ultimate tensile strength by 16.2% and 10.5%, respectively, as well as an increase in the elongation of 43.8%. The main contributions to the overall strengthening of the steel investigated after the different heat treatment regimens produced solid solution strengthening and precipitation hardening, which were independent of the tempering temperature, as well as lath boundary and dislocation strengthening, which was strongly dependent on the tempering temperature. Different approaches for evaluating the strengthening mechanisms and their contributions to the yield strength were applied, and the results are discussed.

Effect of Welding Processes on Mechanical and Microstructural Characteristics of DP780 Steel Welded Joints for the Automotive Industry

Arc welding processes are widely used in the automotive industry among other welding processes. Consequently, laser welding technology is being used instead of arc welding due to the rapid heating and cooling characteristics of the laser. In this study, empirical investigations and comparative study are held out on the arc and laser beam welded joints of DP780 dual-phase steel. Accordingly, weld joint microstructures, hardness distribution, and fatigue properties cross the butt-welded joints were investigated. The results showed that laser beam welding produces narrow fusion and heat-affected zones while gas metal arc welding produced wide welds with incomplete penetration. It was observed that the microstructure of the laser joint weld metal has mainly lath martensite in the ferritic matrix, while microstructure of gas metal arc weld metal relies upon filler type. Heat-affected zone in DP780 steel exhibit hardness softening in both laser beam welding and gas metal arc welding due to martensite tempering, a wider softening region was clearly observed in heat-affected zone welded by gas metal arc welding than laser beam welding. Generally, fatigue ratio, fatigue limit and fatigue life of the welded joints were improved by using laser welding.

Redistribution of Mn between α-Fe matrix and θ cementite during long-term thermal aging in a low alloy steel

The addition of certain amounts of Mn in steel has long been known to retard the growth and coarsening of cementite during tempering, which can increase the tempering resistance of carbon steels. It is now well-established that the retarding effect is inherently correlated with the partitioning of Mn between ferrite(α) matrix and cementite(θ). According to the equilibrium thermodynamics, Mn would diffuse from α-Fe matrix to θ cementite after the initial stage of tempering until equilibrium is reached.However, the manner in which Mn diffuses from α-Fe matrix to θ cementite is unclear, which is key in understanding the mechanism in which the partitioning of Mn can retard the growth and coarsening of cementite. Therefore, the measurement of Mn content across the α-Fe/θ interface is of importance to achieve this goal. In this study, the redistribution characteristics of Mn between α-Fe matrix and θcementite after long-term aging at 370 or 400 °C with quenched–tempered or quenched samples of reactor pressure vessel model steel was investigated by atom probe tomography. Results show that Mn diffuses from the α-Fe matrix and enriches in the θ cementite under all heat treatment conditions. The concentration of Mn in cementite is the highest when the specimen is thermally aged directly after quenching. Moreover, Mn is not distributed uniformly within cementite after long-term aging at 400 °C for 35000 h. Instead, a Mnsegregated zone exists within cementite adjacent to theα-Fe/θinterface,with concentration increasing by aging temperature,which acts as a barrier to the coarsening of cementite by hindering the dissolution of small-sized cementite.The redistribution characteristics of Mn between the two phases is correlated with the difference of diffusivities in theα-Fe matrix andθcementite during thermal aging,and the diffusivity of Mn inθcementite is slower than that inα-Fe matrix.

Nanoindentation response of interphase precipitation in a Nb microalloyed steel

ABSTRACT In this work, dedicated isothermal transformation studies were performed in a Fe–C–Mn–Nb alloy to obtain two types of precipitate morphology embedded in a similar ferritic matrix. Nano-hardness of individual ferrite grain was measured by using an in situ nano-mechanical tester. The peak of nano-hardness distribution from interphase and dispersed precipitate strengthened ferrite corresponds to the nano-hardness range of 2.6–2.8 and 2.5–2.6 GPa, respectively. A wider spread of nano-hardness from interphase precipitate strengthened ferrite was observed and rationalised by considering the variation in inter-particle spacing on slip planes. The ‘pop-ins’ on the load-depth curve was not able to be correlated with the morphology of a precipitate.

Study on the plastic anisotropy of advanced high strength steel sheet: Experiments and microstructure-based crystal plasticity modeling

For their versatile microstructure controlling techniques, advanced high strength steels (AHSSs) show great potential to be tailored for various application scenarios. The unique microstructure of AHSSs renders their complex plasticity behaviors and mechanical properties. In this work, uniaxial tensile tests combined with a 3D digital image correlation system were carried out to capture the mechanical anisotropy evolution of a cold-rolled dual-phase AHSS (DP980) sheet. In contrast to the conventional body centered cubic (BCC) steel sheets, the studied DP980 sheet shows much smaller Lankford coefficients and a mild strength anisotropy. The Lankford coefficients depend significantly on tensile directions and vary obviously with deformation; they increase with deformation first and then decrease after macroscopic strain localization takes place. Postmortem experimental characterizations as well as microstructure based full-field crystal plasticity simulations were employed to uncover the underneath mechanisms. Plasticity heterogeneities and micromechanical interactions between the soft ferrite matrix and hard martensite islands primarily account for the mechanical anisotropy of the studied steel sheet.

Realization of Selective Strengthening of Ferrite by Nb/V Microalloying in a Medium Carbon Lightweight δ-TRIP Steel

The effect of niobium (Nb) and vanadium (V) microalloying on the mechanical properties and microstructure evolution of medium carbon lightweight δ-TRIP steel was investigated. The results indicate that the pristine and microalloyed δ-TRIP steels comprise mainly delta ferrite, bainite, alpha ferrite and retained austenite. With the addition of trace Nb/V elements to δ-TRIP steel, the volume fraction of retained austenite increased and high-density nano-scale Nb/V carbides precipitated as well, for which substantial tensile strength (1028 MPa) and elongation (26.7 pct) were observed. Extensive characterization revealed that the strength-ductility tradeoff in microalloyed δ-TRIP steel was overcome because of the presence of transformation-induced plasticity effect and pinning of dislocations by nano-sized Nb/V carbides in a ferrite matrix, which enhanced the capacity of ferrite grains to accommodate higher density dislocations.

The Influence of Alloying Elements on Damping Capacity of Nodular Cast Irons for Braking System Components

The brake noise is one of the highest incidences of complaints from OEM’s customers, causing problems such as passenger disappointment and warranty costs. With the improvement of the comfort and soundproofing of the automobile, the brake noise has increased customers’ concerns, making them believe that it is a component defect, although it is fulfilling its functions. One of the factors that can attenuate the vibration in a structure is the increase in its inherent damping capacity. Therefore, a material capable of dissipating energy of vibrations may minimize noise, vibration and harshness problems in the braking system. In this work, an experimental study of mechanical properties, microstructural analysis and damping capacity was carried out on different nodular cast irons with variation of the Co, Si, RE contents. The research goal is the achievement of the best compromise between the mechanical properties required for the braking system parts and their damping capacity. In spite of the damping capacity and tensile strength that were likely to be mutually exclusive, a simultaneous increase in both was registered for samples with high Si content, comprising different additions of RE, and for samples alloyed with Co. These results seem to be associated with solid solution strengthening of a fully ferritic matrix, combined with nodule density increase.