Microhardness Of Ferrite Research Articles

Зв'язок хімічної мікронеоднорідності сірого чавуну та структури пластинчатого графіту

The relationship between chemical microheterogeneity of gray cast iron castings, revealed by the method of color etching in sodium picrate, and the structure of flake graphite was studied. The results of microstructural studies indicate that the crystallization of gray cast iron is accompanied by the emergence of several levels of chemical and structural microheterogeneities of the crystallizing melt, affecting the formation of graphite. Microstructural features indicate that the cast iron melt, which is close in carbon content to the eutectic composition, acquires a dendritic and cellular structure before the formation of graphite. The intensity of chemical microheterogeneity and its topology are hereditarily related to the location, size and shape of graphite inclusions. Flake graphite formed inside and on the periphery of melt cells has different structure and thickness of plates. The enrichment of areas of cast iron melt with silicon leads to the formation of dendrites of primary austenite and primary graphite of the largest size in them. The microhardness of ferrite in the dendrites of former primary austenite is approximately 1.5 times greater than the microhardness of ferrite in areas adjacent to inclusions of primary graphite. A close inverse correlation (r = –0.93) was established between the microhardness of ferrite in areas with different liquation intensities and the thickness of graphite flakes located in these areas. The greatest spread of microhardness values and width of graphite flakes is observed in the most liquated areas enriched with silicon in the center of the cells, corresponding to the location of eutectic colonies. In the spaces between the cells, graphite is observed with the smallest thickness of the flakes, which are located at the smallest distance from each other. It is necessary to continue research into the hereditary influence of the structure and physical and chemical properties of high-carbon iron melts on the mechanism of graphite formation and the features of its structure in cast irons.

Physical simulation of low temperature phase separation during multipass welding of super duplex stainless steel

This study aimed to investigate the degree of low-temperature phase separations and 475 °C -embrittlement in physically simulated multipass super duplex stainless steel (SDSS) welds. A single bead weld was produced using gas metal arc welding and multiple thermal cycles with different peak temperatures were applied using a Gleeble physical simulator with fixed and moving jaws to simulate the influence of constrained designs, to mimic the welding of thick SDSS components. The samples were studied using atom probe tomography and the results were correlated with toughness and ferrite microhardness. The results showed that the microhardness of the ferrite in the constrained simulated multipass reheated weld increased from 304 HV to 374 HV after 5 min aging at 475 °C. The amplitude of the Cr concentration of the same sample, showing the level of Fe-Cr phase separation, increased from 1.025 to 1.045 after 5 min aging at 475 °C. Despite the clear development of phase separation in the ferrite, the toughness did not drop. It can be concluded that reheated SDSS welds are not susceptible to 475 °C -embrittlement during short fabrication times of up to 5 min for the austenite level of 50%.

Investigation of Micromechanical Properties of Martensite and Ferrite Microphases in a 35CHGSA Medium‐Si Low‐Alloy Steel

The purpose of this study is to understand in detail the micromechanical behavior of martensite and ferrite microphases in a medium‐Si 35CHGSA dual‐phase (DP) steel by varying the martensite volume fractions (MVF) and ferrite phase constituents. Light microscopy combined with microhardness measurements is supplemented by field‐emission scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X‐ray diffraction in order to evaluate the micromechanical behavior of martensite and ferrite microphases in relation to the evolving microstructure. The experimental observations illustrate that ferrite microhardness diminishes sharply from 352 to 217 HV5g with the change in ferrite volume fraction (FVF) from 3 to 13%, beyond which it increases abnormally to 245 HV5g with a further increase in VF to 15%. The martensite microhardness too varies significantly depending on the progress of ferrite formation in the intercritical range. The hardness drops with further increase in FVF as a result of longer holding times. These observations are rationalized in part to the mutual interaction of martensite with ferrite, and the variation of martensite and ferrite alloying concentrations, developed as a result of ferrite formation during intercritical holding at 720 °C.

Hydrogen Cooling of Turbo Aggregates and the Problem of Rotor Shafts Materials Degradation Evaluation

Changes in the properties of 38KhN3MFA steel, from which the rotor shaft is made, were investigated by comparing the hardness of the shaft surface and hydrogen concentration in the chips and analyzing changes in the morphology of the chips under the influence of various factors. The microstructures obtained from the surface of the rotor shaft samples are presented, and histograms reflecting the parameters of the structural components are constructed. An abbreviated diagram of the “life cycle” of the turbine rotor shaft is given. It was found that, during long-term operation (up to 250 thousand hours), the hardness of the rotor shaft surface decreases from 290 HB to 250 HB. It was recorded that, in the microstructure of the shaft during 250 thousand hours of operation, the amount of cementite decreased from 87% to 62%, and the proportion of free ferrite increased from 5% to 20%. The average values of ferrite microhardness decreased from 1.9 GPa to 1.5 GPa. An increase in the content of alloying elements in carbides was recorded: Cr and V—by 1.15–1.6 times; and Mo—by 2.2–2.8 times. With the help of the developed program (using computer vision methods), changes in their microrelief were detected to study photos of chips.

Abnormal Trend of Ferrite Hardening in a Medium-Si Ferrite-Martensite Dual Phase Steel

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

Prediction of the fatigue limit defining mechanism of nodular cast iron based on statistical microstructural features

The microstructure and nodule count of large-size nodular cast iron components vary spatially. These variables are qualitatively known to affect the fatigue limit, yet no model exists to quantify the effects. Some of the physical aspects, such as the clustering of graphite nodules and the role of ferrite microhardness in ferritic–pearlitic nodular cast iron fatigue, have been unclear in the literature. This paper aims to clarify and quantify these aspects. In the absence of casting defects, the largest ferrite with a crack initiating graphite is shown to be the physical, and statistical, explanation for the mixed grade fatigue limit.

Small-angle neutron scattering study on phase separation in a super duplex stainless steel at 300 °C – Comparing hot-rolled and TIG welded material

The evolution of nanoscale phase separation in the ferrite phase of super duplex stainless steel 25Cr 7Ni (wt%) (SDSS 2507) and two SDSS TIG (tungsten inert gas) weldments have been quantitatively investigated by small-angle neutron scattering (SANS). The results show that the phase separation is more pronounced in the SDSS weldments in comparison to the base metal SDSS 2507, especially after aging for 35,000 h at 300 ° C. These results correlate with the higher ferrite micro-hardness in the aged TIG weldments than in the SDSS 2507. The enhanced phase separation is partly due to the higher contents of Cr and Ni in the ferrite of TIG weldments compared to SDSS 2507 base metal, revealed by energy-dispersive X-ray spectroscopy (EDS). Additionally, the residual strain measurements through focused ion beam and digital image correlation (FIB-DIC), indicate larger residual strains in the ferrite of weldments than in the base metal SDSS 2507. This is also believed to contribute to the enhanced phase separation. • TIG weldments show a faster rate of phase separation at 300 ° C than the base metal of SDSS 2507. • Higher contents of Cr and Ni in the ferrite are partly responsible for the accelerated kinetics of phase separation in the weldments. • Higher heat input during TIG welding leads to higher residual stresses which enhance phase separation

Structures and Mechanical Properties of Some Dual-Phase Steels with Low Manganese Content

In recent years, to reduce cars costs, research has been conducted on dual-phase steels with low manganese content (below 1.0%). This study investigated the influence of technological parameters of heat treatment (heating temperature and cooling medium) on such steels’ structures and mechanical properties. The ferrite-martensitic structures, specific for dual-phase steels, were obtained by intercritical quenching: heating of samples (made of alloys with 0.511% Mn, respectively 0.529% Mn) to temperatures located between critical points Ac1 and Ac3, followed by cooling in water without mechanical agitation and in water activated with ultrasounds at the frequency of 59 kHz. Through metallographic analyses and tensile tests, it was possible to determine the volume fraction of martensite, the ferrite microhardness, the ultimate tensile strength, the total elongation, and with the obtained data, their variations with the heating temperature and the cooling medium were established. Raising the heating temperature (between 760 °C and 820 °C) and using ultrasounds at cooling increased the volume fraction of martensite and the ferrite microhardness. This fact has increased the mechanical strength and reduced the deformability of the studied dual-phase steels. Intercritical quenching in water activated with ultrasounds provided values of structural characteristics and mechanical properties very close to those obtained by quenching in water without mechanical agitation, but was accomplished using a higher-temperature heating. The results obtained were compared with those determined in previous research, performed on dual-phase steel with 1.90% Mn.

Microstructural variation in fatigued interphase arrayed nano-precipitated Ti-microalloyed steel

In the present study, arrayed nano-precipitated TiC in a ferrite matrix was obtained through thermal treatment. The size and shape of the arrayed interphase nano-precipitates were unaffected by fatigue deformation in this Ti-microalloyed steel. In addition, arrayed arrangement of the interphase nano-precipitates was not disrupted by fatigue and still remained only at low cycle numbers, which corresponded to the shortest fatigue life. The interphase nano-precipitated steel was strengthened through a bowing mechanism through which unpinning occurred, dislocation loops were formed, and the microhardness of ferrite was increased. The development of an incipient cell structure in the ferrite matrix was caused by wavy dislocation. After the onset of fatigue, the grain was preferentially rotated toward the {101}α orientation. Higher strain amplitude or lower strain rate led to a more favorable degree of misorientation in the low-angle grain boundaries.

Nitrides Precipitation and Preferential Pitting Corrosion of Ferrite Phase in UNS S39274 Superduplex Stainless Steel

A W-alloyed UNS S39274 was solution treated in three distinct temperatures, 1050, 1100, and 1150 °C, with water quenching, resulting in microstructures with different austenite and ferrite proportions, and chromium nitrides precipitated. The densities of chromium nitrides were evaluated qualitatively by optical (conventional and confocal) and scanning electron microscopy. These particles were found to precipitate inside ferrite grains and in the ferrite/ferrite and ferrite/austenite boundaries. The amount of chromium nitrides increased with the solution temperature, but the microhardness of ferrite was not affected by the increase of nitrides density. As a consequence of nitrides precipitation, ferrite was more susceptible to pitting nucleation and attack than austenite, according to polarization tests in 3.5%NaCl solution at 80 °C and in double loop electrochemical potentiodynamic (DL-EPR) tests.

Aluminium in compacted graphite iron

Compacted graphite was obtained using inmold technology. The effect of aluminium on the crystallization process, microstructure, ferrite microhardness, and hardness of compacted graphite iron was studied. The microscopic and diffraction tests were also performed, and the process of cast iron crystallization was also investigated. Results show that aluminium increases the temperature of the eutectic transformation as well as the transformation temperature in the solid state. It is found that aluminium is a graphite forming element in compacted graphite iron (CGI) at a concentration up to 2.4wt.%. When its concentration is higher than 3.1wt.%, aluminium causes the spheroidization of the carbides in eutectoid mixture. It is also demonstrated that in cast iron with an aluminium content higher than ~8wt.%, AlFe3C0.5 phase crystallizes from the liquid.

Influence of intercritical quenching on the structure and mechanical properties of a dual-phase steel with low manganese content

Abstract In this article are presented the results of studies conducted in order to determine the influence of technological parameters of heat treatment on the structure and mechanical properties of a dual-phase steel with 0.101% C and 0.529% Mn. The structure, specific for a dual-phase steel, was obtained by intercritical quenching, when heating for 30 minutes at 760, 780, 800 and 820 °C, and cooling in water without mechanical agitation and in water activated with ultrasound at the frequency of 59 kHz. The metallographic analyses revealed the volume fraction of martensite and the ferrite microhardness from the structures, while the tensile testing determined the ultimate tensile strength and the total elongation.

Mechanical properties and kinetics of thermally aged Z3CN20.09M cast duplex stainless steel

Cast stainless steels used in nuclear power plants suffer from fracture toughness losses owing to thermal aging after long-term service at temperatures ranging from 280–320°C. To study the thermal aging embrittlement of Z3CN20.09M duplex stainless steel produced in China, accelerated thermal aging experiments were carried out at 350, 380, and 400°C for up to 10000 h. Microhardness and Charpy impact energies were measured at different aging times. The microhardness of ferrite increased drastically over the initial aging time of 2000 h at 380 and 400°C and then slowly reached HV0.01 560. In contrast to this observed change in microhardness, Charpy impact energies sharply decreased after initial aging and then gradually reached a minimum value. Taking the microhardness of the ferrite phase as the parameter describing the thermal kinetics of the stainless steel samples, the activation energy of thermal aging was calculated to be 51 kJ/mol. Correlations between the thermal aging parameter, P, and ferrite microhardness and between P and Charpy impact energy were also analyzed. The results showed that the activation energy calculated from the ferrite microhardness is much more reasonable than that obtained using other parameters, such as chemical composition and impact energy.

Hardening Contribution of G-Phase Nanoparticle Precipitation and Spinodal Decomposition in Aged Duplex Stainless Steel Studied by APT Analysis and Micro-Hardness of Ferrite

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Investigation of intercritical heat treatment temperature effect on microstructure and mechanical properties of dual phase (DP) steel

In the present study, the effect of intercritical heat treatment temperature on the tensile properties and work hardening behavior of ferritic-martensitic dual-phase steel have been investigated utilizing tensile test, microhardness measurement and microscopic observation. Plain carbon steel sheet with a thickness of 2 mm was heat treated at 760, 780, 800, 820 and 840 °C intercritical temperatures. The results showed that martensite volume fraction (Vm) increases from 32 to 81%with increasing temperature from 760 to 840 °C. The mechanical properties of samples were examined by tensile and microhardness tests. The results revealed that yield strength was increased linearly with the increase in Vm, but the ultimate strength was increased up to 55% Vm and then decreased afterward. Analyzing the work hardening behavior in term of Hollomon equation showed that in samples with less than 55% Vm, the work hardening took place in one stage and the work hardening exponent increased with increasing Vm. More than one stage was observed in the work hardening behavior when Vm was increased. The results of microhardness test showed that microhardness of the martensite is decreased by increase in heat treatment temperature while the ferrite microhardness is nearly constant for all heat-treated samples.

MECHANICAL PROPERTIES OF CONTINUOUSLY ANNEALLED Si–FREE P–CONTAINING TRIP STEEL

Since its advent, TRIP steel has been considered as the best automotive steel to improve the automotive safety dramatically because of its excellent mechanical properties. However, TRIP steel was not widely used and the main reason is that the large amount of Si in the steel led to surface quality problems of the plate. In this work a kind of C-Mn-Al-P TRIP steel with Si being replaced by Al and P was designed and subjected to continuous annealing. The macro and micro mechanical performance was analyzed. The results showed that the mechanical properties were excellent with tensile strength between 740 and 790 MPa and elongation above 25%, indicating that the production of TRIP steel without Si addition was feasible. Compared with traditional TRIP steel with Si, the designed steel demonstrated higher yield strength of 550—600 MPa, which was a little higher than that of high Si steel and much higher than that of medium or low Si steel. The micro hardness of ferrite was measured by nano mechanics probe and it was 0.6 GPa higher than that of medium Si TRIP steel. The IF steel whose strength was already known was used as a calibration and the conclusion was that the 0.6 GPa difference in nano hardness could result in 75 MPa difference in yield strength and 80 MPa division in tensile strength. Through further analysis, it was believed that the higher strength of ferrite was mainly caucsed by C, not P.

Study of phase transformation and mechanical properties evolution of duplex stainless steels after long term thermal ageing (>20 years)

The ferrite of thermally aged CF3M duplex stainless steels has been studied at the atomic scale. Accelerated ageing of ingots has been performed at 350 °C in laboratory up to 200,000 h (>20 years). Spatial and chemical evolution of the microstructure of the ferrite has been characterised by 3D atom probe. In addition, micro-hardness has been performed on the same samples. The results obtained have been compared to the microstructural and mechanical characteristics of ferrite of the same ingots aged at 325 °C (close to service temperature) and to the ferrite of an elbow steel aged on-site. This work has shown that: (i) accelerated ageing at 350 °C anticipates the on-site ageing at 323 °C, (ii) the linear relationship found between micro-hardness measurements and the variation V (defined as the integral of the difference between the Cr frequency distribution of the aged sample and the corresponding binomial distribution characteristic of a random solution with the same concentration) is still valid after 200,000 h of ageing at 350 °C, (iii) the activation energy is the same for both spinodal decomposition and G-phase precipitation and finally (iv) the coarsening of G-phase particles has no influence on the relationship between ferrite micro-hardness and the V parameter.

Thermal Aging of Primary Circuit Piping Materials in PWR Nuclear Power Plant

AbstractThe reserved cast austenitic stainless steels (CASS) for primary circuit piping in Daya Bay Nuclear Power Plant were studied. The changes of microstructure, mechanical properties and fracture behavior were investigated using SEM, EPMA, TEM and nanoindentation after accelerated aging at 400°C for up to 10000 h. Microhardness of ferrite increased rapidly in the early stage and then increased slowly later. The impact energy of materials declined with the aging time and reduced to a very low level after aging for 10000 hours. Fracture morphology displayed a mixture of cleavage in ferrite along with dimple and tearing in austenite. Two kinds of precipitations were observed in ferrite by TEM after long periods of aging. The fine Cr-enriched α′ phases precipitated homogeneously in ferrite, and a few larger G phases were observed as well. The precipitation of α′ phases was considered to be the primary mechanism of thermal aging embrittlement in CASS.

THE EFFECT OF CIRCUIT LOADING ON DISTRIBUTION UNIFORMITY OF DEFORMATION DURING THE PLASTIC FLOW OF CARBON STEEL

Investigations of the substructure of cold-rolled low-carbon steels have shown that after 20 % pressurization the already big volumes of ferrite have periodic dislocation structures similar to dislocation cells. After two to four alternating bending cycles, an increase in the homogeneity of distribution of ferrite micro-hardness can be observed, along with a decrease of homogeneity of distribution of dislocations inside individual cells.

Effect of nitrogen on toughness and strain age embrittlement of ferritic steel weld metal

Investigations were carried out to evaluate the influence of dissolved nitrogen in ferritic steel weld metal on its toughness and strain aging behaviour through fracture mechanics as well as conventional impact testing approaches. Manual metal arc C–Mn steel weld metals with nitrogen contents between 80 and 210 ppm (wt) were investigated under four different post-weld conditions, namely, (i) as welded, (ii) stress relieved, (iii) artificially strain aged, and (iv) artificially strain aged and stress relieved. Quantitative metallography and low load microhardness studies of microphases were an integral part of these investigations. The results demonstrate the highly detrimental effect of nitrogen on the toughness behaviour of C–Mn steel weld metal, particularly under strain aging conditions. This is substantiated by the decrease of acicular ferrite content with the accompanying increase in primary ferrite and ferrite with second phases in the microstructures. Also, there is a distinctive increase in acicular ferrite microhardness. Post-weld stress relieving heat treatment under these conditions results in only a marginal improvement in toughness and shifts the fracture behaviour from brittle to ductile or quasiductile only for low nitrogen weld metals. Comparing the results of the crack tip opening displacement and Charpy tests, it is observed that both methods show the influence of nitrogen on toughness behaviour to have the same form but the magnitudes of the effect measured are different, the results obtained using the fracture mechanics method appearing very conservative.