Ferrite Grain Size Research Articles

FIB and TEM Observations of Defects in Hot-Dip Zinc Coatings

The characteristic ability of FIB (focused ion beam) fabrication to remove materials from a very small and/or precisely located slab using an accelerated Ga ion beam were employed to prepare cross-sectional thin films of zinc-coated steel sheets composed of Fe-Zn intermetallic compounds. A few defects observed frequently on the galvannealed (GA) coating surface were analyzed. Streaky marks indicated the rich concentration of oxides at the interface between the galvannealed coating and the steel substrate. The annealing experiment indicated the existence of Mn and Si oxides on the steel substrate. The grain size of ferrite in the extreme surface of the substrate was smaller as compared with conventional IF (Interstitial Free) steels. Furthermore, some superlattice spots were observed at the fine ferrite grain. Defects that are termed 'wavy pattern' and 'dross' were also analyzed by the combination of the FIB and TEM (Transmission Electron Microscope) techniques.

Preparation and Spontaneous Polarization–Magnetization of a New Ceramic Ferroelectric–Ferromagnetic Composite

Ceramic composites with the composition of xPMZNT·(1 –x)NiCuZn have been prepared using a standard ceramic technique, in which x varies as 0, 0.1, 0.2, 0.4, 0.6, 0.9, and 1.0. PMZNT is the abbreviated form of 0.92Pb(Mg1/3Nb2/3)O3·0.04Pb(Zn1/3Nb2/3)O3·0.04PbTiO3 (PMN‐PZN‐PT). NiCuZn is the abbreviated form of Ni0.2Cu0.2Zn0.6Fe2O4. The presence of ferroelectric PMZNT phase and ferromagnetic NiCuZn ferrite phase has been confirmed using X‐ray diffractometry. Ferroelectric hysteresis loops and magnetic hysteresis loops have been observed and studied. In polarization–electric‐field curves, the remnant polarization and coercive fields display little asymmetric characterization because of the existence of the internal electric field. When the amount of NiCuZn ferrite phase increases, the coercive field increases. Meanwhile, the saturation magnetization decreases and the coercivity of the composites increases with the increase of phase fraction of PMZNT, because the interaction between magnetic grains (or magnetic connectivity) is weakened by the existence of nonmagnetic PMZNT phase distributed in the magnetic phases. Under an applied magnetic and electric field, the magnetization and polarization of the composites can be easily tuned. The sintered composites possess high density and fine‐grained microstructure. The average grain size of NiCuZn ferrite grains is slightly larger than that of the PMZNT grains.

The Effect of Delay Time after Hot Rolling on the Grain Size of Ferrite

It is necessary that the yield ratio (YR) of fire-resistant steel for construction is below 0.8. However, during the industrial production of fire-resistant steel, the rapid cooling rate of the laminar cooling configuration makes the ferrite grain become fine and increases the yield ratio (YR>0.8). In order to reduce the YR, by using Gleeble 1500 thermomechanical simulator the experiment of the effect of delay time after hot deformation on the grain size of ferrite was carried out. The results indicate that the hot deformation can increase the system free energy and the ledges generate at the grain boundary of deformed austenite, which leads to the increase of the ferrite nucleation rate and the refinement of ferrite grain. However, with the increase of delayed time, that the release of deformation energy and the reduction of ledges would decrease ferrite nucleation rate and increase the grain size of ferrite. Based on the experimental results, the technology to prolong the delay time after hot rolling is adopted in the industry production. The tested results indicate that the technology is effective to reduce the YR of fire-resistant steel and the YR decreases to 0.79 from initial 0.84.

超狭開先アーク溶接における超細粒鋼熱影響部の特性―超細粒鋼溶接熱影響部の軟化域とミクロ組織（第１報）―

800 MPa high strength steel with a low carbon equivalent has been developed by means of refining ferrite grains. In the welded joints of the 800 MPa steel with ultra-fine ferrite grains (less than 1 μm) and dispersed fine cementite particles, fine ferrite grains coarsen due to the welding heat input, and then remarkable softening occurs in the heat-affected zone. For the ultra-fine grained steel (UFG steel), ultra-narrow gap gas metal arc (UNG) welding was utilized as a low heat input welding process. By the UNG welding process with the cooling time from 1073 K to 773 K of about 4.5 sec, the width of the softened region in the heat-affected zone (HAZ) of the ultra-fine grained steel can be minimized to below 2 mm. In order to clarify the softening behavior in the HAZ of UFG steel, the effect of welding thermal cycle on the microstructures of UFG steel was discussed on both the HAZ of the welded joint of UNG welding and its synthetic HAZ, and the results were also compared with the HAZ of SM490 steel. The peak temperature at each point of HAZ was estimated from thermal conduction theory and the measured data. Slightly coarsened ultra-fine ferrites (fine grain A) and larger polygonal ferrites (fine grain B) can be observed in the regions of the peak temperatures from 920 K to Ac3 (1150 K). It was found that the softening in the HAZ of UFG steel occurs within the peak temperature range of about 920 K (below Ac1=980 K) to 1300 K, and the softening in the regions of the peak temperatures from 920 K to 990 K is dependent on the grain size of the larger polygonal ferrite grains (fine grain B). M-A constituents begin to form when the peak temperature exceeds Ac1, and its volume fraction increases with the peak temperature. As second hard phase, M-A constituents restrain the softening of the HAZ in the peak temperature range from about 1000 K to 1250 K. It is known from the results that the hardness in the HAZ has the lowest value at the peak temperature of 1100 K.

Control of grain size and morphologies of nanograined ferrites by adaptation of the synthesis route: mechanosynthesis and soft chemistry

Nanocrystalline Fe-based spinels with composition Fe 2.5Ti 0.5O 4 can be synthesized using two different routes: soft chemistry and high-energy ball milling. This paper is focussed on the fact that each type of synthesis process can lead to powders with a crystallite size of about 15 nm but with significant differences in the grain size distribution and the agglomeration state. Whereas in the case of mechanosynthesis, the ball-milled powders consist of aggregates, those obtained by soft chemistry are very well dispersed. Moreover the chosen investigated nanopowders present a blocked/superparamagnetic transition depending on the grain size. The grain size morphologies obtained by the two techniques of synthesis can then be fully characterized by complementary experiments: in addition to high-resolution image processing, specific measurements adapted to the study of magnetic relaxation can be used for weighting differently their small and large size tails: namely, magnetization measurements and Mössbauer spectrometry.

The influence of Fe 3+ ions at tetrahedral sites on the magnetic properties of nanocrystalline ZnFe 2O 4

A systematic study on the variation of Mössbauer hyperfine parameters with grain size in nanocrystalline zinc ferrite is lacking. In the present study, nanocrystalline ZnFe 2O 4 ferrites with different grain sizes were prepared by ball-milling technique and characterised by X-ray, EDAX, magnetisation and Mössbauer studies. The grain size decreases with increasing milling time and lattice parameter is found to be slightly higher than the bulk value. Magnetisation at room temperature (RT) and at 77 K could not be saturated with a magnetic field of 7 kOe and the observed magnetisation at these temperatures can be explained on the basis of deviation of cation distribution from normal spinel structure. The Mössbauer spectra were recorded at different temperatures between RT and 16 K. The values of quadrupole splitting at RT are higher for the milled samples indicating the disordering of ZnFe 2O 4 on milling. The strength of the magnetic hyperfine interactions increases with grain size reduction and this can be explained on the basis of the distribution of Fe 3+ ions at both tetrahedral and octahedral sites.

Combustion synthesis and magnetic investigation of nanosized CoFe2O4

Nanosized Co spinel ferrite was prepared by a combustion method. The grain size of resultant ferrite can be controlled by the reaction conditions. Magnetic investigation revealed that the products with an average grain size of 4 nm behave as superparamagnetic materials and have zero coercive force; whereas the samples with an average grain size of 85 nm (near the size of critical single domain) have the maximal coercive force. There were the negative magnetic intergrain interactions which became stronger when the grain size neared that of the single domain.

Distribution of the grain size of ferrite in the deformation zone of 18G2A (A 765-94 ASTM) steel samples after tensile tests and recrystallization

This paper presents a procedure for quantitative image analysis for calculations of the share of the ferrite grains with different scales of grain size. This procedure is described on the basis of a cold-worked and recrystallized 18G2A (A 765-94 ASTM) steel. A method to calculate characteristic values of grain sizes has been developed and the results of experiments are also presented in this paper.

フェライトとカーボンファイバシートの複合体の検討 : フェライト混入モルタルのTV電波吸収特性に関する研究

In city area, electromagnetic interference caused by buildings is a very important problem. For example, a well known ghost phenomenon is caused by interaction between incident and reflected waves caused due to the presence of large and tall buildings. In this study, absorption characteristics in VHF band of ferrite mortar, mixed with four different size of ferrite grain respectively, are measured by using parallel wire line method. Furthermore, absorption characteristics in VHF band of radio wave absorber, combined ferrite mortar with carbon fiber sheet, are measured, and it is considered whether thin radio wave absorber can be realized. As a result, as the size of ferrite grain is small, absorption characteristics in VHF band improve. And the improvement of absorption characteristics in VHF band is possible by combining fertile mortar with carbon fiber sheet.

低炭素鋼の塑性変形されたオーステナイトの粒界から生成するフェライト粒の形,大きさと結晶方位

低炭素鋼の塑性変形されたオーステナイトの粒界から生成するフェライト粒の形,大きさと結晶方位

Influence of Mixed Grain Size Distributions on the Toughness in High and Extra High Strength Steels

Continuously cast high and extra high strength (EHS) steels are investigated with respect to structure coarseness. Large variation in impact resistance between heats is observed, which can not be explained in terms of an average grain size. This has been of major technical concern for the Swedish steel industry. The EHS steels are purely ferritic. The high strength (HS) steels have a complex microstructure consisting of primary ferrite and one or more other phases: pearlite, acicular ferrite, or ferrite with second phase. Primary ferrite grain sizes as well as the cell size of pearlite, acicular ferrite and ferrite with second phase are measured to describe structure coarseness. Image analysis is successfully used to separate primary ferrite and other microstructure constituents. Intercept length based size parameters are utilized and size distributions are given. A generalized grain size is determined from the slope of the size distributions. This grain size is used to interpret the toughness properties. The results show that differences in the size of the larger ferrite grains explain the variation in toughness in both HS and EHS steels.

Effect of Boron Addition on the Microstructure of Hot-deformed Ti-added Interstitial Free Steel.

Effect of boron (B) addition and hot-deformation on microstructure of ferrite was studied using Ti-added interstitial free (IF) steels containing different amount of B. It was clarified that bainitic ferrite having typically lath or plate morphology with high dislocation density can be obtained also in plain IF steels by rapid cooling from austenite region. B addition largely affected the austenite-ferrite transformation and the amount of bainitic ferrite increased with increasing B content. Hot-deformation of austenite enhanced the transformation to polygonal or quasi-polygonal ferrite, resulting in decrease of bainitic ferrite. Interrupting compression tests indicated that enhanced transformation by hot-deformation is mainly due to grain refinement of austenite by recrystallization after hot-deformation. The cooling rate after hot-deformation produced the reversed effect of B on grain size of polygonal or quasi-polygonal ferrite. Ferrite grain size slightly decreased with increasing B content in the case of relatively large cooling rate. When the materials were cooled slowly after hot-deformation, on the other hand, the ferrite grain size increased with increasing B content. The coarse ferrite in B-bearing steels had fairly irregular shape of grain boundaries and inhomogeneous grain size, which suggested the contribution of somewhat discontinuous grain growth. This coarsening of ferrite would be responsible for the previously reported fact that B addition lowers Lankford value of cold-rolled and annealed IF steel sheets.

Fatigue strength of notched specimens having nearly equal sizes of ferrite

Abstract In this study, rotating bending fatigue tests were conducted on circumferential notched large and small specimens of three kinds of carbon steel with nearly equal (20 μm) ferrite grain sizes but different volume fractions of pearlite. The effect of microstructures on the fatigue strengths σws (the limit stress for slip bands), σw1 (the limit stress for macrocrack initiation), and σw2 (the limit stress for fracture) is discussed by comparing these values in three kinds of carbon steel. The main results newly obtained are as follows. (1) Since σws is localized to a highly specified region, variation of Ktσws (maximum stress repeated at the notch root in the limit stress for slip bands) is small, irrespective of notch root radius. (2) If the grain size of ferrite is nearly equal, variation of σw1 is small irrespective of pearlite under the threshold amount of pearlite. The threshold volume fraction of pearlite is about 50%. (3) In medium-carbon steel, σw2 increases with increasing grain size of ferrite. (4) Notch root radius at the branch point, ϱ0, varies depending on the grain size of ferrite. (5) The values of Ktσw1 (maximum stress repeated at the notch root in the limit stress for macrocrack initiation) and Ktσw2 (maximum stress repeated at the notch root in the limit stress for fracture) can be determined by the notch root radius ϱ alone, independent of geometrical conditions.

低炭素鋼の変態域圧延におけるフェライト占積率と粒径の予測

低炭素鋼の変態域圧延におけるフェライト占積率と粒径の予測

Effect of Microstructures on the Fatigue Strength of Notched Specimen in Ferritic-Pearlitic Steels.

Rotating bending fatigue tests were carried out on the circumferential notched specimen of three kinds of carbon steel with nearly equal ferrite grain sizes of 20μm but different volume fractions of pearlite. Fatigue tests were also carried out for 0.15% carbon steel with ferrite grain size of 50μm and 0.45% carbon steel with ferrite grain size of 5μm. The effect of microstructures on the fatigue strengths σws (the limit stress for slip band), σw1 (the limit stress for macrocrack initiation), and σw2 (the limit stress for fracture) is discussed by comparing these values between three kinds of carbon steel. The main results are as follows : (1) Since σws is localized to a highly specified region, variation of Ktσws is small irrespective of notch root radius. (2) If the grain size of ferrite is nearly equal, variation of σw1 is small irrespective of pearlite volume fraction under the certain amount of pearlite. (3) In medium carbon steel, σw2 increases with increasing grain size of ferrite. (4) Notch root radius at the branch point ρo varies with the grain size of ferrite.

Influence of mole ratio of CaO to SiO2 on magnetic properties and grain size of Mn-Zn ferrites

Influence of mole ratio of CaO to SiO2 on magnetic properties and grain size of Mn-Zn ferrites

Strain hardening features of ferrite-martensite steels

A model is suggested for strain hardening of ferrite-martensite steels taking account of the stereology of structural components and redistribution of strains between them during loading. Analytical expressions are obtained making it possible to calculate the yield stress for a two-phase steel at any instant of deformation and the value of true equilibrium strain depending on such structural factors as the volumetric proportion of martensite, its carbon content, and the size and distribution of ferrite grains and martensitic areas. Experimental and calculated data are compared.

A proposal for fatigue crack growth design curve at stages 2a and 2b in steels: Asami, K. J. Soc. Mater. Sci., Jpn. Feb. 1989 38, (425), 124–129

Nanocrystalline grains, whose average size was approximately 35 ± 5 nm, were generated in the outermost layer of a pearlitic wheel steel by dry sliding wear. To explore the evolution of nanocrytalline grains, successive observations from the outermost layer of sliding wear to the undeformed matrix were done by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The investigations showed that the evolution process of nanometer-sized grains in pearlitic wheel steels included four steps: initially, pearlitic plastic deformation took place, lamellar spacing diminished, great amounts of dislocations occurred in ferrite, the thickness of ferrite lamellae decreased and some of ferrite lamellae fractured and dissolved; with increasing strains, considerable cementite started to dissolve into grains, and inside ferrite dislocations accumulated, tangled and formed dislocation walls, which cut ferrite into many short-rod shaped—and even equiaxed shaped dislocation cells; as strains continued to increase, dislocation cells gradually transformed into low-angle sub-boundaries. Accumulation and annihilation of dislocations and rotation of grains induced low-angle boundaries (LABs) to turn into high-angle boundaries (HABs), with the grains having random orientation. At this moment, cementite dissolution was relatively saturated and undissolved cementite particles scattered around the refined ferrite boundaries. Under higher strains and strain rates, refined ferrite grains repeatedly underwent plastic deformation, the formation of dislocation cells and the transformation from LABs to HABs, until finally the size of ferrite grains reached the stable minimum value and the refinement of grains ceased. Meanwhile, cementite continued to dissolve slightly due to dislocation accumulation. When dislocation accumulation rate and annihilation rate reached balanced, the dissolution of cementite ceased.

Superplasticity of Ultra-High Carbon Steels Containing 3-5%Aluminum

ABSTRACTTo study the effect of Al on superplasticity of ultra-high carbon steel (UHCS), 3–5%Al was added to the UHCS, and superplastic behavior was measured at 700–800°C. To produce the fine and superplastic microstructure, an isothermal rolling at 700°C was used, and the grain size of ferrite was less than about 2μm in diameter after the process.Significant improvement in superplasticity was observed; the strain-rate sensitivity was high upto a strain-rate of 10−2 s−1, and 402% elongation was measured at 750°C and at an engineering strain-rate of 5000%/min(i.e., 8.3×10−2s−1). It has been determined that the Al-addition leads to a finer grain size, which, in turn, improved the superplastic properties of the ultra-high carbon steels.

Special features of the mechanism of failure of electroslag welding joints of steel 16GMYuCh

The properties of welded joints are largely determined by their phase composition and by the morphology of the structural components. For instance, in the investigation of welded joints it was established [i] that there is a correlation between the grain size of ferrite in a ferritic-pearlitic structure and the characteristics of failure, in particular the dimensions of transcrystalline cleavage facets. In high-strength steels, which are a promising material for pressure vessels, a predominantly bainitic-type structure forms in the nearseam zone under the effect of the thermal cycle of welding; the influence of this structure on the mechanism of failure has not been sufficiently studied.