A comparative assessment of the hardness of nano-structured bainitic steel affected by using various quenchants

Quenching, in heat treatment, plays a vital role in controlling material properties. It is the most important step in manipulating the strength of steel. It involves cooling the material from the austenitizing temperature at different cooling rates using variations in quenchants to obtain corresponding material properties. The commonly used quenchants are water, oil, and brine. The cooling rate is the rate at which heat is ejected from the material by the quenchant. The effectiveness of the quenchant is judged by its ability to absorb heat from the material and thermally conduct. Because of stringent regulations regarding use and disposal, there is a need to develop new, environmentally friendly quenchants. The experimental design in this study consisted of quenching austenitized nano-structured bainitic steel in four different quenchants, namely, water, oil, brine, and 1 M sodium carbonate solution. This research gives the insight of substituting conventional quenchants with 1 M sodium carbonate solution. The final four samples were characterized using metallography. A comparative study of the hardness of nano-structured bainitic steel quenched in the newly developed quenchant (i.e., 1 M sodium carbonate solution) and of steel quenched with the conventional one is done. All the results are tabulated, and the applicability of the quenchants is discussed.

Effect of δ-ferrite co-existence on hot deformation and recrystallization of austenite

This work evaluates the effect of co-existence of a large volume fraction of δ-ferrite on the hot deformation and dynamic recrystallization (DRX) of austenite using comparative hot torsion tests on AISI 304 austenitic and 2205 duplex stainless steels. The comparison was performed under similar deformation conditions (i.e. temperature and strain rate) and also under similar Zener-Hollomon, Z, values. The torsion data were combined with electron backscatter diffraction (EBSD) analysis to study the microstructure development. The results imply a considerable difference between DRX mechanisms, austenite grain sizes and also DRX kinetics of two steels. Whereas austenitic stainless steel shows the start of DRX at very low strains and then development of that microstructure based on the necklace structure, the DRX phenomena in the austenite phase of duplex structure does not proceed to a very high fraction. Also, the DRX kinetics in the austenitic steel are much higher than the austenite phase of the duplex steel. The results suggest that at a similar deformation condition the DRX grain size of austenitic steel is almost three times larger than the DRX grains of austenite phase in duplex steel. Similarly, the ratio of DRX grain size in the austenitic to the duplex structure at the same Z values is about 1.5.

The formation of ultrafine ferrite through static transformation in low carbon steels

A novel approach was used to produce an ultrafine grain structure in low carbon steels with a wide range of hardenability. This included warm deformation of supercooled austenite followed by reheating in the austenite region and cooling (RHA). The ultrafine ferrite structure was independent of steel composition. However, the mechanism of ferrite refinement changed with the steel quench hardenability. In a relatively low hardenable steel, the ultrafine structure was produced through dynamic strain induced transformation, whereas the ferrite refinement was formed by static transformation in steels with high quench hardenability. The use of a model Ni-30Fe austenitic alloy revealed that the deformation temperature has a strong effect on the nature of the intragranular defects. There was a transition temperature below which the cell dislocation structure changed to laminar microbands. It appears that the extreme refinement of ferrite is due to the formation of extensive high angle intragranular defects at these low deformation temperature that then act as sites for static transformation. © 2008 World Scientific Publishing Company.

Engineered atherosclerosis-specific zinc ferrite nanocomplex-based MRI contrast agents

BACKGROUND: Cardiovascular diseases are the most prevalent cause of morbidity and mortality affecting millions of people globally. The most effective way to counter cardiovascular complications is early diagnosis and the safest non-invasive diagnostic approach is magnetic resonance imaging (MRI). In this study, superparamagnetic ferrite nanoparticles doped with zinc, exhibiting highly enhanced saturation magnetization and T2 and computed tomography (CT) contrast were synthesized. These nanoparticles have been strategically engineered using bovine lactoferrin (Lf), polyethylene glycol (PEG), and heat shock protein (Hsp)-70 antibody specifically targeting atherosclerosis with potential therapeutic value. The nanocomplexes were further validated in vitro to assess their cytotoxicity, internalization efficiency, effects on cellular proliferation and were assessed for MRI as well as X-ray CT in ex vivo Psammomys obesus rat model.

RESULTS: Optimized zinc doped ferrite nanoparticles (Zn0.4Fe2.6O4) with enhanced value of maximum saturation magnetization value on 108.4 emu/g and an average diameter of 24 ± 2 nm were successfully synthesized. Successfully incorporation with bovine lactoferrin, PEG and Hsp-70 (70 kDa) antibody led to synthesis of spherical nanocomplexes (size 224.8 nm, PDI 0.398). A significantly higher enhancement in T2 (p < 0.05, 1.22-fold) and slightly higher T1 (1.09-fold) and CT (1.08-fold) contrast compared to commercial ferrite nanoparticles was observed. The nanocomplexes exhibited effective cellular internalization within 2 h in both THP-1 and Jurkat cells. MRI scans of contrast agent injected animal revealed significant arterial narrowing and a significantly higher T2 (p < 0.05, 1.71-fold) contrast in adult animals when compared to juvenile and control animals. The excised heart and aorta agar phantoms exhibited weak MRI contrast enhancement in juvenile animal but significant contrast enhancement in adult animal specifically at the aortic arch, descending thoracic aorta and iliac bifurcation region with X-ray CT scan. Histological investigation of the contrast agent injected aorta and heart confirmed site target-specific accumulation at the atherosclerotic aortic arch and descending thoracic aorta of the adult animal with severely damaged intima full of ruptured microatheromas.

CONCLUSION: Overall, the study demonstrates the strategic development of nanocomplex based bimodal MRI and CT contrast agents and its validation on Psammomys obesus for atherosclerosis diagnostics.

Formation of nanoscale titanium carbides in ferrite: an atomic study

The formation and evolution of nanoscale titanium carbide in ferrite during the early isothermal annealing process were investigated via molecular dynamics simulation. The atomic interactions of titanium and carbon atoms during the initial formation process explained the atoms aggregation and carbides formation. It was found that the aggregation and dissociation of titanium carbide occurred simultaneously, and the composition of carbide clusters varied in a wide range. A mechanism for the formation of titanium carbide clusters in ferrite was disclosed.

Dynamic strain-induced ferrite transformation (DSIT) in steels

The goal in the heat treatment or thermomechanical processing of steel is to improve the mechanical properties. For structural steel applications the general aim is to refine the ferrite grain size as this is the only method that improves both the strength and toughness simultaneously. For conventional hot rolling and accelerated cooling processes, it is difficult to refine the grain size below 5. μm without extensive alloying. However, it has been found that inducing transformation during deformation (i.e. dynamic transformation) can lead to grain sizes of the order of 1. μm, even in very simple steel compositions. The exact mechanism(s) for this transformation process are still being debated, and this has also been complicated by recent studies where such grain sizes can be obtained by static transformation from austenite that has been heavily deformed at low temperatures prior to the transformation. This chapter reviews the various major studies related in particular to dynamic transformation and considers the contributions from the deformed austenite structure developed prior to the transformation and the potential for dynamic recrystallisation of the ferrite. A key factor is proposed to be the early three-dimensional impingement of the ferrite which also provides an insight into cases where ultrafine grains are achieved statically.

Investigation on the behavior of austenite and ferrite phases at stagnation region in the turning of duplex stainless steel alloys

This paper investigates the deformation mechanisms and plastic behavior of austenite and ferrite phases in duplex stainless steel alloys 2205 and 2507 under chip formation from a machine turning operation. SEM images and EBSD phase mapping of frozen chip root samples detected a build-up of ferrite bands in the stagnation region, and between 65 and 85 pct, more ferrite was identified in the stagnation region compared to austenite. SEM images detected micro-cracks developing in the ferrite phase, indicating ferritic build-up in the stagnation region as a potential triggering mechanism to the formation of built-up edge, as transgranular micro-cracks found in the stagnation region are similar to micro-cracks initiating built-up edge formation. Higher plasticity of austenite due to softening under high strain is seen responsible for the ferrite build-up. Flow lines indicate that austenite is plastically deforming at a greater rate into the chip, while ferrite shows to partition most of the strain during deformation. The loss of annealing twins and activation of multiple slip planes triggered at high strain may explain the highly plastic behavior shown by austenite.

Effect of coiling treatment on microstructural development and precipitate strengthening of a strip cast steel

The effect of a simulated coiling treatment on a strip cast Nb-containing steel has been investigated. A lath ferritic supersaturated microstructure was observed in the as-cast condition with no coiling. The microstructure remained lath like during coiling at high temperature (850 °C) and the formation of chemically complex Nb-rich precipitates containing C, N, Si and S was observed. Coiling at an intermediate temperature (700 °C) caused the formation of polygonal ferrite with a dendritic morphology due to chemical micro-segregation. The polygonal ferrite contained Nb(C,N) precipitates. The microstructure remained lath like at the lowest coiling temperature (600 °C). In the latter case the precipitation of Nb-rich clusters was observed, and atom probe tomography revealed them to be ∼85% Fe. Small angle neutron scattering and transmission electron microscopy were used to quantify precipitation kinetics during coiling and the mechanical properties were evaluated with a shear punch apparatus. A yield strength model was developed to describe the observed mechanical behaviour, and this showed that the two largest contributors to strength were the bainitic microstructure and the Nb-rich precipitates. Strategies to further strengthen these materials are suggested.

Theranostic multimodular potential of zinc-doped ferrite-saturated metal-binding protein-loaded novel nanocapsules in cancers

The present study successfully developed orally deliverable multimodular zinc (Zn) iron oxide (Fe3O4)-saturated bovine lactoferrin (bLf)-loaded polymeric nanocapsules (NCs), and evaluated their theranostic potential (antitumor efficacy, magnetophotothermal efficacy and imaging capability) in an in vivo human xenograft CpG-island methylator phenotype (CIMP)-1(+)/CIMP2(-)/chromosome instability-positive colonic adenocarcinoma (Caco2) and claudin-low, triple-negative (ER(-)/PR(-)/HER2(-); MDA-MB-231) breast cancer model. Mice fed orally on the Zn-Fe-bLf NC diet showed downregulation in tumor volume and complete regression in tumor volume after 45 days of feeding. In human xenograft colon cancer, vehicle-control NC diet-group (n=5) mice showed a tumor volume of 52.28±11.55 mm(3), and Zn-Fe-bLf NC diet (n=5)-treated mice had a tumor-volume of 0.10±0.073 mm(3). In the human xenograft breast cancer model, Zn-Fe-bLf NC diet (n=5)-treated mice showed a tumor volume of 0.051±0.062 mm(3) within 40 days of feeding. Live mouse imaging conducted by near-infrared fluorescence imaging of Zn-Fe-bLf NCs showed tumor site-specific localization and regression of colon and breast tumor volume. Ex vivo fluorescence-imaging analysis of the vital organs of mice exhibited sparse localization patterns of Zn-Fe-bLf NCs and also confirmed tumor-specific selective localization patterns of Zn-Fe-bLf NCs. Dual imaging using magnetic resonance imaging and computerized tomography scans revealed an unprecedented theranostic ability of the Zn-Fe-bLf NCs. These observations warrant consideration of multimodular Zn-Fe-bLf NCs for real-time cancer imaging and simultaneous cancer-targeted therapy.

Grain size effect on behaviour and microstructure of a warm deformed Ti-IF steel

The effect of grain size on the deformation behaviour in the fenite region of a Titanium stabilized Interstitial Free steel was investigated by hot torsion. The initial work hardening regime is followed by a softening regime where a broad peak stress develops. The peak stress and the stress at final strain were relatively insensitive to grain size. However, at low values of the Zener-Hollomon parameter, the strain to the peak stress was strongly dependent on the grain size. A series of microstructural parameters were examined to explain these observations.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

Microstructural evolution of ultrafine grained structure in plain carbon steels through single pass rolling

In the present study, the effect of nominal equivalent strain (between 0 and 1.2), deformation temperature (790– 750°C) and carbon content (0.06 – 0.35%C) was investigated on ferrite grain refinement through dynamic strain induced transformation (DSIT) in plain carbon steels in single pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain, which could be classified into three regions; no DSIT region, DSIT region, and ultrafine ferrite (UFF) grain region. Hence, two critical strains; dynamic strain induced transformation (εC, DSIT) and ultrafine ferrite formation (εC, UFF) were determined. These strains were increased significantly with an increase in carbon content. The critical strain for UFF formation reduced with decrease in deformation temperature. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μm) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite– pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with increase in the nominal equivalent strain.

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pet) steels, with and without the addition of 0.039Nb (in wt pet), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Formation of ultrafine grained structure in plain carbon steels through thermomechanical processing

In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06--0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μ$m) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite-pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with an increase in the nominal equivalent strain.

Grain size effect on the warm deformation behaviour of a Ti-IF steel

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free (IF) steel was investigated using hot torsion. The initial work hardening regime is followed by the development of a broad stress peak after which work softening occurs. The hypothetical saturation stress (Estrin–Mecking model) and the stress at final strain were relatively insensitive to grain size. However, the strain to the peak stress was strongly dependent on the grain size at low values of the Zener–Hollomon parameter. A simple phenomenological approach, using a combined Estrin–Mecking model and an Avrami type equation, was used to model the flow curves. The hypothetical saturation stress, the stress at final strain and the strain to peak stress were modelled using three different hyperbolic sine laws. A comparison with independent data from the literature shows that the apparent activation energy of deformation determined in this work (Q=372 kJ/mol) can be used to rationalize the steady-state stress in compression data found in the literature.