95 research outputs found
Inter-critical Annealing of a Lean Composition Steel under Controlled Cooling to Produce Multiphase Microstructure
No full textME, MEDIn the past years, there has been growth in the search and use of new advanced materials in the transport industry. So far, conventional steel is the main material for car bodies, but there is a growing demand for other materials to decrease the weight of cars and thereby saving costs, energy and environment. Also, the increasing safety requirements in the automotive industry has forced search for new materials. The different parts need to be as light as possible, but with sufficient strength and ductility. Hereby, multiphase steels like dual phase and TRIP (Transformed Induced Plasticity) steels are important because of their high strength in combination with good formability. Various methods have been used for the production of these steels to get best outcomes. However, extremely limited work has been reported for the production of dual/ multiphase steels with controlled cooling and using a lean chemistry. The present experimental work reports on the methods of production of dual/ multiphase steels (tensile strength range 500–800 MPa; ductility in the range 12–33%) with ferrite/ martensite, ferrite/ bainite/ martensite and ferrite/ martensite/ austenite/ bainite multiphase structures. These structures were obtained in a normalized steel of very lean chemistry (0.11C, 1.8Mn, and 0.325 Si) subjected to inter-critical annealing and soaking followed by controlled cooling in an annealing simulator. Dual phase structures were produced by inter-critical annealing followed by direct cooling to room temperature to achieve high strengths. Multiphase microstructures were produced by holding in the bainitic and martensitic range to get various combinations of strength and ductility (multi-functional). The present work also compares the results for annealing process obtained through experimental investigations and through software predictions. Finally, micro-mechanical modeling has been done for steel with dual phase structure The true stress- strain curves obtained through actual tensile experiments closely matched with the true stress- strain curves predicted by micro-mechanical modelling
A new annealing route for industrial processing of dual phase steels to obtain improved mechanical properties
No full textDual-phase (DP) steels offer high potential of weight reduction without sacrificing
mechanical properties for their application in automotive industry. They derive their
properties through the second hard phase (martensite/ bainite) in a ferrite matrix. DP steels
are mostly produced through the continuous annealing process route in the industry because
of the requirement of high production rates, uniformity in properties, and leaner chemistry
design feasibility. Important mechanical properties which are desirable for the final
components include absence of yield point, low yield point to ultimate tensile strength, high
strain hardenability along with high ductility etc.
The main objective of the present work was to improve the mechanical properties of a low
carbon Si based ferrite-martensite DP steel by tailoring the second phase (martensite)
morphology, distribution, and size in the ferrite matrix. An existing conventional continuous
annealing process (CAL) route was modified to develop an improved annealing process route
suitable for industrial usage. A custom designed annealing simulator (capable of simulating
conditions similar to industrial continuous annealing lines) was used to simulate the various
annealing processes. Several combinations of processing routes depending on the governing
mechanisms such as, ferrite recrystallization, pearlite dissolution, and phase transformation
etc. were investigated for their effect on the morphology, and distribution of the martensite
phase and the resulting mechanical properties. The main focus of the current work was to
study the effect of heating rates, isothermal annealing temperatures, and soaking time periods
(with no changes in cooling regime of conventional CAL) on the morphology, and
distribution of martensite phase. Further, the effect of combining thermal cycling as a pretreatment
to conventional CAL processing was also investigated. It was observed that by
varying the above stated annealing parameters, it was possible to trigger the ferrite
recrystallization, pearlite dissolution, and phase transformation at various stages of the
annealing process cycle. All such changes resulted in change of martensite morphology,
distribution, and even grain size and thus affected the final mechanical properties of DP steel.
This entire experimentation effort resulted in the development of a new processing route
called “Continuous Heating Annealing Process (CHAP)” that gave strength levels of 625
MPa with ductility similar to that obtained with the conventional CAL process with a
significant improvement in strain hardening exponent. Thus, the present research provided a
new annealing route (without any major changes in the conventional CAL process) for
processing of DP steels with improved strength-ductility combinatio
Effect of second phase distribution and morphology on the bake hardening behavior of dual phase steels
No full textThe present scenario of rapid boom in automotive industry has ushered tremendous
improvement and growth in the steel processing technology. Advanced High Strength Steels
(AHSS) have fulfilled most of the aspects concerning better utilization and fabrication of
light weight steels for automobile applications. Various grades of AHSS have been processed
over the past few decades, however, the search for even better mechanical properties and
subsequently fuel efficient-light weight structures has deemed a deep research prospect in
dual phase steels (DP steels), a category of AHSS. DP steels is the most commonly used
AHSS grade for automotive industry. Dual Phase steels also known as DP steels consist of a
hard martensite/ or bainite phase embedded within a softer ferrite phase. This peculiar
combination of a hard phase (martensite) and a soft phase (ferrite) provides a perfect balance
of strength and ductility in these steels. The typical production process of DP steels involves
inter-critical annealing of low carbon steels which is followed by a rapid quenching or
cooling techniques to obtain DP microstructure wherein, martensite is distributed along the
grain boundaries of the ferrite grains. DP steels are mostly used for fabricating the exterior
members of automobile bodies like the roof or floor panels and the cross member regions.
The finished or heavily formed (simulated by pre-straining in this research work) auto-body
is given a finishing paint curing treatment which helps in proper curing or adhesion of the
paint coat over the entire exterior panels in the vehicles. This finishing operation is
industrially referred to as the Bake Hardening treatment. The term hardening is associated
with the improvement in final yield strength of the automobile body after this treatment. The
increase in final yield strength is due to the presence of free or available interstitial solute
carbon atoms in DP steels during its processing. These interstitial solute atoms upon
receiving sufficient diffusion energy (during baking treatment) pin or lock the dislocations
created during various forming operations (stamping, bending, extrusion etc.) thus, a rise in
final yield strength is always obtained. Hence, in addition to curing of the paint coat, the dent
resistance of the final component also improves at no extra production cost. In the present
research work, bake hardening characteristics of a conventionally processed DP steel viz.
Continuous Annealing Line (CAL) process was evaluated against a modified Continuous
Annealing Line (mod-CAL) process. A typical industrial continuous annealing line (CAL)
process was employed to anneal a 67% cold rolled steel to obtain the dual phase
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microstructure. Subsequent to this conventional annealing, the steel was now subjected to an
improved process (mod-CAL) with modified initial heating rate and peak annealing
temperature. The processed specimens (through CAL and mod-CAL respectively) were
further pre-strained in the range 1–5 % followed by the bake hardening treatment at 170
for 20 minutes. It was observed that the CAL processed specimen showed a peak of about 70
MPa in bake-hardening index at 2 % pre-strain level. At higher pre-strain values (in excess
of 2 %), a gradual drop in bake-hardening index was observed. On the contrary, the mod-
CAL processed specimens showed near uniform bake-hardening response at all pre-strain
levels and a decrease could be noted above 4% pre-strain. The evolving microstructure at
each stage of annealing process and after bake-hardening treatment was studied using field
emission scanning electron microscopy. The microstructure analysis distinctly revealed the
differences in the martensite spatial distribution and interface morphologies developed by the
two annealing processes. The modified process showed predominant formation of martensite
within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite
was considered responsible for the observed improvement in the bake-hardening response.
Furthermore, along with improved bake-hardening response, negligible loss in tensile
ductility was also noted. This behavior was correlated with delayed micro-crack initiation at
martensite interface due to the serrated nature of the lath martensite
Effect of Inter-critical Annealing Parameters on the Recrystallization, Austenite Formation and Stabilization in a Dual Phase Steel
No full textME, MEDAdvanced high strength steels (AHSS) have been developed in the last few decades especially
for automotive applications due to their remarkable properties combining high strength and good
ductility. DP steels are the most common grade of advanced high strength steels which are
produced through inter-critical annealing of a low carbon steel followed by an accelerated
cooling to obtain the ferrite-martensite (dual phase) microstructure. The strength of DP steels is
manipulated by the amount of martensite and ductility by the size and distribution of this phase.
The fraction of martensite formed depends upon the austenite amount available during the intercritical
annealing process. So, it is always important to understand the kinetics of ferrite
recrystallization and austenite formation during the annealing of cold rolled sheets. The present
work focuses on understanding the effect of intercritical annealing parameters (viz. annealing
temperature, soaking time periods etc.) on ferrite recrystallization, austenite formation and its
stabilization during the production of dual phase steel from a low carbon cold rolled steel. The
main effort of this research work has been to arrive at the combination of annealing process
parameters to produce the desired ferrite-martensite structures. For the study, commercial
software viz. Thermo-Calc (3.0) and JMat-Pro (7.0) have been used to predict the various
annealing parameters. The predicted values have been compared with the experimental values
and a significant variation has been observed in the results. It has been concluded that the
annealing parameters clearly affect the phenomenon of ferrite recrystallization, and austenite
formation and its stabilization during the production of dual phase steels. Hence, this study is
very helpful in understanding the synergistic effect of ferrite recrystallization and austenite phase
transformation behaviour in dual phase steels to tailor design the mechanical properties by
microstructure control
Study on the Rapid Detection of Salmonella by Loop Mediated Isothermal Amplification[Lamp Method].
No full textThis Dissertation / Report is the outcome of investigation carried out by the creator(s) / author(s) at the department/division of Central Food Technological Research Institute (CFTRI), Mysore mentioned below in this page
A Simplified Micromechanical Modeling Approach to Predict the Tensile Flow Curve Behavior of Dual-Phase Steels
No full textMicromechanical modeling is used to predict material’s tensile flow curve behavior based on microstructural characteristics. This research develops a simplified micromechanical modeling approach for predicting flow curve behavior of dual-phase steels. The existing literature reports on two broad approaches for determining tensile flow curve of these steels. The modeling approach developed in this work attempts to overcome specific limitations of the existing two approaches. This approach combines dislocation-based strain-hardening method with rule of mixtures. In the first step of modeling, ‘dislocation-based strain-hardening method’ was employed to predict tensile behavior of individual phases of ferrite and martensite. In the second step, the individual flow curves were combined using ‘rule of mixtures,’ to obtain the composite dual-phase flow behavior. To check accuracy of proposed model, four distinct dual-phase microstructures comprising of different ferrite grain size, martensite fraction, and carbon content in martensite were processed by annealing experiments. The true stress–strain curves for various microstructures were predicted with the newly developed micromechanical model. The results of micromechanical model matched closely with those of actual tensile tests. Thus, this micromechanical modeling approach can be used to predict and optimize the tensile flow behavior of dual-phase steels
A Thermal Cycling Route for Processing Nano-grains in AISI 316L Stainless Steel for Improved Tensile Deformation Behaviour
No full textThe present work significantly improved the mechanical strength of AISI 316L stainless steel by producing nano-sized grains. Steel was subjected to cold rolling followed by repetitive thermal cycling to produce ultra-fine/nano-sized grains. The optimum processing parameters including extent of cold deformation, annealing temperature for thermal cycling, soaking period during each thermal cycle, and number of thermal cycles were determined through a systematic step-by-step procedure. After conducting thermal cycling under optimum conditions, a significant amount of grain size reduction was achieved. The effect of nano-sized grains on tensile deformation behavior was analysed. High cold deformation resulted in increased amount of stored strain energy. The stored strain energy accelerated the re-crystallisation kinetics during the thermal cycling process. Every thermal cycle resulted in irregular dispersal of stored energy. This irregular dispersal of stored energy favoured recrystallisation rather than grain growth and led to refinement of grains, in the absence of strain induced martensite. Repetitive thermal cycling promoted grain refinement and resulted in very significant grain size reduction with resultant grain size in the range of 800-1200 nm as compared to initial size of 90-120 mu m. The resultant microstructure improved tensile strength by 106.8 per cent, from 590 MPa to 1220 MPa
Effect of Isothermal Annealing on Microstructural Morphology of Martensite in a Super-Martensitic Stainless Steel Subjected to Different Prior Conditions
No full textA super-martensitic stainless steel of the Fe-Cr-Ni family was investigated for morphological changes during isothermal annealing after subjecting it to different prior conditions. The key issues during thermomechanical treatment included determination of conditions for austenite stability and reversibility and deciding the appropriate prior treatments for the cold worked alloy before subjecting it to isothermal annealing. The study evaluated the effect of isothermal annealing on the recrystallization kinetics, phase reversion, and microstructural changes in the alloy. Intercritical isothermal annealing was carried out on samples in the range 750-900 degrees C for short time periods in the range of 1-2.5min. The recrystallization behavior and microstructural changes were studied by electron backscatter diffraction, X-ray diffraction, and Vickers's hardness measurements. Martensite morphology showed significant changes during the isothermal annealing process with dependence on prior matrix substrate. The tensile properties were also evaluated. The cold rolled (CR) and isothermally annealed samples provided an improved combination of strength and ductility at the optimum heat treatment parameters
Investigation of Implantable Multichannel Neurostimulators
No full textabstract: There is a strong medical need and important therapeutic applications for improved wireless bioelectric interfaces to the nervous system. Multichannel devices are desired for neural control of robotic prosthetics that interface to remaining nerves in limb stumps of amputees and as alternatives to traditional wired arrays used in for some types of brain stimulation. This present work investigates a new approach to ultrasound-powering of implantable microelectronic devices within the tissue that may better support such applications. These devices are of ultra-miniature size that is enabled by a wireless technique. This study investigates two types of ultrasound-powered neural interfaces for multichannel sensory feedback in neurostimulation. The piezoceramics lead zirconate titanate (PZT) ceramic and polyvinylidene fluoride (PVDF) polymer were the primary materials used to build the devices. They convert ultrasound to electricity that when rectified by a diode produce a current output that is neuro stimulatory to peripheral nerve or the neurons in the brain. Multichannel devices employ a form of spatial multiplexing that directs focused ultrasound towards localized and segmented regions of PVDF or PZT that allows independent channels of nerve actuation. Different frequencies of ultrasound were evaluated for best results. Firstly, a 2.25 MHz frequency signal that is reasonably penetrating through body tissue to an implant several centimeters deep and also a 5 MHz frequency more suited to application for actuation of devices within a less than a centimeter of nerve. Results show multichannel device performance to have a complex inter-relationship with frequency, size and thickness, angular incidence, channel separations, and number of folds (layers connected in series and parallel). The output electrical port impedances of PVDF devices were examined in relationship to that of stimulating electrodes and tissue interfaces. Miniature multichannel devices were constructed using an unreported method of employing state of the art laser cutting systems. The results show that PVDF based devices have advantages over PZT, because of better acoustic coupling with tissue, known better biocompatibility, and better separation between multiple channels. However, the PZT devices proved to be better overall in terms of compactness and higher outputs for a given ultrasound power level.Dissertation/ThesisMasters Thesis Bioengineering 201
An Empirical Evaluation of Social Influence Metrics
No full textabstract: Predicting when an individual will adopt a new behavior is an important problem in application domains such as marketing and public health. This thesis examines the performance of a wide variety of social network based measurements proposed in the literature - which have not been previously compared directly. This research studies the probability of an individual becoming influenced based on measurements derived from neighborhood (i.e. number of influencers, personal network exposure), structural diversity, locality, temporal measures, cascade measures, and metadata. It also examines the ability to predict influence based on choice of the classifier and how the ratio of positive to negative samples in both training and testing affect prediction results - further enabling practical use of these concepts for social influence applications.Dissertation/ThesisMasters Thesis Computer Science 201
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