The aim of this work is focused on nucleation stages with emphasis on the development of intra-granular ferrite morphologies during isothermal austenite transformation in titanium free micro-alloyed steel. Isothermal treatment was carried out in the temperature range 350 to 600οC. These treatments were interrupted at different times between 2 and 1800 s in order to analyze the evolution of the microstructure. Metallographic evaluation was done by using optical and scanning electron microscopy (SEM) enabled determination of the nucleation onset at all treatments and subsequent on the development of intra-granular ferrite of isothermally treated Ti free micro-alloyed steel. The results show that at high temperatures ( 500  C) polygonal intra-granularly nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by an incomplete transformation phenomenon, At intermediate temperatures (450 and 500  C) an interlocked acicular ferrite (AF) microstructure is produced, and at low temperatures (400 and 350  C) the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intra-granularly nucleated were observed. In addition to sheaf type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

Medium carbon V-microalloyed steel continuously cooled from the austenitization temperature at still air, with predominantly acicular ferrite structure, has been investigated by means of four-point bending of notched Griffiths–Owens’s type specimens at liquid nitrogen temperature. Local fracture stress and plastic strain were determined by using finite element analysis and fracture surface examination using scanning electron microscope. It was revealed that cleavage fracture initiation, which takes place close to the notch root in the narrow zone of high plastic strains, is not related to any broken coarse second phase particles. It was assumed that microcracks nucleate by strain induced fracture of pearlite nodules. Two effective surface energy values of 24 and 42 J/m2 were estimated according to the Griffith’s equation, indicating the influence of crystallographic orientation between neighboring grains at the origin of fracture. Lower value was attributed to fracture of coarse ferrite–pearlite units with similar crystallographic orientation and higher value to fracture propagation through fine acicular ferrite matrix.

To reveal the prior-austenite grain boundaries in micro-alloyed steels there is different techniques. This study has been carried out over a wide range of temperature (950-1150 οC) and it has been found that the tested procedure based on the combination of heat treatment and thermal etching (TE) method give excellent results for the Titanium and Titanium free medium carbon micro-alloyed steels at all the austenitization conditions tested.

This work is focused on nucleation stages during isothermal austenite transformation in two types of Vanadium micro-alloyed steels. Isothermal treatment was carried out in the temperature range 350 to 600οC. Metallographic evaluation using optical and scanning electron microscopy (SEM) enabled determination of the nucleation curves of isothermally decomposed austenite. Three curves are found to be relevant to this initiation stage of transformation: first curve is related to grain boundary nucleated ferrite (GBF), second curve is related to intra-granularly nucleated ferrite (IGF) and the third to pearlite (P) curve. GBF and IGF curves are divided into two regions, which represents the high temperature and the low temperature segments as consequence of either displacive nature of transformation or diffusion transformation. Addition of Ti to V – micro-alloyed steel in this work seems to be balanced by a slightly higher C and Mn content, leading to limited effect on nucleation stage of austenite decomposition.

The demand for economical steels has increased over past years. Steels are expected to have as lean chemistry as

possible and to be easier to fabricate. At the same time they are expected to have properties at least equal to steels which

are being substituted. These demands have been, at least in part, met by V microalloyed medium carbon forging steels

in order to substitute traditional quenched and tempered (Q+T) steels. In spite of some lower toughness, V microalloyed

medium carbon forging steels broaden its application due to considerable energy save by eliminating the heat treatment

from the production route. During continuous cooling, dominantly acicular ferrite microstructure is formed. Main

characteristics of acicular ferrite are intragranular nucleation and strongly disorganized microstructure with a larger

ability to deflect cracks. Acicular ferrite is, therefore, widely recognized to be a desirable microstructure due to good

mechanical properties [5]. On the other hand, generally, data related to isothermal decomposition seems to be lacking.

Therefore, the aim of the present study is to clarify the influence of transformation temperature and time on austenite

isothermal decomposition in V-microalloyed forging steels.

Cleavage fracture of the medium carbon V-microalloyed steel with structure consisting of acicular ferrite, pearlite and grain

boundary ferrite has been investigated by means of four-point bending of the notched samples at -196°C. It was found that

cleavage fracture initiation has not been related to the coarse second phase particles cracking. Calculated values of the effective

surface energy of 49 J/m2 and critical cleavage fracture stress normalized by yield stress of 1.84 are in agreement with the results

for the steels with ferrite-pearlite and bainite structures.

This study focuses on methylol functional benzoxazines as precursors to build a network structure utilizing both benzoxazine and resole chemistry. The first part is a review of systems that contain methylol groups which play a role on their crosslinking formation. The polymerization mechanism and properties of resoles will be highlighted as the most abundant polymers that are characterized by polymerization through condensation reaction of methylol group. In the second part, the effect of incorporating methylol group into benzoxazine monomers is studied. Differential scanning calorimetry (DSC) is used to study the effect of methylol group on the rate of polymerization. Kissinger and Ozawa methods using non-isothermal DSC at different heating rates show that methylol monomer exhibits lower average activation energy compared to the un-functionalized monomer. The effect of adding catalysts into the monomers is …

The aim of this work was to establish the deformation behavior of two vanadium mic-roalloyed medium carbon steels with different contents of carbon and titanium by tensile testing at 77 K. Samples were reheated at 1250 C for 30 min and continuously cooled in still air. Beside acicular ferrite as the dominant morphology in both microstructures, the steel with lower content of carbon and negligible amount of titanium contains consi-derable fraction of grain boundary ferrite and pearlite. It was found that Ti-free steel exhibits a higher strain hardening rate and significantly lower elongation at 77 K than the fully acicular ferrite steel. The difference in tensile behavior at 77 K of the two steels has been associated with the influence of the pearlite, together with higher dislocation density of acicular ferrite.

The aim of this work was to determine TTT diagram of medium carbon V-N micro-alloyed steel with emphasis on the

development of intragranular ferrite morphologies. The isothermal treatment was carried out at 350, 400, 450, 500, 550

and 600 οC. These treatments were interrupted at different times in order to analyze the evolution of the microstructure.

Metallographic evaluation was done using optical and scanning electron microscopy (SEM). The results show that at high

temperatures (≥ 500 ○C) polygonal intragranulary nucleated ferrite idiomorphs, combined with grain boundary ferrite and

pearlite were produced and followed by an incomplete transformation phenomenon. At intermediate temperatures (450, 500

○C) an interloced acicular ferrite (AF) microstructure is produced, and at low temperatures (400, 350 ○C) the sheave of

parallel acicular ferrite plates, similar to bainitic sheaves but intragranularly nucleated were observed. In addition to sheaf

type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

The polymerization mechanism of methylol-functional benzoxazine monomers is reported using a series of monofunctional benzoxazine monomers synthesized via a condensation reaction of ortho-, meta-, or para-methylol–phenol, aniline, and paraformaldehyde following the traditional route of benzoxazine synthesis. A phenol/aniline-type monofunctional benzoxazine monomer has been synthesized as a control. The structures of the synthesized monomers have been confirmed by ¹H NMR and FT-IR. The polymerization behavior of methylol monomers is studied by DSC and shows an exothermic peak associated with condensation reaction of methylol groups and ring-opening polymerization of benzoxazine at a lower temperature range than the control monomer. The presence of methylol group accelerates the ring-opening polymerization to give the ascending order of para-, meta-, and ortho-positions in …