Clodualdo Aranas

A metastable phase diagram for the dynamic transformation of austenite at temperatures above the Ae3

Abstract A method is proposed for calculation of the pseudobinary phase diagram associated with the dynamic transformation of austenite to ferrite. Here the driving force is taken as the difference between the austenite flow stress at the moment of initiation and the yield stress of the fresh Widmanstätten ferrite that takes its place. The energy opposing the transformation consists of the Gibbs energy difference between austenite and ferrite at temperatures above the Ae3 and the work of accommodating the shear displacements and dilatation associated with the phase change. A metastable phase diagram is calculated for a 0.30 wt.% Mn-0.01 wt.% Si steel by balancing the driving force against the three obstacles. The results show that, under dynamic conditions, the ferrite phase field extends all the way from room temperature to that for the formation of delta ferrite.

Thermo-mechanical improvement of Inconel 718 using ex situ boron nitride-reinforced composites processed by laser powder bed fusion

Hexagonal boron nitride-reinforced Inconel 718 (h-BN/IN718) composites were fabricated using a laser powder bed fusion (LPBF) technique to treat a nanosheet-micropowder precursor mixture prepared in a mechanical blending process. Tailoring the BN in IN718 enhanced the thermal resistance of the composites, thereby dampening the sharpness of the melting temperature peak at 1364 °C. This is because the presence of the BN reinforcement, which has a low coefficient of thermal expansion (CTE), resulted in a heat-blocking effect within the matrix. Following this lead, we found that the BN (2.29 g/cm3) was uniformly distributed and strongly embedded in the IN718 (8.12 g/cm3), with the lowest alloy density value (7.03 g/cm3) being obtained after the addition of 12 vol% BN. Consequently, its specific hardness and compressive strength rose to 41.7 Hv0.5·cm3/g and 92.4 MPa·cm3/g, respectively, compared to the unreinforced IN718 alloy with 38.7 Hv0.5·cm3/g and 89.4 MPa·cm3/g, respectively. Most importantly, we discovered that the wear resistance of the composite improved compared to the unreinforced IN718, indicated by a decrease in the coefficient of friction (COF) from 0.43 to 0.31 at 2400 s. This is because the BN has an exfoliated surface and intrinsically high sliding and lubricating characteristics.

Improved electrical and thermo-mechanical properties of a MWCNT/In–Sn–Bi composite solder reflowing on a flexible PET substrate

Multi-walled carbon nanotube (MWCNT)/indium–tin–bismuth (In–Sn–Bi) composite nanostructures in which In–Sn–Bi nanoparticles have been penetrated by the MWCNT arrays were synthesized using a chemical reduction method. The incorporation of 0.6 wt% MWCNTs with high electrical conductivity into the In-based solder resulted in low minimum electrical resistivity (19.9 ± 1.0 µΩ·cm). Despite being reflowed at the relatively low temperature of 110 °C, the composite solder nanostructures were able to form mechanically stable solder bumps on a flexible polyethylene terephthalate (PET) substrate due to the MWCNT arrays with a high thermal conductivity of 3000 W/(m·K) and In–Sn–Bi nanoparticles with a low melting temperature of 98.2 °C. Notably, the composite solder bumps exhibited high flexibility (17.7% resistance increase over 1000 cycles of operation in a bending test) and strong adhesion strength (0.9 N average shear strength in a scratch test) on the plastic substrate because of the presence of mechanically flexible and strong MWCNTs dispersed within the solder matrix materials. These overall properties are due to the improved diffusivity of the composite solder nanostructures by the cover of the In–Sn–Bi nanoparticles along the MWCNT arrays and the network structure formation of the composite solder bumps.

Microstructural Evolution of a C-Mn Steel During Hot Compression Above the Ae3

In order to study the microstructural evolution during deformation, hot compression tests were carried out on a 0.06 wt pct C-0.30 wt pct Mn-0.01 wt pct Si steel at temperatures above the Ae3. The volume fraction of ferrite produced dynamically increased with the applied strain and decreased with increasing temperature. The present data are used to generate an isothermal strain–temperature–transformation diagram based on the applied strain. Results of this type can be employed to predict the effect of dynamic transformation during thermomechanical processing.

Dynamic Transformation during the Torsion Simulation of Strip Rolling

Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, Ar3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium Ae3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.

A Modular Solder System with Hierarchical Morphology and Backward Compatibility

A modular solder system with hierarchical morphology and micro/nanofeatures in which solder nanoparticles are distributed on the surface of template micropowders is reported. A core-shell structure of subsidiary nanostructures, which improved the intended properties of the modular solder is also presented. In addition, polymer additives can be used not only as an adhesive (like epoxy resin) but also to impart other functions. By combining all of these, it is determined that the modular solder system is able to increase reflowability on a heat-sensitive plastic substrate, oxidation resistance, and electrical conductivity. In this respect, the system could be readily modified by changing the structure and composition of each constituent and adopting backward compatibility with which the knowledge and information attained from a previously designed solder can offer feedback toward further improving the properties of a newly designed one. In practice, In-Sn-Bi nanoparticles engineered on the surface of Sn-Zn micropowders result in pronounced reflowing on a flexible Au-coated polyethylene terephthalate (PET) substrate even at the low temperature of 110 °C. Depending on their respective concentrations, the incorporation of CuO@CeO2 nanostructures and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymers increases oxidation resistance and electrical conductivity of the modular solder.

Dynamic Transformation during Plate and Strip Rolling

Torsion simulations were carried out of both plate (long interpass times) and strip (short interpass times) rolling. Both isothermal and continuous cooling conditions were employed. The dynamic transformation of austenite to ferrite was observed under all conditions and at all temperatures within the austenite phase field. About 8 to 10 volume percent ferrite was formed in a given pass, leading to about 50 - 70 % ferrite at the end of selected simulations. During the interpass intervals, some retransformation to austenite took place, the amount of which increased with holding time and temperature and decreased with the addition of alloying elements. It is shown that the driving force for the transformation is the softening associated with the replacement of work-hardened austenite grains by the softer alpha phase. The implications with respect to rolling load (i.e. mean flow stress) are also discussed.

Influence of Pushing and Pulling the Electrode Procedure and Addition of Second Layer of Welding on the Wear in Hardfacing of Fe-Cr-C

The aim of this work is evaluate the influence of welding conditions on abrasive wear resistance in coating of Fe-Cr-C. The metal base used in this investigation was the steel SAE 1020 and as welded metal the selfshilded tubular wires of Fe-Cr-C with 1.6 mm of diameter. The welding parameter such as amperage, voltage, welding speed, wire feed speed and the distance between the point and samples were kept constant by varying the electrode inclination and the number of layers deposited. These resulted in four different weld conditions: pulling and pushing the weld pool and hardfacing formed with 1 end 2 layers. Their influences on dilution, microhardness and microstructure were evaluated and correlated with the abrasive wear according to the standard tests methods for abrasion measurements through the usage of dry sand/rubber wheel apparatus, ASTM G-65-04. The results showed that the wear resistance of the four different conditions was affected by dilution, microstructure morphology and carbide volume fraction. The best conditions for hardfacing deposition were for pushing the torch and two layers added.

Dynamic Transformation and Retransformation During the Simulated Plate Rolling of an X70 Pipeline Steel

The controlled rolling of pipeline steels involves pancaking the austenite and then subjecting it to accelerated cooling. However, the formation of ferrite during rolling decreases the amount of austenite available for microstructure control. Here the formation of ferrite during rolling is simulated using a five-pass rolling schedule applied by means of torsion testing. The first and last pass temperatures were 920 and 860 °C with 15° of cooling between passes. All of the rolling was carried out above the Ae3 temperature of 845 °C that applies to this steel. Interpass times of 10 and 30 s were employed, which corresponded to cooling rates of 1.5 and 0.5 °C/s, respectively. Samples were quenched before and after the first, third, and fifth passes in order to determine the amount of dynamic ferrite produced in a given pass. The amounts of dynamic ferrite formed and retained increased with pass number. The amounts of ferrite that retransformed increased with pass number. The simulations indicate that ferrite is unavoidably produced during plate rolling and that the microstructures present at the initiation of accelerated cooling do not consist solely of austenite.

ADVANCES IN THE ASSESSMENT OF HOT CRACKING OF Cu-CONTAINING STEELS

While the hot cracking of Cu-containing steels is a serious and widely known problem for the industry, the literature reports that copper only slightly impairs the hot-ductility measured in laboratory. A distinction is drawn between hot ductility and hot shortness and the respective cracking operating mechanism involved at the respective range of temperature. The representability of the laboratory assessment of hot cracking of Cucontaining steels by hot tensile tests compared with actual results in industrial practice is discussed. The effectiveness of the variable affecting in each cracking mechanism and respective temperature range, focused in improving the hot cracking assessment of Cu-steels and the relative importance of oxidation will also be discussed. It is concluded that the hot tensile test in argon might be a good simulation if properly interpreted and that the role of oxidation is secondary in causing hot shortness. A schematic diagram for the hot tensile test is proposed.