Samuel F. Rodrigues

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.

Prediction of hot flow plastic curves of ISO 5832-9 steel used as orthopedic implants

. The flow stress curves obtained showed two regions where firstly there is a rising on stress characterized as work hardening mechanism acting and secondly a decreasing in work-softening after a peak stress. The flow curves were modeled by adjusting the experimental data with Zener-Hollomom parameter to construct the constitutive equations that describe the plastic behavior in both regions. The first region was described until the peak stress, taking into consideration the competition between work hardening and recovery while the second one was described applying the softening time of 50% and the Avrami equation. In some hot deformation conditions the simulated curves showed good agreement with the experimental ones while in others conditions the simulated showed differences to experimental curves that was discussed and associated with other mechanisms that acted during hot deformation.

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.

Hot Deformation Behavior and Microstructural Evolution of a Medium Carbon Vanadium Microalloyed Steel

Hot forging of steel requires application of large strains, under which conditions, dynamic recrystallization (DRX) is expected to take place. In this study, torsion tests were carried out on a medium carbon vanadium microalloyed steel (38MnSiVS5) to simulate hot forging. Deformations were applied isothermally in the temperature range 900-1200xa0°C at strain rates of 0.1-10xa0s−1 in order to observe for the occurrence of DRX and to investigate for the microstructural evolution during straining. The shape of the flow curves indicated that the recrystallization takes place during deformation. This was supported by optical microscopy performed on the quenched samples which displayed considerable amounts of recrystallized grains. It was shown that the grain size depends on straining conditions such as strain rate and temperature. Finally, it was revealed that these process parameters can considerably affect the evolution of microstructure of industrial grade steels by means of DRX.

Dynamic Transformation During the Torsion Simulation of Plate and Strip Rolling

Torsion simulations of plate and strip rolling were carried out on a plain C–Mn and an X70 Nb steel over the temperature range 900–1000 °C. Pass strains of 0.4 were applied at a strain rate of 1 s−1. Interpass times of 0.5, 1, 1.5, 3, 10, and 20 s were employed to determine the mean flow stresses (MFSs) applicable to plate and strip rolling. By means of double differentiation, critical strains of 0.05 and 0.12 were established for the initiation of dynamic transformation and dynamic recrystallization, respectively. It is shown that the dynamic transformation of austenite to ferrite takes place above the Ae3 temperature, resulting in significantly lower than expected MFS values. The nucleation and growth of ferrite reduces the rolling load and modifies the microstructure. Shorter interpass times produce larger decreases in MFS than longer ones because they do not permit as much retransformation to austenite as takes place during longer interpass intervals.

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.