Hot closed die forging is a regular process applied to manufacture metallic components used in supporting and fixing cable conductors in electric power transmission lines. In this forming process, the final microstructure of the workpiece depends on the control of several processing parameters. In this study, simulation of the industrial forging process of a ball hook component was performed using the finite element software DEFORM-3D following the thermomechanical procedures coupled with microstructure evolution. This simulation allowed the study of the process parameters such as temperature, strain and strain rate and microstructure evolution during hot closed die forging of a low carbon steel. The geometry and grain size evolutions obtained by simulation are compared with those found in the actual process and measured by optical microscopy. The attained data indicated that the simulation is able to describe processing parameters and microstructure evaluation during hot forging of a metallic component.

A ferritic stainless steel, with a chromium/nickel equivalent ratio of 7.4, was deformed in torsion over a temperature range of 900°C to 1200°C and a strain rate of 1 s-1. The flow curves attained showed two different shapes: at higher temperatures, the materials typically softened only by dynamic recovery while, at lower temperatures, the flow curves were characterized by a rapid rise to a peak stress, followed by extensive flow softening. There was a very significant reduction of grain size during deformation under all the conditions studied, suggesting that dynamic recrystallization occurred. These observations are incongruent with the literature, which reports that the restoration mechanisms for ferrite change from dynamic recrystallization to dynamic recovery when the deformation temperature decreases, at a constant strain rate. Observations of the microstructural evolution after interrupting the tests at different levels of strain and analysis of the dependence of the work-hardening rate on the applied stress lead to a discussion about the competition between recovery and recrystallization during deformation of ferritic stainless steel at high temperatures and upon extensive strain.

Um aço inoxidável austenítico, com razão cromo/níquel equivalente de 7,4, foi deformado na faixa de temperaturas de 900°C a 1200°C com taxa de deformação de 1 s-1. As curvas tensão x deformação obtidas mostraram duas formas diferentes: em altas temperaturas a forma típica de materiais que amaciam apenas por recuperação dinâmica enquanto que, em baixas temperaturas, as curvas tensão x deformação foram caracterizadas por um crescimento rápido até uma tensão de pico, seguido por um amaciamento extensivo. Foi observada uma redução significativa no tamanho de grão durante a deformação em todas as condições estudadas, sugerindo que a recristalização dinâmica tenha ocorrido. Estas observações são incongruentes com a literatura, que reporta que os mecanismos de restauração para a ferrita variam de recristalização dinâmica para recuperação dinâmica quando a temperatura de deformação diminui a uma taxa constante. Observações da evolução microestrutural em ensaios interrompidos em diferentes níveis de deformação e a análise da dependência da taxa de encruamento com a tensão aplicada possibilitam uma discussão sobre a competição entre recuperação e recristalização durante a deformação de aços inoxidáveis ferríticos em altas temperaturas e submetidos a grandes deformações.

Dilatometric techniques were used to build the continuous cooling transformation (CCT) diagram for a medium carbon microalloyed steel; the microstructure and hardness were determined at different cooling rates. The mechanical behavior of the steel in the austenite field and at temperatures approaching austenite to ferrite transformation was measured by means of hot torsion tests under isothermal and continuous cooling conditions. The no recrystallization temperatures, Tnr, and start of phase transformation, Ar3, were determined under continuous cooling condition using mean flow stress vs. inverse of absolute temperature diagrams. Interruption of static recrystallization within the interpass time in the austenite field indicated that the start of vanadium carbonitride precipitation occurred under 860 °C. Austenite transformation was found to start at around 710 °C, a temperature similar to that measured by dilatometry, suggesting that interphase precipitation delays the transformation of deformed austenite. Pearlite was observed at temperatures ranging from 650 °C to 600 °C, with the flow curves taking on a particular shape, i.e., stress rose sharply as strain was increased, reaching peak stress at low deformation, around 0.2, followed by an extensive softening region after peak stress.

Two types of duplex stainless steels were deformed by torsion at a temperature range of 900 to 1200 °C and strain rate of 1.0 s-1 and their final microstructures were observed. The austenite volume fraction of steel A (26.5Cr - 4.9Ni - 1.6Mo) is approximately 25% at room temperature, after conventional annealing, while that of steel B (24Cr - 7.5Ni - 2.3Mo) is around 55%. Experimental data show that steel A is ductile at high temperatures and displays low ductility at low temperatures, while steel B has low ductility in the entire range of temperatures studied. At high temperatures, steel A is essentially ferritic and shows dynamic recrystallized grains after deformation. When steel A is strained at low temperatures and displays low austenite volume fraction, microstructural observations indicate that failure is triggered by grain boundary sliding due to the formation of an austenite net structure at the ferrite grain boundaries. At intermediate volume fraction, when austenite forms a dispersed second-phase in steels A and B, failure begins at the ferrite/ferrite boundaries since some of the new ferrite grains may become immobilized by the austenite particles. When steel B is strained at volume fraction of around 50% of austenite and both phases percolate the microstructure, failure occurs after low straining as a consequence of the different plastic behaviors of each of the phases. The failure characteristics of both steels are correlated not only with the volume fraction of austenite but also with its distribution within the ferrite matrix, which limits attainable strain without failure.