Oscar Balancin

Prediction of steel flow stresses under hot working conditions

An austenitic stainless steel was deformed in torsion over a temperature range of 900-1200 °C using strain rates of 1, 5 and 10 s-1. The stress vs. strain curves determined were corrected for deformation heating and the flow stress was found to rise in the initial work-hardening regime, reaching a maximum before dropping to the steady state due to softening brought about by dynamic recrystallization. In order to determine the onset of dynamic recrystallization, diagrams of work-hardening rate vs. applied stress were drawn up for the hardening region of the flow stress curves. The flow stress curves were modeled by adjusting an evolution equation having one internal variable that describes the plastic behavior in the work-hardening regime to the experimental data. The flow stress after the onset of dynamic recrystallization was determined by incorporating the fractional softening into the evolution equation. Describing the effects of temperature and strain rate on the evolution equation through Zener-Hollomon parameters, a database was constructed for use in computer models to predict the roll force of rolling or forging loads under hot working conditions.

Influence o the microstructure of duplex stainless steels on their failure characteristics during hot deformation

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.

Surface integrity analysis when milling ultrafine-grained steels

This paper quantifies the effects of milling conditions on surface integrity of ultrafine-grained steels. Cutting speed, feed rate and depth of cut were related to microhardness and microstructure of the workpiece beneath machined surface. Low-carbon alloyed steel with 10.8 µm (as-received) and 1.7 µm (ultrafine) grain sizes were end milled using the down-milling and dry condition in a CNC machining center. The results show ultrafine-grained workpiece preserves its surface integrity against cutting parameters more than the as-received material. Cutting speed increases the microhardness while depth of cut deepens the hardened layer of the as-received material. Also, deformations of microstructure following feed rate direction were observed in workpiece subsurface.

Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel

Grain re Þ nement is a useful method to improve the strength and toughness of steels without changing their chemical composition. In this study, critical temperatures for the thermomechanical treatment of niobium microalloyed steel were determined experimentally and through thermodynamic data. Simulations of conventional and controlled thermomechanical processing and a thermomechanical treatment to obtain ultraÞ ne-grained microstructures were conducted using torsion tests. The Þ nal microstructures displayed signiÞ cant grain size reÞ nement. Conventional processing produced grains with an average size of 12 .m, while controlled processing led to an average grain size of 4.9 .m and severe plastic deformation at warm temperatures resulted in a grain size of 1.3 .m. The ultra reÞ nement of ferrite grains was associated with strain-induced dynamic phase transformation and dynamic recrystallization of as-transformed ferrite.

Plastic behavior of medium carbon vanadium microalloyed steel at temperatures near gamma alpha transformation

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.

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.

Propriedades mecânicas e de corrosão de dois aços inoxidáveis austeníticos utilizados na fabricação de implantes ortopédicos

ASTM F 138 austenitic stainless steel is extensively used as an orthopedic implant material. However, some aspects, such as low strength in the annealed condition and susceptibility to localized corrosion, limit wider use of this kind of steel. Recently, a high-nitrogen austenitic stainless steel, specified in the standard ISO 5832-9, has been indicated as an alternative to ASTM F 138 steel for more severe loading and permanent application inside the human body. In this work, microstructure, mechanical properties, corrosion resistance and fatigue behavior of both steels were determined and compared. ISO 5832-9 steel displayed better mechanical and corrosion behaviors than did ASTM F 138 steel. The combination of these features lead ISO steel to enhanced fatigue performance in both neutral and aggressive environments. Analyzed were the role of nitrogen in solid solution, combined with niobium in the Z-phase, and the factors that led to superior ISO 5832-9 properties.

Evidence of Strain-Induced Precipitation on a Nb- and N-Bearing Austenitic Stainless Steel Biomaterial

Pilot-scale plate rolling experiments and laboratory thermomechanical processing experiments were carried out to understand the mechanism of microstructural banding in low-carbon microalloyed steels. The microstructural banding originates with large elongated austenite grains, which are present at the roughing stage of rolling. The large austenite grains develop when conditions favour abnormal grain growth during reheat and/or strain induced grain boundary migration (SIBM) in the first few rolling passes. Microstructural banding is eliminated by designing TMP schedules to avoid abnormal grain growth and SIBM.

Influence of deformation on the kinetics of phase transformation in a forging steel during warm working

Dilatometric techniques were used to determine the start and finish transformation heating temperatures for a carbon steel (0.30% C - 1.5% Mn). The mechanical behavior of the steel was measured by torsion testing in the temperature range of 700 to 820 °C with holding times ranging from 1 to 30 min. The flow stress curves presented different shapes and stress levels. These differences were attributed to the ferrite and pearlite, ferrite and austenite, and austenite strained structures. When ferrite and pearlite were deformed together, the flow stress presented a hump with little straining; when the austenitic structure was deformed the shape of the flow stress curve was typical of materials having low stacking fault energy. The microstructural evolution observed by optical and scanning electron microscopy revealed that the evolution of the phase transformation was dependent on the testing temperatures, holding times and amount of straining. Comparisons were made on the kinetics of phase transformation with and without the application of plastic deformation, and evidence of strain-induced dynamic transformation was investigated.

Influência da microestrutura no comportamento plástico de aços inoxidáveis duplex

Two kinds of stainless steels with different ferrite and austenite volume fractions were deformed by torsion at a temperature range of 900 to 1250°C. Steel A (25.5Cr - 4.9Ni - 1.6Mo) has Creq/Nieq = 4.8 and grade B (22.2Cr - 5.6Ni - 3Mo) has Creq/Nieq = 3.5. The results show that the shape of the flow stress curves depends on the material and deformation conditions. Four different shapes of flow curves were observed. At high temperatures, steel A has a typical behavior of ferritic stainless steels. As the straining temperature was decreased, flow curves with peek stresses at low deformation were observed. When the austenite particles are coarsened inside the matrix (steel B), the flow stress displays a peak stress, dividing extensive hardening and softening regions. When the volume fraction of both phase are comparable and the microstructure is characterized by percolation of the both phases present in the samples, the flow stress curve acquires a very particular shape in hot torsion tests.