A. Momeni

Dynamic recrystallization behavior of 13%Cr martensitic stainless steel under hot working condition

In this study, the efiect of hot deformation on martensitic stainless steel was carried out in temperatures between 950 to 1100 ‐ C and strain rates of 0.001, 0.01 and 0.1 s i1 . Two important dynamic recrystallization parameters, the critical strain and the point of maximum dynamic softening, were derived from strain hardening rate vs stress curves. Then the calculated parameters were used to predict the dynamic recrystallized fraction. Our results show that critical stress and strain increase with decreasing deformation temperature and increasing strain rate. The hot deformation activation energy of the steel is also investigated in the present work with 413 kJ/mol. Our experimental ∞ow curves are in fair agreement with the kinetics of dynamic recrystallization model.

Effect of hot working and post-deformation heat treatment on microstructure and tensile properties of Ti-6Al-4V alloy

Abstract The effects of hot compression, hot rolling and post-rolling annealing on microstructure and tensile properties of Ti-6Al-4V were analyzed. Hot compression tests were conducted in the temperature range of 800-1 075 °C and at strain rates of 0.001-1 s−1, and the relations between the characteristic points of flow curve and processing variables were developed. Two passes of hot rolling test with total reduction of 75% were performed in the temperature range of 820-1 070 °C and at constant strain rate of 2 s−1. After hot rolling, some specimens were subjected to heat treatment at 870 °C and 920 °C for 2 h followed by air cooling. Hot rolling in beta phase field resulted in coarse beta grains transforming to martensite by cooling. Otherwise, rolling in the alpha/beta phase filed gave rise to a partially globularized alpha microstructure. The post-rolling heat treatment completed the partial globularization of alpha phase in two-phase region and otherwise broke down the martensitic structure of beta-rolled samples. Tensile tests showed that the strength characteristics as well as elongation decrease significantly with increasing the rolling temperature from the two-phase to the single-phase region. Increasing heat treatment temperature contributed to lower strength for the specimens rolled in two-phase region and higher strength characteristics for the beta-rolled specimens.

Constitutive analysis and processing map for hot working of a Ni-Cu alloy

The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C–1150 °C and at strain rates of 0.001 s−1-1 s−1. Flow curves at low strain rates, up to 0.01 s−1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5, which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10–35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.

Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel (DSS) were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800–1100°C and strain rates of 0.001–1 s−1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-Hollomon parameter (Z) increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6.×1015 (lnZ=35.5), a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction (EBSD) confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.

Dynamic recrystallization and precipitation in high manganese austenitic stainless steel during hot compression

Dynamic recrystallization and precipitation in a high manganese austenitic stainless steel were investigated by hot compression tests over temperatures of 950–1150°C at strain rates of 0.001 s−1-1 s−1. All the flow curves within the studied deformation regimes were typical of dynamic recrystallization. A window was constructed to determine the value of apparent activation energy as a function of strain rate and deformation temperature. The kinetics of dynamic recrystallization was analyzed using the Avrami kinetics equation. A range of apparent activation energy for hot deformation from 303 kJ/mol to 477 kJ/mol is obtained at different deformation regimes. Microscopic characterization confirms that under a certain deformation condition (medium Zener-Hollomon parameter (Z) values), dynamic recrystallization appears at first, but large particles can not inhibit the recrystallization. At low or high Z values, dynamic recrystallization may occur before dynamic precipitation and proceeds faster. In both cases, secondary phase precipitation is observed along prior austenite grain boundaries. Stress relaxation tests at the same deformation temperatures also confirm the possibility of dynamic precipitation. Unexpectedly, the Avrami’s exponent value increases with the increase of Z value. It is associated with the priority of dynamic recrystallization to dynamic precipitation at higher Z values.

Repetitive Thermomechanical Processing Towards Ultra Fine Grain Structure in 301, 304 and 304L Stainless Steels

Thermomechanical processing as a combination of cold rolling and annealing was performed on austenitic stainless steels 301, 304 and 304L. Two cold rolling steps each one up to a reduction of 75% were combined with an intermediate annealing at 800°C for 20 min. The final annealing was performed at the same temperature and time. Cold rolling contributed to martensite formation at the expense of metastable austenite in the studied materials. Austenite in 30l was found to be less stable than that in 304 and 304L. Hence, higher strength characteristics in the as-quenched 301 stainless steels were attributed to the higher volume fraction of martensite. Both α′-martensite and ɛ-martensite were found to form as induced by deformation. However, the intensity of ɛ-martensite increased as the stability of austenite decreased. Annealing after cold rolling led to the reversion of austenite with an ultra fine grained structure in the order of 0.5–1 μm from the strain induced martensite. The final grain size was found to be an inverse function of the amount of strain induced martensite. The thermomechanical processing considerably improved the strength characteristics while the simultaneous decrease of elongation was rather low.

Microstructural evolution through hot working of the single-phase and two-phase Ti-6Al-4V alloy

Abstract The microstructural evolution of the alloy Ti-6Al-4V was analyzed after hot compression tests in the temperature range of 800–1150°C and the strain rate range of 0.001 s−1 to 1 s−1. The slope of work hardening and that of flow softening were conspicuously higher in the two-phase microstructure than those obtained in the single-phase β region. It was found that in the two-phase region the peak strain, the strain for highest rate of flow softening and the steady state strain were almost independent of the Zener–Hollomon parameter, Z. While in the single-phase region, characteristic strains were dependent on Z. The microstructural observations indicated that globularization in the α phase contributed to the flow softening. It was also shown that the kinetics of globularization decreased with strain rate. Otherwise, it was concluded that dynamic recrystallization comes into operation during hot deformation of the single-phase β microstructure. In the single-phase region the recrystallization softening and the average grain size diminished with strain rate and increased with temperature.

Effect of Hot working on Secondary Phase Formation in 2205 Duplex Stainless Steel

In this research, aging treatments at temperatures of 800 and 900 ‐ C for difierent aging time of 5{60 min were conducted on solution treated as well as hot worked samples of 2205 dual phase stainless steel. The efiect of aging treatment on precipitation of intermetallic phases was investigated in undeformed specimens and those subjected to hot deformation with difierent strain rates of 0.001{1 s i1 . It was found that ae precipitation increased by hot working. It was also concluded that the volume fraction of phase increased with deformation temperature and decreased with strain rate. The precipitation of intermetallic phases (i:e: ae and ´) was analyzed by an Avrami-type kinetics equation of %(ae+´)=A(1-exp(-kt n )) and the values of n and lnk were estimated for difierent thermomechanical regimes. The values of n were assessed to increase from 0.4 to 1 with strain rate in the studied range. Otherwise, It was also understood that lnk decreased with strain rate. Microstructural observations by means of optical microscopy and scanning electron microscopy showed that ae particles mostly nucleated at the ferrite-austenite interfaces. But no sign of ´-phase could be seen. This fortifled the idea of certain literature that ´-phase always forms at early stages of aging and consumes through the precipitation of ae.

Hot deformation behavior of Fe−29Ni−17Co alloy

Hot compression tests were carried out on a Fe-29Ni-17Co alloy in the temperature range of 900 °C to 1200 °C and at strain rates of 0.001–1 s−1. Dynamic recrystallization was found responsible for flow softening during hot compression. The flow behavior was successfully analyzed by the hyperbolic sine equation and the corresponding material constants A, n and α were determined. The value of apparent activation energy was determined as 423 kJ/mol. The peak and steady state strains showed simple power-law dependence on the Zener-Hollomon parameter. The dynamic recrystallization kinetics was analyzed using Avrami equation and the corresponding exponent was determined to be about 2.7. This value, higher than 2 reported in the literatures, is associated with the mechanism of continuous dynamic recrystallization in the studied alloy. The flow curve up to the peak was modeled by the Cingara equation and the strain exponent, c, was determined about 0.85. The higher value of c compared with the value of 0.2 which has been reported for some stainless steels fortified the idea of extended dynamic recovery or continuous dynamic recrystallization in the studied alloy.

Influence of hot working on mechanical and physical properties of an Fe-Ni-Co alloy

Abstract The flow behavior, mechanical and physical properties of alloy Fe-29 Ni-17 Co were studied using hot compression and hot rolling tests over the temperature range of 900 °C to1 200 °C and strain rates of 0.001 s−1 to 1 s−1. At temperatures below 1 100 °C, the flow curves were similar to those of dynamic recovery; whereas, at higher temperatures, faint peaks appeared on the flow curves. A modified power-law constitutive equation was used to model the dependences of flow stress on temperature and strain rate in different deformation regimes. The Kocks–Mecking equation was used to model the flow curves and to measure the mean flow stress of the material in hot rolling. The samples hot rolled at 1 100 – 1 200 °C to a reduction of 30 – 40 % showed the best combination of ductility and strength and low thermal expansion coefficient. Microstructures of the hot rolled samples indicated that hot rolling at 1 100 – 1 200 °C to reduction of 30 – 40 % leads to complete dynamic recrystallization.