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The influence of pre-strain on the tension and creep behavior of Al-Mn-Mg containing nano-sized TiC particles at elevated temperatures was investigated. Introduction of TiC nano-particles and the application of pre-strain significantly increase the strength at the ambient temperature and at 473 K, even though the strengthening effect diminishes at 573 K due to the more activated dislocation climb. Small pre-strain (von Mises strain of 0.2) could increase the yield strength at 573 K from 72 MPa to 96 MPa. Larger pre-strain promotes the precipitation during hot deformation at 573 K, and concurrent precipitation suppresses work softening by the pinning effect, leading to a significantly enhanced ductility and creep rupture life. [ABSTRACT FROM AUTHOR]

We investigated the effect of preexisting plastic deformation with the strain of 30% on the creep behavior of TP347 austenitic steel at 650 °C with different applied stresses. The results showed that the creep rupture life of the prestrained sample increased more than that of the unstrained sample tested at 300, 250 and 200 MPa due to the strengthening of the mechanical twins induced by preexisting plastic deformation. Conversely, the prestrained samples were ruptured earlier than that of the unstrained sample tested when the lower creep stresses (150 and 125 MPa) were applied. The shorter creep rupture life of the prestrained sample was due to the reduced amount of time to a minimum creep rate and the shorter tertiary-stage creep time which can be related to the accelerated precipitation of sigma phase at the grain boundaries in the strained sample. [ABSTRACT FROM AUTHOR]

The hot deformation and static softening behavior of various high Mn (20–30 wt%) austenitic steels microalloyed with different V (0.1, 0.2 wt%), C (0.2, 0.6, 1 wt%) and N (0.005–0.025 wt%) contents were investigated. Double-hit torsion tests at temperatures in the range 700–1100 °C were carried out and specimens quenched at selected conditions were examined using advanced microscopy techniques (EBSD-TEM) to characterize the recrystallization and strain-induced precipitation behavior. The results show that precipitation of vanadium at the hot working temperature range is sluggish. It mainly occurs for the combinations of 20%Mn–0.6%C–0.2%V and 30%Mn–1%C–0.1%V. When the carbon content is reduced to 0.2%C, strain-induced precipitation is suppressed at typical hot working temperatures, independently of the N level. The flow stress behavior was affected by the amount of C and by modifying the base composition from 30%Mn to 20%Mn–1.5%Al. However, the effect is complex and depends on deformation conditions. In the absence of strain-induced precipitation, the static softening kinetics was accelerated by increasing C content. However, no effect of Mn or V in solid solution was observed. In those cases where strain-induced precipitation took place, static recrystallization was severely delayed, leading to a major contribution of recovery to softening kinetics. [ABSTRACT FROM AUTHOR]

The effects of deformation at 900 °C and 1050 °C along with various cooling rates on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel are investigated. Without prior high temperature deformation, the microstructures can be polygonal ferrite, pearlite, granular ferrite, acicular ferrite or bainite ferrite, depending on the cooling rate. High temperature deformation moves the transformation curves toward the left side in the continuous cooling transformation (CCT) diagram and promotes the formation of polygonal ferrite, granular ferrite, and acicular ferrite, which leads to a lower hardness. Electron backscattering diffraction (EBSD) results indicated that with a cooling rate of 40 °C/s, high proportion of low angle grain boundary (LAGB), large orientation gradient, and high dislocation density and residual stress can be obtained by deformation at high temperature before cooling. Low cooling rate reduces the proportion of LAGB, orientation gradient, dislocation density, and residual stress due to the release of residual stress and the consumption of dislocations. Deformation before cooling slightly lowers the transformation start temperature (Ar 3) while accelerating the transformation speed of the supercooled austenite. The yield and tensile strength increase with the increase in cooling rate. Furthermore, the impact energy value increases with the increase of cooling rate, and it slightly decreases when the microstructure fully changes to bainite ferrite owing to the significant increase in strength. Impact toughness is more sensitive to strength at low temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The effects of Nb on strain-induced precipitation and softening were quantified by means of the Gleeble-3500 thermo-mechanical simulator and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results show that Nb cannot completely dissolve during the reheating processes in the high Nb steels. The undissolved precipitates act as heterogenous nucleation sites for the precipitates induced by the strain, and prolong the strain-induced precipitation start time shown in the stress relaxation curves. The strain-induced precipitation start time is linearly related to the content of supersaturated Nb (the difference between dissolved Nb content after soaking and dissolved Nb content in equilibrium at corresponding deformation temperature). Increasing the content of supersaturated Nb can shift the nose of the precipitation C-curve to shorter time and higher temperatures. The effect of content of supersaturated Nb on precipitation increases along with the decrease of temperature. The softening ratio is linear with dissolved Nb content before the strain-induced precipitation and with precipitable Nb content after the strain-induced precipitation. The precipitates induced by strain have more strong retarding effect on the static recrystallization than the solid solution of Nb atoms. However, with the increase of the average precipitate radius, the retarding effect of precipitate rapidly decreases. [Copyright &y& Elsevier]

Abstract: The strain-induced precipitation kinetics of TiC in a 0.05% C–0.10% Ti HSLA steel was investigated by two-stage interrupted compression method. The precipitation–time–temperature (PTT) diagram for TiC precipitation was obtained by analyzing the softening kinetics curves of deformed austenite, which was confirmed to be of validity by employing transmission electron microscopy (TEM). Experimental results showed that the PTT diagram for TiC precipitation exhibited a typical “C” shaped and the nucleation of strain-induced TiC precipitation was a very rapid process in the temperature range 900–925°C. The relatively severe deformation applied on the steel was considered to be the main factor resulting in the fast kinetics of TiC precipitation. The TiC precipitates were heterogeneously distributed in either a chain-like or a cell like manner, implying that the precipitates nucleated on dislocations or on dislocation sub-structures, which were produced by deformation. The growth of TiC precipitates approximately followed a parabolic law. In addition, the coarsening of strain-induced TiC precipitates had already started before the completion of precipitation. [Copyright &y& Elsevier]

Abstract: Static recrystallisation of three steels having different Al, V and N contents (one of them without V) has been studied by means of hot torsion tests. It has been found that strain-induced precipitates of AlN in the austenite have a mean size of approximately 86nm. These particles barely inhibit the static recrystallisation, as associated pinning forces are very weak. However, mean size of VCN particles is equal to 11nm. This fine size leads to a temporary inhibition of recrystallisation, revealed by the characteristic “plateau” of the plots of recrystallised fraction versus holding time after deformation. Besides, activation energy for recrystallisation considerably augments when VCN precipitation occurs, but it hardly increases when AlN particles precipitate. On the other hand, diffusion coefficient of Al in austenite is two orders of magnitude higher than for V. Furthermore, according to thermodynamic calculations based on Hillert and Staffanson method, precipitation of AlN particles starts at much higher temperatures than VCN. Aforesaid reasons make AlN particles to be much coarser than VCN precipitates. From the results it can be concluded that low Al contents would lead to more intense precipitation of VCN that is beneficial for V-microalloyed steels. [Copyright &y& Elsevier]

An ultra low carbon Cu bearing HSLA steel, micro alloyed with Nb and Ti was forged in two stages and subsequently cooled at different cooling rates. Variation in microstructures and the mechanical properties at different cooling rates have been studied. Volume fraction of bainitic phase has been found to vary with different cooling rates. Maximum strength was achieved in this steel at an intermediate cooling due to precipitation hardening of very fine microalloying carbides, carbonitrides and ϵ-Cu precipitates. Finer polygonal ferrite and granular bainite structure obtained at the slowest cooling rate resulted in high impact toughness values at ambient and sub ambient temperatures. [Copyright &y& Elsevier]

Nucleation kinetics of the precipitation for MnAl6 and MnAl12 in the aluminum alloy AA3003 alloy have been investigated experimentally and theoretically. The results show that cold rolling enhances the rate of precipitation and this effect increases as the annealing temperature decreases. Micro-segregation of the solute atom Mn at the dislocation network during cold deformation is found to have a significant effect on the nucleation kinetics of the precipitation of MnAl6 and MnAl12 in AA3003 in addition to the effect of dislocations, which increase the nucleation site density and reduce the nucleation barrier. A model to predict the start times of the precipitation during isothermal holding is constructed by considering the effects of dislocations and recovery as well as micro-segregation of Mn on the nucleation kinetics of precipitates. The predictions of the model are in good agreement with the experiment data. [Copyright &y& Elsevier]