Strain-induced Precipitation Research Articles

Unraveling the origin of strain-induced precipitation of M23C6 in the plastically deformed 347 Austenite stainless steel

In this research, the phenomenon of strain-induced precipitation of M23 (M=Cr, Fe, Mo) C6 precipitates in 347 austenite stainless steel was systematically studied. During annealing at 650°C, the experimental results exhibited significantly earlier formation of M23C6 precipitates in the plastically strained specimen, than in the no-strain specimen. The various microstructural characterizations showed that the accelerated formation of M23C6 precipitates occurred around deformation bands that were generated by deformation twinning. Extensive investigation suggested that the strain-induced precipitation of M23C6 is attributed to the formation of strain-induced α′ martensite during plastic deformation near deformation bands.

Effect of niobium clustering and precipitation on strength of an NbTi-microalloyed ferritic steel

The microstructure–property relationship of an NbTi-microalloyed ferritic steel was studied as a function of thermo-mechanical schedule using a Gleeble 3500 simulator, optical and scanning electron microscope, and atom probe tomography. Contributions to the yield stress from grain size, solid solution, work hardening, particle and cluster strengthening were calculated using the established equations and the measured microstructural parameters. With a decrease in the austenite deformation temperature the yield stress decreased, following a decrease in the number density of >20nm Nb-rich particles and ≈5nm Nb-C clusters, although the grain refinement contribution increased. To achieve the maximum cluster/precipitation strengthening in ferrite, the austenite deformation should be carried out in the recrystallisation temperature region where there is a limited tendency for strain-induced precipitation. Based on the analysis of cluster strengthening increment, it could be suggested that the mechanism of dislocation–cluster interaction is closer to shearing than looping.

Modelling the strain-induced precipitation kinetics of vanadium carbonitride during hot working of precipitation-hardened Ferritic–Pearlitic steels

A model for describing the strain-induced precipitation kinetics of vanadium carbonitride in the hot deformed austenite of precipitation-hardened Ferritic–Pearlitic (AFP – Ausscheidungshärtende Ferritisch–Perlitische) steels is presented. The model considers heterogeneous nucleation of precipitates on dislocation nodes and predicts the volume fraction, size and composition of the precipitates as well as the composition of the remaining austenite during the nucleation, growth and coarsening stages of the precipitation process. The effect of static recovery on the precipitation kinetics is also incorporated in the model. A fairly good agreement between the model predictions and experimental observations is demonstrated.

Evolution in Dislocation Configuration of Deformed Fe-40Ni-Ti Alloy during Isothermal Relaxation

The thermo-simulation test and transmission electron microscopy (TEM) were applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation at 850°C in a deformed Fe-40Ni-Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth, and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation cells and finally, the sub-grains can be observed when relaxing to 1000s. The strain induced precipitates occur both onto the dispersed dislocations and dislocation cells. The precipitates pin the dislocations which results in retarding the progress of dislocation configuration evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though bypassing mechanism. Adopting the same simulation test to bainitic steel, the optimum refinement could be obtained at 60-200s during relaxation processing, corresponding to the perfect dislocation cells formation of Fe-40Ni-Ti alloy.

Effects of rolling temperature on the microstructure and mechanical properties of Ti–Mo microalloyed hot-rolled high strength steel

The effects of rolling condition on the microstructure and mechanical properties of Ti–Mo microalloyed hot-rolled high strength steel were investigated by varying rolling temperature from austenite recrystallization region to austenite+ferrite dual phase region. The results revealed that rolling temperature plays a decisive role in the mechanical properties by significantly affecting the characteristics of grain refinement and precipitation. Conducting the rolling in the regions through austenite recrystallization region to austenite non-recrystallization region was able to provide desirable microstructural features for impact properties as well as for strength by inducing a homogeneous and fine grain structure, well developed dislocation structures inside grain, and relatively high number density of fine carbides, although precipitation hardening effect was somewhat reduced by strain-induced precipitation.

Strain-induced Precipitation Behavior of δ Phase in Inconel 718 Alloy

To study the precipitation dynamics of δ phase in Inconel 718 alloy, two-stage interrupted compression method was used in the region of cold deformation temperatures and the temperatures range from 875 to 975 °C. The precipitation-time-temperature (PTT) curve of δ phase was obtained by analyzing the softening kinetics curves. For verifying the type of the precipitates and confirming the validity of the test, the transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersion spectrum (EDS) were employed. Experimental results indicated that the PTT curve for δ precipitation exhibited a typical “C” shape and the nose points of start and finish precipitation were about 5 s at 920 °C and 2815 s at 940 °C, respectively. In addition, the nucleation of δ was heterogeneous. The nucleation sites varied with temperatures, including dislocation, grain boundary and stacking fault within γ′ phase. And δ particles grew quickly at higher temperature with lower density. Moreover, the driving force of nucleation was mainly including chemical free energy, interfacial energy and dislocation distorted energy. And the dislocation distorted energy could decide the density of nucleation in the strain-induced process.

Influence of the Microalloying Elements on the Temporary Inhibition of Static Recrystallization by Strain-Induced Precipitates

The kinetics of static recrystallization of austenite and its transitory inhibition by strain-induced precipitates have been characterized in several microalloyed steels with different compositions. This inhibition can be seen by the formation of “plateaus” in the curves of static recrystallization obtained from isothermal double-deformation tests. The influence of the type of microalloying element (Nb, V, Al) and the mean size of the precipitates on the duration time of the plateau of recrystallization inhibition has been studied and empirical relationships between these variables have been obtained. Al-steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Multipass hot deformation behaviour of high Al and Al–Nb steels

The effect of Nb (up to 0.07wt%) and high Al content (up to 2wt%) on the multipass deformation behaviour of steels with 0.2% C and 2% Mn was studied with the aid of hot torsion simulations. From the tests, the critical Non-Recrystallisation (Tnr), Recrystallisation Limit and Stop Temperatures (RLT and RST) and the ferrite phase transformation start temperature (Ar3) were determined. It was observed that an increase in Al content from 1% to 2% or a microalloying addition of 0.03% Nb to 1% Al steel both led to a significant increase in the recrystallisation critical temperatures, which is greater than 100°C in the case of the Tnr. However, the value of the Tnr was not affected when 0.03% or 0.07% Nb was added to the 2% Al steel. Specimens quenched after several deformation passes were examined by optical and TEM means in order to study the interaction between static recrystallisation, strain-induced precipitation and γ→α phase transformation, and determine the mechanisms leading to strain accumulation in the steels investigated. The results suggest that for the 1% Al steels, the Al and Nb solute drag effect is the main mechanism leading to the increase in the critical recrystallisation temperatures, while for the 2% Al steels the occurrence of γ→α phase transformation at temperatures close to the Tnr is the main mechanism involved in softening retardation, with a limited contribution of Nb. However, γ→α phase transformation taking place at temperatures close to the Tnr resulted in a loss of hot ductility, which can limit the industrial applicability of the 2% Al steels.

Dynamic and Static Recrystallization Behaviour of Coarse-grained Austenite in a Nb-V-Ti Microalloyed Steel

The dynamic recrystallization (DRX) and static recrystallization (SRX) behaviour of coarse-grained austenite in a Nb-V-Ti microalloyed steel were studied by using a Gleeble thermomechanical simulator. Continuous and interrupted compression tests of coarse-grained austenite were performed in the temperature range of 1000–1150 °C at a strain rate of 0.1–5 s−1 . The peak and critical strains for the onset of DRX were identified with strain hardening rate analysis, and the ratio of critical strain to peak strain was found to be consistent with the one reported for finegrained austenite. An equation of the time for 50% softening was proposed by considering the activation energy of steel without microalloying elements and the solute drag effect of microalloying elements. Strain-induced precipitation may not take place at the deformation temperature above 1000 °C, which indicates that SRX of coarse-grained austenite is mainly retarded by coarse grain size and Nb in solution during rough rolling.

Characterization of Initial Structures, Texture and Precipitates in Strip-cast 3wt%Si Steel Sheet

A Fe-3wt%Si strip was produced using a vertical type twin-roll strip casting process, and two typical initial structures were observed. One was the solidification structure consisting of coarse columnar grains and the other was the deformed structure which was composed of outer inclined columnar grains and inner fine equxied grains. The solidification structure was characterized by {111}//ND fiber while the deformed structure shows a strong Goss texture. In addition, the size distributions of MnS precipitates, which were used as grain growth inhibiters, were investigated in the as-cast strip. The size of these particles in the deformed grains was smaller than that in the solidified grains, which could be attributed to the strain-induced precipitation mechanism. The differences of the initial structures, texture and precipitation behaviors resulted from the different superheating along the transverse direction of the as-cast strip, which originated from the inhomogeneous flowing of liquid steel.

Dynamic Recrystallization and Precipitation in 13Cr Super-Martensitic Stainless Steels

The influence of precipitation on the kinetics of static and dynamic recrystallization (DRX) was investigated in AISI 403 and 403Nb martensitic stainless steels. Hot compression tests were performed in the temperature range of 1073 K to 1473 K (800 °C to 1200 °C) and strain rates of 0.001 and 0.1 s−1 to study DRX and precipitation behaviors. In parallel, stress relaxation tests were conducted with pre-strains of 0.1, 0.15, 0.2, and 0.25, a strain rate of 0.1 s−1, and in the 1073 K to 1473 K (800 °C to 1200 °C) temperature range to study the kinetics of precipitation and recrystallization. Samples of hot compression and stress relaxation tests were quenched and the evolution of the microstructure was examined using optical and scanning electron microscopy. The results indicated that DRX interacts with dynamic precipitation (DP) over the temperature range of 1173 K to 1273 K (900 °C to 1000 °C). Hot compression testing results, confirmed by EBSD analysis, indicated that partial DRX occurs before precipitation in 403Nb, at 1073 K (800 °C). By contrast, no DRX was observed in 403 steel. At higher temperatures, i.e., over 1273 K (1000 °C), DRX preceded DP in both steels. Increasing the strain rate raised the temperature range of interaction between DRX and DP up to 1373 K (1100 °C). Strain-induced precipitation (SIP) was observed over the entire range of investigated test temperatures. Static recrystallization (SRX) took place predominantly in the temperature range of 1173 K to 1373 K (900 °C to 1100 °C), at which SIP significantly delayed the SRX finishing time. The results are analyzed in the framework of the classical nucleation theory and the underlying mechanisms are identified.

Static Recrystallization and Precipitation Behavior of a Weathering Steel Microalloyed with Vanadium

The static recrystallization (SRX) and precipitation behavior of a weathering steel microalloyed with vanadium were investigated through double-pass compression tests under controlled conditions using the MMS-300 thermal-mechanical simulator. The deformation temperatures ranged from 800 °C to 1000 °C, and the inter-pass time from 1 s to 500 s. The simulation results showed that SRX occurred after 5–10 s at the first compression deformation. The softening fraction of SRX was found to increase with increasing the deformation temperature and the pre-strain. However, the softening fraction scarcely changed during the process of strain-induced precipitation. In addition, the kinetics of SRX was described by the Avrami equation, and the Avrami exponent appeared to be closely associated with the deformation temperature. The microstructure evolution was investigated at the initiation and completion of recrystallization. The amount and distribution of the precipitates were analyzed. The relationship between the driving force of SRX and the pinning force of precipitation was discussed. Besides, the recrystallization inhibition was detected at the early stage of precipitation, and the pinning forces were found to be of a magnitude comparable to the driving force. Moreover, the pinning forces were found to increase with the degree of precipitation and reach a peak at the intermediate stage of precipitation, and finally reduce as the particles coarsened.

Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

Based on Dutta and Sellars’s expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate, and grain size. Although the equation given by these authors reproduces the typical “C” shape of the precipitation start time (P s) curve well, the expression is not reliable for all cases. Recrystallization–precipitation–time–temperature diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V, and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (K s) and constant B, which is no longer a constant, but a function of K s when the latter is calculated at the nose temperature (T N) of the P s curve. The value of parameter B is deduced from the minimum point or nose of the P s curve, where ∂t 0.05/∂T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters are derived from the latest studies undertaken by the authors and this work represents an attempt to improve the model. The expressions are now more consistent and predict the precipitation–time–temperature curves with remarkable accuracy. The model for strain-induced precipitation kinetics is completed by means of Avrami’s equation.

Physical Modelling of the Interaction between Softening and Nb (C,N) Strain-Induced Precipitation in High-Mn Steels

The applicability of a physical model to describe the austenite microstructure evolution after hot deformation in High-Mn steels was investigated. Double-hit torsion tests were carried out to determine the softening behaviour of two High-Mn steels, one of them microalloyed with 0.11 wt% Nb. The values of the unknown parameters included in the model were determined by fitting experimental results. The model describes adequately the softening evolution of the steels. At high temperatures recovery and recrystallization contribute to mechanical softening, the latter having the main contribution. In contrast, when strain-induced precipitation occurs recovery has a larger effect.

Study of the Effect of Thermomechanical Processing on Grain Refinement in HSLA Steels

The present study highlights the approach to multiscale analysis of the grain refinement during thermomechanical processing of microalloyed steels. HSLA steels for pipe-line use are introduced in which strain-induced precipitation and strain-induced transformations are controlled to produce ultra-fine grained microstructures. Multiscale modeling combining Digital Material Representation and Crystal Plasticity enables to gain a better fundamental understanding of mechanical response and microstructure evolution of precipitation strengthened austenite and ferrite of three microalloyed steel grades. The opportunities for the development of new thermomechanical processing schedules, focused on the grain refinement, are enhanced and the proposed models identifications procedure on the basis of the torsion tests at various forming conditions are presented and discussed. In the pipe-line grade steels, the combination of microalloying elements, accelerated cooling and transformation temperature, has led to much higher grain refinements and increased amount of bainitic ferrite microstructures than in the standard thermomechanical processing. Sensitivity analysis of studied microalloyed steel grades in the light of the discussed problems using microstructural analysis of obtained microstructures was also performed. Finally, the main challenges related to the multiscale modeling of proposed ideas are addressed.

Control of Microstructure by MaxStrain Device

Grain refinement should increase strength of metallic materials in a predictable manner. However, in applications of severe plastic deformations for this purpose, limits have been observed due to self-recovery and strain-induced precipitation assisted by generation of adiabatic heat. Pure metals and single-phase alloys have not been the best candidates for achieving ultrafine-grained microstructures therefore more often precipitation-hardening multi-phase alloys have been used in SPD experiments. To generate ultrafine-grained microstructures by accumulated multiple compressive strains executed at various strain rates during programmed thermal cycles the MaxStrainTM device was developed for GleebleTM physical simulator. This paper deals with processing of Al-6061 wrought alloy and Al-319 cast alloy by the MaxStrain device, and describes obtained microstructures.

Novel Approach to Model Static Recrystallization of Austenite during Hot-Rolling of Nb-Microalloyed Steel: Effect of Precipitates

Microalloying additions are critical for grain size control during thermo-mechanical processing. The addition of niobium is known to delay the onset and growth of recrystallization. A physically-based model for the interaction of strain-induced precipitation, recovery and recrystallization is presented. A key feature of the model is the incorporation of the effect of precipitation on the nucleation of recrystallization. Quantitative agreement between the experimental measurements and the model predictions has also been demonstrated. The model offers valuable insight into the relative contributions of solute and precipitate Nb as well as the optimum conditions for strain accumulation.

Influence of (Al, Nb, V) Precipitates on the Recrystallization Inhibition in Microalloyed Steels

Under certain conditions of temperature, time and deformation, static recrystallization of austenite in microalloyed steels can be temporarily inhibited by means of the strain-induced precipitation of nanoparticles that cause a pinning effect on austenite grain boundaries in motion. This inhibition can be seen by the formation of a “plateau” in the curves of static recrystallization of austenite obtained from double-deformation tests carried out under isothermal conditions. In this work, several microalloyed steels with different compositions are studied by hot torsion tests in order to characterize the kinetics of recrystallization and its inhibition. The precipitation state in austenite is studied in several samples by means of transmission electron microscopy. The influence of the type of microalloying element (Al, Nb, V) and the mean size of the precipitates on the duration time of the plateau is studied and relationships between these variables can be obtained. Particularly, it is seen that Al-alloyed steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Influence of Deformation Temperature on γ-α Phase Transformation in Nb–Ti Microalloyed Steel during Continuous Cooling

The dynamic continuous-cooling-transformation (CCT) diagrams of Nb–Ti microalloyed steel for four different deformation temperatures of 850, 900, 950 and 975°C were plotted by means of a combined method of dilatometry and metallography. The influence of deformation temperature on continuous cooling transformation behavior and transformation microstructure was elucidated. The reason that the polygonal ferrite transformation field is shifted to the right and moved down as the deformation temperature is increased and the progress of transformation for lowest continuous cooling rate of 0.5°C/s was obviously retarded at the lowest deformation temperature of 850°C was discussed in details based on transformation kinetics and strain-induced precipitation kinetics, respectively. Moreover, the polygonal ferrite grain size for lowest continuous cooling rate of 0.5°C/s can be significantly refined from approx. 15.4 μm to 9.4 μm as the deformation temperature is reduced from 975°C to 850°C. In addition, the empirical equation to calculate γ-α phase transformation start temperature was established, and the calculated Ar3 temperatures are in good agreement with measured ones.

Microstructures and Precipitates in High Purified Fe- 17 wt% Cr Alloy Steels

In the present paper, the influences of rolling temperatures on microstructure and precipitate in high purified Fe- 17 wt% Cr alloy steels were investigated by conventional process and low temperature rolling process. The hot rolled bands were annealed, cold rolled and annealed in the same processes. By means of optical microscopy observations, transmission electron microscopy analyses and thermodynamic calculations, the microstructural evolutions were studied and compared. It was shown that low temperature rolling process resulted in the refined rolling and annealing microstructure; Low temperature rolling process could form more fine TiC precipitates than in the conventional process due to the occurrence of strain-induced precipitation. These results have been confirmed by the thermodynamic calculations.