Dynamic Recrystallization Kinetics Research Articles

Construction of a Predictive Model for Dynamic and Static Recrystallization Kinetics of Cast TC21 Titanium Alloy

In this study, hot compression experiments were conducted on cast TC21 titanium alloy using a Gleeble-1500D thermal simulation compression tester, and the hot-compressed specimens were heat-treated. The data obtained after analyzing the thermal compression of cast TC21 titanium alloy were analyzed to construct a thermal machining diagram with a strain of 0.8 and to optimize the machining window. This study investigated the microstructure of the alloy after hot pressing experiments and heat treatment, applying the study of the microstructure evolution law of cast TC21 titanium alloy. The analysis of the tissue evolution law established the dynamic and static recrystallization volume fraction as a function of heat deformation parameters. The results show that the optimal processing window for cast TC21 titanium alloy is a deformation temperature in the range of 1373 K–1423 K and a strain rate of 0.1 s−1. The increase in deformation volume and deformation temperature both favor recrystallization and make the recrystallization volume fraction increase, but the increase in strain rate will inhibit the increase in the recrystallization degree to some extent. The dynamic and static recrystallization equations for the cast TC21 titanium alloy at different temperatures were constructed. The experimental measurements of recrystallization volume fraction are in good agreement with the predicted values.

(FeCoNi)75Cu25-xSix high entropy alloys prepared by mechanical alloying and spark plasma sintering: Microstructure, deformation behavior and dynamic recrystallization kinetics

The present research aimed to produce (FeCoNi)75Cu25-xSix (x = 0, 5, 10, 15, 20, 25 at.%) high-entropy alloys (HEAs) using mechanical alloying (MA) and spark plasma sintering (SPS). The HEAs containing 0 to 25 at.% of Si are designated as Si0, Si5, Si10, Si15, Si20, and Si25 alloys. The microstructure, phase evolution, and deformation behavior of the HEAs were investigated before and after SPS. Compression tests were conducted on SPSed alloys at room temperature and 800 °C. XRD examination revealed a single-phase FCC structure in Si0 alloy, with Si5 and Si10 alloys consisting of distinct FCC phases. Si15 alloy contained BCC phase and intermetallic compound Ni74Si26. Si20 alloy had BCC and two intermetallics Ni74Si26 and FeSi, while Si25 alloy consisted of intermetallic Ni31Si12 and BCC phases. For Si0, Si5, Si10, and Si15 alloys, cold pressing resulted in ductile failure; for Si20 and Si25 alloys, it caused brittle failure. The Si20 alloy had the highest strength at 1520 MPa, while the Si25 alloy had the lowest strength at 810 MPa. Dynamic recrystallization (DRX) occurs at 800 °C in all alloys with a peak in flow stress followed by a gradual decrease to a steady state stress. Stress-strain curves at 800 °C were analyzed using DRX kinetics, and the Avrami exponent n was between 1 and 2 for all alloys.

Characterization of hot workability of a Cu containing high strength low alloy steel using processing map

Hot workability of an experimental Cu containing martensitic high strength low alloy (HSLA) steel was characterized by conducting isothermal hot compression tests at different temperatures (900–1100 °C) and strain rates (0.0003–1 s−1). Flow curves exhibits predominantly single peak behaviour at temperatures for a strain rate > 0.01 s−1. At temperature > 950 °C and for low strain rates of 0.0003 and/or 0.001 s−1, multiple peaks characterised by oscillations in flow stress was observed. The strain rate sensitivity (m), temperature sensitivity (S) of the material was calculated and the processing map was developed to identify optimum condition for thermo-mechanical processing. The material exhibits three suitable processing windows in temperature – strain rate space viz., (1) 1050–1100 °C; 0.1–0.01 s−1 (2) 980–1020 °C; 0.1–1 s−1 and (3) 980–1020 °C; 0.001–0.0003 s−1. The microstructure of the material deformed under all the above mentioned processing conditions exhibited defect free equiaxed dynamically recrystallized grains with an average grain size of 30–40 μm. Any or combination of these processing conditions can be used by the industry for designing suitable forging process (roughing, finishing etc.) based on the infrastructure available. Further, using Avrami approach, the effect of strain, strain rate and temperature on the kinetics of dynamic recrystallization (DRX) was also evaluated. The results obtained are presented and discussed.

Microstructure, deformation behavior, and dynamic recrystallization kinetics of FeNiMnCu-based high entropy alloys prepared by mechanical alloying and spark plasma sintering

In the present research, FeNiMnCuX (X: Co, Cr, Mo, Ti, W) high entropy alloys (HEAs) were successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). SPS transformed the base and Co-containing alloys into FCC structures, Cr- containing alloys changed into a mix of FCC and BCC structures, while other alloys formed a combination of FCC/BCC structure and an intermetallic compound. The base alloy and alloys with Co, and Cr additions exhibited plastic deformation in cold compression tests, whereas other alloys exhibited brittle behavior. During hot compression tests, dynamic recrystallization occurred with a peak in flow stress followed by a gradual decrease to a steady state stress in alloys with Co, Mo, Ti and W additions. The highest peak stresses were observed at 800 °C for Ti- and W- containing alloys. The dynamic recrystallization and fractional softening kinetics are explained by the Avrami kinetic model. An Avrami exponent around 1 was obtained based on the kinetics of dynamic recrystallization for the alloys doped with Co, Mo, Ti, and W.

Shift of dynamic recrystallization kinetics in thermal compression induced by prior cold deformation treatment at Ni80A superalloy

To achieve the uniform and fine-grained microstructures during thermal deformation of nickel-based superalloys, a novel two-stage deformation approach was employed, involving prior cold deformation treatment (PCDT) and subsequent thermal deformation. Experimental investigations concerning this two-stage deformation method were carried out on Ni80A superalloy, encompassing various combinations of cold and thermal compression parameters. The analysis of microstructures revealed that PCDT significantly enhances dynamic recrystallization (DRX) process, resulting in fine-grained microstructures during thermal compression and expanding the processing-parameter domains for grain refinement. To clarify the shift of DRX kinetics in thermal compression induced by PCDT, the DRX kinetics models considering this pre-treatment were derived. These models demonstrated a linear inverse correlation between the activation energy within DRX kinetics models and prior stored energy from PCDT. This suggested that PCDT promotes the DRX process by reducing the activation energy during thermal compression. Following, the kinetics curves depicting DRX volume fraction during thermal deformation were drawn. Results indicated that under an identical thermal compression condition, the kinetics curve shifts towards decreasing strain with increasing of prior cold deformation. Under various thermal compression conditions, these kinetics curves for Ni80A superalloy processed with PCDT get shifted to the direction of decreasing strain when compared to the alloy without PCDT. These shifts underscore a pronounced acceleration of DRX process induced by PCDT. Finally, a finite element model incorporating DRX kinetics models was developed to further illustrate the influence of PCDT on the shift of DRX in an upsetting process, which shows a promising potential for theoretical calculations.

An improved cellular automaton model of dynamic recrystallization and the constitutive model coupled with dynamic recrystallization kinetics for microalloyed high strength steels

This study analyzed the deformation behavior of Nb–Ti microalloyed high strength steel at strain rates of 0.001–0.1 s−1 and deformation temperatures of 1050–1150 °C using hot compression tests. The deformation activation energy of 321.1 kJ/mol was calculated based on the stress-strain curve, and the characteristic points on the flow stress curve were extracted. In contrast, the relationship equation between the characteristic points and the Zener-Hollomon parameter was established. A modified dynamic recrystallization (DRX) kinetics model was proposed based on the stress-strain data, considering the strain rate and deformation temperature. A high-precision constitutive model was established based on the dislocation density evolution phenomenological model, which couples with the modified DRX kinetics model. Furthermore, an improved DRX cellular automaton (CA) model was constructed, which comprehensively took into account a series of processes such as dislocation density evolution, recrystallization nucleation, and grain growth. Notably, the model also embedded the pinning effect of second phase (SP) particles on the DRX process. The improved CA model can control the pinning force by adjusting the size and volume fraction of SP particles and can accurately predict the evolution characteristics of DRX and the average austenite grain size of Nb–Ti microalloyed high strength steels during thermal deformation. The development of an improved CA model can be extended to apply to the DRX evolution of other microalloyed steels, and the model provides a powerful tool and method for understanding the thermal deformation behavior and microstructural evolution of microalloyed steels.

High temperature performance of additively manufactured Al 2024 alloy: Constitutive modelling, dynamic recrystallization evolution and kinetics

The current study investigates the high-temperature deformation behaviour, and microstructural evolution of the laser powder bed fused, Ti modified Al 2024 alloy. The high-temperature performance was evaluated using a hot compression test performed in the temperature range of 200–350 °C and strain rate range of 0.1–10 s−1. The flow behaviour at elevated temperatures at different strain rates can provide us insight into the high temperature application and can also be utilized for optimization of deformation-based post processing technique. The deformation-based post processing method utilizes work hardening to improve the mechanical properties and to reduce the inherent defect of additively manufactured parts, such as pores and lack of fusion. The optimal deformation conditions for these processes can be obtained from the processing map. The flow stress during different deformation conditions (strain, strain rate and temperature) was predicted using different phenomenological models such as Johnson-Cook (JC) model, strain compensated Arrhenius equation, and artificial neural network (ANN). The JC model was observed to be the least suited method in the current investigation, whereas the ANN method was observed to be best suited for predicting flow stress with an average absolute relative error of 0.5% and a correlation coefficient of 0.9998. Different deformation mechanisms such as dynamic recovery (DRV) and dynamic recrystallization (DRX) were investigated for different deformation conditions using empirical models, finite element analysis (FEA) and microstructural characterization using TEM, and EBSD. The primary DRX mechanism in the current study was observed to be continuous dynamic recrystallization or CDRX mechanism.

On the modified kinetics model describing the dynamic recrystallization behavior during hot deformation of Incoloy 925

The isothermally hot compression tests of Incoloy 925 were conducted over a temperature range of 900–1150 °C and strain rate of 0.01–10 s−1 to investigate the dynamic recrystallization (DRX) kinetics behavior and its effect on the microstructure evolution. The values of DRX fraction under different deformation conditions were evaluated through electron backscatter diffraction analysis combined with artificial neural network approach. The DRX behavior was also investigated based on the traditional Avrami-DRX kinetics model and the strain-rate dependent Avrami-DRX kinetics model modified by genetic algorithm & BiHill function, respectively. The results reflected that the modified model shows a better robustness to describe the DRX behavior under the deformation condition with a relatively low DRX fraction or at the domain with higher strain rate. Besides, the crystallographic orientation parallel to the compressive direction tends to form the <101> fiber highlighted by the deformed grains, while the predominant orientations of DRX grains show a tendency towards <001> fiber. Moreover, the transition rule of the main DRX mechanism was further studied by discussing the linear relationship between the Z parameter and DRX grain size.

Influence of vanadium, niobium and boron on kinetics of austenite recrystallization of steels with different strength levels under hot deformation conditions

Summarized results of research of the dynamic and static austenite recrystallization kinetics of low-carbon low-alloy and alloyed steels of strength classes 420, 620, 690, 750 and 890 and medium-carbon steels of strength class 1700 containing different amounts of vanadium, niobium and boron are presented. Studies were carried out by the plasmetric method under deformation conditions close to hot rolling. It was found that vanadium has a weak effect on recrystallization, and niobium in all the studied steels significantly slows it down in the hot rolling temperature range, regardless of the total doping level; microalloying of steels with boron leads to acceleration of austenite recrystallization.

Modeling of Dynamic Recrystallization Kinetics in Ce Containing Mg Alloys

Modeling of Dynamic Recrystallization Kinetics in Ce Containing Mg Alloys

Dynamic Recrystallization Kinetics of As-Cast Fe-Cr-Al-La Stainless Steel during Hot Deformation

To investigate the dynamic recrystallization (DRX) behavior of as-cast Fe-20Cr-5.5Al-0.64La stainless steel, a series of compression tests were carried out on a Gleeble-3500 thermal simulator in the temperature range of 1000~1150 °C and the strain rate range of 0.001~1 s−1. The true stress-true strain curves were obtained and their characteristics were analyzed. Using regression analysis, the apparent activation energy for the Fe-20Cr-5.5Al-0.64La stainless steel was estimated to be 300.19 kJ/mol, and the constitutive equation was developed successfully with a hyperbolic sine equation as: &lt;!-- MathType@Translator@5@5@MathML2 (no namespace).tdl@MathML 2.0 (no namespace)@ --&gt;&lt;math&gt;&lt;mrow&gt;&lt;mover accent="true"&gt;&lt;mrow&gt;&lt;mi&gt;ε&lt;/mi&gt;&lt;/mrow&gt;&lt;mo&gt;˙&lt;/mo&gt;&lt;/mover&gt;&lt;mo&gt;=&lt;/mo&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mi&gt;e&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mn&gt;21.91&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;msup&gt;&lt;mrow&gt;&lt;mfenced open="&amp;#x005B;" close="&amp;#x005D;" separators="&amp;#x007C;"&gt;&lt;mrow&gt;&lt;mrow&gt;&lt;mrow&gt;&lt;mi mathvariant="normal"&gt;sinh&lt;/mi&gt;&lt;/mrow&gt;&lt;mo&gt;⁡&lt;/mo&gt;&lt;mrow&gt;&lt;mfenced separators="&amp;#x007C;"&gt;&lt;mrow&gt;&lt;mn&gt;0.035&lt;/mn&gt;&lt;mi&gt;σ&lt;/mi&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mn&gt;3.18&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;mi mathvariant="normal"&gt;e&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;x&lt;/mi&gt;&lt;mi mathvariant="normal"&gt;p&lt;/mi&gt;&lt;mo&gt;⁡&lt;/mo&gt;&lt;mfenced separators="&amp;#x007C;"&gt;&lt;mrow&gt;&lt;mfrac&gt;&lt;mrow&gt;&lt;mo&gt;&amp;#x2212;&lt;/mo&gt;&lt;mn&gt;300190&lt;/mn&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;R&lt;/mi&gt;&lt;mi&gt;T&lt;/mi&gt;&lt;/mrow&gt;&lt;/mfrac&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;!-- MathType@End@5@5@ --&gt;. The critical strain, the peak strain and the strain for the maximum softening rate were identified based on the work hardening rate curves and expressed as a function of the Zener−Hollomon parameter. The kinetic model of DRX was established using the stress−strain data. According to the analysis of the kinetics model and microstructure evolution, the evolution of DRX volume could be described as follows: the volume fraction of DRX grains increased with an increase in strain; at a fixed deformation temperature, the DRX volume fraction was larger at a lower strain rate for the same strain; and the size of DRX grains increased with an increase in temperature or a decrease in strain rate.

Supersolvus Recrystallization and Grain Growth Kinetics for the Fine Tuning of Grain Size in VDM Alloy 780 Forgings

VDM Alloy 780 is a new polycrystalline nickel-based superalloy developed for aeronautical applications. In most of the targeted applications, grain size after forging must be precisely controlled to meet the targeted mechanical properties and in-service life requirements. Grain size in forgings is the direct consequence of the recrystallization and grain growth kinetics which are addressed in this paper at high temperatures, above the solvus temperature of γ′ and η/δ phases. The dynamic and post-dynamic recrystallization kinetics as well as the grain growth kinetics of VDM Alloy 780 are detailed over a range of thermomechanical conditions. Dynamic recrystallization appears to be limited, with only 30 pct recrystallized at quite high strain of 1.7 applied at 1050 °C and 0.01 s−1 for instance, but this is compensated by fast post-dynamic evolution. Within the investigated thermomechanical range, recrystallization is completed with 5 minutes of post-deformation hold in VDM Alloy 780 independent of the prior strain, strain rate and dynamic recrystallization fraction. For a strain as low as 0.08, an isothermal annealing of 30 minutes at 1050 °C generates a homogenous and fully recrystallized microstructure. Capillarity driven grain growth following recrystallization is also relatively slow, for instance an exposure at 1050 °C (50 °C above the solvus temperature) for 2 hours results in an increase in average grain size from 20 to 70 μm. This opens the possibility to fine tune the grain sizes by subsequent heat treatments within a time scale that is compatible with industrial conditions. The high cobalt content (25 pct) is suspected to play a role in the control of microstructure evolution kinetics. It is noteworthy that VDM Alloy 780 is shown here to not undergo the heterogeneous grain growth phenomenon reported in low strain regions for other nickel-based superalloys, which is also an asset for applications requiring strict control of grain sizes and grain size distributions.

Characterizing local distribution of microstructural features and its correlation with microhardness in resistance spot welded ultra-low-carbon steel: Experimental and finite element characterization

This study investigated the influence of microstructural features on the heterogeneous distribution of hardness in resistance spot welded (RSWed) 340BH steel. The microstructure and texture across the weld region was investigated using optical microscopic and electron backscattered diffraction (EBSD) techniques. Finite element analysis (FEA) was used to simulate the RSW process, which helped in predicting the nugget dimensions as well as the cooling rate in different weld regions. Rapid cooling in the fusion zone (FZ) led to the formation of martensite with typical packet and block substructures. Numerical reconstruction of the prior austenite grain (PAG) was employed to understand the orientation relationship between martensite and austenite. The results showed that child laths obey a Kurdjumov-Sachs orientation relationship (K-S OR) with respect to the austenite matrix with a slight deviation (~3° angular deviation) from a conventional K-S OR. Moreover, microhardness was measured across the weld zone. An increase in the hardness was observed throughout the weld zones. The hardness increase in the heat-affected zone (HAZ) was due to a finer (smaller) grain size and an increase in the dislocation density, whereas in the FZ, the improvement in hardness was attributed to the phase change during the solidification process. The microhardness of martensite formed during RSW was strongly dependent on the block size according to the Hall-Petch relationship. • A heterogeneous distribution of microstructure and hardness in RSWed 340BH steel was characterized using EBSD and FEA. • Lower ZHP values and fast kinetics of dynamic recrystallization resulted in fine structure of ferrite grains in ICHAZ. • FZ hardness was mainly driven by martensite formation, while grain size distribution and dislocation density were mainly responsible for higher hardness in HAZ. • The block size in the martensite substructure followed Hall-Petch type relationship with microhardness of lath martensite.

Establishment of Constitutive Model and Analysis of Dynamic Recrystallization Kinetics of Mg-Bi-Ca Alloy during Hot Deformation.

The flow behavior of the solution-treated Mg-3.2Bi-0.8Ca (BX31, wt.%) alloy was systematically investigated during hot compression at different deformation conditions. In the present study, the strain-related Arrhenius constitutive model and dynamic recrystallization (DRX) kinetic model were established, and the results showed that both two models had high predictability for the flow curves and the DRX behavior during hot compression. In addition, the hot processing maps were also made to confirm a suitable hot working range. Under the assistance of a hot processing map, the extrusion parameters were selected as 573 K and 0.5 mm/s. After extrusion, the as-extruded alloy exhibited a smooth surface, a fine DRX structure with weak off-basal texture and good strength-ductility synergy. The newly developed strong and ductile BX31 alloy will be helpful for enriching low-cost, high-performance wrought Mg alloy series for extensive applications in industries.

The Flow Stress-Strain and Dynamic Recrystallization Kinetics Behavior of High-Grade Pipeline Steels.

The hot deformation behavior of high-grade pipeline steels was studied in the strain rate range of 0.001~0.1 s−1 and the temperature range of 1050~1200 °C by using hot compression tests on a Gleeble 3500 thermomechanical simulator. The flow stress increases with the increase in strain rate and the decrease in deformation temperature, and the deformation activation energy is about 358 kJ/mol. The flows stress–strain behavior of the work-hardening and dynamic recovery (DRV) was calculated using the Estrin–Mecking equation, and the kinetics model of the dynamic recrystallization (DRX) was established based on the Avrami equation through characteristic strains. Furthermore, the flow stress–strain behavior of high-grade pipeline steels was predicted by the established model based on the coupling effects of DRV and DRX. The corresponding predicted results are in good agreement with the experimental results according to standard statistical parameters analysis. Finally, the economic strain (ε3) is proposed by the third derivative of the given kinetic model. Based on these calculation results, when the economic strain (ε3) is reached, uniform and refined DRX grains can be obtained, the energy consumption reduced, and the production costs controlled, which is of great significance to actual factory production.

Assessment of the post-dynamic recrystallization effects on the overall dynamic recrystallization kinetics in a Ni-base superalloy

Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.

The effect of carbon on the restoration phenomena during hot deformation of carbon steels

Abstract A study has been made to determine the influence of the carbon content on the kinetics of dynamic and static recrystallization during and after hot deformation of carbon steels. For this purpose, single- and double-hit hot compression experiments at various strain rates and temperatures together with the Avrami-type kinetics equation and Bergstrom approach have been utilized to investigate recrystallization behavior. The results show that the apparent activation energy of hot deformation decreases with increasing carbon content and this phenomenon results in a faster dynamic recrystallization at high temperatures and/or low strain rates. Also, increasing carbon content leads to a higher rate of static recrystallization at low strain rates while at high strain rates it causes a slower kinetics of static recrystallization.

Formation of austenite in ferrite-beinite, beinite-martensite and martensitic shipbuilding steels and its influence on the transformed structure

This article investigates the kinetics of austenite grain growth during heating and the features of phase transformations depending on the austenite grain size in ferrite-bainitic, bainite-martensitic and martensitic shipbuilding steels. The kinetics of dynamic and static recrystallization is studied depending on the holding time at a given temperature. The studies carried out made it possible to determine the effect of the austenite grain size in shipbuilding steels of manganese, manganese-nickel and nickel alloying composition on the transformed structure.

Effects of surface roughness on interfacial dynamic recrystallization and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo alloy joints produced by hot-compression bonding

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo hot-compression bonding joints was systematically investigated. It is found that for the bonding interface of rough surfaces, elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area. As the surface roughness increases, the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area. Meanwhile, high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation. A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces. In contrast to that of rough surfaces, bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions. While with the increase of deformation strain, small grain nuclei form along the bonding interface, which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries. Moreover, the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces. This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces, leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

Interactions Between Dynamic Softening and Strengthening Mechanisms During Hot Forging of a High-Strength Steel

Open-die forging is a critical step in the manufacture of large numbers of components used in the transportation and energy industries. Dynamic recrystallization, dynamic transformation, and dynamic precipitation take place during the hot deformation process and significantly affect microstructure conditioning, which ultimately influences the service properties of the component. In the present work, using a Gleeble 3800 thermomechanical simulator, the open-die forging of a large-size ingot made of a modified AISI 6140 medium carbon high-strength steel is investigated. Deformation temperatures ranging from 950°C to 1,250°C and strain rates ranging from 0.01 to 1 s−1, representative of the actual process, are considered in the analysis. The generated true stress–true strain curves are used as a basis for the development of a constitutive model predicting the occurrence of softening and strengthening phenomena as a function of thermomechanical conditions. The corresponding activation energy is determined to be about 374 kJ mol−1 and is compared against the values reported in the literature for other high-strength steels. Dynamic recrystallization kinetics is studied using the t50 model, and the influence of temperature and strain rate is quantified and discussed. The interaction between dynamic precipitation and dynamic recrystallization is discussed, and the deformation conditions under which such interactions occur are determined. The thermomechanical results are validated by microstructure examination, including laser confocal microscopy, field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. The present study focuses on reproducing the deformation cycle applied during the open-die forging process of a vanadium-containing high-strength steel used in the industry with special attention to the interaction between dynamic recrystallization and precipitation processes.