Hot deformation characteristics of INCONEL alloy MA 754 and development of a processing map

The characteristics of hot deformation of INCONEL alloy MA 754 have been studied using processing maps obtained on the basis of flow stress data generated in compression in the temperature range 700 °C to 1150 °C and strain rate range 0.001 to 100 s^-1. The map exhibited three domains. (1) A domain of dynamic recovery occurs in the temperature range 800 °C to 1075 °C and strain rate range 0.02 to 2 s^-1, with a peak efficiency of 18 pct occurring at 950 °C and 0.1 s^-1. Transmission electron microscope (TEM) micrographs revealed stable subgrain structure in this domain with the subgrain size increasing exponentially with an increase in temperature. (2) A domain exhibiting grain boundary cracking occurs at temperatures lower than 800 °C and strain rates lower than 0.01 s^-1. (3) A domain exhibiting intense grain boundary cavitation occurs at temperatures higher than 1075 °C. The material did not exhibit a dynamic recrystallization (DRX) domain, unlike other superalloys. At strain rates higher than about 1 s^-1 the material exhibits flow instabilities manifesting as kinking of the elongated grains and adiabatic shear bands. The material may be safely worked in the domain of dynamic recovery but can only be statically recrystallized.     

Hot deformation mechanisms in a powder metallurgy nickel-base superalloy IN 625

The hot working behavior of the nickel-base superalloy IN 625 produced by hot extrusion of a powder metallurgy (P/M) compact has been studied by compression testing in the temperature range 900 °C to 1200 °C and true strain rate range 0.001 to 100 s⁻¹. At strain rates less than about 0.1 s⁻¹, the stress-strain curves exhibited near steady-state behavior, while at higher strain rates, the flow stress reached a peak before flow softening occurred. The processing maps developed on the basis of the temperature and strain rate and strain dependence of the flow stress exhibited three domains. (1) The first domain occurs at lower strain rates (<0.01 s⁻¹) and temperatures higher than about 1050 °C. The peak efficiency and the temperature at which it occurs have increased with strain. The microstructure of the specimen deformed in this domain exhibited extensive wedge cracking. (2) The second domain occurs in the intermediate range of strain rates (0.01 to 0.1 s⁻¹) and temperatures lower than 1050 °C, and in this domain, microstructural observations indicated dynamic recrystallization (DRX) of γ containing δ precipitates and carbide particles resulting in a fine-grained structure. (3) The third domain occurs at higher strain rates (> 10 s⁻¹) and tempe ratures above 1050 °C, with a peak efficiency of about 42 pct occurring at 1150 °C and 100 s⁻¹. Microstructural observations in this domain revealed features such as irregular grain boundaries and grain interiors nearly free from annealing twins, which are typical of DRX of homogeneous γ phase. The instability map revealed that flow instability occurs at strain rates above 1 s⁻¹ and temperatures below 1050 °C, and this is manifested as intense adiabatic shear bands. These results suggest that bulk metal working of this material may be carried out in the high strain rate domain where DRX of homogeneous γ occurs. On the other hand, for achieving a fine-grained product, finishing operations may be done in the intermediate strain rate domain. The wedge cracking domain and the regime of instability must be totally avoided for achieving defectfree products.     

Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy

The hot deformation behavior of hot isostatically pressed (HIP) NIMONIC AP-1 superalloy is characterized using processing maps in the temperature range 950 °C to 1200 °C and strain rate range 0.001 to 100 s^•1. The dynamic materials model has been used for developing the pro-cessing maps which show the variation of the efficiency of power dissipation given by [2m/ (m + 1)] with temperature and strain rate, withm being the strain rate sensitivity of flow stress. The processing map revealed a domain of dynamic recrystallization with a peak efficiency of 40 pct at 1125 °C and 0.3 s^•1, and these are the optimum parameters for hot working. The microstructure developed under these conditions is free from prior particle boundary (PPB) de-fects, cracks, or localized shear bands. At 100 s^•1 and 1200 °C, the material exhibits inter-crystalline cracking, while at 0.001 s^•1, the material shows wedge cracks at 1200 °C and PPB cracking at 1000 °C. Also at strain rates higher than 10 s^•1, adiabatic shear bands occur; the limiting conditions for this flow instability are accurately predicted by a continuum criterion based on the principles of irreversible thermodynamics of large plastic flow.     

Dynamic Recrystallization in Sintered Cobalt during High-Temperature Deformation

The constitutive flow behavior of sintered cobalt in the temperature range 873 to 1473 K (600 to 1200 °C) and at strain rates from 0.001 to 10 s⁻¹ has been studied using constant true strain rate hot compression tests. On the basis of these data, a processing map has been generated that depicts the variation of strain rate sensitivity with temperature and strain rate. The processing map reveals a domain of dynamic recrystallization (DRX) with an optimum condition of processing at 1273 K (1000 °C) and at 10⁻³ s⁻¹. When deformed within the domain, the stress-strain curves exhibit a single peak followed by flow softening, leading to steady-state behavior. In addition to this, a recently developed approach based on flow curve analysis is used to study the DRX kinetics, which is found to follow an Avrami-type relation.     

Characterization of hot deformation behavior of brasses using processing maps: Part II. β Brass and α-β brass

The hot deformation behaviors of β brass in the temperature range of 550°C to 800°C and α-β brass in the temperature range of 450°C to 800°C have been characterized in the strain rate range of 0.001 to 100 s⁻¹ using processing maps developed on the basis of the Dynamic Materials Model. The map for β brass revealed a domain of superplasticity in the entire temperature range and at strain rates lower than 1 s⁻¹, with a maximum efficiency of power dissipation of about 68 pct. The temperature variation of the efficiency of power dissipation in the domain is similar to that of the diffusion coefficient for zinc in β brass, confirming that the diffusion-accommodated flow controls the superplasticity. The material undergoes microstructural instability in the form of adiabatic shear bands and strain markings at temperatures lower than 700°C and at strain rates higher than 10 s⁻¹. The map for α-β brass revealed a wide domain for processing in the temperature range of 550°C to 800°C and at strain rates lower than 1 s⁻¹, with a maximum efficiency of 54 pct occurring at about 750°C and 0.001 s⁻¹. In the domain, the α phase undergoes dynamic recrystallization and controls the hot deformation of the alloy, while the β phase deforms superplastically. At strain rates greater than 1 s⁻¹, α-β brass exhibits microstructural instabilities manifested as flow rotations at lower temperatures and localized shear bands at higher temperatures.     

Hot Deformation Behavior and Microstructure Evolution of 2Cr12Ni4Mo3VNbN Steel Used for Last-Stage Large Blades of Steam Turbine

The hot deformation behavior and microstructure evolution of a recently developed 2Cr12Ni4Mo3VNbN martensitic stainless steel are examined through hot compression tests conducted within the temperature range of 900–1200 °C and the strain rate range of 0.01–10 s⁻¹. The constitutive equation and processing maps corresponding to hot deformation are established. The activation energy for hot deformation of 2Cr12Ni4Mo3VNbN steel is determined to be ≈457491.77 J mol⁻¹. Simultaneously, the microstructure evolution during hot deformation is studied. Based on the processing maps and microstructure evolution analysis, it is concluded that the optimal windows for hot processing are within the temperature range of 1106–1150 °C and the strain rate range of 0.01–2.7 s⁻¹, as well as at 1200 °C within the strain rate range of 1–2.7 s⁻¹, exhibiting a power dissipation efficiency of 0.32. As the temperature increases and the strain rate decreases, the degree of dynamic recrystallization escalates.     

Hot workability analysis and processing parameters optimisation for 20CrMnTiH steel by combining processing map with microstructure

The processing map of 20CrMnTiH steel is developed by using the dynamic material model according to the hot compression experiments, performed on a Gleeble-3500 thermal simulator at the temperature range of 850–1150°C and the strain rate of 0.01–1?s^?1. Hot workability characteristics of 20CrMnTiH steel are analysed based on the developed processing map. The safe deformation regions with higher power dissipation efficiency η exhibit the dynamic recrystallisation (DRX) mechanism and show fine and homogeneous microstructure. The unstable regions with negative instability coefficient ξ occur at both lower temperature with all strain rates and at high temperature with high strain rate at the strain of 0.2. The area of instability gradually decreases with the increasing strain and only appears at lower temperature and higher strain rate when the strain is above 0.2. The unstable regions indicate the flow localisation by microstructure analysis. Combining with the developed processing map with DRX behaviour, the optimal values of hot processing parameters for 20CrMnTiH steel are obtained to achieve good hot workability and small grains sizes at the process parameters ranged at 1036–1070°C/0.1–1?s^?1 and 918–985°C/0.01–0.014?s^?1.     

High-temperature deformation behaviour and microstructural evolution of low carbon steel based on processing map

The high-temperature deformation behaviours of low carbon steel QD08 were investigated by hot compression tests over temperatures from 1000 to 1200°C and strain rates from 0.1 to 10 s^?1. The processing map was obtained by superimposition of the power dissipation and the instability maps and the regions having the lowest strain rate sensitivity added for more clarification of low and high workability regions. The results show that the security domain mainly of hot deformation with a higher powder dissipation factor and maintain a smooth variation, by the metallographic observations, the grain refinement by DRX under the stable deformation conditions. On the basis of processing map and microstructure evolution, the optimal deformation processing parameters are the hot deformation temperature range from 1070 to 1100°C, and strain rates range from 5 to 10 s^?1.     

Characterization of hot deformation behavior of brasses using processing maps: Part I. α Brass

The constitutive flow behavior of α brass in the temperature range of 500°C to 850°C and strain rate range of 0.001 to 100 s⁻¹ has been characterized with the help of a power dissipation map generated on the basis of the principles of the Dynamic Materials Model. The map revealed a domain of dynamic recrystallization in the temperature range of 750°C to 850°C and in the strain rate range of 0.001 to 1 s⁻¹, with a maximum efficiency of power dissipation of about 54 pct. The optimum hot working conditions are 850°C and 0.001 s⁻¹, and these match with those generally employed in industrial practice. In the temperature range of 550°C to 750°C and strain rates lower than 0.01 s⁻¹, the efficiency of power dissipation decreases with decreasing strain rate, with its minimum at 650°C. In this regime, solute drag effects similar to dynamic strain aging occur to impair the hot workability. The material undergoes microstructural instabilities at temperatures of 500°C to 650°C and at strain rates of 10 to 100 s⁻¹, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands.     

The Study of Hot Deformation Behavior of Mechanically Milled and Hot Extruded Al–BN Nanocomposite

Hot deformation behavior of mechanically milled and hot extruded Al–BN nanocomposite is investigated by hot compression test in the temperature range of 350–500 °C and strain rate of 0.001–1 s^?1. The plastic flow of the nanocomposite as a function of temperature and strain rate is described using a constitutive equation. Based on dynamic materials model, the processing map is developed at the strain of 0.7 representing stable and instable domains. The stable and instable domains in the processing map are verified by microstructural evaluation using transmission and scanning electron microscopes. The results show that the flow instability domains including micro voids and surface cracks have occurred in the range of: (1) T = 350–380 °C, \(\dot{\varepsilon }\) = 0.001–0.015 s^?1, (2) T = 370–430 °C, \(\dot{\varepsilon }\) = 0.1–1 s^?1, and (3) T = 460–500 °C, \(\dot{\varepsilon }\) = 0.001–0.03 s^?1. The safe and stable domains for hot deformation of nanocomposite have occurred in the range of: (1) T = 350–370 °C, \(\dot{\varepsilon }\) = 0.1–1 s^?1, (2) T = 390–450 °C, \(\dot{\varepsilon }\) = 0.003–0.05 s^?1, and (3) T = 440–500 °C, \(\dot{\varepsilon }\) = 0.1–1 s^?1. Finally, the investigation shows that the best processing parameters for this new nanocomposite are within the temperature range of 390–450 °C and strain rate range of 0.003–0.05 s^?1.     

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Newly Developed Austenitic Heat-Resistant Alloy

The hot deformation characteristics, microstructure evolution, and dynamic recrystallization (DRX) mechanism of the newly developed austenitic heat-resistant steel Fe–18Cr–10Ni–0.3Nb–2.5Cu were systematically investigated by thermal compression tests combined with microstructure characterizations. The activation energy (Q) map, Zener–Hollomon parameter (Z) map, and processing map were plotted according to the stress–strain curves to reveal the inherent connection between the three maps and the hot deformation characteristics of this alloy. The high η region in the processing map does not precisely correspond to the region where DRX developed. Nevertheless, the flow instability map accurately predicts the microstructure. The variation pattern of Z corresponded more closely to the hot deformation microstructure evolution than did the variation pattern of Q. The degree of DRX increases with decreasing Z. The optimal process parameters are 1000 °C/0.01 s⁻¹/0.8 and 1100 °C/10 s⁻¹/0.8 (temperature/strain rate/strain), and they result in complete DRX and a narrow range of Z values. The DRX mechanism at high strain rate is characterized by the combined enhancement of discontinuous DRX (DDRX), continuous DRX (CDRX), and twin-DRX (TDRX). The dominance of the particle-stimulated nucleation (PSN) mechanism at intermediate strain rate results in the formation of incompletely recrystallized microstructures with approximate orientation. Sufficient time at low strain rate promotes the development of DDRX and CDRX.

     

Processing map for hot working of powder

The constitutive flow behavior of a metal matrix composite (MMC) with 2124 aluminum containing 20 vol pct silicon carbide particulates under hot-working conditions in the temperature range of 300 °C to 550 °C and strain-rate range of 0.001 to 1 s^-1 has been studied using hot compression testing. Processing maps depicting the variation of the efficiency of power dissipation given by [2m/(m + 1)] (wherem is the strain-rate sensitivity of flow stress) with temperature and strain rate have been established for the MMC as well as for the matrix material. The maps have been interpreted on the basis of the Dynamic Materials Model (DMM). [3] The MMC exhibited a domain of superplasticity in the temperature range of 450 °C to 550 °C and at strain rates less than 0.1 s^-1. At 500 °C and 1 s^-1 strain rate, the MMC undergoes dynamic recrystallization (DRX), resulting in a reconstitution of microstructure. In comparison with the map for the matrix material, the DRX domain occurred at a strain rate higher by three orders of magnitude. At temperatures lower than 400 °C, the MMC exhibited dynamic recovery, while at 550 °C and 1 s^-1, cracking occurred at the prior particle boundaries (representing surfaces of the initial powder particles). The optimum temperature and strain-rate combination for billet conditioning of the MMC is 500 °C and 1 s^-1, while secondary metalworking may be done in the super- plasticity domain. The MMC undergoes microstructural instability at temperatures lower than 400 °C and strain rates higher than 0.1 s^-1.     

Hot Deformation Behavior of a Novel Cr–Mo–V Hot Work Die Steel

Herein, high-temperature hot compression tests are carried out to investigate the hot deformation behavior of a novel Cr–Mo–V hot work die steel. Meanwhile, a strain compensation Arrhenius constitutive equation is created, and the fitting correlation coefficient R between the experimental and predicted values is 0.98921, with a mean average absolute relative error (AARE) of 3.82%. According to the results, the hot deformation constitutive equation of the novel Cr–Mo–V steel can predict the flow stress of hot deformation with high accuracy. Furthermore, the equations for critical stress (strain), peak stress (strain), and Zener–Holomon parameters are constructed, and they demonstrate a satisfactory linear fit. Additionally, a hot processing map of the novel Cr–Mo–V steel is developed at strains of 0.4, 0.5, 0.6, and 0.7. By analyzing the hot processing map and viewing the microstructure, this study concludes that the optimal hot processing parameters of the novel Cr–Mo–V steel are 1050–1150 °C and 0.8–1 s⁻¹ at a strain of 0.7, with a peak power dissipation efficiency of nearly 0.68.     

Microstructural control in hot working of IN-718 superalloy using processing map

The hot-working characteristics of IN-718 are studied in the temperature range 900 °C to 1200 °C and strain rate range 0.001 to 100 s⁻¹ using hot compression tests. Processing maps for hot working are developed on the basis of the strain-rate sensitivity variations with temperature and strain rate and interpreted using a dynamic materials model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring at 950 °C and 0.001 s⁻¹ with an efficiency of power dissipation of 37 pct and the other at 1200 °C and 0.1 s⁻¹ with an efficiency of 40 pct. Dynamic recrystallization in the former domain is nucleated by the δ(Ni₃Nb) precipitates and results in fine-grained microstructure. In the high-temperature DRX domain, carbides dissolve in the matrix and make interstitial carbon atoms available for increasing the rate of dislocation generation for DRX nucleation. It is recommended that IN-718 may be hot-forged initially at 1200 °C and 0.1 s⁻¹ and finish-forged at 950 °C and 0.001 s⁻¹ so that fine-grained structure may be achieved. The available forging practice validates these results from processing maps. At temperatures lower than 1000 °C and strain rates higher than 1 s⁻¹ the material exhibits adiabatic shear bands. Also, at temperatures higher than 1150°C and strain rates more than 1s⁻¹, IN-718 exhibits intercrystalline cracking. Both these regimes may be avoided in hotworking IN-718.     

Flow Stress and Critical Dynamic Recrystallization Behavior of Cu–Fe16Mn0.6C High Manganese TWIP Steel

The true stress–strain curve of Cu–Fe16Mn0.6C twinning induced plasticity (TWIP) steel was studied with a compression test on Thermecmastor‐Z thermal simulator at a temperature range of 850–1150°C and strain rate range of 0.03–30 s^?1. The influence of deformation temperature and strain rate on high‐temperature flow stress and critical recrystallization behavior of the TWIP steel was investigated. It is concluded that the peak flow stress of Cu–Fe16Mn0.6C under high‐temperature deformation decreases as the temperature increases but increases with the strain rate. Meanwhile at strain rate of 0.03 and 30 s^?1 obvious peak stresses are observed which demonstrates the dynamic recrystallization. The constitutive equation of Cu–Fe16Mn0.6C under high temperature deformation is calculated by linear regression method. The activation energy is 505 kJ mol^?1. The relationship between critical strain of dynamic revrystallization and Zener–Hollomon parameter is determined by the curve between strain‐hardening rate and flow stress.     

Optimization of hot workability in stainless

The hot working behavior of 304L stainless steel is characterized using processing maps developed on the basis of the Dynamic Materials Model and hot compression data in the tem- perature range of 700 °C to 1200 °C and strain-rate range of 0.001 to 100 s♪-1. The material exhibits a dynamic recrystallization (DRX) domain in the temperature range of 1000 °C to 1200 °C and strain-rate range of 0.01 to 5 s_-1. Optimum hot workability occurs at 1150 °C and 0.1 s_-1, which corresponds to a peak efficiency of 33 pct in the DRX domain. Finer grain sizes are obtained at the lower end of the DRX domain (1000 °C and 0.1 s^-1). The material exhibits a dynamic recovery domain in the temperature range of 750 °C to 950 °C and at 0.001 s"1. Flow instabilities occur in the entire region above the dynamic recovery and recrystallization domains. Flow localization occurs in the regions of instability at temperatures lower than 1000 °C, and ferrite formation is responsible for the instability at higher temperatures.     

Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures （850- 1200 C） and strain rates （0. 001- 10 s^- 1 ） on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By re gression analysis, the activation energy （Q） in the whole range of deformation temperature is determined to be 367. 562 kJ/mol. The effects of the temperature and the strain rate on mierostructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstrueture.     

Hot Deformation Behavior of a New 9%Cr Heat Resistant Steel G115

The hot deformation behavior of a new heat resistant steel G115 designed for 650 °C ultra-supercritical (USC) power plants was experimentally studied. Hot compression test was carried out in the temperature range of 900 — 1200 °C and the strain rate range of 0.1 — 20 s^?1 by using Gleeble-3800 thermal-mechanical simulator, and the corresponding flow curves were obtained. Experimental results show that the flow stress increases with the decrease of deformation temperature and the increase of strain rate. The hot deformation activation energy of G115 steel was determined to be 494 kJ/mol and the constitutive equation was also obtained. For convenience of the practical application, a good approximate equation was obtained for calculating the peak stress values of G115 steel under different deformation conditions. At the strain value of 0. 9, natural logarithm of the critical Zener-Hollomon parameter Z_e of G115 steel was determined to be in the scope of 49. 67 and 50. 65, above which there will be no dynamic recrystallization (DRX). And natural logarithm of the critical Zener-Hollomon parameter Z_s of G115 steel was determined to be in the scopes of 45. 58 and 46. 27, below which full DRX may occur. Then, the status diagram of dynamic microstructures of G115 steel was established. In addition, the strain rate sensitivity of G115 steel is not constant during the test temperature range and it increases linearly from 900 to 1200 °C. Therefore, hot deformation at higher temperatures would obtain better workability.     

Hot Ductility and Compression Deformation Behavior of TRIP980 at Elevated Temperatures

The hot ductility tests of a kind of 980 MPa class Fe-0.31C (wt pct) TRIP steel (TRIP980) with the addition of Ti/V/Nb were conducted on a Gleeble-3500 thermomechanical simulator in the temperatures ranging from 873 K to 1573 K (600 °C to 1300 °C) at a constant strain rate of 0.001 s^?1. It is found that the hot ductility trough ranges from 873 K to 1123 K (600 °C to 850 °C). The recommended straightening temperatures are from 1173 K to 1523 K (900 °C to 1250 °C). The isothermal hot compression deformation behavior was also studied by means of Gleeble-3500 in the temperatures ranging from 1173 K to 1373 K (900 °C to 1100 °C) at strain rates ranging from 0.01 s^?1 to 10 s^?1. The results show that the peak stress decreases with the increasing temperature and the decreasing strain rate. The deformation activation energy of the test steel is 436.7 kJ/mol. The hot deformation equation of the steel has been established, and the processing maps have been developed on the basis of experimental data and the principle of dynamic materials model (DMM). By analyzing the processing maps of strains of 0.5, 0.7, and 0.9, it is found that dynamic recrystallization occurs in the peak power dissipation efficiency domain, which is the optimal area of hot working. Finally, the factors influencing hot ductility and thermal activation energy of the test steel were investigated by means of microscopic analysis. It indicates that the additional microalloying elements play important roles both in the loss of hot ductility and in the enormous increase of deformation activation energy for the TRIP980 steel.     

Developing the Processing Maps Using the Hyperbolic Sine Constitutive Equation

Hot compression tests were performed on a duplex stainless steel at temperatures ranging from 1223 K to 1473 K (950 °C to 1200 °C) and strain rates from 0.001 to 100 s^?1. The constitutive analysis of flow stress was carried out using the hyperbolic sine function, and the material constants were determined at two typical strains of 0.3 and 0.7. The power dissipation map, instability map, and processing map for the material were developed for strains of 0.3 and 0.7. The developed processing maps were based on the hyperbolic sine as well as the conventional power-law constitutive equations. The efficiency of power dissipation (η) varied from 12 to 60 pct over the studied temperature and strain rate. The highest value of η was obtained at strain rates below 0.01 s^?1, whereas the lowest value of η was observed at the intermediate strain rates. The instability region in sin h-based processing map was only observed in the range of 1423 K to 1473 K (1150 °C to 1200 °C) and at a strain rate of 100 s^?1, while the conventional processing map did not predict any instability region. Optical microscopy observations were more consistent with the results of the sin h-based processing map and indicated that the instability regime at high temperatures and high strain rates was due to the development of adiabatic shear bands.