Application of neural networks to predict the elevated temperature flow behavior of a low alloy steel

In order to study the workability and establish the optimum hot forming processing parameters for 42CrMo steel, the compressive deformation behavior of 42CrMo steel was investigated at the temperatures from 850 °C to 1150 °C and strain rates from 0.01 s⁻¹ to 50 s⁻¹ on Gleeble-1500 thermo-simulation machine. Based on these experimental results, an artificial neural network (ANN) model is developed to predict the constitutive flow behaviors of 42CrMo steel during hot deformation. The inputs of the neural network are deformation temperature, log strain rate and strain whereas flow stress is the output. A three layer feed forward network with 12 neurons in a single hidden layer and back propagation (BP) learning algorithm has been employed. The effect of deformation temperature, strain rate and strain on the flow behavior of 42CrMo steel has been investigated by comparing the experimental and predicted results using the developed ANN model. A very good correlation between experimental and predicted result has been obtained, and the predicted results are consistent with what is expected from fundamental theory of hot compression deformation, which indicates that the excellent capability of the developed ANN model to predict the flow stress level, the strain hardening and flow softening stages is well evidenced.     

Hot deformation behavior and microstructure evolution of twin-roll-cast Mg–2.9Al–0.9Zn alloy: A study with processing map

The hot deformation behavior and microstructure evolution of twin-roll-cast of Mg–2.9Al–0.9Zn–0.4Mn (AZ31) alloy has been studied using the processing map. The tensile tests were conducted in the temperature range of 150–400 °C and the strain rate range of 0.0004–4 s⁻¹ to establish the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (i) the continuous dynamic recrystallization (CDRX) domain in the range of 200–280 °C/≤0.004 s⁻¹ with fine grains which provides a potential for warm deformation such as deep drawing; (ii) the discontinuous dynamic recrystallization (DDRX) domain around 400 °C at high strain rate (0.4 s⁻¹ and above) with excellent elongation which can be utilized for forging, extrusion and rolling; (iii) the grain boundary sliding (GBS) domain at slow strain rate (below 0.004 s⁻¹) above 350 °C appears abundant of cavities, which result in fracture and reduce the ductility of the adopted material; and (iv) the flow instability region which locates at the upper left of the processing map shows the metallographic feature of flow localization.     

High temperature deformation behavior of a near alpha Ti600 titanium alloy

The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s⁻¹. The apparent activation energy of deformation was calculated to be 620.0 kJ mol⁻¹, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s⁻¹ and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s⁻¹ and the entire temperature range investigated.     

Microstructural evolution in 42CrMo steel during compression at elevated temperatures

In order to study the workability and to optimize the processing parameters for 42CrMo steel hot forming, the hot compressive tests of 42CrMo steel were performed in the temperature range of (850-1150) °C at strain rates of (0.01-50) s^− 1 and deformation degrees of (10-60)% on Gleeble-1500 thermo-simulation machine. The effects of processing parameters, including the strain rate, forming temperature and deformation degree, on the microstructures of 42CrMo steel were investigated by metallurgical analysis. The results show that the average grain size of the deformed 42CrMo steel increases with the forming temperature and decreases with the deformation degree and strain rate. In a word, the grain size of hot compressive 42CrMo steel is dependent on the forming temperature, strain and strain rate, also on Zener-Hollomon parameter.     

Study on hot deformation characteristics of 12%Cr ultra-super-critical rotor steel using processing maps and Zener–Hollomon parameter

The hot deformation behavior of a 12%Cr ultra-super-critical rotor steel was investigated in the temperature range of 900–1200 °C and strain rate range of 0.01–10 s^? 1. The processing maps of the material were derived by using dynamic materials model and Prasad instability criterion. The deformation mechanisms of different regions in the processing maps and corresponding microstructures were discussed in combination of the change in Zener–Hollomon parameter. The results showed that for 12%Cr ultra-super-critical rotor steel, the flow stress, microstructural evolution and efficiency of power dissipation are closely related to the Zener–Hollomon parameter. The lower the Zener–Hollomon parameter is, the larger the extent of flow softening becomes, the more easily the dynamic recrystallization may occur, and the bigger the dynamic recrystallization grain size is. Moreover, the higher the Zener–Hollomon parameter, the lower the power dissipation rate. Flow instability occurring under high Zener–Hollomon parameter is manifested as flow localization. During the hot tension at 1 s^? 1, the thermoplasticity of 12%Cr ultra-super-critical rotor steel is the worst at 950 °C.     

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300–550 °C and 0.001–1 s^–1, respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg₂Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300–350 °C and at strain rate 1 s^–1.     

Hot working behavior of 2205 austenite-ferrite duplex stainless steel characterized by constitutive equations and processing maps

The hot deformation characteristics of the 2205 duplex stainless steel were analyzed using constitutive equations and processing maps. The hot compression tests were performed at temperature range of 950-1200 °C and strain rate of 0.001-1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. However, the stress exponent and strain rate sensitivity were different at low and high deformation temperatures where austenite and ferrite are dominant, respectively. It was recognized that strain at the peak point of flow curve increases with the Zener-Hollomon parameter, Z, at low temperature deformation while at high temperature deformation it actually decreases with Z. The power dissipation map, instability map and processing map were developed for the typical strain of 0.3. It was realized that dynamic restoration mechanisms could efficiently hinder the occurrence of flow instability at low and medium strain rates. Otherwise, the increase in strain rate at low and high temperatures could increase the risk of flow instability.     

Constitutive equations to predict high temperature flow stress in a Ti-modified austenitic stainless steel

The experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1523 K) and strain rates (10⁻³–10² s⁻¹), were employed to develop constitutive equations in a Ti-modified austenitic stainless steel. The effects of temperature and strain rate on deformation behaviors were represented by Zener-Holloman parameter in an exponent type equation. The influence of strain was incorporated in the constitutive analysis by considering the effect of strain on material constants. The constitutive equation (considering the compensation of strain) could precisely predict the flow stress only at 0.1 and 1 s⁻¹ strain rates. A modified constitutive equation (incorporating both the strain and strain rate compensation), on the other hand, could predict the flow stress throughout the entire temperatures and strain rates range except at 1123 K in 10 and 100 s⁻¹. The breakdown of the constitutive equation at these processing conditions is possibly due to adiabatic temperature rise during high strain rate deformation.     

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s⁻¹ to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10⁻⁴ or 1 × 10⁻³ s⁻¹. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s⁻¹ and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from 900 °C (near the Ae₃ temperature) to 1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.     

The workability evaluation of wrought AZ80 magnesium alloy in hot compression

Workability, an important parameter in metal forming process, can be evaluated by means of processing maps on the basis of dynamic materials model (DMM), constructed from experimentally generated flow stress variation with respect to strain, strain rate and temperature. To obtain the processing maps of wrought AZ80 magnesium alloy, hot compression tests were performed over a range of temperatures 523–673 K and strain rates 0.01–10 s⁻¹. As the true strain is 0.25, 0.45, 0.65, 0.85 respectively, the response of strain-rate sensitivity (m-value), power dissipation efficiency (η-value) and instability parameter (ξ-value) to temperature and strain rate were evaluated. By the superimposition of the power dissipation and the instability maps, the stable, metastable and unstable regions were clarified clearly. In further, in the stable area the regions having the highest efficiency of power dissipation were identified and recommended. The optimal working parameters identified by the processing maps contribute to designing the hot forming process of AZ80 magnesium alloy without any defect.     

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s⁻¹. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s⁻¹ in the hot processing of 8% Cr steel.     

Constitutive analysis to predict high-temperature flow stress in modified 9Cr–1Mo (P91) steel

Constitutive analysis for hot working of modified 9Cr–1Mo (P91) ferritic steel was carried out employing experimental stress–strain data from isothermal hot compression tests, in a wide range of temperatures (1123–1373 K), strains (0.1–0.5) and strain rates (10⁻³–10² s⁻¹). The effects of temperature and strain rate on deformation behaviour were represented by Zener–Hollomon parameter in an exponent-type equation. The influence of strain was incorporated in the constitutive equation by considering the effect of strain on different material constants. Activation energy was found to vary with strain in the range 369–391 kJ mol⁻¹. The developed constitutive equation (considering the compensation of strain) could predict flow stress of modified 9Cr–1Mo steel over the specified hot working domain with very good correlation and generalization.     

Constitutive modeling for elevated temperature flow behavior of 42CrMo steel

In order to study the workability and establish the optimum hot formation processing parameters for 42CrMo steel, the compressive deformation behavior of 42CrMo steel was investigated at the temperatures from 850 to 1150 °C and strain rates from 0.01 to 50 s⁻¹ on Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit a peak stress at a small strain, after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The flow stress obtained from experiments consists of four different stage, i.e., Stage I (Work hardening stage), Stage II (Stable stage), Stage III (Softening stage) and Stage IV (Steady stage). The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in an exponent-type equation. A revised model describing the relationships of the flow stress, strain rate and temperature of 42CrMo steel at elevated temperatures is proposed by compensation of strain and strain rate. The stress–strain values of 42CrMo steel predicted by the proposed model well agree with experimental results, which confirmed that the revised deformation constitutive equation gives an accurate and precise estimate for the flow stress of 42CrMo steel.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

Development of processing maps for a Ni-based superalloy

The hot deformation characteristics of a Ni-based superalloy were studied in the temperature range 1050–1180 °C and strain rate range 0.01–10 s^− 1 using hot compression tests. Processing maps for hot working were developed on the basis of the variations of efficiency of power dissipation with temperature and strain rate, interpreted using a dynamic materials model. A hot deformation equation is given to characterize the dependence of peak stress on the temperature and strain rate. A hot deformation apparent activation energy of the Ni-based superalloy is about 496 kJ/mol. The processing maps of the Ni-based superalloy obtained in a strain range of 0.1–0.7 are essentially similar, which indicates that strain does not have a significant influence. The maps exhibit a clear domain with its peak efficiency at about 1140 °C and 0.01 s^− 1; the domain has its peak efficiency of about 36–41% for different strains. On the basis of hot deformation microstructural observations, the full recrystallization region can be identified in the processing map at a strain of 0.7.     

Hot deformation behavior of Ti-15-3 titanium alloy: a study using processing maps,activation energy map,and Zener–Hollomon parameter map

The hot deformation behavior of Ti-15-3 titanium alloy was investigated by hot compression tests conducted in the temperature range 850–1150 °C and strain rate range 0.001–10 s⁻¹. Using the flow stress data corrected for deformation heating, the activation energy map, processing maps and Zener–Hollomon parameter map were developed to determine the optimum hot-working parameters and to investigate the effects of strain rate and temperature on microstructural evolution of this material. The results show that the safe region for hot deformation occurs in the strain rate range 0.001–0.1 s⁻¹ over the entire temperature range investigated. In this region, the activation energy is ~240 ± 5 kJ/mol and the ln Z values vary in range of 13.9–21 s⁻¹. Stable flow is associated with dynamic recovery and dynamic recrystallization. Also, flow instabilities are observed in the form of localized slip bands and flow localization at strain rates higher than 0.1 s⁻¹ over a wide temperature range. The corresponding ln Z values are larger than 21 s⁻¹. The hot deformation characteristic of Ti-15-3 alloy predicted from the processing maps, activation energy map, and Zener–Hollomon parameter map agrees well with the results of microstructural observations.     

Investigation on hot deformation of 20Cr–25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

Abstract

The deformation behaviour of a 20Cr–25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000–1200°C and strain rates of 0·01–10 s^?1. It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516·7 kJ mol^?1. Based on the dynamic materials model and the Murty’s instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025–1120°C and 0·01–0·03 s^?1 or 1140–1200°C and 0·08–1 s^?1, where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

Compressive deformation behavior of Al 2024 alloy using 2D and 4D processing maps

The hot deformation behavior of Al 2024 was studied by isothermal hot compression tests in the temperature range of 250–500 °C and strain rate range of 10⁻³ to 10² s⁻¹ in a computer-controlled 50 kN servo-hydraulic universal testing machine (UTM). The results show that the flow stress of Al 2024 alloy increases with strain rate and decreases after a peak value, indicating dynamic recovery and recrystallization. The processing map exhibits two domains of optimum efficiency for hot deformation at different strains, including the low strain rate domain at 500 °C and between 10⁻² and 10⁻¹ s⁻¹ and the high strain rate domain in 250 and 300 °C in the strain rate range of 10¹ to 10² s⁻¹. An attempt has been made in this article to generate a new hybrid 4D process map which illustrates contours of power dissipation and instability in the 3D space of strain rate, temperature, and strain.     

Study of the processing map and hot deformation behavior of a Cu-bearing 317LN austenitic stainless steel

The hot-working behavior of a Cu-bearing 317LN austenitic stainless steel (317LN–Cu) was investigated in the 950–1150 °C temperature and 0.01–10 s^− 1strain rate range, respectively. The effects of different deformation parameters and optimum hot-working window were respectively characterized through analyzing flow stress curves, constitutive equations, processing maps and microstructures. The critical strain for dynamic recrystallization (DRX) was determined by the inflection point on θ-σ and −∂θ/∂σ-σ curves. The peak stress was found to increase with decrease in temperature and increase in strain rate. Typical signs of DRX over a wide range of temperatures and strain rates were observed on the flow stress curves. The power dissipation maps in the strain range of 0.1–0.4 were basically similar, indicating the insignificant effect of strain on the power dissipation maps of 317LN–Cu. However, the instability maps showed strong strain sensitivity with increasing strain, which was attributed to the flow localization. The optimum hot-working window for 317LN–Cu was obtained in the temperature range 1100–1120 °C and strain rate range 0.01–0.018 s^− 1, with a peak efficiency of 38%. Microstructural analysis revealed fine and homogenized recrystallized grains in this domain.