Hot Deformation Characteristics of 34CrMo4 Steel

The 34CrMo4(AISI 4130)steel is extensively utilized in the compressed natural gas cylinders.Due to the importance of thermomechanical processing in the production of these cylinders,the dynamic recrystallization(DRX)characteristics of 34CrMo4 steel were investigated.The effect of hot deformation parameters such as temperature and strain rate on the dynamic restoration processes of a 34CrMo4 alloy was studied.Hot compression tests were performed in the temperature range of 900 to 1100 ℃ and the strain rate r...     

Hot Deformation Behavior of As-cast AISI M2 High-speed Steel Containing Mischmetal

The hot deformation behavior of as-cast AISI M2high-speed steel containing mischmetal(RE)has been investigated on a Gleeble-3500simulator in the temperature range of 1 000-1 150℃and strain rate range of 0.01-10 s-1 at true strain of 1.0.The mechanical behavior has been characterized using stress-strain curve analysis,kinetic analysis,processing maps,etc.Metallographic investigation was performed to evaluate the mechanism of flow instability.The results show that the deformation activation energy decreases with increasing deformation temperature; the efficiency of power dissipation increases with decreasing strain rate and increasing temperature;flow instability is observed at low-to-medium temperature and higher strain rate region when the strain is smaller,but extends to lower strain rate and high temperature regions with the increment of strain,in which it is manifested as flow localization near the grain boundary.Hot deformation equations and processing maps are obtained.The optimal processing window is suggested and the deformation mechanism is dynamic recrystallization(DRX).     

Serration Behavior in Zr-Cu-Al Glass-forming Systems

The metallic glass matrix composites(MGMCs)and bulk metallic glasses(BMGs)were studied by statistical analysis during plastic deformation at the strain rates of 2×10~(-2),2×10~(-3),and 2×10~(-4) s~(-1),respectively.No serration events occur in both MGMCs and BMGs during compression tests at the strain rate of 2×10~(-2) s~(-1).When deformed at the strain rate of 2×10~(-3) s~(-1),the BMG displays a larger plasticity,which is due to the larger serration events followed by a series of small serrations caused by the continuous movement of free volume.The amplitudes and elastic-energy densities increase with increasing the strain rates owing to many serrations in MGMCs.It is deduced that the Young′s modulus decreases from the normalized stress drop and fluctuations are observed on stressstrain curves,which is attributed to a lower coefficient according to the stick-slip model.     

Continuous bending and straightening technology of Q345c slab based on high-temperature creep deformation

A new continuous bending and straightening casting curve with the aim of full using of high-temperature creep deformation was proposed.The curvature of bending and straightening segment varies as sine law with arc length.The basic arc segment is shortened significantly so that the length of bending and straightening area can be extended and the time of creep behavior can be increased.The distance from solidifying front in the slab was calculated at 1 200°C by finite element method.The maximum strain rate of new casting curve at different locations inside the slab is 6.39×10~(-5) s~(-1) during the bending segment and it tends to be 3.70×10-5 s~(-1) in the straightening segment.The minimum creep strain rate is 7.45×10~(-5) s~(-1)when the stress is 14 MPa at 1 200°C.The strain rate of new casting machine can be less than the minimum creep strain rate.Thus,there is only creep deformation and no plastic deformation in the bending and straightening process of steel continuous casting.Deformation of slabs depending on creep behavior only comes true.It is helpful for the design of the new casting machine and improvement of old casting machine depending on high temperature creep property.     

Hot Deformation Behavior of Nickel-Based Superalloy GH4720Li

 The hot deformation characteristics of GH4720Li alloy were studied at the temperature of 1100-1170 ℃ and strain rate of 001-1 s-1 using Gleeble hot compression tests. True stress-true strain curves and deformation microstructures were investigated. Constitutive equation was established using the hyperbolic law. Processing map for hot working was also developed on the basis of the variations of efficiency of power dissipation with temperature and strain rate. The results show that dynamic recrystallization is the dominant softening mechanism during hot deformation. Fully recrystallized grain is obtained at strain of 07 above 1130 ℃, and coarsening occurs above 1150 ℃. The mean deformation activation energy is determined to be 512 kJ/mol. According to the low activation energy value, high dissipation efficiency parameter and fine recrystallized microstructure, 1130 ℃ is chosen as the hot working temperature for GH4720Li alloy.     

Hot Deformation Behavior of Vanadium-microalloyed Medium-carbon Steel for Fracture Splitting Connecting Rod

Single compression tests were carried out with a Gleeble-3800 thermal simulator to investigate hot deformation behavior of two vanadium-microalloyed medium-carbon steels for fracture splitting connecting rod.The tests were performed to a total true strain of 0.92 at true strain rates ranging from 10-2 to 10s-1 and deformation temperature of 900-1 150 ℃.The results show that hot deformation behavior of the tested steels is similar to that of conventional medium-carbon microalloyed steels and dynamic recrystallization is easier to occur at higher deformation temperature and lower strain rate.The austenite deformation resistance and activation energy of deformation increase with increasing vanadium content from 0.15%to 0.28% and thus the starting time of dynamic recrystallization was delayed.Finer recrystallized austenite grain could be obtained at higher strain rate,lower deformation temperature and higher vanadium content.TEM observation of the specimens quenched just before and after deformation reveals that vanadium is mainly in dissolved solute condition in austenite and thus affects the dynamic recrystallization behavior of the tested steels mainly through solute-drag effect.     

Hot Deformation Behavior and Processing Map of Spray Formed M3∶ 2 High Speed Steel

Hot deformation behavior of a new type of M3∶ 2 high speed steel with niobium addition made by spray forming was investigated based on compression tests in the temperature range of 950-1 150 ℃ and strain rate of 0. 001-10 s~(-1). A comprehensive constitutive equation was obtained,which could be used to predict the flow stress at different strains. Processing map was developed on the basis of the flow stress data using the principles of dynamic material model. The results showed that the flow curves were in fair agreement with the dynamic recrystallization model. The flow stresses,which were calculated by the comprehensive constitutive equation,agreed well with the test data at low strain rates( ≤1 s~(-1)). The material constant( α),stress exponent( n) and the hot deformation activation energy( Q_(HW)) of the new steel were 0. 006 15 MPa~(-1),4. 81 and 546 kJ·mol~(-1),respectively. Analysis of the processing map with an observation of microstructures revealed that hot working processes of the steel could be carried out safely in the domain( T = 1 050-1 150 ℃,ε = 0. 01- 0. 1 s~(-1))with about 33% peak efficiency of power dissipation( η). Cracks was expected in two domains at either lower temperatures( 1 000 ℃) or low strain rates( 0. 001 s~(-1)) with different cracking mechanisms. Flow localization occurred when the strain rates exceeded 1 s~(-1) at all testing temperatures.     

Constitutive Equations for Flow Behavior of Powder-forged Fe-0.5C-2Cu Steel under Hot Compression

To investigate the hot deformation behavior of powder-forged(P/F)Fe-0.5C-2Cu steel,the hot compression tests were conducted at temperatures ranging from 900to 1 000 ℃ and strain rates from 0.1to 10s-1 using Gleeble-1500thermal simulator.The true stress-true strain curves at different temperatures and strain rates of P/F steel were obtained.It is found that dynamic recovery only occurs as strain rate is 10s-1 at 900℃,and the dynamic recrystallization is the main softening mechanism.The flow stress increases with decreasing temperature and increasing strain rate.The experimental data are employed to develop constitutive equations on the basis of the Arrheniustype equation by introducing the strain with nonlinear fitting.The flow stresses predicted by the proposed constitutive equations are in good agreement with the experimental values,and the correlation coefficient(R2)and the average absolute relative error(AARE)are 0.995 25and 3.07%respectively.These results indicate the proposed constitutive equations can effectively describe the hot deformation behavior of the material.     

Prediction of Critical Conditions for Dynamic Recrystallization in 316LN Austenitic Steel

Hot compression experiments conducted on a Gleeble-3500thermo-mechanical simulator and metallographic observation tests were employed to study the critical conditions of dynamic recrystallization(DRX)of 316 LN austenitic stainless steel.The true stress-true strain curves of 316 LN were obtained at deformation temperatures ranging from 900℃to 1 200℃and strain rates ranging from 0.001s-1 to 10s-1.Based on the above tests,the critical conditions of DRX were determined and compared with those obtained from work-hardening theory and the Cingara-McQueen flow stress model.Furthermore,the microstructure was observed to validate the calculated results.The ratio of critical strain to peak strain(εc/εp)for 316 LN was determined,and the quantitative relationship between the critical strain and the deformation parameters of 316 LN was elucidated.The results demonstrated that the onset of DRX corresponds to the constant normalized strain hardening rate(Γ),namely,the critical strain hardening rateΓcfor316LN is equal to 0.65.     

Hot Deformation Behavior and Flow Stress Prediction of Ultra Purified 17％ Cr Ferritic Stainless Steel Stabilized with Nb and Ti

The hot deformation behavior of ultra purified 17% Cr ferritic stainless steel stabilized with Nb and Ti was investigated using axisymmetric hot compression tests on a thermomechanical simulator.The deformation was carried out at the temperatures ranging from 700 to 1 100℃ and strain rates from 1to 10s-1.The microstructure was investigated using electron backscattering diffraction.The effects of temperature and strain rate on deformation behavior were represented by Zener-Hollomon parameter in an exponent type equation.The effect of strain was incorporated in the constitutive equation by establishing polynomial relationship between the material constants and strain.A sixth order polynomial was suitable to represent the effect of strain.The modified constitutive equation considering the effect of strain was developed and could predict the flow stress throughout the deformation conditions except at800℃in 1s-1 and at 700℃in 5and 10s-1.Losing the reliability of the modified constitutive equation was possibly ascribed to the increase in average Taylor factor at 800℃in 1s-1 and the increase in temperature at 700℃in 5and10s-1 during hot deformation.The optimum window for improving product quality of the ferritic stainless steels was identified as hot rolling at a low finisher entry temperature of 700℃,which can be achieved in practical production.     

Constitutive Modeling for Flow Behaviors of Superaustenitic Stainless Steel S32654 during Hot Deformation

Hot deformation behavior of superaustenitic stainless steel S32654 was investigated with hot compression tests at temperatures of 950-1 250 ℃ and strain rates of 0.001-10s~(-1).Above 1 150 ℃,with strain rate lower than 0.1s~(-1),the flow curves exhibit nearly steady-state behavior,while at higher strain rate,continuous flow softening occurs.To provide a precise prediction of flow behavior for the alloy,the constitutive modeling considering effect of strain was derived on the basis of the obtained experimental data and constitutive relationship which incorporated Arrhenius term and hyperbolic-sine type equation.The material constantsα,n,Q and lnA are found to be functions of the strain and can be fitted employing eighth-order polynomial.The developed constitutive model can be employed to describe the deformation behavior of superaustenitic stainless steel S32654.     

Hot deformation behavior and processing map of Mg-2Zn-1Al-0.2RE alloy

In this study,uniaxial hot compression tests were carried out between 200 and 400℃ over strain rates of0.001-1 s~(-1) to investigate the hot deformation behavior of Mg-2 Zn-1 Al-0.2 RE alloy with coarse grains.The average activation energy was measured to be 174.51 kJ/mol.In addition,a constitutive relation based on the Arrhenius equation was established.Dynamic recrystallization(DRX) kinetics were studied by Avrami equation to characterize the evolution of DRX volume fraction.DRX was favored at high temperatures of 300-400℃ and low strain rates of 0.001-0.01 s~(-1).According to dynamic material model and Prasad's instability criterion,a maximum power dissipation of 38% and 32% occurs at 400℃/0.001 s~(-1) and 400℃/0.01 s~(-1),respectively.According to the proce ssing map,330-400℃/0.001-0.01 s~(-1)was determined as the optimum deformation parameter range.     

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture,chip formation is a severe plastic deformation process including large strain,high strain rate and high temperature.And the strain rate in high speed cutting process can be achieved to 105 s~(-1).30CrMnSiNi2Asteel is a kind of important high-strength low-alloy structural steel with wide application range.Obtaining the dynamic mechanical properties of30CrMnSiNi2Aunder the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining.The dynamic compressive mechanical properties of30CrMnSiNi2Asteel were studied using split Hopkinson pressure bar(SHPB)tests at 30-700°C and3000-10000s~(-1).The stress-strain curves of 30CrMnSiNi2Asteel at different temperatures and strain rates were investigated,and the strain hardening effect and temperature effect were discussed.Experimental results show that 30CrMnSiNi2Ahas obvious temperature sensitivity at 300°C.Moreover,the flow stress decreased significantly with the increase of temperature.The strain hardening effect of the material at high strain rate is not significant with the increase of strain.The strain rate hardening effect is obvious with increasing the temperature.According to the experimental results,the established Johnson-Cook(J-C)constitutive model of 30CrMnSiNi2Asteel could be used at high strain rate and high temperature.     

Hot deformation behavior of GH4945 superalloy using constitutive equation and processing map

The hot deformation behavior of GH4945 superalloy was investigated by isothermal compression test in the temperature range of 1 000-1 200°C with strain rates of 0.001-10.000s~(-1) to a total strain of 0.7.Dynamic recrystallization is the primary softening mechanism for GH4945 superalloy during hot deformation.The constitutive equation is established,and the calculated apparent activation energy is 458.446kJ/mol.The processing maps at true strains of 0.2,0.4and 0.6are generally similar,demonstrating that strain has little influence on processing map.The power dissipation efficiency and instability factors are remarkably influenced by deformation temperature and strain rate.The optimal hot working conditions are determined in temperature range of 1 082-1 131°C with strain rates of 0.004-0.018s~(-1).Another domain of 1 134-1 150°C and 0.018-0.213s~(-1) can also be selected as the optimal hot working conditions.The initial grains are replaced by dynamically recrystallized ones in optimal domains.The unsafe domains locate in the zone with strain rates above 0.274s~(-1),mainly characterized by uneven microstructure.Hot working is not recommended in the unsafe domains.     

Static Recrystallization Behavior of SA508-III Steel during Hot Deformation

The static recrystallization behavior of SA508-III steel was investigated by isothermal double-hit hot compression tests at the deformation temperature of 950-1 250 ℃,the strain rate of 0. 01-1 s~(-1),and the inter-pass time of 1-300 s.The effects of deformation parameters,including forming temperature,strain rate,degree of deformation( pre-strain) and initial austenite grain size,on the softening kinetics were analyzed. Experimental results show that static recrystallization kinetics is strongly dependent on deformation temperature and degree of deformation,while less affected by the strain rate and initial grain size. The kinetics and microstructural evolution equations of static recrystallization for SA508-III steel were developed to predict the softening behavior and the statically recrystallized grain size,respectively. Based on the comparison between the experimental and predicted results,it is found that the established equations can give a reasonable estimate of the static softening behavior for SA508-III steel.     

Effect of Thermomechanical Parameters on Σ3~n Grain Boundaries and Grain Boundary Networks of a New Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 800-1 100℃and strain rate range of 0.1-10s-1 using a Gleeble 3500thermomechanical simulator to investigate the influence of hot deformation parameters(temperatures,strain rates and strains)on the grain boundary network evolution of a new grade Fe-Cr-Ni superaustenitic stainless steel.The results showed that a dominant effect of deformed temperature isΣ3n(n=0,1,2,3) boundaries population increased with decreasing temperature,while they first increased and then reduced with increasing strain and strain rate.Interestingly,besidesΣ3n(n=1,2,3)twin grain boundaries,someΣ1boundaries could interrupt grain boundaries network effectively,which enhance material performances.But they are scarcely reported.The misorientation of some segments LAGBs in the deformed microstructure(pancaked grains)increased and slid to high angle grain boundaries with increasing the fraction of recrystallized grains during hot deformation.     

Hot workability of high manganese transformation induced plasticity steel

The hot deformation behavior and microstructure evolution of high manganese transformation induced plasticity steel(Fe - 20Mn - 3Si - 3Al) were investigated by using hot compression test in a temperature range from 800℃to 1 050℃and strain rate ranging from 0.01 s~(-1) to 5.0 s~(-1).The effects of temperature,strain rate,and true strain on the flow behavior and microstructures of high manganese transformation induced plasticity steel were discussed.The results show that the dynamic recrystallization occur...     

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

The dynamic recrystallization （DRX） and static recrystallization （SRX） behaviour of coarse-grained aus- tenite 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 1000-1 150 ℃ 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 fine- grained 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 ℃, which indicates that SRX of coarse-grained aus- tenite is mainly retarded by coarse grain size and Nb in solution during rough rolling.     

Effect of Initial Microstructure on Warm Deformation Behavior of 45 Steel

 In order to investigate the effect of initial microstructure on warm deformation behavior, some specimens of 45 steel were annealed and some quenched. Then the specimens were isothermally compressed on a Gleeble 3500 machine. The deformation temperature range was 550 to 700 ℃ and the strain rate range was 0.001 to 0.1 s-1. An optical microscope (OM) and a transmission electron microscope (TEM) were used to study the microstructures. The results show that the microstructure of annealed specimens is ferrite and pearlite and that of quenched specimens is martensite. The flow stress of quenched specimens is higher than that of annealed ones at 550 ℃ when strain rates are greater than 0.001 s-1. However, at 600 to 700 ℃ and strain rate of 0.001 s-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the tempering and dynamic recrystallization easily occur in the specimens with martensite during warm compression, which results in the above phenomena.     

Predicting the critical conditions and stress-strain curves for dynamic recrystallization in SPHC steel

 The hot compression tests were carried out on a SPHC steel at the temperature range of 900～1150℃ and strain rate range of 0.1～10s-1, which the maximum true strain is 0.8. The activation energy of tested steel was calculated, which was 299.4 KJ /mol. The critical stresses and strains for initiation of dynamic recrystallization were determined based on changes in the work hardening rate ( ) as functions of the flow stress ( ) or strain ( ), respectively. The dependence of the peak strain ( ), the peak stress ( ), and the steady state stress ( ) were determined based on the Zener-Hollomen parameter. The mathematical models of the flow stress evolution were established in the hardening and dynamic recovery region and dynamic recrystallization region, respectively. The average error between experimental and predicted curves was around 3.26%.