Comparison of dynamic softening in 301, 304, 316 and 317 stainless steels

The mechanical torsion data in the form of flow curves and strain hardening rates from both as-cast and worked 300 series austenitic stainless steels, tested in the range 1200-900°C and 0.1 to 5.0 s-1, have been analysed to deepen understanding of dynamic softening mechanisms. The critical strain for dynamic recrystallization (DRX) is determined from the downward inflection of the strain hardening rate-stress curves, and completion of DRX is taken from the start of the steady-state regime. The rate of softening can be described by means of the Avrami equation with a mean k value of 1.27. These conclusions, based upon mechanical data, have been confirmed by optical metallographic methods. The peak strain (e _p) at which there is about 30% DRX is shown to be a function of the Zener-Hollomon parameter (Z) and the original grain size (D₀). The transition from multiple-peak grain coarsening to single-peak grain refinement behaviour has been determined. While the DRX grain size is a linear function of the steady-state flow stress with a power of -1.23, the subgrain diameter function has a power of -1. The stress and strain for subgrain formation were determined from changes in slope of the strain hardening-stress curves.     

Effect of Nb and C on the hot flow behavior of Nb microalloyed steels

The compressive deformation behaviors of a C–Mn steel (0.36C–1.42Mn) and two Nb microalloyed steels (0.35C–1.41Mn–0.044Nb and 0.055C–1.42Mn–0.036Nb) were investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.005 s⁻¹ to 10 s⁻¹ on Gleeble-1500 thermo-mechanical simulator. It was found that the flow stress of the C–Mn steel is the lowest among the experimental steels, indicating that Nb microalloying in HSLA steels can effectively increase the hot deformation flow stress, and the 0.055C–1.42Mn–0.036Nb steel has a higher flow stress than that of the 0.35C–1.41Mn–0.044Nb steel, indicating that C addition generates a softening effect. The flow stress constitutive equations of hot deformation were developed for the experimental steels, the activation energy Q about 360 kJ/mol for the 0.055C–1.42Mn–0.036Nb steel was higher than that for the 0.35C–1.41Mn–0.044Nb steel (347 kJ/mol) and the C–Mn steel (278 kJ/mol). Characteristic points of flow stress for the three steels were analyzed. The results showed that Nb addition can effectively increase the peak strain and the steady state strain of steels, thus delay distinctly the occurrence of dynamic recrystallization, while C addition can reduce the peak strain and the steady state strain of Nb microalloyed steels, thus promote the occurrence of dynamic recrystallization.     

Metadynamic recrystallization in C steels

Metadynamic recrystallization has been investigated in three plain carbon steels (ENIA, EN2 and EN24) through the use of hot interrupted compression tests on a wedge plastometer. Holding time was 0.5 s between passes. Strain rates of 0.05 and 0.12/s and small strain increments of 3, 5 and 7% were employed. Test temperatures were varied between 800 and 1100°C. Various incremental and continuous stress strain curves were highlighted at different temperatures and strain rates for 3 steels, ENIA, EN2 and EN24, resulting in varying flow stresses and strains. Highest peak stress was 180 MPa for EN24 at peak strain of 0.25 and 900°C, with a strain rate 0.12/s. Peak strain values for all steels at 1100°C was 0.133 at a strain rate of 0.05/s and 0.15 at a strain rate of 0.12/s. Strain accumulation resulted in dynamic and metadynamic recrystallization with refinement to about 15 μm for dynamic and 22 μm for metadynamic recrystallization. Fractional softening,X, decreased from 0.27 to 0.12 as recrystallization times in metadynamic recrystallization increased from 0.9 s to 1.5 s at 1100°C. Time for 50% metadynamic recrystallization was also reduced as temperature increased. For ENIA, a drop from 10000 s to 20 s, as temperature increased from 800 to 1100°C was observed. For EN24 and EN2 steels, a drop from 4000 s to 6 s for similar temperature rise was observed. Metadynamic recrystallization (at strains higher than critical strain) is observed to be a strong function of strain rate and a very weak function of temperature and strain. It significantly refined the austenite grain size prior to transformation.     

Modelling extrusion of 2618 aluminium alloy and 2618-1 10%AI203 and 2618-20%AI203 composites

Abstract

The metal matrix composites 2618-10%Al₂O₃ and 2618-20%Al₂O₃ have been subjected to hot torsion testsfor temperatures T from 300 to 500°C and strain rates ε from 0.1 to 5 S^-l. The flow stresses decrease with increasing temperature and decreasing strain rate and the composites have greater strength than the matrix alloy. The strength of the extruded 2618-10%Al₂O₃ is higher than that of the as cast 2618-20%Al₂O₃. A constitutive analysis has been carried out with the sinh Arrhenius equation for the composites and the alloy. Plots of log ε versus log(sinh a(ασ_p) and log(sinh α?_p) versus 1/T are approximately linear over the range of conditions tested. To improve the deformation processing, which is requiredfor many discontinuously reinforced metal matrix composites, the extrusion of the three materials has been modelled using the finite element software Deform and the constitutive laws determined above. Of particular significance are the dependences of peak load and maximum temperature on billet temperature and ram speed for an extrusion ratio of 31.     

Dynamic recrystallisation of D2 and W1 tool steels

Abstract

Hot torsion continuous tests were performed on a high carbon, high chromium cold work die steel (D2) and a water hardenable carbon tool steel (W1) at strain rates of 0·1, 1, and 4 s^-1 in the temperature ranges of 900 to 1150°C for D2 and 900 to 1200°C for W1. The stress–strain (σ–?) curves rose to a peak stress σ _p , then declined to a steady state value σ _ss , typical of dynamic recrystallisation (DRX). The commencement and effective completion (99%) of DRX are obtained from θ–σ and σ–? curves respectively where θ is the strain hardening rate dσ/d?. The kinetics of DRX are assumed to follow an Avrami equation whereas the time t _ss for 99% DRX is related to σ _ss and temperature by a sinh function. The equilibrium recrystallised grain size D _s decreases with increase in σ _ss and Z, the Zener–Hollomon temperature compensated strain rate. Due to the presence of carbides, which stimulate nucleation, D2 generally has faster DRX kinetics than W1.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

Fließkurven metallischer Werkstoffe im Bereich hoher Formänderungsgeschwindigkeiten

Flow Curves of Metallic Materials in a Range of High Deformation Velocity The flow curves of aluminium, copper and steels are determined in the strain rate range of 10³ up to 10⁴ s^?1 in the indirect impact tension test as well as in the impact torsion test. In order to take the influence of the wave character of the dynamic loading into consideration, the tests were numerically simulated and the determind accuracy of flow curves and its parameters is examind. In the range ε < 10³ s^?1, the relation between flow stress and strain rate can be described by the laws of thermal activated glide processes, whereas for ε > 2000 s^?1 a linear relationship between flow stress and strain rate is determined, which is related damping processes dominating in this range.     

Flow curves up to peak strength of hot deformed D2 and W1 tool steels

Abstract

A high carbon, high chromium cold work die steel (D2) and a water hardenable carbon tool steel (W1) were hot deformed in torsion between 900 and 1150°C for the alloy steel and 900 and 1200°C for the carbon steel, at strain rates from 0·1 to 4 s^-1. The slope of the stress–strain curves, which represents strain hardening, decreased linearly then changed gradually to a slower linear decline before decreasing quickly to zero at the peak stress. On further strain, stress decreased to a steady state regime, indicative of dynamic recrystallisation. Kocks–Mecking analysis provided an activation enthalpy with an average comparable to the activation energy derived from a sinh modified Arrhenius analysis of peak stress. The alloy carbides of the D2 steel have the effect of raising its strength to over twice that of the carbon steel, accelerating the onset of dynamic recrystallisation but drastically lowering its ductility.     

Constitutive modeling for flow behavior of GCr15 steel under hot compression experiments

The thermal compressive deformation behavior of GCr15 (AISI-52100), one of the most commonly used bearing steels, was studied on the Gleeble-3500 thermo-simulation system at temperature range of 950–1150 °C and strain rate range of 0.1–10 s⁻¹. According to the experimental results, the stress level decreases with increasing deformation temperature and decreasing strain rate. The peak stresses on the true stress–strain curves suggest that the dynamic softening of GCr15 steel occurs during hot compression tests. To formulate the thermoplastic constitutive equation of GCr15 steel, Arrhenius equation and the Zener–Hollomon parameter in an exponent-type equation were utilized in this paper. In addition, a modified Zener–Hollomon parameter considering the compensation of strain rate during hot compression was employed to improve the prediction accuracy of the developed constitutive equation. Analysis results indicate that the flow stress values predicted by the proposed constitutive model agree well with the experimental values, which confirms the accuracy and reliability of the developed constitutive equation of GCr15 steel.     

Fe-8Mn-3Al-0.2C轻质高强钢的热变形行为

为了探究Fe-8Mn-3Al-0.2C轻质高强钢的热变形行为,在变形温度为1 123~1 423 K,应变速率0.01,0.1,1,10 s-1,真应变为0.6的条件下利用Gleeble-1500热模拟实验机进行热压缩模拟实验,通过实验机记录温度、真应力与真应变的关系,观察组织形貌演变规律.结果表明:流变应力曲线分为3个阶段,即加工硬化、动态软化及稳定流变应力;当变形温度升高和应变速率下降时,峰值应力及其所对应的临界应变减小,说明更容易发生动态再结晶;在变形初期ε0.1时,流变应力曲线出现应变增加而应力几乎保持不变的类屈服平台;压缩后的组织为奥氏体/铁素体双相组织,动态再结晶先在铁素体内部发生,随后由奥氏体承担;随着变形温度的升高和应变速率的下降,晶粒尺寸细化并趋于均匀,说明动态再结晶完成的更充分;本实验钢在本文处理工艺及0.6真应变下的最佳热加工工艺参数区间为1 250~1 400 K,应变速率为0.03~0.3 s~(-1);受合金元素影响,实验用钢的表观应力指数和热变形激活能分别为4.588 9和250.6 k J/mol,本构方程为ε·=6.20×10~9[sinh(0.009σ)]~(4.588 9)exp(-(250 601)/(8.314T)).     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Comparison of static softening in 300 series stainless steels under iso- and anisothermal conditions

Abstract

Multistage torsion tests in the temperature range 1200–900°C and strain rate range 0·1–2·0 s ^?1 were carried out to study the static recrystallisation (SRX) kinetics of 301,304,316, and 317 steels under iso- and anisothermal conditions. The fractional softening decreased with decreasing temperature, pass strain ε_i, and strain rate. The fraction recrystallised was plotted according to the Avrami expression. The temperature compensated timefor 50% SRX was a function of ε to the power of ?2 and of the Zener-Hollomon parameter Z to the power of ??. The activation energy for SRX Q_SRX was determined and compared with extensive results from the literature. The effect of accumulated strain is reflected in the decrease of Q_SRX for all alloys. The SRX grain size is fitted by the Barraclough expression, in which it is a function of original grain size, ;amp;epsilon;, and Z. Finer grain sizes were obtained from accumulated straining in multistage anisothermal tests than from continuous straining, in which only dynamic recovery and dynamic recrystallisation were operative.

MST/1249     

Analysis of high flow stress and microstructural evolution of TC6 titanium alloy during isothermal forging

Abstract

Elevated temperature true stress – strain curves have been determined for the isothermal deformation of a TC6 titanium alloy using hot compression testing in the deformation temperature range 800 – 1040°C, strain rate range 0.001 – 50 s^-1 and reduction in height of 30 – 50%. The experimental results show that the flow stress of TC6 titanium alloy is strongly dependent on process parameters, especially on the deformation temperature and strain rate. The peak stress and steady stress of such an alloy have the same characterisation, which increases with higher strain rate and lower deformation temperature. During isothermal forging, microstructural characterisation, including volume fraction, grain size, and grain pattern of prior α phase, varies with different temperatures, height reductions, and strain rates.     

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.     

THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS

Abstract The tensile yield and flow stresses of aluminium, A1-2.63Mg alloy and A1-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall-Petch relation. The same relation describes the flow stress-grain size dependence for A1-2.07Li alloy at temperatures ranging from - 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ₀, and slip band stress intensity parameter, k_ε, at yield, k_y, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile-ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of k_ε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.     

Modeling of flow stress of 42CrMo steel under hot compression

The compressive deformation behavior of 42CrMo steel was investigated at temperatures from 850 °C to 1150 °C and strain rates from 0.01 s^?1 to 50 s^?1 on a Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit peak stresses at small strains, then the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. 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 the 42CrMo steel at elevated temperatures is proposed by compensation of strain. The stress–strain relations of 42CrMo steel predicted by the proposed models agree well with experimental results.     

Hot flow behavior of boron microalloyed steels

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hot-compression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 °C) and constant true strain rates (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure.     

Thermodynamic model for precipitation of carbonitrides in high strength low alloy steels containing up to three microalloying elements with or without additions of aluminium

Abstract

To control austenite grain growth in high strength low alloy (HSLA) steels, additions of micro alloying elements showing chemical affinity for interstitial elements (C and N) are used. These elements have a significant effect on the mechanical properties of the steels. The mechanical properties of HSLA steels depend both on the amounts of the microalloying elements dissolved in the austenite and on undissolved carbonitrides. In the present work, a thermodynamic model enabling calculation of the composition of the austenite as well as the composition and quantity of the carbonitrides in HSLA steels containing up to four microalloying elements is presented. This model can be a useful tool in determining the optimum chemical compositions for steels as well as the heat treatment parameters required to maximise hardenability and to control austenite grain growth.

MST/1471     

Einfluss von Temperatur und Vorverformung auf das plastische Werkstoffverhalten von modernen Karosseriestählen

Influence of temperature and prestraining on the plastic material behaviour of modern sheet steels for autobody applications Within the scope of a common research project of the automotive and steel industry, characteristic values describing the plastical behaviour of 20 sheet steels have been determined. In detail, quasistatic tensile tests at the testing temperatures ‐40 °C, 23 °C and 100 °C were carried out to obtain flow curves for the as delivered materials as well as for steels after a defined prestraining or heat treatment. Additionally, sheet metal testing led to forming limit diagrams and limiting drawing ratios including the working ranges for deep drawing. The results of the tensile tests showed significant differences between steel groups with regard to their strain hardening behaviour, which can be described by the ratio of yield and tensile strength R_p0,2/R_m or the Θ_IV‐value, and their temperature sensitivity. Within one steel group, consisting of steels with similar strain hardening behaviour, it might be possible to determine flow curves of one steel in a defined condition in order to calculate the flow curves of other steels with different strength. An advantage would be a lesser number of experimental tests which have to be performed in order to supply reliable input data for numerical material and component modelling.     

On the evaluation of parameters of the constitutive equation for 7475 Al alloy

From the mechanical data on 7475 Al alloy, it is evident that flow stress is significantly dependent on the strain during superplastic flow. This is due to its ability to strain-harden during superplasticity. The rate of increase in the flow stress is much higher at 457° C than at 517° C. This gives rise to non-unique values for the parameters of the constitutive equation. At 457° C, whereas the stress exponent (n) and activation energy for superplastic flow at 1 × 10^–4 sec^–1 increase only slightly with strain, the grain size sensitivity parameter (p) and structure parameter (A) decrease significantly with strain. These changes in the constitutive parameters are associated with dislocation activity occurring within the grain interior, leading to grain elongation without significant changes in the grain size, through the parameter, (b/d) ^p , of the constitutive equation.