Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

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.     

Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract

The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10^?3–3×10^?5 s^?1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol^?1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Effect of nitrogen on tensile flow behaviour of type 316 LN austenitic stainless steel

Abstract

The influence of nitrogen content on the tensile flow behaviour of type 316 LN austenitic stainless steel has been studied. Nitrogen content in the steel has been varied in the range 0·07 to 0·22 wt-%. Tensile tests were carried out over the temperature range of 300–1123 K at a nominal strain rate of 3×10^?3 s^?1. The tensile flow behaviour of the steels has been analysed based on the constitutive equation proposed by Voce. The Voce’s parameters of initial stress (σ_i) and saturation stress (σ_s) were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile properties of the steels were predicted from Voce constitutive equation parameters.     

Quantitative Analysis of Work Hardening and Dynamic Softening Behavior of low carbon alloy Steel Based on the Flow Stress

In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950 °C, 1050 °C, 1150 °C and 1250 °C with strain rate of 0.01 s⁻¹, 0.1 s⁻¹ and 10 s⁻¹ on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V^* of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01 s⁻¹ and 0.1 s⁻¹. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions.     

Processing map for hot working of Ni–16Cr–8Fe alloy (IN 600)

Abstract

The hot deformation characteristics of IN 600 nickel alloy are studied using hot compression testing in the temperature range 850–1200°C and strain rate range 0·001–100 s^?l. A processing map for hot working is developed on the basis of the data obtained, using the principles of dynamic materials modelling. The map exhibits a single domain with a peak efficiency of power dissipation of 48% occurring at 1200°C and 0·2 s^?1, at which the material undergoes dynamic recrystallisation (DRX). These are the optimum conditions for hot working of IN 600. At strain rates higher than 1 s^?1, the material exhibits flow localisation and its microstructure consists of localised bands of fine recrystallised grains. The presence of iron in the Ni–Cr alloy narrows the DRX domain owing to a higher temperature required for carbide dissolution, which is essential for the occurrence of DRX. The efficiency of DRX in Ni–Cr is, however, enhanced by iron addition.

MST/1856     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

Abstract

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ? · behaviour, the nature of the stress versus strain ? curves, ductility, and microstructure upon failure. The log σ–log ? · plot for the ? · range ～5 × 10^-6–3 × 10^-2 s^-1 at T = 1173–1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol^-1, respectively. All the σ–? curves, obtained at ? · = 1 × 10^-4 s^-1 for the temperature range 1173–1273 K, and at T = 1198 K for the strain rate range 1 × 10^-4–1 × 10^-2 s^-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III.     

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.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

Abstract

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσ_y and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

The rate of dynamic recrystallization in 17-4 PH stainless steel

The hot working behavior of 17-4 PH stainless steel (AISI 630) was studied by hot compression test at temperatures of 950–1150 °C with strain rates of 0.001–10 s⁻¹. The progress of dynamic recrystallization (DRX) was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the reciprocal of the time corresponding to the DRX fraction of 0.5% or 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. The Zener-Hollomon parameter (Z) was found to be inappropriate for analysis of DRX kinetics. Therefore, the dynamic recrystallization rate parameter (DRXRP) was introduced for this purpose. The DRXRP may be determined readily from the Avrami analysis and can precisely predict the rate of DRX at hot working conditions.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Assessment and modelling of isothermal forging of intermetallic compounds Part 1 – TiAl

Abstract

In this, the first of four papers concerned with the isothermal forging of intermetallic compounds, Ti–48Al–2Mn–2Nb (at.-%), an alloy based on the γ-TiAl intermetallic phase, has been deformed over the temperature and strain rate ranges 1050–1125°C and 3·0 × 10^-4–3·0 × 10^-2 s^-1 respectively. Examination of the stress–strain curves shows an increase in flow softening behaviour with increasing temperature and decreasing strain rate, contrary to what might have been expected. Forged microstructures indicate that grain refinement via dynamic recrystallisation has occurred, resulting in a fine, almost fully γ microstructure. Constitutive data calculated from initial stress–strain curves (for example activation energy of deformation and strain rate sensitivity) have been used to model deformation behaviour with a reasonable degree of success.     

Compressive behaviour capable of predicting hot tensile ductility behaviour in a 0·1%C–Mn steel

Abstract

Compression testing was carried out on 0·1%C–0·9%Mn steel at various temperatures and strain rates. Peak strain behaviour was investigated as function of temperature and strain rate. Normalisation of flow stress in the stress–strain curves by peak stress exhibits an interesting trend that the normalised value changes as strain, strain rate and temperature vary. Three regions could be characterised based on the way that the softening factor defined as the difference in normalised value between the peak stress and the flow stress at a strain of 0·5 changes with temperature. It was shown that tensile elongation behaviour could be predicted based on the softening factor determined in compression tests. As the softening factor increases, tensile ductility increases.     

Research on the dynamic recrystallization behavior of GCr15 steel

The dynamical recrystallization (DRX) of GCr15 steel was investigated at deformation temperatures of 950–1150 °C and strain rates of 0.1–10 s^?1 on a Gleeble-3800 thermo-mechanical simulator. The stress–strain curves at lower strain rates are typical of the occurrence of DRX and exhibit a peak in the flow stress before reaching steady state. The flow stress at higher strain rates increases rapidly to the maximum too, but followed by a steady region. The microstructures after deformation certify that DRX takes place in all specimens. And the results show that DRX occurs more easily with the decrease of strain rate and the increase of deformation temperature. Using regression analysis, the DRX activation energy of the steel, the relationship of critical strain and deformation conditions were determined. In order to determine the recrystallized fraction under different conditions, an approximate model based on the stress–strain curves was investigated, and the kinetic model for DRX was established.     

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.     

Hot deformation behaviour and constitutive modelling of P92 heat resistant steel

Abstract

Hot compression tests are conducted in the present paper to investigate hot deformation behaviour of the newly developed heat resistant steel P92, which is used to fabricate main steam pipes for ultra supercritical power plants. Stress–strain curves at elevated temperatures and different strain rates are obtained. It is found that dynamic recrystallisation happens only when the temperature is above 1100°C and strain rate is below 0·1 s^?1. Otherwise, dynamic recovery is the main softening mechanism. Constitutive modelling with the hyperbolic sine, including an Arrhenius term, is used to predict peak and saturated stresses. Material constants for this model are determined. Results show that the model can be used to predict peak and saturated stresses. However, the model would fail in predicting flow stress with respect to strain; thus, a model containing nine independent parameters and the complete form of Spittel equation are utilised to predict flow stress curves softened by dynamic recrystallisation and dynamic recovery respectively since there are no unified equations. The predicted stress–strain curves are in good agreement with experimental results, which confirmed that the models developed in the present paper are effective and accurate for P92 steel.