Values Of Strain Hardening Exponent Research Articles

Low strain hardening enables improved water droplet erosion performance through deep rolling

The present work investigates the role of deep rolling surface treatment on the water droplet erosion behavior of 17–4 PH stainless steel. Standard heat treatments were first applied to samples of 17–4 PH stainless steel to achieve three states, solution treated and two different aging conditions. The mechanical properties of these conditions were determined using hardness and tensile tests. Then, deep rolling surface treatments were applied to the various conditions of the 17–4 PH. Water droplet erosion tests were subsequently performed at impact velocities of 250 m/s and 300 m/s, and the erosion performance in terms of incubation period and maximum erosion rate before and after the deep rolling treatment is compared. It was found that deep rolled samples exhibited considerably longer incubation periods compared to only heat-treated samples. The improvement in erosion resistance of the 17–4 PH stainless steel is attributed to its lower strain hardening exponent values, which reduces embrittlement that is usually associated with the work hardening due to deep rolling. Hence, it is concluded that low strain hardening exponent is essential to the effectiveness and success of mechanical surface treatments to combat water droplet erosion.

Formability evaluation of tailor weld blanks of austenitic stainless steel (AISI 304)

ABSTRACT Tailor weld blank (TWB) is preferred in different components of automobiles to reduce weight of the vehicle. However, TWBs have lower formability than sheets because of increased hardness and strength of weld region. Hence, in this work, detailed formability analysis of TWB of AISI304 is done and compared with as-received sheets along with microstructural correlation. Forming limit diagram for as-received sheet, TWBs are determined. Strain hardening exponent value of as-received sheet is 0.381, unweld region and weld region of TWB is observed to be 0.362 and 0.301, respectively. Numerical simulations were also done for predicting the FLC of TWB of AISI304 with the help of PAM STAMP 2 G solver. TWB showed lower formability than as-received sheets. Similarly, predicted FLD for TWB is almost coinciding with the experimental value. Similarly, optical micro-structure studies were also analysed for as-received sheet, unweld region and weld region of TWB, which showed fully austenitic structure with random grain orientations. At the same time, it was found that strain path chosen for FLD determination has a strong influence on microstructure evolution.

Improvement of stretch formability and weakening the basal texture of Mg alloy by corrugated wide limit alignment and annealing process

In this paper, a process of corrugated wide limit alignment (CWLA) was used to improve the stretch formability of large-dimension AZ31 Mg alloy sheets by inducing tensile twins. The effects of {10 1¯ 2} tensile twins on microstructure evolution, mechanical properties and stretch formability of AZ31 Mg alloy sheet was studied. The results indicated that the CWLA samples exhibited a weakened basal texture and a high fraction of twin boundaries due to the twinning and recrystallization behavior. After CWLA and annealing for 30min at 300 °C, the strain-hardening exponent value increased from 0.2 to 9.4, the Lankford value decreased from 2.85 to 0.37, and the Erishen value was increased by ∼95% compared to the as-received samples. The present study would provide a new insight into the development of Mg alloys having excellent stretch formability.

Hot deformation characteristics of ASS-304 austenitic stainless steel by tensile tests

Sheet metal forming depends on the key material properties namely strain hardening exponent (n) and planar/normal anisotropy. In this work, a comparative study has been performed between the room temperature and elevated temperature tensile tests of ASS 304 steel samples. Uniaxial tensile tests were conducted on along three directions, along rolling direction (RD) (0° to RD), across (90° to RD) and diagonally (45° to RD) and corresponding numerical values of anisotropy were determined. Tensile tests were performed from room temperature (27 °C) to 300 °C at an interval of 100 °C. Results indicated that yield strength YS of decreased initially and then suddenly increased at 300 °C temperature specimen. The ultimate tensile strength UTS of different temperature specimen values are decreased simultaneously. Elongation (%) and strain hardening exponent values also showed higher values for 300 °C temperature specimen compared to other samples. It was observed that better material forming properties could be obtained around 300 °C deformation temperature wherein lower loads are required to produce sheet metal components.

Experimental Investigations on Plastic Deformation and Densification Characteristics of P/M Fe–C–Cu–Mo Alloy Steels Under Cold Upsetting

Powder metallurgy (P/M) alloy steel containing copper (Cu) and molybdenum (Mo) has tremendous potential in various industrial sectors. The present work investigates the effects of Cu and Mo addition on plastic deformation/densification characteristics of P/M plain carbon (Fe–0.5%C) steel. Sintered preforms of 1 and 2% addition of Mo and Cu to the plain carbon steel were subjected to cold upsetting by applying uni-axial load on the preforms. The compressive stress was gradually applied and increased on the alloy steel preforms till fine crack appears on lateral surface of the deformed preforms. Various analytical stresses useful for die design are evaluated using conventional plasticity correlations. The plain carbon steel is observed to exhibit better densification and deformation properties and higher strain hardening exponent value compared to other alloy steels. The basic ferrite–pearlite microstructure coupled with the presence of pore-free copper and Mo particulates is understood to influence the deformation and densification of Cu- and Mo-added alloy steels, respectively. The Cu- and Mo-added alloy steels require more axial compressive stress to attain same level of plastic deformation and densification of the plain carbon steel. Addition of Cu and Mo to the plain steel is observed to reduce the plastic flow and densification level during cold upsetting.

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Hot tensile properties and strain hardening behaviour of Super 304HCu stainless steel

Super 304HCu austenitic stainless steel containing 2.3–3 (wt.%) of Cu is mainly used in superheaters and reheaters tubing of ultra super critical boilers which operates over 600°C of steam temperature. Tensile tests were carried out on Super 304HCu, using nominal strain rate of 1×10−3s−1, at room temperature, 550°C, 600°C and 650°C. The tensile strength and elongation were found to decrease with increase in test temperature. The stress strain curves were fitted using Hollomon equation to determine the strain hardening exponent value. Differential Crussard–Jaoul (C–J) analysis of the tensile curve is used to determine the variation in strain hardening exponent. Kocks–Mecking (K–M) type plots were used to determine the stages of strain hardening during tensile loading of the specimen. The strain hardening capacity of the Super 304HCu is found to decrease with increase in test temperature.

Microstructure evolution and mechanical properties changes in the weld zone of a structural steel during low-cycle fatigue studied using instrumented indentation testing

Room-temperature low-cycle fatigue (LCF), instrumented indentation, tensile tests, finite element (FE) analysis, and optical microscopy examinations were carried out to investigate the mechanical properties and microstructures in the weld zone of a structural steel, SS400, in cyclic loading conditions. Five indentation arrases were performed across the weld zone of five specimens, including one virgin specimen and four specimens subjected to LCF loading at different strain levels. The mechanical properties, such as elastic modulus E, yield strength σy, and strain hardening exponent n, were then estimated from indentation testing and FE analysis results. The obtained results and optical microscopy examinations show that the mechanical properties and microstructures in the individual region of the weld zone are not only decided by the metallurgical conditions resulting from the welding but also were highly influenced by the strain level applied during cyclic loading. In the as-welded condition, the values of elastic modulus, yield strength, and strain hardening exponent in the weld metal (WM) are higher than those in the base metal (BM), while materials in heat affected zone (HAZ) reveal a gradual decrease of elastic modulus, yield strength, and strain hardening exponent values from the WM to BM region. It is also shown that with increasing strain amplitudes during cyclic loading, the elastic modulus E displays decrease in all regions of the weld zone, both yield strength σy and strain hardening exponent n exhibit decreasing characteristic in WM and increasing characteristic in BM region, while the yield strength in HAZ region increases, the strain hardening exponent in this region keeps almost unchanged. The dependence of the mechanical properties on the applied strain level is also discussed in terms of microstructural changes in fatigue failure specimens.

In-plane plane strain formability of adhesive-bonded steel sheets: influence of adhesive properties

In the present work, the influence of adhesive properties on the formability of adhesive-bonded sheets (deep drawing quality (DDQ) steel and SS 316L) has been studied through in-plane plane strain (IPPS) formability tests. The adhesive properties were modified by different hardener to resin ratios and by having epoxy and acrylic adhesives. The formability is quantified by load–extension behaviour, limit strains and strain hardening exponent of adhesive-bonded sheets. It is observed from the work done that with increase in hardener to resin ratio, the elongation of adhesives is found to improve during tensile tests. This forms the basis for the actual formability change of adhesive-bonded sheets at different hardener/resin (H/R) ratios. During IPPS formability tests of adhesive-bonded steel sheets, the total elongation is found to improve with increase in hardener/resin ratio. Likewise, limit strains also improve with increase in hardener/resin ratios. The improvement in elongation and limit strains is due to the conversion of a resin-rich formulation to a hardener-rich formulation, making the sample more ductile. Both the acrylic and epoxy adhesive-bonded sheets show an equal amount of improvement in limit strain. The strain hardening exponent (n) of adhesive-bonded blanks has improved with increase in hardener/resin ratio in all the three regions of deformation. The adhesive-bonded blanks have larger strain hardening exponent values as compared to double sheet in the corresponding regions of deformation, indicating formability improvement as compared to double sheet.

Plastic characterization of metals by combining nanoindentation test and finite element simulation

Materials with the same elastic modulus E and representative stress and strain (σr, ɛr) present similar indentation–loading curves, whatever the value of strain hardening exponent n. Based on this definition, a good approach was proposed to extract the plastic properties or constitutive equations of metals from nanoindentation test combining finite element simulation. Firstly, without consideration of strain hardening, the representative stress was determined by varying assumed representative stress over a wide range until a good agreement was reached between the computed and experimental loading curves. Similarly, the corresponding representative strain was determined with different hypothetical values of strain hardening exponent in the range of 0–0.6. Through modulating assumed strain hardening exponent values to make the computed unloading curve coincide with that of the experiment, the real strain hardening exponent was acquired. Once the strain hardening exponent was determined, the initial yield stress σy of metals could be obtained by the power law constitution. The validity of the proposed methodology was verified by three real metals: AISI 304 steel, Fe and Al alloy.

Effect of Microalloying Elements (B, Nb, V and Ti) on the Strain Hardening Behavior of High-Manganese TWIP Steels.

ABSTRACTThe present research work analyses the influence of microalloying elements (B, Nb, V and Ti) on the tensile strength and the strain hardening behavior of a high-manganese TWIP steel. The analysis was carried out by means of true stress-true strain curves derived from uniaxial tension tests. The work hardening exponent was determined by using the Hollomon and differential Crussard-Jaoul models. Metallographic characterization was carried out to determine the metallurgical changes associated with n values. The results indicate that the Hollomon analysis results in strain hardening exponent values up to 0.46. On the other hand, the differential Crussard-Jaoul analysis establishes a clear distinction of n value for two stages of plastic deformation which are determined by a sharp slope change in the plot of ln(dσ/dε)-lnε.

Tensile Behaviour of Al-Si Alloy and Al-Si/Graphite Composites at Elevated Temperatures

The high temperature tensile behaviour of Al-Si alloy and two of its composite systems with graphite as major reinforcement were investigated. The composites were developed by the stir casting method, wherein a mixture of graphite (3 wt %) and Cu / Ni (2 wt% each) were added into the molten Al-Si alloy to fabricate two systems such as Al-Si-Cu/graphite composite and Al-Si-Ni/graphite composite. The properties of composites were better than that of the matrix alloy. Tensile behaviour of alloy and composites were studied at different temperatures from room temperature to 300°C. It is found that the tensile strength of the alloy and composites were decreasing with increase in temperature. The transition elements (Cu / Ni) have played the key role in improving the ultimate tensile and yield strength of the composites over the alloy. The flow stress of the composite is more than that of the alloy. The strain hardening exponent value continuously drops with the increase of tensile temperature due to the thermal softening effect. The % elongation of the alloy is more than that of the composites. Fracture surfaces of the samples are analyzed by scanning electron microscope to understand the fracture mechanisms. Fractography reveals that the fracture behaviour of the alloy changes from cleavage mode at room temperature to complete ductile mode at high temperature.

Effect of Annealing Process and Cold Reduction Ratio on Microstructure and Mechanical Property of 350MPa Grade Structural Hot Dip Galvanized Steel Sheets

Hot rolled 350MPa Grade structural hot dip galvanized steel sheets was used to study the effect of annealing process and cold reduction ratio on the microstructures and mechanical properties. The results showed that the experimental steel was recrystallize finished after cold rolling follow different cold reduction ratio and Salt-bath annealing (annealing temperature 750°C and soaking time 60s) under experimental condition. When the cold reduction ratio was increased, annealing temperature was above 720°C, the yield strength and tension strength had no significant change. The annealing temperature was 750°C,the elongation had no significant change . when the cold reduction ratio was increased,the plastic strain ratio value reduced, and the strain hardening exponent value was gradually increase. In the end, that suggested the optimal annealing temperature is 750°C,and cold reduction ratio is 55% to 65%, consequently that obtained good match of mechanical properties and punching property.

Effect of strain rate on tensile and compression behaviour of Al–Si/graphite composite

Effects of strain rate (10 −4–10 −2 s −1) on tensile and compressive strength of the Al–Si alloy and Al–Si/graphite composite are investigated. The strain hardening exponent value of the composite was more than that of the alloy for all strain rates during tensile and compressive loading. The yield stress of the composite was more than that of the ultimate tensile strength of the alloy for all strain rates. Tensile and compressive properties of the alloy and composite are dependent on strain rates. The negative strain rate sensitivity was observed for the composite and alloy at lower strain rates during the compression and tension loading respectively.

Using direct hot-rolling approach to obtain dual-phase weathering steel Cu–P–Cr–Ni–Mo

A weathering steel Cu–P–Cr–Ni–Mo has been developed which exhibits special continuous cooling transformation characteristics which permit the desired dual-phase (DP) microstructure to be obtained by direct hot-rolling. Hot-rolling procedures to obtain DP microstructures have been designed based on the continuous cooling transformation diagram of weathering steel Cu–P–Cr–Ni–Mo. The results show that the microstructures of DP weathering steels Cu–P–Cr–Ni–Mo are characterized by an irregular distribution of island-shaped martensite–austenite in the matrix of polygonal ferrite grains. DP weathering steel Cu–P–Cr–Ni–Mo with favorable corrosion resistant property, weldability and mechanical properties, such as, high strain hardening exponent values, a lower ratio of yield to tensile strength, and higher strengths; and is obtained successfully by direct hot-rolling.

Effect of low nitrogen content on work hardening and microstructural evolution in Hadfield steel

Nitrogen content in austenitic manganese steels is usually not reported nor standardized. The present work has found that it typically ranges between 130 ppm and 200 ppm in solution. Microstructural evolution has been studied by optical microscopy, using mainly differential interference contrast mode. Work hardening was analyzed under compression conditions using Hollomon's fitting and Vickers microindentation hardness measurements at different deformation levels. Even low contents of nitrogen in solution exhibit a significant effect on the slope of stress–strain curves, with strain-hardening exponent values of 0.7, 0.57 and 0.41 for 720 ppm, 130 ppm and 71 ppm of nitrogen, respectively. It was also found that the nitrogen content influences the microstructure of deformed coarse grained polycrystalline austenite, leading to twining and shear bands at the upper level of nitrogen, while it was not clearly found in the samples with lower nitrogen content.

Effects of constitutive parameters on adiabatic shear localization for ductile metal based on JOHNSON-COOK and gradient plasticity models

By using the widely used JOHNSON-COOK model and the gradient-dependent plasticity to consider microstructural effect beyond the occurrence of shear strain localization, the distributions of local plastic shear strain and deformation in adiabatic shear band(ASB) were analyzed. The peak local plastic shear strain is proportional to the average plastic shear strain, while it is inversely proportional to the critical plastic shear strain corresponding to the peak flow shear stress. The relative plastic shear deformation between the top and base of ASB depends on the thickness of ASB and the average plastic shear strain. A parametric study was carried out to study the influence of constitutive parameters on shear strain localization. Higher values of static shear strength and work to heat conversion factor lead to lower critical plastic shear strain so that the shear localization is more apparent at the same average plastic shear strain. Higher values of strain-hardening exponent, strain rate sensitive coefficient, melting point, thermal capacity and mass density result in higher critical plastic shear strain, leading to less apparent shear localization at the same average plastic shear strain. The strain rate sensitive coefficient has a minor influence on the critical plastic shear strain, the distributions of local plastic shear strain and deformation in ASB. The effect of strain-hardening modulus on the critical plastic shear strain is not monotonous. When the maximum critical plastic shear strain is reached, the least apparent shear localization occurs.

Evaluation of the Elastic-Plastic Mixity Parameters on the Base of Different Crack Propagation Criteria. Part 2. Solution and Results

We propose a new scheme of mixed-mode problem solution based on the deformation theory of plasticity with a power-law hardening stress-strain response and on application of elastic and plastic mixity parameters. Depending on the mixed-mode loading conditions and the initial crack line direction, this approach allows one to analyze a wide range of possible crack propagation paths controlled by shear and tensile mechanisms. The equilibrium equation with Airy function is used for a two-dimensional problem in the polar coordinate system. The Ramberg-Osgood model is applied to a material with power-law hardening behavior. Using the finite difference method we obtained a numerical solution of the mixed-mode loading problem with boundary conditions corresponding to two cases of crack propagation. Within the framework of the proposed approach we estimated the dependencies between mixity parameters and various loading parameters and crack inclination angle for a range of strain hardening exponent values, which dependencies closely fit the experimental data.

Influence of a novel two-step austempering process on the strain-hardening behavior of austempered ductile cast iron (ADI)

An investigation was carried out to examine the influence of a novel two-step austempering process on the strain-hardening behavior of austempered ductile cast iron (ADI). Strain-hardening exponent ( n value) of specimens austempered by conventional single-step austempering process as well as the novel two-step process were determined over the entire plastic deformation regions of the stress–strain curves. Optical microscopy and X-ray diffraction analysis were performed to examine mechanisms of strain-hardening behavior in ADI under monotonic (tensile) loading. Test results show that this novel two-step process has resulted in improved microstructural variables in the ADI matrix, and higher hardness, yield strength and tensile strengths, but lower ductility and strain-hardening exponent values compared to the conventional single-step austempering process. Test results also indicate that strain-hardening exponent of ADI is a function of amount and morphology of microstructural constituents and interaction intensities between carbon atoms and dislocations in the matrix.

The effect of strain-hardening on frictional behavior in tip test

In previous investigation of the tip test using aluminum alloy AL6061-O, it was found that the distance of radial tip from the external side surface of the workpiece deformed has a linear relationship with the maximum forming load measured at a certain punch stroke. In this study, further experiments with aluminum alloys AL2024-O, AL5083-O, AL6061-O, AL7075-O, annealed carbon steel AISI 1010 and pure copper C12100 were carried out to investigate the effect of material properties on frictional behavior by the tip test for the materials commonly used in bulk metal forming. To characterize the relationships among radial tip distance, maximum forming load and shear friction factor, finite element analyses were employed. It was observed that the linear relationships among these three were maintained for the various tested materials. Also, it was found that the friction condition at the punch was always higher than that at the lower die interface such that the ratio of shear friction factors at the die and punch interfaces should be less than one. By examining the material properties currently tested, it was determined that this ratio could be estimated from a logarithmic equation from the value of strain-hardening exponent, depending on the material used.