Influence of PreTransformed Martensite on WorkHardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

 The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work hardening behavior under the uniaxial tensile condition. The X ray diffractometer (XRD) was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work hardening curve of prestrained specimens observably changes with true strain. The work hardening exponent n of stainless steel decreases with the increase of pre transformed martensite. The achievement is a significant contribution to the process design during pressing.     

The influence of biaxial prestrain on the tensile properties of three aluminum alloys

We have investigated the influence of planar biaxial prestrain on the subsequent tensile behavior of three aluminum alloys. The flow stability encountered in the uniaxial stage is explained in terms of the reduced tensile hardening following biaxial prestraining. A critical stress value approximately equal to the nonprestrained ultimate tensile strength is shown to be the controlling factor for the onset of failure.     

Effect of prestrain on aging and bake hardening of cold-rolled, continuously annealed steel sheets

To understand the bake hardening (BH) behavior in an actual automotive part, 40 tensile specimens were machined from the actual press-formed outer-door panel of a compact car and both bake and work hardenability distribution data were determined. Strain applied by actual press forming was estimated from the work hardening data. Finally, the effects of prestraining mode and amount on ambient aging and BH response were also investigated. The BH widely ranged from 10 to 54 MPa and the work hardenability was between 17 and 82 MPa, depending on panel location. Bake hardening in the outer-door panel decreased as the work hardening increased, indicating that the BH steel must be applied to the shallow drawn parts in order to maximize the BH effect in dent resistance. In order to establish the effects of prestrain and ambient aging time on the age and subsequent BH, the specimens were prestrained and aged at ambient temperature for various time intervals, and then baked at 170 °C for 20 minutes. In the as-temper-rolled and press-formed condition, the steels were extremely resistant to ambient aging. However, it was found that a 0.3 pct tensile prestrain was sufficient to initiate ambient aging within 1 day, and the effect was accelerated with greater prestrain. With 8 days of ambient aging, all prestrained steels exhibited 20 to 25 MPa of age hardening. Irrespective of prestrain amount in the range of 0.5 to 5.0 pct, the BH decreased as the aging time increased.     

Influences of cell walls and grain boundaries on transient responses of an if steel to changes in strain path

The effects of changes in strain path on plastic behaviour in sheets of an interstitial-free steel with two widely different grain sizes were investigated. The sheets were prestrained in rolling and, apart from supplementary tests, they were tested in uniaxial tension at 90° to the rolling direction. The results support the following conclusions. The magnitude of the increase in reloading yield stress and amplitude of the subsequent reduction in work hardening depend on the strength of dislocation walls generated in the prestrain rather than the grain size. The walls are more effective barriers to dislocation glide in freshly activated slip systems than to glide in the original slip systems operating in the prestrain. The primary cause of the subsequent reduction in hardening rate is disruption and partial dissolution of the original dislocation substructure. The final recovery in hardening rate is caused by generation of a new substructure compatible with the new deformation mode.     

Plastic behavior of dual phase steel following plane-strain deformation

Dual-phase, high-strength steel sheet has been prestrained in plane-strain tension. Residual hardening and ductility properties were evaluated by performing subsequent uniaxial tensile tests in either co-axial or noncoaxial principal strain axis orientations. In contrast to similar work on aluminum-killed 1008/1010 steel sheet, only minor changes were found in the subsequent flow behavior of dual-phase steel, and no significant difference was found between the two orientations. The small effect of an abrupt strain path change observed in this study is consistent with the low work hardening rate of this alloy.     

Measurement and analysis of plane-strain work hardening

A new technique for measurement of plane-strain work hardening has been developed which uses tensile loading and computer analysis for interpretation, and which eliminates the experimental uncertainties of large strain gradients, friction, and out-of-plane bending inherent in the usual plane-strain deformation modes. Plane-strain and tensile work-hardening curves have been measured for 2036-T4 aluminum alloy using several types of sheet specimens. The work-hardening rate in plane strain is lower than that in uniaxial tension. In each case a Voce-type empirical work hardening law represents the data well. Hill’s theories cannot account for these data because the isotropic hardening assumption is violated. A method of analysis was introduced to determine Hill’s newm parameter as a function of strain andm was found to vary from 1.6 to 2.0 in the strain range 0.02 ≤ ε ≤ 0.18.     

Effect of prestrain on the ductile fracture behavior of an interstitial-free steel

The effect of tensile prestrain on the ductile fracture behavior of an interstitial-free (IF) steel has been studied using primarily (1) the analysis of void density by optical microscopy, (2) characterization of the size of dimples by scanning electron microscope (SEM) and image analyzer, and (3) estimation of strain hardening behavior of a series of prestrained tensile specimens, loaded until fracture. The variation of void density with local plastic strain around the necked region of the specimens indicated the existence of two types of void nucleation pertaining to inclusions and precipitate particles. The critical strain for void nucleation (ε{inn}) for the precipitate particles initially increases and then decreases with the increase in percentage prestrain. This phenomenon has been explained using the strain hardening exponent and nature of dislocation-particle interaction. The nature of variation of the average size of the dimples and that of ε{inn} with prestrain are found to be similar. The dimple size thus bears a proportional relationship with the void, nucleation strain ε{inn} and hence the former can be used to predict (ε{inn}) for IF steel.     

Effect of prestrain and baking on the secondary deformation properties of cold-rolled ultra-high-strength steel sheets

Two experimental steels with tensile strength above 980 MPa were prepared to investigate the effect of prestrain and baking on their mechanical and fracture behaviors. The experimental results reveal that,for both experimental steels,with increases in the prestrain level,the bake hardening value increases before reaching a maximum point,and then decreases with further increases in the prestrain level. The results of a "bending-baking-secondary bending"test indicate that the secondary bendability deteriorates at a high level of prestrain. The yield strength of the experimental steels was found to increase and the elongation to decrease after high levels of prestrain and bake hardening. Fracture morphology images indicate that a high prestrain level is associated with shallow dimples and more and larger local cleavage areas.     

Influence of prestrain history on fracture toughness properties of steels

The effects of prestrain history on fracture toughness properties (J _Ic values andJ _R curves) of 4340 steel and 316 stainless steel were investigated. It was observed that monotonic prestrain decreased fracture toughness of both steels regardless of prestrain level. Although cyclic prestrain elevated fracture toughness of 4340 steel, it degraded that of 316 stainless steel. The effects of cyclic prestrain on fracture behavior of 4340 steel and 316 stainless steel were found to be related to cyclic softening and cyclic hardening characteristics, respectively. Moreover, material strengths rationalized the influence of prestrain history on fracture toughness properties of these two steels. Formerly with the Westinghouse Electric Corporation     

The influence of strain path on subsequent mechanical properties—Orthogonal tensile paths

Several aluminum alloys have been subjected to two stage tensile straining, an initial prestrain followed by a subsequent tensile strain at 90 deg to the initial direction. In AA1100-0 and AA3003-0 the prestrain produces dislocation tangling and diffuse cell walls resulting in an enhanced flow stress and decrease in ductility when the material, is subsequently strained in the orthogonal direction. In a fine grained experimental Al−Fe−Ni alloy the prestrain is accompanied by a very low accumulation of dislocations and in this case the flow stress is reduced and ductility enhanced in subsequent orthogonal straining. The commercial alloys AA2036-T4 and AA5182-0 are unaffected by the two stage tensile strain path. The results are considered in terms of the forming limit curve and it is also shown that the behavior is consistent with the concept of an “alien” dislocation distribution being generated during the prestrain.     

Effect of changing strain paths on

The effect of changing strain paths on forming limits of aluminum alloy 2008-T4 has been investigated by determining forming limit diagrams (FLDs) after prestraining. Sheets were pre- strained to several levels in uniaxial, biaxial, and plane-strain tension parallel and perpendicular to the prior rolling direction (RD). Abrupt changes in the strain path can be used to increase the forming limits. Prestraining in biaxial tension generally lowers the forming limits for the entire FLD, whereas prestraining in uniaxial tension raises the limits on the right-hand side of the FLD without much effect on the left-hand side. Prestraining in plane-strain tension raises both sides away from the minimum. Finally, it was found that after prestraining, the amount of the additional plane-strain deformation possible depends on the effective strain during prestrain.     

plane-strain work hardening and transient behavior of interstitial-free steel

Stress transients resulting from abrupt changes in strain path have been shown to be important to subsequent formability. In order to investigate whether these transients result from strain aging or related interstitial effects, two-stage experiments were performed on Armco interstitial-free steel After a prestrain in plane-strain tension, the material was strained in uniaxial tension in the direction of zero initial extension. The stress-strain curve in plane strain was found to deviate markedly from that predicted by usual plasticity theory (Hill’s theory withM = 2.0). Comparison of monotonie curves from uniaxial and plane-strain tension using a newly-developed, self-consistent calculation suggested that IF steel follows Hill’s new theory with constantM ≈ 2.9. After the change from plane strain to uniaxial tension, positive stress transients (flow stress exceeds the monotonie flow stress) were measured. This form of transient agrees with ones measured for other steels. It therefore appears that the origin of the transient phenomenon is independent of interstitial content, and that static strain aging is not the mechanism by which these stress transients occur. Formerly A. E. Browning, Graduate Research Associate, Department of Metallurgical Engineering, The Ohio State University     

Constitutive Behavior of Commercial Grade ZEK100 Magnesium Alloy Sheet over a Wide Range of Strain Rates

The constitutive behavior of a rare-earth magnesium alloy ZEK100 rolled sheet is studied at room temperature over a wide range of strain rates. This alloy displays a weakened basal texture compared to conventional AZ31B sheet which leads to increased ductility; however, a strong orientation dependency persists. An interesting feature of the ZEK100 behavior is twinning at first yield under transverse direction (TD) tensile loading that is not seen in AZ31B. The subsequent work hardening behavior is shown to be stronger in the TD when compared to the rolling and 45 deg directions. One particularly striking feature of this alloy is a significant dependency of the strain rate sensitivity on orientation. The yield strength under compressive loading in all directions and under tensile loading in the TD direction is controlled by twinning and is rate insensitive. In contrast, the yield strength under rolling direction tensile loading is controlled by non-basal slip and is strongly rate sensitive. The cause of the in-plane anisotropy in terms of both strength and strain rate sensitivity is attributed to the initial crystallographic texture and operative deformation mechanisms as confirmed by measurements of deformed texture. Rate-sensitive constitutive fits are provided of the tensile stress–strain curves to the Zerilli–Armstrong[1] hcp material model and of the compressive response to a new constitutive equation due to Kurukuri et al.[2]     

Isotropic and kinematic hardening in a dispersion-strengthened aluminum alloy

The room temperature mechanical behavior of a dispersion strengthened aluminum alloy was examined in tension, compression, and in fully reversed loading. The alloy, 8009, is characterized by a high volume fraction of 50–100 nm dispersoid (25%) and 0.5 mm grain size. In tension, 8009 exhibits low strain to UTS and large post uniform elongation; in compression, near steady state deformation is observed after 2–3% strain. The Bauschinger effect was quantified as a function of prestrain in the forward direction. The experimental reverse loading curves were compared to those expected for ideal isotropic hardening and ideal K1 type kinematic hardening. The alloy exhibits nearly pure kinematic hardening of the K1 type. Based on the microstructure and the fully reversed loading behavior, the monotonic deformation behavior is explained.     

Evaluation of Strain Hardening Parameters

The plane-strain compression test for three kinds of materials was carried out in atemperature range between room temperature and 400 ~C. The a- e curves and strain-hardeningrate at different temperatures were simulated and a reasonable fit to the experimental data wasobtained. A modified model created by data inference and computer simulation was developed todescribe the strain hardening at a large deformation, and the predicted strain hardening are in agood agreement with that observed in a large range of stress. The influences of different param-eters on strain hardening behaviour under large deformation were analysed. The temperatureincrease within the test temperatures for stainless steel 18/8 Ti results in dropping of flowstress and strain-hardening rate. For favourable r-fibre texture to obtain high r, the cold roll-ing was applied at large reduction. In the experimental procedure, the X-ray diffraction test wascarried out to compare the strain hardening and microstructure under large deformation for abcc steel (low carbon steel SS-1142). The results indicate that the high strain-hardening ratepossibly occurs when the primary slip plane {110} is parallel to the rolling plane and the strain-hardening rate decreases when lots of {110} plane rotate out from the orientation {110}//RP.     

Yield surfaces in prestrained aluminum and copper

The analysis of strain-hardening materials subjected to multiaxial states of stress requires more detailed experimental information about the effects of previous plastic deformation on the yield surfaces of real materials than is presently available. To provide insight into some of these effects thin walled tubular specimens of 1100-0 aluminum and annealed OFHC copper were subjected to biaxial stresses through the application of simultaneous axial tension and internal pressure, and the effects of the magnitude, direction, and sequence of prestraining operations on subsequent yield surfaces were determined. It was found that the yield surface behavior depends greatly upon the definition of yielding employed. Use of small proof strain definitions resulted in very anisotropic yield surface characteristics which reflected the effect of previous deformation. On the other hand, use of large proof strains resulted in isotropic yield surface characteristics which were devoid of previous deformation influence. The small proof strain yield curves were found, in general, to expand and translate in the direction of prestrain and, for biaxial prestrains, to be distorted in the vicinity of the loading point. Multiple prestrain sequences in normal directions induce a large negative cross effect similar to Bauschinger effect observed under reversed loading. Such anisotropic behavior was found to contradict the two most commonly used continuum mechanics predictions, the isotropic and kinematic hardening rules.     

Influence of Pre-Transformed Martensite on Work-Hardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work-hardening behavior under the uniaxial tensile condition. The X-ray diffractometer (XRD) was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre-transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work-hardening curve of prestrained specimens observably changes with true strain. The work-hardening exponent n of stainless steel decreases with the increase of pre-transformed martensite. The achievement is a significant contribution to the process design during pressing.     

Comparison of plane-strain and tensile work hardening in two sheet steel alloys

Extended tensile tests and newly-devised plane-strain tests have been performed on two commercial sheet steels: an aluminum-killed (A-K) steel and a cold-rolled, dual-phase (D-P) steel. Up to uniform strain in uniaxial tension, the average work-hardening rate of both steels was found to be independent of strain state (isotropic hardening). The planestrain behavior of both steels was in good agreement with predictions of Hill’s old theory of normal anisotropic plasticity. At larger strains, the work-hardening rate in tension is greater than in plane strain. All of the tests were adjusted to reflect constant strain rates and all results were well represented by work hardening of the Hollomon type.     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Latent hardening behavior of monocrystalline Al-Mg solid solution

The latent hardening behavior of dilute Al-Mg single crystal was investigated in this study. We performed the latent hardening tests on five systems, one in each of the five system groups. The latent hardening ratios (LHR) and the hardening rates were calculated. The LHR of systems that form attractive junctions is highest in this investigation. The LHRs of systems that form Lomer-Cottrell sessile locks, Hirth locks, or cross-slip systems are in the middle range. The coplanar system has the lowest LHR, which is in agreement with the theoretical prediction. An equation was developed that correlates the LHR with the dislocation densities at various prestrain values. The secondary deformation curve is predicted qualitatively in accordance with the interaction strength of the latent system with the primary system. Based on such a model, a prediction of the shapes of the secondary deformation curves in the strongest and weakest latent systems can be made.