In this paper, a comparative study has been made on the capability of Johnson Cook (JC), modified Zerilli–Armstrong (ZA), and strain-compensated Arrhenius-type constitutive models for representing ...the elevated temperature flow behaviour of modified 9Cr–1Mo steel. The experimental stress–strain data from isothermal hot compression tests over a wide range of temperatures (1123–1373
K), strains (0.1–0.5) and strain rates (0.001–1
s
−1) were employed to evaluate the material constants of these constitutive models. Suitability of these models were evaluated by comparing the correlation coefficient and absolute average error of prediction, ability to describe the deformation behaviour, number of material constants involved, and the computational time required to evaluate these constants. It is observed that the JC model is inadequate to provide good description of flow behaviour of modified 9Cr–1Mo steel in the above hot working domain. Predictions of the other two models are in good agreement with the experimental data. However, strain-compensated Arrhenius-type equation could track the deformation behaviour more accurately though it requires more number of material constants and more computational time to evaluate these constants than the modified-ZA model.
This article discusses the role of twinning on dynamic recrystallization (DRX) and microstructural evolution during moderate to high strain rate (0.1 to 100 s
−1
) hot deformation (1173 to 1373 K ...(900 to 1100 °C) range) in a Ti-modified austenitic stainless steel (alloy D9). The extent of DRX increased with increasing strain rate and temperature in the range of hot working parameters employed in the present study. The acceleration of DRX with strain rate is attributed to increased rate of dislocation accumulation during high strain rate deformation as well as adiabatic temperature rise. The DRX grains were found to be twinned and a linear relationship was observed between the area fraction of DRX grains and the fraction of Σ3 boundaries. Analysis of misorientations revealed that the majority of these Σ3 boundaries are newly formed coherent twin boundaries during DRX. Interaction of pre-existing Σ3 boundaries that may regenerate new Σ3 boundaries did not seem to occur frequently during DRX. The majority of the twin boundaries are found within the DRX grains, signifying that these annealing twins are mainly formed by “growth accidents” during the expansion of the DRX grains. It is suggested that annealing twins play an important role during nucleation and subsequent expansion of the DRX process in alloy D9.
A modified 9Cr–1Mo steel has been exposed to three separate normalization treatments i.e. at 950°C, 1025°C and 1100°C from As-received (normalized at 1050°C and tempered at 750°C) condition. After ...subsequent tempering treatment the impact toughness in terms of upper shelf energy (USE) and ductile-to-brittle transition temperature (DBTT) has been evaluated using Charpy impact testing (with 10mm×10mm×55mm specimens) following ASTM E23 procedures. A substantial improvement in USE (~20J) and reduction in DBTT (~20°C) has been noticed for the 1025°C treatment. The result has been analysed in terms of change in yield strength and ductility evaluated with tensile tests. Further, effects of microstructure, precipitate and texture on the change of DBTT have been studied with TEM and EBSD analyses. The results have been attributed to the smaller ‘effective grain size’, higher fraction of high-angle boundaries, presence of beneficial γ-fibre texture and dissolution of the pre-existing coarse precipitates in the reheated sample as compared to As-received material.
In this study, the microstructures of the three principal heat-affected zones (HAZs) of the P91 steel weld joint were simulated using appropriate heat treatments at different soaking temperatures ...ranging from 865 °C to 1200 °C. Low cycle fatigue tests were conducted on the samples of each microstructural zone of the weld joint at 550 °C employing a constant strain amplitude of ±0.6% and strain rate of 3 × 10−3 s−1. In the weld joint’s microstructures, the weld metal exhibits the lowest fatigue life, followed by the inter-critical (IC). However, the actual weld joint specimen failed at the interface between the ICHAZ and the base metal.
Studies on creep-fatigue interaction behaviour of Alloy 617M Goyal, Sunil; Mariappan, K.; Shankar, Vani ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
07/2018, Letnik:
730
Journal Article
Recenzirano
In this investigation, response of Alloy 617 M to creep-fatigue interaction loading has been studied. The creep-fatigue interaction tests were conducted in air environment at a constant strain rate ...of 3 × 10−3 s−1 and strain amplitude of ± 0.4% with hold periods of 1 min, 10 min and 30 min at peak tension or compression at 973 K. The material showed initial hardening to a saturation followed by reduction in stress initiated by crack nucleation, propagation and final failure. The fatigue life decreased in presence of hold period. The decrease in fatigue life was more in tensile hold than in compressive hold. The fatigue life showed saturating trend for higher hold periods. Fractographic studies revealed significant intergranular creep cavitation under tensile hold whereas compressive hold exhibited transgranular fatigue striations in association with intergranular creep cavitation. Significant stress relaxation was found to take place during hold and was more in compressive hold than under tensile hold. The creep and fatigue damage have been assessed based on linear damage summation rule. Creep damage fraction was assessed from the stress relaxation considering the average stress, minimum stress and integrated stress over the relaxation curve. The creep-fatigue damage assessment considering integration of creep damage during stress relaxation showed most conservative compared to the other two approaches.
The experimental stress–strain data from isothermal hot compression tests over a wide range of temperatures (1073–1473K), strains (0.1–0.5) and strain rates (0.001–1s−1) were employed to formulate a ...suitable constitutive model to predict the elevated-temperature deformation behaviour in a Ti-modified austenitic stainless steel (alloy D9). It was observed that the Johnson–Cook (JC) model in its original form is inadequate to provide good description of flow behaviour of alloy D9 in the above hot working domain. This has been attributed to the inadequacy of the JC model to incorporate the coupled effects of strain and temperature and of strain rate and temperature. A modified constitutive model based on the Zerilli–Armstrong model has been proposed for considering the effects of thermal softening, strain rate hardening and isotropic hardening as well as the coupled effects of temperature and strain and of strain rate and temperature on flow stress. The proposed modified constitutive model could predict the elevated-temperature flow behaviour of alloy D9 over the specified hot working domain of alloy D9 with good correlation and generalization.
► Constitutive analysis is performed on compression flow stress data for P91 steel. ► A new relation between stress multipliers
α
g
and
α
ER is proposed as (
α
g
/
α
ER)
=
0.27977
+
0.01531(
n
p
)
2. ...► The adjustable
α
g
is obtained by force fitting the Garofalo equation
ε
˙
=
constant
sinh
(
α
g
σ
)
n
g
. ►
α
ER
=
β
e
/
n
p
is obtained from the plots of power
(
ε
˙
∝
σ
n
p
)
and exponential
(
ε
˙
∝
exp
(
β
e
σ
)
)
laws. ► Implications of the proposed relation to constitutive analysis are discussed.
Constitutive analysis is performed following the Garofalo sine–hyperbolic equation on the true stress–strain data obtained from isothermal hot compression tests on modified 9Cr–1Mo (P91) steel over a wide range of temperature (1123–1373
K) and strain rate (0.001–100
s
−1). We propose a new relationship between the stress multipliers
α
g
and
α
ER for P91 steel as (
α
g
/
α
ER)
=
0.27977+0.01531(
n
p
)
2; where the adjustable stress multiplier
α
g
is obtained by force fitting Garofalo equation (
ε
˙
=
constant
sinh
(
α
g
σ
)
n
g
) and
α
ER is obtained as
α
ER
=
β
e
/
n
p
from the data fitted to power
(
ε
˙
∝
σ
n
p
)
and exponential
(
ε
˙
∝
exp
(
β
e
σ
)
)
laws for the entire stress/strain rate regime.
α
g
was easily obtained knowing
α
ER and following this, the other constitutive parameters
n
g
,
Q and ln
A
g
were determined and were found to be strain dependent except
n
g
. The successful prediction of flow stress has been supported by a higher correlation coefficient (
R
=
0.994) and a lower average absolute relative error (5.03%) for the entire investigated hot working domain.
Constitutive analysis for hot working of modified 9Cr–1Mo (P91) ferritic steel was carried out employing experimental stress–strain data from isothermal hot compression tests, in a wide range of ...temperatures (1123–1373
K), strains (0.1–0.5) and strain rates (10
−3–10
2
s
−1). The effects of temperature and strain rate on deformation behaviour were represented by Zener–Hollomon parameter in an exponent-type equation. The influence of strain was incorporated in the constitutive equation by considering the effect of strain on different material constants. Activation energy was found to vary with strain in the range 369–391
kJ
mol
−1. The developed constitutive equation (considering the compensation of strain) could predict flow stress of modified 9Cr–1Mo steel over the specified hot working domain with very good correlation and generalization.
▶ Constitutive analysis is performed on hot compression flow data for P91 steel. ▶ The data is examined in terms of modulus, lattice and pipe diffusion coefficients. ▶ P91 steel obeyed rate equation ...ε˙/DL =constant sinh(αLσ/G)nh; αL, Ah, nh were strain dependent. ▶ Deviation in prediction was observed at lower strain rates 0.001 and 0.01s−1. ▶R=0.99 and AARE=7.3% revealed satisfactory prediction for the entire domain.
True stress–strain data from isothermal hot compression tests on modified 9Cr-1Mo steel over a wide range of temperature (1173–1373K) and strain rate (0.001–100s−1) were employed for constitutive analysis following the sine-hyperbolic Arrhenius equation. The correction for shear modulus and diffusivity in the constitutive equation showed a clear deviation from power-law at higher stresses and this was accounted for by considering the contribution from pipe diffusion. The stress dependence was found to obey rate equation of the form ε˙/DL =constant sinh(αLσ/G)nh, where DL is lattice diffusivity, G is shear modulus and, αL and nh are constants. After incorporating the influence of strain on material constants, the developed constitutive equation could predict flow stress in the strain rate range 0.1–100s−1 at all temperatures with very good correlation and generalization. Though deviation in prediction was observed at lower strain rates 0.001 and 0.01s−1, a higher correlation coefficient (R=0.99) and a lower average absolute relative error (7.3%) for the entire investigated hot working domain revealed that the prediction of flow stress was satisfactory.
► Intrinsic workability of P91 steel has been studied in the hot working domain. ► Experimental stress–strain data have been generated using hot compression tests. ► Processing map based on DMM have ...been developed using the experimental data. ► The domains of the processing map have been validated using the optical micrographs. ► Activation energy map has been used to finalize the optimum hot working parameters.
Intrinsic workability of modified 9Cr–1Mo steel has been studied in a wide range of temperatures (1123–1373
K) and strain rates (0.001–10
s
−1). Using the experimental data obtained from isothermal hot compression tests, processing map at 0.5 true strain has been developed employing dynamic material model (DMM) approach. The activation energy map has been developed to substantiate the results obtained from processing map and to finalize the optimum processing parameters. Microstructural studies have been carried out to validate the domains of the processing map. The material shows localized deformation bands in the temperature range of 1150–1373
K at strain rates above 1
s
−1 and exhibits abnormally elongated martensite laths at higher temperature (1373
K) and lower strain rates (0.001–0.01
s
−1). The optimum domain for the hot deformation is found to be in the temperature ranges of 1250–1350
K and strain rate ranges 0.015–0.3
s
−1 with a peak efficiency of 38%. In this domain, apparent activation energy is found to be 400
kJ/mol. The microstructure of the specimens deformed in this region exhibits defect free equiaxed grains.
