2022 Volume 62 Issue 5 Pages 963-969
Microscale changes in the microstructure of and stress distribution in a polycrystalline duplex stainless steel comprising ferrite and austenite caused by plastic tensile deformation are characterized to understand the microscopic processes of the crystal plasticity of dual-phase steel. Because the ferrite of the body-centered-cubic structure and the austenite of the face-centered-cubic structure exhibit different mechanical properties, texture changes in the ferrite and austenite caused by uniaxial tensile deformation are investigated based on the electron backscatter diffraction pattern. Residual stresses formed by tensile deformation are characterized using a two-dimensional method based on X-ray diffraction. The results show that the steel exhibits a banded microstructure of ferritic coarse grains and austenitic fine grains, and that the texture is changed by tensile deformation. The grains in the steel are rotated by tensile deformation depending on their orientation with respect to the tensile axis. Residual stress measurements demonstrate that compressive stresses remain in the elastically hard ferrite after tensile deformation, whereas tensile stresses remain in the austenitic phase. The formation of the residual stresses are discussed based on the characteristic microscale plasticity of ferrite and austenite.
Because the mechanical and chemical properties of steels are affected by the microstructure, texture, and chemical composition of steels, the microstructures and relevant microscale phenomena in steels must be characterized to control their overall properties. For instance, the characteristic mechanical properties of duplex stainless steels are attributed to constituent phases with different structures.1,2,3,4,5,6,7) In these studies, it was recognized that the mechanical properties of duplex stainless steels are generally interpreted based on the fractions and characteristics of the constituent phases, which are characterized by various techniques such as in situ neutron diffraction and synchrotron radiation diffraction. Neutron diffraction measurements demonstrated that differences in the evolution of lattice strains during loading are affected by the deformation mode of tension and compression, and an elastoplastic self-consistent model was proposed to predict the evolution of internal stresses during loading.1) Diffraction experiments of duplex stainless steels using white X-ray synchrotron radiation suggested that the hard phase in the steel changed from ferrite to austenite during tensile deformation.4) In addition, it was shown in a lean duplex stainless steel that the yield stress and tensile strength increased as the temperature decreased and an the strain rate increased, which may be caused by the characteristics of ferrite with a body-centered-cubic (bcc) structure instead of austenite with a face-centered-cubic (fcc) structure.3) This phenomenon is speculated to be associated with the transformation-induced plasticity (TRIP) effect, because X-ray diffraction experiments showed that austenite was transformed to stress-induced martensite at lower temperatures.3)
The difference in thermal properties between ferrite and austenite is important in mechanical processing at high temperatures because it causes residual stresses in the steels. For example, because the volume fraction and chemical composition of duplex stainless steels change during solidification and heat treatments, complicated residual stresses remain at room temperature and phase transformation may be enhanced.6,7) Regarding residual stresses, in situ synchrotron X-ray diffraction experiments and computational modeling have been performed to quantify lattice strains in different grains with specific orientations and associated intergranular residual stresses in an austenitic stainless steel under uniaxial tension.8) In this study, microscale internal stresses were characterized as intergranular and intragranular residual stresses in the steel and were associated with the mechanical properties.
In the abovementioned studies, residual stresses in the individual phases of the steels were primarily measured by the sin2ψ method using X-ray diffraction.9,10) Because the detailed elastoplastic properties and characteristics of dislocations in the microstructure of ferrite and austenite in duplex stainless steels were not necessarily considered in previous studies, the residual stress tensor in duplex stainless steels should be characterized coupled with the concept of crystal plasticity to understand the microscale elastoplastic properties of the steels. In particular, dislocations are typically vital to the mechanical properties of metallic materials; dislocations generated by external stresses are assumed to form internal or residual stresses in metals owing to complicated long-range interactions of dislocations.11)
Therefore, it is speculated that residual stresses remaining in the steels may affect the precision of metal forming and shaping of steels, in which stresses formed by external loading are transferred as inhomogeneous stresses into the steels in a complicated manner. Even if uniaxial loading is applied to an iron alloy, microscale inhomogeneous stresses are distributed in polycrystalline alloys.12,13) This implies that microscale stresses, referred to as Type III stress, are complicated, even in a small portion. In addition, it was reported that small portions of the microstructure are affected easily by external loadings, as observed in the stress-induced transformation of retained austenite in TRIP steels; this is because the transformation occurs easily at small grains owing to the effects of interfaces.14) This prompted us to characterize microscale changes in the microstructure and residual stresses in duplex stainless steels caused by plastic deformation. The specimen used in this study is a typical duplex stainless steel, in which the fractions of ferritic and austenitic phases are comparable and the banded microstructure is exhibited.15) The deformation texture in the microstructure and stress distribution were investigated to understand the relationship between the microstructural changes and residual stresses occurring during tensile plastic deformation.
The specimens used in this study were duplex stainless steel, which was as-received SUS329J3L manufactured by Nippon Yakin Kogyo Co., Ltd.. The chemical composition of the steel was Fe–0.013C–5.25Ni–22.42Cr–3.18Mo–1.00Mn–0.35Si–0.06Cu–0.0164N (in mass%). The microscopic composition of ferrite and austenite depends on the treatment and location; nevertheless, using electron microprobe analysis, the representative compositions were determined to be 28Cr–3.8Ni–3.0Mo and 22Cr–6Ni–1.5Mo, respectively.15)
The as-received specimens and tensile-deformed specimens were characterized using electron backscatter diffraction (EBSD) and residual stress measurements. The tensile-deformed specimens were prepared by deforming the as-received specimens by up to 15% via tensile tests. After successive tensile deformations, the crystal orientations with respect to the tensile axis of the specimens were measured via EBSD (Oxford HKL Channel5) attached to a scanning electron microscope (Hitachi SU6600).
Residual stresses after deformation up to 10% nominal strain in ferrite and austenite in the specimens were measured via the X-ray two-dimensional (2D) method (Bruker, D8 Discover with GADDS) using a 2D detector.16,17) The X-ray used was Cr Kα radiation, and the total reflected collimator for the X-ray was about 1.0 mm in diameter. In stress measurements, the diffraction peak at 156.0 degree from {211} of ferrite and the diffraction peak at 128.5 degree from {220} of austenite were used. Debye rings obtained via X-ray diffraction were employed to calculate the residual stress tensor in ferrite and austenite. The measured Debye rings were divided to twenty parts and fitted to representative values using the least-squares method. The residual stresses were calculated by assuming that the Young’s modulus was 193/210 GPa in ferrite/austenite and the Poisson’s ratio was 0.28/0.272 in ferrite/austenite.16) Strain tensors for the measured specimens were calculated from the measured normal stresses for ferrite and austenite.
Figure 1 shows inverse pole figure (IPF) maps of ferrites before deformation and after 15% deformation, which are represented as orientation with respect to the tensile direction. The IPF map images were obtained from the normal direction of the specimen. The microstructure of ferrite exhibited a banded structure with a thickness of 10–30 μm, and the grain size of ferrite was comparable to that of the band thickness. Portion A of ferrite in the microstructure was primarily surrounded by austenitic grains. The grain size of ferrite in duplex stainless steels is generally larger that of austenite, as far as they are heat-treated under a given condition. This is because the diffusivity of substitutional elements is higher in ferrite than in ferrite18) and the recrystallization rate is also high in ferrite.19)
IPF maps of (a) ferrite before deformation and (b) ferrite after 15% deformation, represented as orientation with respective to rolling or tensile direction, together with stereo-triangle showing crystal orientation in pseudo-color along tensile direction.
The yield stress of the duplex stainless steel was approximately 500 MPa, and work hardening was observed with deformation. The crystal orientation of the ferritic grains in portion A was almost unchanged by the 15% tensile deformation, and the microscale plastic strains of the ferrite appeared to be constrained by the surrounding austenite. By contrast, the ferritic grains in portion B were divided into different small grains by the 15% tensile deformation. The grains were rotated by uniaxial external loading, as will be discussed later.
If the duplex steel is plastically deformed by external loading, then the orientations of the polycrystalline grains in ferrite and austenite are assumed to be rotated to form a texture. Figure 2 shows the IPFs of the deformed specimen with respect to the tensile direction, together with the scale of the intensity of the orientation distribution. The measured area of the IPFs is the same as that of the ferritic grains shown in Fig. 1, which were deformed by 0%, 4.0%, 5.9%, 8.2%, and 15% nominal strains by tensile deformation. These results indicate that the distribution of the crystal orientations of the ferritic grains was gradually changed by deformation. In particular, the texture component near <103> increased with 15% tensile deformation, and the <101> texture component was slightly decreased by the tensile deformation. In plastic deformation of bcc metals and alloys, there are primary slip of (101)[111] and secondary slip of (101)[111], in which {101} planes exhibit relatively low resolved shear stress. If bcc crystalline grains have the initial tensile direction oriented close to [001], the grains are eventually rotated to [013] direction through these different slip systems during tensile deformation.
IPF changes in ferrite grains shown in Fig. 1 with respect to tensile direction due to crystal rotation by tensile deformation. <103> texture component in bcc structure is increased.
By contrast, the IPF maps of austenite before deformation and after 15% deformation in the duplex stainless steel are shown in Fig. 3, which was taken in the same portion as that shown in Fig. 1. The grains in austenite were finer than those in ferrite, as shown in Fig. 1, which corresponds to the relatively high recrystallization temperature of austenite. In the steel deformed more than 15%, it was difficult to identify the orientations of austenitic grains from Kikuchi patterns obtained via EBSD. Because grains close to phase boundaries between ferrite and austenite are plastically deformed in a complicated matter during tensile deformation. Although KAM maps by EBSD are sometimes effective to visualize the plastic strains, significant contrasts were not observed in this case. This is due to an increase in the dislocation density near the phase boundaries, which implies that the crystallinity of the grains was reduced by plastic deformation and prevented the observation of IPF maps via EBSD. A similar phenomenon of crystallinity reduction was observed in the ferritic grains.
IPF maps of (a) austenite before deformation and (b) austenite after 15% deformation, represented as orientation with respective to rolling or tensile direction, together with stereo-triangle showing crystal orientation in pseudo-color along tensile direction.
To investigate the texture changes caused by tensile deformation, the changes in the orientation of the austenite grains with respect to the tensile direction are shown in Fig. 4. The steels were strained because of tensile deformation by 0%, 4.0%, 5.9%, 8.2%, and 15% strains. These results were obtained from the area shown in Fig. 3. The results indicated that the <111> texture component in the austenite was slightly increased by tensile deformation up to 15%, which indicates that grains with orientations other than <111> were rotated to <111> by deformation.
IPF changes in austenite with respective to tensile direction due to crystal rotation by tensile deformation. <111> texture component in fcc structure increased slightly by tensile deformation up to 15%.
To demonstrate the crystal rotation and division of grains, a stereo-triangle was used to visualize the orientation changes of the grains. Figure 5 presents a stereo-triangle showing the crystal rotation of ferrite at portion B in the duplex stainless steel before deformation and after 15% deformation. These orientation changes indicate that the ferritic grains in portion B (ferrite) were rotated to the [111] orientation, whereas the austenitic grains in the neighboring portion B were distributed. It was speculated that this occurred because the slip direction of ferrite was the [111] orientation, and the maximum resolved shear stress was relatively low in the orientation of the tensile axis near portion B. Hence, the crystal orientation or texture change of austenite by plastic deformation is likely to be different from that of ferrite, since their slip systems are different. These microscale deformation characteristics in ferrite and austenite affect the residual stresses formed after tensile tests through mutual interactions of deformations in ferrite and austenite.
Example of stereo-triangle showing crystal rotation of ferrite at portion B in duplex stainless steel by 15% tensile deformation. (Online version in color.)
The phase fractions in the duplex stainless steel are important for the discussion of the mechanical properties because they affect the overall mechanical properties.8) The mechanical properties of duplex stainless steel have been investigated to determine the volume fraction of ferrite and austenite in steels using neutron diffraction measurement.20) The accuracy of the phase fraction determined by time-of-flight neutron diffraction was evaluated using model samples comprising ferritic and austenitic stainless steels. The analyzed volume fractions were almost consistent with the expected fractions with a small error. Hence, the technique used to evaluate the volume fraction is useful for discussing the properties. In addition, the present observation of the microstructure suggests that the size, shape, and morphology of the ferrite and austenite are important factors that affect plastic deformation. In this case, the initial fraction of ferrite and austenite was estimated to be approximately 56% and 44%, respectively. However, as complicated strains are evolved in the vicinity of phase boundaries by plastic deformation, the fraction of unidentified phase was increased by deformation.
When the mechanical properties of duplex stainless steel are discussed, the work hardening of ferrite is different from that of austenite.21) For instance, to investigate the characteristics of dislocation evolution in ferritic and austenitic stainless steels under tensile deformation, neutron diffraction line-profile analysis was performed.21) The experimental results showed that the work hardening of austenitic steel was higher than that of ferritic steel. The difference in the work hardening between the two steels was interpreted using the dislocation density estimated by line-profile analysis. The higher dislocation density of the austenitic steel was thought to originate from its lower stacking fault energy, and the difference in dislocation substructures between ferrite and austenite contributed to the work hardening.
Moreover, it has been reported that characteristic slips and thermally activated processes occur in ferritic alloys or bcc iron alloys.22,23) The orientation and temperature dependences of the flow stress of ferritic alloys have been extensively investigated in single crystals of solid-solution iron alloys over a wide temperature range. In these studies, it was clarified that the work hardening rate of ferrites is lower than that of austenite. Additionally, low work hardening was indicated in high-purity iron single crystals, which revealed a significant orientation and temperature dependence.24,25) In these studies, the characteristics of the low work-hardening rate were explained by the cross slips of screw dislocations and characteristic differences of screw dislocations and non-screw dislocations.
Therefore, because of the difference in the microstructure and microscale plastic deformation between ferrite and austenite, the residual stresses in duplex stainless steel should be investigated. In this study, the residual stresses after plastic deformation were characterized based on the residual stress tensor and stress ellipsoid.
3.2. Residual Stress Characterization Using 2D MethodFigure 6 exemplifies a diffraction pattern of the duplex stainless steel before deformation taken by Cr Kα radiation. Diffraction peaks at 156.0 degree from ferrite and at 128.5 degree from austenite were used in the present stress analysis. The strains measured by the 2D method were converted to the stress tensors coupled with parameters such as the elastic stiffness. The stresses measured using the 2D method were normal stresses of σxx, σyy, and σzz for the ferrite and austenite phases in the x-, y-, and z-axes, respectively. Here, σxx is defined as the stress in the x-direction, where the normal stress exerts on the plane parallel to the x-axis. As the stress values of σzz were small, plane stresses were assumed in the present case. The stress tensor data for ferrite and austenite of the specimens subjected to 0%, 5% and 10% strain obtained in this study are shown in Table 1. The σxx values measured in this study qualitatively showed that residual stresses in ferrite were compressive, whereas residual stresses in austenite were tensile with respect to the tensile direction. However, because the axis of the maximum stress is unknown, a method for visualizing the directions and magnitudes of normal stresses should be established.
X-ray diffraction pattern of the duplex stainless steel before deformation.
| strain | Stress tensor in ferrite (MPa) | Stress tensor in austenite (MPa) |
|---|---|---|
| 0% | ||
| 5% | ||
| 10% |
The measured stresses were converted to principal stresses σ1, σ2, and σ3 using matrix calculations. The Debye ring was divided into twenty parts and fitted to a value using the least-squares method. A three-dimensional residual stress tensor can be calculated by substituting the values of the diffraction angle at each point. The three-dimensional residual stress tensor was calculated using the values of the diffraction angle at each point and the diffraction angle. Table 2 summarizes the three principal stresses in ferrite and austenite for specimens deformed at 0%, 5%, and 10% nominal strain.
| Strains | Principal stress in ferrite (MPa) | Principal stress in austenite (MPa) | ||||
|---|---|---|---|---|---|---|
| σ1 | σ2 | σ3 | σ1 | σ2 | σ3 | |
| 0% | −25.9 | 9.8 | −1 | 63 | 19.3 | −0.4 |
| 5% | −269.6 | 129.8 | −6.2 | 142.3 | −50.5 | 0.5 |
| 10% | −246.8 | 122.8 | −7 | 151.1 | −47.6 | −5.2 |
Subsequently, a stress ellipsoid was drawn based on the residual stress tensor and principal stress value.16,17) The stress ellipsoid exhibited the magnitude (axis length of the ellipsoid), direction (arrow direction), and type (inward and outward arrows showed the compressive and tensile stresses, respectively) of three principal stresses σ1, σ2, and σ3. As the principal stress σ3 was almost negligible, the stress ellipsoid was approximated as an ellipse. Figure 7 shows the ellipses of principal residual stresses in ferrite in the duplex stainless steel after 0%, 5%, and 10% deformation. Similarly, residual stresses in austenite strained by plastic deformation were obtained, as shown in Fig. 8, which represent ellipses of principal residual stresses in austenite after 0%, 5%, and 10% deformation. The principal stresses σ1 in ferrite were compressive, whereas those in austenite were tensile. Although the direction of σ1 in ferrite differed from that of σ1 in austenite, the stress balance between ferrite and austenite appeared to be reasonable. These residual stresses were likely to be due to the microscale plastic deformation of grains in ferrite and austenite.
Ellipsoidal representation of principal residual stresses (in MPa) in ferrite after 0%, 5%, and 10% deformation. The tensile direction and transverse direction are x-axis and y-axis, respectively. (Online version in color.)
Ellipsoidal representation of principal residual stresses (in MPa) in austenite after 0%, 5%, and 10% deformation. The tensile direction and transverse direction are x-axis and y-axis, respectively. (Online version in color.)
In a recent investigation, the crystal plasticity modeling of image-based ferritic–austenitic duplex steels was performed to predict cleavages using micromechanical modeling.26) The multiphase crystal plasticity implementation successfully reproduced the experimental data from macroscopic tensile tests, which were constrained by microstructure scale measurements. Although dislocation characteristics were described in this study, crystal rotations in ferrite and austenite were not characterized in detail. By contrast, residual stresses in centrifugally cast duplex stainless steels were measured by pulsed neutron diffraction, and the mechanism of occurrence of the phase stress distributed in the steels was discussed. The measured stresses in this study are likely to correspond to Type I or Type II residual stresses in more wide ranges, where dislocation characteristics were not sufficiently considered.7) Therefore, although the approach in the present study is not similar to those of previous studies, it indicated that the residual stress ellipsoid or ellipse in different phases can conveniently depict the residual principal stresses in ferrite and austenite. Furthermore, it can facilitate the discussion of the deformation mechanism from the viewpoint of lattice defect, which is one of the possibilities to interpret the residual stresses in steels with different phases. Hence, multiscale or interdisciplinary characterization is also required to understand the mechanical properties of steels in the future, as dislocation characteristics were taken into in studies of high-strength pearlitic steels27,28) and stainless steels.29,30)
EBSD and residual stress measurements were performed to characterize microscale changes in a duplex stainless steel composed of ferrite and austenite caused by plastic deformation. The main conclusions obtained were as follows:
(1) The duplex stainless steel exhibited a banded microstructure comprising coarse ferritic grains and fine austenitic grains. These grains were plastically deformed depending on the orientation of the phases with respect to the tensile axis.
(2) The orientation distribution of the ferrite was changed to increase the <103> texture component by uniaxial tensile deformation. This is primarily due to the slip characteristics of ferritic iron alloys.
(3) The orientation distribution of austenitic grains changed and slightly increased the <111> texture component. This is contrary to the microscopic plastic deformation or slip of dislocations in ferrite.
(4) A procedure was established to visualize residual stress ellipsoids from residual stress tensors measured using the 2D method. It was shown that compressive stresses remained in ferrite after tensile deformation, whereas tensile stresses remained in austenite with respect to the tensile direction. These principal residual stresses were reflected by the characteristics of dislocations distributed in ferrite and austenite, which may be due to the difference in work hardening between the phases.
The authors acknowledge the support from Grant-in-Aid for Scientific Research by the Japan Society for the Promotion of Science (20H02574). They also would like to thank Dr. H. Todoroki for providing the duplex stainless steel for this study.
