Abstract
The strain induced martensitic transformation (SMT) of the austenite stainless steel (SUS 304) under cyclic loading and static loading was investigated directly using electron backscattered diffraction. Two different SMT characteristics are observed, which are attributed to the differences of plastic and twinning deformation. The maximum cyclic stress has a strong influence on the SMT. The total area fraction of the Fe-α′ phase increases significantly when the maximum cyclic load is >80 σUTS. In other words, the SMT is apparently absent when the samples are loaded with less than σmax = 70 σUTS, although such samples are fractured completely. Moreover, there is a clear R ratio effect on the SMT. For example, the loading condition R = −1 gives rise to a strong SMT compared to R = 0·1 due to the more severe strain caused by the compressive stress. In contrast, no clear frequency effect (1 versus 30 Hz) on the SMT is detected, which may be attributed to the same maximum cyclic stress. Like the SMT characteristics under cyclic loading, the proportion of Fe-α′ phase shows no clear increase until the sample is loaded statically to a tensile stress <70 σUTS.
Keywords
Introduction
In recent years, austenitic stainless steels have been employed for various engineering applications on account of their excellent machinability, high corrosion resistance, high strength and high ductility. In this instance, austenitic stainless steels have been utilised for the components in power plants and automobile industries. Many components made of stainless steel are produced by the severer plastic deformation via stretching, drawing and bending processes. Holmberg and Thilderkvist 1 have investigated the influence of material properties and stamping conditions on the stiffness and static dent resistance of various steels, including stainless steels. With plastic deformation of stainless steels, grain slipping and twinning deformation occur. Furthermore, twins play an important role in the nucleation and growth dynamic recrystallisation during hot deformation. 2 It is considered that a study of the strain characteristics in stainless steels is of considerable importance. To date, a number of experiments have been carried out to observe the strain region in stainless steels using electron backscattered diffraction (EBSD), 3 etching techniques and photoelastic stress analysis. 4
Austenite structures in the stainless steels are metastable and caused by the presence of Cr (18) and Ni (8), both of which lead to a strain induced martensitic transformation (SMT) when they are subjected to plastic deformation. In this case, face centred cubic to α prime martensitic transformation occurs. 5 Müller-Bollenhagen et al. 6 reported that metastable austenitic stainless steels are known to undergo a partial transformation of austenite into martensite as a consequence of plastic deformation, and this transformation process can be attributed to the chemical composition, the accumulative strain as well as the strain rate, the temperature and the microstructure.
Because the material properties of the stainless steel are altered by SMT, an examination of the SMT characteristics could be significant. Until now, several researchers have systematically studied the SMT characteristics of austenitic stainless steels, along with relevant information used to explain their material characteristics. The volume fraction of the martensite transformation depends on the strain behaviour. 7 The mechanisms of α prime martensite transformation have been investigated on the basis of a comparison of the cohesive energy, in which the energy of the transient lattices is estimated by use of the embedded atom method potential for Fe. 8
It has been reported that the SMT of SUS 304 stainless steel occurs during the loading process, and this occurrence is verified using the EBSD technique. The occurrence of SMT takes place under static tensile loading accompanied by low level plastic strain. 9 In addition, the area fraction of SMT does not increase significantly until the sample is loaded to its ultimate tensile strength (UTS). After the UTS point, the total fraction of SMT increases rapidly to the fracture point. The effect of the stress state on the transformation kinetics of stainless steel 301LN sheets at room temperature has been investigated by Beese and Mohr. 10 It appeared that the martensitic transformation kinetics cannot be described solely by a monotonically increasing function of stress triaxiality. For instance, less martensite is developed under equibiaxial tension than under uniaxial tension for the same increment in equivalent plastic strain. 10 Eto et al. 11 examined the influence of residual compressive stress on the SMT after a shot peening process; however, no clear effect was detected. The reason for this is the reduction in the residual stress caused by the shot peening. The effects of SMT on the fatigue behaviour (rotating bending fatigue) of a type 304 stainless steel in laboratory air and in 3NaCl solution were investigated by Nakajima et al. 12 In their work, specimens subjected to tensile prestrains at ambient temperature and −25°C were employed, and SMT occurred to a much greater degree at −25°C than at ambient temperature.
Because many engineering components are cyclic loaded in service, information about the fatigue properties is of special significance in the process of designing components for engineering applications. 13 This is especially true, since >90 of failure is caused by fatigue. Although the fatigue properties of the stainless steels have already been thoroughly investigated, there appears to be little information regarding their SMT characteristics, particularly under cyclic loading. Thus, the aim of the present study is to investigate the SMT characteristics of an austenite stainless steel under cyclic loading at various conditions.
Experimental
The present work uses a commercial austenitic stainless steel (SUS 304) plate, 1 mm thick, made by hot rolling process. The chemical composition of the SUS 304 is Fe–Ni8–Cr18–S0·03–C0·08 (wt-). Specimens were machined into the form of rectangular dumbbell shapes with two notches, 0·6 mm deep (see Fig. 1). The notches were machined by a wire electric discharge machine. The specimens were annealed at 1150°C for 2 h and air cooled to room temperature before the investigation of the SMT characteristics.
Schematic illustration of test specimen
In order to examine the properties of SMT, mechanical loading was applied to the samples under various conditions using an electroservo hydraulic system with 50 kN capacity. In this approach, tensile static loads and tensile–tensile cyclic loads were applied. The static loads were applied at a loading speed of 1 mm min−1 to several specific loading points, e.g. σ: 50–100σUTS, σn: [(σUTS−σf)/2+σf] and σf: (fracture point). Figure 2 shows the relationship between the tensile stress (σ/σUTS) and the tensile strain (ϵ/ϵf) for the SUS 304 stainless steel, obtained using the specimen described. Several stress levels, indicated on the stress–stain curve by the arrows, are the measurement points at which the SMT characteristics were examined. This examination was carried out beside the notch tip, i.e. in the high stress concentration area (see Fig. 1). In the fatigue tests, cyclic loading was performed with various values of maximum stress, frequency and R ratio (σmin/σmax), namely, σmax of 50–100 σUTS, frequency of 1 and 30 Hz and R ratios of −1 and 0·1. The applied load and strain values were measured by a commercial load cell and a strain gauge respectively.
Tensile stress–tensile strain curve for SUS 304 stainless steel (σ/σUTS versus ϵ/ϵf), showing SMT measurement points
The SMT characteristics were examined by EBSD analysis using a high resolution electron JSM-7000F microscope (JEOL Ltd). In the present analysis, the phase textures (Fe-α′ and Fe-γ phases) were examined using HKL Channel 5 software. Before the measurements, the sample surfaces were polished for ∼2 h to mirror level in a vibropolisher using colloidal silica. The main measurement conditions for the present analysis were as follows: accelerating voltage of 15 kV, beam current of 5 nA and measurement step size of 1 μm.
Results and discussion
Static loading tests
Figure 3 shows the inverse pole figure (IPF) maps of the specimens after loading to the specific stress levels as shown in Fig. 2. The colour level of each pixel in the crystal orientation map is defined according to the deviation of the orientation measured from the normal direction. The large austenite phases are the dominant feature in the sample when the sample is loaded to an extent less than its UTS. Interestingly, twin deformation and a more severe plastic deformation zone are observed, especially in the 80 and 100 σUTS samples. More severe plastic deformation is observed in the σn and σf samples, where the grains are seen to have collapsed completely. Such a change of microstructure could result in different SMT characteristics.
Inverse pole figure (IPF) maps of specimens after tensile stress, loaded to specific stress levels, as shown in Fig. 2
Figure 4 displays the orientation imaging microscopy (OIM) maps after the tensile tests, showing the Fe-α′ phase and Fe-γ phase. On the basis of the OIM maps, variations of the area fraction of the Fe-α′ and Fe-γ phases as a function of the tensile stress are summarised in Fig. 5. It is clear that the texture of the Fe-γ is mainly formed in the sample near the notch tip when it is loaded with <70 σUTS; in this region, the total area fraction of Fe-γ is ∼80. Those phase formations are very similar to that in the samples after the annealing process without any applied loads. From this, even if a relatively high stress is present in the sample, the stress induced martensite transformation is weak. A slight increase in the SMT was obtained in the 80 σUTS and 100 σUTS samples, where several Fe-α′ phase colonies are affected as shown in Fig. 4. This is due to more severe deformation and twinning deformation (Fig. 3). 6 The total area fraction of the Fe-α′ phase apparently increases for the samples when loaded to the σn and σf points. A particularly strong texture of the Fe-α′ phase is detected, especially in the σf sample (50Fe-α′). From the IPF maps, the SMTs in the σn and σf samples can be attributed to more severe plastic deformation. It is briefly concluded from this result that the strain induced martensite formation occurs with two different processes, i.e. plastic deformation and twinning deformation.14,15 In addition, this occurrence is quite clear in samples loaded statically with >80 σUTS. Similar SMT characteristics have been observed in a previous report on SUS 304 stainless steel. 9
Orientation imaging microscopy maps after tensile tests, showing Fe-α′ and Fe-γ phases
Variation of area fraction of Fe-α′ and Fe-γ phases as function of tensile stress, obtained on basis of results of Fig. 4
Cyclic loading tests
Figure 6 shows the relationship between the stress amplitude and cyclic number to failure for the samples tested at R = 0·1 and 30 Hz, i.e. the S–N curve. Note that the arrows in this curve indicate specimens that did not fail within 10
7
cycles, reaching the endurance limit. In this approach, the specimens were fractured at the different values of maximum stress based on its UTS, σmax = 40, 50, 60, 70, 80 and 100 σUTS, as pointed out in Fig. 6. The S–N relationship was also evaluated using 
(the power law dependence of cyclic stress and cycles to failure), where σa is the stress amplitude, Nf represents the cycle number to the final fracture, σf is the fatigue strength coefficient and b is the fatigue exponent. The values of σf and b, obtained with σf = 1072·9 MPa and b = −0·163, are relatively similar to those for the related stainless steel (AISI304).
16
Relationship between stress amplitude and cyclic numbers to failure (30 Hz, R = 0·1)
Figures 7 and 8 present the IPF maps and OIM maps of the specimens after the fatigue tests, with cyclical loading at R = 0·1 and 30 Hz. In addition, on the basis of the OIM maps, variations of the area fraction of Fe-α′ phase and Fe-γ phase are indicated in Fig. 9. Although all the samples are fractured completely by the cyclic loading, weak SMT is obvious, especially in the sample fractured at the maximum stress of <70 σUTS. In contrast, more severe SMT is obtained in the samples loaded with >80 σUTS. This trend may be similar to that for the samples loaded under static tensile loading (Fig. 5). It is considered from the IPF maps (Fig. 7) that the SMT is affected by the more severe deformation (plastic and twinning deformation). To verify the severity of plastic deformation, a microstructural observation was carried out. Figure 10 depicts optical micrographs of the samples near the fatigue crack, which fractured at the maximum cyclic loads of 60 σUTS and 80 σUTS, as pointed out on Fig. 6. The slip planes can be seen clearly in the grains for both samples (dark region), although those slips are most evident in the sample loaded to the higher applied stress of 80 σUTS. This result suggests that the extent of plastic deformation directly reflects the severity of the SMT, as shown in Fig. 9. A similar approach has been carried out by Nakajima et al. 12 In their work, the strain induced martensite formation in SUS 304 stainless steels was investigated in the associated sample, beside fatigue cracks, but no clear SMT was detected. This may be affected by the lower maximum cyclic stress, <70 σUTS.
Inverse pole figure (IPF) maps of specimens after fatigue tests
Orientation imaging microscopy maps after fatigue tests
Variation of area fraction of Fe-α′ and Fe-γ phases as function of tensile stress, obtained on basis of results of Fig. 8
Optical micrographs of samples 60 σUTS and 80 σUTS after fatigue tests
The influence of the cyclic stress on the SMT was further examined at σmax: 60 σUTS and 80 σUTS with different R ratios (−1 and 0·1) and frequency F (1 and 30 Hz). Figure 11 displays the OIM maps for the samples fractured using the different loading conditions. It is clear that the strong texture of the Fe-γ phase (>84·0Fe-γ) is detected for all samples with σmax = 60 σUTS whatever loading conditions of R ratio and frequency were applied to them. On the other hand, the overall phase texture has clearly become Fe-α′ phase for the 80 σUTS samples, especially at R = −1. The area fraction of the Fe-α′ phase in the sample with 80 σUTS and R = −1 is 50·2, which is ∼32·6 higher than that for the sample with R = 0·1. Such a strong SMT in this sample (80 σUTS and R = −1) is affected by the compressive loading. In our previous work, 4 more severe SMT is detected in the related stainless steel around the dent zone, formed from the high compressive stress.
Orientation imaging microscopy maps after fatigue tests under various cyclic loading conditions
Conclusions
An SMT in SUS 304 stainless steel was examined under various loading conditions using EBSD analysis. The results obtained were as follows.
The total area fraction of the Fe-α′ phase increases when the static tensile stress applied is >80 σUTS of the appropriate sample value. Twinning and plastic deformation are affected. After tensile loading with more than σUTS, i.e. σn and σf, the area fraction of Fe-α′ phase increases rapidly and extensive plastic deformation is detected.
The SMT cannot be detected clearly in the sample fractured by cyclic loading at the maximum stress <70 σUTS, whereas more severe SMT is obtained in the samples cyclically loaded with more than σmax = 80 σUTS. This trend is similar to that obtained in the samples loaded by the static tensile stress.
The influences of fatigue conditions (R ratio and frequency) on the SMT characteristics have been examined. There is an R ratio effect, especially at R = −1, on the SMT, due to the compressive stress. On the other hand, there are no clear frequency effects (1 versus 30 Hz) on the SMT.