High-cycle fatigue behavior of ultrafine-grained austenitic stainless and TWIP steels

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Abstract

High-cycle fatigue behavior of ultrafine-grained (UFG) 17Cr–7Ni Type 301LN austenitic stainless and high-Mn Fe–22Mn–0.6C TWIP steels were investigated in a reversed plane bending fatigue and compared to the behavior of steels with conventional coarse grain (CG) size. Optical, scanning and transmission electron microscopy were used to examine fatigue damage mechanisms. Testing showed that the fatigue limits leading to fatigue life beyond 4 × 106 cycles were about 630 MPa for 301LN while being 560 MPa for TWIP steel, and being 0.59 and 0.5 of the tensile strength respectively. The CG counterparts were measured to have the fatigue limits of 350 and 400 MPa. The primary damage caused by fatigue took place by grain boundary cracking in UFG 301LN, while slip band cracking occurred in CG 301LN. However, in the case of TWIP steel, the fatigue damage mechanism is similar in spite of the grain size. In the course of cycling neither the formation of a martensite structure nor mechanical twinning occurs, but intense slip bands are created with extrusions and intrusions. Fatigue crack initiates preferentially on grain and twin boundaries, and especially in the intersection sites of slip bands and boundaries.

Introduction

In recent years, advanced ultrafine-grained (UFG) materials with a mean grain size as small as 1 μm have increasingly captured the industry's attention due to their highly enhanced mechanical properties [1], [2]. Of the several techniques that have been employed to produce ultrafine-grained materials, one of the most widely applied techniques for the refining of grain size is equal-channel angular pressing (ECAP) that involves severe plastic deformation. However, even though ECAP has been extensively used to produce UFG non-ferrous materials with medium and high stacking fault energies (SFE) at a laboratory scale, such as Cu [3], Ti [4] and Al [5], the processing of high strength steel materials is more difficult and rare.

In order to develop fine-grained steels for a mass production, reversion treatment of strain-induced martensite has been applied to obtain nano/submicron-grained austenitic Cr–Ni stainless steels from commercial grades such as Type 304L, 301 and 301LN [6], [7], [8], [9], [10]. This simple route involves heavy cold rolling (50–75%) of a metastable stainless steel to create a considerable fraction of stress-induced martensite, followed by a short annealing treatment to revert the martensite into fine-grained austenite either through a martensitic shear or diffusional reversion mechanisms [6], [7], [8], [11].

Austenitic steels with a high Mn content (15–30%, all concentrations hereafter are in wt%) and various amounts of C, Al, and Si may possess the twinning-induced plasticity effect (TWIP) resulting in excellent tensile strength–ductility combinations due to intense mechanical twinning in straining. Therefore these steels have the potential to provide applications for structural and safety components in the automotive industry [12], [13], [14], [15], [16]. Several investigations have been carried out to determine the mechanical properties of high-Mn TWIP steels in quasi-static and dynamic tensile loading at low and ambient temperatures. Investigations on the hot deformation mechanisms and recrystallization kinetics of these steels bearings with various Al content have recently been performed by Hamada et al. [17], [18], [19]. Furthermore, the present authors have studied the fatigue behavior of three TWIP steels with a Mn content between 16% and 22% and Nb or Al alloying, each having a relatively large grain size [20]. In TWIP steels, the grain refinement to a submicron level can be obtained quite readily by controlling the recrystallization kinetics of cold rolled steels, as shown by Ueji et al. [21] and Bracke et al. [22]. The fine-grained structure of Fe–31Mn–3Al–3Si steel with a 1.8 μm grain size was obtained after 88% cold rolling and subsequent annealing at 700 °C for 30 min [21]. Bracke et al. [22] found that annealing at 700 °C for 2 min of the 50% cold rolled Fe–22Mn–C steel resulted in a grain size of 2.5 μm. Recently, de las Cuevas et al. [23] manufactured a fine grained structure of the 0.6C–22Mn steel with a equiaxed grain size of 1.5 μm by heating 60% cold rolled sheet at 700 °C for 9 min. The impaired ductility is a serious problem in UFG carbon steels, but both UFG 301LN and UFG TWIP steels possess enhanced strength while retaining their excellent ductility [9], [16].

Fatigue resistance is one of the key properties concerning the practical utilization of ultrafine-grained steels. To date, a considerable amount of research has been focused on the fatigue behavior of nanocrystalline and UFG non-ferrous materials (e.g. [3], [4], [5], [24], [25], [26]). It is well known that a small grain size will have a positive effect on the yield and tensile strength while generally improving fatigue strength, particularly in low SFE materials, by retarding the crack initiation. The enhanced resistance to crack initiation is considered as a result of high strength of fine grains that act as potential obstacles for dislocation movement, and thereby the formation of microcracks is sluggish [27], [28]. Otherwise, fine grain size may adversely affect the resistance of the fatigue crack growth, as cracks can sometimes propagate along grain boundaries [29].

Furuya et al. [28] investigated the fatigue strength of a series of UFG ferrite–cementite steels with a ferrite grain smaller than 1 μm. It was found that the fatigue limit ratios (fatigue limit FL/tensile strength TS) increased from 0.43 to 0.53. The fatigue behavior of UFG austenitic steels on the other hand has rarely been investigated, however. Di Schino et al. [30], [31] studied the influence of grain refinement between 1 and 50 μm on the fatigue strength of two austenitic stainless steels, Type 304 and high-N Cr–Mn. It was found that grain refining has a strong effect on the fatigue strength of Type 304 steel. However, grain refining has only a slight effect on the fatigue resistance of high-N Cr–Mn steel due to the formation of slip bands promoted by nitrogen alloying.

Other studies have shown surface nanocrystallization to improve the fatigue strength of materials [27], [29], [32], [33], [34], [35], [36], especially in a high-cycle regime. For instance, Roland et al. [33] investigated AISI 316L steel after surface mechanical attrition treatment (SMAT) and measured a 21% improvement in the fatigue limit. Uusitalo et al. [35] reported a significant improvement in the fatigue resistance of 316L and 301LN after SMAT. These investigations showed the fatigue limit increasing from about 300 MPa (Type 316L) and 350 MPa (Type 301LN) to around 500 MPa. However, SMAT generates compressive residual stress in the surface layer and therefore corrosion-performance of stainless steels can be impaired in corrosive environments [37]. On the other hand, the reversion annealing technique can produce bulk material of homogeneous UFG structure with the absence of internal stresses. Consequently, the corrosion resistance can be improved, as reported in a previous study [38].

To date, very few studies focusing on the fatigue damage mechanisms of fine-grained stainless and TWIP steels have been published. Hamada et al. [20], [39] have investigated the behavior of CG high-Mn TWIP steels in a high-cycle regime (fatigue life beyond 104 cycles). Low-cycle behavior has been investigated by Niendorf et al. [40], [41]. A relatively high fatigue strength was observed, even though it seems that the TWIP mechanism does not operate in cyclic straining. The motivation of the current study is to obtain more data and create an understanding on fatigue behavior of these two types of steel allowing comparisons to be made between their conventional CG counterparts.

Section snippets

Material preparation

Two types of austenitic steels were used in this study. The first consisted of a commercial Type 301LN sheet in an annealed-state with 5 mm thickness, supplied by Outokumpu Oyj, Tornio Works (Tornio, Finland). The second was a high-Mn TWIP steel that was melted down at a laboratory scale within vacuum and then hot rolled to a thickness of 5 mm at Centro Sviluppo Materiali (Rome, Italy). The compositions of the two steels are given in Table 1. The TWIP steel is coded as 6C22Mn based on its C and

Microstructures and grain sizes

After 50% cold rolling reduction, the martensite content of 301LN measured by a Ferritescope was 87%. Upon annealing at 800 °C for 10 s, the martensite reverted back to fine-grained austenite. In this steel, the reversion transformation is essentially a diffusion-controlled process as confirmed by previous studies [9]. Fig. 1a presents a typical microstructure of the annealed 301LN. This microstructure consists of a mixture of fine reverted austenite grains; with an average size of 0.75 μm;

Conclusions

High-cycle fatigue of two ultrafine-grained austenitic steels, 17Cr–7Ni (Type 301LN) stainless steel and high-Mn (Fe–0.6C–22Mn) TWIP steel, was investigated using reversed bending loading and compared to the behaviors of coarse-grained counterparts. The main results and conclusions are as follows:

  • (1)

    The reversion annealing (800 °C, 10 s) of 50% cold rolled 301LN steel is very efficient in promoting an ultrafine-grained structure with a grain size of about 0.75 μm. In TWIP steel, the average grain

Acknowledgements

Mr. Tero Oittinen is gratefully thanked for his assistance in SEM-EBSD observations. The section involving work on the TWIP steel was carried out within the project RFSR-CT-2005-00030 which was funded by the Research Fund for Coal and Steel, The European Commission, Brussels.

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