PSI - Issue 23

Ladislav Poczklán et al. / Procedia Structural Integrity 23 (2019) 269–274 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

270

2

Nomenclature ε a,eq

total equivalent strain amplitude axial strain amplitude torsional strain amplitude total crack length on surface

ε a γ a

2a

N number of cycles da/dN crack growth rate a i

extrapolated crack length to zero cycle

k g

crack growth coefficient

1. Introduction Austenitic stainless steel 316L is widely used in many structural applications (e.g. food processing, oil and chemical industry, pharmaceutical equipment etc.). The main reasons of the success of this material are its excellent corrosion resistance and good mechanical properties at wide range of temperature. Since numerous structural components are subjected to cyclic loading, it is necessary to know the cyclic behaviour of the material. Most of the fatigue studies have been done in uniaxial tension/compression mode (e.g. Vogt et al., 1991, Li et al., 1994 and Alain et al., 1997), whereas most of the components are loaded by combined forces. It is therefore important to study fatigue response in other modes, too (J acquelin et al., 1983, Mazánová et al., 2017) . The understanding of fatigue crack nucleation and propagation can be very useful for life prediction of the material as well as for assessment of maintenance intervals. A standard for the chemical composition of 316L allows sort of some variation of an element content (e.g. an amount of Ni is allowed to be between 10 – 14 wt. %). Since deformation mechanisms of 316L steel are strongly dependent on its chemical composition, there can be differences in deformation mechanisms among 316L steels with different chemical compositions. A planar dislocation glide is usually observed. When the stacking fault energy (SFE) is higher, dislocation cross slip leads to an evolution of a low energy 3D dislocation structures (Gerland et al., 1989, Lu et al., 2016). When the SFE is lower, a deformation twinning likely appear. T etragonal α ` martensite can be developed as well and finally, laths of hexagonal ε martensite were also observed in 316L steel at low temperature (Kruml et al., 2000).

2. Material and methods

Experimental material in form of hot rolled sheet was provided by Acerinox Europa. The microstructure was formed by equiaxial austenitic grains with average size 40 µ m and by delta ferrite bands. The chemical composition is given by Tab. 1.

Table 1. The chemical composition of 316L steel in wt. %. Cr Ni C Mo Mn

Si

N

P

S

Fe

16.631

10.000

0.018

2.044

1.261

0.380

0.042

0.032

0.001

bal.

Specimens used in this study have hollow cylindrical gauge section (see schematics in Fig. 1). The inner hole was fabricated by spark erosion. The first type of specimens, shown in Fig. 1a), had a smooth surface and was used for studying fatigue crack initiation mechanism. The specimen used for measuring the crack growth rate is shown in Fig.1 b). This one had a notch in form of artificial hole in the middle of the gauge part. This notch was fabricated by spark erosion and its diameter was approximately 300 µm . Both types of specimens were machined from the plates without any following heat treatment. For reaching a surface suitable for further observations, the external surface of the gauge part was mechanically and electrolytically polished.