Thermal fatigue crack growth in stainless steel M. Kadlec a, * , P. Hau  sild a , J. Siegl a , A. Materna a , J. Bystrianský b a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Materials, Trojanova 13,120 00 Praha 2, Czech Republic b Institute of Chemical Technology in Prague, Faculty of Chemical Technology, Department of Metals and Corrosion Engineering, Technická 5,166 28 Praha 6, Czech Republic article info Article history: Received 12 January 2011 Received in revised form 12 December 2011 Accepted 11 July 2012 Keywords: Austenitic stainless steel Thermal fatigue Crack growth model abstract A judgment of residual service life of engineering parts exposed to thermal fatigue makes it possible to deal with economic and safety issues in power plants. The aim of this study is to analyze a fatigue crack initiation and propagation in A321 stainless steel bodies subjected to repeated thermal shocks. For this purpose, various methods of crack propagation monitoring were used. The first stage of experiments included mechanical cyclic loading of specimens with the central notch at fixed temperatures ranging from 20  C to 410  C. The crack growth rate was only minimally influenced by temperature in this case. Thermal loading of the same specimens with DT varying from 150  C to 340  C showed very rapid crack initiation in the notches and its asymmetric growth. Metallographic and fractographic analyses of failed specimens were carried out after 1000, 3000 and 6000 thermal cycles. The comparison of the fracture surface micromorphology confirmed the similarity in the mechanism of the thermal and mechanical fatigue crack growth. Stress analysis using the finite element method consisting of transient thermal and mechanical solutions was performed in order to simulate the experiments. Thermal fatigue crack growth assessment was carried out on the basis of the experiments and the computed thermally induced stress intensity factors. This model successfully confirms the discussed analogy of thermal and mechanical stress induced damage. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Several parts of power plants and chemical facilities are exposed to repeated thermal shocks. These shocks occur, for example, when a low temperature fluid strikes a hot steam generating surface. The restraint of thermal expansions and contractions of material caused by boundary conditions results in strains and stresses in the material. Depending on the size and speed of thermal changes a fatigue crack can be initiated. Extensive work [1] studied thermal fatigue of different austen- itic and duplex stainless steels and confirmed striation forma- tions as a major crack growth mechanism. Cracks initiated from persistent slip bands. The surface residual stresses changed markedly during the first load cycles. The dislocation density was lower than expected based on the mechanical fatigue data. Ref. [2] investigated differences between mechanical uniaxial and thermal fatigue loading conditions and showed that initia- tion is faster in thermal fatigue than in uniaxial iso-thermal fatigue for the same levels of strain. Ref. [3] dealt with thermally induced crack networks investigated by an image analysis. The work proved shielding effect of the longer cracks. Furthermore, the surface can be damaged by a corrosion which can be involved in the crack initiation [4]. Authors in work [5] studied three AISI steels and proved no material differences in the terms of crack initiation nor the fracture micromorphology. Study [6] indicated that iso-thermal fatigue propagation data can be used to predict crack propagation rates provided that iso- thermal data are taken from the temperature at which the mechanical load peaks are known. Modeling of experiments using finite element method (FEM) studies were carried out as well, e.g. in [7], where authors reliably estimated the number of cycles to initiation, using standard iso-thermal fatigue life curves. Use of an analytical method based on handbook K solutions for elastic stress distributions produced overly conservative estimations. This paper presents the results from an experimental and numerical investigation of thermal fatigue cracks in the specimens exposed to repeated thermal shocks. The work continues with studies which are described in Ref. [8,9]. The experimental data and the thermal fatigue crack growth model based on the FEM simu- lations can be a useful guide for the permissible crack size assess- ment of the engineering parts exposed to the thermal fatigue. * Corresponding author. Tel.: þ420 225 115317. E-mail address: martin.kadlec@fjfi.cvut.cz (M. Kadlec). Contents lists available at SciVerse ScienceDirect International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp 0308-0161/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijpvp.2012.07.005 International Journal of Pressure Vessels and Piping 98 (2012) 89e94