Fatigue and cyclic deformation behavior of brazed steel joints M. Koster n , C. Kenel, A. Stutz, W.J. Lee, A. Lis, C. Affolter, C. Leinenbach Empa—Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland article info Article history: Received 22 March 2013 Received in revised form 17 May 2013 Accepted 22 May 2013 Available online 14 June 2013 Keywords: Brazing Fatigue Martensite Electron microcopy Finite element method Strain measurement abstract To investigate the fatigue assessment of brazed steel joints, stress controlled fatigue tests were conducted with specimens of the steel AISI CA 6-NM (1.4313) and with its brazed joints. Brazing was performed in a shielding gas furnace under H 2 atmosphere with Au 18wt% Ni as filler metal. Experiments were performed at a load ratio of R ¼0.1 with different specimen geometries to compare their fatigue behavior and to investigate the failure mechanism. The results of the experiments—based on a lifetime oriented approach—show the existence of two different regimes depending on the number of cycles to fracture (N f ). For N f o10 4 the maximum tolerable loads for all specimens approach the ultimate tensile strength of the substrate material, whereas for N f 410 4 the substrate material provides the highest strength, followed by the brazed round specimens and by the brazed T-joint specimens. Investigations on the failure mechanisms revealed that for brazed specimens, fatigue and residual fracture occurred always in the interface of the brazing zone. The crack path is characterized by interfacial jumps, accompanied by ductile deformation features. The analysis of the strain evolution during the cyclic loading experiments shows that the cyclic deformation behavior is significantly influenced by cyclic creep. Furthermore, the experiments show that brazed round specimen exhibit higher strains at similar loading amplitudes, compared to the substrate material. These new findings were also confirmed by FE-calculations, showing an inhomogeneous distribution of local stresses and strains in the proximity of the braze layer. The archived results show the complex interactions of a braze layer on the cyclic deformation behavio—compared to its bulk material—and lead to a better understanding of the fatigue assessment of brazed steel joints. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In the recent years, brazing has gained increasing importance as a joining technology for many seminal applications as e.g. in chemical engineering, power generation and for the production of power electronic components [1–3]. Brazing generally plays an essential role as a favorable joining technology because the thermal stresses of the joining partners are significantly reduced compared to e.g. welding. Furthermore, brazing allows joining dissimilar materials as e.g. metals and ceramics at fast process times. Generally, brazing is performed by heating an assembly over the melting point of a filler metal, which is placed between two plates of substrates material, without reaching the melting point of the substrate material. The liquid filler metal wets the surfaces of the substrate material and fills the joint gap. Subsequent adhesion and diffusion processes during the cooling of the assembly significantly influence the final joint strength. The general differentiation between soldering and brazing is made according to the process temperatures used for the joining process. Joining at T o450 1C is referred to as soldering, whereas using filler metals with T m 4450 1C is named brazing. With the use of advanced furnace brazing methods, as e.g. high temperature (HT) brazing at T 4900 1C in vacuum or with a shielding gas, especially brazing of steel structures becomes more economical and efficient [4]. The specimens investigated in the current work represent classical HT brazed components. They consist of Au-18 wt% Ni as filler metal and of the steel AISI CA 6-NM (X3 CrNiMo 13-4) as substrate material. The substrate material is a typical representative of the group of soft martensitic steels. Generally, soft martensitic steels are characterized by a low carbon content of around 0.05 wt% and up to 6 wt% nickel as alloying element. The low carbon content leads to a decrease of hardness and to an increasing fracture toughness and corrosion resistance. Due to their chemical composition, soft martensitic steels generally underlie a martensitic transformation even when cooled in air [5,6]. Additional heat treatments can be performed to optimize the mechanical properties, leading to a microstructure that consists mainly of martensitic, austenitic and ferritic phases. The favorable—so called “tempered” martensitic microstructure—combines high strength with high strain at failure and provides a high resistance against H 2 S-induced stress corrosion cracking [4–8]. The steel investigated in this work contains besides 0.05 wt% carbon and 4 wt% nickel, 13 wt% chromium, and small amounts of molybdenum. Due to their favorable properties, soft martensitic steels are often used for demanding applications as e.g. for the production of pumps, Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.083 n Corresponding author. Tel.: +41 58 765 4512; fax: +41 58 765 1122. E-mail address: Michael.Koster@empa.ch (M. Koster). Materials Science & Engineering A 581 (2013) 90–97