PSI - Issue 13

Anke Schmiedt et al. / Procedia Structural Integrity 13 (2018) 22–27

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A. Schmiedt et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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stainless steel joints using a gold-base filler metal were investigated by Manka et al. (2013). The aim of this study is to present the time-dependent influence of a condensate corrosion on the tensile and fatigue properties of brazed AISI 304L/BAu-4 joints. The pre-corrosion is conducted acc. to VDA test procedure 230-214, as already investigated for AISI 304L/BNi-2 joints by Schmiedt et al. (2018b). The optical measurement technique of the digital image correlation (DIC) allows a 3-dimensional, non-contact and full-field image analysis, aiming at a visualisation of strain distributions. Up to now, the DIC technique with standard digital cameras is mainly utilised within quasi-static tests (Chen et al., 2018), as the application within fatigue tests at common frequencies of 10 Hz requires a complex triggered image acquisition. Koster et al. (2013) already discussed the high potential of DIC to characterise fatigue damage mechanisms in brazed steel joints. In the current study, the local deformation and damage behaviour of AISI 304L/BAu-4 brazed stainless steel joints is investigated within tensile and fatigue tests, using a DIC system with a triggered image acquisition. The influence of the gauge length on the strain values at the brazed specimens is evaluated by various virtual strain gauges, which are computed after the test in the area of the brazing seam. Fatigue tests with a stepwise load increase enable an assessment within the total stress range, leading to elastic and plastic material deformations. Further, the effect of the surface condition is analysed using brazed specimens with corrosion-dependent local circumferential notches. First results of the current study were already published by Schmiedt et al. (2018a). The austenitic stainless steel AISI 304L (X2CrNi18-9; 1.4307), acc. to specification DIN EN 10088-3, and the gold-base brazing alloy BAu-4 (Au 827, Au82Ni18 ), applied as a 50 µm amorphous brazing foil, were used to manu facture the brazed butt joints. For metastable austenites, a deformation-induced phase transformation to martensite can influence the fatigue behaviour (Nebel et al., 2003, Hahnenberger et al., 2014). It is well known that the transformation is sensitive to the test temperature (Nikitin et al., 2008). The brazing process was carried out in a vacuum furnace with a vacuum of 10 -5 to 10 -6 mbar, using a brazing temperature of 1050 °C in conjunction with a short dwell time of 2 min, a heat treatment for 2 hrs at 950 °C and a subsequent free cooling to room temperature. Fig. 1a shows the microstructure of the brazed joint. Tensile and fatigue test specimens were produced from brazed cylindrical bars acc. to the technical drawing in Fig. 1b. In contrast to standard tensile specimens, the used specimen geometry shows a shorter gauge length. Therefore, an influence on the strain values during tensile tests is expected if necking occurs. For a direct comparison to the results of brazed stainless steel joints using nickel-base filler metals (Schmiedt et al., 2018b), the brazed AISI 304L/BAu-4 specimens were pre-corroded acc. to the VDA test procedure 230-214. The alternating corrosion exposure consists of daily periods of semi-immersion, drying and a steam phase in combination with a weekly heat ageing at 400 °C. The pre-corrosion was applied over six time spans from 1 to 6 weeks, leading to a local circumferential metal dissolution in the area of the brazing seam as well as to depositions of corrosion products on the specimen’s surfaces, Fig. 1c . For the DIC analysis, a white speckle pattern was generated on the black painted specimens, Fig. 1d. (Schmiedt et al., 2018a) 2. Materials and experimental procedure 2.1. Materials and material conditions

Fig. 1. a) Microstructure of the brazing seam (BS) in the as-received condition; b) Specimen geometry (all dimensions in mm); c) Specimen after 6 weeks of pre-corrosion; d) Experimental setup for fatigue tests. (Schmiedt et al., 2018a)