Brazing is considered as an efficient and economic joining technology with significantly reduced thermal stresses of the joining components in comparison to welding, and therefore it is established for a wide range of industrial applications, such as automotive parts. During the operation of e.g. exhaust gas heat exchangers (EGR-coolers), the tensile and fatigue loading of brazed components is superimposed by corrosive attack due to aggressive exhaust gases.
In the present study the influence of condensate corrosion on the microstructure and the depending mechanical properties under quasi-static and cyclic loading of austenitic stainless steel AISI 304 (X5CrNi18-10) brazed at 1050 °C with nickel based filler metal BNi-2 (Ni 620) was investigated. The corrosive damage results from exposure to synthetic exhaust gas condensate in conjunction with alternating heat ageing at 600 °C in daily and weekly cycles according to VDA test sheet 230-214, which is established for evaluation of condensate corrosion in exhaust gas-carrying components. Brazed AISI 304/BNi-2 joints were tested in initial condition as well as after six grades of pre-corrosion due to weekly ageing durations of up to six weeks to assess the influence of increasing corrosive damage on tensile and fatigue properties. Therefore, in addition to quasi-static tensile tests, cyclic stepwise load increase tests complemented with strain and electrical resistance measurements were applied to estimate the fatigue as well as corrosion fatigue properties. The results have been validated in constant amplitude tests until 2E6 cycles. Corrosion- and deformation-induced microstructural changes of base materials and joining zones were evaluated using light and scanning electron microscopy.
In this context, a significant reduction of tensile strength as well as fatigue strength due to increasing grade of pre corrosion caused by synthetic exhaust gas condensate was determined and quantified for investigated brazed AISI 304/BNi-2 joints. Local strain measurements using optimized extensometers as well as deformation-induced changes in electrical resistance have proven to be appropriate for a precise corrosion fatigue assessment.