Thermal fatigue damage of steel joints brazed with various nickel filler metals

Nowadays, the effect on the thermal fatigue properties of the complex structure of borides arising in steel joints [1–3] obtained using industrial boron-containing nickel filler metals has been poorly stu died. Studies of borides effect on fatigue mechanisms in the brazed seam by the action of temperature cyclic changes will help to optimize the filler metal compositions, based on Ni – Cr – Si – B system, and obtain the most homogeneous structure. The general trend of increasing the fatigue characteristics of joints can be illustrated by works [4–5]. The resistance of materials, including joints, to fatigue damage is increasingly of interest from the point of view of tests that simulate operating conditions. High operating temperatures and resistance to thermal cyclic loads are a mandatory requirement for many energy-stressed units operating under temperature change conditions [6–9] In such products as rocket nozzles, heat exchangers, gas turbine engine blades, pistons of internal combustion engines, components of atomic technology and energy turbines – the efficiency depends on the operating temperature. Thermal cycling test allows to establish the dependence of thermal fatigue and destruction mechanisms on the microstructure [10–12]. This method is suitable for the qualitative assessment of factors affecting the strength, including fatigue, such as, for example, the elemental composition of the filler metal, the parameters for obtaining a joint or the distribution of stresses. Thus, this method based on a small number of tests may allow an assessment of the fatigue processes and the development of damage in the brazed joint. Thermal fatigue is a destruction of the internal stresses created by cyclically changing the temperature field in the material. Thermal fatigue is possible both in total with external loads and without it. Stresses arise from the irregularity of heating or cooling over the cross section with changes in operating temperature. Stresses can also be structural, as a result of phase transformations and phase hardening. In massive structures heated from the surface, thermal deformations are localized in the surface layer, where the crack grows. Thin-walled structures are destroyed by thermal fatigue where they are tougher: near the corner joints, Thermal fatigue damage of steel joints brazed with various nickel filler metals*