(1) If the vibration frequency and amplitude are too large, the longitudinal resonance of the entire metal bellows may occur under certain conditions. After a certain period of time, the metal bellows may be damaged. The failure mode is mostly fatigue annular crack.
(2) If the stiffness value of the metal bellows is too high, fatigue cracks will occur after a period of reciprocating motion.
(3) Embrittlement of Austenitic Stainless Steel
Austenitic stainless steel is the most widely used type of stainless steel, which is typically divided into three series: one is 18-8 steel represented by Cr18Ni8, such as 0Cr19Ni9, 1Cr18Ni9Ti; One is 25-20 steel represented by Cr25Ni20, such as 2Cr25Ni20Si2, 4Cr25Ni20 and 00Cr25Ni22Mo2; The other is represented by 25-35 steel, such as 4Cr25Ni35. Among them, 18-8 series steel is most commonly used. 1.1 Austenitic stainless steel σ Phase embrittlement Due to the small ferrite content, the tendency of coarse grain embrittlement and 475 ℃ embrittlement of austenitic stainless steel is not obvious. Research [1] shows that the embrittlement mechanism is σ Phase embrittlement is dominant. On the one hand, due to the large welding residual stress generated during welding, the yield strength of austenitic weld is low, and the cold work hardening tendency is large. Under the effect of shrinkage stress, the austenitic weld will produce "autogenous hardening", which will increase its strength and hardness and decrease its plasticity; On the other hand γ The microstructure of welds mainly composed of columnar crystals is uneven. Sometimes, in order to prevent hot cracks, a small amount of δ Phasic γ+δ The low temperature toughness of the weld is reduced due to the duplex structure of the weld. Under certain temperature range (such as 600~850 ℃), austenitic stainless steel weld metal will γ → σ Change, and σ The phase mainly precipitates along the grain boundary, resulting in severe embrittlement of the joint.
(4) Weld cracking: most metal corrugated pipes are longitudinally welded pipes. Due to the quality defects of longitudinal seam welding itself, the pipe strength is not enough, leading to cracking and damage at the weld.
