Why do seamless stainless steel elbows crack?
The occurrence of cracks in stainless steel seamless elbows is a serious hidden danger in industrial pipelines, which not only affects work but also poses serious hazards. Below, we will analyze the causes of the occurrence of cracks.
1. Stress corrosion
Static stress corrosion+corrosive medium: The inner bending side of the seamless stainless steel elbow in the oxygen pipeline is subjected to a static pressure of 8.5MPa for a long time, and undergoes electrochemical reaction with the medium, causing the surface oxide film to rupture and form micro cells, and the anode metal is dissolved and cracked. It mainly occurs in chemical pipelines that transport oxygen and chloride ionic liquids, and cracks often appear as branching branches (intergranular corrosion).
2. Hidden mines due to process defects
Cold work hardening: Local deformation of stainless steel seamless elbow exceeds 30% during cold forming, resulting in residual stress that has not been eliminated (requiring annealing treatment)
Material adulteration: Poor quality 201 stainless steel seamless elbows contain impurities of scrap steel, and the metal ductility is insufficient during bending, leading to direct tearing.
During manufacturing, the surface metal layers overlap, accelerating corrosion penetration in acidic media.
3. Extreme working condition test
Thermal stress type:>150 ℃ rapid cooling and rapid heating cycle
Low temperature brittle fracture: impact load below -50 ℃
Vibration fatigue: High frequency vibration of pulsating airflow in compressors
Finally, I will teach you how to identify cracks:
Macro characteristics:
Centipede like longitudinal cracks appear on the inner curved side (stress concentration zone).
There are brown corrosion products at the edge of the crack (different from the silver bright fracture caused by mechanical damage).
Microscopic inspection:
Metallographic microscope → observe whether the crack extends along the grain boundary (typical feature of stress corrosion).
Ultrasonic flaw detection → detecting internal cracks with a depth greater than 0.5mm.
Eddy current testing → Capture surface folding defects.
Why do seamless stainless steel elbows crack?
The occurrence of cracks in stainless steel seamless elbows is a serious hidden danger in industrial pipelines, which not only affects work but also poses serious hazards. Below, we will analyze the causes of the occurrence of cracks.
1. Stress corrosion
Static stress corrosion+corrosive medium: The inner bending side of the seamless stainless steel elbow in the oxygen pipeline is subjected to a static pressure of 8.5MPa for a long time, and undergoes electrochemical reaction with the medium, causing the surface oxide film to rupture and form micro cells, and the anode metal is dissolved and cracked. It mainly occurs in chemical pipelines that transport oxygen and chloride ionic liquids, and cracks often appear as branching branches (intergranular corrosion).
2. Hidden mines due to process defects
Cold work hardening: Local deformation of stainless steel seamless elbow exceeds 30% during cold forming, resulting in residual stress that has not been eliminated (requiring annealing treatment)
Material adulteration: Poor quality 201 stainless steel seamless elbows contain impurities of scrap steel, and the metal ductility is insufficient during bending, leading to direct tearing.
During manufacturing, the surface metal layers overlap, accelerating corrosion penetration in acidic media.
3. Extreme working condition test
Thermal stress type:>150 ℃ rapid cooling and rapid heating cycle
Low temperature brittle fracture: impact load below -50 ℃
Vibration fatigue: High frequency vibration of pulsating airflow in compressors
Finally, I will teach you how to identify cracks:
Macro characteristics:
Centipede like longitudinal cracks appear on the inner curved side (stress concentration zone).
There are brown corrosion products at the edge of the crack (different from the silver bright fracture caused by mechanical damage).
Microscopic inspection:
Metallographic microscope → observe whether the crack extends along the grain boundary (typical feature of stress corrosion).
Ultrasonic flaw detection → detecting internal cracks with a depth greater than 0.5mm.
Eddy current testing → Capture surface folding defects.