Metallurgy Failure Analysis for an Abnormal Fracture Exhaust Valve Made by 6Cr21

摘要

The working environment of the exhaust valves is very bad.Valves not only bear high-frequency impact,alternate tension-compression load and thermal stress,but also bear high temperature corrosion and flushing action of high-speed gas.The plate-rod transition region of valves is the second hot spot,which is the and important failure region,because it is very sensitive to corrosion and fatigue.The fracture of valves often causes devastating damage to gas mechanism of engine.The deep metallurgy failure analysis for an early fracture engine exhaust valve was performed through macroscopical and microcosmic as well as chemical composition aspects.The valve ruptured at the transition area between rod and head.The head was made of the austenitic material of 6Cr21Mnl 0MoVNbN (6Cr21).The SEM results showed that the fracture surface consists of fatigue source area,fatigue spreading area and transient breaking area.Obvious shell lines and secondary cracks were found on the fatigue spreading area,so we it was concluded that the valve belonged to fatigue fracture.The loose structure of fatigue source area was the direct reason of fatigue fracture.In addition,the TEM observation of the slice cut from the region closing to the fatigue source of fracture surface showed that deep areas near the surface of the valve were oxidized severely,and the oxidized area was full of micro cracks and holes.This resulted in the declining of the intensity,ductility and bearing capacity,etc.leading to the fracture of the valve.