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Failure Analysis on Superheat Section Connecting Pipe Flange of Ethylene Cracking Furnace

Author: LiJian
Tutor: MaXiaoMing；ZhaoChanXing
School: South China University of Technology
Course: Mechanical Engineering
Keywords: Austenitic Stainless Steel Failure Analysis Stress Corrosion Cracking
CLC: TQ221.211
Type: Master's thesis
Year: 2011
Downloads: 8
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Abstract

A large numbers of cracks near the weld area were found in the superheat section connecting pipe flange during inspection, which put off the overall production of ethylene cracking furnace system. So a project was set up. By using the Physical Testing methods and the Finite element method to find out the failure causation, and put forward relevant prevention measures.The macroscopic observation of the samples show that, the crack propagation and fractographic details accord with the features of austenitic stainless steel’s SCC(Stress Corrasion Cracking). Almost no plastic deformation characteristics were observed on both sides of the crack, both internal and external surface of the flange had crack propagation source. The main penetrated crack originated from the flange surface, but most of the cracks propagated from inside to outside of the wall. The fractures which were covered by corrosion products present river pattern in microscopic observation. Corrosion pit and secondary cracks could also be observed. The results of the energy spectrum analysis and determination of chloride in thermal insulation cotton show that, the existence of chlorine formed the stress corrosion environment in high temperature steam.The metallographic structure in the weld, HAZ(Heat affected zone) and base metal reveal that, Many microscopic cracks and banded ferrite structure were found around the weld, while no micro-cracks were found from the base metal away from the weld. There also had a large number of intergranular carbide precipitation in HAZ. Hardness test results also reveal that, materials in vicinity of the weld had certain improver, which leads to the anticorrosion performance dropped. In addition, residual stress determination found that, although the device had treated by the local post-weld heat treatment annealing, there remains high level of residual stress near the weld, which provided a stress condition for SCC.FEM(Finite element method) soft(ANSYS) was used to calculate the structural stress distribution of the flange based on the simplification of structure and boundary condition. The results reveal the structure stress of the flange is low in steady-state operation, while welding residual stress levels are high, which caused the stress concentration on the areas near the weld lines. So welding residual stress is the main stress factor that leads to the failure of the flange, the structural stress caused by the internal pressure is a secondary stress factor.The results of comprehensive analysis showed that: the failure system of the flange included three main components of SCC. So in this case, 304H austenitic stainless occurred SCC in the high-temperature, high-pressure steam and chlorine environment.

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