Failure Analysis And Mechanism Research For Super Heater Of Steam Boiler

A steam super heater furnace had been built in a Petrochemical plant. Final rupture of furnace super heater tubes in unit occurred in addition of amount of hydroxide solution while boiling boiler trial operation. The diameter of super heater tubes is 4> 89 X 6.5 with material Cr-Ni austenitic stainless steel. In order to investigate cause of cracking and avoid its occurring once again, we have carried on failure predication:Firstly, fractography study and fractography analysis on the mechanism is hydroxide embitterment type Iran granular stress corrosion cracking of Cr-Ni austenitic stain less steel. Corrosive residuals are oxides and certain amount of sodium element.Secondly, the tube material used in super heater have been examined Testing items include chemical compositions of the steel, mechanical and photomicrographs and hardness etc. The results show that the characteristics of Cr-Ni austenitic stainless steel tubes to meet the requirements national standard.The third, after theoretical analysis and test verification to the stress, it is found that the residual stresses and thermal stresses are very high within the welding affected region for both 180 deg.elbows and the connections between the straight tubes and the manifolds.It is concluded that final rupture of super heater tubes is the stress corrosion cracking of Cr-Ni austenitic stainless steel caused by the presentation of hydroxide solution in tubes while trial operation of the furnace .Therefore, it is suggested to take the relevantprevention measures of corrosion. Those include proper material selections, stress relaxation and relief, structural modification and improvement conditions of operation procedures.