Correlation between Precipitation and Impurity Segregation in 347H Heat Resistant Austenitic Steel

Abstract

Generally, premature cracking at coarse grained HAZ in 347H heat resistant austenitic steel always shows IG (InterGranular) fracture mode. The mechanism for the formation of voids at grain boundaries and their development to the IG cracking in coarse-grained HAZ region is still not clear. Many theories explaining elevated IG cracking cannot clarify why premature cracking always occurs within the coarse-grained HAZ region in heat resistant austenitic steel boiler tube. A number of mechanisms of elevated temperature IG cracking are explained premature cracking in coarse-grained HAZ region in terms of : 1) Void nucleation & growth, GBS (Grain boundary sliding), 2) Brittle intermetallic phase (Inter Metallic Compound, Laves phase(Fe2Nb), Sigma (Cr,Fe)), 3) Sensitization (Cr, C – depleted zone at GB), Residual stress, SIPH, 4) Impurity segregation. Each theory has strong points and weak points for explaining premature cracking in coarse-grained HAZ region. The purpose of this research is try to explain the effect of correlation between precipitation and impurity segregation on IG cracking mechanism. Among common phenomenon at elevated temperature in heat resistant steel, precipitation and impurity segregation is investigated due to considerable factor for mechanical property in this research. The IG cracking mechanism is supposed by two factors : matrix strength (which is related with dislocation pile up) and impurity segregation (which is related with GB cohesion stress). Based on brittle fracture mechanism of Griffth’s theory, when the increasing dislocation pile up stress reach to GB cohesion stress, GB fracture happened. The two kinds of condition (without heat treatment and with heat treatment) is prepared for investigating different precipitation behavior then it affect to different matrix strength and impurity segregation kinetics. First, the correlation between precipitation and impurity segregation behavior is analyzed. The MC carbides and M2P phosphides affect each other due to sharing Nb atom. M2P phosphides is retardated due to non-equilibrium precipitation of MC carbides. The different precipitation behaviors depending on stress condition of MC carbides and M2P phosphide are discussed. And the effect of precipitation on impurity segregation is investigated. The segregation factors related precipitation are supposed as the amount of [P] which is related with M2P phosphide in matrix and incoherent MC interface as impurity trap site in matrix and total GB precipitates interface area as segregation site at GB. Finally the effect of correlation between precipitation and impurity segregation to IG cracking is discussed. The IG cracking of two different condition (without heat treatment and with heat treatment) is discussed by two factors which are dislocation pile up and GB cohesion stress.