Evolution Of High-temperature-transformed Secondary Austenite And Its Influence On Properties Of S32750 Duplex Stainless Steel

S32750 duplex stainless steel combines the superior characteristics of both ferritic stainless steel and austenitic stainless steel,which is applied widely in marine engineering,petroleum,oil,gas,paper-making,transportation,construction and other fields.So far,major second phases(Cr2N,σ phase,χ phase)in S32750 duplex stainless steel have been well studied.However,the research on secondary austenite transformed at high temperature(1100℃～1300℃)is insufficient.With improved performance of duplex stainless steel,secondary austenite has become one of the key factors restricting its applications.Thus,it is urgent to reveal the evolution of secondary austenite in duplex stainless steel at high temperature and its influence on properties,which could establish elimination mechanisms of secondary austenite on this basis.In view of the issues mentioned above,S32750 duplex stainless steel was taken as the research object to analyze the nucleation and growth behavior,evolution law and elimination method of secondary austenite during high temperature transformation systematically.Meanwhile,by combining various experimental methods,the influence of secondary austenite on mechanical properties and corrosion resistance of S32750 duplex stainless steel were revealed with main research results as follows:(1)As a sub-stable product,secondary austenite could be regarded as the combined product of "Temperature-Cooling rates",which presented a near K-S or N-W orientation relationship with ferrite and had N segregated.When Cr2N is involved in the growth process,Cr2N provided essential N and exhibited a synergistic-competitive growth mechanism with secondary austenite.Secondary austenite was divided into needle-shaped austenite and isolated austenite.Needleshaped austenite precipitated at the phase boundary first then grain boundary of ferrite and finally from the inner ferrite grain.Isolated austenite precipitated at ferrite grain boundary and inner ferrite grain.On the basis of the research above,the optimization process of eliminating secondary austenite was established as follows:1150℃ for 1 hour with furnace cooling to 1100℃ and preservation for 1 hour,then water-quenched.(2)Needle-shaped austenite reduced the impact energy of S32750 duplex stainless steel,shrunk the ductile-brittle transition zone,decreased the elongation and enhanced the tensile strength.The enrichment of N in needle-shaped austenite promoted the brittle transformation.The specific morphology of needle-shaped austenite reduced the hindrance to impact cracks,destroyed the plastic-toughness combination of ferrite-austenite,and facilitated dislocation accumulated at the phase boundary.(3)Needle-shaped austenite degraded the fatigue behavior of S32750 duplex stainless steel.Needle-shaped austenite transformed the "orientation relationship"with the ferrite,which led to the dislocation passing through the phase boundary and increased the probability of the fatigue cracking from ferrite.Another major factor of fatigue cracking in duplex stainless steel was the non-metallic inclusion.(4)The impact toughness of S32750 duplex stainless steel was slightly improved by the connection of isolated austenite.Partially pinned in ferrite,isolated austenite enhanced the yield strength of duplex stainless steel.(5)Needle-shaped austenite deteriorated pitting corrosion resistance and stress corrosion resistance of S32750 duplex stainless steel.Needle-shaped austenite reduced corrosion resistance of the original austenite,increased the difference of element concentration gradient and local corrosion tendency,and promoted the cracks propagating transgranularly during stress corrosion process due to high nitrogen content.The morphology of needle-shaped austenite provided an easier way for the propagation of the stress corrosion cracks.(6)Isolated austenite weakened the corrosion resistance of S32750 duplex stainless steel.The segregation of N in isolated austenite added the difference of element concentration gradient and the quantity of N-depleted zones,and promoted the formation of corrosion pits and stress corrosion cracks.Isolated austenite enhanced the probability of stress corrosion cracking by acting as the stress concentration sites.