Study On Solidification And Precipitation Behavior Of Duplex Stainless Steel

Duplex stainless steel(DSS)combines the high strength of ferritic stainless steel with the excellent toughness of austenitic stainless steel.In addition,DSS has good pitting corrosion resistance and chloride stress corrosion resistance.Because of these excellent properties,as a weldable structural material,duplex stainless steel has developed rapidly.Since the advent of duplex stainless steel in the 1930s,duplex stainless steel has developed to the fourth generation,which is mainly divided into two directions:economical and super duplex stainless steel.For the moment,there are few reports on the mechanism of microstructure evolution of DSS during heating and solidification process.Moreover,brittle phase precipitation behavior of cast-rolled DSS needs to be studied urgently.In this paper,high temperature melting experiments,solution treatment experiments and performance tests were designed to solve these problems.The heating and solidification process and precipitate formation rule of DSS were studied by means of CSLM(comfocal scanning laser microscope),EPMA(electron probe microanalysis),and TEM(transmission electron microscrope),etc.The main work and innovation are as follows:(1)The in-situ observation of the heating and solidification process of LDX2101 and SAF2507 was carried out by high temperature melting experiments.The results show that during the heating process,the phase transition of the two steels is ??? process and nucleation-growth mechanism.After the microstructure completely evolves into ?,there exhibits a grain boundary migration growth mechanism.(2)The LDX2101 chromium nickel equivalent ratio is 2.289,and SAF2507 is 2.095.Solidification in situ observation experiments confirmed that the solidification mode of both steels is F mode.In the solidification process of the lean duplex stainless steel LDX2101,when solidified at a small cooling rate,8 grows in the form of columnar dendrites,and y grows in the? in the form of dots;when solidified at a large cooling rate,8 is nucleated in a large amount and grows rapidly in the form of cellular crystal.y grows in the ? in a short strip form.In the solidification process of super duplex stainless steel SAF2507,when solidified at a small cooling rate,? grows in the nucleation of the cellular dendrites,and the nucleation of y is sufficiently grown as snowflake dendritic or not fully grown into dot shape;When solidified at a large cooling rate,the ? cell dendritic size decreases,and y grows in a fine point in the ?.After observing the solidified microstructure,a large number of cracks were found inside LDX2101 at a small cooling rate.Extended crack and dendritic crack models were proposed to explain the causes of cracks.As a low melting point element,copper element segregates at the grain boundary during solidification,which reduces the interfacial bonding energy,and the tensile stress generated by the liquid phase shrinkage during solidification leads to the generation of extended cracks.The residual liquid phase among the closed dendrites during solidification has no liquid phase replenishment in the subsequent solidification,thereby generating dendrite cracks.(3)The brittle phase precipitation of super duplex stainless steel SAF2507 has been studied by isothermal aging experiment.It is found that the ? is the intermediate phase of ?,and its formation time is earlier than ?,but the growth rate is very slow.The ? is mainly nucleated in the ferrite or at the phase boundary,has a very high molybdenum content,and nitrides are often present at the nucleation.The x appears as a bright white block under the electron probe.The a is the main precipitation phase.Due to the high alloying characteristics of 2507 duplex stainless steel,formation temperature of ? is around 900?.The ? mainly nucleates at the ferrite phase boundary and extends to the inside of the ferrite.The final microstructure is a reticulate structure occupying the original ferrite position,with a small amount of secondary austenite formation.The? has high chromium and molybdenum content,and the electron probe image is white bulk structure.The ? phase has two kinds of structures at different temperatures:reticular and lamellar.Tensile and electrochemical experiments show that the formation of a small amount of a will cause a serious decline in toughness and corrosion resistance.Cold deformation promotes the precipitation of ?.The precipitated TTT curve of cast-rolled SAF2507 super duplex stainless steel is obviously shifted to the right compared with the hot-rolled state,indicating that twin-roll strip cast process can effectively inhibit the formation of brittle phase,not only delaying the precipitation of ?,but also reducing the precipitation temperature.