Microstructural Control Of 2205 Duplex Stainless Steel And Its Mechanical Properties

2205 duplex stainless steels(DSS)with a microstructure composed of approximately equal amount of ferrite and austenite have been widely used in shipping,chemical transportation and oil transportation due to their excellent mechanical properties and resistance to chloride stress corrosion.However,the ratio of ferrite to austenite in HAZ and the morphology of austenite are prone to change,which seriously affects the mechanical properties of welded joints,due to the dissolution of austenite,coarsening of ferrite grains and reformation of austenite during the welding process.Hence,it's of great theoretical value and practical significance for the control of microstructures and mechanical properties of 2205 DSS HAZ to study the formation mechanism of reformed austenite and its effect on the mechanical properties of 2205 DSS.In this paper,the microstructural evolution of 2205 DSS during welding thermal simulation and hot deformation was studied in detail.The factors affecting the reformation behavior of austenite were studied by rapid heat treatment and the reformation mechanism of austenite was studied systematically.On the basis of rapid heat treatment processes,different single ferrite phase region+dual phase region two-step heat treatments were performed on 2205 DSS,and the effect of two-step heat treatment parameters on the microstructure and mechanical properties of 2205 DSS were studied.The formation mechanism of grain-shaped orange peel on the surface of 2205 DSS tensile specimens after two-step heat treatment was analyzed in depth and its elimination method was proposed.The following conclusions were obtained.During the welding thermal simulation,the residence time of simulated HAZ in single phase region and dual-phase region increased gradually,and the retained austenite decreased while the Widmanstatten austenite and grain boundary austenite increased in the microstructure with the increasing in heat input;the Widmanstatten austenite formed at relatively high temperatures in dual-phase region was unstable and decomposed into austenite fragments due to partition behavior of alloy elements.Under the condition of ultra high heat input,no Widmanstatten austenite and grain boundary austenite were found in the simulated HAZ of 2205 duplex stainless steel,and all the austenite was equiaxed austenite with fine and uniform grain size.The microstructure of 2205 duplex stainless steel was insensitive to deformation rate,however,it was greatly affected by deformation temperature during hot deformation.In the process of hot deformation,the ratio of different types of grains in ferrite phase and austenite phase was very different due to their different softening behavior.Ferrite is mainly composed of recrystallized grains and substructures while austenite is mainly composed of deformed grains and substructures.The increasing of peak temperature holding time inhibited the formation of grain boundary austenite and large intragranular austenite effectively while promoted the formation of small equiaxed austenite.With the decreasing in dual phase treatment temperature,the nucleation density of reformed austenite increased while the grain size decreased gradually,and the morphology of reformed austenite changed from Widmanstatten austenite into acicular austenite and at last into equiaxed austenite.K-S or N-W orientation relationship existed between the reformed austenite and ferrite matrix.With the increasing of dual phase region treatment temperature,the N-W orientation relationship between reformed austenite and ferrite gradually changed into the K-S one.The preferred growth direction of reformed austenite is parallel to the stacking direction of its close packed plane.The in situ observation of the dissolution and reformation of austenite indicated that the dissolution mechanisms of austenite could be divided into two types:ferrite-austenite phase boundary migration type and ferrite nucleation and grow-up type,in which the former type was the dominant mechanism while the latter type accounted for only a small proportion.During the process of austenite reformation,acicular austenite formed first at the grain boundaries of ferrite.The nucleation incubation period of intragranular austenite was longer than grain boundary austenite.With the prolongation of isothermal time,large numbers of austenite grains formed within ferrite grain.The high nucleation density of intragranular austenite grains restrained their own growth effectively.Ferrite and austenite dual phase microstructures were obtained after two-step heat treatments.The volume fraction of austenite phase increased with the increasing in the second step solution treatment temperature.With the second step solution treatment temperature of 1000 ? the volume ratio of ferrite to austenite in the microstructure was the closest to 1:1.Different two-step heat treatment parameters had little effect on the tensile and yield strength of 2205 DSS.Some cleavage surfaces and micro-cracks appeared in the fracture surface of tensile specimens with the second step solution treatment temperature of 850? and 900? due to the straight phase boundaries between reformed austenite and ferrite,while only equiaxed dimples with uniform size were observed in the fracture surface of tensile specimens with the second step solution treatment temperature of 950C and 1000?.The phenomenon of orange peel on the surface of 2205 DSS tensile specimens after two-step heat treatment became more and more severe and the surface roughness increased gradually with the increasing in tensile strain.Under the K-S and N-W orientation relationship,the slip systems activated in austenite and ferrite were parallel or close to parallel.Dislocations slipped in the manner of stimulated slip,or synchronous slip to across the phase boundaries.The phase boundaries within ferrite grains were not able to prevent the slip of the dislocations effectively.Strain could transfer all the way to the original ferrite grain boundaries leading to strain concentration.Ferrite grains with different orientations tilted in different directions,which resulted in the formation of ferrite grain-shaped orange peel on the surface of tensile specimens.When the second step solution temperature was lower than 950?,the precipitation of ? phase was promoted by cold rolling process.The microstructures were composed of ferrite and near equiaxed austenite and ? phase was unable to precipitate when the second step solution temperature was above 950?.The cold rolling process caused the orientation relationship between austenite and ferrite matrix to deviate from the K-S and N-W relationships,which made it possible that the phase boundaries in ferrite grains could bear part of the strain to reduce the strain concentration at ferrite grain boundaries,and thus avoiding the appearance of ferrite grain-shaped orange peel on the surface of deformed 2205 DSS.The highest tensile strength,yield strength and elongation of 2205 DSS were greatly improved after cold rolling process was added between the two steps of two-step heat treatment.The strength-elongation product of 2205 DSS increased by more than 50%.