Effect Of Heat Treatment On Microstructure And Properties Of 4A Duplex Stainless Steel

In this paper,4A duplex stainless steel smelted by intermediate frequency furnace are used as the research object.The effects of solution treatment on the microstructure,mechanical properties and pitting corrosion resistance of 4A duplex stainless steel were studied by means of optical microscopy(OM),scanning electron microscopy(SEM),EDS analysis,transmission electron microscopy,electrochemistry and mechanic test.In addition,we have made a corresponding discussion on the erosion corrosion behavior of 4A duplex stainless steel in liquid-solid flow.The 4A duplex stainless steel was solution-treated at different temperature.The results show that the content of ?-phase is increased with the solution temperature in the range of 950~1150 ?.The strength(?b)and hardness decreases to the lowest values at 1050 ?and then increase with the increase of the solution temperature.In addition,the alloying elements are redistributed and gradually tend to be uniform in the ? phase and the ? phase.The ? phase occurs at the interface of ?/? solution-treated at 950 ?,which is the main reason for the decrease of impact energy and pitting potential.A suitable volume fraction of ?/? is distributed uniformly after solution treated at 1100 ? for 2h,thus the steel exhibits the best mechanical properties and pitting corrosion resistance.The 4A duplex stainless steel after the solution treatment(1100 ?)was subjected to aging treatment at different temperatures(750~900 ?).The precipitation behavior of ? phase at 850 ? for different time(1 h,2 h,4 h)was analyzed.Furthermore,the mechanical properties and corrosion resistance of 4A duplex stainless steel were also studied.The results show that ? phase is rich in Cr,Mo but Ni-depleted,its crystal structure is tetragonal and it is formed by the decomposition of ?.The amount of ? phase increased first and then decreased with the increase of aging temperature and it reached a peak at 850 ?.When the aging time is extended,the precipitation of ? phase increase.? phase increases the hardness of the material,but its tensile strength,impact toughness and corrosion resistance is on the decline as a whole.The impact toughness is particularly sensitive to the precipitation of ? phase.The effects of heat treatment on erosion corrosion behavior of 4A duplex stainless steel in flowing 3.5% NaCl solution with 5% sand were studied by using erosion testing machine.The results show that the corrosion rate of the material increases with the increase of the flow rate,and there is a critical flow rate which affects the erosion corrosion.During the solution treatment,the proportion between the ? phase and the ? phase in the steel,the distribution of the alloying elements in two phases and the precipitation have an effect on the erosion corrosion of the material.The erosion corrosion rate of 4A duplex stainless steel after solution and aging treatment is lower than that without aging treatment.