Study On Microstructure And Properties Of Friction Stir Welds On Reduced Activation Ferritic/Martensitic Steels

Reduced activation ferritic/martensitic(RAFM)steel is considered to be the primary candidate structural material for experimental blanket module of nuclear fusion reactor due to its excellent thermophysical properties and preparation technology of tons steel ingot.In this paper,friction stir welding(FSW)was used to carry out welding process tests for RAFM steel with thickness of 5 mm to obtain many well-formed and defect-free FSW joints.The microstructure evolution of FSW joints was studied by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electron back scatter diffraction(EBSD)and transmission electron microscope(TEM).Moreover,Vickers hardness,high temperature tensile properties,impact properties and high temperature creep resistance of FSW joints were tested to discuss the relationship between microstructure and properties.Main conclusions are as follows:(1)High-quality FSW joints of RAFM steel could be fabricated by FSW in the process parameters of 200～400rpm welding speed,60mm/min welding speed and 10 kN axial pressure.(2)Microstructure analysis showed that,in stir zone(SZ),a large number of lath martensite and some retained austenite formed between the laths could be observed.The heat engine affected zone(TMAZ)and the fine grain heat affected zone(FG-HAZ)are mainly composed of lath martensite,while the grain size in TMAZ is relatively coarse.The heat affect zone(IC-HAZ)exhibits a mixed microstructure of lath martensite and recrystallized ferrite.(3)FSW joints of RAFM steel exhibit good impact properties,high temperature tensile properties and high temperature creep resistance.At-20 ℃,the highest impact absorbed energy of SZ can reach 24.56 J,and impact fracture was characterized by uniform equiaxed dimples by SEM observation.The tensile strength of FSW joints at 600 ℃ was equivalent to that of the base metal(BM),and the total elongation is between 11.5 and 25.5%.Moreover,the creep rupture time of the joint at the condition of 600 ℃/200 MPa could reach 1173.8 h,and creep specimens were fractured at the edge of HAZ at the advancing side of the weld(AS-HAZ).(4)The tensile strength at 600 ℃ of SZ in RAFM steel FSW joint was obviously improved,which was 150～230MPa higher than that of the BM,but its elongation was reduced to 10～15.5%.At the same time,SZ also exhibits excellent high temperature creep resistance,and its creep rupture time at 600 ℃/240 MPa would be as long as 4968 h.The tempering treatment can improve the impact toughness and total elongation of SZ in the FSW joint,but the high temperature tensile strength and creep rupture time were reduced to 500～525MPa and 1319 h,respectively.