Effect Of Post Weld Heat Treatment And Ta Element On The Structures And Properties Of CLF-1 Steel Joints Using Laser Welding

In this study,a 10Kw laser welding platform was used for the 10mm thick CLF-1 steel,the structural material of China’s controllable fusion experimental reactor project.Under the optimal process parameters(laser power 10kW,defocus amount 0mm,focus diameter 0.3mm,welding speed 2m/min and shielding gas(Ar)flow 20L/min),I-groove laser self-fusion welding and I-groove laser welding with different amounts of Ta added to the upper part of the groove were realized.After calculation,the added Ta content accounted for 0.128%,0.256%,and 0.384%of the weld volume,respectively.After inspection,welded joints with good front and back shapes and no metallurgical defects were obtained.After welding,welding joints without Ta were subjected to two different post-weld heat treatments(1)740℃/1.5h/AC(referred to as PWDT)and(2)980℃/45min/AC+740℃/1.5h/AC(referred to as PWNT)And the microstructure and mechanical properties of the weld metal under various conditions were analyzed systematically.At the same time,the influence of Ta on the structure and properties of weld metal was studiedIt was found that the base material of CLF-1 steel is composed of well-divided tempered lath martensite,and M23C6 type carbides precipitate at the austenite grain boundaries and lath martensite boundaries.There are dislocations growing inside the lath martensite and the precipitation of MX type carbides.Various strengthening methods such as fine grain strengthening,precipitation strengthening,dislocation strengthening,etc.make CLF-1 steel have excellent physical and chemical properties.Due to the rapid heating and cooling speed of laser welding,it belongs to the non-equilibrium phase transition process.The weld metal of the as-weld state without Ta is mainly caused by a large amount of lath martensite and a small amount of delta ferrite that is left by the insufficient process of the peritectic reaction.The structure of the PWDT weld seam is composed of a large amount of tempered martensite,a small amount of residual δ-Fe,and a small amount of M23C6 carbide with a dispersed distribution.The metal structure of the PWDT weld metal is composed of a large amount of tempered martensite and a small amount of M23C6 carbides with a small distribution.Although the residual δ-Fe in the weld metal disappears,the sizes of tempered martensite and M23C6 carbides have grown compared to the base metal,resulting in less impact energy than the base metal.Due to the limited stirring effect of the molten pool,the Ta element is unevenly distributed in the thickness direction.The welds with the three contents of Ta show the highest Ta content in the upper part,the second highest Ta content in the middle and the lowest Ta content in the bottom part.With the increase of Ta content,the ferrite in the upper and middle of the weld increases and the grains become larger,a small amount of δ-Fe remains in the lower part of the weld.More MX-type carbides were found in the upper weld structure of 0.128%Ta.When the Ta content was 0.384%,the upper structure martensite disappeared and consisted of coarse ferriteThe weld hardness without Ta as-welded state,PWDT state,PWDT state is 416HV,272HV and 200HV respectively.In 0.128%Ta,0.256%Ta and 0.384%Ta welds,the bottom part has the highest hardness value,the middle part has the second highest hardness value,and the upper part has the lowest hardness value.As the Ta content increases,the hardness of the weld metal gradually decreased.The tensile samples at room temperature of the as-welded state,PWDT state,PWDT state and Ta-added welded joints are all broken at the base material away from the center of the weld,indicating that their mechanical properties exceed the base material.The impact energy of the as-weld metal without Ta addition(-40J)is about 17%of the base material(～235 J),the PWDT state(～117.5J)is about 50%of the base material and the PWDT state(～154J)is about 65%of the base material.When the amount of Ta added is 0.128%,the precipitation amount of MX-type carbides on the upper part of the weld metal increases,which exerts a dispersion strengthening effect,but due to the large amount of δ-Fe remaining,the room temperature impact energy(27J)of the weld metal is lower than that of the weld metal without Ta.And when the amount of Ta added was 0.256%and 0.384%,the impact energy of the weld metal at room temperature was only 12J and 8J.