Study On Creep Properties For TIG Welding Joints Of CLAM Steel At High Temperature

China low activation martensitic(CLAM) steel, with China independent rights, is generally considered as the primary candidate structural material for blanket components because of its inherent geometrical stabilities, excellent thermo-physical properties. Besides, the CLAM is suitable for commercial production because of its low activation composition and the advanced and mature research technology base. In view of the working conditions of CLAM steel, it has important theoretical significances and practical values to study high temperature mechanical properties of the welded joints working in the high temperature conditions so as to improve the welded joints’ creep limit and endurance strength. Considering the difficulties of testing its practical longterm creep rupture performance, it’s very practical and significant to predict its longterm creep properties and the long-term rupture strength by building mathematical models and suitable predictive parameters.In this paper, the experiment and research method was made after studying the research methods of other RAFM at home and abroad. The 5 mm-thick CLAM steel plates were welded along rolling direction by double-layer TIG welding process, and the welding wire was designed and made with as the filler materials. The samples were divided into two groups. One was post weld heat treated(PWHT) for 30 min at 760℃ and the other one was not. The microstructure, hardness, impact absorbing energy and the high-temperature tensile strength were obtained by modern analytic methods. Moreover, the high-temperature creep tensile tests of the PWHT samples were conducted on the electronic lasting creep relaxation testing machine. The creep curve was plotted and the creep parameter was calculated. The appearance of fracture and distribution of the dislocation at different point of time during the process of creep tests were observed with the help of SEM and TEM technology. At last, the long-term breaking strength of the welding joints was predicted by using Larson-Miller parameter.It is found that the TIG welding joints of CLAM has the typical three-stage creep curves, and at constant temperature, the steady creep rate increased and the rupture time decreased with stress. The stress exponent n=15, obtained from creep equation, was higher than the typical values about 3–7, and activation energy Q was about 436 KJ/mol. K, also higher than the self-diffusion energy of Fe(300 k J/mol.K), indicating that the deformation mechanism is dislocation creep deformation.According to the TEM images, at 550℃ and 180 MPa applied stress, with creeping process going, the dislocations pile up in the martensite lath grain boundary, which hinders the dislocation motion. Consequently, the steady creep rate decreases, slowing down the creep deformation. Fracture surface topography shows that the fracture mechanism of TIG welded joints of CLAM steel is typical ductile fracture and the spherical particles at the dimple on the fracture surface indicates that they are typical equal-axis ductile voids.In addition, the long-term rupture strength(at 550℃ for 100000h) of the TIG welding joints of CLAM steel was predicted using Larson-Miller parameter. Its breaking strength is 106 MPa, closing to 140 MPa,(the value for the base metal-CLAM), suggesting that the creep property of welded joints is qualified.