Research On Stress Relaxation Behaviour Of High Temperature Bolts In Power Plants

Stress relaxation at elevated temperature is not only an indispensable part for structural integrity, but also a key issue for the design of bolted flanged connections in steam turbine of power plant. Therefore, it is of great theoretical and practical importance to investigate the stress relaxation of the flange-bolt joints. In order to guarantee the sealing, the flanged bolts in high and medium pressure cylinder of steam turbine are often refastened during periodic or non-periodic overhaul. The stress and strain distributions in the process of bolt fastening are very complex, and the repeated loading may eventually cause permanent unrecoverable deformation, or even blot fracture. Hence, it is very essential for the safety operation of power plant to study the relationship between the stress relaxation and deformation, to accurately establish an mechanical model describing the stress relaxation behaviors during repeated loading.Focusing on the candidate materials of high pressure cylinder flange bolts, i.e.12Cr-1Mo-1W-1/4V(12CrMoV), 1Cr-1/2Mo-1/4V (1CrMoV)and nickel base alloy Nimonic 80A, uniaxial creep experiment and a single loading stress relaxation experiment were performed. The relationship between creep and stress relaxation was investigated. On basis of creep mechanism, the mechanical model of stress relaxation was established. By introducing a creep damage parameter, the damage evolvement model of stress relaxation was set up. Then based on this model, calculation was performed to simulate stress relaxation behaviors in the process of repeated loading, and the results agreed well with the experiments. The main research work is as follows:(1) Basd on the microstructure evolvement, the relationship between the primary and secondary stages of creep and stress relaxation was analyzed; The creep, single loading and reloading stress relaxation behaviors of 12CrMo4V and 1CrMoV steels were investigated; The creep and repeated loading creep experiments of Nimonic 80A were performed.(2) Based on the Altenbach-Gorash-Naumenko(AGN) creep models, the stress relaxation damage model was built; By introducing an anti-relaxation variable R, a modified model which could be able to describe repeated loading stress relaxation was established as well.(3) Combining the improved Euler algorithm with differential evolution algorithm with adaptive step size, a model parameters identification method for the primary and secondary stages relaxation was proposed, with which the parameters could be able to determined simultaneously.(4) On the basis of the modified model, the mechanical behaviors of repeated loading of full size Ni-based alloy (Nimonic 80A) bolt was studied, which provides a theoretical basis for the turbine cylinder bolt fastening and initial stress selection.