Design Methodology Of Steam Turbine HP-IP Rotors Based On Creep Damage Analysis

Steam turbine HP-IP rotors are subjected to creep condition in a long term of service.The accumulative damage on microstructure of rotor steel will lead to degradation of its mechanical behavior and threaten the security of turbine operation.So the research of the effect of creep damage on the mechanical properties of rotor steel can be helpful to the improvement of design technology and remaining life prediction of the rotors.The creep damage factor has not yet been taken account into HP-IP rotor creep design specifications because the current creep damage calculation method is not perfect and the damage factor is difficult to be applied to the rotor life prediction.In order to complete the creep strength design specification of HP-IP rotors,a design methodology based on the damage analysis is required and the creep damage theory is also need to be improved.The HP-IP rotor made by 30CrlMol V steel is studied in this work.At first,a rotor serviced for 140000 hours is utilized and dissected as sample materials with different creep damage degree.Then,experiments of high temperature mechanical property,creep and creep crack growth are carried out on these materials to identify the damage characteristics in different rotor positions.The influence of damage on creep/creep crack growth behaviors is observed.And the corrective creep damage constitutive equations as well as the creep crack growth rate predicted formula are derived based on thermodynamics damage theory.Finally,a creep damage design method for high temperature turbine rotors is proposed.Its feasibility is verified by a real rotor case from industry.The progresses have been made in the following aspects:(1)The regularity of creep damage and its microscopic characters are revealed by metallographic tests.It shows that creep damage appears in diffusion and dissolution of impurity elements,carbides precipitation,grain growth,grain boundary cavities and some other forms.Metallurgy variation is the main creep damage performance during most of the creep time.(2)The hot tensile and creep tests results show that creep damage induced by microstructure changes weakens the strength and ductility of rotor steel.The significance of metallurgy change has been ignored in previous creep damage theory.In this work,the creep deformation energy and environment heat are both taken as the damage driving force and a set of creep damage constitutive equations are deduced based on the thermodynamics damage theory.The new equations can describe the entire creep damage procedure that contains metallurgy changes and the cavity nucleation and growth.It can be applied to the creep damage calculation of material after long-term service at high temperature.Compared to the other creep damage models,the results by new equations are in good agreement with the creep experimental curves.New creep damage constitutive equations lay a theoretical foundation for completing the creep damage design approach to high temperature rotors.(3)The relationship between damage factor and creep crack growth rate is obtained by the creep crack growth tests and the microstructure analysis around creep crack tip.Based on the damage analysis,the predictive formula of high temperature fracture mechanics parameter C*is improved by adding the creep damage factor.And the improved formula predicts the creep crack extension of damaged rotor steel more accurately.(4)Based on the damage theory,a creep damage equivalent stress-strain design principle and a creep damage failure criterion for turbine rotors are proposed.It improves the creep strength design method,overcoming the environmental limitation on the design method.It also simplifies the rotors design flow and is suitable for the over-term serviced turbine rotors.A 30CrlMolV HP-IP rotor is taken as the computation example,and the creep damage equivalent stress-strain control criterion and the creep damage failure criterion are introduced into the design procedure.The design result based on the creep damage theory is accordant with the practical situation.