Full Description

The material 1CrMoV is a low alloy ferritic steel widely used in the power generation industry for the manufacture of high temperature turbine components, in particular steam turbine rotors operating up to ~565C at the inlet stages of high and intermediate pressure (HP/IP) steam turbines. In this thesis work is described an extensive project, which has been undertaken in order to
(i) investigate the cyclic thermal fatigue behavior of 1CrMoV turbine rotor steel under service-like loading conditions,
(ii) improve the procedure for lifetime calculation of high temperature steam turbine rotors and
(iii) gather further understanding of the creep-fatigue damage interaction occurring at a turbine rotor stress increasing feature during operation.

The research project was developed in three phases, in which different types of 1CrMoV specimens have been subjected to thermal fatigue cycles in an effort to reproduce in the laboratory specimens the real thermo-mechanical loading conditions experienced at a critical feature of a steam turbine rotor in service.

In the first part of the research project, the thermal fatigue behavior of 1CrMoV uniaxial plain specimens under service-like loading cycles was studied by means of thermo-mechanical fatigue (TMF) cycles, characterized by low strain rates, hold periods at the peak temperature and strain and temperature histories representative of the conditions at a steam turbine rotor feature in operation. A relatively simple constitutive model was developed based on an elastic-plastic stress-strain law with kinematic hardening for time independent plasticity and the Norton-Bailey creep law for time dependent plasticity.

The second part of this research focused on the creep-fatigue behavior of 1CrMoV circumferentially notched bars with isothermal fatigue (N-LCF) and thermo-mechanical fatigue (N-TMF) experiments. For the N-TMF tests the temperature and loading histories at the notch root were similar to the service-like TMF cycle type applied in uniaxial tests. The endurances from these tests could therefore be directly compared with uniaxial data.

In the last phase of the project, service-like thermal fatigue experiments with component-like test pieces were investigated. A novel thermal fatigue experiment was implemented, involving the application of thermal cycles with severe temperature gradients to two large (approximately 200 x 200 x 200 mm ) 3D feature specimens with a complex 10 mm deep groove. The experiment aimed to reproduce at specific notch locations loading conditions as close as possible to those of a high temperature steam turbine rotor feature in service also in terms of multi-axial stress state and significant strain gradients at the notch.

Valuable thermal fatigue endurance data for 1CrMoV steel were gathered by means of this experimental campaign and were taken as benchmark to verify the capabilities of different lifetime assessment methodologies for predicting the exact number of cycles to crack initiation in all specimens subjected to complex service-like thermal fatigue cycles. Metallographic post-test inspection was performed on all specimens in order to establish the character of the thermal fatigue damage and the respective mechanisms of failure associated to each TMF cycle type.

Adopted experimental methods, results of the thermal fatigue tests and achievements of the research project are described in this work.

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Published: 2014 Number of Pages: 152 File Size: 1 file , 2.9 MB