Many steam turbine blade failures during the past few decades are known to have initiated from corrosion pits. Despite the amount of research that has been undertaken worldwide, no predictive methodology has been developed for defining the risk of corrosion fatigue (CF) failure. This paper describes the research undertaken toward the development of such a methodology.
Tests with dual certified 403/410 12 percent Cr martensitic steel were performed to quantify the influence of corrosion pits on the fatigue life. Threshold stress intensity factors, ΔKth, and fatigue limits, Δσ0, were determined in air and two aqueous solutions. Additionally, stress-life tests were performed with pre-pitted specimens in air and aqueous solutions. The data for transition from a pit-to-a-crack have been correlated using the Kitagawa Diagram. This presentation of the data relates the steady stress, cyclic stress and pit size to the prediction of fatigue failure. Ultrasonic fatigue testing was an essential aspect of this program.
Based on these test results, a comprehensive methodology has been developed to quantify the risk of corrosion-fatigue failure at a pit. This paper is based on research supported by the Electric Power Research Institute Inc.  The authors are grateful for the support provided by EPRI and their permission to submit this paper.
Fracture mechanics and Kitagawa Diagram
Based on extensive testing, the Kitagawa-Takahashi Diagram  (abbreviated as the Kitagawa Diagram) has been proven to be an appropriate way to correlate pit-to-crack fatigue data. That data was obtained for specific values of R (ratio of minimum to maximum stress for combined steady and cyclic stress cycles). Once the Kitagawa Diagram parameters were defined to be a function of R, these diagrams could then be applied to operating steam turbine blades to define a critical pit size (i.e. when the pit is at risk of forming a propagating fatigue crack).
Prior to defining the research program, it is important to explain fracture mechanics as related to crack initiation, growth and eventual failure. The intensive investigation of cracks has led to a widely accepted method of presenting such data, as shown in Figure 1. The abscissa scale is the cyclic stress intensity factor, and the ordinate is crack growth rate per cycle. The cyclic stress intensity factor is defined in Equation 1.
ΔK is the cyclic stress intensity factor, MPa√m,
Δσ is the cyclic stress range, MPa,
c is the instantaneous crack size, meters and
Y is a geometry factor (for this case Y = 0.65).
Note that the definition of cyclic stress intensity factor, ΔK, incorporates both cyclic stress and crack size. The data presented in Figure 1 have been obtained as a part of this project for the 403/410 SS material. These data are from multiple specimens, and the plotted results demonstrate data reproducibility.
A three-segment line has been drawn through the crack growth data. This characteristic shape is consistent with data for many materials. The three-component straight line is defined as follows:
- Region 1: This is the slow growth portion of the curve. The fatigue crack threshold (ΔKth), corresponds to the stress intensity factor range, below which cracks do not propagate.
- Region 2: This is the power law growth region (also known as the “Paris” region).
- Region 3: Rapid unstable crack growth just prior to failure.