High Temperature Low Cycle Fatigue of IN 738 and Application of Strain Range Partitioning
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I.
INTRODUCTION
COMPONENTS of various machines and structures operating at elevated temperatures are subject to cyclic thermal stresses resulting from start-up and shut-down. Therefore the problem of high temperature low cycle fatigue (HTLCF) is of great importance to the design of the components. There are several studies on (HTLCF) behavior of various blade alloys for gas turbines. 2-6 It has become apparent that the fatigue lives depend on testing temperature, environment, strain rate, and strain wave shape. 3'5-1EIn studies on creep-fatigue interaction, it is important to quantify and predict the fatigue lives from an engineering point of view. Various methodologies for predicting fatigue life have been proposed. 4'13-~6 Among them the strain range partitioning method proposed by Manson, Halford, and Hirschberg is of interest since it is possible to explain the variation in fatigue lives tested with different strain wave shapes.lE'13'17-20 Thepresent investigation has been carded out to study the effects of strain rate and strain wave shape on (HTLCF) of cast IN 738 nickel-base superalloy and to apply the strain range partitioning (SRP) method to the observed results. The Ostergren method which involves the tensile stress will be also considered in this investigation. 4 Fig. 1 - Low cycle fatigue specimen (mm).
II.
EXPERIMENTAL PROCEDURE
HTLCF specimens were machined from fully heat treated IN 738 casting to the shape shown in Figure 1. Final machining was carded out by low stress grinding to remove machining related surface defects. The composition of the alloy (wt pet) was 0.09 C, 8.25 Co, 15.95 Cr, 0.011 B, 1.6 Ta, 3.5 A1, 3.45 Ti, 0.7 Nb, 2.48 W, 0.5 Zr, 1.62 Mo, Ni bal. The grain size of this material was found to be 2 mm. The microstructure of this alloy in the fully heat treated condition consisted of y ' with a bimodal distribution, carbide particles of M23C6 type at the grain boundaries, and blocky carbide of MC type distributed at grain boundaries as well as throughout the grains, as shown in Figure 2. M.Y. NAZMY is Scientist with Brown Boveri Research Center, Baden/D~ittwil CH-5405, Switzerland. Manuscript submitted July 23, 1982. METALLURGICAL TRANSACTIONS A
A detailed microcharacterization of this material in the fully heat treated condition has been documented elsewhere. E1 The (HTLCF) behavior was investigated under fully reversed longitudinal strain control conditions using a servo-hydraulic testing machine. Testing was done in air at a temperature of 1123 K. Seven types of strain wave shape shown in Figure 3 were used to examine the effect of strain wave shape on the fatigue life. The fast-fast and the slow-slow triangular wave shapes were done at a strain rate of 10 -E, and 10-5s -~, respectively. Most of the tests with slow-fast and fast-slow sawtooth waves were done at a tensile strain rate of 10-SS -1 and a compressive strain rate of 10-Vs -1 , and vice versa. In tests with truncated waves, the hold times were varied from 400 to 700 seconds, and the ramp rate was always 10-Es - l .
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