The Effect of Temperature, Specimen Size, and Geometry on the Fracture Toughness of a 3 Pct NiCrMoV Low Pressure Turbine
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I.
INTRODUCTION
THE investigation of the
catastrophic failure of a steam turbine at Hinkley Point 'A' Power Station identified the cause as spontaneous brittle fracture of a low pressure turbine disc initiated by a stress corrosion crack. ,,2 The Central Electricity Generating Board subsequently initiated a wide ranging metallurgical investigation of cracking in other nonreheat machines. 3 In order to assess the integrity of these machines fracture toughness data were needed over the range of temperature experienced by the bore of a shrunk-on LP disc, typically 10 ~ to 130 ~ The present paper describes a series of fracture toughness measurements made on a grade of 3 pct NiCrMoV disc steel used extensively in the CEGB. Preliminary Charpy tests indicated that with practicable sizes of test pieces some of the fracture toughness tests would be invalid on ASTM E399 size requirements. To facilitate a study of post yield behavior it was therefore decided to test a series of geometrically similar specimens of varying size, in two different geometries, over a temperature range which would span all modes of failure from fully brittle to fully ductile. The results have been analyzed using established post yield fracture mechanics techniques. The limitations of these techniques and the applicability of this study to integrity assessments of components in service are discussed.
II.
EXPERIMENTAL PROCEDURES
The material used came from first and second stage LP discs. Table I shows the range of chemical analysis and details of the mechanical and impact properties of the discs used. The two geometries tested were compact tension (CT) N, B. SHAW and G.M. SPINK are Research Officers, Central Electricity Generating Board, Scientific Services Department, North Eastern Region, Harrogate, England. Manuscript submitted June 16, 1981. METALLURGICALTRANSACTIONS A
and three point bend (SENB) span to width ratio 4: 1. Three sizes were tested in both geometries at each temperature. Details of specimen sizes and test temperatures are given in Tables II and III. In addition to conventional specimens, a number of three point bend specimens were machined from 'half-discs' as removed from LP rotors. These varied slightly in size but typical dimensions are shown in Figure 1. A stress intensity calibration for this test piece was determined experimentally Table I. Material Composition and Mechanical Properties Element C Si Mn Ni Cr Mo V S P
Material Discs used for the compact tension specimens 45 mm T singleedge-notch bend specimens 25 mm T and 12.5 mm T singleedge-notch bend specimens
Typical Composition Wt Pct 0.23 to 0.30 0.16 to 0.27 0.39 to 0.49 2.57 to 2.94 0.56 to 0.71 0.18to0.28 0.10to0.13 0.01 to 0.038 0.01 to 0.031 Mechanical Properties 0.2 Pct PR Stress U.T.S. Elongation FATT MN m-2 MN m -2 5.65X/APct ~ 644 770 19 40 613 780 18 50 627
763
17
23
720
840
17
58
Tensile properties measured at 20 ~
VOLUME 14A, APRIL 1983--751
Table II.
Results for Compact Tension Specimens
Table III.
(a) Width W = 100 mm, Thickness B = 50
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