Development of an analysis to determine the intragranular distortion during creep and its application to assessing the creep life of 2.25Cr-1Mo steels.

摘要

The intragranular creep distortion within ferrite grains of a 2.25Cr-1Mo steel (heat A) deformed at 843K and 106 MPa has been investigated by transmission electron microscopy (TEM). Intragranular distortion detected by shifts in Kikuchi patterns was observed quite early during creep deformation (;A standard procedure for the distortion measurements was developed so that an accurate assessment of the distortions could be obtained with a minimum of measurements. It was found that the distribution of distortions in 13 grains of a specimen strained 2.6% obeyed a normal probability distribution with an average distortion of 0.068 degrees/;The relationship between the creep strain and average intragranular distortion was examined in two 2.25Cr-1Mo steels under a variety of creep conditions. It was found that a linear relationship existed between the strain and the intragranular distortion during tensile test conditions as well as under stress accelerated creep test conditions. Accumulation of the distortion was reduced, however, if the creep test was conducted at 893 K, 50 K higher than the designed service operation temperature. A temperature accelerated creep test condition at 943 K reduced the distortion even more; this occurred at the same time that a reduction in the Mo;Creep damage in 2.25Cr-1Mo steel heat exchange tubes used for 80,000, 190,000 and 221,000 hours (against a design service life of 100,000 hours) in a fossil power plant, was quantitatively evaluated in a TEM by means of the intragranular distortion (IGD) analysis developed in this thesis. At the outer region of these tubes, intragranular distortions were calculated as 0.022, 0.032, and 0.058 (degrees/