Title

ON THE EFFECTS OF STATIC AND CYCLIC LOADS ON FERRITIC STEELS AT ELEVATED TEMPERATURES (CREEP-FATIGUE INTERACTION)

Date of Completion

January 1986

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

A study inquiring into the nature of the creep-fatique interaction in ferritic steels was carried out on five steels with compositions of 2 1/4Cr-1Mo, 9Cr-1Mo, 9Cr-2Mo, 12Cr-1Mo, and 12Cr-2Mo. This study concentrated on understanding the microstructural stability of these steels during exposure to elevated temperature and creep loading. The role of microstructure in controlling the high temperature mechanical properties was investigated as well. The creep-fatigue interaction was studied by performing a series of load control creep, cyclic creep, and fatigue tests at 538(DEGREES)C.^ The studies on the microstructural stability found that mechanical properties were adversely affected by a sixty four thermal preexposure to the test temperature of 538(DEGREES)C. Creep lifetimes were decreased by as much as forty percent, and low cycle fatigue and tensile properties were affected as well. The loss of mechanical properties was due to the loss of the elements chromium and molybdenum from solid solution which in turn led to a softening of the alloy matrix. Microhardness measurements and electron microscopy were utilized to study this process. Electron microscopy revealed precipitates formed during the preexposure and this led to the depletion of chromium and molybdenum from solution.^ Cyclic creep tests were performed with load cycles ranging from one hour down to one minute. The shorter cycle times had the tendency to increase the creep lifetime, especially for the 9Cr-2Mo and 12Cr-2Mo alloys which precipitated a fine dispersion of Fe(,2)Mo precipitates that strengthened these alloys. Fatigue failure mechanisms did not operate during the cyclic creep tests, so additional load control fatigue tests were performed with different R ratios in order to determine the conditions necessary to initiate fatigue cracks in these materials. Interesting observations from these experiments were that fatigue failure did not occur when the R ratio was -1, but did occur when it was -1.05. From these experiments, it was determined that creep and ratchetting deformation mechanisms were the dominate processes leading to failure if the mean stress during cycling was equal to or greater than zero. ^