MICROSTRUCTURAL CHANGES DURING CREEP AND LIFE ASSESSMENT OF MOD.9CR-1MO STEEL

Several microstructural changes take place in a material during the co urse of creep. These changes can be a measure of creep life. In this p aper, microstructural changes in Mod.9Cr-1Mo steel were studied and it was examined which is a good measure of creep life. Microscopic struc tural changes, such as void growth, lath structure uniformly oriented to the tensile axis and elongation of grains, are evident only in the necked portion of ruptured specimens. These macroscopic structural cha nges are not useful for creep life assessment. Lath width increases an d dislocation density within lath decreases with increasing creep dura tion. These changes in dislocation substructure start in the early sta ge of creep life, and cause the increase of strain rate in the tertiar y creep stage. The lath width and the dislocation density reach a satu rated value before rupture. The saturated values are independent of te mperature, and uniquely related to creep stress normalized by shear mo dulus. The extent of these microstructural changes are greater at lowe r stresses under which the material is practically used. These facts s uggest that the lath width and the dislocation density within lath can be a useful measure of creep life. Hardness of crept specimens is clo sely related to the lath width and the dislocation density within lath , The changes of these microstructural features can be evaluated by th e measurement of hardness.