Development of Alloy Design and Evaluation Systems for High Temperature Materials Based on Non-Steady-State Creep Concept

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Long term creep properties of high temperature materials are evaluated from short term creep tests. The objective of the present research is to improve ways of formulation of short term creep data and evaluation of long term properties. The results are summarized as follows :1.Improvement of rupture life prediction : The following creep equation was proposed based on OMEGA method :epsilon=(1/OMEGA){In(1+zetat)-In(1-etat)} (1)where epsilon is the creep strain, t is the time, and OMEGA, zeta and eta are material constants. This equation can predict more accurate rupture life from shorter creep tests.2.Relevant condition of short term tests : Short term test conditions are selected based on deformation mechanism maps so that creep mechanism does not change from service condition of engineering plant. Long term creep data of 2.25Cr-lMo steel was analyzed to clarify how to select the test conditions. It was found that deformation mechanism maps presently available have several misunderstandings. A correct mechanism map of the steel was proposed.3.Examination of prediction methodology : Several methods proposed for rupture life prediction were examined by long term creep date over 10^5h. More relevant methods were suggested among the proposed methods. Another finding to be mentioned is the concept of inherent creep strength. Namely, all the low alloy steels gives similar creep rupture strength when test duration is long enough.4.Correlation between microstructural degradation and creep life consumption : Degradation of martensitic lath structure was studied in detail during creep of martensitic steels. A linear relation was found between relative growth of lath width and creep strain. A nondestructive life assessment method was proposed based on the linear relation.5.Modification of database system : An object-oriented database system was modified to the form applicable to creep data.