The role of primary and tertiary creep in defining the form of the Monkman-Grant relation using the 4-θ methodology: An application to a 1CrMoV rotor steel

It is important to be able to predict the life of materials at high temperatures and the Monkman-Grant relation offers potential for reducing the development cycle for new materials. This paper uses the 4-θ methodology to i. identify and explain the form of this relation in terms of creep mechanisms and ii. to discover whether this form was compatible with development cycle reduction. The Monkman-Grant proportionality constant (M 2 ) was found to fall into three groupings depending on the amount of damage and the rate at which this occurred. Only once this was considered did the exponent on the secondary creep rate equal -1 - as predicted by 4-θ methodology. Only once such a grouping is undertaken does the relation accurately predicted lives close to operating conditions. • The normally measured minimum creep rate be replaced by a theoretical minimum rate given by θ 3 θ 4 . • The exponent on this minimum creep rate ρ was predicted to equal -1. • The constant M 2 is related to the amount of damage at failure and to the rate of its accumulation. • The constant M 2 fell into three groupings depending on the amount and rate of damage accumulation. • The constant ρ equalled -1 in each group and one group is relevant for longer term life assessment.