A physically-based creep damage model for effects of different precipitate types

Abstract

The development of a new precipitate coarsening continuum damage mechanics (CDM) model to simulate the multi-precipitate strengthening mechanisms present in 9Cr steels under high temperature creep deformation is presented here. The key strengthening and degradation associated with the different coarsening kinematics and volume fractions associated with M 23 C 6 and MX precipitates in 9Cr steels are simulated within a CDM framework for the first time. The new CDM creep model is implemented in a uniaxial code and successfully applied to 9Cr steels across a range of temperatures via physically-based steady-state creep constants. The role of increasing Al content on the high temperature creep behaviour of 9Cr steels is simulated via varying the volume fraction of MX carbonitrides. The results highlight (i) the important role of MX carbonitrides on creep strength of 9Cr steels and (ii) the requirement to simulate steady-state creep behaviour in 9Cr steels from a physical basis.

A precipitate evolution-based continuum damage mechanics model of creep behaviour in welded 9Cr steel at high temperature

“…The hyperbolic sine flow rule has been demonstrated by Barrett et al 4 to give agreement with experimental data for 9Cr steels tested at both high-and low-stress regimes. The present implementation includes a multiple precipitate strain-and temperature-induced coarsening state variable, D P , with details on the derivation presented in Ó Murchu´et al 5 The creep strain rate is defined following the approach of Dyson and Osgerby, 20 Perrin and Hayhurst 21 and Hyde et al 22 for precipitate strengthened steels as follows…”

“…Figure 1 shows a schematic of the microstructure of 9Cr steels, with details of these features described elsewhere. 4,5 It should be noted that during service, this elongated tempered lath martensite microstructure is rearranged to a structure of gradually coarsening subgrains, with boundaries containing carbides which also experience coarsening.…”

“…Laves phase particles) and cavitation damage. The requirement to simulate the contributions of both M 23 C 6 and MX precipitates to creep strength is demonstrated in the continuum damage mechanics (CDM) creep model of Ó Murchu´et al, 5 where an 80% reduction in MX volume fraction is predicted to reduce creep life by a factor of 6. M 23 C 6 precipitates are known to be less thermally stable than MX precipitates during creep, along with experiencing enhanced strain-induced coarsening.…”

Through-process modelling of welding and service of 9Cr steel power plant components under load-following conditions

“…The present work does not attempt to simulate creep strain, which would require its own constitutive model, e.g. a power-law model, such as in [15], or a hyperbolic sine model, such as in [16]. Instead, the creep-strain data was correlated with microstructure and microstructureevolution data and is presented here as an idealised creep-strain, as follows:…”

The effect of ferrite phases on the micromechanical response and crack initiation in the intercritical heat‐affected zone of a welded 9Cr martensitic steel

“…Modern modelling techniques provide the opportunity to gain unique insight into, and quantify, specific microstructure effects on constitutive response. For example, a continuum damage mechanics model was used to account for multiple precipitate types within a 9Cr steel, and the results indicate that the volume fraction of MX carbide is a vital factor for creep behaviour of 9Cr steel at elevated temperature . Crystal plasticity finite element (CPFE) approaches have provided information on inelastic deformation at the microscale .…”