Session: DA-08-01 Developments in FFS Techniques

Paper Number: 105941

105941 - Effect of Grain Size on the Creep Residual Life Evaluation in the Framework of the Api 579-1 Standard 

Creep is a classic inelastic-strain accumulation mechanism of pressure components exposed to high temperature. Due to the long-term exposure, the residual life is usually checked accounting for the steady-state creep, namely the dominant regime in terms of contribution to the accumulated creep strain. The creep strain accumulation directly depends on the stress field via a constitutive equation that directly reflects the underlying microstructural phenomenon, namely diffusion and dislocation glide+climb. Moreover, creep diffusion, dominant at low stress, is known to be grain size dependent. Commonly, the grain size effect is neglected for two reasons: I) the diffusive regime is completely ignored; II) the structure is considered to have a uniform texture and the same average grain size. Unfortunately, every time a welding joint is made, the HAZ undegoes a microstructural change resulting often in a local refinement of the grain size.
In two previous papers by two of the authors, the impact of a diffusive plus dislocational creep formulation on the API 579-1 creep residual life was studied for a low-alloy ASME SA-335 P22 butt-weld. In this work the analysis was extended in order to include the impact of the microstructure in terms of grain size, considering that it directly affects the diffusive creep rate. A FE inelastic model was built up for a butt-weld and the weld zones were characterized with respect to their grain size. The creep constitutive equations were set up for diffusive and dislocational secondary creep, including the grain size contribute to the former. The custom user-subroutine employed in the FEA was able to determine the creep strain accumulation and the API 579-1 creep damage based on the Larson-Miller theory within the different weld zones and the results indicated a damage concentration in the fine-grained intercritical HAZ which is consistent with the Type IV cracking of welds in the creep regime.
 

Presenting Author: Lorenzo Scano S.S.I. s.r.l.

Presenting Author Biography: Lorenzo Scano earned his M.S. in Mechanical Engineering in 2003 and a Ph.D. in mechanical engineering in 2008 both from the University of Udine (Italy).
From 2003 to 2007 he worked as quality inspector and steel structure engineer in the field of heavy metal carpentry. Since 2007 he is the owner of S.S.I. s.r.l., a consultancy in the field of pressure equipment design and integrity structural analysis.

Authors:

Lorenzo Scano S.S.I. s.r.l.
Francesco Piccini S.S.I. s.r.l.
Salvatore Palomba S.S.I. s.r.l.
Matteo Bruno University of Naples “Federico II”
Luca Esposito University of Naples “Federico II”