PSI - Issue 2_B

Rahul Unnikrishnan et al. / Procedia Structural Integrity 2 (2016) 3501–3507

3506

Unnikrishnan et al./ Structural Integrity Procedia 00 (2016) 000 – 000

6

An appropriate colour coding in the misorientation maps could help to reveal the extent of plastic deformation within each grain (Jin et al., 2013). Misorientation maps may also be helpful in predicting the direction of crack propagation but this needs further investigation. EBSD and micro-hardness are simple graphic methods for studying reheat cracks which illustrate the local dislocation density, but it is difficult to quantify the degree of deformation present as a reference unstrained condition must be defined (Wilkinson et al., 2010). There is considerable variation in the values given by both methods due to such factors as the crystal orientation, the grain geometry, the step size used and any strain induced by metallographic preparation.

Figure 9. Intragranular misorientation (GROD) maps of the reheat crack at (a) location 1, (b) location 2, (c) location 3 and (d) magnified GROD map of grains at the crack tip on a different colour scale.

Figure 10. Frequency distribution of Grain Reference Orientation Deviation (GROD) values at different locations.

4. Conclusion

Plastic strain around a reheat crack in an ex-service type 316H austenitic stainless steel steam generator component has been studied using both EBSD and hardness mapping. The KAM and GROD maps revealed higher levels of lattice misorientation towards the weld region, where the crack initiated, with strain particularly concentrated at grain boundaries. The pattern of deformation shown by the EBSD measurements was confirmed by the hardness survey. Also KAM and GROD maps seem effective in identifying cavity nucleation sites in SEM images. Acknowledgements