Reheat cracking in 316H stainless steel
Figure 1: Reheat crack near a weld in 316H stainless steel.
Introduction
Creep cavitation is an important failure mechanism in components operating at high temperature. Reheat cracking is associated with creep cavitation in weldments, which occurs either during high temperature service or during post-weld-heat-treatment. This cavitation phenomenon can be driven by applied, residual stresses or a combination of both.
Challenges
StressMap, The Open University and PhD candidate Rahul Unnikrishnan have been working with EDF Energy UK to investigate plastic deformation and creep cavitation around reheat cracking in order to provide a better understanding of reheat cracking and creep damage development mechanisms. This enables the improvement of models for predicting the life and structural integrity of susceptible welded components operating at high temperatures.
Study of creep damage around reheat cracking in a 316H austenitic stainless steel weldment
This study carried out on a 316H austenitic stainless steel weldment, which has developed reheat cracking after prolonged service.
The electron backscatter diffraction (EBSD) technique has emerged as a promising tool to quantify strain induced lattice misorientations due to development of geometrically necessary dislocations (GNDs) during deformation in polycrystalline materials.
Figure 2 – Macrographs of as-deposited (top-left), cold-rolled (top-right) and cold-rolled with laser processing (bottom).
Kernel average misorientation (KAM) and low angle boundary fraction (LABF) maps were used to quantify inelastic strain around the reheat crack. The EBSD studies (Figures 3 and 4) revealed higher levels of lattice misorientation towards the weld region where the crack initiated. In this region, strain is particularly concentrated at grain boundaries. The pattern of deformation shown by the EBSD measurements was confirmed by the hardness survey.
The SANS2D small-angle scattering instrument (SANS) at the ISIS Pulsed Neutron Source (STFC Rutherford Appleton Laboratory, Didcot, UK) was deployed in order to measure cavity distribution around the around reheat cracking, in particular along a line normal to the crack (Figure 5a). Comparing SANS cavitation measurements data with EBSD data, shows that the areas with higher cavity densities had higher local misorientations (Figure 5b).
Figure 3 (a-c) Orientation maps and (d-f) grain boundary maps of reheat crack at location 1 (a, d), location 2 (b, e) and location 3 (c, f). Position of each location is marked in Figure 2(a).
Figure 4 – Kernel average misorientation (KAM) maps of the reheat crack at (a) location 1, (b) location 2, (c) location 3 and (d) magnified map of grains at the crack tip.
Figure 5(a) SANS data showing the increase in fractional size distribution of cavities and (b) EBSD data showing higher values of KAMa lognormal, as approaching the crack, along a line normal to the crack.
Findings
- EBSD studies on reheat crack shows higher local misorientation in areas closer to crack.
- The KAM 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.
- SANS technique was successfully used to give an absolute quantitative description of cavitation damage up to about 350 nm in size near the reheat cracking. Cavitation is found to increase significantly as the crack path is approached, near the crack mouth.
- KAM was higher at areas with high density of cavities as confirmed by SANS.
References
[3] Rahul Unnikrishnan, PhD Thesis, The Open University, 2018, “Characterization of inelastic strain in type 316H austenitic stainless steel using electron backscatter diffraction”
