An EBSD Study of the Deformation of Service-Aged 316 Austenitic Steel

Abstract

Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297 K and 823 K (24 °C and 550 °C) at strain rates from 3.5 × 10−3 to 4 × 10−7 s−1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains <0.1 and a gamma distribution at strains >0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain of up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10−5 s−1, all metrics were insensitive to deformation temperature, mode (tension vs compression), and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both the aged and unaged materials. Lattice misorientations develop with strain faster in aged steel than in unaged material, and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements, and good agreement was established between the two techniques of assessing plastic strain in aged 316 steel.