Crack Initiation in Carburised 316H Stainless Steels

: with Relevance to High Temperature Low-cycle Fatigue

Abstract

Crack initiation mechanism changes induced by environmentally driven carburisation are poorly
understood. Physical restrictions on practical sample production volumes and a historical focus
on macroscopic phenomena has limited the data available for practical use in mechanistic investigations. Following preliminary studies conducted by EDF Energy, experiments are presented
that were undertaken to address this knowledge gap.
In this thesis microstructural changes caused by oxidation in a simulated reactor gas, previously
developed by EDF Energy, are presented for two 316H stainless steels formed via different processing routes. The oxidation process leads to the formation of a thick duplex oxide layer and
carburisation of the steel behind the oxidation front. Because of the complicated interactions
and property variations introduced at the surface of the steel, focus is given to chemical distributions in the oxides formed on a cast steel along with grain boundary morphology changes in
the carburised layer on a sample taken from a forged bar. Auger electron spectroscopy measurements conducted on a transverse section through the oxidised surface compliment previous
observations of carburisation in Type 316H. Observations on the cast steel show the complex
chemistry and structure of the spinel oxide along with the elevated levels of dissolved carbon in
the near surface. Similar oxide depths are identified in samples from the cast and forged materials, however, measurements of carbide morphology changes in the forged material indicate
that it is thermal ageing, a consequence of prolonged exposure to elevated temperature during
oxidation, that drives these changes rather than the carburisation itself.
Laboratory and synchrotron X-ray diffraction measurements are utilised to interrogate stresses
generated during thermal cycling and mechanical loading of conditioned samples. During thermal cycling stresses measured in the oxide show disagreement with simple models, that are
based on estimated oxide properties. Synchrotron measurements appear to indicate the presence of a compressive triaxial stress field in the carburised layer behind the oxide. Currently, it
is not known if this stress state persists at elevated temperatures. Understanding of the stress
state in the carburised layer is important for making meaningful predictions of component life
and critical to the understanding of potential cracking mechanisms operating in the near surface
of a component during operation.
The diffraction results are supported by post-test microscopic investigations conducted on the
cracked surfaces of the carburised layer. During repeated thermal cycling cracks appear at
defects in the oxide layer are thought to be initiated from tensile stresses generated from local creep of the substrate. From room-temperature mechanical tests, oxide fracture initiates
at an applied bulk strain between 0:45% to 0:9%. The cracks generated being identified as
intergranular cracks that propagate to a depth just beyond the carburised zone.
The results of a micro-tensile test conducted at 500 °C, with in-situ observations conducted on
the outer oxide, confirm that a critical strain must be applied to the sample before cracking
occurs. The value of the critical strain reported at this temperature is between 0:06% to 0:1%,
i
ii
much lower than at ambient temperature, presumably due to the absence of high compressive
stresses that protect the oxide at lower temperatures. Uncertainty on the initiation strain
values are expected to be high because of complications posed by the experimental apparatus.
The combination of high temperature micro-scale testing, in conjunction with other analytical
techniques, appears the logical future avenue for investigating the mechanical properties of the
carburised layer in conditioned components.

Date of Award	23 Mar 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Matthew J Peel (Supervisor) & David M Knowles (Supervisor)