[Thesis]. Manchester, UK: The University of Manchester; 2016.
Laser cladding is a process that is used to improve the properties of a metal surface.
The properties in question may include hardness, wear-, corrosion- and/or fatigue-resistance.
The process involves fusing a thin layer of additional metal to the original surface,
using a laser as the heat source. Unfortunately, residual stresses are generated due
to the rapid and highly localised thermal expansion and contraction that occur during
the heating-melting-solidification-cooling cycle. These residual stresses can have
a detrimental effect on the final performance of the clad component, especially with
respect to corrosion resistance. Detrimental tensile residual stresses can be mitigated
through the use of post-processing techniques such as laser shock peening (LSP). LSP
is a process that uses a pulsed laser to generate intense spots of recoil pressure
on a surface, thereby introducing compressive residual stresses. Post weld heat treatment
(PWHT) is another process that could be also used in laser cladding in order to relieve
tensile residual stresses. In this work, laser cladding was carried out by depositing
a clad layer of AISI grade 316L stainless steel on to either a S275 steel substrate
or an AISI grade 316L stainless steel substrate, using different process parameters.
The hardness and residual stresses in the overlay and substrate were assessed for
each laser clad sample before and after being treated with LSP and PWHT. The corrosion
rate and microstructure were also assessed in each case. The novelty of this work
is two-fold. Firstly, to the author’s knowledge, it is the first study that attempts
to link process parameters to both the residual stresses and the corrosion performance
of austenitic stainless steel overlays deposited by laser cladding. The second novel
aspect is based on the application of both LSP and PWHT to the deposited overlay in
order to investigate whether an improvement in the mechanical properties and the corrosion
resistance can be realised. In this study, tensile residual stresses were generated
in the clad layers. However, the magnitude of the residual stresses did not appear
to be particularly sensitive to the deposition parameters. Indeed, it was found that
the number of layers that is deposited is more important than the choice of process
parameters. LSP was effective in reducing the tensile residual stresses and in fact
it introduced compressive stresses to all the samples that were treated. In contrast,
PWHT only led to satisfactory stress relief when the AISI grade 316L stainless steel
was deposited on to a matching substrate material. This was related to the fact that
a difference between the thermal expansion coefficients of the overlay and substrate
led to the development of significant tensile residual stresses on cooling down after
PWHT. The corrosion tests on the clad coupons led to the development of pits and cracks.
However, after LSP only pits were found, without any sign of cracking, for the test
durations that were investigated owing to the fact that compressive stresses were
generated. Similar results were found after PWHT for the clad samples in which the
overlay material matched the substrate material. However, signs of cracking were observed
after PWHT in samples where AISI grade 316L stainless steel was deposited on to an
S275 steel substrate due to tensile residual stresses remaining within the overlay.
This result suggests that there may be little benefit in carrying out PWHT for components
in which grade 316L stainless steel is deposited on to a steel substrate. In contrast,
there appear to be clear benefits associated with carrying out LSP in order to mitigate
the residual stresses and retard the onset of cracking.