A dissertation submitted to the Faculty of Engineering, University of the
Witwatersrand, Johannesburg, in fulfillment of the requirements for the
degree of Master of Science in Engineering
Johannesburg, 2018 / The use of austenitic stainless steels in harsh environments at elevated temperatures
has increasingly become a global problem, these alloys can fail unpredictably when
subjected to tensile stresses and chlorides. Hence the study was focused on
understanding the environmentally assisted cracking of Ru enriched laser alloyed layers on
304L stainless steel in a corrosive environment at elevated temperatures. The Ru
composition of laser alloyed samples was 0, 0.96, 1.96, 4.74 and 9.2 wt%.
Microstructural analysis and microhardness measurements were performed in order to
understand the grain orientation and resistance to indentation respectively. The bend beam
SCC test was conducted by stressing the samples to 350 MPa and exposing them to 50 ppm
sodium chloride with 10 ppm dissolved oxygen at 160°C for 172 hours. The results revealed
a significant improvement in the SCC resistance. The samples with lower Ru content (0,
0.98 and 1.96 wt%) were less susceptible to SCC when compared to as-received 304L
stainless steel. Cracks initiated from pits and propagated transgranularly on the alloyed layer.
The crack growth rate decreased as the Ru content was increased. The samples with 4.74 and
9.2 wt% Ru were immune to SCC. Electrochemical test results showed improved corrosion
resistance when the Ru level was increased to 1.96 wt%. Thereafter, there was a
gradual increase in corrosion rates for samples with 4.74 and 9.2 wt% Ru. However, these
corrosion rates were lower when compared to as-received 304L stainless steel. Another
SCC test was conducted to investigate fractography of vacuum remelted samples alloyed
with Ru. The results showed ductile failure for most of the samples and the maximum
stress threshold of 580 MPa was archived on samples with 1.07 wt% Ru. There was a
sudden increase in failure time, % elongation and % reduction in area when the Ru content
was increased to 1.07 wt%.
In essence, laser surface alloying 304L stainless steel with higher Ru content (more than
2wt%) improves SCC resistance, but does not improve the general corrosion
resistance, therefore a careful selection for any application is necessary. However, the
cost analysis revealed the laser surface alloying of 304L stainless steel with Ru to be more
efficient over other corrosion resistant materials. / MT 2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/25976 |
| Date | January 2018 |
| Creators | Tshilwane, Nick Nonofo |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xvi, 137 leaves), application/pdf, application/pdf |
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