Student Number : 9800022T -
PhD thesis -
School of Mechanical, Industrial and Aeronautical Engineering -
Faculty of Engineering and the Built Environment / For many applications, it may be useful to be able to estimate creep
properties of a material from simpler testing procedures such as tensile tests
than the conventional creep testing procedures. Most alloys used for creep
service conditions are in a hardened condition and thus tertiary creep,
controlled by micro structural degradation, is dominant. The object of the
study was to investigate a reasonably simple method for estimating the creep
behavior of a low alloy 1% Cr, 0.25 % Mo steel from tensile yield data. The
study involved performing of series of investigations, including age hardening,
tensile and creep tests.
Microstructural degradation was monitored from specimens held in a furnace
for different times and temperatures, which were then tested in tension at
room temperatures. Tensile tests were carried out at different temperatures
and strain rates and the data used to determine material parameters for use
in kinetic equations describing deformation. For comparison, creep curves
were obtained from both creep tests and tensile tests results. Tests on
furnace aged specimens were used to quantify softening due to material
degradation and formulate a structure evolution and kinetic expressions used
to determine creep curves.
The modified equation by Dorn was used to determine the material parameters
and to predict flow characteristics. Two sets of mechanisms were observed. At
low temperature and high stress (above 550MPa) dislocation by glide
mechanism was investigated. At higher temperatures and low stress (below
550MPa), some form of power law creep was observed. Glide mechanism was
investigated and material parameters σ ) , n and activation volume v, were
calculated. The calculated value of σ ) was assumed for both plastic deformation
and the softening kinetics.
A reasonably good estimate of the creep behavior of the low alloy steel used
in this investigation in which tertiary creep dominates can be calculated from
tensile yield stress values. Furthermore, the creep rate and recovery have
similar stress dependences, with the stress and temperature dependence similar to that predicted by recovery theory. The value of activation energy
observed for creep for this alloy is in line with the processes which could be
related to self diffusion.
In order to justify the significance of this study, four existing empirical models
are discussed, highlighting their merits and demerits with respect to the models
used in this study. These are θ-Projection, Damage Mechanics, Estrin-Mecking
and the Internal Stress Methods. Generally, in this class of alloys, recovery
process occurs under an effective stress (i.e. an applied stress less the internal
stress). Thus the possibility of using tensile data obtained in this study in the
internals stress model was explored. The model could replicate the one used in
this study if the change in internal stress value o σ is assumed to be negligible.
This could be assumed to be true for tensile data at high stresses and low
temperature especially during secondary creep rate when the internal stress
approximates to the applied stress and at short test durations.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/2159 |
| Date | 28 February 2007 |
| Creators | Jamiru, Tamba |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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