Stress relaxation tests on a low alloy steel.
Date
2011-10-11
Authors
Mokolobate, Kenosi Norman
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Abstract
The deformation behaviour and dislocation substructure of crystalline solids is attributed
to the physical processes of dislocation passing obstacles. The energy needed to
overcome an obstacle, i.e. an impurity, atom, etc. is provided by activated dislocation
caused by either mechanical or thermally activated processes. To verify which
deformation mechanism is active in low alloy steel, tensile tests interrupted by stress
relaxation periods were studied. These tests were performed over temperatures ranging
from 288K to 873K and at constant strain rate.
Strain rates before and after stopping the cross-head and deformation mechanisms were
determined. The results of these experiments and how they correlate with the effects of
temperature were also discussed. Rather than interpreting the difference to be due to
mechanically activated deformation, as has been considered elsewhere, deformation was
considered to be thermally activated with a component responsible for hardening and a
component unaccompanied by hardening. Creep of low alloy ferrite steel is recovery
controlled. Recovery in this material was caused by complex structures of carbides
precipitates while dislocation tangles recovery was attributed both to carbide growth and
to tangled dislocations re-arrangement and annihilation. Mechanical activation of
dislocation glide has not been detected in these experiments.
Flow stresses were calculated from the strain rate differences together with the strains
estimated to be due to the hardening component of deformation using a rate equation that
describe thermally activated glide controlled deformation. Flow stress–strain curves that
were plotted were found to be independent of testing temperature. These curves were
compared to hardening curves determined previously from constant force creep tests and
were found to be similar.