The existing approaches to the detection of precipitation are largely based
on room temperature measurements on quenched samples. However, the direct
measurement of precipitation kinetics at high temperatures is also possible
through analysis of the mechanical properties of the specimens. In this wo
rk, a creep method was developed and applied to the detection of aluminum n
itride precipitation in a dual phase 3% silicon steel containing 0.038% C.
Prior to loading, the specimens were solution treated for 20 min and then c
ooled to the test temperature. A constant stress was applied to each sample
by means of a computerized MTS machine and the strain was recorded continu
ously during testing. Microstructural examination revealed that the austeni
te fraction and morphology and the microstructure of the ferrite matrix are
quite different depending on whether samples are directly heated or heated
and then cooled to the test temperature. The resulting creep rate is sensi
tive both to the occurrence of precipitation as well as to phase transforma
tion; when the microstructure remains fixed, the slope of the true strain-l
og(time) curve decreases immediately after the initiation of precipitation.
The precipitation-time-temperature diagrams determined in this way are of
classical C shape.