City Research Online

Abstract

Hot tensile tests have been performed on a variety of plain carbon and micro-alloyed steels, in order to determine the influence of such factors as phase transformation, grain size and precipitation on hot ductility. The γ-∝ phase transformation has been shown to produce a significant ductility trough in the high temperature tensile behaviour of plain C steels, and factors which alter the A3 temperature, such as cooling rate and C content, have been shown to produce a change in the temperature at which the ductility trough occurs. This ductility trough is due to strain concentration in the ferrite films surrounding the grains, leading to intergranular failure. It has also been shown that an increase in grain size can increase the depth and width of this ductility trough. For plain C steels with a C content of 0.35% or above, and for some micro-alloyed steels, a ductility trough may also be present in the single phase austenite region. For the plain C steel, this is. believed to be due to the increase in activation energy for deformation associated with increasing C contents. In microalloyed steels, the trough is due to the retardation of dynamic recrystallization associated with the presence of fine carbide and/or nitride precipitates, which allows intergranular cracks to develop. The depth and width of this ductility trough is primarily dependent on the size and amount of precipitates present, although it has been shown that grain size has a secondary effect. In C-Mn-Nb-Al steels, factors which tend to reduce hot ductility by reducing precipitate size and/or increasing the amount of precipitate present include the reheating of tensile samples cast 'in-situ', the introduction of temperature oscillations during COOling from solution temperature, and the presence of large amounts of'dynamic' precipitates formed during tensile testing. In C-Mn-V-Al steels, 'dynamic' precipitates do not have such an adverse effect. Strain rate was also shown to have an important influence on hot ductility, and decreasing strain rates have been Shown to reduce hot ductility in both plain carbon and micro-alloyed steels.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > Engineering

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