Tensile Behavior of Ti,Mo-added Low Carbon Steels with Interphase Precipitation

Abstract

Tensile behavior and structure-property relationship of ferritic steels with nano-sized carbide dispersion were invesigated using Ti-added steel and Ti,Mo-added low carbon steels. By austenitizing followed by isothermal heat treatment at 700°C, polygonal ferrites containing very fine carbides of TiC and (Ti,Mo)C were obtained in the Ti-added and the Ti,Mo-added steels, respectively. The size of such carbides was finer in the Ti,Mo-added steel than in the Ti-added steel at the same isothermal holding. The results of tensile tests for these samples showed that the strength is higher as the carbide size is smaller. The structure-based strength calculation led to a good agreement with the experiments, when it was assumed that the Ashby-Orowan mechanism is dominant for precipitation strengthening of nano-sized alloy carbides. It was also suggested that a relatively large tensile ductility is related to enhanced recovery during the tensile deformation, accompanied with promotion of secondary slips or cross slips in a finer scale due to the nano-sized particles.

A Review on Nano-Scale Precipitation in Steels

“…Orowan looping mechanism also does not mean a strength reduction, as Gutierrez-Urrutia and Raabe [27] and Sun et al [39] showed a great Orowan strengthening but at a great expense of ductility. Kamikawa et al [29] published a work on strength improvement through the Orowan looping mechanism without the loss of ductility. Yet, such strength increment (200-300 MPa) is minute when compared to the strength enhancement (1 GPa) by the precipitate shearing mechanism.…”

“…Again, no reduction of the uniform elongation was observed, suggesting a small increment (0.6 nm in diameter) in the size of precipitates while maintaining a high number density (in the order of 10 24 m −3 ) through nano-scale-co-precipitation can produce a large strength improvement (43% increase from 434 MPa to 622 MPa) at no expense of ductility. On the contrary, Ni3Ti [14] Cu + NiAl -4%Ni [1] Cu + NiAl -2.5%Ni [1] NiAl -3%Mn [2] NiAl -no Mn [2] NiAl [3] Cu + NiAl [15] Cu + (Ti,Mo)C [16] (Ti,Mo)C [16] TiC [29] (Ti,Mo)C [29] k carbide [7] k carbide [27] NiAl [39] (b) Figure 3. A compilation of (a) yield strength increment and (b) ductility change against precipitate size of various precipitate strengthened steels.…”

“…Different models that are based on chemical strengthening [2], modulus strengthening [2][3][4]6,33], coherency strengthening [2,6], and order strengthening [2,3] have been used to predict the interaction force between the dislocations and coherent precipitates in steels. On the other hand, models based on Orowan looping [27,39] or Asby-Orowan looping [29] are used to describe the dislocation interaction with incoherent precipitates.…”

“…A compilation of (a) yield strength increment and (b) ductility change against precipitate number density of various precipitate strengthened steels. TiC [40] Ni3Ti [14] Cu + NiAl -4%Ni [1] Cu + NiAl -2.5%Ni [1] NiAl -3%Mn [2] NiAl -no Mn [2] NiAl [3] Cu + NiAl [15] Cu + (Ti,Mo)C [16] (Ti,Mo)C [16] Cu + M2C [17] TiC [29] (Ti,Mo)C [29] Cu [33] K carbide [26] k carbide [7] Cu -no Ni [4] Cu -0.75%Ni [4] k carbide [27] NiAl [39] (a)…”

“…The strengthening of precipitate strengthened steels relies on the pinning of dislocations, mainly through two mechanisms, the precipitate shearing mechanism [1,3,4,23,33,41] and the Orowan looping mechanism [27,29,39]. Precipitate shearing mechanism occurs when the precipitates are small in size so that dislocations can cut through (shearable or penetrable) the precipitates easily, while the Orowan looping mechanism takes place when the precipitates are large and impenetrable by dislocations [42][43][44][45][46][47].…”