Effect of Carbon Content on Bainite Transformation Start Temperature in Low Carbon Fe–9Ni–C Alloys

Abstract

Bainite in steel is an industrially useful structure. However, the controlling factor of its transformation start point is not known clearly. In this study, to clarify the effect of carbon content on the bainite transformation start temperature (B s ), we evaluated the dilatation curve and the microstructure in low carbon Fe-9Ni alloys. As a result, B s decreased with increasing of carbon content. Furthermore, the driving force of partitionless transformation from fcc to bcc at B s , which was calculated considering nickel segregation, was approximately constant at 400 J/mol in all alloys. This value is consistent with the driving force required for partitionless growth of ferrite, as reported in a previous study. This consistency suggests that B s depends on the martensitic growth behavior of lath-shaped ferrite, which is determined by the supercooling starting from the T 0 line.

Effect of Carbon Content on Bainite Transformation Start Temperature in Middle–High Carbon Fe–9Ni–C Alloys

“…2 correspond to B s . 3,4,6,13) These TTT diagrams indicate that B s in these alloys does not move or moves little with increasing carbon content. It differs from B s in low carbon alloys which Table 1.…”

“…It differs from B s in low carbon alloys which Table 1. Chemical compositions and T0 temperatures of the alloys used in the current and previous 4) studies. decrease with increasing carbon content.…”

“…Experimental Procedure Table 1 shows the chemical compositions of the nine Fe-9Ni alloys used. In a previous study, 4) we used alloys A, B, C, D, and E to evaluate B s in low carbon region. In this study, we used alloys F, G, H, and I, which contain 0.3-0.9 mass% carbon.…”

“…4) In this paper, we focus on middle-high carbon alloys. We defined that bainite, which transforms just below B s , contains a group of lath-shaped ferrites with/without cementite.…”

“…[3][4][5][6] Bhadeshia 3) proposed that B s is parallel and below T 0 , at which fcc and bcc systems of the same composition have the same free energy. In a previous study, 4) we confirmed this suggestion in Fe-9 mass%Ni alloys with 0.003-0.10 mass% carbon. This phenomenon suggests that B s depends on the driving force of partitionless transformation from fcc to bcc, which corresponds to the supercooling from T 0 .…”