Recommedation Tech |
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Air-cooled bainite steel |
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In
the late 1920's, J M Robertson first found
a mid-temperature transformation product named
bainite later. Bainite structure exhibits
good combination of strength with toughness.
However, the development of bainite steel
was limited due to the complicated technique
of isothermal treatment.
In the late1950's,
Pickering and Irvine developed an air-cooled
low carbon bainite steel of Mo-B series.
In the early 1970's,
Chinese Hongsheng Fang found that a certain
content of manganese may change the shape
of TTT-curve. Manganese has a special redistribution
rule in overcooling austenit transformation.
To illustrate this rule, Rα/γ was defined
as enrichment factor of manganese in α/γ inter-phase
and Rα/γ=XMn (interphase)/XMn(average), where
XMn (interphase) represents the manganese
content in α/γ interphase, and XMn (average)
is the average manganese content in alloy.
Correspondingly Rα and Rγ denote the enrichment
factors of manganese in ferrite and austenite
respectively.
The experimental
results of manganese enrichment factors in
certain Mn-B steel showed that no visible
difference in manganese concentration between
α and γ phases is found, while there is a
distinct enrichment in the α/γ interphase,
in accord with negligible partition local
equilibrium(NP-LE).
The concentration spike
of manganese is bound to cause pinning-up
effect on α/γ interphase movement, i.e, solute
drag effect, which greatly delays ferrite
growth. In addition, manganese enrichment
in the α/γ interphase results in the decrease
of carbon activities and activity gradient
in the austenite matrix around the α/γ interphase,
and the decrease of carbon diffusion rate
in austenite, and restrains ferrite growth,
that is so-called solute drag-like effect.
The solute drag and drag-like effects prompt
the formation of “bay"shape at about
600℃of γ→αtransformation curve, depress bainite
transformation temperature and driving force,
and refine bainite grain size.
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