Austenite to bainite phase transformation in the heat-affected zone of a high strength low alloy steel

Analysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0·1–1·9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and Ms temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the Ms temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter αm controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of Ms and αm, the volume fraction of retained austenite at room temperature has been calculated for Fe–C a...

Bainite and martensite start temperature calculated with exponential carbon dependence

The International Institute of Welding (IIW) microstructure classification scheme for ferrous weld metals has been investigated as a basis for the quantification of complex microstructures in steels. The aim has been to cover the full range of microstructures observed in plain carbon and low alloy steel products, as well as ferritic weld metals and parent plate heat affected zones. The mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials have been briefly reviewed. The classification and terminology used for intragranular as well as austenite grain boundary microstructural constituents have been considered, and also the way in which transformation products are orientated in space. Problems encountered in relating microstructural constituents to principal structures have been discussed in detail and solutions proposed. The microstructure classification and terminology...

http://mseresearch.persiangig.com/document/s1.pdf

Classification and quantification of microstructures in steels

http://www.chem.ucla.edu/~wunch/Chiou_Surface_Science_article.pdf

Structure and stability of Fe3C-cementite surfaces from first principles

We describe here the effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels that were processed as structural beams at three different cooling rates. Nb-microalloyed steels exhibited increase in yield strength with increase in cooling rate during processing. However, the increase in the yield strength was not accompanied by loss in toughness. The microstructure at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while at intermediate cooling rate besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite and lath-type ferrite. At higher cooling rate, predominantly, lath-type (acicular) or bainitic ferrite was obtained. The precipitation characteristics were similar at the three cooling rates investigated with precipitation occurring at grain boundaries, on dislocations, and in the ferrite matrix. The fine scale (∼8–12 nm) precipitates in the ferrite matrix were MC type of niobium carbides. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels with increase in cooling rate is related to the change in the microstructure from predominantly ferrite-pearlite to predominantly bainitic ferrite.

Effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels

The processes of upper bainite formation, the crystallographic aspects and the internal structure in a 1.83% silicon steel have been investigated. The T-T-T diagram is separated into two C-curves as in the case of steels containing strong carbide forming elements. In the early stage of upper bainitic transformation, very fine needlelike ferrite subunits with the parallelogram cross sections elongated in a γ // α direction form on a{111} γ sheet in a side-by-side fashion. In the later stage of transformation, these subunits are coalesced on a{111} γ plane and produces upper bainite laths. This bainite is always related to the parent austenite with the Kurdjumov-Sachs relationship and accompanies the surface reliefs. e carbide particles precipitate mainly after the ferrite subunits formation or after the coalescence of them. Therefore, the carbide precipitation is not a fundamental characteristic of bainite transformation but is a secondary effect. e carbide needles precipitating within a bainite lath by long time holding are aligned almost parallel to the α with almost constant intervals and are related to the ferrite by the Jack relationship. These aspects are exactly similar to those in tempered martensite. Such an e carbide configuration is thought to arise from the precipitation on the dislocations introduced during transformation.

Bainite Transformation in a Silicon Steel

Morphology of bainite and Widmanstatten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstatten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstatten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowB
 s and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.

https://link.springer.com/content/pdf/10.1007/BF02649046.pdf

Morphology of bainite and widmanstätten ferrite

Bainite transformation and the diffusional migration of bainite/austenite broad interfaces in Fe-9%Ni-C alloys

The formation mechanism of intermediate transformation products and the effects of B as well as carbide forming elements such as Cr, Nb and Ti on the isothermal transformation characteristics of low carbon 3 wt% Mn steels have been investigated and the following results have been obtained.(1) Widmanstatten ferrite and bainite exibit both the diffusional and the displacive aspects in their formation processes.(2) The addition of the above described elements markedly retards the formation of primary ferrite, Widmanstatten ferrite and bainite in the specimens directly quenched to isothermal transformation temperatures from 1200°C, while the precipitation of coarse carbide particles in austenite accelerates the fololowing transformations.(3) Ferrite nucleation on coarse carbide particles can be explained in terms of the decrease in the activation energy for nucleation by the lattice correspondence at the interface.(4) The coarse carbide particles precipitated at austenite grain boundaries are related to the ferrite enclosing these particles with Kurdjumov-Sachs relationship.

Effects of Small Amounts of B, Nb and Ti Additions on Nucleation and Growth Processes of Intermediate Transformation Products in Low Carbon 3% Mn Steels

The morphologies and the formation mechanisms of isothermally transformed Widmanstatten ferrite and bainite in various steels were investigated. Widmanstatten ferrite often grew from the grain boundary ferrite allotriomorph formed by a diffusional mechanism in the temperature range of the upper C-curve, whereas, bainite grew directly from an austenite grain boundary showing its own C-curve in a TIT diagram at temperatures between Bs (bainite starting temperature) and Ms (martensite starting temperature). Both structures accompanied well-defined surface reliefs of the invariant plane strain-type, and were in the shape of a lath or a plate consisting of several parallel needle-like ferrite subunits with parallelogram cross sc:ctions. The crystallographic properties of Widmanstatten ferrite and upper bainite were similar to those of lath martensite. Therefore, it was concluded that the difference between bainitic ferrite and Widmanstatten ferrite existed only in the nucleation events, where both structures grew by the same mechanism.

Yun Chul Jung

Papers

Bainite Transformation in a Silicon Steel

Morphology of bainite and widmanstätten ferrite

Bainite transformation and the diffusional migration of bainite/austenite broad interfaces in Fe-9%Ni-C alloys

Effects of Small Amounts of B, Nb and Ti Additions on Nucleation and Growth Processes of Intermediate Transformation Products in Low Carbon 3% Mn Steels

Morphology and growth process of bainitic ferrite in steels