Steel Bainite Transformation
As a mixture of ferrite and carbide, and in most cases cementite, decomposed from austenite, bainite steels were originally discovered following the invention of isothermal heat treatment.
Bainite is transformed from austenite through a heat-treating process, and through this transformation it is possible to create its useful characteristics of relatively high strength with good formability and toughness.
Bainite in steels is usually referred to as a mixture of ferrite and carbide, in most cases cementite, decomposed from austenite. Discovery of bainite in steels was associated with the invention of isothermal heat treatment, which initiated many discoveries of decomposition of austenite in 1920-1930s. Hultgren in 1920, using isothermal heat treatment, discovered what he called “secondary ferrite” in a matrix of martensite. Robertson in 1929, published a detailed metallographic work, in which he demonstrated how decomposition of austenite proceeds at different temperatures during isothermal treatment. However, in this publication he did not discuss the time dependence. In 1930, Davenport and Bain improved the isothermal technique with efficient quenching, by using thin specimens, and published micrographs of partially transformed specimens. Their method later leads to the construction of time-temperature transformation (TTT) diagrams, which remains one of the most useful tools for steel research. As they observed units of bainite, they described the structure as “acicular, dark etching aggregate”, which is similar to pearlite and martensite in same steels. They called the structure “martensite-troostite”, since it etched in a way different from both martensite and troostite (fine pearlite). Later, it was reported that such structure is tougher than tempered martensite with the same hardness. This promising mechanical property soon inspired many works. In honour of Bain, his colleagues proposed the name “bainite” in 1934. The proposal was widely accepted after some years and the term “bainite” is used since.
Bainite is transformed from austenite through a heat-treating process. By heating steels or cast irons above their eutectoid reaction temperature, austenite can be obtained through the austenitizing process. When austenite cools down to a temperature below the eutectoid point, and then is held at a temperature below the pearlite transformation point, usually between 600°CC and 200°CC, austenite will start to decompose into bainite.
Bainite in steel is a useful structure because it has a relatively high strength with good formability and toughness. Many types of high-strength steels contain bainite with various carbon content; therefore, it is important to know the bainite transformation start temperature (Bs) with various carbon content.
Bainite Transformation Start Temperatures
The temperature at which bainite transformation starts is referred to as the Bs temperature, and several empirical equations that show the effect of alloying elements on Bs have been determined. Steven and Hayes established the following equation for Bs as a function of composition (in wt.%) for hardenable low-alloy steels containing from 0.1-0.55% carbon:
Bs(°CC)= 830 - 270(%C) - 90(%Mn) - 37(%Ni) – (%70Cr) – 83(%Mo)
For low-carbon bainitic steels, containing between 0.15 and 0.29%C, for high-temperature applications in the electric power industry. Bodnar et al. established the following equation, with composition of the alloying elements in wt%:
Bs(°CC)= 844 - 597(%C) - 63(%Mn) - 16(%Ni) – (%78Cr)
The microstructure of bainite is similar to that of martensite in steel. The shape of bainitic ferrite (BF) transfers from lath to plate with decreasing transformation temperature between Bs and the martensite transformation temperature (Ms). The bainite consisting of lath-shaped ferrite is called upper bainite (UB). Figure 1 shows the schematic image of the crystallographic and morphological features of UB. The orientation relationship between prior austenite and BF is near the Kurjumov-Sachs relationship. The prior austenite grain divides to some packets, which consists of parallel elongated ferrite laths. And these packets are subdivided to some blocks, which consist of BF laths having almost the same crystallographic orientation. These morphological and crystallographic features of UB are similar to those of lath martensite in steel. This conformity suggests that their transformation mechanisms are similar.
Figure 1: The schematic image of the morphological features of the upper bainite structure