1 Introduction
Bainitic steels are used in applications with high requirements on the mechanical properties, hence, detailed knowledge regarding the microstructure of bainite is a key aspect in order to optimize the mechanical properties for different applications.
The first stage of the bainitic transformation is the formation of acicular ferrite and whether this occurs through a diffusional[
1‐
6] or diffusionless[
7‐
12] transformation has been debated for a very long time. It is often referred to as “the bainite controversy”. According to the diffusionless hypothesis bainitic ferrite is formed through diffusional nucleation and diffusionless growth. The bainitic ferrite formed will thus, according to this hypothesis, be supersaturated with carbon from which carbides can form within the ferrite or if the formation of carbides is prevented by
e.g., a Si addition, the carbon will diffuse into the residual austenite.[
7,
8,
11] On the other hand, according to the diffusional hypothesis the first stage of the bainitic transformation occurs through the formation of thin bainitic ferrite plates that grow parallel to each other, constituting a packet,[
13‐
15] while the second stage can be regarded as an eutectoid transformation, which can be either cooperative or degenerate in its nature as discussed by Hillert,[
1,
16] Aronson[
17] and more recently by Yin
et al.[
13] The cooperative eutectoid growth is characterized by ferrite and cementite growing together forming a lamellar structure if the carbon content is high enough. They grow preferentially at an angle to the primary ferritic plate and, moreover, both ferrite and cementite are in contact with the parent austenite during growth.[
2] The degenerate eutectoid growth, on the other hand, results in cementite in-between the ferritic plates with the same growth direction as the ferrite.[
18] Degenerate growth of bainite is more common at higher temperatures and the cooperative growth becomes more dominant towards lower temperatures.
Bainite is generally divided into upper and lower bainite relating to the temperature it is formed at. The terms originate from Mehl[
19] who observed two different morphologies of bainite, feathery and acicular. Over the years the ways to distinguish between the two has varied depending on whether it is based on the ferrite or carbide morphology and several definitions have been proposed.[
20‐
23] The different definitions were recently reviewed by Furuhara.[
24] It has been known since long that the transition temperature between different types of bainite varies with the carbon content of the steel and also that there is no sharp transition related to the morphology. This was observed already by Pickering[
25] and has been discussed recently by Yin
et al. in relation to cooperative and degenerate eutectoid growth.[
15,
18]. Ohmori
et al.[
20] related upper and lower bainite to the ferrite morphology and could also observe gradual changes with temperature. The only definition indicating a sharp transition of the bainite morphology is the one by Mehl. Mehl’s description is based on the nucleation sites and specifies that upper bainite is related to intergranular nucleation, whereas lower bainite is related to intragranular nucleation.
It has been shown that the orientation relationship between the bainitic ferrite and the parent austenite follows the well-known Kurdjumov–Sachs (K–S), Greninger-Troiano (G–T) or Nishiyama–Wassermann (N–W) orientation relationships (ORs).[
26‐
31] This is similar to martensite even though the bainitic transformation is considered distinct from the martensitic transformation. Nonetheless, proponents for both hypotheses of bainite formation agree that bainite is formed through a displacive mechanism like martensite. For bainite, K–S is the most frequently reported OR and will be used as a reference also in this study. According to K–S there are 24 different possible crystallographic relations between the ferrite and the austenite, each called a variant, whereof each normally corresponds to one packet in the bainitic structure. These variants can be grouped into four close-packed plane groups referred to as CP-groups, where the variants in each group share the same parallel plane. Within each CP-group, six different relations between ferrite and austenite are possible considering the combination of three close-packed directions in FCC and two parallel close-packed planes in BCC, thus resulting in six different variants within each CP-group. Furthermore, the relation between neighboring variants is called variant pairing, and can be used to describe crystallographic aspects of the microstructure.[
32]
From works performed on both martensite and bainite it may be concluded that the carbon content and the transformation temperature have significant effects on the variant pairing.[
31‐
36] However, experimental studies on the effect of austempering temperature on the bainitic microstructure have mainly been conducted at high temperatures, above 400 °C, and/or on samples with a carbon content lower than 0.5 wt pct. The choice of low carbon steel and high austempering temperatures in previous studies can, at least partially, be explained by the effect of carbon on the martensite start temperature,
i.e., it is not possible to study the bainitic transformation at a low temperature in a low carbon steel without interference from the martensitic transformation. Hence, in the literature, studies on local crystallography, preferred variant pairing, and morphology over a wide temperature range in low alloy bainitic steels with higher carbon content is lacking. Therefore, we here perform such a study. The effect of temperature on the variant pairing for a low alloy medium carbon bainitic steel and its relation to the microstructural morphology as well as upper and lower bainite is investigated. The results are utilized to discuss the nature of the bainitic transformation regarding its diffusional/diffusionless and displacive character. The martensitic microstructure of the same alloy has also been included for comparison.
2 Experimental
The investigated material was received in soft annealed condition with the chemical composition given in Table
I. Samples of dimension 1.1 × 6 × 6 mm
3 were cut and subsequently austenitized in a tube-furnace at 880 °C for 20 min in a protective argon atmosphere. Directly after austenitization, austempering for one hour was conducted in a Bi-Sn metal bath at different temperatures from 275 °C to 450 °C to achieve a fully bainitic material followed by quenching in brine. Shorter durations of austempering were also applied in order to achieve samples partially transformed to bainite and fully martensitic samples were obtained by direct quenching in brine after austenitization. The martensite start temperature for the material has previously been determined with dilatometry to 259 °C.[
37] The transformation time for the formation of bainite was also determined with dilatometry, and all samples were confirmed to be fully transformed after 1 hour of isothermal holding.
Table I
Material Composition in Mass Pct
0.61 | 0.21 | 0.36 | 0.10 | 0.90 | max 0.05 | 0.0007 | 0.0090 | 0.0040 | 0.01 | bal. |
The microstructures were examined by light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Prior to microstructural characterization by SEM backscatter electron (BSE) imaging and EBSD, samples were mechanically polished until final polishing with 0.02 μm colloidal silica; whereas, for SEM secondary electron (SE) imaging and LOM, samples were etched with picric acid after the mechanical polishing. The SEM work was conducted in a field emission gun scanning electron microscope (FEG-SEM) JEOL JSM-7800F with a Bruker e-FLASH
HD EBSD attachment. The SEM was operated at 12 kV with a working distance (WD) of 7 mm for imaging, while for EBSD 12 kV, 20 mm WD and step size 30-50 nm were used. The EBSD post-processing was conducted using the QUANTAX CrystAlign software and MTEX, version 5.2.[
38,
39]
The methodology developed by Nyyssönen
et al.[
40‐
43] was applied for the orientation relation analysis. The method was originally developed for orientation analysis of martensite but can be directly applied to the bainitic structure, since both microstructures have similar orientation relation between parent and product phases,
i.e., the K–S relation.[
40] The algorithm applied uses the variant pairing definitions from Refs. [
32,
44,
45] To illustrate the combinations of variant pairing it is usually related to variant V1 according to Table
II. This relation can be extended to cover each of the four CP-groups, however, when presenting results, all CP-groups are summarized as if they would have been in the first CP-group
e.g., V1-V6 is thus equivalent to V7-V12 and V13-V18, etc. It can be noted that the most common relations between variants is usually within the same CP-group.[
33,
46] However, variant pairs belonging to different CP-groups are possible and can even be the most dominant variant pairing which is seen, for example, in high carbon martensite.[
33] In addition to the CP-groups, the variants may be grouped according to the Bain correspondence,
i.e. three groups, referred to as Bain groups.[
46] Variants within the same Bain group have a relatively low misorientation towards each other. Bain groups are also given in Table
III. To identify the variant boundaries, the fast multiscale clustering (FMC) method in MTEX was used. The FMC method made it possible to also distinguish the more diffuse variant boundaries at higher temperatures. The variant boundaries were imported into the graphical user interface developed by Nyyssönen to calculate the variant pairing.
Table II
The Variant Pairing Within One CP-Group Related to V1 and How the Different Combinations are Categorized[
47]
V1-V2 | V1–V2, V3–V4, V5–V6 | 0.186 |
V1-V3 or V1-V5 | V1–V3, V3–V5, V5–V1, V2–V4,V4–V6, V6–V2 | 0.228 |
V1-V4 | V1-V4, V2-V5, V3-V6 | 0.123 |
V1-V6 | V1–V6, V3–V2, V5–V4 | 0.049 |
Table III
Theoretically and Experimentally Determined Misorientation Angle Between Different Variants Pairs Related to V1 as Well as the Parallel Planes, CP-Groups, Bain Groups and Parallel Directions
V1 | (111)γ || (011)α | CP1 | 1 | [− 1 0 1]γ || [− 1 − 1 1]α | — | — | — | — | — | — | — | — |
V2 | 2 | [− 1 0 1]γ || [− 1 1 − 1]α | 60 | 60.2 | 60.2 | 60.2 | 60.2 | 60.2 | 60.2 | 60.1 |
V3 | 3 | [0 1 − 1]γ || [− 1 − 1 1]α | 60 | 60 | 60 | 60 | 60 | 60 | 60 | 59.9 |
V4 | 1 | [0 1 − 1]γ || [− 1 1 − 1]α | 10.5 | 5.8 | 5.6 | 5.3 | 5.5 | 5.4 | 5.5 | 7 |
V5 | 2 | [1 − 1 0]γ || [− 1 − 1 1]α | 60 | 60 | 60 | 60 | 60 | 60 | 60 | 59.9 |
V6 | | 3 | [1 − 1 0]γ || [− 1 1 − 1]α | 49.5 | 54.3 | 54.4 | 54.8 | 54.6 | 54.8 | 54.6 | 53.2 |
V7 | (1− 11)γ || (011)α | CP2 | 2 | [1 0 − 1]γ || [− 1 − 1 1]α | 49.5 | 51.1 | 50.6 | 50.9 | 51.2 | 51.5 | 51.6 | 51.9 |
V8 | 1 | [1 0 − 1]γ || [− 1 1 − 1]α | 10.5 | 9.8 | 10.3 | 10.2 | 9.8 | 9.6 | 9.3 | 8.6 |
V9 | 3 | [− 1 − 1 0]γ || [− 1 − 1 1]α | 50.5 | 52.2 | 52.2 | 52.4 | 52.4 | 52.5 | 52.6 | 52.1 |
V10 | 2 | [− 1 − 1 0]γ || [− 1 1 − 1]α | 50.5 | 51 | 50.8 | 50.9 | 51.1 | 51.2 | 51.4 | 51.6 |
V11 | 1 | [0 1 1]γ || [− 1 − 1 1]α | 14.9 | 13.3 | 13.5 | 13.3 | 13.1 | 12.9 | 12.7 | 12.9 |
V12 | 3 | [0 1 1]γ || [− 1 1 − 1]α | 57.2 | 57.8 | 57.7 | 57.5 | 57.7 | 57.7 | 57.9 | 58.6 |
V13 | (− 111)γ || (011)α | CP3 | 1 | [0 − 1 1]γ || [− 1 − 1 1]α | 14.9 | 13.3 | 13.5 | 13.3 | 13.1 | 12.9 | 12.7 | 12.9 |
V14 | 3 | [0 − 1 1]γ || [− 1 1 − 1]α | 50.5 | 51 | 50.8 | 50.9 | 51.1 | 51.2 | 51.4 | 51.6 |
V15 | 2 | [− 1 0 − 1]γ || [− 1 − 1 1]α | 57.2 | 56.5 | 56.2 | 56.2 | 56.4 | 56.6 | 56.8 | 57.4 |
V16 | 1 | [− 1 0 − 1]γ || [− 1 1 − 1]α | 20.6 | 16.6 | 16.6 | 16.3 | 16.3 | 16 | 15.9 | 16.8 |
V17 | 3 | [1 1 0]γ || [− 1 − 1 1]α | 51.7 | 51.1 | 51.1 | 51.1 | 51.1 | 51.2 | 51.5 | 51.7 |
V18 | 2 | [1 1 0]γ || [− 1 1 − 1]α | 47.1 | 51 | 50.8 | 51.2 | 51.2 | 51.5 | 51.5 | 50.9 |
V19 | (11− 1)γ || (011)α | CP4 | 3 | [− 1 1 0]γ || [− 1 − 1 1]α | 50.5 | 52.2 | 52.2 | 52.4 | 52.4 | 52.5 | 52.6 | 52.1 |
V20 | 2 | [− 1 1 0]γ || [− 1 1 − 1]α | 57.2 | 57.8 | 57.7 | 57.5 | 57.7 | 57.7 | 57.9 | 58.6 |
V21 | 1 | [0 − 1 − 1 ]γ || [− 1 − 1 1]α | 20.6 | 18.4 | 18.7 | 18.5 | 18.2 | 17.9 | 17.6 | 17.5 |
V22 | 3 | [0 − 1 − 1]γ || [− 1 1 − 1]α | 47.1 | 51 | 50.8 | 51.2 | 51.2 | 51.5 | 51.5 | 50.9 |
V23 | 2 | [1 0 1]γ || [− 1 − 1 1]α | 57.2 | 56.5 | 56.2 | 56.2 | 56.4 | 56.6 | 56.8 | 57.4 |
V24 | 1 | [1 0 1]γ || [− 1 1 − 1]α | 21.1 | 18.8 | 19 | 18.8 | 18.6 | 18.3 | 18 | 18.2 |
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