High-strength bainitic steels have properties that give them advantages over conventional steels. This has facilitated the research on these materials for their application in several technological fields.
To study: Viewpoints on technological aspects of advanced high-strength bainitic steels. Image Credit: M. Kosal/Shutterstock.com
The concept of these materials has been revisited in a study published in Metals with the aim of highlighting the challenges and limitations of the materials and exploring other directions of research and development.
What are bainitic steels?
Conventional bainitic steels contain high levels of cementite in their microstructures. Bainitic steels are formed by the process of isothermal holding. Bainite steels have several morphologies including lower bainite, upper bainite, and granular bainite. Depending on the processing temperature, cementite occurs either inside the bainitic ferrite plates (lower bainite) or at their boundaries (upper bainite).
Granular bainite differs from lower and upper bainites. Unlike conventional bainites, the microstructure of granular bainite is devoid of the presence of carbide. The final microstructure of granular bainite contains both high carbon austenite and marstenite (which is harmful) in solid solution. With the addition of silicon, carbide-free bainitic steels that possess different morphologies and with higher carbon content can be made due to inhibition of cementite precipitation.
Secondary electron SEM images of Nital etched metallographic surface of: (a) Granular CFB obtained by continuous cooling (b) nanostructured bainite from 1C-2.5Si wt% steel (among other elements) treated at 250°C for 16 h (vs) nanostructured bainite from 0.7C-1.4Si wt% steel (among other elements) treated at 220°C for additional time. BF stands for bainitic ferrite. Image Credit: Morales-Rivas, L, Metals
Material properties comparable to those of quenched and tempered steel can be obtained when isothermal heating is used until complete bainitic transformation by inhibiting martensitic transformation during final cooling. Recent research has indicated that wetsuit can help heal microcracks induced by cementite precipitation during quenching.
High-strength nanostructured bainitic steels
These types of bainitic steels are also called super bainitic steels and low temperature bainitic steels. Based on silicon, they have a high tensile strength (over 2000 MPa) and an elongation of up to 20%. These steels contain large amounts of carbon and silicon. An advantage of these steels is the lower starting temperature for the bainitic conversion. The isothermal resistance results in a very fine microstructure, with nanometric subunits of bainitic ferrite.
In these steels, the bainitic phase is kept at room temperature because the remaining austenite is carbon-rich in solid solution. Therefore, upon final cooling, the martensitic transformation is impeded, creating a high strength steel compared to conventional bainitic morphologies.
Other alloying elements are used in the manufacture of nanoscale bainitic steels. These are manganese, chromium and molybdenum. Other elements such as vanadium and aluminum are used depending on manufacturing processes and applications.
The advantages of these elements include lower bainitic and martensitic starting temperatures, improved hardenability and minimized chemical segregation problems. The ultra-high strength values of these steels are due to the nano-size characteristics of bainitic ferrite. Additionally, the material has improved ductility due to the presence of residual austenite.
Mechanical performance of conventional and AHSS steels. Image Credit: Morales-Rivas, L, Metals
The article published in Metals studied these high-strength nanostructured bainitic steels. The main part of the article is divided into three parts which explore different bainitic carbide-free steels. These are high/medium carbon nanostructured bainitic steels formed at low temperature with isothermal behavior, medium/low carbon steels made into flat products and medium-low carbon steels generally made into long products, which are continuously formed. cooling.
The main objective of the study is to examine and highlight the main opportunities and limitations of advanced bainitic high strength steels. The study examines different alloying and microstructure strategies for the fabrication of high-strength bainitic steels.
A research direction highlighted in the study is the development of nanostructured bainitic bearing steels. This material has a surface layer of nanostructured bainite with the core made of tempered martensite. The nanocrystalline surface layer has a carburizing effect which gives the hardened steel superior surface properties.
The wear and rolling contact behaviors of this material are superior to conventional high carbon chromium steels, which typically contain coarse carbides in their microstructures. Nanostructured bainitic bearing steels have been proposed for bearing components, which are very demanding.
The study highlighted several other interesting research directions for high-strength nanostructured bainitic steels and constitutes an important addition to the knowledge base in this area of research.
Schematic illustration of advanced high-strength bainitic steels (inside the dotted blue line) as a function of C content and cross-sectional size of the part. Image Credit: Morales-Rivas, L, Metals
Future research directions
The authors indicated that future research directions should include the use of simple and standard chemical compositions as alloying elements for these steels. This would improve recyclability, which is more in line with the concept of circular economy. Fine-tuning the microstructure will greatly benefit material design.
Furthermore, the authors indicated that efforts should be made to reduce the mass carbon content, which may require the integration of thermochemical steps during manufacture. This is more viable for parts that have a smaller cross section. The development of thermally stable microstructures using continuous air cooling will be advantageous for both fabrication and assembly.
Morales-Rivas, L (2022) Viewpoints on technological aspects of advanced high-strength bainitic steels [online] Metals 12(2) 195 | mdpi.com. Available at: https://www.mdpi.com/2075-4701/12/2/195