Steel is one of the most extensively used alloys composed of iron and carbon and sometimes has other alloying elements for improved and enhanced quality for desired applications.
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The properties of steel can be improved in terms of fatigue, weldability, hardness, wear and corrosion resistance, ductility, and tensile strength by either alloy addition or heat treatment processes. This article discusses the effects of heat treatment on steel and the microstructural changes, the equipment and procedures used, and recent relevant studies.
Microstructure of Steel
The microstructure of steel is composed of grains, each with its own crystal lattice structure. Heat treatment subjects steel to controlled heating and cooling processes to alter its microstructure, resulting in changes to hardness, strength, toughness, and other mechanical properties.
The three primary phases of steel microstructure are ferrite, cementite, and austenite, and their transformations play a pivotal role in determining the material's final characteristics.
Heat Treatment Processes
Annealing is a heat treatment process designed to relieve internal stresses, improve machinability, and refine grain structure by heating steel to a critical temperature and then slowly cooling it with control, which allows a finer and more uniform grain structure formation.
In contrast to annealing, quenching is a rapid cooling process that involves immersing hot steel into a cooling medium, like water, oil, or polymer solutions, preventing equilibrium phase formations and resulting in a hardened microstructure characterized by martensite.
Sometimes, after quenching, when the requirement is to have steel with reduced brittleness and enhanced toughness while maintaining a certain hardness, another process called tempering is done by controlled reheating quenched steel to a temperature below its critical point, which decomposes martensite into ferrite and cementite microstructures.
Another heat treatment process similar to annealing is normalizing, which involves air cooling instead of controlled furnace cooling.
In the normalizing process, steel is heated to a critical temperature, held for a specific duration, and then allowed to cool in ambient air, refining the grain structure and enhancing its mechanical properties, making it particularly useful for improving its machinability.
An example of these heat treatment processes is covered in one study that investigated the microstructural changes in AISI 1050 steel under various heat treatments, including annealing, normalizing, and spheroidizing.
Results indicated that heat treatments influenced grain size, yielding altered mechanical properties. Annealing led to increased grain size, reduced strength, and enhanced ductility, while normalization resulted in a slight grain size increase with higher strength. Spheroidizing showed a significant decrease in grain size, yielding softer material with improved ductility.
Equipment to Treat and Analyze Steel Microstructures
The most obvious requirement for the heat treatment of steel is a furnace, usually designed to control the heating process in terms of temperature, humidity, and duration for steel to achieve desired mechanical and metallurgical properties.
Similarly, for analyzing microstructures of steel, several technologies can be implemented, including scanning electron microscopes (SEM), transmission electron microscopes (TEM), X-Ray Diffraction (XRD), and Differential Scanning Calorimeter (DSC).
For instance, a 2019 study used transmission electron microscopy and electron backscatter diffraction (EBSD) for microstructure analysis. The researchers focused on enhancing the microstructure of economical duplex stainless steel with and without tungsten (W) addition to achieve high tensile strength, elongation, and pitting resistance.
The study optimized the heat treatment process to control the comparison and distribution of the two phases (ferrite and austenite).
The highest product of tensile strength and elongation was achieved for Cr19 series duplex stainless steel following solution treatment for five minutes at 1050 °C. The microstructure analysis revealed an excellent transformation-induced plasticity (TRIP) effect, primarily attributed to the existence of a more unstable austenite phase.
Recent Developments
Microstructural Changes in EN353 Steel
In a 2023 study on EN353 grade steel, researchers investigated the impact of heat treatment and microstructural changes on steel properties.
The researchers predicted the continuous cooling transformation behavior of EN353 steel using JMat-Pro software, revealing phases like bainite, perlite, and carbide inclusions in microstructural examinations. Specifically, a specimen of size 40×40×40 mm underwent heat treatment at 870 °C for 2 hours, followed by isothermal heating at 600 °C for 73 minutes and air cooling.
The study addressed the challenges of achieving a fine pearlitic microstructure through normalizing alone, highlighting the necessity of combining isothermal heat treatment for optimal results.
This research has practical implications for industries relying on EN353 steel, as it enhances understanding and control of microstructural changes to improve mechanical properties for various applications, such as gear manufacturing.
Influence of Quenching Temperature on Proto-Austenite in Steel
In another 2023 study, researchers investigated the impact of heat treatment on the microstructure of high-vanadium (V) content quenched and tempered (Q&T) steel. They analyzed proto-austenite grains at different quenching temperatures utilizing metallographic and scanning electron microscopes and studied the precipitation behavior and matrix microstructure characteristics during tempering.
The results showed that higher quenching temperatures increased proto-austenite grain size, with a significant rise beyond 920 ℃.
An increase in quenching temperature also resulted in smaller precipitates, while higher tempering temperatures increased precipitate size. The optimal comprehensive mechanical properties were achieved when quenched at 920 ℃ for 1 hour and tempered at 630 ℃ for 1.5 hours, yielding a tensile strength of 1233 MPa and a low-temperature impact value of 64 J.
This study addresses critical factors influencing steel microstructures, crucial for applications such as mooring chain steel in marine engineering equipment.
Conclusion
The microstructural changes in steel under various heat treatments give rise to changes in the properties of steel.
Different heat treatment techniques, like annealing, normalizing, and tempering, are used for the heat treatment of steel, each having its unique advantage, allowing the steel to have the required properties in accordance with desired applications. These microstructural changes can be observed using analysis tools like SEM, TEM, XRD, and DMC.
More from AZoM: How Does Heat Treatment Change the Properties of Steel?
References and Further Reading
Jia, Y., Yin, X., Xu, Y., Wang, G. (2022). Effects of Heat Treatment on Microstructure and Mechanical Properties of a Transformation-Induced Plasticity-Aided Economical Duplex Stainless Steel. Metals. doi.org/10.3390/met12122019
Sharma, L., & Chaubey, S. K. (2023). To Study the Microstructural Evolution of EN353 Steel under Different Heat Treatment Conditions. Archives of Metallurgy and Materials. doi.org/10.24425/amm.2023.142418
Sivam, S. S. S., Loganathan, G. B., Umasekar, V. G., Saravanan, K., Kumar, P. S., Raja, S. (2019). Study on Microstructural Characteristics and Mechanical Behaviour of AISI1050 Steel under Various Heat Treatments. International Journal of Vehicle Structures and Systems. doi.org/10.4273/ijvss.11.1.04
Wang, T. (2023). Effect of Heat Treatment on Microstructure and Properties of Quenched and Tempered Steel with High Vanadium Contents. In Journal of Physics: Conference Series. IOP Publishing. doi.org/10.1088/1742-6596/2541/1/012056
Yang, D. P., Du, P. J., Wu, D., Yi, H. L. (2021). The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process. Journal of Materials Science & Technology. doi.org/10.1016/j.jmst.2020.10.032
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