Steel grade 45 is a medium carbon steel that has a good balance of strength and ductility for many applications. This steel grade is often used for automotive components, axles and crankshaft components, plates, bars, and a variety of other structural and engineering components. The steel grade is also well suited for welding and parts that must be machined.
The microstructure of steel grade 45 is ferrite-pearlite, while the chromium content is 0.2 to 0.3 wt.%. The average carbon content of the steel is 0.45 wt.%, resulting in an average hardenability of HRC 32. Alloying elements in the steel are commonly controlled through the production process.
The normalizing heat treatment involves heating the steel to 830- 860 °C and cooling in the air. Normalizing reduces the stress caused by hot working, surface or local treatments and improves the properties of the steel. It increases its ductility, tensile strength, and impact properties, as well as prevents the formation of retained austenite. The normalized steel grade 45 can then be air cooled, or oil/water quenched to achieve desired hardness and increased strength.
The tempering treatment is typically applied after quench hardening in order to improve the toughness of the steel grade. Tempering involves tempering the steel to a lower temperature (500-650 °C) in order to reduce the hardness, reduce brittleness, and reduce the risk of cracking. The results of tempering will reduce the hardness and increase the yield strength of the steel grade.
The annealing treatment is applied to the steel grade 45 to soften the material and improve its machinability. This is achieved through heating the steel to a temperature (730-790 °C) and then air cooling. The heat helps to relieve stress and deformation, while the air cooling, often referred to as an ‘unknown anneal’, increases its formability.
The metalurgists have to consider various parameters such as the strength, ductility, hardness, and machinability of the steel grade before using it for various applications. A metalurgist would also require the microstructure along with details on composition and performance of the steel in order to decide the best course of action for a particular application. The microstructure of steel grade 45 is studied through a metallurgical technique known as optical microscopy.
Optical microscopy involves passing light rays through a specimen, with the aim of obtaining information about its composition and structure. The microscopic observation allows the metalurgist to obtain detailed images of the microstructure of the steel and identify the various phases present. In the case of steel grade 45, the optical microscope reveals the presence of ferrite, pearlite, and bainite. The detailed observations made through microscopes help metalurgists make the right decision for a particular application.
The microstructure of steel grade 45 can also be examined using a scanning electron microscope (SEM). This technique involves passing a high-energy electron beam through a specimen that has been coated with a thin film of conductive material. The SEM produces images of much higher resolution as compared to optical microscopy and also provides detailed information about the microstructure of the steel. This technique can be used to identify the presence of austenite, martensite, and various other phases in the microstructure of the steel grade.
For welding applications, the metalurgists would require the use of a higher carbon version of the steel which is the steel grade 45 (H). This version of the steel has a higher carbon content (0.45 to 0.55 wt.%) and is often used for applications such as hardfacing, welding and heavy-duty components. The higher carbon content also increases the microstructure complexity, making it a difficult steel to weld.
Steel grade 45 (H) can be tempered in order to improve its weldability and formability. This is achieved through heating the steel to a lower temperature (500-650 °C) and then air cooling. The tempering treatment reduces the hardness, brittleness and risk of cracking, while also improving its mechanical properties by increasing its ductility, tensile strength and impact properties. The result is a steel that can be more easily machined and welded.
In conclusion, steel grade 45 (H) is a medium carbon steel that has a good balance of strength and ductility for many applications. It can be normalized, quench hardened, tempered, and annealed in order to achieve desired properties. For weld-ability and formability, the steel is tempered in order to improve mechanical properties. The microstructure of the steel can be observed through optical and scanning electron microscopy.
