Metallographic diagram of Q235 carburizing treatment

Q235 Carbon Steel Heat Treatment Microstructure

Q235 is a low carbon structural steel with a carbon content of 0.14 % to 0.22 %. It has relatively low tensile strength and yield strength compared to other structural steels, but it has good ductility and toughness. Q235 steel is highly resistant to corrosion and possesses excellent weldability. Heat treatment is one of the most important components of steel microstructure and properties, and the carbon content of Q235 steel impacts the heat treatment process.

In order to produce desired microstructure and properties, different microstructures of Q235 steel can be obtained by careful and controlled heat treatment. The primary microstructure of Q235 before heat treatment is ferrite and pearlite. Its as-rolled microstructure is also roughly pearlite and ferrite. Heat treatment of Q235 steel can be conducted to produce various different types of heat-hardened microstructures, such as pearlite, martensite, bainite, and tempered martensite.

Heat treatment of Q235 steel involves heating and cooling processes. Most frequently, a quench and temper process is employed that involves austenitizing the steel, followed by a quench in oil or water and tempering in an oven at a controlled temperature. Heating the Q235 steel to an austenitizing temperature just above the normal eutectoid point of 727°C (1341°F) transforms the microstructure of the steel, while cooling at a rapid rate in a quenching medium quenches the ferrite-pearlite structure into a hard martensitic structure. Tempering then increases toughness and strength while minimizing brittleness. This process results in a hard, but ductile microstructure of tempered martensite.

Through heat treatment, the microstructure of Q235 steel can be tailored to meet various requirements. Microstructure of Q235 steel can also be tailored to create a compromise between strength and toughness for specific parts and applications, such as this Q235 shaft for use in a crankshaft. This requires a temperature-time combination of austenitization and tempering processes to induce bainite microstructure, which gives desired hardness and strength combined with improved ductility and toughness.

The carbide for heal treatments of Q235 steel is heavily dependent on time and temperature, as well as cooling medium used in the quenching process. In order to achieve desired mechanical properties, it is important that the heating process is precisely controlled and temperature-time relationships are closely monitored. Ultimately, it is the choice of pre-treatment and heat-treatment parameters, as well as the type of cooling medium and quenching temperature, which determine the final microstructure and ensure that desired Q235 steel mechanical properties are achieved.

Q235 is a medium carbon steel and contains 0.12–0.20% carbon, 0.30–0.70% manganese, 0.040% phosphorous, 0.045% sulfur and the silicon content is 0.30%. It has a moderately low ductility, good weldability and a relatively low tensile strength.

Q235 is commonly used in structural steel applications such as bridges, buildings, vehicles, and other structures because of its weldability and formability. The material can also be used for a variety of other applications such as energy, automotive components, racking systems, machinery and cutlery, kitchen sinks and storage tanks.

Carbon steels are sensitive to the occurrence of precipitation during welding and other thermal processes. The precipitation of metals such as copper and chromium, which are not normally present in Q235, can lead to severe corrosion if the steel is not properly heat treated or coated.

To prevent corrosion in Q235 steel, regular carbon steel is heat treated according to a low-hydrogen process and cooled very slowly the to improve its strength and ductility. A metallurgical inspection process known as a ‘carburizing’ process is also performed on Q235, where metal carbon is added to the steel surface to provide an additional layer of protection from corrosion, wearing, pitting and other forms of mechanical damage.

In addition to these processing methods, a metallurgist may also recommend using a steeping or ‘salt bath’ annealing technique to increase the carbon and grain size of the steel. This process is known as ‘carbonitriding’ and is typically used to produce a harder and more durable product. Finally, core microstructures can be analyzed with a process known as ‘electron magnification’ to ensure that the steel is functioning as expected.

To help protect the surface of Q235 steel, a ‘steady seepage’ galvanizing process can be used. In this process, the steel is drenched with a zinc-based alloy and then put into an oven. The zinc alloy will cover the steel with a thin deposit and prevent rust and corrosion.

By following these processes and treatments, a Q235 steel will maintain its strength and durability over time. It can endure the stresses of wear, strain and heavy usage, and is an ideal material choice for economic and reliable construction applications.