Heat treatment influence on mechanical properties of structural steels

The engineering properties of the structural steel such as yielding point Fy or it’s hardness can be changed and modified by heat treatment process so that the properties can be specified to resist the desired operating conditions.

Heat treatment technique can be defined as changing the physical and mechanical properties of the structural steel by heating and cooling which occurs at extreme temperatures without any change in the shape of the metal.

Steel manufacturing

The process of Heat Treatment

Hardening

Hardening process includes keeping the steel at a suitable temperature where the temperature ensures that the whole core transforms into austenite which formed from pearlite, and then quickly cool in water or sometimes oil is used instead of water.

Tempering

In order to use the steel for structural purposes it has to reduce the brittleness, this property is gained in this stage. So, after cooling and quenching the steel in water in hardening stage, this quenched metal passes through another heating stage to gain the desired strength and hardness. However, higher tempering temperatures increase the ductility but reduce the hardness, and lower tempering temperatures increases the hardness properties but results in low ductility. In this stage accuracy should be given to the temperature.

Annealing

Structural steel must undergo large strains before it reaches the failure point. In this stage the steel which passed through the two processes above passes through another treatment by heating into high temperatures and then cooling it gradually slow to 20 °C. This process gives the metal high ductility but results into low toughness, which can be treated to gain the toughness properties by another step below.

Normalizing process

Normalizing is performed on structural steel to gain higher strength and hardness by heating the steel and keeping it at a constant high temperature for a certain period of time. This process mainly used when the steel needs to be subjected to cutting and machining.

Carburization and surface hardening process

In several projects for some engineering applications the steel required to be with a hard surface to adequate the erosion, for example in bridges projects which is built with foundations under water. In this case local austentitizing process is used and then rapid quenching with the presence of carbon the steel will gain hard surface properties.