Quenching and tempering of steel | Heat treatment | Hardening | Martensite

In this video, we will cover quenching and tempering of steels as one of the most important heat treatments. Quenching and tempering is divided into a total of 3 steps. First, the steel is austenitized, i.e. transformed into the face-centered cubic austenite lattice, so that the carbon becomes completely soluble in the gamma iron. Subsequent rapid cooling, known as quenching, prevents the carbon from having time to diffuse out of the austenite lattice. In the process, the body-centered cubic unit cells of the ferrite (alpha-iron) are tetragonally expanded. A distorted lattice structure is formed, which is also known as martensite. Martensite is very hard and largely responsible for the increase in hardness and strength after quenching. A disadvantage is the high brittleness of martensite, so that the steel must then be heat treated again. For this purpose, the steel is heated in a third step at moderate temperatures. This process is called tempering. During this process, the martensite recedes somewhat by diffusion. This increases the toughness of the steel. If a high hardness is desired after tempering, the tempering temperature is selected to be relatively low. This is referred to as hardening - the steel is called hardened steel. If, on the other hand, the aim is to achieve high toughness combined with high strength, the tempering temperatures are selected to be higher - the steel is called quenched and tempered steel or sometimes just tempered steel.

00:00 Annealing process
01:01 Process steps
02:03 Difference to the annealing processes
02:46 Heating (austenitizing)
04:18 Quenching
05:45 Martensite: hard but brittle
06:53 Tempering
08:09 Tempering diagram
09:22 Stress-strain diagram
11:02 Prerequisites for the formation of martensite
13:18 Influence of alloying elements on martensite formation
15:24 Example