Martensitic microstructure
Martensite structure consists of distorted crystals with high hardness and stress, formed by rapid cooling.
Physical-Chemical Mechanism of Action
The martensitic structure forms when a metal is cooled rapidly. The fast cooling prevents the complete rearrangement of the crystal lattice into a lower temperature zone. This results in highly disordered, lens-shaped or needle-shaped crystals with high internal stress.
Metallurgical Relevance & Welding Risks
Martensite is very hard and tough, which makes it suitable for special applications like tool steel. However, it is also prone to cracking and fracture because the high stress makes the metal less ductile. Too much martensite can lead to hot cracks and welding defects.
Shop Practice & Action Guidelines
To achieve the desired martensitic structure, it is important to control the cooling rate (t8/5) precisely. A t8/5 of 5 to 10 seconds is typical for a sufficient martensite content. If cooling is too fast (t8/5 < 5 s), too much martensite forms and the weld becomes glass-hard, promoting cracks. If cooling is too slow (t8/5 > 20 s), too much ferrite forms, which reduces hardness.