Residual Carbon Content in Martensite
The residual carbon content in martensite determines its hardness and distortion. Too much carbon leads to a glass-hard, crack-prone structure.
Physico-Chemical Mechanism of Action
In martensite, carbon is forced into solution in the tetragonal lattice and causes distortion of the crystal lattice. The more carbon present, the stronger the lattice distortion and the harder the material becomes.
Metallurgical Relevance & Welding Risks
Too high a residual carbon content (above 0.8 %) leads to strong martensite with high hardness and low ductility. This can cause hot cracks and fracturing of the weld. Too low a carbon content (below 0.4 %) coarsens the microstructure and reduces hardness.
Workshop Practice & Instructions
To get good martensite, the residual carbon content should be between 0.6 % and 0.8 %. This can be achieved by proper cooling and selecting the right steel. When welding high-performance metals like HSS or martensitic steels, rapid cooling (t8/5 < 10 s) is essential to create a homogeneous martensite.