Redness - the property of metals to crack during hot pressure treatment ( forging , stamping , rolling ) in the temperature range of red or yellow heat (850–1150 ° C).
Reasons
Red fragility is mainly due to the distribution of certain impurities ( copper , sulfur ) along the grain boundaries of the metal. In the surface layer of steel containing more than 0.4-0.5% copper, local accumulations of structurally free copper sometimes form at high temperatures, as a result of which surface deformations and cracks can occur during metal deformation .
Red fragility is also observed in steel with a high sulfur content and a low manganese content. In this case, sulfur is found in steel not in the form of a relatively refractory manganese sulfide MnS , but in the form of iron sulfide FeS , which forms a eutectic with iron located at the grain boundaries. At 988 ° C, this eutectic melts, which disrupts the connection between the grains and, when deformed, causes cracking.
Elimination of Red Breakage
To weaken the harmful effects and eliminate red brittleness, elements ( aluminum , titanium , zirconium , manganese, etc.) are introduced into the steel, forming refractory sulfides . The concentration of copper at the grain boundaries can be prevented to some extent by alloying ( nickel , molybdenum , boron ).