As it is quenched, the structure is martensite. Therefore, it is necessary to soften the steel to a predetermined hardness by tempering and to impart toughness at the same time. Since martensite is in an unstable state in which C is supersaturated as a solid solution, it tends to precipitate as carbide when heated at a low temperature. The precipitation of carbides softens the steel and at the same time improves its toughness. Heating also reduces the internal stress of the steel.
The precipitated carbides are very fine at first, but gradually become larger as the temperature rises. When martensite is tempered at 300-400°C, a structure called troostite is formed, and when further tempered at 500-600°C, the structure formed is called sorbite.
Troustite and sorbite are martensite that decomposes and precipitates extremely fine carbides. . Therefore, this is also called fine perlite. Tempering temperature is generally 550 to 650°C for structural steel, but low temperature tempering around 200°C is sometimes applied.
Hardness distribution after quenching is U-shaped, but after tempering, this U-curve generally becomes shallow and uniform. Figure 1 shows this situation. As can be seen from this figure, as far as the hardness is measured, it is difficult to recognize the effect of incomplete quenching. Care must be taken as this can make a big difference.
Kommentarer