The Principle of Elevating Tempering Temperature of Electric Furnace Heat Treatment Tempered Brittle Quenched Steel

When tempering brittle quenched steel, with the increase of tempering temperature, its strength, toughness, and hardness usually increase. However, when plasticity is tempered within certain temperature ranges, the impact toughness of the steel not only does not increase, but also significantly decreases. This embrittlement phenomenon in tubular electric furnaces is called tempering brittleness. Tempering is generally not carried out at 250-350 degrees Celsius, as quenched steel undergoes temper brittleness during tempering within this temperature range. This type of temper brittleness is called low-temperature temper brittleness or the first type of temper brittleness. Once the first type of temper brittleness occurs, it cannot be eliminated, so tempering is generally not carried out within this temperature range in production.
This brittleness can be eliminated by rapid cooling after re tempering above the embrittlement temperature. Alloy steel containing elements such as chromium, manganese, and chromium nickel is quenched and tempered in the embrittlement temperature range (400-500 ℃). The brittleness produced by slow cooling through the embrittlement temperature range in a box type electric furnace after being returned to a higher temperature is called the second type of temper brittleness, also known as high-temperature temper brittleness. The reason for low temperature tempering brittleness in the box furnace is generally believed to be caused by the precipitation of carbide along the interface of martensite sheets or martensite strips in the form of discontinuous flakes. This hard and brittle flake carbide has weak bonding with martensite, which reduces the strength at the martensite grain boundary, so the impact toughness will decrease instead.

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